Choline carboxylates (ChCm with m = 12-18) are simple biocompatible anionic surfactants with very low Krafft temperatures, possessing a rich aqueous phase behavior. In the present work, we have investigated the thermotropic mesomorphism of anhydrous ChCm salts for m = 12-18. Transition temperatures and enthalpies determined by differential scanning calorimetry reveal that all investigated compounds exhibit three different phases between -20 and 95 °C. The phases were further characterized by optical polarizing microscopy, NMR spin-spin relaxation, and X-ray scattering measurements. The nature of the phases was identified with increasing temperature as crystalline, semicrystalline, and liquid-crystalline lamellar. Even long-chain choline carboxylates (m = 18) were found to melt into a lamellar liquid-crystalline phase below 100 °C. Accordingly, with choline as counterion in simple fatty acid soaps, not only the water solubility is considerably enhanced but also the melting points are substantially reduced, hence facilitating thermotropic mesomorphism at temperatures between 35 and 95 °C. Thus, simple choline soaps with m = 12-18 may be classified as ionic liquids.

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Surfactants self-associate in aqueous solutions to form micelles. Less well-known is that they form a wide range of liquid crystals — through self-association. These liquid crystals often occur in consumer products where they play an essential role in product stability and function. Some products are marketed in a liquid crystalline state although they are not recognized by the consumer (or, on occasion, by the manufacturer). This review describes the formation of micelles and the various liquid crystalline phases. These include lamellar, hexagonal, cubic and gel phases which have different long range structures but are based on micelles. The key factors linking surfactant molecular structure to liquid crystal architecture have been elucidated. These are the sizes of the surfactant hydrophobic tail(s) and head groups, together with the head group charge and the presence of any additives. Examples of liquid crystals in emulsion stabilization, household cleaners, conditioners, detergent liquid and some food are described.

Surfactant formulators are faced with many challenges during product processing and manufacture, with increased problems from the use of commercial surfactants. Commercial surfactants have a distribution of EO sizes which can dramatically affect the product properties. The gel (L b )/lamellar (L a ) phase transition of a commercial surfactant C 12-14 EO 2.9 /water system is shown to split into two, with the sequence L b $ L a þ L 0 b $ L a $ L a being observed, where the L a þ L 0 b region occurs over a ca. 10 C temperature range. In addition, both the gel and lamellar phases can incorporate at least 2-3 times more water than the pure surfactants before the formation of a separate aqueous phase. So while commerical surfactants could pose problems for formulators, they could also offer the possibility of enhanced solubilization of actives in the lamellar regions coupled with the enhanced emulsion stabilization by the gel phase.