Some Anomalies between the Surfactants comprising Linear and Branched Hydrocarbon Chains

lCI Explosives Group Technical Centre, 801 Richelieu Boulevard, McMasterville, Quebec J31I 1T9. Canada

Centre de Recherche en Photobiophysique, Universite du Quebec a Trois Rivieres, C.P. 500, Trois Rivieres. Quebec, G9A 5117, Canada

Centre for Surface Science and Engineering, University of Florida, Gainesville, Fl 32611, U.S.A.

Centre for Advanced Materials Processing, Clarkson University, Postdam, New York 13699-5814, U.S.A.

Manuscript received 25 January 1993

This paper deals with the differences observed in the surface properties of two types of surfactant molecules of similar head groups and chain lengths of which one is comprised of linear alkyl chains and the other of branched hydrocarbon chains. The marked difference in the interfacial properties and liquid crystalline behaviour of these surfactants (viz. diethanolamine derivatives of n-alkyl as well as polyisobutylene succinic anhydride) are attributed to the conformational changes in the molecules caused by branching.

The comparison of refractive indices 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 linear alkyl chains. Similarly, the surface viscosity measured by deep channel surface viscometry showed that contrary to the surfactants with linear hydrocarbon chains, the monolayers of the surfactants with branched hydrocarbon chains were more liquid like. The surfactants with linear hydrocarbon chain exhibited increased viscosity in the presence of NaCl in the aqueous sub-phase whereas a significant drop in viscosity was observed with the surfactant comprising branched hydrocarbon chains. Similarly, a drop in viscosity was observed in the presence of ammonium nitrate. Such effects were reflected in the water evaporation rates through their monolayers.

Both of these surfactants formed lamellar liquid crystals alone and also with added water. The repeat distance between the layers of the liquid crystalline phase, measured by low angle X-ray diffraction (LAXD), differed significantly between the surfactants with linear and branched hydrocarbon chains. The repeat distance for a surfactant with a polyisobutylene hydrocarbon chain was higher than that of a surfactant with linear alkyl chain. Higher values of water penetration were observed in the hydrocarbon zone of lamellar liquid crystals of a surfactant comprising polyisobutylene chain.