Particle size analysis of latex suspensions and microemulsions by photon correlation spectroscopy

The particle size in microemulsions and other highly dispersed systems was determined by means of photon correlation spectroscopy (PCS). As PCS cannot be applied to highly concentrated dispersed phases, the measurement accuracy was tested for its dependence on the particle concentration using latex suspensions. The data obtained by clipping and scaling were compared. The particle size determination was expected to provide information about the influence of the structure of the surfactant system on microemulsions, using a homologous alcohol series as cosurfactant and potassium oleate as surfactant. In this system the region of solubilization is characteristically divided from the region of microemulsification by a zone of instability. Furthermore, there are distinct differences in mean particle sizes between microemulsions (9-30 nm) and micellar solutions (4-6 nm).

Particle size distributions and particle size alterations in microemulsions

Several component diagrams for different alcohols as cosurfactants and potassium oleate as the surfactant were investigated. Between the regions of the water-in-oil microemulsion and the micellar solution (reverse micelles), and diagrams showed a zone of instability which was determined by particle size analysis by means of photon correlation spectroscopy (PCS). The polydispersity of the internal phase was determined at the instant of microemulsion formation and after fixed intervals of storage. At the edge of the microemulsion region, a rapid increase in particle size due to coalescence followed by breaking was observed. If a slow transition toward the region of solubilization could be seen by a slow decrease of the droplet size, coalescence was observed after storage. In the middle of the microemulsion region, the particle radius, however, was almost constant for a long interval. The velocity of the microemulsion formation depended on the alcohol used. In some cases a very fast formation (milliseconds) was observed; in others, macroemulsions were formed which became transparent within a few hours or days. PCS was used to follow the dynamic process of formation and breaking of such systems with droplet diameters of 5-200 nm by obtaining the mean hydrodynamic diameters. Distribution curves were calculated by the Laplace transform of the correlation function. The practicability of the method was demonstrated with mono- and polydispersed latex suspensions and microemulsions.