Experimental Evidence of Fractal Aggregates in Dense Microemulsions
The phase in which a microemulsion is of water in oil type, as shown by S.A.N.S. data, can be considered as a colloidal suspension. The pair potential presents a repulsive hard-core plus a Yukawa tail representing the attractive interaction1) V(r) = VA(r) + VR(r). This potential form, similar to that used in the DLVO (Derjaguin, Landau, Verwey and Overbeek)2 theory for colloids, shows two minima with a barrier and gives origin to interesting phenomena such as a phase transition with an upper cloud point temperature and a percolation-like transition 1,3 that suggest aggregation processes can be present in our system. Furthermore, the packing fraction of the droplets, keeping constant their sizes, can be easily changed1, giving rise to a very dense liquid. From a microscopic point of view, the motion of the individual droplet is constrained by the interaction among its neighbours. At normal densities, the probability of an entrapment of the particle, in a cage formed by its nearest neighbours, is low and the particle can diffuse over large distances. For high concentrations (very high packing) the diffusional motion of the particle is dominated by a continuous and, for long time trapping into structural cages, translational motion is possible only if a hole is opened in these cages (for high dense systems the probability of a hole to be opened is very small). This latter process, which corresponds to a slowing-down in the density correlation function, is the configurational or structural arrest, well described by mode-mode coupling theories4 on glassy state. The glass-transition can be studied by dynamic light scattering as a function of the microemulsion concentration; in particular we measure the dynamic structure factor S(k,τ) proportional to the autocorrelation function of the scattered field g1(k,τ). Its initial slope is the mean linewidth (Γ) of spatial fluctuations of wavevector k. Care measurements5 of this latter quantity have shown in the system AOT-water-decane the slowing-down of the density-density correlation function supporting the idea of large clustering effects among the spherical droplets in agreement with structural models of simple glasses generated by the assembly of hard spheres. Different theories6, in particular the well-known free volume theory, indicate that glass transition is a cooperative phenomenon where all particles are involved; in particular several models invoke the presence of clusters. Molecular Dynamics7 experiments in densely packed hard spheres give evidence that large clusters can arise spontaneously. Therefore, we have several suggestions that at high concentrations large clusters can originate from the droplets aggregation and light scattering (elastic and quasi elastic) experiments can give a direct way of studying the system, particularly in the region where it presents a glass transition verifying if ordered structures are present, and of measuring their dimensions and the kinetics of the aggregation. Also in an indirect way viscosity measurements can give information about such a process; in particular as shown in the final part of this work the data of this quantity as function of temperature and of volume fraction ø present a well pronounced peak that in the frame of the current theories can be ascribed to an aggregation process.
KeywordsColloidal Suspension Pair Correlation Function Attractive Potential Dynamic Structure Factor Cloud Point Temperature
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