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

•Stern potential was measured in molecular sieves (MMS) by pH-sensitive nitroxides (NR).•The titration curves of the “slow-motional” NR in MMS were used to measure the shifts in pKa.•Negative (positive) Stern potentials were found at pH greater (lesser) than pzc of the MMS.•The near-zero charge of SBA-15 and MCM-41 was measured at pH 4.1–4.7 and 3.4–3.7, respectively.•The constant pHloc in SBA-15 and МСМ-41 were measured at pH 5.5–6.5 and 6–8.5, respectively.

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.

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