P. Codastefano

Aggregation kinetics in model colloidal systems: a light scattering study

Abstract We report an extensive set of quasi-elastic light scattering measurements of the hydrodynamic radius RH of aggregating polystyrene latices. The experiments have been performed in the transient regime in order to follow the kinetics of the aggregation process induced by the addition of a simple salt. We are able to distinguish various regimes of aggregation in different physical situations. We observe reversible flocculation and diffusion limited cluster aggregation (DLCA) in latices of 481 nm diameter, but also reaction limited cluster aggregation (RLCA) followed by DLCA for particles of 91 nm. The growth laws of RH in time have been verified and give exponential RLCA kinetics for short times and power law DLCA kinetics for long times. Both the exponential and the power law behaviors are observed in the intermediate time regime. In the DLCA case, for the largest particles we used, deviations from the asymptotic law have been obtained and can be described in terms of the Smoluchowski model for Brownian aggregation, together with the fractal nature of the resulting aggregates.

Electrical conductivity of colloidal systems during irreversible aggregation

The low-frequency electrical conductivity of dielectric colloids (polysterene and silica) and metallic (silver) ones in aqueous solutions shows a power-law behavior in time during the salt-induced cluster-cluster aggregation. We interpret this fact using current theories of heterogeneous dielectric systems. The fractal nature of the clusters, checked by light scattering, determines the exponent of the power law.

Kinetics of Aggregation in Colloidal Systems. Electrical Conductivity Measurements

We report low-frequency measurements of the electrical conductivity of charged colloids in an aqueous solution containing a simple electrolyte as aggregating agent. The measurements are performed as a function of time in a regime where the aggregation is diffusion limited. The results for the relative increase of conductivity can be described in terms of an asymptotic power law in time with an exponent 0.73???0.01. A possible connection of this exponent with the fractal dimension df of the clusters formed by aggregation is given.

Electrical conductivity of concentrated polystyrene latices in the fluid to crystalline phase transition

Mesure de la conductivite electrique des suspensions concentrees de particules spheriques de polystyrene

Experimental and theoretical studies of critical phenomena in supramolecular fluid systems

We report on critical phenomena in multicomponent fluids consisting of three‐component Water/Oil/Surfactant microemulsions and on nonionic surfactant/water/binary mixtures. Due to the existence of aggregates of quasi‐macroscopic sizes, static and dynamic background effects become important. A method of analysis of experimental data, involving both mode coupling equations and the linear model equation of states, is developed. All the available experimental results can be very well explained by the mode‐coupling theory involving 3‐d Ising critical exponents.

Static Electrical Conductivity at the Droplet-Lamella Phase Transition of a Water-in-Oil Microemulsion

We study the static electrical conductivity of microemulsions in the single-phase region below the percolation threshold, where the data are interpreted in terms of a fluctuating charge model, and in the lamellar phase above percolation, where conductivity is due to randomly oriented water layers.