Max Leo Robbins

Model for microemulsions. I: Effect of sulfonate surfactant cation and chain size and concentration on phase behavior

One/one aqueous NaCl/decane systems containing C12 o-oxylene sulfonates neutralized with mono-, di-, and triethanol amines were equilibrated and phase volumes determined. The systems phase split into 1 to 3 surfactant-rich phases in equilibrium with excess brine and/or decane. Both birefringent and optically homogeneous surfactant phases showed trends in phase volumes with varying salinity, head group size, chain length, and surfactant concentration. Monoethanol ammonium (MEA) and triethanol ammonium (TEA) sulfonates showed opposite trends with increasing concentration; optimal salinity rose with increasing TEA sulfonate and dropped with MEA sulfonate concentration. The diethanol ammonium (DEA) sulfonate showed an optimal salinity essentially independent of surfactant concentration as did equal weight mixtures of MEA and TEA sulfonates. Optimal salinity increased in the order MEA < DEA = MEA/TEA (11) < TEA indicating that surfacant hydrophilicity increases with increasing head group size. These trends and those with varying chain length are consistent with a proposed model mutually relating water and oil uptakes, interfacial curvature, and head and chain size. The model focuses on the structure of the surfactant at the brine/oil interface and relates structure to surfactant capacity to hold brine and oil together in the microemulsion. It examines the influence of surfactant head and chain volumes and water oil interfacial “solubility” on the direction and extent of interfacial curvature. The effect of these variables on interfacial curvature is in turn related to water and oil uptake. Water uptake increases and oil uptake decreases with increasing head/chain volume ratio, oil alkane carbon number and temperature (sulfonates), and decreasing salinity and aromaticity. The model postulates the coexistence of domains of water and oil continuity and applies spherical geometry to oil and water droplets in these domains. The resulting equations relate water and oil uptakes to the relative thicknesses and volumes of surfactant heads and chains. These structural parameters at balance are evaluated from surfactant heads and chains molecular olumes and interfacial areas. The model provides a physical explanation for the well-known empirical hydrophile/lipophile balance (HLB) system. It extends the HLB concept to include the influence of salinity, temperature, and oil composition.

Model for microemulsions: III. Interfacial tension and droplet size correlation with phase behavior of mixed surfactants

Abstract We have developed a model for microemulsions which relates surfactant structure to microemulsion droplet size, interracial tension, and phase behavior. The concept of curvature parameter, developed to describe microemulsion phase behavior (M. L. Robbins and J. Bock, J. Colloid Interface Sci.124, 462 (1988)), is expanded to cover interfacial tension, droplet size, and phase behavior correlations. The curvature parameter is made up of contributions from the thickness and volumes of surfactant polar heads and hydrocarbon chains at the water/oil interface. A set of equations is developed interrelating water and oil uptakes, droplet interfacial tensions, and droplet sizes via the curvature parameter and applied to equilibrated microemulsions prepared with mixtures of ethoxylated dinonyl phenol and the monoethanol amine salt of dodecyl o-xylene sulfonic acid in 1 1 decane/aqueous NaCl of varying salinity. The model predicts that droplet interfacial tensions should vary inversely with water and oil uptakes and that the square root of droplet tensions should vary linearly with the curvature parameters. Curvature parameters calculated from water and oil uptakes were found to plot linearly against salinity (see previous reference), suggesting that the square root of interracial tension should also plot linearly against salinity. Measured bulk interfacial tensions when plotted as √γ vs salinity gave linear plots as opposed to the conventional semilogarithmic plots which are highly curved. Measured bulk interfacial tensions varied inversely with water and oil uptakes and correlated very well with calculated droplet interfacial tensions using a sifigle empirical constant. Based on limited data, this constant appears to be universal for the systems tested. The current paper extends testing of theoretical correlations to include microemulsion droplet size. Droplet sizes in lower- (water-continuous) and upper- (oil-continuous) phase anionic microemulsions were measured as a function of aqueous salinity using dynamic (laser) light scattering on microemulsions prepared with the surfactant combination of monoethanol ammonium dodecyl ortho-xylene sulfonate/tertiary amyl alcohol (C12∗XS-MEA/TAA) in 1 1 oil/aqueous NaC1. The model predicts that micro-emulsion droplet size should be inversely proportional to the square root of the interfacial tension (√γ), which is in turn inversely related to water and oil uptakes ( V w V s and V o V s . The value of the proportionality constant depends on the thickness and compressibility of the surfactant layer at the water/oil interface. Using the length of the C12∗XS-MEA molecule measured from molecular model projections, droplet sizes calculated from water and oil uptakes agree very well with those determined by dynamic light scattering experiments, lending further support to the model.

