Static and dynamic light scattering studies of water-in-oil microemulsions in the critical region. Evidence of a crossover effect

Abstract

Extensive light scattering measurements, including the intensity, turbidity, and linewidth, on a three-component microemulsion system consisting of mixtures of water, decane, and a surfactant sodium di-2 ethylhexylsulfosuccinate (AOT) (WDA), have been made. The critical and several off-critical mixtures have been studied along constant microemulsion droplet volume fraction lines in the one-phase region over a very large temperature range. In the vicinity of the lower phase separation temperature Tp the intensity data are very well accounted for by the standard theory of critical binary fluids using a single value for the short range correlation length ξ0=(13.5±1.5) Å. By combining a mode-coupling theory including the background effects and a linear model equation of state applicable in the critical region, we have been able to fit the dynamic light scattering data using a Debye cutoff length q−1D which is equal to the constant average diameter of microemulsion droplets. Furthermore, we find clear evidence for a crossover from critical to single particle behavior in both static and dynamic light scattering data. A crossover temperature Tx has been identified at which qDξ(Tx)=1. Analyses of the dynamic light scattering data show that qD, which can only be measured far away from Tp, in fact plays a decisive role in controlling the critical dynamics in the whole temperature range.

Electrical conductivity and percolation phenomena in water-in-oil microemulsions

“…As far as conductivity in the lamellar phase is concerned we model the system as a collection of capacitors coupled in series. The overall conductivity of the microemulsion in this phase is 2 1 &I &w o= -[cri(Iy )+o y ]+- 3 3 cr (I -P )+cr~P where oi and o are the conductivities of the surfactant coated layer and the water layer, respectively, and p the volume fraction of water. The above equation is a simplified version of the effective-medium theory expression which takes into account the fact that there was a random distribution of lamellae orientation.…”

“…8, the radius of the surfactant coated water droplet is calculated to be 67 A. The measured hydrodynamic radius by light scattering indicates that Rh =85 A [3]. Small-angle neutronscattering experiments show that these microemulsion droplets are spherical on the average and with a size distribution of about 30% around the mean [2].…”

The coexistence curves for quarternary microemulsion of {water+KCl+sodium di(2-ethyl-1-hexyl)sulphosuccinate+decane} in the critical region

“…The study of critical phenomena in fluids has been extended recently to micelle solutions and multicomponent microemulsion systems [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. There are some controversies about whether the microemulsion systems and micelle solutions near the critical points belong to the 3D-Ising universality class [1][2][3][4][5][6][7][8][9][10].…”

“…There are some controversies about whether the microemulsion systems and micelle solutions near the critical points belong to the 3D-Ising universality class [1][2][3][4][5][6][7][8][9][10]. It has been pointed out by Fisher [8] that the critical exponents of a binary solution become renormalized when a third component as a impurity is added into it, and the coexistence curve should be characterized by an exponent b * = b/ (1 À a) = 0.367 with a = 0.11 and b = 0.3265.…”

Small-angle light scattering in dense microemulsions, transition from droplet to bicontinuous phase

“…In the vicinity of room t e m p e r a t u r e its phase diagram is dominated by an L2 phase (a (*) Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4-8, 1994. thermodynamically stable single phase of water-in-oil microemulsions). Extensive scattering measurements, light scattering [2,3] and in particular small-angle neutron scattering (SANS) [4] have shown conclusively that in this phase microemulsions are made of water droplets coated by a monolayer of surfactant molecules suspended in the oil. The average radius of the droplet is a linear function of the water-surfactant molar ratio w = [water]/ [AOT].…”