The spring constant of microfabricated cantilevers used in scanning force microscopy (SFM) can be determined by measuring their resonant frequencies before and after adding small end masses. These masses adhere naturally and can be easily removed before using the cantilever for SFM, making the method nondestructive. The observed variability in spring constant-almost an order of magnitude for a single type of cantilever-necessitates calibration of individual cantilevers in work where precise knowledge of forces is required. Measurements also revealed that the spring constant scales with the cube of the unloaded resonant frequency, providing a simple way to estimate the spring constant for less precise work.

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-

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Living organisms construct various forms of laminated nanocomposites through directed nucleation and growth of inorganics at self-assembled organic templates at temperatures below 100°C and in aqueous solutions. Recent research has focused on the use of functionalized organic surfaces to form continuous thin films of single-phase ceramics. Continuous thin films of mesostructured silicates have also been formed on hydrophobic and hydrophilic surfaces through a two-step mechanism. First, under acidic conditions, surfactant micellar structures are self-assembled at the solid/liquid interface, and second, inorganic precursors condense to form an inorganic-organic nanocomposite. Epitaxial coordination of adsorbed surfactant tubules is observed on mica and graphite substrates, whereas a random arrangement is observed on amorphous silica. The ability to process ceramic-organic nanocomposite films by these methods provides new technological opportunities.