1. Direct and quantitative measurements of the concentration and temperature dependence of hydrophobic forces
By virtue of its importance for self-organization of biological matter the hydrophobic force law and the range of hydrophobic interactions (HI) have been debated extensively over the last 40years. We directly measure and quantify the hydrophobic force-distance law over large temperature and concentration ranges. In particular, we study the HI between molecularly smooth hydrophobic self-assembled monolayers, and similarly modified gold-coated AFM tips (radii∼8-50nm). We recently generated quantitative and direct evidence that the hydrophobic force is both long-ranged and exponential down to distances of about 1-2nm. Therefore, we introduced a self-consistent radius-normalization for atomic force microscopy data. This approach allows quantitative data fitting of AFM-based experimental data to the recently proposed Hydra-model. With a statistical significance of r2 ⩾ 0.96 our fitting and data directly reveal an exponential HI decay length of 7.2 ± 1.2 Å that is independent of the salt concentration up to 750mM. As such, electrostatic screening does not have a significant influence on the HI in electrolyte concentrations ranging from 1mM to 750mM. In 1M solutions the observed instability during approach shifts to longer distances, indicating ion correlation/adsorption effects at high salt concentrations. With increasing temperature the magnitude of HI decreases monotonically, while the range increases slightly. We compare our results to the large body of available literature, and shed new light into range and magnitude of hydrophobic interactions at very close distances and over wide temperature and concentration regimes.
2. Influence of Molecular Dipole Orientations on Long-Range Exponential Interaction Forces at Hydrophobic Contacts in Aqueous Solutions
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. We recently described 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 compared 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.
3. Hydrophobic Forces, Electrostatic Steering, and Acid–Base Bridging between Atomically Smooth Self-Assembled Monolayers and End-Functionalized PEGolated Lipid Bilayers
A molecular level understanding of interaction forces and dynamics between asymmetricapposing surfaces (including end-functionalized polymers) in water plays a key role in the utilization of molecular structures for smart and functional surfaces in biological, medical, and materials applications. To quantify interaction forces and binding dynamics between asymmetric apposing surfaces in terms of their chemical structure and molecular design we developed a novel surface forces apparatus experiment, using self-assembled monolayers (SAMs) on atomically smooth gold substrates. Varying the SAM head group functionality allowed us to quantitatively identify, rationalize, and therefore control which interaction forces dominated between the SAM surfaces and a surface coated with short-chain, amine end-functionalized polyethylene glycol (PEG) polymers extending from a lipid bilayer. Three different SAM-terminations were chosen for this study: (a) carboxylic acid, (b) alcohol, and (c) methyl head group terminations. These three functionalities allowed for the quantification of (a) specific acid–base bindings, (b) steric effects of PEG chains, and (c) adhesion of hydrophobic segments of the polymer backbone, all as a function of the solution pH. The pH-dependent acid–base binding appears to be a specific and charge mediated hydrogen bonding interaction between oppositely charged carboxylic acid and amine functionalities, at pH values above the acid pKA and below the amine pKA. The long-range electrostatic “steering” of acid and base pairs leads to remarkably rapid binding formation and high binding probability of this specific binding even at distances close to full extension of the PEG tethers, a result which has potentially important implications for protein folding processes and enzymatic catalysis.