A.D. Nikolov

Ordered Micelle Structuring in Thin Films Formed from Anionic Surfactant Solutions I. Experimental

Reflected light micro-interferometry was used to observe stratification (i.e., the kinetics of layered structuring) in thinning foam films formed from micellar solutions of sodium n-alkyl sulfates at concentrations much lower than those at which liquid crystalline structures form in bulk solutions. The effects of surfactant concentration and chain length, added electrolyte, and capillary pressure on the step-wise jump transition thicknesses have been investigated. We have further conducted a model study with films formed from aqueous dispersions of latex particles. In similar fashion to those formed from micellar solutions, the thinning films changed thickness with regular step-wise jump transitions, and the films exhibited a number of metastable equilibria. These observations verify that the stratification of thin liquid films can be explained as a layer-by-layer thinning of ordered structures of micelles or colloidal particles formed inside the film. The stratification-nonstratification phase diagram is presented for anionic micelles and is similar to the order-disorder phase diagram for latexes. In the accompanying paper (Part II) we show that the phenomenon of stratification and ordered micellar structures is governed by the long range electrostatic repulsion beteen the ionic micelles and the restricted volume of the film. This present study provides evidence, for the first time, for the presence of structural forces in thin films with thicknesses on the order of 100 nm and strongly suggests that the free liquid film dynamics may be used as a probe to study the ordering in, and stability of, microheterogeneous systems.

Thin liquid film structure and stability: The role of depletion and surface‐induced structural forces

Film stability and structure formation inside a liquid film containing colloidal particles are investigated by Monte Carlo (MC) numerical simulations and by analytical methods. The effective pair interaction between particles is calculated from the Ornstein–Zernike theory with Percus–Yevick closure. Consistent with the recent experimental observations and theoretical studies, these MC simulations reveal the phenomena of internal particle layering as well as inlayer structure formation. In particular, an ordered two‐dimensional hexagonal structure is observed at a particle concentration of 37 vol% (instead of 43 vol% for the hard‐sphere potential) when the effective pair interaction between particles is taken into account. Furthermore, the particles inside a layer ‘‘condense’’ due to the attractive depletion force which leads to the formation of voids. The formation of such void structures results in the formation of ‘‘dark spots’’ which have been observed in film thinning experiments. The calculated film ...

Universality in film stratification due to colloid crystal formation

Abstract Experimental results with different colloidal systems (ionic and non-ionic micelles, swollen micelles, latex suspensions) are presented to demonstrate the universality of the phenomenon termed film stratification (stepwise thinning of liquid films). A general explanation of the phenomenon is proposed which assumes that the stratification is caused by the layer-by-layer expulsion of a colloid crystal-like structure formed by spherical particles (micelles, latex particles, etc.) inside the film. The effect of different factors on the stratification such as surfactant and electrolyte concentrations, temperature, etc. is explained from this viewpoint.

Foams, thin films and surface rheological properties

This overview paper highlights the role of thin liquid film dynamics in understanding foam structure and stability, and summarizes the recent contributions of the fundamental research performed in our laboratory. The significance of the surface rheological properties such as surface elasticity or surface tension gradient, and surface shear and dilatational viscosities in the hydrodynamics of thin foam films as well as in foam bulk rheological behavior is illustrated. The phenomenon of stratification (i.e., stepwise thinning) in foam films and a new mechanism for the stabilization of foams due to the formation of long-range, “ordered” micellar structures inside the film, over distances of the order of 100 nm or 1,000A, are discussed. The stabilizing/destabilizing effects of oil for aqueous foam systems and the importance of the “pseudoemulsion” film in controlling stability in such three-phase foam systems are explained.

DEMULSIFICATION OF WATER IN OIL EMULSIONS USING WATER SOLUBLE DEMULSIFIERS

ABSTRACT The possibility of using a water soluble,as opposed to the conventional oil soluble demulsifier, to destabilize a w/o emulsion in crude oil has been explored. It was found experimentally that a surfactant soluble in the water (dispersed) phase could destabilize the emulsion. Polymer molecules with varying HLBs and molecular weights and structure were synthesized and these compounds were added to the water phase to destabilize the water/crude oil emulsions. Molecules with a high percentage of hydrophilic groups and low molecular weights showed very good demulsifying abilities.

Mechanisms of Oil Removal from a Solid Surface in the Presence of Anionic Micellar Solutions

Abstract We use a differential interference microscope to observe the separation of crude oil droplets from a solid silica surface in the presence of a micellar solution. Surfactant adsorption lowers interfacial tension, resulting in a receding oil/water/solid contact line. Because water and surfactant molecules diffuse between the solid and oil phases, a process that occurs ahead of the (outer) moving contact line, the conventional three-phase contact line is preceded by a second contact line. The mechanism of oil droplet separation appears to be a combination of both a rolling-up mechanism and a diffusional mechanism.

Interfacial effects in solids-stabilized emulsions: measurements of film tension and particle interaction energy

Abstract The extent of the stability of solids-stabilized emulsions depends on the resistance offered by the interfacial layer of particles to coalescence. An experimental technique is described to study the effect of interfacial layers of hydrophobic particles on the process of coalescence of a water drop at a planar oil/water interface. The influence of interfacial solids packing density on the film tension and particle interaction energy was investigated for two hydrophobic solids. Dust particles filtered from shale oil formed periodic structures at the interface and the film tension and particle interaction energy exhibited multiple maxima and minima. Spherical glass beads (4 μm diameter) coated with asphaltic matter, on the other hand, displayed a single maximum in particle interaction energy around monolayer coverage.

The Effect of Oil on Foam Stability: Mechanisms and Implications for Oil Displacement by Foam in Porous Media

Foam stability in the presence of Salem crude oil and pure hydrocarbons is investigated as a function of chain length of ..cap alpha..-olefin sulfonates and electrolyte concentration. Interactions between aqueous foam films and emulsified oil droplets are observed using transmitted light, incident light inteferometric and differential interferometric microscopic techniques. Foam destabilization factors are identified including the pseudoemulsion film tension and the surface and interfacial tension gradients. Results from foam-enhanced oil recovery experiments in Berea Sandstone cores are presented using the combined gamma ray/microwave absorption technique to measure dynamic fluid saturation profiles.

CONTACT ANGLE MEASUREMENTS WITH SESSILE DROPS AND BUBBLES

Abstract In this note a simple and precise method for measurement of contact angles in the range 0°–180° on solid surfaces is proposed. In the case when the three-phase contact line is irregular due to surface roughness and inhomogeneity, the method provides an effective averaged value of the contact angle. The method is based on a numerical solution of the Laplace equation for the interfacial profile. The input parameters used are the liquid-gas surface tension and a pair of the following parameters of the drop (bubble) shape: volume, height, contact radius, and equatorial radius. Depending on the magnitude of the contact angle different pairs of parameters are to be measured for the purpose of best accuracy.

Demulsification of water-in-crude oil emulsions : effects of film tension, elasticity, diffusivity and interfacial activity of demulsifier individual components and their blends

This is the first study to report on the effects of the individual components and blends of an emulsion breaker package on dynamic film and interfacial tensions, film elasticity and diffusivity. These parameters were then correlated with the demulsifier performance. The measurements were made on systems of known crude oil, water and emulsion breaker chemistry. Results indicate that good demulsifier components must create low interfacial and film elasticity, high film diffusivity, high interfacial activity, and low dynamic film tension.