Figure 11.

Surface active compounds at the air-water surface and in bulk aqueous phase. (a) Soluble Monolayer: Long-chain alcohols, such as Octanol, can spread at the air-water (blue background) surface to make a soluble monolayer (monomer can transfer between monolayer and bulk). The monolayer has a certain orientation, i.e., hydrophilic OH-headgroup (white circle) towards the water and hydrophobic C8 alkyl chain (black curvy line) towards the air. Above its solubility limit in bulk aqueous solution, octanol can behave like pure hydrophobic oil, i.e., forming its own oil emulsion with octanol also adsorbed at this interface. (b) Soluble Monolayer: Detergents, such as the anionic sodium dodecyl sulphate (SDS), can spread at the air-water surface to form a soluble monolayer. The orientation is the same as the long-chain alcohol with the negatively charged hydrophilic polar group facing the aqueous phase, (yellow circle) and the hydrophobic C12 alkyl chain towards the air (black curvy line). Above its solubility limit in bulk, monomers form micelles. (c) Insoluble Monolayer: Lipids, such as phospholipids, can spread at the air-water surface to form an insoluble monolayer, again with headgroups in the water phase (green circles) and double acyl chains in air (double black curvy lines), with relatively little molecular lipid (bracketed) in solution (water solubility, S_w ~10 nM or less). In the bulk aqueous phase monomer lipids self-assemble into vesicles. (d) Surface-Stabilised-Droplets. Micro- or nano-emulsion droplets can be stabilised with, for example, the anionic detergent SDS.