Large-Scale Molecular Systems pp 537-543 | Cite as

# Experimental Evidence of Fractal Aggregates in Dense Microemulsions

## Abstract

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 interaction^{1}) V(r) = V_{A}(r) + V_{R}(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 changed^{1}, 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 theories^{4} 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 g^{1}(k,τ). Its initial slope is the mean linewidth (Γ) of spatial fluctuations of wavevector k. Care measurements^{5} 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 theories^{6}, 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 Dynamics^{7} 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.

## Keywords

Colloidal Suspension Pair Correlation Function Attractive Potential Dynamic Structure Factor Cloud Point Temperature## Preview

Unable to display preview. Download preview PDF.

## References

- 1.M. Kotlarchyk, S.H. Chen, J.S. Huang and M.W. Kim, Phys. Rev. Lett. 53 941 (1984); Phys. Rev. A 29, 2054 (1984).CrossRefGoogle Scholar
- 2.E.J. Verwey and J. Th. Overbeek, “Theory of the stability of lyofobic colloids” (Elsevier, Amsterdam, 1948).Google Scholar
- 3a.M.A. van Dijk, Phys. Rev. Lett. 55, 1003 (1985);CrossRefGoogle Scholar
- 3b.M.W. Kim and J.S. Huang, Phys. Rev. A 34, 719.(1986)CrossRefGoogle Scholar
- 4a.U. Bengtzelius, W. Götze and A. Sjölander, J. Phys. C, 17, 5915 (1984).CrossRefGoogle Scholar
- 4b.U. Bengtzelius, Phys. Rev. A, 34, 5059 (1986).CrossRefGoogle Scholar
- 5a.S.H. Chen and J.S. Huang, Phys. Rev. Lett. 55, 1888 (1965);CrossRefGoogle Scholar
- 5b.E. Sheu, S.H. Chen, J.S. Huang and J.C. Sung, Phys. Rev. A. 39, 5867 (1989).CrossRefGoogle Scholar
- 6.see J. Jackie, Rep. Prog. Phys., 49, 171 (1986).CrossRefGoogle Scholar
- 7.H. Jonsson and H.C. Andersen, Phys. Rev. Lett.,60, 2295 (1988).CrossRefGoogle Scholar
- 8.J. E. Martin and D.W. Shaefer, Phys. Rev. Lett. 53, 2457 (1984).CrossRefGoogle Scholar
- 9.D. W. Shaefer and C.C. Han, in Dynamic light Scattering edited by R. Pecora (Plenum, New York, 1985).Google Scholar
- 10.J. E. Martin and J. Ackerson Phys. Rev. A 31, 1180 (1985)CrossRefGoogle Scholar
- 11.S.H. Chen and J. Teixeira, Phys. Rev. Lett. 57, 2583 (1986).CrossRefGoogle Scholar
- 12.D.A. Weitz, J.S. Huang, M.Y. Lin and J. Sung, Phys. Rev. Lett., 54, 141 (1985).CrossRefGoogle Scholar
- 13.M.J. Grimson and G.C. Barker, Europhys. Lett., 3, 511 (1987).CrossRefGoogle Scholar
- 14.R.F. Berg, M.R. Moldover. and J.S. Huang, J. Chem. Phys., 87, 3687 (1987).CrossRefGoogle Scholar
- 15.See for example “Scaling phenomena in disordered System” edited by R. Pynn and A. Skjeltorp (Plenum, New York 1985).Google Scholar
- 16.S. Chandrasekhar, Rev. Mod. Phys., 1943, 15, 1 (1943).CrossRefGoogle Scholar
- 17.D. Eagland in “Water a comprensive treatise” edited by Franks F., Vol. 5 (Plenum, New York, 1975).Google Scholar
- 18.R. Botet, R. Jullien and M. Kolb J. of Phys. A, 17 L75 (1984).CrossRefGoogle Scholar
- 19.P. Mills, J. Phys. (Paris) Lett. 46, L301 (1985).Google Scholar