EFFECT OF THE SPREADING PROCEDURE ON THE FORMATION OF CATIONIC-ANIONIC MIXED MONOLAYERS

Mixed monolayers of dioctadecyldimethylammonium bromide (DODAB) with s odium hexadecylsulfate (SHS) can be obtained by spreading both compone nts at the interface, co-spread (I) or separately spread (), or by inc orporation of the water soluble surfactant SHS from the subphase (3). The most efficient procedure is co-spreading, when equimolar amounts a re used. On the other hand, when the components are spread separately at the interface, or when SHS is adsorbed from the aqueous subphase, i t is not possible to obtain an efficiency as high as that of co-spread ing, even after 5 h. The equimolar composition at the interface can al so be immediately obtained after solvent evaporation when SHS is added to the subphase in large excess to DODAB (9:1); alternatively, the sa me composition can be obtained from smaller excesses of SHS to DODAB, but only after larger time intervals. The composition of the mixed mon olayers obtained by procedures 2 and 3 was estimated from the correlat ion compressibility-composition, obtained independently by co-spreadin g. (C) 1998 Elsevier Science S.A. All rights reserved.