Structure and composition of dodecane layers spread on aqueous solutions of tetradecyltrimethylammonium bromide: neutron reflection and surface tension measurements

Abstract

Neutron reflection and surface tension measurements have been used to study the composition and structure of a mixed (alkane + surfactant) monolayer at the air-water surface, formed either by adsorption from vapor or by spreading dodecane from small lenses placed on the surface of an aqueous solution of tetradecyltrimethylammonium bromide (C,,TAB). The highest concentration of dodecane in the mixed monolayer occurs when the solution concentration of surfactant is about one-third of the critical micelle concentration (cmc), at which point the area per surfactant molecule is about 60 A*, some 20% higher than its value at the cmc but similar to its value in the absence of dodecane. At lower bulk surfactant concentrations (and hence lower surface concentrations), the amount of dodecane in the mixed layer also decreases. At a surfactant concentration of 2.6 mM (cmc = 3.7 mM) the surfactant chain is found to be slightly more extended (20 A when modeled as a uniform layer) in the presence of dodecane than in its absence, while the dodecane forms a layer significantly thicker (22 A) than its fully extended chain length (17 A). The separation of the centers of the surfactant chain and dodecane distributions at this concentration was found to be 5 f 2 A, which together with the separation of each component from the water subphase, 7.5 and 13 A, respectively, indicates that the oil is completely out of the water and that the surface of the mixture is very similar to that of pure dodecane.

Determination of Water-Oil Interfacial Area during 3-Phase Gravity Drainage in Porous Media

“…This is because is larger at the water-air interface (1.30 × 10 −7 kg/m 3 ) than at the water-oil interface (1.22 × 10 −7 kg/m 3 ) for a surfactant concentration of 115 mg/l (18). While a spreading film of alkane at the water-air interface can decrease by up to 10% (24,25), this is not sufficient to explain the results. One would expect to lie between the value for a clean water-air interface and for water-oil as the thickness of any spreading film of oil increased.…”

The intrinsic interfacial structure of ionic surfactant monolayers at water–oil and water–vapour interfaces

“…methylammonium bromide-stabilized water-in-oil (cyclohexane) MEs (Eastoe et al 1996a). On the other hand, neutron reflectivity experiments of planar interfaces indicate higher degrees of penetration of the oil phase (dodecane) in tetradecyltrimethylammonium bromide monolayers (Lu et al 1992). For surface areas per surfactant similar to those investigated in the ME experiments, it was found that the oil did penetrate the surfactant layer, reaching a ratio of about 0.8 oil molecules per surfactant.…”

Wetting of Surfactant Solutions by Alkanes

“…[16,17] The more negative values for DC·HCl compared to those of DTAB and TTAB arise because the dielectric constant of dibucaine at optical frequencies (2.38) is higher than that of the hydrocarbon tails of DTAB and TTAB ( % 2.05), giving rise to a greater contrast in the dielectric profile across the interface.…”

“…Neutron reflectivity studies of monolayers of DTAB, TTAB, hexadecyltrimethylammonium bromide (CTAB), and monododecyl pentaethylene glycol (C 12 E 5 ) at the air-solution interface in the presence of dodecane lenses have shown that the mixed layers are of monolayer thickness. [16,17] The first unambiguous evidence for a wetting transition in alkanes on surfactant solutions was provided by Aratono and co-workers, who measure the surface tension of varying concentrations of tetramethylammonium dodecylsulfate (TMADS) solutions in contact with a reservoir of hexadecane. [18] The firstorder nature of the wetting transition was confirmed for hexadecane on DTAB solutions by ellipsometry.…”

“…Aveyard and co-workers have measured the surface tension isotherms of several different surfactants with and without dodecane lenses. [15,16] Their studies focussed on the variation of the dodecane uptake and the composition of the mixed interfacial layers with surfactant concentration. Neutron reflectivity studies of monolayers of DTAB, TTAB, hexadecyltrimethylammonium bromide (CTAB), and monododecyl pentaethylene glycol (C 12 E 5 ) at the air-solution interface in the presence of dodecane lenses have shown that the mixed layers are of monolayer thickness.…”