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 2\,s. The short penetration depth of the probe laser into solution (${\sim}100$\,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 (${\sim}2$\%\ of the 2\,mM 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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:542499 |
| Date | January 2011 |
| Creators | Woods, David Alexander |
| Publisher | Durham University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.dur.ac.uk/3277/ |
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