Interactions between molecularly smooth gold and mica surfaces across aqueous solutions

Surface diffusion enhanced ion transport through two-dimensional nanochannels

Fast ion permeation in nanofluidic channels has been intensively investigated in the past few decades because of their potential uses in separation technologies and osmotic energy harvesting. Mechanisms governing ion transport at this ultimately small spatial regime remain to be understood, which can only be achieved in nanochannels that are controllably fabricated. Here, we report the fabrication of two-dimensional nanochannels with their top and bottom walls consisting of atomically flat graphite and mica crystals, respectively. The distinct wall structures and properties enable us to investigate interactions between ions and interior surfaces. We find an enhanced ion transport within the channels that is orders of magnitude faster than that in the bulk solutions. The result is attributed to the highly dense packing of adsorbed cations at mica surfaces, where they diffuse in-plane. Our work provides insights into surface effects on ion transport at the nanoscale.

Aqueous Lubrication with the Molecularly Confined Films of Silicone-Based Amphiphilic Block Copolymer Aggregates

The confined film structures and tribological properties of the dilute aqueous solution of a silicone-based amphiphilic block copolymer, bis-isobutyl poly(ethylene glycol) (PEG)-14/amodimethicone (BIPA) copolymer, between mica surfaces were investigated. The BIPA copolymer existed as positively charged water-soluble aggregates in the solution. The adsorption behavior of the BIPA copolymer aggregates on a mica surface from solution was studied using an atomic force microscope (AFM); the result showed the immediate formation of a uniform adsorbed BIPA copolymer layer, followed by the gradual deposition of BIPA aggregates on the top of the adsorbed layer. Friction measurements were carried out using the surface forces apparatus (SFA) for the confined films of BIPA copolymer solution between mica surfaces, which revealed two different sliding film structures depending on the elapsed time after surface preparation. The sliding film consisting of two adsorbed BIPA copolymer layers was obtained for a relatively short elapsed time (not longer than 3 h), which had an extremely low friction coefficient μ (of the order of 10^-5). The sliding film on the following day (elapsed time of approximately 24 h) had the structure of a deposited/kinetically trapped BIPA aggregate layer confined between the opposing adsorbed layers, and the μ values were within the range from 10^-4 to 10^-3. Our results suggest that the different elapsed time ranges and resulting absence or presence of the intervening layer of trapped aggregates between the absorbed layers determine the tribological properties of the confined films. Molecular friction mechanisms are discussed for the two sliding structures, which give insight into using amphiphilic block copolymer aggregates for a new class of aqueous lubrication system to design extremely low friction interfaces.

Modifying surface forces through control of surface potentials

Combining direct surface force measurements with in situ regulation of surface potential provides an exceptional opportunity for investigating and manipulating interfacial phenomena. Recently, we studied the interaction between gold and mica surfaces in water with no added salt, while controlling the metal potential, and found that the surface charge at the metal may vary, and possibly even change its sign, as it progressively approaches the (constant-charge) mica surface [Langmuir, 2015, 31(47), 12845-12849]. Such a variation was found to directly affect the nature of the contact and adhesion between them due to exclusion of all mobile counterions from the intersurface gap. In this work, we extend this to examine the potential-dependent response of the adhesion and interaction between gold and mica to externally applied voltages and in electrolyte solution. Using a surface force balance (SFB) combined with a three-electrode electrochemical cell, we measured the normal interaction between gold and mica under surface potential regulation, revealing three interaction regimes - pure attraction, non-monotonic interaction from electrostatic repulsion to attraction (owing to charge inversion) and pure repulsion. Accordingly, the adhesion energy between the surfaces was found to vary both in no added salt water and, more strongly, in electrolyte solution. We justify this potential-dependent variation of adhesion energy in terms of the interplay between electrostatic energy and van der Waals (vdW) interaction at contact, and attribute the difference between the two cases to the weaker vdW interaction in electrolyte solution. Finally, we showed that through abruptly altering the gold surface potential from negative to positive and vice versa, the adhesion between gold and mica can be reversibly switched on and off. We surmise that the process of bringing the surface into contact is associated with the formation of a strong electric field O (10⁸ V m^-1) in the intersurface gap.

