Particulate Formation, Evolution, and Fate- Hope Michelson Lecture Day 1 Part 3

Summary

The script delves into the complex mechanisms of molecular weight growth and particle inception in flames, focusing on the Haka mechanism for modeling these processes. It introduces the concept of hydrogen abstraction and carbon addition, explaining how these reactions contribute to the formation of multi-ring hydrocarbons. The speaker also explores different inception mechanisms, contrasting thermodynamically driven nucleation with kinetically controlled covalent bond formation. The discussion includes recent theories and experimental evidence supporting the role of resonance-stabilized radicals (RSRs) in particle inception through radical chain reactions. The presentation aims to provide clarity on these intricate topics and stimulate further investigation into the underlying chemical processes.

Takeaways

• 🔬 The Haka mechanism, introduced by Michael Frenklach's group, is used to model molecular weight growth and surface growth, involving hydrogen abstraction and acetylene addition to hydrocarbon species.
• 🔍 The Gibbs free energy for the hydrogen abstraction carbon addition mechanism shows sequential growth from benzene to larger hydrocarbons like pyrene through the abstraction of hydrogens and addition of acetylene.
• 🔴 Molecular weight growth occurs at high temperatures in flames, leading to the simultaneous growth of various hydrocarbons, while inception is the process of gas-phase hydrocarbons clumping together to form particles.
• 🌡️ Inception is distinguished from molecular weight growth by the clumping of hydrocarbons into particles, which can be thermodynamically driven (like nucleation) or kinetically controlled (involving covalent bond formation).
• 🌟 The thermodynamically driven inception may involve large species, possibly larger than 11 aromatic rings, which could nucleate at flame temperatures, but their high carbon to hydrogen ratio and low concentration in flames make this less likely.
• 🚫 Kinetically controlled particle inception is favored at lower temperatures and involves reactions that form covalent bonds, leading to the growth of particles that cannot easily be vaporized.
• 🛑 The difference between dimerization and inception is highlighted, with dimerization being a step towards inception but not the complete process, which also involves continued growth and attraction of more species.
• 🧬 The potential role of resonance-stabilized radicals (RSRs) in particle inception is suggested, with the hypothesis that RSRs could lead to rapid polymerization and particle formation through radical chain reactions.
• 🧪 Pyrolysis experiments with ethylene and indene indicate that RSRs can seed particle formation at lower temperatures, supporting the hypothesis that RSRs are involved in the inception process.
• 📉 The mass spectrometry analysis of particles from flames and pyrolysis experiments show a distribution of species with varying carbon to hydrogen ratios, suggesting the presence of aliphatic character and potential branching in the particles.

Q & A

What is the Haka mechanism?

The Haka mechanism, introduced by Michael Frenklach's group, is a multi-step process used to model molecular weight growth and surface growth in flames. It involves the abstraction of a hydrogen atom from carbon, followed by the addition of acetylene, leading to the formation of larger hydrocarbon species.

How does molecular weight growth differ from inception in the context of flames?

Molecular weight growth refers to the process where hydrocarbons grow in size through chemical reactions at high temperatures in a flame. Inception, on the other hand, is the process where gas-phase hydrocarbons clump together to form particles.

What are the two main classes of inception mechanisms?

The two main classes of inception mechanisms are thermodynamically driven mechanisms, such as nucleation and condensation, and kinetically controlled mechanisms, which involve reactions that lead to covalent bond formation.

What is the significance of the carbon to hydrogen (C/H) ratio in understanding particle formation in flames?

The C/H ratio is significant as it helps determine the types of hydrocarbon species present in a flame and their potential to form particles. A high C/H ratio indicates larger, less volatile species that might be more prone to nucleation, while a lower ratio suggests smaller, more reactive species.

What is the role of dispersion forces in the thermodynamically driven inception mechanisms?

Dispersion forces, such as van der Waals forces, play a role in the thermodynamically driven inception mechanisms by providing an attractive force between nonpolar molecules, which can lead to the clumping of gas-phase species into droplets or particles.

How does the kinetically controlled mechanism differ from the thermodynamically driven mechanism in terms of particle formation?

In the kinetically controlled mechanism, particle formation is driven by reactions that lead to the covalent bonding of hydrocarbon species, creating a solid-like structure that cannot be easily vaporized. This is different from the thermodynamically driven mechanism, which relies on the physical clumping of species through dispersion forces.

What is the significance of the experiment where particles were extracted directly from a flame?

The experiment where particles were extracted directly from a flame provides valuable insights into the types of hydrocarbon species present in the particles and their growth patterns. It helps researchers understand the processes of molecular weight growth and inception by analyzing the mass spectrum of the extracted particles.

What evidence suggests that resonance-stabilized radicals (RSRs) may play a role in particle inception in flames?

Evidence from experiments, such as pyrolysis studies and mass spectrometry, show the presence of RSRs in the gas phase and their potential to react with closed-shell hydrocarbons like ethylene. This suggests that RSRs could be involved in radical chain reactions that lead to particle inception.

How do the concepts of molecular weight growth and inception relate to the formation of soot in flames?

Molecular weight growth and inception are precursor steps to soot formation in flames. As hydrocarbons grow in size through molecular weight growth and clump together through inception, they eventually form larger aggregates that can lead to the development of soot particles.

What challenges do researchers face in modeling the inception and growth of particles in flames?

Researchers face challenges such as the lack of kinetic data for the reactions involved in inception, the need to accurately represent the complex chemistry of hydrocarbon growth, and the difficulty in measuring the properties of incipient particles at the onset of formation.

Outlines

00:00

🔬 Introduction to Mechanisms and Concepts in Flame Chemistry

The speaker begins by introducing the topic of different types of mechanisms and concepts used in the study of flame chemistry. They discuss the Haka mechanism, which is used to model molecular weight growth and surface growth, and is attributed to Michael Franklock's group. The mechanism involves hydrogen abstraction and carbon addition, particularly in the presence of acetylene, a common species in flames. The process is illustrated with a stepwise chemical reaction that results in the formation of multi-ring hydrocarbons. The concept of molecular weight growth is differentiated from inception, where hydrocarbons clump together to form particles. The speaker uses an analogy of people joining a group at tables to explain the clumping process in inception.

