Molecular Organization of Surfactants at Solid-Liquid Interfaces

Abstract

Interactions between surfactant solutions and solid surfaces play a key role in technologically important processes such as colloidal stabilization, ore flotation, and soil removal; however, the interfacial aggregation of surfactant molecules is not yet well understood. Direct images of surfactant aggregates at solid surfaces in aqueous solutions were obtained with atomic force microscopy. The resulting structures for quaternary ammonium surfactants (above the critical micelle concentration) are consistent with half-cylinders on crystalline hydrophobic substrates, full cylinders on mica, and spheres on amorphous silica. These structures-surprisingly different from earlier models-appear to result from a compromise between the natural free curvature as defined by intermolecular interactions and the constraints imposed by specific surfactant-surface interactions. Such interfacial aggregates can potentially be used to pattern surfaces at nanometer-length scales.T h e self-assembly of amphiphilic molecules into spheres, cylinders, bilayers, and bicontinuous ~hases in free solution is fairlv well understood (1-3), but it is not clear how these aggregates are affected by the presence of a solid boundary surface. Pioneering research into interfacial aggregation with adsorption isotherms (4) and the surface force apparatus (SFA) (5) has provided quantitative measures of adsorption but little information on aggregate s&cture. Recent efforts to probe this structure have used fluorescence decay (6) and neutron reflection (7). Here we report direct images of surfactant aggregates at a variety of hydrophilic and hydrophobic substrates with the use of atomic force microscopy (AFM) (8). We restrict this study to (i) atomically flat substrates, where surface comeation is less than the -surfactant dimensions; (ii) cationic surfactants with auatemarv ammonium headgroups [typically tetradecyl trimethylammonium bromide (C,,TAB)], whose aggregation properties have been characterized (9); and (iii) surfactant concentrations above (typically twice) the critical micelle concentration (CMC). In general, the AFM images show interfacial aggregates with a high degree of curvature and periodic structure, in contrast to previously assumed models of flat monolayers and bilayers (see below).AFM imaging of surface aggregates was performed not in the usual contact mode but by means of precontact repulsive forces as described (1 0, 11 ). The imaging tip and sample surface were immersed in surfactant solution in the AFM fluid cell and conse-

Surfactant Aggregates at Rough Solid−Liquid Interfaces

“…The aggregates of many commonly studied surfactants on smooth surfaces are close-packed micelles of cylindrical, hemicylindrical, or spherical shape with typical center-to-center distances of 5-7 nm. 36 We aim to estimate the influence of the size of the AFM probe on the obtained image of such samples, which proves relatively difficult. The micellar aggregates are soft structures that are removed from the surface by energies as little as 10-20 kJ/ mol.…”

“…35,36 Pioneered by Manne and co-workers, this technique revealed that surfactants form surface aggregates in the shape of full 36 or half [35][36][37][38] cylinders or full 36 or half 39 spheres, as well as flat layers, 36,40 depending on the system under investigation. These findings have been confirmed by computational investigations.…”

Surfactant and Hydrocarbon Aggregates on Defective Graphite Surface: Structure and Dynamics

“…Indeed, surfactant adsorption and the resulting surface structures have been studied by various experimental techniques such as the streaming potential method, 3 calorimetry, 4 neutron reflection, 5-7 ellipsometry [8][9][10] and fluorescence spectroscopy, 11,12 and atomic force microscopy (AFM). [13][14][15][16][17][18][19][20][21] Of these, AFM provides the most detailed information on the topology of the assembled surfactant structures at molecular scales: Manne et al first provided evidence of hexadecyltrimethylammonium bromide (C 16 TAB) organization in parallel stripes at the interface between graphite and aqueous solution. 17 In subsequent studies, similar morphologies have been observed for various other surfactants and hydrophobic surfaces, 13,18,20 among them sodium dodecyl sulfate (SDS) on graphite.…”

Force measurements with the atomic force microscope: Technique, interpretation and applications

“…The adsorbed stripes were generally observed in three orientations. Based on observations like these Manne et al [734,793] concluded that CTAB forms cylindrical hemimicelles on graphite (HOPG), full cylinders on mica and spherical micelles on silica. Meanwhile the structure of several surfactants on various surfaces has been revealed.…”