Electrical potential near hydrated surface of ordered mesoporous molecular sieves assessed by EPR of molecular pH-probes

Abstract

Some of the most important parameters of mesoporous materials are Stern potential of double electrical layer (DEL) and surface charge because of the pivotal role they play in the adsorption of charged molecular species. There are not any conventional and easy-to-perform techniques used to measure Stern potential in porous systems. The purpose of this work is to measure Stern potential in a specific case, i.e., practically cylindrical nanosized hydrated channels of the mesoporous molecular sieves (MMS) MCM-41 and SBA-15 with channel diameters ranging from 2.3 to 8.1 nm using pH-sensitive nitroxide radicals (NR). A technique for direct measurement of the near-surface (Stern) potential using pH-sensitive NR was developed. Negative and positive values of Stern potential for the positively and negatively charged surface of MMS channels were found. The ranges of pH of external solution for the near-zero charge, boost-charging of the channel surface and for dissociation of functional groups in the MMS studied were found. © 2014 Elsevier Inc. All rights reserved.