05:01

🌡️ Exploring Molecular Weight Growth and Inception Mechanisms

The speaker delves into the two classes of inception mechanisms: thermodynamically driven nucleation and kinetically controlled covalent bond formation. The former is likened to water vapor forming droplets during humid conditions, while the latter is compared to the phase transition in baking a cake. The speaker references Hai Wong's paper from 2011, which outlines different types of mechanisms, including covalent addition and thermodynamically driven nucleation. The challenges in achieving nucleation at flame temperatures are discussed, along with the potential factors that could influence the clumping of gas-phase species.

10:03

🔍 Analyzing Hydrocarbon Growth and Inception Theories

The speaker continues to explore the theories behind hydrocarbon growth and inception, discussing the evidence and challenges associated with each. They mention the difficulty in observing certain mechanisms and the need to understand the conditions that lead to particle formation. The speaker also discusses experiments that extracted particles from flames and analyzed them using mass spectrometry, revealing interesting patterns in the mass spectrum that suggest the presence of larger, layered structures on particles. The potential for covalently binding different species to form particles is also considered.

15:04

🧩 Investigating Kinetically Controlled Particle Formation

The speaker focuses on the kinetically controlled mechanism for particle formation, which they believe to be a likely candidate for the inception process. They discuss a review paper by Martin Edel and Marcus Crafts that examines various mechanisms published in the literature. The speaker argues against certain mechanisms due to their slow chemistry and high carbon-to-hydrogen ratios, and supports the kinetically controlled mechanism as a more plausible explanation for particle inception in flames.

20:12

🔬 The Role of Chemistry in Slow Inception Processes

The speaker investigates why certain chemical processes are too slow to contribute to inception. They discuss the sequential growth of molecular weight through mechanisms like the Haka mechanism, which involves repeated addition and abstraction of hydrogen and carbon. The speaker points out that reactions involving stable species with closed shells, such as soot precursors, are relatively slow, which may hinder the inception process at lower temperatures where such reactions are necessary.

25:14

🌟 The Importance of Dimerization and Inception in Particle Formation

The speaker clarifies the difference between dimerization and inception, emphasizing that dimerization alone does not equate to inception. They discuss various mechanisms that have been proposed in the literature for chemical covalent particle inception, including radical reactions that generate new species and potentially lead to particle formation. The speaker also addresses misconceptions in modeling papers that confuse dimerization with inception and the importance of distinguishing between the two processes.

30:15

🔎 Examining Radical Species and Their Role in Particle Inception

The speaker explores the role of radical species in particle inception, referencing gas phase measurements of radical species in flames. They discuss experiments that extracted particles from flames and used collision-induced dissociation to analyze them, revealing the presence of rsrs (resonance-stabilized radicals). The speaker proposes a hypothesis that these rsrs could be involved in a chain reaction that leads to rapid polymerization and particle formation, suggesting a potential mechanism for inception.

35:15

🧪 Pyrolysis Experiments and Particle Formation

The speaker presents results from pyrolysis experiments that provide insights into particle formation. They discuss the use of smps (Scanning Mobility Particle Sizer) to measure particle size distributions at various temperatures and the addition of indene, a direct precursor to rsrs, to ethylene flames. The experiments show particle formation at lower temperatures with the addition of indene, supporting the hypothesis that rsrs may play a role in particle inception. The speaker also discusses mass spectrometry analysis that reveals the presence of rsrs in the particles.

40:18

📊 Mass Spectrometry Analysis of Propane and Propene Pyrolysis

The speaker analyzes mass spectrometry data from pyrolysis experiments involving propane and propene. They note the presence of aliphatic character and extra hydrogens, suggesting the formation of branched structures. The data shows distributions of rsrs at particle onset temperatures, with a significant peak at mass 165. The speaker also discusses the collapse of mass spectra peaks to the lowest mass as temperature increases, indicating a shift towards more saturated species. The findings suggest that radical chain reactions may be driving inception, and the speaker invites further questions and discussion on the topic.

Keywords

💡Hydrogen Abstraction C2H2 Addition (HACA)

HACA, which stands for Hydrogen Abstraction C2H2 Addition, is a chemical mechanism introduced by Michael J. P. Frankland's group. It is pivotal for modeling molecular weight growth and surface growth in flames. The process involves the abstraction of a hydrogen atom from a carbon molecule, followed by the addition of acetylene, leading to the formation of larger hydrocarbon species. In the script, HACA is discussed as a multi-step mechanism that significantly contributes to the growth of hydrocarbons in a flame, illustrating the complexity of chemical reactions in such environments.

💡Molecular Weight Growth

Molecular weight growth refers to the process by which hydrocarbon molecules increase in size, typically in high-temperature environments like flames. This concept is central to understanding the progression of hydrocarbon chemistry in combustion processes. The script explains that molecular weight growth occurs through mechanisms like HACA, where sequential addition of acetylene to hydrocarbon species results in the formation of multi-ring hydrocarbons, thereby increasing their molecular weight.

💡Inception

Inception in the context of the video refers to the initial stage of particle formation in a flame, where gas-phase hydrocarbons begin to clump together to form particles. This is a critical distinction from molecular weight growth, as it represents a transition from individual hydrocarbon growth to the formation of collective structures. The script discusses the difference between molecular weight growth and inception, highlighting the need to understand both processes for a comprehensive study of flame chemistry.

💡Thermodynamically Driven Nucleation

Thermodynamically driven nucleation is a process where particles form due to thermodynamic instability, similar to the condensation of water vapor into droplets during rainfall. In the script, this concept is explored as a potential mechanism for particle inception in flames, where hydrocarbon species clump together due to thermodynamic forces, such as dispersion forces or van der Waals forces, rather than forming covalent bonds.

💡Kinetically Controlled Reactions

Kinetically controlled reactions are those that are influenced by the rate of the chemical reaction rather than the thermodynamic favorability of the products. The script suggests that particle inception may be driven by kinetically controlled reactions that lead to covalent bond formation, resulting in the creation of stable, non-volatile particles that cannot easily revert to the gas phase.

💡Resonance-Stabilized Radicals (RSRs)

Resonance-stabilized radicals, or RSRs, are radicals that have their unpaired electron delocalized over multiple atoms, providing stability. In the script, RSRs are highlighted as potentially playing a significant role in the inception and growth of particles in flames. The presence of RSRs in the gas phase and their reactivity with other species, such as ethylene, is discussed as evidence supporting the hypothesis that RSRs may drive particle formation through radical chain reactions.

