Spin probe dynamics of n-hexadecane in confined geometry

A combined study of the rotational dynamics of the stable free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and the phase behavior of n-hexadecane (n-HXD) in the bulk and the confined states in a series of silica gels (SG) by means of ESR and DSC is presented. A slow to fast motion transition of the spin probe TEMPO in the bulk n-HXD occurs at T50 G,bulk ≪ Tm,bulk, i.e., well below the melting temperature due to its trapping and localized mobility in the interlamellar gap of the crystallites [J. Bartoš, H. Švajdlenková, M. Zaleski, M. Edelmann, M. Lukešová, Physica B 430, 99 (2013)]. On the other hand, the dynamics of the TEMPO in the confined systems is strongly slowing down with T50 G (Dpore) >Tm(Dpore) and slightly increases with the pore size Dpore = 60, 100 and 300 Å of the SG’s. At the same time, both the corresponding melting temperature, Tm (Dpore), and melting enthalpy, ΔHm (Dpore), decrease with Dpore together with the mutual anti-correlation between T50 G and Tm as a function of the inverse of pore diameter, 1/Dpore. Moreover, the dynamic heterogeneity of the TEMPO in the confined state below T50 G (Dpore) is closely related to the phase transformation. The strong slowing down of the spin probe motion likely results from its preferential localization at the interface layer of the matrix pore due to specific interaction of TEMPO molecules with the polar silanol groups of the SG matrix. This is supported by special study on a series of the variously filled n-HXD/SG systems, other similar experimental findings as well as by theoretical spectral argument.