Effects of amino-acid functionalization and pH value on temperature-dependent water dynamics in silica confinement

Abstract

2H nuclear magnetic resonance (NMR) field-cycling relaxometry and broadband dielectric spectroscopy (BDS) studies show that water dynamics in silica pores with similar diameters (∼6 nm) strongly depend on the functionalization of the inner surfaces. In all studied confinements, we observe two prominent changes in the temperature dependence of water reorientation. Specifically, the activation energy of Ea ∼ 0.3 eV in the fully liquid state more than triples to Ea ∼ 1.0 eV upon partial crystallization at Tm ∼ 258 K. Furthermore, in the partially crystallized state, the liquid fraction shows a dynamical crossover at ∼185 K, where the common low-temperature behavior of confined water with Ea = 0.4–0.5 eV is established. However, the correlation times of water reorientation are up to two orders of magnitude longer in amino-acid functionalized silica pores than in pristine ones. Comparing the results for different functional groups, NMR and BDS consistently show that the slowdown is strongest for basic lysine followed by neutral alanine and, finally, acidic glutamic acid. Based on this order, one may speculate that the changed dynamics are a consequence of different pH values of water in confinements with different functional groups. Although pH measurements confirm that the pH value strongly depends on the amino-acid functionalization, this speculation must be rejected due to the observation that water with very different pH values does not show diverse reorientation dynamics when enclosed in identical pores.