💡Pyrolysis

Pyrolysis is the thermal decomposition of a substance in the absence of oxygen. In the context of the video, pyrolysis experiments with ethylene and other hydrocarbons are conducted to study particle formation at different temperatures. The script describes how pyrolysis of hydrocarbons like ethylene and propene can lead to the formation of RSRs and the subsequent inception of particles, providing insights into the chemical pathways involved in particle formation.

💡Aromatic Hydrocarbons

Aromatic hydrocarbons, also known as polycyclic aromatic hydrocarbons (PAHs), are hydrocarbons containing fused aromatic rings. They are significant in the context of the video as they are intermediates in the formation of larger hydrocarbon structures and soot in flames. The script discusses the growth of aromatic hydrocarbons through mechanisms like HACA and their potential role in particle inception.

💡Carbon to Hydrogen Ratio

The carbon to hydrogen ratio is a measure of the relative amounts of carbon and hydrogen in a hydrocarbon molecule. It is an important parameter in understanding the composition and stability of hydrocarbon species. The script uses the carbon to hydrogen ratio to differentiate between possible mechanisms of particle inception, noting that certain mechanisms result in species with high carbon to hydrogen ratios that are not typically observed in flames.

💡Surface Growth

Surface growth refers to the process by which particles in a flame increase in size through the addition of hydrocarbon species to their surface. This concept is integral to understanding the development of soot particles in combustion processes. The script mentions surface growth in the context of the HACA mechanism, where acetylene adds to the surface of hydrocarbon radicals, leading to the formation of larger, more complex structures.

Highlights

Introduction to the Haka mechanism for modeling molecular weight growth and surface growth in flames.

Explanation of the hydrogen abstraction carbon addition (HACA) mechanism involving acetylene.

Gibbs free energy illustration for the HACA mechanism showing the growth of multi-ring hydrocarbons.

Differentiation between molecular weight growth and inception in flames.

Discussion on thermodynamically driven nucleation and condensation type mechanisms.

Kinetically controlled reactions leading to covalent bond formation in particle inception.

Review of different inception mechanisms and their plausibility based on Hai Wong's paper.

Analysis of the carbon to hydrogen ratio in particle formation and its implications.

Investigation into the role of radicals and their potential impact on particle inception.

Experiments extracting particles from flames and analyzing their mass spectra.

The Turkerbest hypothesis for particle inception involving radical chain reactions.

Evidence from pyrolysis experiments supporting the Turkerbest hypothesis.

Mass spectrometry analysis revealing the presence of RSRs in particle formation.

Theoretical and experimental challenges in verifying the Turkerbest hypothesis.

Observations of particle formation in propene and propine pyrolysis experiments.

The unique behavior of propargyl (propine) in particle formation despite higher barriers.

The presence of aliphatic character in the mass spectra of particles and its significance.

Final thoughts on the potential role of resonance-stabilized radicals in driving inception.

