π−A Isotherms for Triple-Chain Amphiphiles Bearing Two or Three Hydroxyl Groups. Effect of the Backbone Structure on the Adsorption Behavior of the Molecules on the Surface

Three types of triple-chain surfactants bearing three sulfonate groups showed unusual behavior; that is, their critical micelle concentration measured by the Wilhelmy method for their homologous series increased with an increase in the hydrophobic alkyl chain length. Thus, the difference in the backbone structure of these surfactants, whether glycerol type (glycerol or 2-methylglycerol) or 1,1,1-tris(hydroxymethyl)ethane (i.e., trimethylolethane) type, significantly affects their surface properties. To clarify this unusual behavior, the adsorption manner of triple-chain amphiphiles bearing two or three hydroxyl groups, which are synthetic precursors of triple-chain surfactants bearing two or three anionic headgroups, was studied by measuring pressure−area ( π − A ) isotherms with a computer-controlled film-balance technique. Some clear-cut profiles with respect to the relationship between the structure of these amphiphiles and their adsorption behavior on the surface were revealed as follows: (1) The packing of hydrophobic alkyl chains of triple-chain diols was tighter than that of the corresponding double-chain diols with the same alkyl chain length; (2) as to both triple-chain diols and triple-chain triols, the π − A isotherms were greatly changed depending on their backbone structure, whether glycerol, 2-methylglycerol, or trimethylolethane; (3) three additional isolated oxyethylene units connecting to the backbone of triple-chain triols contribute significantly to the increase in hydrophilicity of the molecule. These results indicate that the choice of the backbone structure of a triple-chain surfactant is important to predict and to understand the packing of hydrophobic chains, which directly relates to its surface properties in water.