SOME ANOMALIES BETWEEN THE SURFACTANTS COMPRISING LINEAR AND BRANCHEDHYDROCARBON CHAINS

This paper deals with the differences observed in the surface properti es of two types of surfactant molecules of similar head groups and cha in lengths of which one is comprised of linear alkyl chains and the ot her of branched hydrocarbon chains. The marked difference in the inter facial properties and liquid crystalline behaviour of these surfactant s (viz. diethanolamine derivatives of n-alkyl as well as polyisobutyle ne succinic anhydride) are attributed to the conformational changes in the molecules caused by branching. The comparison of refractive indic es obtained by optical ellipsometry at the air-water interface showed negative anisotropy for the molecules with branched hydrocarbon chains during film compression which is in contrast to the molecules with li near alkyl chains. Similarly, the surface viscosity measured by deep c hannel surface viscometry showed that contrary to the surfactants with linear hydrocarbon chains, the monolayers of the surfactants with bra nched hydrocarbon chains were more liquid like. The surfactants with l inear hydrocarbon chain exhibited increased viscosity in the presence of NaCl in the aqueous sub-phase whereas a significant drop in viscosi ty was observed with the surfactant comprising branched hydrocarbon ch ains. Similarly, a drop in viscosity was observed in the presence of a mmonium nitrate. Such effects were reflected in the water evaporation rates through their monolayers. Both of these surfactants formed lamel lar liquid crystals alone and also with added water. The repeat distan ce between the layers of the liquid crystalline phase, measured by low angle X-ray diffraction (LAXD), differed significantly between the su rfactants with linear and branched hydrocarbon chains. The repeat dist ance for a surfactant with a polyisobutylene hydrocarbon chain was hig her that that of a surfactant with linear alkyl chain. Higher values o f water penetration were observed in the hydrocarbon zone of lamellar liquid crystals of a surfactant comprising polyisobutylene chain.