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 indicates the coexistence of complex structures. The (*) marked 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. A noticeable ‘structure-making’ effect is exerted on the whole system in the 40-60 % DPSO/H2O w/w range, due to the co-existence of H-bonds, dipole-dipole and hydrophobic interactions. Thus, this effect causes the glass-forming properties responsible of the cryoprotective effect of the sulfoxides.