Investigating hydration dependence of dynamics of confined water: Monolayer, hydration water and Maxwell–Wagner processes

Abstract

The dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21 and 36 Å, respectively, is examined by broadband dielectric spectroscopy ͑10 −2 -10 9 Hz͒ and differential scanning calorimetry for a wide temperature interval ͑110-340 K͒. The dynamics from capillary condensed hydration water and surface monolayer of water are separated in the analysis. Contrary to previous reports, the rotational dynamics are shown to be virtually independent on the hydration level and pore size. Moreover, a third process, also reported for other systems, and exhibiting a saddlelike temperature dependence is investigated. We argue that this process is due to a Maxwell-Wagner process and not to strongly bound surface water as previously suggested in the literature. The dynamics of this process is strongly dependent on the amount of hydration water in the pores. The anomalous temperature dependence can then easily be explained by a loss of hydration water at high temperatures in contradiction to previous explanations.

Comment on “Investigating hydration dependence of dynamics of confined water: Monolayer, hydration water, and Maxwell–Wagner processes” [J. Chem. Phys. 128, 154503 (2008)]

“…Therefore, we disagree with the point of view presented in Ref. 1. Moreover, we performed calculations according to the protocol described in the paper, accepting the authors' assumptions about the temperature dependencies of dielectric permittivity and conductivity of dispersed phase ͑water͒, w Ј and w , the dielectric permittivity value of the surrounding medium ͑SiO 2 matrix͒ s Ј, and the normalization condition v w + v s = 1 for the dimensionless volume fractions of water and the silica matrix ͑v w and v s ͒.…”

“…1 This assumption is very doubtful. Instead, it is indubitable that the authors definitely changed the impact of the electrode polarization effect by implementing the Teflon film.…”

“…IV E of the paper͒ the approach, which we employed in our earlier works for similar porous glass systems 3,4,10 and in a number of subsequent publications including systems of different nature and composition. [10][11][12] Since the criticisms of Sjöström et al 1 touch the fundamentals of the mentioned phenomenon, we feel ourselves obliged to response with following comment.…”

“…The subject of water in confined porous matrices has been an area of our interests for about a decade. [2][3][4][5][6] Therefore, the study of Sjöström et al 1 attracted our attention and we would like to point out some questionable, from our point of view, aspects of their investigation.…”

“…The recent paper by Sjöström et al 1 presents a study of the hydration dependence of the dynamics of water confined in silica porous matrices with different pore sizes. The authors of the paper used several techniques ͑Fourier transform infrared spectroscopy, differential scanning calorimetry, and broadband dielectric spectroscopy͒ in order to generate experimental material and they received some interesting experimental results.…”