Evidences for charged hydrogen bonds on surfaces bearing weakly basic pendants: The case of PMMA–ran–PDMAEMA polymeric films

Positively charging a solid surface usually confers on it a bactericidal action that increases with the charge density. The latter may be modulated by chemically specific interactions between surface groups such as charged hydrogen bonds. As this possibility has not yet been extensively investigated, we studied the surface electrolytic properties of PEG–b–(PMMA–ran–PDMAEMA)n plaques by means of contact angle titrations, ATR–FTIR spectroscopy, DFT electronic structure calculations, and Monte Carlo titration simulations. Contact angle titrations highlighted the impact that polymer composition and film preparation have on plaques surface energy upon changing the pH. Following the relative increase in IR N–H bending absorption due to ammonium formation via FTIR experiments suggested, instead, an increase of surface groups pKb compared to free DMAEMA monomers. Interestingly, the N–H bending absorption for polymers with high DMAEMA contents appeared one pH unit below than for the low DMAEMA content cases. Basing on electronic structure harmonic frequencies analysis and Monte Carlo simulations, we rationalize the difference in apparent pKb as related to a shift in N–H bending frequency (+ 120 cm−1) due to a more likely ammonium–amine dimerization. We predict that a sufficient density of free N–H groups ought to appear at lower pH values the higher is the DMAEMA contents.