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 i maging fluid systems in their natural state without prior specimen pre paration. As such, it is potentially a very powerful tool for studying emulsion structures at a resolution significantly higher than that fo r conventional optical microscopy. We present data on several water-oi l emulsions that demonstrate the capability of the instrument. Separat e preparations of vegetable oil and polybutene were used, emulsified w ith nonionic polysaccharide surfactants. Rather large contrast variati ons were found between the oil and water phases, with water always app earing brighter whether it was in the dispersed or continuous phase. T he observed contrast is attributed to variations in the secondary elec tron emission characteristics arising from the dielectric response fun ction of water compared with hydrocarbons. We propose that delocalized electrons in pi-bonds provide an energy-absorbing mechanism that lead s to a reduction in secondary electron emission from unsaturated hydro carbon oils, relative to that of water, by lowering the energy gap. We present a qualitative description of an ''energy window'' model, affo rding 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.