The synthesis and characterisation of novel long chain dimethyl siloxane surfactants.

A basic introduction to liquid crystals and surfactants has been given, along with a description of the main techniques employed in the study of liquid crystals (in particular optical polarising microscopy, differential scanning calorimetry, nuclear magnetic resonance spectroscopy and x-ray diffraction studies). Conventional surfactants comprise a polar head group and a hydrophobic, hydrocarbon chain ie. they are amphiphilic. Because long chains have high melting points the length of the alkyl chain in these compounds is limited to <ca C[18], as long chain surfactants are usually insoluble. Therefore, in this project the alkyl group has been replaced by a long, hydrophobic polydimethylsiloxane chain. Polydimethylsiloxanes are low melting materials (glass transition at ca -120&deg;C) with very flexible chains, hence surfactants based on them might be readily soluble in water. This project involves chemical attachment of amphiphilic mesogens to alpha-SiH terminated siloxanes of varying lengths and the examination of their surfactant properties. The following type of structure was successfully synthesised: CH[3]CH[2]CH[2]CH[2](Si (CH[3]) [2]O)n Si(CH[3])[2] - m where n = integer; m = amphiphilic mesogen. The amphiphilic head groups of these novel surfactants contained the salts of either a mono-, or a dicarboxylic acid. After the synthesis of these surfactants, the liquid crystal and micelle properties of the sodium and calcium salts, were investigated utilising a number of physical techniques eg. optical microscopy and differential scanning calorimetry. Finally, some work on synergism has been described. When different types of surfactants are purposely mixed, what is sought is synergism, the condition when the properties of the mixture are better than those attainable with the individual components by themselves.