Dynamics of pH-sensitive nitroxide radicals in water adsorbed in ordered mesoporous molecular sieves by EPR Spectroscopy

• pH-sensitive nitroxides (NR) were used to study mesoporous sieves (MMS).
• There were observed the “fast” and “slow” motions of NR inside the hydrated MMS.
• At pH > 10 NR are fully removed from the MMS with small volumes of water solutions.
• Fractions of the “fast-motional” NR (q) in the MMS vs. pH are cup-shaped.
• q-Values can be used to measure pH inside MMS and near-surface electrical potential.

A spin pH probe technique was used to study the influence of the channel diameter on the EPR spectra of pH-sensitive nitroxide radicals (NR) located in the channels of the mesoporous molecular sieves MCM-41 and SBA-15 with diameters ranging from 2.3 to 8.1 nm. From EPR spectra analysis and the results of the NR retention by the mesoporous molecular sieves upon washing with an aqueous KCl solution, the regularities of NR molecular location inside the channels were studied. The obtained dependence of the fraction of the radical molecules in the fast motional regime (with the rotational correlation times, τc = 2 × 10−11 s–9 × 10−11s) in the channels of the mesoporous molecular sieves as a function of pH indicates that both NR in the fast and slow motional regime (with τc = 8 × 10−9s–7 × 10−10s) may be used for estimation of the solution acidity inside the channels and of the near-surface electrical potential.

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