We prepared a thermodynamically unstable microemulsions in which a droplet does not change in size for over one year. The phase diagram showed these microemulsions to exist in the two-phase region. These microemulsions may be regarded as microemulsions in a broad sense by Rosano. They were obtained by the rapid cooling of the homogeneous phase in nonionic surfactant-oil-water systems, to room temperature. The droplet mean radius was related to the ratio of oil to surfactant by a linear function. The droplet size of these microemulsions can thus be controlled by changing this ratio.
The stability of the microemulsions depends on the nature of the oil and surfactant. The stability rises dramatically with increase in the carbon number of the oil. The droplet having liquid paraffin (Lp) or squalane (Sq) undergoes no appreciable change in size for over a year at 25°C.Small quantities of Lp or Sq increase the stability of the unstable microemulsions containing a short chain hydrocarbon oii, The stability of the microemulsions increases with the size of the surfactant molecule. The main cause of instability in such systems thus appears to be related to Ostwald ripening rather than droplet coalescence.