pi-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 show ed unusual behavior; that is, their critical micelle concentration measured by the Wilhelmy method for their homologous series increased with an incre ase in the hydrophobic alkyl chain length. Thus, the difference in the back bone structure of these surfactants, whether glycerol type (glycerol or 2-m ethylglycerol) or 1,1,1-tris(hydroxymethyl)ethane (i.e., trimethylolethane) type, significantly affects their surface properties. To clarify this unus ual behavior, the adsorption manner of triple-chain amphiphiles bearing two or three hydroxyl groups, which are synthetic precursors of triple-chain s urfactants bearing two or three anionic headgroups, was studied by measurin g pressure-area (pi-A) isotherms with a computer-controlled film-balance te chnique. Some clear-cut profiles with respect to the relationship between t he structure of these amphiphiles and their adsorption behavior on the surf ace were revealed as follows: (1) The packing of hydrophobic alkyl chains o f triple-chain diols was tighter than that of the corresponding double-chai n diols with the same alkyl chain length; (2) as to both triple-chain diols and triple-chain triols, the pi-A isotherms were greatly changed depending on their backbone structure, whether glycerol, 2-methylglycerol, or trimet hylolethane; (3) three additional isolated oxyethylene units connecting to the backbone of triple-chain triols contribute significantly to the increas e in hydrophilicity of the molecule. These results indicate that the choice of the backbone structure of a triple-chain surfactant is important to pre dict and to understand the packing of hydrophobic chains, which directly re lates to its surface properties in water.