Ak. Chattopadhyay et al., SOME ANOMALIES BETWEEN THE SURFACTANTS COMPRISING LINEAR AND BRANCHEDHYDROCARBON CHAINS, Journal of the Indian Chemical Society, 70(4-5), 1993, pp. 353-364
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.