We have used a custom-built ultrasound velocimeter to carry out high-precision velocity measurements of reverse micelle solutions, made of ionic (AOT) and nonionic (C₁₂E₄) surfactants in oil, as a function of water concentration. We show that the observed velocity variation as a function of increasing water concentration differs from the characteristics of the surfactant polar headgroups. The complex profile of compressibility curves obtained from velocity and densimetric measurements can be accounted for by the relation existing between the surface polar headgroup of each surfactant and the number of interacting water molecules. At the highest water concentration, the compressibility parameters obtained are different from those reported for “bulk” water and reflect the peculiar properties of confined water.

The self-diffusion coefficient of amphiphilic probes in the L₃ phases of a surfactant system has been measured by fringe pattern photobleaching (FRAPP) experiments. The variation of the diffusion coefficients with the surfactant volume fraction is in agreement with a model for diffusion in cubic phases, which allows to determine approximately the topology of the phases. The observed variation of the diffusion coefficients with the probe lengths agrees with the free-area model for surfactant diffusion.

The microstructure of a ternary system of a non-ionic surfactant (n-dodecyl pentaoxyethylene glycol ether, C ₁₂E ₅), water and a water-soluble random heteropolymer, poly (styrene-r-Na styrene sulfonate) with degrees of sulfonation ranging from 30% to 90% is investigated at room temperature. Using small angle X-ray scattering and freeze-fracture electron microscopy we focus on the lamellar phase region and study samples along four different paths of constant surfactant-to-water ratio with increasing polymer content. We demonstrate that for samples with low water content (S/W ratios 3.4 and 4.9), regardless of the sulfonation degree of the polymer, the lamellar phase persists up to several weight percent of polymer and the polymer molecules are confined inside the surfactant bilayers. With increasing quantity of polymer the lamellar phase Bragg peak shifts in a manner consistent with the increase of the lamellar period, which indicates increased bilayer thickness. For mixtures less concentrated in surfactant first order phase transitions are observed for all polymer charge contents when the amount of polymer is increased. For low charge contents of polymer (30% and 45%) the lamellar stack of the polymer-doped surfactant bilayers coexists with either cubic or another lamellar phase and microscopic phase separation is only observed. For large charge contents (65% and 90%) a surfactant-rich lamellar phase coexists with a polymer-rich water solution. It is suggested that the observed phase behaviour is governed by the concentration number of free water molecules.

We introduce a new method to measure the energetics and range of weak biochemical bonds using functionalized vesicles. Large bilayer regions are held in molecular proximity by osmotic depletion forces to enable rapid specific bonding. By fixing an electrical charge to the tethering site of the functional group on one surface, persistent adhesion of the vesicles after removal of the depletion stress is titrated against the clamped electrostatic potential of the opposite surface. We demonstrate the method with DNA bases and obtain new information on the range of their specific interactions.

Grazing incidence x-ray surface scattering has been used to investigate liquid surfaces down to the molecular scale. The free surface of water is well described by the capillary wave model (<z(q)z(-q)> ~ q-2 spectrum) up to wavevectors > 10^8 m^-1. At larger wavevectors near-surface acoustic waves must be taken into account. When the interface is bounded by a surfactant monolayer, it exhibits a bending stiffness and the bending rigidity modulus can be measured. However, bending effects generally cannot be described using the Helfrich Hamiltonian and the characteristic exponent in the roughness power spectrum can smaller than 4. Finally, upon compression, tethered monolayers formed on a subphase containing divalent ions are shown to buckle in the third dimension with a characteristic wavelength on the order of 10^8 m^-1.