Proton Activity in Nanochannels Revealed by Electron Paramagnetic Resonance of Ionizable Nitroxides: A Test of the Poisson-Boltzmann Double Layer Theory

Chemical and physical processes occurring within the nanochannels of mesoporous materials are known to be determined by both chemical nature of the solution inside the pores/channels as well as the channel surface properties, including surface electrostatic potential. Such properties are important for numerous practical applications such as heterogeneous catalysis and chemical adsorption including chromatography. However, for solute molecules diffusing inside the pores the surface potential is expected to be effectively screened by counter ions for the distances exceeding the Debye length. Here we employed EPR spectroscopy of ionizable nitroxide spin probes to experimentally examine the conditions for the efficient electrostatic surface potential screening inside the nanochannels of chemically similar silica-based mesoporous molecular sieves (MMS) filled with water at ambient conditions and a moderate ionic strength of 0.1 M. Three silica MMS having average channel diameters of D = 2.3, 3.2 and 8.1 nm (C12MCM-41, C16MCM-14, and SBA-15, respectively) were chosen to investigate effects of the pore diameter at the nanoscale. The results are compared with the classical Poisson-Boltzmann (PB) double layer theory developed for diluted electrolytes and applied to a cylindrical capillary of infinite extent. While the surface electrostatic potential was effectively screened by the counter ions inside the largest channels of 8.1 nm in diameter (SBA-15) the effect of the surface electrostatic potential on local effective pH was significant for the 3.2 nm channels (C16MCM-14). The smaller pores of C12MCM-41 (2.3 nm in diameter) provided the most critical test for the PB equation that is based on a continuum electrostatic model and demonstrated its inapplicability likely due to discrete nature of molecular systems at the nanoscale and nanoconfinement effects leading to larger spatial heterogeneity.