Water Affects Contact At Hydrophobic Interfaces

Energetics controlling the way two hydrophobic surfaces make contact and slide past one another play a central role in protein folding, lock-and-key type enzyme reactions, and other processes influenced by forces of adhesion and friction. Theoretical studies have shown that as hydrophobic surfaces come in contact even underwater, water is excluded from the interface, leading to dry contact. But until now, that scenario has not been probed directly by experiment. University of Akron polymer scientists Adrian P. Defante and Ali Dhinojwala and coworkers used a surface-sensitive spectroscopy method to directly examine the contact interface between hydrophobic polydimethylsiloxane (PDMS) and films of two materials regarded as equally hydrophobic—octadecyltrichlorosilane (OTS) and poly(vinyl-n-octadecyl carbamate) (PVNODC). In the absence of water, OTS and PVNODC respond identically to contact with PDMS. In water, however, PVNODC’s hydrophobic side chains rearrange and trap water at the interface, lowering the adhesion energy by 50% compared with the OTS-PDMS interface (Langmuir 2015, DOI: 10.1021/la504564w). Standard mathematical models used for calculating adhesion thermodynamics fail in this case, highlighting the need to better understand interfacial contact phenomena.