Transcripts

okay are we ready

okay so um

yeah so let's oh we've been talking

about different types of mechanisms or

you know what do we know what don't we

know

um let me just introduce

um some kind of like

um mechanisms and Concepts we've been

you know working with for a while and

um hakka the Haka mechanism you probably

heard about it it is the mechanism like

the main mechanism people have been

using to model

um molecular weight growth and to talk

about surface growth and all these and

it's a highly useful mechanism so um

this is a so this is a mechanism where

you like um it was introduced by Michael

franklock's group

um many years ago decades ago

um

and it basically is so is it the name of

it the acronym is hydrogen abstraction

the original paper was C2 H2 addition

which is acetylene Edition so you could

even call it hydrogen abstraction carbon

addition okay so

um it's a a multi-step but not that many

steps a couple step um mechanism where

you take a hydrogen off of a carbon

and then acetylene um as is a really

um uh is is one of the most

um concentrated species in a flame you

generate a lot of acetylene uh so

acetylene is usually floating around in

a flame so you once you abstract a

hydrogen from your carbon the acetylene

can attach to your species where you

abstracted the hydrogen where you have

that radical site

and add two carbons with the hydrogen

coming off so basically the chemical

reaction looks like something like your

whatever hydrocarbon species you have

plus a hydrogen you abstract it and you

end up with that radical

plus I said H2 right that radical the

acetylene and the Second Step plus an

acetylene attaches to that

um carbon and with the release of a an H

atom you end up

um in this case you generate a ring some

cases you just generate the the chain

and then a second acetylene comes in and

makes a six-membered ring so

um in this case you did the addition in

um one of these sites that's like a boat

okay so um so that's the basic uh Haka

mechanism

um this is uh a figure that shows kind

of like what you'd get for the Gibbs

free energy for this hydrogen

abstraction

um carbon Edition

mechanism where you'd actually showing

the growth from say a Benzene starting

from a Benzene and then sequentially

adding acetylene to the Benzene and

growing the species to Benzene and then

the two ring thing is anaphylene and

then you grow to

um

phenanthrene that three-ring structure

to a pyrene so just by sequentially

abstracting hydrogens and adding

acetylene you start to grow these these

multi-ring hydrocarbons okay so this is

molecular weight growth

um

this is not Inception right this is

molecular so this is like you you when

in a flame what you're you're if when

this is happening it's usually happening

at higher high temperature and you're

you're growing basically all of the

hydrocarbons

um at once right your your each of those

um say bent so you have a whole bunch of

benzene Rings each Benzene is growing

more and more rings so you have

basically distributed molecular weight

growth of the different gas phase

species in the flame so that's called

molecular weight growth

when you have Inception

now the difference between molecular

weight growth and Inception is

um

the Inception is when you take now all

those hydrocarbons and you Clump them

so you now are taking gas phase and

turning it into a particle

by like basically combining them all

together so it so you can kind of think

about it as

um say you're sitting out on the mall in

you know wherever you live Boston right

and you have a whole bunch of tables out

there and people are you know different

restaurants in the middle of summer it's

really fun okay let's say Paris and

you're and people are sitting out and

you know someone walks by and they join

your group then you're like so each each

little table is getting more and more

people just kind of like chatting and

having a nice time and then a street

performer comes out and everyone rushes

over to the street performer there's

your Inception okay so you're going from

growth of your individual little groups

too like like what is it that's causing

everyone to Clump to one spot okay

that's what you're trying to figure out

why is this happening so so we're trying

to figure out both how do you get

molecular weight growth and how you get

Inception

okay so so there are two classes of

inception mechanisms like you can think

of this as

um two kind of like

um ends and in between there are other

you know like can be a whole bunch of

other mechanisms but there too like

really distinct classes one is this um

thermodynamically driven like nucleation

condensation type of you know the same

way you'd get a droplet forming from

um when it rains when it's humid outside

and it starts to rain your water vapor

is coming up into droplets right that's

your thermodynamically driven type of

mechanism so what you're trying to

figure out is why are your gaseway

species clumping up into your droplets

and in the flame right

um what is causing them their bat not

they're kind of like water you you can

vaporize it back into a vapor right this

would be the same type of thing you're

just like physically binding their

electrostatic bonds

um you're not actually covalent

violently binding them with sharing

electrons between bonds right

um so we call that dispersed Bound by

dispersion forces

so dispersion forces like a van der

waals force between nonpolar

molecules okay so and then the other

class is um this kinetically controlled

type of reactions causing gum covalent

bond formation

um that in the end you have so you have

uh reactions that are making covalent

bonds so your particle at the end is now

covalently bound and you can't just

vaporize it it's more like the phase

transition that happens when you bake a

cake and you're going from a liquid to a

solid you're not going to go back to a

liquid right so this is like a very

different type of of mechanism so the

question is

what are we having kinetic or

thermodynamically driven

um

particle formation

so let's look at the different

um

uh types of mechanisms this is from Hai

Wong's paper is review paper this is a

great paper actually it's written in

2011 came out in 2011 from the

proceedings

um one of the review papers

um

here are three different types of

mechanisms

on the bottom an a is um uh you

basically have this covalently like

adding adding adding probably CO2 maybe

Haka type formation of these like curved

type of structure it's possibly going

into Bucky Balls or other fullering type

structures

and then in the second so in that one

we can pretty well say that it's not

happening it's too slow that mechanism

is too slow we don't see this type of

product normally and um the carbon

dehydrogen ratio for this type of one

particle formation is way too high so

carbon hydrogen ratio would be way way

over two and that's not what we see okay

the second mechanism is now our new are

the one I said was thermodynamically

driven you basically condense ph's or

something

into particles

so that's or nucleate you know we we

might call it nucleating those into

particles

um

the and

um

theory has shown that in order to get

this type of nucleation at flame

temperatures where you see Inception

it's going to take pretty large species

usually larger than 11 aromatic rings to

form this so the question is can that be

happening in the flame do we have

species large enough or is there

something else that's causing those

species to be less volatile so that they

want to

nucleate so and that could be something

like you know it doesn't have oxygen

does oxygen want to cause these things

to be attracted to each other is it

aliphatic side chains is causing them to

be attractive are they radicals are the

