Abstract

There is interest in developing responsive, surface-active species that can be used to confer functionality and sensing capabilities to surfaces. Controlling structure, organization and material properties of thin film coatings requires an understanding of the nature of intermolecular interactions taking place at the air-water and air-solid interfaces. With the objective of combining differential diol (sugar) binding capabilities of boronic acids with surface activity of silicones, a series of boronic acid-derived bola amphiphiles (silicone boronic acid, SiBA) have been characterized at the air-water interface as a function of silicone chain length and subphase composition. Isotherms of SiBA show similar phase transitions as poly(dimethyl)siloxane but display a sharp increase in pressure before film collapse, indicating strong tethering of boronic acid headgroups to the subphase. For comparison, hydride and amine-terminated silicones were studied. Boronic acids occupy a relatively larger molecular area at the interface than other polar headgroups, suggesting they adopt a planar orientation on the subphase possibly forming boronic acid-boronic acid complexes rather than being submerged into the subphase. SiBA showed low sensitivity for sugars in the subphase at pH 5.5 which appears to be improved by addition of amines. Testing if amines alone are responsible for the observed shifts, a series of amines in the absence of diols were added to the subphase. The addition of amines in the subphase led to isotherm shifts to higher molecular areas and decreased film stability. While generally small molecule crosslinkers in the subphase increase film stability, for SiBA, they may be disrupting the boronic acid self-complexations, making the film less resistant to collapse.