Static and dynamic properties of water-in-oil microemulsions near the critical and percolation points

Abstract

AbstmcL The three-component ionic microemulsion system consisting o f ~o~/water/decane shows an U~USUBI phase behaviour in the vicinity of room temperature The phase diagram in the temperaturevolume-fra~tio" (of the dispened phase) plane exhibits a lower coiisolute critical point at about 40 'C and 10% t'olume fraction. A percolorion line, stmtmg from the vicinity of the critical point. cuts across the plane. extending to the high-volume-fraction side at progrecsively lower temperatures. This phase behaviour can be understood in t e r m o f a system o f polydispersed spherical water droplets, each coated by a monolayer of AOT, dispersed in a continuum of oil. These droplets interact with each other via n hard-core plus a shon-range attractive interaction. the strength of which increases with temperature. We rhow thal Baxter's sticky-sphere model CM account for the phase behaviour, including the percolation line, quantitatively provided th3t the stickiness pmmeter is a suitable function of temperature. We use the structure factors measured by small-angle neutron scattering (SANS) below the critical temperature to determine this functional dependence. We 3150 invstigate the dynamics of droplets. below and approaching the critical and pcrcolalion points, by dynamic light scattering. The first cumulant and time evoluuon of the droplet density correlation function can be quantitatively calculared by assuming the existence o f polydispersed fractal clusters formed by the microemulsion droplets due to attraction. The relaxation phenomena obselved in an extensive set of measurements o f electrical conductivity and permittivity close to percolalion can also be inrerpreted through the same cluster-forming mechanism, which reproduces the most relevant features o f the frequency-dependent complex dielectric constant o f this system.

Ideal glass-glass transitions and logarithmic decay of correlations in a simple system

“…With this aim we study an extension of the hard sphere system (HSS), the sticky hard spheres sys-tem (SHSS) introduced by Baxter [17] and used extensively, in particular in colloid physics [18,19]. The HSS is the archetype of a simple system and its only control parameter is the packing fraction φ = πnσ 3 /6, with σ denoting the hard sphere diameter.…”

Clusters in simple fluids

“…What is more, percolation lines are frequently observed in attractive colloidal suspensions. According to the particular physical situation, the percolation threshold is associated with a sharp change in the following quantities ): i) Electrical conductivity in water-in-oil microemulsions (Chen et al, 1994;Weigert et al, 1997), ii) Viscosity of triblock copolymer micellar solutions (Mallamace et al, 1999(Mallamace et al, , 2001, iii) Elastic modulus of dispersions of silica spheres grafted with polymer chains (Grant and Russel, 1993), and iv) Correlation functions, measured by dynamic light scattering, of dispersions of silica spheres coated with stearyl alcohol (Verduin and Dhont, 1995). It is worth noting that in all these cases the percolation line passes in the vicinity of the critical point.…”

Phase diagram of the adhesive hard sphere fluid

“…Percolation can be an observable phenomenon in colloidal systems, and may sometimes be detected directly, such as by a sudden increase in electrical conductivity across the sample. 5 The Baxter model is particularly appealing in the light of recent theoretical 6 and experimental 7 work that predicts and confirms the existence of a re-entrant glass-liquid-glass transition in colloidal systems with short-range attractive forces. For very short-range attraction there is also a glass-glass transition between a caged ''repulsive'' glass and an energetic ''attractive'' glass.…”