Structure and composition of dodecane layers spread on aqueous solutions pf dodecyl and hexdecyl trimethylammonium bromides studied by neutron reflection

Neutron reflection measurements have been used to study the composition and structure of mixed dodecane-surfactant monolayers at the air/water surface. The surfactants were dodecyltrimethylammonium bromide (C12TAB) and the corresponding C16TAB. Partially isotopically labeled C16TABs were used to localize the position of the oil in the layer with greater precision. The surfactant concentrations were chosen to be at their critical micelle concentrations (cmc) of 14 × 10-3 M (C12TAB) and 9.1 × 10-4 M (C16TAB). The area per molecule for both surfactants increased on the addition of oil from 48 to 54 ± 3 Å2 for C12TAB and 43 to 52 ± 3 Å2 for C16TAB. In each case the surfactant layer was found to be slightly thicker in the presence of dodecane than in its absence, although the increase in area would normally cause the thickness to decrease. The area per molecule of the dodecane was found to be 76 and 67 ± 7 Å2 on C12TAB and C16TAB, respectively, to be compared with earlier surface tension estimates of 78 and 56 Å2. On C12TAB the thickness of the dodecane layer is slightly greater (18.5 ± 2 Å) than its fully extended chain length of dodecane (18 Å), but on C16TAB it is significantly greater (20.5 ± 2 Å). The separations of the centers of the surfactant and dodecane distributions at the corresponding cmc's were found to be 6.5 and 8.5 ± 1 Å. Labeling of the C6 fragments at either end of the C16TAB made it possible to locate the oil distribution more accurately. The center of the outer C6 (hexyl) group (i.e., the air side) of the surfactant tail was found to coincide exactly with the center of the oil distribution, whereas the inner C6 (hexamethylene) group was centered 8.5 ± 1 Å from the center of the oil distribution. © 1995 American Chemical Society

Adsorption of oil into surfactant monolayers and structure of mixed surfactant and oil films

We describe a tensiometric method for determining the adsorption isotherm of an oil on a surfactant monolayer adsorbed at the air-water surface. The method involves measuring the surface pressure of oil, π(oil), as a function of its activity, a(oil), varied by changing the relative vapour pressure. We compare the isotherm of dodecane adsorption onto a C12E5 monolayer determined in this way with that measured directly using neutron reflectivity. The agreement between the two allows us to conclude that, at least for this system, addition of oil does not result in a change in the chemical potential of the surfactant. Structural analysis of the dodecane + C12E5 mixed film has been performed with neutron reflectivity using two contrasts. In one, only the surfactant chain region is highlighted, whilst in the other only the oil film is visible. We document, for the first time, changes in thickness and packing density of both the oil and surfactant chains in the mixed surfactant + oil layer upon increasing oil content. For a diverse range of other oil + surfactant systems, we have determined the initial (S(i)) and equilibrium (S(e)) spreading coefficients of oil by measuring π(oil) following addition of liquid oil (at unit activity) to surfactant solution surfaces. Those systems in which S(e) is close to zero display a repulsive van der Waals component of the disjoining pressure-oil film thickness isotherm, whilst with toluene as oil the calculated isotherm is attractive, consistent with non-spreading observed for this oil

Neutron reflectivity studies of AOT monolayers adsorbed at the oil/water, air/water and hydrophobic solid/water interfaces

The structures of monolayers of Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate) adsorbed at the oil/water, air/water and hydrophobic solid/water interfaces have been determined using neutron specular reflection in conjunction with isotopic substitution. The areas per molecule were found to be 78±2 Å and 80±5 Å at the critical micelle concentration (CMC=2.5 x 10-3 M), increasing to 132±5 Å at 8.0 x 10-6 M and 21 aut 3 ± 20 A2 at 2.5 x,10-6 M at the air/water and hydrophobic solid/water interfaces respectively. The thicknesses of the AOT layer at the air/water and solid/water interfaces were found to be 18±2 Å and 15±2 Å with no significant change with concentration. At the oil/water interface the thickness is much larger than the fully extended molecule and is 26±2 Å at an area per molecule of 68±2 Å2. This is probably caused by roughening of the oil/water interface by thermal fluctuations

The structure of monododecyl pentaethylene glycol monolayers with and without added dodecane at the air/solution interface: A neutron reflection study

Neutron reflectometry and surface tensiometry have been used to study the composition and structure of a mixed monolayer of dodecane and the nonionic surfactant pentaethylene glycol monododecyl ether (C12E5) and the structure of the C12E5 on its own at the surface of aqueous solutions of C12E5. Partially labeling of the surfactant with deuterium gave the surface normal distributions of the ethylene glycol and hydrocarbon chains of the surfactant and of the water, and these results are compared with those from other members of the C12Em series. The C12E5 layer has a higher capacity for oil than that of the comparable hydrocarbon chain length cationic surfactant, dodecyl trimethylammonium bromide (C12TAB), and the structure of the composite layer is also different. The separation of the centers of the surfactant chain and dodecane distributions is only 3 Å in the C12E5 system in comparison with 6.5 Å for C12TAB. In this respect, the C12E5/dodecane system more closely resembles the sodium dodecyl sulfate/dodecanol system, and this is attributed to a favorable interaction between the dodecane and the ethylene glycol chain, which encourages a closer approach of the oil to the water surface. The hydrocarbon chain in the C12Em series where m is 6 or less is tilted at about 40° away from the surface normal. Upon incorporation of dodecane into the C12E5 layer, the hydrocarbon chain changes to an almost perpendicular orientation