Adsorption of polyvinylalcohol on paraffin-water interfaces and the properties of paraffin-in-water emulsions stabilized by polyvinylalcohol : an experimental study

The adsorption of polyvinylalcohol (PVA) on the paraffin-water interface has been studied by interfacial tension measurements and by direct measurement of the adsorption with emulsions. The aim of the study was to provide more insight into the factors which influence the adsorption of polymers at phase interfaces. The interfacial properties of polymers, some of which find application as adhesives, flocculating agents and emulsion stabilizers, are of great practical importance. This helps to explain why much fundamental research, both theoretical and experimental, has been performed in recent years on polymer adsorption. Only a small proportion of this research, however, has been devoted to the adsorption of polymers at liquid- liquid interfaces, and so this was chosen as the subject for our investigation. To this end, the interface of a well defined model system has been studied by several different techniques.

The interfacial tension was measured with the static Wilhelmy plate method as a function of the concentration of PVA (0.1-4000 ppm) and time (up to 30 h) for PVA samples differing in degree of hydrolysis, molecular weight and origin, Experiments were also done in a Langmuir trough with spread and adsorbed PVA monolayers at the paraffin-water interface. Interpretation of the interfacial tension measurements in terms of polymer adsorption is complicated by the irreversible nature of the latter. The irreversibility of polymer adsorption can be deduced from the fact that the measured interfacial tension depends not only on the concentration of PVA in the aqueous phase but also on the history of the interface. As a result, the Gibbs law of adsorption cannot be applied without further preface to polymers.

Characteristic differences were noted between the interfacial activities of PVA of different degrees of hydrolysis. The discontinuity found in the plots of interfacial tension (γ) against the logarithm of polymer concentration ( c_p ) for PVA with a low content of acetate groups (about 2 %) was absent from the corresponding plots for PVA with a higher acetate content (>12 %). The discontinuous nature of the former γ-log c_p curves was explained in terms of kinetic factors, namely the relative rates of diffusion and unfolding of the polymer, which determine how the interface is occupied. Differences in the mechanism of interfacial occupation will be reflected by differences in the configuration of the adsorbed polymer molecules. This picture was confirmed by the time-dependence measurements,

The molecular weight of the PVA was found to have no obvious effect on the interfacial tension, whereas the adsorption measured directly on the emulsion interface clearly increased with increasing molecular weight. One may conclude therefore that there is no well-defined relationship between the reduction in interfacial tension and the amount of polymer adsorbed. In this respect, the behaviour of polymer contrasts with that of low-molecular-weight surfactants.

In the adsorption experiments with emulsions, the specific area of each emulsion was determined by measurement of the turbidity. The degree of dispersion went through a maximum with increasing PVA concentration when the acetate content of the latter was ≥12 %, This maximum corresponded to a discontinuity in the adsorption isotherm. The concentration at which the maximum was situated decreased with increasing molecular weight and with decreasing volume fractions of paraffin. An explanation for this was again sought in terms of kinetic factors influencing the formation of the stabilizing polymer layer during emulsification.

The properties of the polymer layer of the emulsions were markedly different, depending on whether the emulsion had been prepared with a polymer concentration below or above that corresponding to the maximum specific area. The emulsion area was large when the amount of polymer per ml paraffin was kept low. A number of tentative experiments were done to enable the mechanism of emulsion formation to be studied more closely.

All emulsions obtained were found to be extremely stable to coalescence right down to low polymer concentrations.

Comparison of the results of the different measurements with the same system shows that the way in which the interface is formed has a great influence on its properties. This underlines the irreversible nature of the adsorption, which is one of the reasons why none of the existing theories is adequate to explain the measured effects quantitatively. By virtue of the many different measurements, however, this tudy serves as a basis for further theoretical and experimental work on the surface- active properties of polymers. In addition this investigation helped to show which factors determine the emulsifying capacity of polymers.