title: Influence of molecular dipole orientations on long-range exponential interaction forces at hydrophobic contacts in aqueous solutions.
abstract: Strong and particularly long ranged (>100 nm) interaction forces between apposing hydrophobic lipid monolayers are now well understood in terms of a partial turnover of mobile lipid patches, giving rise to a correlated long-range electrostatic attraction. Here we describe similarly strong long-ranged 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 compare the interaction forces measured at very different length scales using atomic force microscope and surface forces apparatus measurements. Both systems gave a long-ranged exponential attraction with a 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 stronger 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 on dipolar and hydrophobic surfaces at interfaces.
authors.affiliation: Department of Chemical Engineering, §Materials Research Laboratory, and ∥Materials Department, University of California , Santa Barbara, California 93106, United States.
authors.forename: Kai
authors.initials: K
authors.lastname: Kristiansen
authors.forename: Philipp
authors.initials: P
authors.lastname: Stock
authors.forename: Theodoros
authors.initials: T
authors.lastname: Baimpos
authors.forename: Sangeetha
authors.initials: S
authors.lastname: Raman
authors.forename: Jaye K
authors.initials: JK
authors.lastname: Harada
authors.forename: Jacob N
authors.initials: JN
authors.lastname: Israelachvili
authors.forename: Markus
authors.initials: M
authors.lastname: Valtiner
keywords: hydrophobic; molecular dipole; self-assembled monolayer; surface forces apparatus