A small angle neutron scattering study of the interface between solids and oil-continuous emulsions and oil-based microemulsions

Philip A. Reynolds, Mark J. Henderson, John W. White*

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    7 Citations (Scopus)


    We have measured the small angle neutron scattering (SANS) from slurries of powder in contact with surfactant solutions and emulsions to determine the fluid/solid interfacial structure. The slurry solids consisted either of graphite or pyrites particles; and the fluids were hexadecane containing the robust commercial polyisobutylenesuccinamide (PIBSA) surfactant, or a high internal phase emulsion of aqueous ammonium nitrate in hexadecane stabilised by PIBSA. To resolve the interfacial structure for both systems, combinations of deuterated and protonated materials were used. At low concentration in hexadecane, PIBSA forms a complete monolayer on graphite with a footprint per molecule of 103Å2 and a layer thickness of 19Å. At higher concentrations, the complete monolayer of footprint is 61Å 2 and 30Å thick indicating compression of the PIBSA chain coil structure. Geometric exclusion effects caused by the stacking of the graphite particles also results in an excess of oil for ca. 160Å above the surfactant monolayer. For pyrites in contact with surfactant in hexadecane, the oxidised surface layer, while smooth at the oil interface, is diffuse and/or rough at the interface with the bulk sulphide below. There is again a complete monolayer of surfactant adsorbed at the oxide surface, in a relatively compressed state with a footprint of 70Å2, more tightly bound than on graphite. The excess of oil phase above the adsorbed surfactant monolayer is observed for samples with larger pyrites particle sizes but not for a sample with smaller particles. This suggests that the oil excess does arise from purely geometric solid particle packing, but that the local particle surface curvatures are significantly higher than the overall particle size would suggest. The scattering from the pyrites/emulsion interface was modelled by a 30Å thick monolayer of surfactant coating an oxide surface with a molecular footprint of 123Å2. For the larger particle size samples, there is a 30Å thick layer of oil above the pyrites particle surface before a bulk emulsion/pyrites mixture is reached. These results extend previous reflectometry experiments on the silicon/emulsion interface, indicating that for stable emulsions the structures are qualitatively similar for three dissimilar solid surfaces. They show that useful results on surfactant structure and emulsion layering at the solid/emulsion and other solid/fluid interfaces can be simply obtained by SANS on powder samples variously contrasted by deuteration. SANS can be applied to a much greater range of solid interfaces than reflectometry since large neutron-transparent single crystals are not required, although the variety of faces in a powdered material degrades the quality of the information.

    Original languageEnglish
    Pages (from-to)55-65
    Number of pages11
    JournalColloids and Surfaces A: Physicochemical and Engineering Aspects
    Issue number1
    Publication statusPublished - 12 Jan 2004


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