radicals the ones that are want to be

attracted so there's a ton of work in

this area trying to figure out what's

going on there

so in order to have something that's

larger if we just have plain

pahs that are kind of nucleating so if

you go back to our stabilimer grid this

would be way over so I know remember I

put the number of six member rings at

the top in blue this would be way over

to the right hand side of our figure

here our chart and that has a really

high carbon to hydrogen ratio these are

large species and we actually don't even

see them in Flames normally right so so

this indicates this doesn't mean that it

doesn't happen but it indicates at least

under atmospheric pressure it's not

happening

um so

um the carbon hydrogen ratio is greater

than 2.4 as probably not the main

mechanism for shift formation for

Inception

okay

so

um

in addition as you go let's see

as you go up

um

The Binding so this is why people think

it's over that over larger larger than

11 Rings why the big you know part

particle masses are good for this type

of mechanism as you go up in number of

aromatic Rings The Binding energy

increases so you basically have more

attraction between these these molecules

so that's on the top curve plot on the

on the bottom plot

we see

um the uh

the

red curve is The Binding sublimation

temperature and that increases because

these things are attracted to each other

right this is thermodynamics because the

larger species are more attracted to

each other The Binding energy is higher

than the sublimation temperature is

going to be higher okay or the

vaporization temperature is going to be

higher

okay

um

so that increases with the number of

carbon atoms

um and but on the other hand the

concentration of these species for each

additional ring you add it decreases

exponentially

so these larger species are just not as

concentrated in the flame so even if you

could you somehow get one of these these

two things together they're not going to

find each other very well and they're

probably not going to be responsible at

least this mechan is not going to be

responsible fully responsible for

Inception

okay

so

um

is so here's a figure if you've been

studying so you've probably seen this

figure multiple times this is an

experiment where they sucked the

particles right out of the flame right

um and they did the mess back on the

particles themselves and they see this

interesting these interesting humps and

the Mass Spectrum of the particle

and it kind of indicates that you have

on the left hand side you have all these

like what we would typically see if

you're going to vaporize the particle

you see like all those Peaks

and then you see like almost like you're

adding layers onto the onto the particle

this doesn't mean that you're seeing all

those individual

um

um sheets in your Mass Spectrum you're

actually adding them onto the particles

this is seeing the entire particle

itself so this is a really interesting

experiment

um but I'm not sure if I've seen it

reproduced

um it would be really nice if someone

reproduced it and and delved into what

was going on

um it's a really interesting experiment

uh so this is the the same group and

what they saw is if they ionize so on

the left hand

um side a graph this is ionizing with um

248 nanometers and a Mass Spectrum from

the particle so the particle and the

ionize the 248 nanometers and then they

see actually what what you normally see

in a Mass Spectrum aerosol Mass Spectrum

or laser desorption ionization Mass

Spectrum where you collect the sample

and then you like vaporize it somehow

and then ionize that's very typical of

what you see but if you ionize it a

shorter wavelength where you might be

actually more able to ionize the

particle you actually see these larger

masses and I think that's because you're

actually starting to see the entire

you're seeing the entire particle almost

like grow between and it would be nice

if you could have these data at

different heights in the flame like this

would be a really cool experiment but it

I I don't they didn't seem to do that

measurement yeah

okay so

um

so let's go back to this guy

um so okay so this one isn't we don't

think is is happening

um and this one may be happening but we

need to figure out a way that it could

be happening but it you know our data

seem to indicate that it's not

um and this one

um at the top this is covalently binding

um different so this is the kinetic

mechanism the kinetically controlled

mechanism so that one

may be happening and

um and we think that it probably is at

least I do so this is I'm going to say

like this is this is Hope's Theory

um that um we we're gonna that it's it I

think it's a kinetically control room

okay so

um because I think the other ones seem

to have a lot of issues and we have a

lot of evidence that this is what's

happening so let me just go into a

little bit more detail on that

okay so here's another paper a review

paper that just came out last year by um

Martin edel

this um Marcus crafts group kind of

going into all the different mechanisms

that have been published in the

literature okay so

um so they're they went there are a

whole bunch of them so

um the top one we just talked about

that's the nucleation one it seems like

the Carbon High generation is too large

um and the density is too low for these

species to be large species to be around

unless there's some other way to make

them more less volatile

um and then the second one to the right

is the the kinetically controlled one

that um that I'm voting for okay so

um the ones over to the left

um those are just too slow like the

bottom left one is the one that was I

showed you from Hai Wong's paper

um the first one we talked about that

you know make the curve you know

probably through a Haka type mechanism

um we think that's just too slow

um and these other ones the carbon to

hydrogen ratio if you get species that

are like huge your carbon hydrogen ratio

is just going to be too large right we

just don't see that in a flame

um okay so uh

so let's talk about the now the ones

over to the uh lower right

so where it says c p a h that means

curved pah so the fpah up at the top

right that means flat pah

the so so the question is if you have a

curved pah can you have a dipole moment

because your pah is curved

so how do you think you could get a

curved pah

any ideas

I know you know this

because you've looked at a soccer ball

before

a football

yes you have five membered rings if you

embed yes thank you what was your name

again Tanner

Dalton okay don't okay I am it's okay

I'm gonna remember Dalton yes

I should remember that that's a good

name

um okay so so you're gonna as as soon as

you embed a five-membered ring you can

get curvature right like you look at a

soccer ball it has five membered rings

embedded in all that those six-membered

Rings right okay

um so so here's the theory you have like

a dipole moment associated with this

curvature

um and here's a theoretical study

um that you know kind of assesses what

is The Binding energy for these

different

um molecules whether they're flat or

curved okay so they went through and let

me just point out a couple of them so

this one's corny and that's one's flat

okay and then

um this one is coranuline and that one

is curved

um they're not exactly the same size but

they're close

um and you notice that actually

curvature doesn't seem to have a big

effect even though even though they

calculate that the dipole moment for

karanuline should be comparable to water

um and higher than for chlorine so well

then you know it was a good idea but it

didn't actually work so it turns out

that um

that The Binding energy is is similar

for the two

um and uh and it just didn't help on top

