Chapter 22
Nonideality at Interfaces in Mixed Surfactant Systems Paul M. Holland
Downloaded by UNIV OF GUELPH LIBRARY on August 10, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch022
General Research Corporation, Santa Barbara, CA 93111
Nonideality at various solution interfaces is examined using a model based on the pseudophase separation approach, regular solution approximation for treating nonideality at solution interfaces, and a simplified method for taking changes in molar areas on mixing into account. Comparison with experimental results for mixed anioniccationic surfactant systems show this approach to provide useful predictions for surface tensions both above and below the critical micelle concentration, and useful predictions of contact angles on Teflon and Parafilm.
The adsorption of surfactants from solution onto interfaces leads to most of the practical benefits derived from mixed surfactant systems. Among the major effects observed are interfacial tension lowering and changes in the wettability of surfaces as reflected in contact angle changes. Since the chemical potential of surfactant molecules in bulk solution effectively controls the composition at solution interfaces at equilibrium, a mixed surfactant model for interfaces can be viewed as a natural extension of solution models for mixed surfactant systems. Theory At concentrations far below the mixed CMC, the interface is only sparsely covered by surfactant and a significant amount of "bulk-like" water is present at the interface. As the concentration increases, the surface becomes "saturated" with an adsorbed mixed surfactant monolayer. This regime is indicated by a constant slope in the plot of surface tension versus the logarithm of activity or total surfactant concentration. Here, the Gibbs equation „, = - * r ^ L
0097-6156/92/0501-0327S06.00/0 © 1992 American Chemical Society
In Mixed Surfactant Systems; Holland, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
(i)
Downloaded by UNIV OF GUELPH LIBRARY on August 10, 2012 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0501.ch022
328
MIXED SURFACTANT SYSTEMS
applies, and can be used to determine the surface area per mole at the interface. In the case of mixed surfactant systems the average area per mole at the interface is obtained. Eventually as the surfactant concentration continues to increase, the CMC or a phase boundary is surpassed and the surfactant activity tends to level off sharply. At or above concentrations where the Gibbs equation holds, a "surface pseudophase" consisting of a "saturated" monolayer of adsorbed surfactant aggregate can be defined. This approach diverges from the standard surface solution approach (see refl) because the presence of water at the interface is not explicitly included and the sum of mole fractions of surfactant at the interface is therfore assumed to be unity. In this treatment, any "residual" solvent effects at the interface are now either accounted for in the standard state chemical potentials for the pure components, or in a surface interaction parameter accounting for nonideality in mixed systems. This provides the basis for developing a tractable and generalized nonideal mixed surfactant model for interfaces (2-4). The mixed monolayer model can be designed for use above the CMC, below the CMC where the Gibbs equation holds, for contact angles, and for extension to multicomponent systems (at least in principle). This can be developed as follows. For concentrations at or above the CMC in a pure surfactant system, the chemical potential of surfactant / in the adsorbed monolayer can be expressed as
χ
where μ™ is a standard state chemical potentialat the surface and π™ ω a force field term containing the maximum (constant) surface pressure at or above the CMC and the molar area at the interface. The chemical potential of surfactant component / in solution is given by {
μ. = μ* + RT ln C"
3
()
where μ° is a standard state chemical potential and CJ" the monomer concentration. At or above the CMC in the pure system a similar expression results μ
= μ*
(
= μ? *Γ1η
