The thermodynamics of colloidal surfactant solutions
Although surfactant selective electrodes have been employed for several decades, those electrodes "of the second kind" and those incorporating a liquid membrane suffered severe limitations. A large improvement in the reliability of such electrodes was achieved by Cutler (Ph.D. Thesis, University of Aberdeen, 1975) using a liquid plasticized PVC membrane in which the surfactant was directly complexed to the PVC. However, such electrodes still had several drawbacks including the solubilisation of the liquid plasticiser from the membrane and the influx of water through the membrane after only a few hours in solution. It was the purpose of this study to reproduce and perhaps improve upon the electrodes developed by Cutler and to use them to investigate the thermodynamics of colloidal ionic surfactant solutions. Initially, the electrodes developed for both cationic and anionic surfactants by Cutler were reproduced and during the course of the study were improved upon by replacing the liquid plasticizer content of the membranes by a high molecular weight PVC-compatible terpolymer. This polymeric blended membrane is much tougher than the liquid plasticized one and has a much higher resistance to the uptake of water. Thus, the useful lifetime of a membrane was extended from a period of hours to a period of weeks. In addition the polymer blended membrane has no liquid components and hence the solubilisation of membrane component in micellar solution is no longer a problem. The surfactant selective electrodes was used in conjunction with electrodes reversible to other common species in solution (Na+, Cl-, Br-, etc.) to construct cells without liquid junction for potential measurements. In addition to getting rid of the unavoidable error associated with potential measurements using cells with liquid junction, this experiment allowed for a direct determination of the surfactant monomer concentration above the CMC. This is, to the authors knowledge, the first time that such a measurement has been experimentally possible. Initially a well documented system was chosen for investigation: the micellisation of sodium dodecyl-sulphate in aqueous salt (NaBr) solutions. The well known depression of the CMC with increasing salt concentration was observed and from this the degree of dissociation of counterions from the micelles was calculated. This corresponded closely to other values reported in the literature. Recently a thermodynamic theory of the micellisation of ionic surfactants has been reported by Hall (1981) and using data obtained by the electrode measurements, three tests of the theory were possible. In each case the theory was found to correspond with the experimental measurements. Additionally using the electrode measurements it was possible to estimate the interaction or 'Harned' coefficient between the dodecylsulphate and sodium ions in aqueous salt solutions below the CMC. The use of the electrodes was extended to other 'non detergent' amphiphilic molecules and the aggregation of BDPH, a cationic drug, was investigated. These molecules were found to form pre-micelles before the onset of true micellisation and an iterative simulation procedure indicated that these pre-micelles were probably dimers. The effect of salt upon the CMC was also determined and hence, α, the degree of dissociation of counterions from the micelles was calculated. This value agreed well with other literature reports. The electrodes were also used to investigate the aggregation of ionic surfactants in mixed solvent media e.g. ethylene glycol + water and ethanol + water mixtures. The variation of a with solvent composition was observed and additionally it was demonstrated that the electrode measurements could be used to determine conveniently the solubility product of ionic surfactants in aqueous or mixed solvent systems. Mixed micellar systems were also studied. These included anionic-cationic, ionic-nonionic and ionic-zwitterionic systems and the composition of the mixed micelles in many cases was found to follow a very simple model based upon the regular mixing of the micellar components. The phase behaviour of these systems was also observed since many of them were found to form stable micelles only at certain compositions. Phase separation including precipitation, coacervation or liquid crystalline phase formation was found in many systems over certain solution compositions. The electrodes were used to calculate the α value of a mixed micellar system as a function of micelle composition. In the final chapter the binding of ionic surfactants to synthetic macromolecules is discussed and the results of electrode measurements are reported. The measurement of the adsorption isotherm of the surfactant into the polymer was found to be very quick and convenient and the systems investigated included both anionic and cationic surfactants binding onto either neutral polymers or highly charged polyelectrolytes. The effect of the solution composition variables (i.e. salt, surfactant and polymer concentrations) and the molecular weight of the polymer upon the binding process was determined for one system, sodium dodecylsulphate binding to polyvinylpyrollidene. A recent theory by Hall to account for the binding of ionic surfactant to neutral polymers is discussed and experiments were carried out to test this theory.