Experimental mapping of a pH gradient from a positively charged micellar interface to bulk solution

Abstract The nanoscale pH gradients from a charged membrane interface to the bulk solution, determine fundamental properties of chemical and biological systems. We estimated the proton distribution on micelle surfaces experimentally measuring the apparent pKa of a weak acid linked at varying distances to a micelle anchoring moiety and relating these values to the electrical potential at the micellar surface. The pH probes consisted of benzoic acid connected to chains with variable number (1–5) oxyethylene (−OCH2CH2–) units. The methylene of the terminal oxyethylene was connected to a quaternary nitrogen containing a long hydrocarbon chain. We calculated the apparent pKa (pKap) of the benzoic acid derivatives with surfactant micelles by UV spectroscopy. The difference between the pKa in solution and the pKap in micelles (ΔpK) of all probes was very similar with negatively charged sodium dodecyl sulphate (SDS) or zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) micelles, indicating that the dissociation of the substituted benzoates occurred at the micellar interface in both SDS and HPS. In contrast, the ΔpK of the probes decreased as a function of the benzoate's distance to the micelle interface in hexadecyltrimethylammonium chloride (CTAC) micelles, The variation of ΔpK with distance, from the CTAC micellar interface to bulk solution, was quantitatively correlated to H+ distribution from the interface. The experimental results coincided with independently calculated Poisson Boltzmann H+ distribution. These results demonstrate that that it is possible to experimentally map H+ gradients from a charged interface.