The ellipsometric isotherms of diethanolamine derivatives of n-alkylsuccinic anhydride (where the alkyl group comprised C12, C14, and C16 hydrocarbon chains) and polyisobutylene succinic anhydride (where the average backbone carbon numbers of polyisobutylene chains were 14, 22, and 34) at the air-water interface were compared with a view to the development of an understanding of the structural organisation of their monomolecular films. The ellipsometric angle, Δδ, has proven to be very sensitive to the nature of the physical state of the film that occurs during film compression. At 20 ± 1°C, the two series of surfactants exhibited very different behavior upon compression. A rodlike model was used to describe the different molecular behaviors exhibited by the surfactants comprising linear and branched hydrocarbon chains. Equations relating variations of refractive indexes, film thicknesses, and molecular areas were used for the determination of the molecular properties of the films during monolayer compression. Calculated refractive indexes and film thicknesses were obtained from experimental δΔ values. The results indicated that the change in δΔ values during film compression is more sensitive to the refractive indexes than to the film thicknesses. For the surfactants with n-alkyl chains the refractive index component increased linearly, whereas a reverse trend was observed with the surfactant molecules comprising polyisobutylene chains. Such an anomaly in refractive index can be explained by the effect of substituents in the hydrocarbon chains and subsequent differences in packing behavior of monolayers. The |δΔ| of n-alkyl derivatives increased upon compression and reached a steady state at collapse pressure, whereas a sharper rise in |δΔ| was observed with the derivatives comprising polyisobutylene chains. Such phenomena indicated multilayer formation by the molecules with polyisobutylene chains in the collapse region.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry