D. M. Bloor

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.

Kinetic and equilibrium studies associated with the formation of inclusion compounds involving n-butanol and n-pentanol in aqueous cyclodextrin solutions

The equilibrium constants associated with the inclusion compounds of n-butanol and n-pentanol with α-cyclodextrin in aqueous solutions have been measured using the head-space analysis technique involving gas chromatography. These data confirm that a 1:1 inclusion complex is formed. The kinetics associated with the formation of these inclusion compounds have been studied using the ultrasonic relaxation method. From a detailed analysis of relaxation data it is clear that the kinetics of the process do not conform to the simple behaviour suggested by the equilibrium experiments. In addition there are also discrepancies between values of the volume change of the reaction determined independently from relaxation and density data. The significance of these results is discussed in terms of a two-step mechanism for the formation of the complex.

Adiabatic compressibility of surfactant micelles in aqueous solution

The compressibilities of micellar solutions of the surfactants sodium dodecyl sulphate (SDS), sodium decyl sulphate (SDeS), sodium octyl sulphate (SOS), sodium decyl sulphonate (SDeSon), decyltrimethylammonium bromide (DeTABr), decylammonium bromide (DeABr) and dodecyltrimethylammonium bromide (DTABr) have been determined from sound-velocity measurements. A theoretical treatment has been introduced which enables the micellar compressibility (Kmic) to be evaluated from these data. From a systematic study of the above compounds it was found that a significant factor which controls the overall compressibility of the micellar aggregate is the nature of the surfactant head group. The overall picture is that the micelle resembles an aggregate with a compressible core surrounded by a less compressible surface structure.

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.

The binding of sodium hexadecyl sulphate to poly(N-vinylpyrrolidone) in aqueous solutions. Kinetic and equilibrium studies

Binding isotherms and kinetic measurements associated with the adsorption/desorption of the surfactant sodium hexadecyl sulphate (SHS) onto the neutral polymer, poly(N-vinylpyrrolidone)(PVP), at two polymer concentrations are reported. The surfactant monomer concentrations were estimated in situ for all solutions from e.m.f. measurements of a cell containing the surfactant ion-selective electrode. The binding isotherms are very similar to those reported for shorter-chain alkyl sulphates in the presence of PVP. In addition, spectral data associated with the 3 : 1 intensity ratio of the vibronic fluorescence bands of pyrene have also been measured at the same polymer concentrations as a function of increasing SHS. The purpose of these measurements was to use the spectral data on the pyrene as a probe to monitor the hydrophobic environment in the polymer–surfactant formulation. Finally, kinetic (relaxation) data were measured with the pressure-jump relaxation technique and revealed a single relaxation process at all concentrations. The results indicate that the driving force during the binding process is a self-aggregation of the surfactant ions which occur as a result of the initially bound surfactant, creating a nucleus for the promotion of small aggregates on the polymer chain. Application of the linear phenomenological theory to the binding and kinetic data shows clearly that the adsorption rate increases with increasing amount of bound surfactant and that the desorption rate is proportional to the amount of bound surfactant. The desorption rate constant measured for the two formulations agree well. The relaxation data were also analysed using the Aniansson and Wall kinetic treatment associated with monomer aggregate exchange.

Kinetic and equilibrium studies associated with the solubilisation of n-pentanol in micellar surfactants

Ultrasonic relaxation studies in mixed micellar solutions of the surfactants sodium decylsulphate and dodecyltrimethylammonium bromide with n-pentanol have shown two well defined relaxations associated with the exchange process involving the partitioning of alcohol and also surfactant monomer with the mixed micelle. The monomer concentration of n-pentanol has been measured using head-space analysis involving gas chromatography, and the surfactant monomer concentrations have been determined from specially fabricated surfactant-selective electrodes. The combined relaxation/equilibrium data have been analysed using a phenomenological treatment in which the kinetics and thermodynamics associated with both exchange processes have been evaluated. The main conclusion from this work is that the association of both alcohol and surfactant monomer to the mixed micelle is almost a diffusion-controlled process and that the aggregation numbers of the mixed micelles apparently decrease when alcohol is added.