Glass-forming microemulsions

Pseudoternary microemulsions, in which the alcohol is polyhydric and is probably contained almost wholly within the aqueous phase, are described. Separate O/W and W/O clear-phase regions exist. These, like the O/W macroemulsions in this system which were reported earlier, are capable of being cooled without separation or crystallization into the vitreous state. To determine the extent to which microscopic droplet systems retain the bulk liquid characteristics, glass transition temperatures T_g, which are determined by the achievement of liquid structural relaxation times of ∼100 s, have been measured during reheating of the vitrified samples. T_g for the dispersed droplet microemulsion state of o-xylene is almost the same as for the macroemulsion or bulk liquid phase. This is interesting since each individual system in the microdispersed state is no larger than one of the "clusters" often imagined to make up the subunits of the vitreous-phase structure. Vitrified microemulsions, in which the dispersed phase has a lower T_g than that of the matrix, offer a means of preparing noncommunicating, truly ultramicroscopic liquid systems, for which a number of interesting possible applications exist. As one example, we have been able to vitrify p-xylene which according to the T_b/T_m > 2 rule for glass-forming ability should be almost impossible to vitrify (T_b/T_m < 1.43). This implies that, in microemulsion form, crystallization of molecular liquids during normal cooling will tend to be an unusual phenomenon.