The structure and kinetics of oil-in-water microemulsions stabilised by nonionic surfactants
This thesis is concerned with the behaviour of the single phase (10) alkane oil-in- water (01W) microemulsions stabilised by nonionic surfactants of the general structure H-(CH2)n(OCH2CH2)m-OH, abbreviated to CnEm. The aims were to attempt to characterise their behaviour in terms of the effect of the different constituents n, m, and alkane oil chain length (x).Initially the limiting temperature phase boundaries of the 10 region were established by measurements of turbidity. Turbidity was found to be at a minimum at the lower temperature phase boundary (the solubilisation phase boundary, SPB), and at a maximum at the upper temperature phase boundary (UTPB). The temperature position of the 10 region was found to be increased by increasing m or x, and decreased by increasing n. The width of the temperature range was generally found to be decreased by increasing the ratio of oil to surfactant (R). For a series of decane-inwater microemulsions stabilised by surfactants having a constant ratio n/m=2, the SPB was found to be little affected by an increased overall surfactant length, whereas the UTPB was found to be increased.The microemulsion droplet sizes were determined by both static and dynamic light scattering (turbidity and PCS respectively). The particle size was measured at varying temperatures and was found to be at a minimum at the SPB and to increase with temperature (explaining the increase of turbidity with increasing temperature). The increased particle size was assumed to correspond to clustering or growth of the microemulsion droplets. The size measured at the SPB was assumed to be that of the individual droplets at their preferred size. The hydrodynamic radius (rh) of the droplet at the SPB was used to calculate the area (As) occupied per surfactant molecule at the interface between the droplet core and the surfactant monolayer. As was found to be increased by increasing n, and decreased by increasing x, whereas m was found to have little effect.Turbidity measurements were found to be a simple method of measuring the extent of clustering or growth of the droplets with increasing temperature. From this information the equilibrium constant (K) and the associated standard enthalpy, entropy and Gibbs free energy changes were obtained. A simple scheme of droplet aggregation was postulated in which K for the addition of one droplet to an existing cluster (equating to growth by the equivalent of one droplet at the preferred size) was constant regardless of cluster size. The model was found to fit well to a middle range droplet concentration of 0.04 - 0.10 M. Measurements at lower droplet volume fractions were found to be too inaccurate, and K was found to decrease for fractions greater than this range. A model which would accommodate the decrease in K with higher droplet volume fraction would be highly complex and include further estimated parameters. It was therefore deemed appropriate to employ the simple model for comparison within the relevant range of droplet volume fraction. As expected K was found to increase with increasing temperature for all systems. The large positive enthalpy (of the order of 1000 kJmol-1) obtained for all systems accounts for the strong temperature dependence of the behaviour of these microemulsions. The positive entropy change (of the order of 3 JK4mol-1) was attributed to the increasing disorder of the water molecules following dehydration from the surfactant head group region. The removal of water from the head groups has the additional effect of increasing inter-head group and therefore interdroplet attractions thus promoting clustering or growth. The resulting negative Gibbs free energy (of the order of -14 kJmol-1) indicates that the clustering/growth process is spontaneous and entropically driven. The clustering/growth process was found to be dependent on the packing density of the surfactant head groups at the droplet monolayer surface. Assuming clustering occurs the thermodynamic parameters were calculated per mole of surfactant molecules involved in the droplet contact zone of the clusters. Droplets having the more widely spaced (less closely packed) head groups were found to have a larger enthalpy and entropy change, probably as a result of the greater requirement for dehydration of the head groups.A temperature jump method was employed to study the kinetics of the clustering/growth process. The results were found to be consistent with the equilibrium data, in that the rates of the clustering/growth process were found to be dependent on the packing density of the surfactant head groups. Droplets having the less closely packed head groups were found to cluster more slowly, again an indication of the greater difficulty in dehydrating the head groups. The activation energy was also calculated for the clustering/growth process, and was found to be of the order of a few hundred kJmol4, but no discernible trends with molecular structure of the components were observed.