The phenomenon of solubilization of less polar material into micelles and other surfactant aggregates has been well studied but little is known about its equivalent at interfaces. The possibilities range from adsorption of mixed (surfactant and non- or weakly polar) aggregates at the hydrophilic/aqueous interface to incorporation of non- or weakly polar material into monolayers of surfactant. The former is expected to resemble the bulk behaviour to some extent but the range of possibilities for the latter seems to be large. The importance of the process is that it is crucial in the delivery of additives to interfaces, e.g. perfume in detergents and the delivery of pesticides and fungicides to non-wetting surfaces.
Some data (unpublished, done in collaboration with Professor F.Tiberg, now at Camurus) on the adsorption of a microemulsion at the silica/aqueous interface is shown in the two diagrams below. This experiment was done at several different contrasts and concentrations. There are some difficulties in the interpretation because some differences in coverage are seen with different isotopes and because the coverage in an earlier ellipsometric measurement was 100% whereas here (see lower diagram) it is less that 100% with the discrepancy increasing as the oil fraction decreases. Nevertheless, the neutron give well resolved distributions of the individual components at the interface and these show clearly that the bulk structure is retained in a distorted form. The other interesting result is the discrepancy between the surface and bulk compositions, written on the lower diagram. As far as we know, noone has yet tried to model a system of this kind.
The incorporation of non- or weakly polar materials into a monolayer at the air/water interface can occur via the vapour phase, contact with an insoluble liquid, or from solubilized or weakly soluble material in the bulk solution. Spreading an oil on the surface of a surfactant solution leads to a situation where there is a lens in equilibrium with a film of molecular thickness consisting of oil and surfactant. By labelling the oil and surfactant the relative positions of oil and surfactant to each other and to the water surface can be determined. The diagram shows the results of such a study for dodecane spread on hexadecyl trimethyl ammonium bromide (C16TAB). By labelling just the terminal hexyl fragment of the C16TAB it could be shown that the centres of the oil (dodecane) and hexyl fragment distributions are located at the same position in the interface, i.e. the oil occupies the outer part of the layer. The surfactant also changes its orientation when oil is added with the hexyl, group of the surfactant moving outwards by 4.5 $Aring; in the presence of the oil. This work was done in collaboration with Bernie Binks and Paul Fletcher at http://www.hull.ac.uk/chemistry/index.html.
For more detail, see
Structure and composition of dodecane layers spread on aqueous solutions of dodecyl and hexadecyltrimethyl ammonium bromides studied by neutron reflection, J. Phys. Chem., 99, 4113 (1995) and Structure of monododecyl pentaethylene glycol monolayers with and without added dodecane at the air/solution interface: a neutron reflection study, J. Phys. Chem. B, 102, 5785-5793 (1998)
Any non-surfactant material dissolved or solubilized in the bulk solution will be at least weakly polar. It will then be more closely integrated into the surfactant layer than the oil discussed above. In particular, it will tend to compete for surfactant head group space and it becomes much more difficult to predict the extent of its adsorption. From a technological point of view, solution additives are very widely used and it is therefore this situation that is most important to understand. We have made a number of studies in this area,
The composition and structure of sodium dodecyl sulphate-dodecanol mixtures adsorbed at the air-water interface J. Coll. Int. Sci., 174, 441 (1995), The influence of sorbitol on the adsorption of surfactants at the air/liquid interface, J. Coll. Int. Sci., 184, 391-8 (1996), The structure and composition of the mixed monolayer of hexadecyl trimethyl ammonium bromide and benzyl alcohol adsorbed at the air/water interface, Langmuir, 14, 2139-2144 (1998), Comparison of the coadsorption of benzyl alcohol and phenyl ethanol with the cationic surfactant, hexadecyl trimethyl ammonium bromide, at the air-water interface, J. Coll. Int. Sci., 247, 397-403 (2002), Impact of Model Perfumes on Surfactant and Mixed Surfactant Self-Assembly, Langmuir, 24, 12209-12220 (2008).
To illustrate how the position of such a cosurfactant can be determined with good accuracy, the example of hexadecyltrimethylammonium p-tosylate, which is slightly different from the above in that the counterion is really part of the surfactant, although it has many features of some of the important additives, is shown in the figure below. Here the hexadecyl chain was labelled in groups of four carbon atoms and then the partical structure factor method used to determine the distance of the labelled tosylate from each labelled fragment in the hexadecyl chain. Since the partial structure factor has a very simple form in this case, it can be seen by inspewction that the tosylate ion is located in more or less exactly the same position as the second block of four carbons from the head group. this work was done with Colin Bain's group and is published in
Monolayers of hexadecyltrimethylammonium p-tosylate at the air/water interface, J. Phys. Chem. B, 102, 9473 (1998).
This project is currently continuing as a CASE project with Unilever.