What is so special about aerosol-OT? 1. Aqaueous systems

Tb identify why Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate) is such a versatile surfactant, adsorption and aggregation in aqueous solutions have been investigated with 11 different AOT-related compounds. The first set, denoted di-CnSS with n = 4, 5, 6, 7, and 8, contain linear chains. In addition six different branched chain systems have been studied, including Aerosol-OT itself. These surfactants span extremes of branching, from 3,5,5-trimethyl-1-hexyl, through 2-ethyl-1-hexyl (AOT), to 1-ethyl-2methyl-1-pentyl (see Figure 1), and therefore it is possible to delineate structure - performance relationships. For one model compound, di-C6SS, adsorption studies were made with two complementary techniques, drop volume tensiometry (DVT) and neutron reflection (NR). The tensiometrically derived area per molecule at the critical micelle concentration A(cmc) was 60 +/- 3 Angstrom (2), which is in excellent agreement with that obtained directly by neutrons, 59 +/- 2 Angstrom (2). The procedures for obtaining a sufficiently high surface chemical purity and the application of the Gibbs equation, to achieve agreement between DVT and NR, are discussed. Partial structure factor experiments with di-C6SS gave quantitative information on the interfacial structure in terms of surfactant and solvent distributions, and the results are compared with those obtained previously for AOT (Prog. Colloid Polym. Sci. 1995, 98, 243 and J. Phys. Chem. B 1997, 101, 1615). For the di-CnSS systems classic effects of chain carbon number on surface tension behavior, critical micelle concentrations (cmc's), and A(cmc) were observed. With the branched chain compounds a significant increase in A(cmc) was found, between 10 and 20 Angstrom (2), over those for equivalent carbon number straight chain systems. Furthermore, slight variations in A(cmc) were detected, reflecting changes in packing owing to differing extents of chain branching. The efficiency of packing was correlated with chain structure by introducing an empirical branching factor. In dilute aqueous systems regular Aerosol-OT behaves in a readily understandable fashion, fitting into the general pattern of behavior for all these sulfosuccinates: no special effects due to the 2-ethyl-1-hexyl chains were noted.