DI-n-propylsulfoxide/water mixtures: structure and cryoprotective properties.

Liquid di-n-propylsulfoxide (DPSO) exhibits strong self-associative effects both pure and in its water mixtures, due to the existence of intense intermolecular DPSO-DPSO and DPSO-H2O interactions, also involving S=O….H-C hydrogen bonds, that cause the very strong deviations from ideality found in calorimetric behaviour. The Raman spectra of DPSO-water mixtures were recorded at different molar ratios in order to study the behaviour of both S=O and C-H stretching modes. The aqueous mixtures were obtained by direct mixing of DPSO to required molar ratio (χ) and examined by FT-Raman spectroscopy as well as DSC technique. Fig. 1 reports the Raman spectra of the νSO region and the noticeable changes observed suggest the coexistence of complex structures. The bands, found even in the diluted solutions, are attributable to monomeric ‘free’ DPSO molecules (1094 cm-1), cyclic dimer (1067 cm-1), polymeric linear chains (1038 cm-1) and DPSO molecules interacting via H-bond (1021 cm-1). We attributed the 1015 cm-1 component to the DPSO/H2O 1:1 complex and the 1004 cm-1 to the DPSO/H2O 1:2 complex. The shift of the latter component from 1004 to 981 cm-1 by dilution is due to the increasing size of the hydration shell. On the contrary, the Raman spectra of the νCH region (fig. 2) reveals changes only in a less extent, supporting the existence of SO···HC hydrogen bonds in pure DPSO that gradually breaks in the presence of water. DSC measurements show liquid/solid or glassy/crystalline transitions only in the solutions with DPSO content is lesser of 40 % or greater of 75 % w/w, suggesting that in the 45-70 % w/w DPSO range the system exhibit a noticeable glass-forming tendency and a great stability of the amorphous state to the reheating. All measurements suggest that a noticeable ‘structure-making’ effect is exerted on the whole system in the 45-70 % DPSO/H2O w/w range, due to the co-existence of H-bonds, dipole-dipole and hydrophobic interactions. Consequently, we can explain the strong glass-forming properties, responsible of the cryoprotective effect found in presence of DPSO, like it was observed for other sulfoxides.