Question

"How does a positive, neutral (zwitter) and negative amino acid effect a cationic surfactant?" Can someone please explain in details using Arginine, Isoleucine and Glutamic acid as explain? Also, how each can effect the Critical Micelle Concentration of a cationic surfactant?

This is for my PCHEM lab btw.

Answer 1

A cationic surfactant possess a hydrophilic head and a hydrophobic(alkyl chain) tail. This hydrophilic head is positively charged. Surfactants form micelles (aggregates), and the lowest concentration where the micelle formation occurs is known as critical micelle concentraion (CMC). A positively charged amino acid such as arginine will show repulsive interactions with the cationic part of surfactant. A negatively charged amino acid such as glutamic acid will show attraction towards the cationic part of surfactant and will bind to it strongly. A neutral amino acid such as isoleucine will show hydrophobic interactions with the surfactant.

During micelle formation, hydrophobic interactions dominate the cation-solvent or hydrophilic interactions. When hydrophobic interactions are more (as when concentration of surfactant is high), micelles are formed.

So, the main thing is to make the hydrophobic interactions dominate.

If glutamic acid is present, it will get attracted to cationic part. Now, the surfactant is binded to the solvent as well as the amino acid through ionic interactions. This, in turn will increase the hydrophilic interactions and we now need a higher concentration (of surfactant ) than normal to make the hydrophobic interactions dominate. This, in turn, will increase the critical micelle concentration.

Whereas, there are less hydrophilic interactions due to cation-cation repulsions in case of arginine. At low concentration of surfactant only, micelle formation happens. Hence, cmc decreases.

Isoleucine will increase the hydrophobic interactions, and hence cmc will decrease.