Surface Aggregate Phase Transition Erica J. Wanless, † Tim W. Davey, and William A. Ducker* ,‡ Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Received December 12, 1996. In Final Form: May 7, 1997 X A surface aggregate phase transition is described. Atomic force microscopy has been used to image the equilibrium association of sodium dodecyl sulfate (SDS) and 1-dodecanol molecules at the interface between graphite and aqueous solutions. In pure SDS solutions, the molecules associate into long, parallel hemicylindrical surface aggregates over a concentration range from about one-third to at least 10 times the critical micelle concentration (cmc). Above the cmc, dodecanol has little influence on the surface aggregate structure, probably because dodecanol is partitioned into the bulk micelles. Below the cmc, dodecanol causes a transition from hemicylindrical aggregates to a two-phase mixture in which flat sheets coexist with swollen hemicylindrical aggregates. In this mixture, the hemicylindrical aggregates are preferentially located at and parallel to steps on the underlying graphite substrate. Under conditions where hemicylinders and flat sheets coexist, an increase in bulk dodecanol concentration results in an increase in surface coverage by flat sheets. No bulk solution changes were detected by NMR in the region where the surface phase transition was observed. Introduction In aqueous solution, surfactants self-assemble into a variety of aggregate shapes including spherical and rod like micelles and planar bilayers. Reasonable predictions of aggregate shape can be made on the basis of the forces acting on the surfactant monomer, and phase transitions can be understood in terms of changes in these forces. For example, an increase in electrolyte concentration in ionic surfactant solutions reduces the electrostatic repulsion between headgroups and lowers the curvature of the aggregate. For certain ionic surfactants, this is manifest as a sphere-to-rod transition. 1 Addition of a long-chain alcohol to surfactant solutions swells the hydrocarbon component of the aggregate and results in a lower curvature aggregate. 2 For example, several n-alcohols induce the sphere-to-rod transition of sodium dodecyl sulfate (SDS) in NaCl solutions. 1 The forces on the monomer are sometimes summarized in terms of a packing parameter, 3 which is the ratio of the hydrocarbon volume to the product of the area occupied by the headgroup and the length of the hydrocarbon chain. The packing parameter is higher for surfactants which form lower curvature aggregates. When surfactants are present in solution, they often also spontaneously aggregate at solid-liquid interfaces and these surface aggregates can be studied directly with the atomic force microscope (AFM), after the method of Manne et al. 4 Previously the equilibrium surface ag- gregation of cationic, 4,5 anionic, 6-9 and zwitterionic 8,10 surfactants in aqueous solution has been investigated using this method. These studies have emphasized that the surface aggregate morphology is frequently different from the bulk solution aggregate shape, with the substrate playing a dominant role in determining the surface aggregate structure. To date, different surface aggregate structures have been observed on different substrates 10 and for different surfactants on the same substrate. 8 The zwitterionic surfactant, (dodecyldimethylammonio)pro- panesulfonate, forms spherical micelles on mica whereas the cationic surfactant, dodecyltrimethylammonium bro- mide, forms cylindrical micelles. Mixtures of the two surfactants form structures of intermediate length, so a transition from one structure to another is observed. A surface phase transition for a single adsorbate on a single substrate has not yet been observed. In this work we describe a phase transition for a surfactant-cosurfactant system. AFM is used to show the effect of 1-dodecanol on the aggregation of dodecyl sulfate at the graphite-solution interface. The surface aggregation of dodecyl sulfate on graphite in the absence of dodecanol has been thoroughly characterized: 6 above 2.8 mM dodecyl sulfate assembles in an organized periodic structure consisting of long parallel aggregates. The aggregates appear as stripes in the AFM images and are observed when the tip and the solid substrate are slightly separated. These aggregates are believed to be hemi- cylindrical structures and can be characterized by their period and the thickness of the adsorbed layer. 7 The hemicylindrical structure is retained even when the surfactant concentration and electrolyte concentration are varied significantly. 6,7 Although the addition of electrolyte to SDS solutions induces a sphere-to-rod transition in bulk, there is no clear evidence for a salt-induced transition at the graphite-solution interface. Since n-alcohols are known to be more effective than electrolyte in inducing bulk phase changes, 1,11 we have investigated the influence of an n-alcohol on surface aggregation. Long-chain alcohols are sparingly soluble in water. This solubility is increased in aqueous surfactant solutions, with a particularly large increase above the critical micelle concentration (cmc) where the alcohol is partitioned into the micelles. 2 The surfactant cmc is also lowered in the presence of n-alcohols. 12,13 This decrease is the result of an increase in the entropy of mixing and a decrease in the * To whom correspondence may be addressed: e-mail, duck@ alkali.otago.ac.nz. † Now at Department of Chemistry, University of Newcastle, Australia. ‡ Now at Department of Chemistry, Virginia Tech. X Abstract published in Advance ACS Abstracts, July 1, 1997. (1) Nguyen, D.; Bertrand, G. L. J. Colloid Interface Sci. 1992, 150, 143-157. (2) Hunter, R. J. Foundations of Colloid Science, Vol. II; Oxford University Press: Oxford, 1991. (3) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd eds.; Academic Press: London, 1992; Chapter 17. (4) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (5) Manne, S.; Gaub, H. E. Science 1995, 270, 1480-1482. (6) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 3207- 3214. (7) Wanless, E. J.; Ducker, W. A. Langmuir 1997, 13, 1463-1474. (8) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915-5920. (9) Ducker, W. A.; Lamont, R. J. Colloid Interface Sci., in press. (10) Ducker, W. A.; Grant, L. M. J. Phys. Chem. 1996, 100, 11507- 11511. (11) Candau, S.; Zana, R. J. Colloid Interface Sci. 1981, 84, 206- 219. (12) Shinoda, K. Bull. Chem. Soc. Jpn. 1953, 26, 101. 4223 Langmuir 1997, 13, 4223-4228 S0743-7463(97)00146-7 CCC: $14.00 © 1997 American Chemical Society