Development of Corexit 9580—A Chemical Beach Cleaner

ABSTRACT Chemical beach cleaners can facilitate cleanups of oiled shorelines by improving the efficiency of washing with water. The improvement is a result of reduced adhesion of the oil coating, which makes it easier to remove from shoreline surfaces, thereby reducing washing time and lowering the temperature of the wash water needed to clean a given area. The criteria established for use of chemical beach cleaners in the Exxon Valdez spill cleanup included demonstrating enhanced cleaning with low levels of toxicity to marine biota and with minimal oil dispersion. Since no commercially available products satisfactorily met these criteria for use in Alaska, a new product, Corexit 9580, was specifically developed in response to this need. This paper describes the successful development of this chemical, including both laboratory testing and field testing in Prince William Sound.

Model for microemulsions: II. Hydrophile—lipophile balance (HLB)—salinity—oil molar volume phase maps

Abstract The previously described model for microemulsions (10) has been expanded to generate theoretical phase maps defining the boundaries for transitions in microemulsion types. Winsors I a III a II transition boundaries (lower a middle a upper phase microemulsions) are defined for alkyl o-xylene sulfonates with simultaneously varying cation size, chain length, and aqueous phase salinity. Variation in cation size and chian length is reduced to a single variable, the hydrophile/lipophile ( H L ) volume ratio (Vr), which also includes interfacial water and oil. Interfacial water and oil is shown to depend on the packing area of surfactant molecules in the interface relative to the areas occupied by surfactant heads and chains, respectively. Theoretical 1 a m and m a u transition boundaries are evaluated and plotted in H L ratio-salinity phase space together with the locus of optimal salinities. Trajectories crossing these phase boundaries are evaluated and plotted for specific surfactants. This theoretical phase map, when used with the model equations, contains all the information necessary to calculate water ( V w V s ) and decane ( V o V s ) uptakes at constant surfactant concentration with varying salinity and H L ratio for the entire homologous series of alkyl o-xylene sulfonates containing ethanol amine cations of varying size. The concepts embodied in the H L ratio-salinity phase map are qualitatively expanded to describe transitions and water and oil uptakes in multidimensional phase space, e.g., H L ratio-salinity-oil alkane carbon number space.

IMPROVED DISPERSANT BASED ON MICROEMULSION TECHNOLOGY

ABSTRACT An initial basic study focused on the interaction between dispersant surfactants and the oil-water interface. In essence, the study identified criteria to explain why a good dispersant is effective and why a poor dispersant is ineffective. The dynamic behavior of the oil-water interface, after the addition of the dispersant, was continuously monitored by a modified Wilhelmy plate device. This procedure provided much insight on the impact of the dispersant at the oil-water interface. One key finding of this study concerned the conditions for achieving very low interfacial tensions. It is known in microemulsion technology that a microemulsion formed by specific surfactants exhibits ultra-low interfacial tension against either oil or water. Microemulsion phase behavior studies then established that some specific surfactants, which form a certain type of microemulsion, are also highly effective dispersants, more effective than current state-of-the-art products. This improvement results in the formati...