Denver G. Hall

Electrostatic effects in dilute solutions containing charged colloidal entities

The Donnan osmotic pressure of a solution containing charged colloidal entities is divided into electrostatic and non-electrostatic contributions. The expression adopted for the former contribution is obtained from a general thermodynamic treatment of the diffuse double layer. Expressions are derived which allow for specific binding and which also provide a definition of amounts bound electrostatically. When the approximation is made that the electrostatic binding of indifferent coions Γcoions is negligible the equations which result have the same form as those that apply to non-interacting uncharged systems and can be handled in the same way. The treatment provides a basis for a successful but somewhat arbitrary approach developed previously. In particular an expression for the activity coefficient of unbound counterions and coions is justified. The prediction (based on the application of the Poisson—Boltzmann equation to cell models) that Γcoions is negative is contradicted by work concerning the significance of single ionic chemical potentials.

Ionic activities in sodium dodecyl sulphate solutions from electromotive force measurements

Measured surfactant ion activities (a1) and counterion activities (a2) in solutions of sodium dodecyl sulphate (SDS) in the presence and absence of NaCl are reported. The results show that premicellar association is insignificant. Mean ionic activities of surfactant (a±) above the c.m.c. (critical micelle concentration) agree well with estimates from light scattering. This shows that the surfactant ion electrodes work well in the presence of micelles and that solubilisation of membrane constituents is unimportant. Comparison of a± values with estimates from surface tension studies suggests increasing surfactant adsorption above the c.m.c.At the c.m.c. in the presence of NaCl we find log a1+ 0.795 log a2=–3.185. (i), By putting (c′)2=m1[m1+0.2(c–m1), (ii), where c and m1 denote total and monomeric surfactant concentration and c′ is the concentration that would give the same activity as that in the solution of interest if no micelles formed, we found that increasing c leads to decreasing m1 above the c.m.c. An attempt to obtain liquid junction potentials in solutions above the c.m.c. was unsuccessful. Differences between present and previous results are probably attributable to the non-Nernstian response of previous surfactant electrodes.

The use of ion-selective electrodes to determine the effective degree of micelle dissociation in tetradecylpyridinium bromide solutions

A surfactant membrane electrode selective to the tetradecylpyridinium cation has been constructed and used with various ion-selective electrodes to evaluate the effective degree of micelle dissociation of tetradecylpyridinium bromide micelles. Four different approaches have been used to evaluate this parameter and in three cases the results are consistent.

Electrochemical studies of micelle–counterion interactions in mixtures of ionic and non-ionic surfactants

For solutions containing a 1 : 1 ionic surfactant plus a 1 : 1 electrolyte with a common counterion the expression α2±=C3[C3+m1+α(C1–m1)]γ2 is used to estimate the effective micellar degree of dissociation α from experimental values of a±. The mean ionic activity of supporting electrolyte in situations where m1C3C1(C3, C1 and m1 denote concentrations of added salt, total ionic surfactant and ionic-surfactant monomer, respectively, and γ is an appropriate activity coefficient). The method is free from uncertainties due to liquid-junction effects. For pure ionic surfactants α is found to depend only weakly on C1 and agrees well with values obtained from the effect of salt on the c.m.c. For mixtures of ionic + non-ionic surfactants 1/α is found to vary linearly with micellar mole fraction of the non-ionic xN and aproach unity as xN→ 1. The implications of this finding on the validity of a recent theory describing the c.m.c. of such mixtures is discussed.

The construction and characteristics of drug-selective electrodes. Applications for the determination of complexation constants of inclusion complexes with α- and β-cyclodextrins including a kinetic study

Ion-selective membrane electrodes selective to the drugs chlorpromazine, dicyclomine, imipramine, desipramine and propranolol hydrochlorides have been constructed using a modified poly(vinyl chloride) membrane which has ionic end-groups as ion-exchange sites and which was cast using a solid polymeric plasticiser. These drug electrodes show excellent Nernstian responses in the concentration range 10–2–10–7 mol dm–3 and their selectivity coefficients with respect to each other, as well as their workable pH range have been investigated. The electrodes have also been used to determine the complexation constants of chlorpromazine and dicyclomine hydrochlorides with both α- and β-cyclodextrins. In all cases a 1:1 complex was observed. The kinetics associated with the formation of the complex involving α-cyclodextrin and dicyclomine hydrochloride have also been investigated using the ultrasonic technique.

