Dynamics of Surfactant Adsorption at Solid--Liquid Interfaces

Abstract

The adsorption kinetics of surfactants at the solid--liquid interface is of fundamental
interest to a wide variety of process including detergency, wetting of solid surfaces, agricultural sprays
and paper processing. Accordingly, a significant body of work
has been carried out to understand this field.
Much of this work has used the optical techniques of ellipsometry
and optical reflectometry or mass measurements from the quartz crystal microbalance.
These methods have the time resolution to measure surfactant adsorption kinetics but are
insensitive to chemical composition and thus produce limited information on the adsorption
of surfactant mixtures.

The technique I adopt here, total internal reflection (TIR) Raman spectroscopy, provides detailed
information about the chemical composition of the surface with a time resolution of 2s.
The short penetration depth of the
probe laser into solution (nm) provides surface sensitivity. The different components
of the adsorbed film are distinguished by their vibrational Raman spectra. The
Raman signal from a component in the adsorbed layer is linearly proportional to the amount of that
component present, allowing straightforward interpretation of the acquired data. I use principal component
analysis to deconvolute the recorded spectra.

First I look at the equilibrium and kinetic aspects of the adsorption of two model surfactants
to a flat silica surface as single component systems: the
cationic surfactant cetyltrimethylammonium bromide (CTAB) and the non-ionic surfactant Triton X-100.
Use of the well-defined wall jet geometry provides known hydrodynamics allowing the mass transport to
the surface to be modelled. The mass transport model is coupled with a kinetic model consistent with
the Frumkin isotherm allowing the whole adsorption process to be captured. The fit between the
model and the experimental results helps to understand interactions on the surface.

Secondly I look at
the two model surfactants adsorbing to silica as a mixed system. The adsorption isotherm shows strong synergistic behaviour with the addition
of small amounts of CTAB (of the 2mM total surfactant concentration) doubling the adsorbed amount
of Triton X-100. This synergism has a marked influence on the kinetics: for example, when
Triton X-100 replaces CTAB the Triton X-100 surface excess overshoots its equilibrium value
and returns only very slowly to equilibrium. For systems above the cmc, the repartitioning
of surfactant between micelles and monomers results a local increase in the monomer concentration of Triton
X-100 resulting in a temporary spike in the Triton X-100 surface excess during the rinsing of
a mixed layer.

Finally I study alternative model surfaces to silica. The adsorption to CTAB and Triton X-100 to a cellulose
surface is studied, and detailed equilibrium isotherms obtained by slow variation of the bulk
concentration controlled with a continuous stirred tank mixer. The preparation of the model cellulose
surface is also followed spectroscopically. Spectra are also acquired from mica surfaces in
optical contact with silica hemispheres; it is unfortunately not yet possible to acquire
useful data on adsorption at the mica--water interface.