Effect of the spreading procedure on the formation of cationic-anionic mixed monolayers

Mixed monolayers of dioctadecyldimethylammonium bromide (DODAB) with sodium hexadecylsulfate (SHS) can be obtained by spreading both components at the interface, co-spread (I) or separately spread (), or by incorporation of the water soluble surfactant SHS from the subphase (3). The most efficient procedure is co-spreading, when equimolar amounts are used. On the other hand, when the components are spread separately at the interface, or when SHS is adsorbed from the aqueous subphase, it is not possible to obtain an efficiency as high as that of co-spreading, even after 5 h. The equimolar composition at the interface can also be immediately obtained after solvent evaporation when SHS is added to the subphase in large excess to DODAB (9:1); alternatively, the same composition can be obtained from smaller excesses of SHS to DODAB, but only after larger time intervals. The composition of the mixed monolayers obtained by procedures 2 and 3 was estimated from the correlation compressibility-composition, obtained independently by co-spreading. (C) 1998 Elsevier Science S.A. All rights reserved.