Molecular wet adhesion

Abstract

Adhesion is a phenomenon that incorporates a number of physical and chemical forces. While it is seen as an interfacial effect, the complex contributions of physico-chemical forces lead to a range of interaction over which two surfaces influence each other. At the smallest length-scale, the ubiquitous dispersion forces and specific chemical groups dominate adhesion. Roughness confounds these contributions to adhesion and their influence reduced rapidly away from the surface. Real surfaces pose challenges to maintain adhesive association, and both nature and science have developed polymer-based solutions to restore adhesion. A device frequently employed is to adsorbed polymer layers, used to {u0300}bridge' across surfaces thus increasing the range of interaction. We have called this distal adhesion, of which two subtypes have been reported in the literature, one elastic and viscous in nature. While the elastic behaviour is relatively well understood, viscous distal adhesion remains a contentious issue. The overarching aim of this thesis is to investigate the relationship between polymer adsorption and distal adhesion at the molecular level using Atomic Force Microscopy (AFM). The roles of the surface, the polymer and the solvent are tested by systematically varying several chemical properties including the polarity and hydrogen bonding capability of the surfaces and polymers; chain architecture; and chemical composition and properties of the solvent. Solvents studied include pure water and various binary aqueous mixtures of aprotic and protic organic solutes. It is shown that the conformation of adsorbed polymer molecules determines the type of distal adhesion, with the elastic behaviour resulting from the swollen conformation characterised by surface extended polymer loops and tails. Viscous behaviour, on the other hand, requires the flat conformation generally attained when the adsorption density is low. It is demonstrated that viscous distal adhesion is independent of the interfacial chemistry between the polymer and the surface, contradicting the current paradigm and prompted a fresh investigation of the hitherto inconspicuous role of the solvent. Our experimental results showed that wet viscous distal adhesion attains the highest strength in pure water, and the presence of solutes generally weakened it. A new model is proposed to rationalise the results whereby the difference in connectivity of water molecules in the bulk and at the interface is the key determinant in the magnitude of wet viscous distal adhesion. Water has a high propensity to form an elaborate hydrogen-bonded network of molecules in the bulk. The removal of a polymer chain from a non-hydrogen bonding substrate simultaneously tests the connectivity of water in the two environments. The dissolved alcohols act to dilute the number of water-water hydrogen bond, leading to a situation where, at a certain alcohol concentration, bulk and interfacial water molecules become equivalent in terms of connectivity and the distal adhesion vanishes. The application of our results is not limited to wet molecular adhesion phenomenon, but is also relevant to solute solvation, protein-water interaction, molecular recognition and other phenomena driven by properties of liquid water. A refinement of our model is contingent on the availability of further computational and experimental studies. Show more