Influence of Molecular Dipole Orientations on Long-Range Exponential Interaction Forces at Hydrophobic Contacts in Aqueous Solutions

Using atomic force microscope (AFM) and surface forces apparatus (SFA) we measured similarly strong long-ranged (>100nm) attractive forces between self-assembled monolayers of carboranethiols, with dipole moments aligned either parallel or perpendicular to the surface, and hydrophobic lipid monolayers deposited on mica. We observed a long-ranged exponential attraction with decay length of 2.0 ± 0.2 nm for dipole alignments perpendicular to the surface. The effect of dipole alignment parallel to the surface is larger than for perpendicular dipoles, likely due to greater lateral correlation of in-plane surface dipoles. The magnitudes and range of the measured interaction forces also depend on the surface area of the probe used: At extended surfaces, dipole alignment parallel to the surface leads to a longer-range attraction due to electrostatic correlations of freely rotating surface dipoles and charge patches on the apposing surfaces. In contrast, perpendicular dipoles at extended surfaces, where molecular rotation cannot lead to large dipole correlations, do not depend on the scale of the probe used. Our results may be important to a range of scale dependent interaction phenomena related to solvent/water structuring dipolar and hydrophobic at interfaces.