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

Tb identify why Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate) is suc h a versatile surfactant, adsorption and aggregation in aqueous solutions h ave been investigated with 11 different AOT-related compounds. The first se t, denoted di-CnSS with n = 4, 5, 6, 7, and 8, contain linear chains. In ad dition six different branched chain systems have been studied, including Ae rosol-OT itself. These surfactants span extremes of branching, from 3,5,5-t rimethyl-1-hexyl, through 2-ethyl-1-hexyl (AOT), to 1-ethyl-2methyl-1-penty l (see Figure 1), and therefore it is possible to delineate structure - per formance 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 molecul e at the critical micelle concentration A(cmc) was 60 +/- 3 Angstrom (2), w hich is in excellent agreement with that obtained directly by neutrons, 59 +/- 2 Angstrom (2). The procedures for obtaining a sufficiently high surfac e chemical purity and the application of the Gibbs equation, to achieve agr eement between DVT and NR, are discussed. Partial structure factor experime nts with di-C6SS gave quantitative information on the interfacial structure in terms of surfactant and solvent distributions, and the results are comp ared 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 cl assic 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 sy stems. Furthermore, slight variations in A(cmc) were detected, reflecting c hanges in packing owing to differing extents of chain branching. The effici ency of packing was correlated with chain structure by introducing an empir ical branching factor. In dilute aqueous systems regular Aerosol-OT behaves in a readily understandable fashion, fitting into the general pattern of b ehavior for all these sulfosuccinates: no special effects due to the 2-ethy l-1-hexyl chains were noted.