Tensoactivo Aot

Langmuir 2000, 16, 8733-8740

8733

What Is So Special about Aerosol-OT? 1. Aqueous Systems†
Sandrine Nave and Julian Eastoe*
School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.

Jeff Penfold
ISIS-CLRC, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, U.K.
Received March 7, 2000. In Final Form: May 11, 2000
To 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, from3,5,5-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 micelleconcentration Acmc was 60 ( 3 Å2, which is in excellent agreement with that obtained
directly by neutrons, 59 ( 2 Å2. 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 structurein 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 Acmc were observed. With the branched chain compounds asignificant increase in Acmc was
found, between 10 and 20 Å2, over those for equivalent carbon number straight chain systems. Furthermore,
slight variations in Acmc 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 regularAerosol-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.

Introduction
One of the great myths of surfactant science is about
the molecular structure of Aerosol-OT (AOT, sodium bis(2-ethyl-1-hexyl) sulfosuccinate). The rich phase behavior
of AOT, and itsability to form microemulsions, is often
loosely attributed to a “conelike molecular structure”. The
purpose of this study is to find whether, indeed, there is
anything unique about the AOT structure. In pursuit of
this aim 11 homologues, as shown in Figure 1, have been
synthesized, and relevant properties characterized. By
comparing these structural variants, it is possible to
pinpoint theeffects of chain branching in AOT itself.
Aqueous-phase behavior is described in part 1, while part
2 (following paper in this issue) deals with threecomponent water-in-oil microemulsion systems.
The relationship between surfactant molecular structure and phase behavior is a central question in colloid
science, especially with regard to adsorption and aggregation. The critical micelleconcentration (cmc), surface
excess (Γ), and limiting surface tension (γcmc) are characteristic of any given surfactant, and so these are useful
quantities. For homologous surfactants an array of such
values can be helpful in identifying structure-performance relationships. In particular, interfacial packing can
be related to the length and nature of hydrophobic chains,
as well as the size, charge,...