Physical aspects - Surface active material

8 important questions on Physical aspects - Surface active material

What are 6 surface active materials?

  1. Fatty acids
  2. Surfactants (esters)
  3. Globular proteins
  4. Flexible proteins
  5. Polymers (macromolecules)
  6. Polysaccharide gum arabic

How do fatty acids act as surface active materials?

Small molecules with hydrophilic head and hydrophobic tail. As they have both, they can adsorb at oil-water interfaces. Hydrophilic group is very small, so often more in the oil than at the interface. So slightly surface active, but not very suitable to really stabilise the interface.

How do surfactants (esters) acts as surface active materials?

Chemically modified. Hydrophilic and hydrophobic part can be altered in size. Hydrophilic part larger, so more surface active than fatty acids.

How do globular proteins act as surface active materials?

Hydrophobic core and hydrophilic outside. Hydrophobic sides should be in oil, hydrophilic should be in aqueous phase. Only possible when large distinctive areas. At the interface, the protein will adapt to their conformation so the correct side is towards oil and water. Proteins may unfold a bit.
Large proteins are very surface active, smaller proteins (and peptides) are not surface active as their hydrophobic/hydrophilic sides are not distinctive and large enough.

How do flexible proteins act as surface active material?

They have a hydrophobic and hydrophilic side. Linear chain of peptides without secondary structure. Molecules rearrange in such a way that the correct side is correct.

How do polymers act as surface active materials?

Have hydrophobic and hydrophilic groups. Not in nature (too little hydrophobic groups), only chemically prepared block co-polymers are surface active (with increased hydrophobicity).

How does the polysaccharide gum arabic act as surface active material?

It has carbohydrates groups (hydrophilic) and a protein (hydrophobic) backbone. Protein will adsorb at oil, carbohydrates at water.

What can the charge of surface active materials be and what does it result in?

  1. Neutral
  2. Anionic (negatively charged)
  3. Cationic (positively charged)
  4. Amphoteric (both positive and negative charges, e.g. Proteins)

Charge leads to electrostatic repulsion, it may prevent aggregation in emulsions.

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