of the fact that when we do the

experiments when we look at the mass for

coranuline and chlorinine we actually

don't see karanuline or chlorine I don't

know what they actually are but they're

not those species so they're not the

stabilimers yes

so why do you think that we do have

kinetics or don't have kinetics

kinetically

yes

yes

I yeah I think that it is kinetic I

think it's chemistry

right

why is the chemistry so oh here's why

the chemistry is slow is because

um uh for okay let's go back to the one

where I said chemistry is too slow for

that

yeah that's a good question

um this one at the bottom left like why

you mean why is that chemistry too slow

okay look at that particle that you're

actually forming on the right hand side

if you say okay we want to get to the

point where I have a particle

that chemistry that would cause that

formation is probably something like the

Haka mechanism

okay so now you're taking a molecule and

you're repeatedly so now instead of

doing like you're repeatedly adding

you're trying you're extracting a

hydrogen you're adding a carbon you're

attraction hydrogen adding a carbon

stretch you know so you're repeatedly

doing this

um so in order for that to so you have a

whole bunch of steps is causing that to

happen we call that sequential growth

molecular weight growth

um on top of the fact that a lot of the

species that are like have these like

closed um shells six-membered rings are

pretty stable so to get over a barrier

the barriers are relatively High to get

over the barrier the reactions are

relatively slow

where you have should Inception

temperatures

this will this like these are these um

the soot uh these Hawker type mechanisms

are faster at high temperatures

uh this is after where you have

Inception

it's lower yeah where you see Inception

it's lower temperatures

not not the high not like 2000 Kelvin or

1500 Kelvin yeah yeah yeah it's right

it's like in that high temperature

regime

yeah so it's it you need to be faster at

lower temperatures

an Inception yeah okay yeah good

question that's an excellent question

okay so so we think this is not

happening

um the density is too low for these to

begin with and

um the carbon hydrogen ratio is just too

high and and they the people who did

this you know Marcus's group

um concluded that like I'm not

judging their work they they actually

conclude made that conclusion

okay

um so

um and I want to okay here so

um

so uh okay so here's another set they

had in that same paper right

um the upper left hand corner with

polyline

um so you basically have

um a whole bunch of

um I guess settling like lumping

together type of mechanism that that we

don't see

um it's that's just not observed because

so we can rule that guy out

um

the bottom two left ones are a

kinetically controlled type mechanisms I

think those are possible

um the um

uh the upper the those three that on the

right those are not Inception mechanisms

those are dimerization mechanisms so yes

those you could have dimerization

mechanisms but that's not the same as

Inception Inception would be dimer plus

growth plus you know plus you know it's

it's um it's kind of like you're sitting

at your tables and and okay two two

groups actually

um notice each other and go hey look

who's over there and then they get

together that doesn't mean everyone else

is going to come around and get together

with them right so so you have to be

careful when someone says they see

dimerization that does not unless they

have a follow-on mechanism that says oh

we're going to keep growing we're going

to keep sucking in all the carbons

be careful because people sometimes

confuse dimerization with Inception

okay and you see this over and over and

over and over again in modeling papers

like we do Inception by pyrene

dimerization

so you just have to be very careful with

with like think be be look at it

carefully when you see that

okay so um and then the right hand is a

possible the right hand is basically the

kinetically controlled for if it were

smaller species okay so let's let's um

talk a little bit more about that

um okay we're back to here our two kind

of classes of mechanisms

so I think that here on the left hand

side the species are too small to

condense or nucleate or whatever you

however you want to say it

um at these Inception temperatures

actually Inception temperatures I think

this 1400 to 1700 Kelvin comes from

measurements and I think those are

really those are really hard

measurements

how do you know when the particles have

been when you get Inception you have to

be able to measure the incipient

particles and that's a really hard

measurement to make so I think actually

it's at low much lower temperatures

where you see particle Inception

um so that's another thing figure out

how to measure incipient particles

um uh and then um reactions between

stable precursors are too slow

um and remember dimerization is not the

same as Inception okay

so what are the what are the

possibilities now let's go back in the

literature and see what other people

what people have proposed for

um chemical

um covalent

um particle Inception okay here's a nice

mechanism where you start out with those

rsrs up at the upper left-hand Corners

cycle pentadieneal and

um

into Neil right those have radical so

it's radical radical reactions generate

another species and then that continues

to react and in in the process you end

up generating a whole bunch of different

radicals

um and this one this is this is

molecular weight growth but you can

imagine that maybe that mechanism could

keep going but in the end what you end

up with here is

um two closed shell very stable species

that are probably not all that reactive

right so now you've hit a dead end right

so now you have to figure out how to

excite those again they're not they're

not prone to be like they're not in the

configuration to generate an rsr so

these are hard to like activate again to

get them going

okay

um

so here's another mechanism again

um we're starting with naphthalene

pretty stable species you know

generating a radical out of it reacting

with a snaffling

um and then generating but now you can

see like maybe in this case you can

regenerate a radical

um it's not an an rsr but but maybe this

is a way to keep the reaction going if

you have you end up with a radical

product right okay so um so this is

another potential Inception mechanism

um here's a mechanism that was just

published recently this comes from Mani

sarathi's group this is published like

last week I think so it's not in your

notes because I added it because it

wasn't you know just came out

um so uh so this is pyrene dimerization

um so it's it's like and

um so on the upper left hand

um Cur uh is a is a mechanism for

physical powering dimerization what

people talk about all the time is is

pyrene nucleating which we know doesn't

happen like it's not nucleating in a

flame at these temperatures

um it's just not thermodynamically

stable the upper right hand one is a

mechanism that was proposed by

um Andrea Diana's group

um with a

propaneal so a radical Pro sorry radical

parine perineal

physically

dimerizing with a pyrene so there's an

attraction because one of them is a

radical right so that that's a little

bit that's hard to do because it's

pretty stable to pull off a hydrogen but

if you're a high enough temperature

maybe you have it happen enough times

and then

um Mani sarathi's group proposes that

you generate this you know attraction

and then you have enough time for them

to react so now you have a covalently

bound pyrene parine type of dimer

so can you think of any reason why this

is not

a really important mechanism as a

formation

just based on what I've told you

[Music]

