I saw the PEGs (for ex PEG200) in surfactant classification list, but these materials can not form a micelle. What is the reason?
PEG has no charge. Because of the large dipole moment, it is strongly hydrated with water. To form micelles in it, hydrophilic groups and hydrophobic groups should alternate. Copolymerization of ethylene oxide and propylene oxide produces a copolymer with alternating hydrophobic and hydrophilic groups.
Dear Dr. Mirgorod,
Thank you so much for your complete answer!
Best Regards
Are you confusing PEGs and fatty acids of PEGS? The second are certainly surfactants but PEGs are not. For a molecule to be a surface active, there has to be a thermodynamic reason. PEGs like water. Fatty acids hate water. PEGs want to remain in water, fatty acids don't. If there's a hydrophobic surface nearby, the fatty acid part will try to adsorb on it. Whether it does or not is determined by the balance between PEG remaining in water and the fatty acid adsorbing. The same argument applies to whether a surface active material will self-associate to form micelles. Generally, the molecules have to order themselves - e.g., the hydrophobic chains may have to fully straighten out. This creates order (as opposed to chaos or being random) which is thermodynamically undesirable but takes the chains away from the water, which is desirable. So, again, there's a balance. This is often referred to as the hydrophilic/lipophilic balance (HLB). Personally, I don't like the term but it can be useful to help with selection of surfactants during formulation development.
- Badges
- Science topic
- Similar topics
- Engineering
- Chemical Engineering