Factors influencing the structure of microemulsions

What are the most important factors influencing the type of microemulsion Here again we have to distinguish between nonionic and ionic surfactants. For nonionic surfactants, often alkylethylene glycols, temperature is the dominating parameter for the structure of a microemulsion. For ionic surfactants, mostly SDS or CTAB, the salt concentration dominates 

With increasing temperature the oil drops become larger and larger because the spontaneous curvature decreases. Accordingly, the volume of the oil phase decreases until at 23°C all oil is incorporated into relatively large oil drops in water and we reach a one-phase region (often denoted Li) of an oil-in-water microemulsion. This point is ideally suited to determine the spontaneous radius of curvature because, here, the radius of the oil drop can be calculated from the added volume fractions with Eq. (12.21). Raising the temperature decreases Co further and from 29°C on lamellar structures are formed (La). At 32°C the PIT is reached and the spontaneous curvature is zero. 

Going higher in temperature results in a negative curvature (towards the hydrophilic head groups) and the surfactants start to favor water-in-oil emulsions. Now the whole sequence is repeated only with water drops in oil. Between 35°C and 37°C we have large water drops which decrease in size with increasing temperature (L2). Then, above 37°C, two phases appear a water phase underneath a water-in-oil microemulsion. 

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