yeah and and

um what's one reason why it wouldn't

um react a lot when

yeah and you actually have to have

pyrene rather than fluoranthine too

right like especially if you like we

might have some priorine but not a lot

of pyrene if you if a lot of us

floranthem

um but it may be that at some you know

some of them may like do that and you

end up with a dimer right so this isn't

something we should rule out the fact is

that we don't even see Mass 404 in our

Mass Spectra so we don't see this dimer

but you know it's possible that it can

happen but um but you're right I think

that the fact that you're not seeing not

seeing a lot of the pyrene you're not

you know it and it's also pretty stable

Coast shell very stable molecule

so but it's possible that it does happen

yeah

okay

um

and then oh yeah this is what they and

it turns out that when they do the

calculations it happens at pretty high

temperature if it happens it happens at

pretty high temperature

um so you're like but not super high

right 1100 Kelvin you could that's

probably not too far from the Inception

region right so you know it's possible

that it does happen but

okay

so rsrs so these are gas phase

measurements of radical species of of

species in a flame and you see there are

a couple of rsrs here in the gas phase

so you actually can see them in the gas

phase they're there they're there so the

question is how important are they in in

formation and should we start to think

about how to to incorporate them okay so

here's your fluorineal and you have

endoneal so fluorineal is as we see we

see this Mass huge like it's huge for us

165 we all we see it all the time and

it's huge and this is one of the main

Peaks that other people see so if you're

looking for a mechanism if you're trying

to think of something this is a good

mask to start with like what's happening

with this mask why do we always see it

and what's it doing

okay

um

and this is an experiment that was done

by Niels Hansen's group

um where they extracted from a flame

um I know I'm just throwing a lot of

data at you I so like absorb as much as

you can and start thinking about how to

put it all together and make a story

because that's where we are right now

um so this from Niels Hansen's group

where they extract it from a flame

and did

um this Mass Spec where they did

um Collision induced dissociation so

they they basically extracted from a

flame

um uh ionized and then slammed their

ions into the gas gas gas in a gas cell

and they could

um control how fast you know how much

kinetic energy they used to slam their

molecules into the gas phase

so here they did one to five EV

Collision energy so I was

um somewhere in the range of one to five

EV of energy of the ions that they

generated slamming into these um uh

argon or whatever gas the camera which

gas they used in this gas cell and you

can see a whole bunch of Peaks so this

is relatively low energy Collision

energy but they they saw a whole bunch

of Peaks some of those Peaks were rsrs

I've labeled them okay they're in the

dark the darker ones the blue ones

if they used higher Collision Energies

boom a lot of those other Peaks went

away and look what happened the rsrs

popped out right

what does that mean to you

have any ideas

I know slightly in the day it's been

many hours of people talking at you

but if you're if you're if you saw the

these this result like you slammed your

your molecules into these gas and you

saw like a lot of the piece went away

but these ones stuck

okay were you yawning or raising your

hand

okay so just think about it say say you

had like

um two benzenes stuck together

um and they were Mass what

141 like 156 right and then

um low energy they're 156 and then you

slam them harder into

um an argon and boom

right then you'd basically get fennel I

should have done an rsr okay an rsr like

psychopended psychopendidino right

um two C5 uh so that's what 65 Mass 65

two of them 130 140. okay so you see

Mass 40 in your Mass Spectrum you know

low energy you slam into it nothing

happens high energy boom and you break

apart and now you have cyclopedicineal

it kind of so the way they interpreted

the results is it indicated that what

they had in the particles they extracted

were bound

um species that looked like when they

broke apart it they could they could had

larger they had larger curves of

covalently bound things that they could

break apart into

um rsrs

um so they concluded that they had a lot

of like

um bound you know covalently linked

species of rsrs

or actually I think they might have said

other things but I interpret it as rsrs

so I think there are as ours um but they

um interpret as bridged and and branched

things okay that broke apart when the

energy was high enough

okay so there's more evidence that you

have rsrs that are bound together in

these particles okay

so what's the mechanism that would allow

you to do this so here's a proposal

and this is just a hypothesis at this

point this is a paper we published a few

years ago

um where we said okay we have we see all

these rsrs what we think might be

happening is I we have an rsr it reacts

with something and this is an example

just we did fennel but we did a whole

bunch of other like types of examples

and

um in the paper

um

but this uh um so in denial is an rsr

reacts with phenol

um it generates this adduct

um this adduct

um so remember how you stabilize why

aromatics are stable is because they

have they are able they have to have to

share their electron density with with

um they're with bonds with double bonds

the pi orbitals and double bonds right

so that stabilizes and make these things

really like Rock Solid you know it's

hard to break them apart okay so in the

middle here

um

in the at the attic between the um in

denial and the fennel

um is a bond that is not

um is what we call C CP3 so it's a

carbon with three atoms on it so it's

singly bonded to two carbons and two

hydrogens

if you pull off a hydrogen now you're

going to have a radical with an electron

that can pop into a the p orbital on

that carbon and then

interact with all the pi orbitals so

basically

popping off hydrogen on that c

um that tetrahedral or CP3 hybridized

carbon

will give you an rsr back

so what this does is a raction that

regenerates an rsr

and it looks like the Bears are low

enough that that hydrogen can just come

off on its own it doesn't even have to

be abstracted so the barriers act so the

hydrogens can go off on their own so now

you're generating hydrogens hydrogen

atoms which are radicals you're

generating a new rsr radical in this

whole reaction and so this is basically

a chain reaction so it's like a

polymerization so we're kind of like

rapidly polymerizing the carbon that's

um in the The Flame

and generating a particle and this is

how you could get a disordered structure

okay so this is just a hypothesis and

now we're trying to figure out if it's

right so now we're trying to work on

okay how do we verify it we have some

theoretical estimate you know I you know

what okay what we don't have so we have

some theory that suggests it should be

work we have some experiments that show

that we have all these rsrs in the OR

associated with the particles

um we don't have a Model A kinetic model

because in order to run a kinetic model

you have to have rate constants right we

don't have rate constants for any of

these reactions that could be happening

right so now we have to go figure out

how to generate these rate constants so

so we'll be talking a little bit more

about that but

um and we also need experiments like is

is this even okay we have

um we see the rsrs but is there a way we

can kind of really test this this

hypothesis

okay this height this hypothesis called

the turkerbest mechanism which is a

clustering of hydrocarbons by radical

chain reactions

okay so

um and what we think is we have these

um uh

resonance stabilize radicals they get

you have a chain reaction they quickly

generate an um an incipient particle and

then we also think that um we have rsrs

that are associated with the particle

itself and maybe that's how we're

getting surface growth

that is total total speculation total

hypothesis out there

um

so we we need to figure out all of this

um and and we're we're working on other

people a lot of other people are working

on this

um a lot of people are trying to prove

us wrong

um that's okay oh that's okay

um okay

so what's our evidence I don't want to

run over like this is uh we're we end at

5 30 I think okay so we're gonna

so if I if I get close just start waving

because I get too excited about this

stuff okay so let's do pyrolysis

experiment remember smps allows us to

get on particle size distributions right

okay so this is a figure that shows if

we take ethylene so we normally run

ethylene flames it's a really good

sitting fuel it's been studied a lot a

lot of people in our community study it

um so uh so we do paralysis of Ethylene

it's been done 100 times before no one's

really done the looking at um the mass

spec so where I'll show you some Mass

back in a minute

um but so here's the smps

at a number of temperatures so that's on

the axis that's going back over to the

right Mobility size so there's size

distributions as a function of

temperature so the temperature is color

coded okay so you see that there are no

particles we don't see particles in the

smps at temperatures below about 1100

okay so we don't we just don't see

particles for ethylene at those

temperatures but we do see them and they

grow in the size distribution like you

see that the particles are growing in

size as you go up in temperature okay

okay so

what we did is we added so we did this

experiment also with with ending we just

took Indian ending remember makes

