Attractive forces between two hydrophobized mica surfaces immersed in water were measured using the surface force apparatus (SFA) over the temperature range 25-50 °C. Monolayers of dihexadecyldimethylammonium (DHDA), dioctadecyldimethylammonium (DODA), or dieicosyldimethylammonium (DEDA) surfactants were deposited onto mica by dipping cleaved mica into a cyclohexane solution containing the surfactant. The SFA measurements confirm the presence of a very long range attractive force reported in previous studies. A surprising result is that the magnitude of this attractive force depends on the surfactant chain length and changes dramatically with temperature. The molecular structure of the monolayer in air and in water at 25 °C was obtained from atomic force microscopy (AFM) images. The monolayers are uniform and continuous over microns, and the spacing between individual methyl groups was determined. For aqueous solutions, the differences in the AFM images for the three surfactants correlate with the differences in the SFA curves. These observations may explain the disagreement between the published force curves from different laboratories. Analysis of the observed temperature dependence leads to the following main conclusions: (1) the magnitude of the force depends critically on the state of the hydrocarbon chains on the surface. A very long range force is observed only when the chains show crystalline order; (2) the force has a stronger temperature dependence than that associated with hydrophobic interaction between molecules or the hydrocarbon-water surface tension; (3) cavitation mechanisms are ruled out; and (4) we still do not understand the origin of this anomalous long-range attractive force between hydrophobic surfaces in water.

Trimers with one attractive bead and two repulsive beads, similar to recently synthesized trimer patchy colloids, were simulated with flat-histogram Monte Carlo methods to obtain the stable self-assembled structures for different shapes and interaction potentials. Extended corresponding states principle was successfully applied to self-assembling systems in order to approximately collapse the results for models with the same shape, but different interaction range. This helps us directly compare simulation results with previous experiment, and good agreement was found between the two. In addition, a variety of self-assembled structures were observed by varying the trimer geometry, including spherical clusters, elongated clusters, monolayers, and spherical shells. In conclusion, our results help to compare simulations and experiments, via extended corresponding states, and we predict the formation of self-assembled structures for trimer shapes that have not been experimentally synthesized.

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