David C. Steytler

Structural studies of microemulsions stabilised by aerosol-OT

Abstract In this article we review structural studies of AOT-stabilised water-in-oil microemulsions. Nanometer-sized spherical droplets of water coated by a curved monolayer of surfactant are preferred over a wide range of pressure and temperature conditions, as well as in very different physical environments, for example a supercritical or a solid alkane. Within the one-phase region variation of pressure and temperature essentially serves to fine-tune the strength of the attractive interactions between droplets.

Fluoro-surfactants at air/water and water/CO2 interfaces

Aqueous phase behaviour and water-in-carbon-dioxide (w/c) microemulsion formation were studied with various fluorinated sulfosuccinate surfactants. For aqueous solutions of two different compounds surface coverages measured by neutron reflection and surface tension were consistent, giving reliable values for mean areas per molecule at the air/water (a/w) interface. At 20°C and pressures above about 250 bar, seven different surfactants were found to be effective at stabilising w/c microemulsions. With reference to recent work (J. Eastoe, A. Downer, A. Paul, D. C. Steytler and E. Rumsey, Prog. ColloidPolym. Sci., 2000, 115, 214) it is possible to identify a structure–performance relationship for these surfactants in water–CO2 systems. Comparison of the phase behaviour shows that sodium bis(1H,1H-nonafluoropentyl)-2-sulfosuccinate (di-CF4) forms microemulsions at the lowest pressure, e.g. for w = 20 ([water]added/[surf]) at 30°C the w/c phase was stable down to 120 bar. High-pressure FTIR spectroscopy indicates a fraction of the added water partitions out of microemulsion droplets, thereby saturating the bulk CO2 . Furthermore, high-pressure small-angle neutron scattering (SANS) is characteristic of a simple spherical droplet structure in the microemulsions. SANS data also indicate temperature-induced changes in radius, and this is consistent with partitioning of water to maintain saturation of the bulk CO2. For three different surfactants it has been possible to compare adsorption at a/w and w/c surfaces, highlighting differences in packing requirements to stabilise these two different interfaces.

Fluorinated surfactants in supercritical CO2

Abstract Liquid or supercritical carbon dioxide has important environmental and economic advantages over petrochemical solvents currently used for industrial processes. However, low solubility in CO2, particularly of polar compounds, is a hurdle to its implementation as an acceptable alternative. These solubility problems have been overcome by employing specialised fluorinated surfactants to stabilise water nano-droplets as water-in-supercritical/liquid CO2 microemulsions. Such novel microemulsions can now facilitate innovative ‘green-and-clean’ applications of carbon dioxide technology.

Scattering studies of microemulsions in low-density alkanes

Using small-angle neutron scattering and dynamic light scattering we have studied the structure and dynamics of water-in-oil microemulsion droplets stabilised by the surfactant Aerosol-OT in the low-density alkanes propane and n-butane. The phase stability, interdroplet structure and droplet diffusion coefficient are dependent on the alkane density. At densities of ca. 0.6 × 103 kg m–3 surfactant-coated, essentially monodisperse, randomly distributed, weakly interacting, spherical water droplets are present. At lower densities enhanced droplet–droplet attractions promote large, transient clusters of droplets. The intradroplet structure (i.e. droplet size) appears to be independent of alkane chain length and density.

Spectrophotometric studies of food colors in near-critical carbon dioxide

The feasibility of using pressurized carbon dioxide as an extraction solvent for food colors has been investigated by semi-quantitative solubility tests in liquid C02 on a variety of permitted food colors. Tests included the effect of using non-toxic entrainers at low levels. The solubilities of some of the colors in liquid and supercritical C02 have been more accurately determined spectrophotometrically using a high-pressure optical cell: the carotenoids and chlorophylls show limited solubility, which is enhanced by increasing the pressure or the temperature or both. The spectra of the carotenoids indicate that CO2 is a weakly interacting solvent, but their solutions in CO2 display pressure- and temperature-dependant solvatochromism.