indones so it's a what we call Direct

precursor to an rsr so notice that we

see

um particle formation at much lower

temperatures for in for ending

so we generate indino and it generates

it starts to generate particles right

okay so right there it's like okay

that's good we kind of predicted that

something like that would happen if we

did in Indy you know according to our

Theory we should have particles starting

at it more easily form particles with

ending

than with Ethylene okay

um so and so yeah just that's what you

just said and that's an RZR right okay

so now what does it look like with the

mass spec right why are we so now we can

go to the mass spec the aerosol Mass

Spec and say oh why why is this

happening can we can we show what's

happening okay so here's a mass spec on

the left for ethylene notice there are

no particle when there are no particles

we don't see signal okay when we have

particles we see signal okay so we don't

see particles at the lower temperatures

only at the higher temperatures ending

we see particles at all these

temperatures we see signal at all those

temperatures but we have isolated little

clumps of signals one right there and

one right there it's hard to see this on

this slide

um but but let me show you the mass

Spectra at these temperatures

okay

okay so ethylene on the left ending on

the right at 1073 Kelvin okay no signal

of ethylene we have signal for ending on

the right hand side

in danil is mass 115 okay and you see

the Indian opaque

okay

um

so let's see oh yeah oh wait I was going

to show you this

yeah

um if we go up to higher temperatures

now we see ethylene pyrolysis

um at 1173 we see a whole bunch of Peaks

because it's now starting to form

particles

um and ending actually internal ending

looks the same about the same

um okay

if we take a little tiny bit of ending

and put it into ethylene so now we seed

the ethylene with indine so we see that

in in ethylene with a little bit of rsr

we start to see particles at the lower

temperature

so

um so it's like oh well that's that's

kind of a good thing like compared to

like if we're trying to see that our

theories right like oh that means that

just a little bit of rsr is starting to

seed particle formation with ethylene

okay so we have this little bit of

mixture and what's interesting is

if you see it 1073 Kelvin for the ending

alone and for the ethylene plus indine

it's not just ending

thus reacting the Indian has to be

reacting with the Ethylene

with with Ethylene ethylene isn't

falling apart yet so in denil has to be

reacting with ethylene to generate

particles because we see other Peaks

that are coming up in there that we

don't see under just ending alone so

it's not just ending reacting with

itself okay so so that's actually kind

of fascinating

um so we see also under for the ending

we see the 115 Peak

we see

two endonils making the 230 Peak we see

three endonos we actually see four into

Nils two I just didn't put it on here

um and then our Mass Spec you know

doesn't see anything else

um uh but you know we also see those

same Peaks actually at

um for the mixture

um but they're not actually all that big

right so it's the endonil is reacting

with the ethylene we think in that

mixture okay

cool so that kind of is really kind of

convincing that maybe something like

this is working

um okay

and and that the radicals are are

reacting with a closed shell species the

radicals are reacting with ethylene

which is closed shells not a radical

so that means it could be sucking up a

whole bunch of the hydrocarbons yeah

oh um it's like

um uh it's uh

you would ask something like so hard

um uh is

um

it's like a maybe five percent by

um a number of of ending in ethylene and

then it's diluted in in um argon yeah so

it's massively diluted it's only it's

actually not that much ethylene it's

massively diluted and argon yeah

okay

so that's that's kind of a fun

experiment so what happens I don't I'm

not going to run I'm not going to run

over so um if we have uh other rsers so

we can do this with propene and propine

propine is nice because it makes

a very important molecule in combustion

you've probably heard about perpardulin

Benzene and it's and and the the drama

associated with it so per partial plus

propardial is likely what what people

think causes Benzene formation in a

flame so perpartual is a nice ours it is

an rsr so we can take propane

generate per partial propane also

generates an rsr called allyl so they're

very similar

the barrier to generating

um propardial from propine is higher

than the barrier for generating aloe

propane okay so you might expect more

particles easier formation from propane

than propine

um okay so

um

sorry

so um here it is

even though like okay so propine has uh

lower

um particle formation temperature than

propene even though the barrier for Aloe

formation is lower for propane than for

propylene so there's something magical

about propagal propagal is is a happy

molecule

um it so so this is this is a an

interesting

um uh observation and we don't

understand it okay so this is another

one of those things that we're working

on

um and other people are working on so

it's kind of a fun fun result

um but we were we're gonna we're gonna

nail it we're gonna figure it out

um okay so just looking in more detail

at the mass Spectra

um this is at at three different

temperatures the lowest of the top

highest at the bottom and notice that on

on these Mass Spectra for each number of

carbons they're at the lowest

temperature there's a distribution of

Peaks

um so the

um Peak with the highest carbon to

hydrogen ratio would be on your left

because each of those Peaks on the right

is adding a hydrogen atom to the that

mass whatever the number of carbons so

it'd be like kind of like 18 carbons

with a distribution of numbers hydrogens

okay and then as you go up in

temperature

those collapse to the lowest

to the least number of carbons does that

make sense

you're supposed to go yeah yeah it does

yeah because remember as we're going up

in temperature like we saw this result

before right we saw this result like as

we go up 100 and and temperature we

eliminate those hydrogens and we come

collapse back down to like pericondensed

um pH probably something like that we

rearrange and get rid of hydrogens and

we start to be a happy pH okay

so that's for propane for propine

um

uh this is for propane see the same

things

um so uh this exact same thing you have

clusters of Peaks up at the top at the

lowest temperatures go up in temperature

and we collapse that back down to the

lowest Mass

the highest carbon to hydrogen at a

particular carbon okay

so that's all making sense that's all

good but the interesting thing is we

actually have a lot of aliphatic

character right what is causing it's

extra hydrogens how would you get that

aliphatic branching right all those

extra you know

um branches that add hydrogen onto your

your whatever your molecule okay

um so we think that these are are

examples of that of that happening

okay

um

and now we see for propane and propine

the same rsrs right this is a lower

lower

um masses we see the same rsrs

um and um so at the particle onset

temperatures

um we see the same types of

distributions of not exactly the same

rsrs we see in the flame like all the

Flames like many of the Flames but we

see a lot of them again 165 is huge so

if you figure that one out let me know

let me know what's going on there

um okay so let's see

um

okay I'll go for a couple minutes but

I'm not going to go over I'm not going

to go over okay so we saw this one

before

um

again this is the number of carbon

hydrogen carbons

um and the carbon to hydrogen ratio is

the lowest for the largest masses right

so you get that distribution for the

large masses right because you can have

that

um carbon you you have a lot more

probably a lot more branching at these

higher masses I mean I think that might

be what's going on

um and that dashed line is if you took

the stabilimers and you added up the

carbon to hydrogen ratio for the

stabilimers for each of the numbers of

carbons okay okay

um oh yeah that's what I just said that

the large

um larger species are more saturated

that have more hydrogen right saturated

means having more hydrogen

um okay uh the uh and it decreases with

temperature as we've said before right

we keep seeing this is an interesting

thing

um and that it's not driven by entirely

by thermodynamics right it's not that

most of thermodynamically stable species

by a long shot at these um at these

different masses okay so so there's

something that's going on that's keeping

us from sinking into those thermodynamic

Wells and staying there where we have

some kind of mechanism that's like

Dynamic it's exciting things are

reacting right so we're making particles

um okay so uh hey sorry if I'm yelling

at you

um okay so these residently stable

stabilized radicals may be driven

um the Inception may be driven by

radical chain reactions

okay so um I think I'll stop there

because it's a good stopping place for

today and then we'll pick up on how do

you model this like the next step is

like how would you put this into a model

what would be the first step because

right now no one's modeling it

um because we haven't actually gotten to

the point where we can

okay so are there any questions on any

of this

um

you're and if you just really want to

speak into this that microphone

okay so remember to meet each other

remember to say hi if you haven't if

there's someone in here you don't know

walk over and say hi my name is

um these are your colleagues for the

future

um these are people who are going to

help you and you can help them it's

gonna be fun

okay