In aqueous solutions---such as physiological fluids, seawater, or detergent solutions---both adhesion and cohesion between surfaces are severely limited by high salt concentrations, oxidizing pH levels, and bound hydration layers at the solid-liquid interface. While problematic, these limitations do not entirely prohibit adhesion: certain synthetic polymers and biological molecules exhibit adaptive amphiphilic interactions that strongly bind to wet surfaces and lead to robust adhesion. Identifying the intra- and inter-molecular interactions between adhesive molecules and surfaces---including hydrogen bonding, electrostatic interactions, solvation interactions, polymer dynamics, and synergistic interactions---is imperative for the future design of high-performance wet adhesives and materials. In the following thesis, a surface forces apparatus (SFA) and other surface characterization techniques are used to study how unique molecules display concurrent adhesive mechanisms, adapt, self-assemble, and partition between chemically heterogeneous surfaces (either hydrophobic or hydrophilic) to achieve durable wet adhesion.

This thesis is divided into chapters on surfactant and polyelectrolyte self-assembly and adhesion (Chapter 2) and bio-inspired peptidyl adhesion (Chapters 3 and 4). Chapter 2 explores the behaviors of aliphatic surfactants and silicone polyelectrolytes as they self-assemble at hydrophobic interfaces and mediate strong adhesion to mineral surfaces. In this section, the molecular geometries of surfactants and polyelectrolytes are shown to control the self-assembled structures that form at aqueous surfaces, as well as the overall adhesion between surfaces in solution.

Chapters 3 and 4 investigate the molecular origins of peptide-based wet adhesion. Certain sessile marine organisms, particularly mussels, robustly attach to wet and chemically heterogeneous surfaces through protein adhesive plaques that contain high concentrations of the catecholic amino acid 3,4-dihydroxyphenylalanine (Dopa). In Chapter 3, SFA measurements and molecular dynamics simulations reveal how Dopa and other amino acids in mussel foot proteins (Mfps) or peptides partition between hydrophobic or hydrophilic organic surfaces, leading to an adaptive adhesion mechanism. Chapter 4 explores the synergistic interaction between catechol and cationic amino acids (such as lysine and arginine) in surface adhesion. Through SFA measurements with siderophores---bacterial iron-chelators that consist of paired catechol and cationic moieties---it is shown that adjacent catechol-cation placement provides a "1-2 punch", whereby cationic amino acids evict hydrated salt ions from a mineral surface, allowing catechol binding to underlying oxides. Overall, these results provide a compelling rationale for the >20 mole% of cationic residues in Dopa-rich Mfps and establish a set of design parameters for future bio-inspired synthetic polymers.