Direct observation of water-oil emulsion systems in the liquid state by environmental scanning electron microscopy

Environmental scanning electron microscopy is a technique capable of imaging fluid systems in their natural state without prior specimen preparation. As such, it is potentially a very powerful tool for studying emulsion structures at a resolution significantly higher than that for conventional optical microscopy. We present data on several water-oil emulsions that demonstrate the capability of the instrument. Separate preparations of vegetable oil and polybutene were used, emulsified with nonionic polysaccharide surfactants. Rather large contrast variations were found between the oil and water phases, with water always appearing brighter whether it was in the dispersed or continuous phase. The observed contrast is attributed to variations in the secondary electron emission characteristics arising from the dielectric response function of water compared with hydrocarbons. We propose that delocalized electrons in π-bonds provide an energy absorbing mechanism that leads to a reduction in secondary electron emission from unsaturated hydrocarbon oils, relative to that of water, by lowering the energy gap. We present a qualitative description of an "energy window" model, affording secondary electrons a range of energies over which interactions with the sample are minimal, and discuss how variations of the energy window are likely to influence secondary electron emission.