Thermodynamic and kinetic parameters associated with the exchange process involving alcohols and micelles

The partition coefficients of the alcohols butan-1-ol → hexan-1-ol between water and cetyltrimethylammonium bromide micelles have been obtained from solubility experiments. These results have been combined with previously obtained ultrasonic relaxation data to derive the kinetic and thermodynamic parameters associated with the exchange process involving alcohol molecules and cetyltrimethyl ammonium bromide micelles. Bearing in mind the simplicity of the approaches taken to interpret the equilibrium and kinetic measurements, there is remarkable self-consistency in the data.

Thermodynamics of solutions of polyelectrolytes, ionic surfactants and other charged colloidal systems

Thermodynamic arguments are used to derive a simple expression for the Donnan equilibrium between a solution of charged colloid plus electrolyte and a solution of electrolyte alone. From it, expressions for the solute chemical potentials and the osmotic pressure are obtained. Polydispersity, excluded volume effects and interactions between the various solute species are allowed for. The treatment has features in common with approaches based on the Poisson–Boltzmann equation but is simpler and is not restricted by considerations of particle geometry. It predicts successfully the characteristic behaviour of dilute solutions of polyelectrolyte plus salt and agrees with experimentally obtained activity coefficients of supporting electrolyte better than a prominent existing model. When the theory is applied to solutions of ionic surfactants the dubious assumptions inherent in some previous approaches are avoided, yet similar equations emerge which allow better for solution non-ideality and give good agreement with experiment. The treatment also permits the application to charged micelles of the methods successfully applied previously to non-ionic micelles. The application to colloid stability is discussed. Finally, the application to polyacid titration behaviour is outlined. This exposes some limitations of the theory and suggests appropriate modifications.

Equilibrium and kinetic studies associated with the interaction between sodium dodecyl sulfate and polyvinylpyrrolidone in aqueous solution

Binding isotherms for various sodium dodecyl sulfate–polyvinylpyrrolidone mixtures have been constructed from electrochemical measurements involving a dodecyl sulfate-selective membrane electrode. It was found that once the surfactant starts to bind to the polymer, small surfactant aggregates are formed. These aggregates grow in size until the polymer becomes fully saturated after which ‘free’ micelles are formed in solution. The degree of counterion dissociation was also measured using an electrode selective to the sodium counterion. The surfactant concentration at the start of binding and at the onset of the formation of free micelles was also determined using gel filtration. Finally the kinetics of the binding process were measured using ultrasonic relaxation measurements.

Analysis of the fast relaxation times for micelle kinetics taking into account new EMF data

The analysis of the concentration dependence of the fast relaxation time associated with monomer/micelle exchange in several ionic surfactants has been carried out using different modifications of the original Annianson and Wall relaxation equation and taking into account new emf data from surfactant-selective electrode measurements. In most of the published work describing the analysis of the fast relaxation time it has been normal practice to assume that in the micellar region the monomer surfactant concentration is constant and equal to the c.m.c. On the other hand, the electrode measurements have clearly shown that the surfactant monomer concentration decreases significantly in the micellar region. The micellar parameters that can be derived from the analysis of the fast relaxation times are very different when these two conditions are realised.

Chemical relaxation and equilibrium studies associated with binding of anionic surfactants to neutral polymers

The binding of the surfactants sodium octyl and decyl sulphates (SOS and SDeS) to poly (N-vinylpyrollidone)(PVP) and polyethylene oxide (PEO) has been examined. Equilibrium binding isotherms were obtained from e.m.f. studies using surfactant-ion-selective electrodes. The kinetic data were obtained from measurements of ultrasound absorption. Estimates of the binding rates were obtained by applying a phenomenological theory. It is concluded that, on balance, the surfactant aggregates at concentrations below their c.m.c. and the desorption rate of bound surfactant qualitatively obeys first- order kinetics. The interaction of the surfactant sodium hexadecyl sulphate (SHS) with PEO was also investigated using the pressure-jump apparatus.