Rod-like micelles thicken CO2

A new approach to thicken dense liquid CO(2) is described using the principles of self-assembly of custom-made CO(2) compatible fluorinated dichain surfactants. Solutions of surfactants in CO(2) have been investigated by high-pressure phase behavior, small-angle neutron scattering (HP-SANS) and falling cylinder viscosity experiments. The results show that it is possible to control surfactant aggregation to generate long, thin reversed micellar rods in dense CO(2), which at 10 wt % can lead to viscosity enhancements of up to 90% compared to pure CO(2). This represents the first example of CO(2) viscosity modifiers based on anisotropic reversed micelles.

Rotational dynamics of AOT reversed micelles in near-critical and supercritical alkanes

Time-resolved fluorescence depolarisation studies of Aerosol-OT (AOT) reversed micelles containing the hydrophilic probe perylene tetracarboxylate in ethane and propane at high pressure are reported. Such measurements have been made at sub-nanosecond resolution and support the concept of a simple spherical structure for AOT-stabilised aggregates in supercritical and near-critical fluids at high pressure. The results show a negligible pressure dependence for both the micellar rotational correlation time, ϕmic, and the rotational correlation length, ξr. For w= 3 (w=[water]/[AOT]) reversed micelles far from the one-phase (L2)→ two-phase boundary in both supercritical ethane and near-critical propane, the results obtained for the hydrodynamic radius of the reverse micelles, rh, are consistent with those obtained from other techniques. As the microemulsion phase boundary is approached by lowering the pressure both the solvent density and viscosity decrease, but the rotational correlation length, ξr, increases only slightly. As w increases in the region between w= 3 and w= 10, the internal motion of the probe molecule, 3,4,9,10-perylene sodium tetracarboxylate (PTC) becomes dominant, and the technique becomes less sensitive to overall droplet rotation and therefore to the determination of the droplet size.

Ion reactivity in reversed-micellar systems. Kinetics of reaction between micelles containing hydrated nickel(II) and murexide-containing micelles in the system aerosol–OT + water + heptane

A stopped-flow kinetic study has been made of a metal–ligand substitution process taking place in Aerosol–OT stabilised water pools in n-heptane. In a typical stopped-flow experiment, reversed micelles containing solubilised metal ion are mixed with micelles containing ligand such that the total concentration of micelles is unchanged on mixing. Micelle sizes are dependent on the water: surfactant mole ratio “R”(where R=[H2O]/AOT]) and the system has been characterised by ultracentrifugation methods over the range R= 0.26–16.9. The kinetic results for complexation in the reversed micellar system support a mechanism involving rapid exchange of reactants between aqueous pools followed by rate-limiting loss of a solvated water molecule from the metal-ion in a water pool. Rate constants for ligand release and water-exchange in the water pools are close to the bulk water values and are not very dependent on pool size.

Influence of pressure and temperature on microemulsion stability

For three different microemulsion systems the stability of the one-phase region in pressure–temperature (P/T) space has been studied. We also report P/T stability studies of the sodium bis(2-ethylhexyl) sulphosuccinate (AOT)–water–n-alkane microemulsion droplet system using the straight-chain alkanes from the homologous series propane to n-decane as oil. On progressively decreasing the carbon number of the oil the P/T phase boundaries of these systems exhibit a reversal of behaviour. By plotting microemulsion stability on the oil density–temperature surface we have normalised the phase behaviour observed in P/T space for AOT–water–n-alkane systems. Further, the oil density–temperature plots suggest that a minimum oil density of ca. 0.5 × 103 kg m–3 is required for AOT-stabilised microemulsion droplets to exist as discrete entities.

Branched trichain sulfosuccinates as novel water in CO2 dispersants

A series of highly branched trichain sulfosuccinate surfactants have been synthesized and studied in condensed CO2 and aqueous environments. Aqueous critical micelle concentrations (CMCs) showed a general trend of increasing CMC with decreasing chain length, whereas increased branching appeared to increase solubility in CO2 and aid the dispersion of water. Near infrared spectra confirmed observed cloud with a large increase in solubility above the cloud pressures in this solvent.