Series 10 Colloids VI Variable or pH Dependent Charge I

Summary

This video from Cornell University explores the pH-dependent charge variability in soil colloids, particularly focusing on hydroxyl groups. It explains how soil pH affects the charge of colloidal materials, like humic substances and phyllosilicates, through the interaction of hydroxyl groups and protons or hydroxides. As pH increases, these colloids acquire more negative charge, enhancing their ability to hold onto cations. Conversely, lower pH results in positive charge. The discussion highlights how different colloidal materials, like smectite and humic materials, exhibit varying charge behaviors based on their structure and surface area exposure to pH changes.

Takeaways

• 😀 pH-dependent charges on colloidal materials are influenced by the presence of hydroxyl groups.
• 😀 Hydroxyl groups on humic materials, phyllosilicates, and oxide materials can carry both positive and negative charges depending on the pH of the solution.
• 😀 Increasing the pH of the soil solution results in more negative charge on the colloid, enhancing its cation exchange capacity (CEC).
• 😀 Decreasing the pH leads to proton binding, reducing the negative charge on colloidal materials and resulting in more positive charge.
• 😀 The charge on colloids is variable and depends on the exposure of hydroxyl groups to the soil solution.
• 😀 Colloidal materials like phyllosilicates have hydroxyls on their edges, but the majority of their hydroxyl groups are inside the structure, limiting their pH-dependent charge effects.
• 😀 Colloids with large surface areas and exposed hydroxyls show stronger pH-dependent charge changes.
• 😀 Phyllosilicates (e.g., smectites) have a less pronounced increase in CEC with rising pH compared to humic materials due to fewer exposed hydroxyl groups.
• 😀 Humic materials increase their ability to retain cations (CEC) as pH increases and negative charge accumulates on their surface.
• 😀 At higher pH levels, colloids tend to hold more cations, while at lower pH, they can attract and hold anions.
• 😀 The structural arrangement of colloidal materials (e.g., 1:1 or 2:1 phyllosilicates) significantly impacts their pH-dependent charge behavior.

Q & A

What is the role of hydroxyl groups in the pH-dependent charge of humic materials?

Hydroxyl groups are the key source of pH-dependent charge in humic materials. These groups can either gain or lose protons depending on the pH of the surrounding solution, leading to either a negative or positive charge on the material.

How does increasing the pH affect the charge of a colloid?

Increasing the pH results in the addition of hydroxide ions to the soil solution, which bind with protons from hydroxyl groups. This process removes protons and increases the negative charge of the colloid.

What happens to the charge on a colloid when the pH is lowered?

When the pH is lowered, protons are added to the solution, which bind with the negative charge on the colloid. This results in a positive charge on the colloid.

What is the effect of pH-dependent charge on cation and anion exchange capacity?

As the pH increases, and the colloid becomes more negatively charged, its capacity to hold cations increases. Conversely, lowering the pH leads to a decrease in negative charge and reduces the ability to hold cations.

Why does the charge variability depend on the exposure of hydroxyl groups?

The charge variability depends on whether hydroxyl groups are exposed to the soil solution. If hydroxyl groups are hidden inside a material, they cannot interact with the pH changes, and thus do not contribute to pH-dependent charge.

How do phyllosilicates differ from humic materials in terms of pH-dependent charge?

Phyllosilicates, such as smectite, typically have hydroxyl groups only at their edges, making their pH-dependent charge less pronounced compared to humic materials, which have more exposed hydroxyl groups across their structure.

What happens to the charge on smectite at higher pH levels?

At higher pH levels, smectite shows a slight increase in cation exchange capacity due to an increase in negative charge, but this effect is much smaller compared to humic materials because of fewer exposed hydroxyl groups.

How does the structure of phyllosilicates affect their pH-dependent charge?

Phyllosilicates with a tetrahedral-octahedral structure can have exposed hydroxyl groups at their edges, but the internal hydroxyl groups are less affected by pH changes. The pH-dependent charge is therefore primarily influenced by the exposed edges.

What is the general trend in charge variation as pH increases?

The general trend is that as pH increases, the negative charge on colloids also increases due to the exposure of more hydroxyl groups that bind with protons, removing them from the structure.

What determines whether a colloid is predominantly pH-dependent or permanent in charge?

The structural arrangement of the colloid determines this. Colloids with more exposed hydroxyl groups, such as humic materials, exhibit more pH-dependent charge, while others, like certain phyllosilicates, show a more permanent charge due to limited exposure.