A synergetic lubricating effect between porcine gastric mucin (PGM) and chitosan based on their mucoadhesive interaction is reported at a hydrophobic interface comprised of self-mated polydimethylsiloxane (PDMS) surfaces. In acidic solution (pH 3.2) and low concentrations (0.1 mg mL(-1)), the interaction of PGM with chitosan led to surface recharge and size shrinkage of their aggregates. This resulted in higher mass adsorption on the PDMS surface with an increasing weight ratio of [chitosan]/[PGM + chitosan] up to 0.50. While neither PGM nor chitosan exhibited slippery characteristics, the coefficient of friction being close to 1, their mixture improved considerably the lubricating efficiency (the coefficient of friction is 0.011 at an optimum mixing ratio) and wear resistance of the adsorbed layers. These findings are explained by the role of chitosan as a physical crosslinker within the adsorbed PGM layers, resulting in higher cohesion and lower interlayer chain interpenetration and bridging.

Measurements have been made of the adhesion and friction forces between organic monolayers in heptane/acetone mixtures using an atomic force microscope (AFM). It has been found that the contact mechanics are best modeled by treating the friction force as the sum of a load-dependent term (attributed to "molecular plowing") and an area-dependent term attributed to shearing (adhesion). The relative contributions of plowing and shearing are determined by the coefficient of friction, μ, and the surface shear strength τ. The transition from adhesion- to load-determined friction is controlled by the solvation state of the surface: solvated surfaces represent a limiting case in which the shear term approaches zero, and the friction-load relationship is linear, while in other circumstances, the friction-load relationship is nonlinear and consistent with Derjaguin-Muller-Toporov mechanics. A striking correlation has been observed between the concentration-dependence of the association constant (K(a)) for the formation of 1:1 hydrogen-bonded complexes and the pull-off force F(a) and surface shear strength τ for the same molecules when one partner is immobilized by attachment to an AFM probe and the other is adsorbed to a surface. Analysis of the concentration-dependence of F(a) and τ enables the prediction of K(S) with remarkably high precision, indicating that for these hydrogen bonding systems, the tip-sample adhesion is dominated by the H-bond thermodynamics. For mixed monolayers, H-bond thermodynamics dominate the interaction even at very low concentrations of the H-bond acceptor. Even for weakly adhering systems, a nonlinear friction-load relationship results. The variation in τ with the film composition is correlated very closely with the variation in F(a). However, the coefficient of friction varies little with the film composition and is invariant with the strength of tip-sample adhesion, being dominated by molecular plowing and, for sufficiently large concentrations of hydroxyl terminated adsorbates, the disruption of intramonolayer hydrogen bonding interactions.

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Lubrication of soft polydimethylsiloxane (PDMS) elastomer interfaces was studied in aqueous mixtures of porcine gastric mucin (PGM) and branched polyethyleneimine (b-PEI) at neutral pH and various ionic strengths (0.1-1.0 M). While neither PGM nor b-PEI improved lubrication compared to polymer-free buffer solution, their mixtures produced a synergistic lubricating effect by reducing friction coefficients by nearly two orders of magnitude, especially at slow sliding speed in the boundary lubrication regime. An array of spectroscopic studies revealed that small cationic b-PEI molecules were able to strongly bind and penetrate into large anionic PGM molecules, producing an overall contraction of the randomly coiled PGM conformation. The interaction also affected the structure of the folded PGM protein terminals, decreased the surface potential and increased light absorbance in PGM:b-PEI mixtures. Adding an electrolyte (NaCl) weakened the aggregation between PGM and b-PEI, and degraded the lubrication synergy, indicating that electrostatic interactions drive PGM:b-PEI complexation.