March 2021 • Vol. 27 • Issue 3
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K.J. Lissant’s Fundamental Studies with High Internal Phase Ratio Emulsions and Incremental Advances and Refinements of this Technology Over the Past 50 Years …by Paul Thau
ntil the early 1970s, little attention was given to high internal phase ratio emulsions (often referred to as HIPEs). This was probably because few emulsifying agents known at that time had the capacity to produce and stabilize such emulsions. Shortly later, polymeric emulsifiers became available. It then became possible to prepare stable water-in-oil emulsions containing up to 95% internal phase. These emulsions were called high internal phase ratio emulsions. It had long been known that if a monodisperse system of rigid spheres is packed in the most advantageous manner into an emulsion, the spheres would occupy a little over 74% of the total volume of the system. Experience had shown that when conventional soap-type emulsifiers were used in attempts to prepare emulsions with over 70% internal phase, they usually inverted at about 74%. Lissant’s group at Petrolite Inc. found that it was possible to prepare emulsions with well over 90% aqueous internal phase that were relatively monodisperse.1-3 The droplets of these emulsions were not spherical. They must assume some transitional form between spheres and polyhedra. For this reason, they conducted a study of the restrictions placed on such systems by the pure geometry of the situation. For simplicity they first assumed that the droplets are truly monodisperse, that is, all droplets have equal volume. They also assumed that the specialty emulsifier essentially prevents coalescence. High internal phase ratio emulsions are defined as emulsions comprising greater than 75% by volume of aqueous internal phase.4 The droplets present in high internal phase ratio emulsions are deformed from the usual spherical shape into polyhedral shapes and are locked in place. The high internal phase ratio emulsions normally display unusual rheological properties, which are generally attributed to the existence of the polyhedral droplets. The main reason that classical water-in-oil emulsions were greasy and occlusive was because in most instances the classical formulation contained a high concentration of oil, wax, or petrolatum, plus a much lower concentration of water. The breakthrough with high internal phase ratio emulsions is that the aqueous concentration with the use of polymeric emulsifiers can be anywhere from 70% to 95%. (continued on Page 3)
A C E L E B R AT I O N O F W O M E N I N S C I E N C E
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...see pages 4-6 for more information.
MARCH 8th-31st