Probing the Surface Properties of Gold at Low Electrolyte Concentration

Using the surface force balance (SFB), we studied the surface properties of gold in aqueous solution with low electrolyte concentration (∼10(-5) M and pH = 5.8), i.e., water with no added salt, by directly measuring the interaction between an ultrasmooth gold surface (ca. 0.2 nm rms roughness) and a mica surface. Under these conditions, specific adsorption of ions is minimized and its influence on the surface charge and surface potential of gold is markedly reduced. At open circuit potential, the electrostatic interaction between gold and mica was purely attractive and gold was found to be positively charged. This was further confirmed by force measurements against a positively charged surface, poly-l-lysine coated mica. Successive force measurements unambiguously showed that once gold and mica reach contact all counterions are expelled from the gap, confirming that at contact the surface charge of gold is equal and opposite in charge to that of mica. Further analysis of adhesion energy between the surfaces indicated that adhesion is mostly governed by vdW dispersion force and to a lesser extent by electrostatic interaction. Force measurements under external applied potentials showed that the gold-mica interaction can be regulated as a function of applied potential even at low electrolyte concentration. The gold-mica interaction was described very precisely by the nonlinearized Poisson-Boltzmann (PB) equation, where one of the surfaces is at constant charge, i.e., mica, and the other, i.e., gold, is at constant potential. Consequently, the gold surface potential could be determined accurately both at open circuit potential (OCP) and under different applied potentials. Using the obtained surface potentials, we were able to derive fundamental characteristics of the gold surface, e.g., its surface charge density and potential of zero charge (PZC), at very low electrolyte concentration.

Direct Observation of Confinement-Induced Charge Inversion at a Metal Surface

Surface interactions across water are central to areas from nanomedicine to colloidal stability. They are predominantly a combination of attractive but short-ranged dispersive (van der Waals) forces, and long-ranged electrostatic forces between the charged surfaces. Here we show, using a surface force balance, that electrostatic forces between two surfaces across water, one at constant charge while the other (a molecularly smooth metal surface) is at constant potential of the same sign, may revert smoothly from repulsion to attraction on progressive confinement of the aqueous intersurface gap. This remarkable effect, long predicted theoretically in the classic Gouy-Chapman (Poisson-Boltzmann) model but never previously experimentally observed, unambiguously demonstrates surface charge reversal at the metal-water surface. This experimental confirmation emphasizes the implications for interactions of dielectrics with metal surfaces in aqueous media.

Molecular-scale quantitative charge density measurement of biological molecule by frequency modulation atomic force microscopy in aqueous solutions

Surface charge distributions on biological molecules in aqueous solutions are essential for the interactions between biomolecules, such as DNA condensation, antibody-antigen interactions, and enzyme reactions. There has been a significant demand for a molecular-scale charge density measurement technique for better understanding such interactions. In this paper, we present the local electric double layer (EDL) force measurements on DNA molecules in aqueous solutions using frequency modulation atomic force microscopy (FM-AFM) with a three-dimensional force mapping technique. The EDL forces measured in a 100 mM KCl solution well agreed with the theoretical EDL forces calculated using reasonable parameters, suggesting that FM-AFM can be used for molecular-scale quantitative charge density measurements on biological molecules especially in a highly concentrated electrolyte.

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 asymmetric apposing 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 pK(A) and below the amine pK(A). 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.

Measuring the influence of solution chemistry on the adhesion of au nanoparticles to mica using colloid probe atomic force microscopy

Engineered nanoparticles are used increasingly in numerous commercial products, leading to concerns over their environmental fate and ecotoxicity. We report the adaptation of colloid probe atomic force microscopy (AFM) to quantitatively determine the adhesive behavior of gold nanoparticles (Au NPs) with mica, chosen as a model for sand, in various water chemistries. Au NP-covered polystyrene (PS) beads were prepared by a combined swelling-heteroaggregation (CSH) technique prior to attachment to tipless AFM cantilevers. Force measurements were performed over a range of solution conditions (pH, ionic strength (IS), and natural organic matter (NOM) content). Plain PS beads with no Au NPs were used as controls. In general, adhesion of Au NP-PS beads to mica were found to increase as IS increased while a rise in pH led to a decrease in adhesion. Plain PS beads were not observed to adhere to mica in any of the experimental solution conditions, and the PS force curves were unaffected by changes in the pH and electrolyte concentrations. In the presence of NOM, pull-off forces for Au NP-PS beads increased in magnitude when NaCl was added. In addition, the experimental approach force curves were not successfully described by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability. To reconcile the discrepancy between theory and experiment, an extended DLVO (xDLVO) empirical model was used to account for the contribution of non-DLVO interactions (known collectively as structural forces) between the Au NPs and mica surfaces.