J. Phys. Chem. 1994,98, 3222-3225
3222
Thermal Expansivity, Molar Volume, and Heat Capacity of Liquid Dimethyl Sulfoxide-Water Mixtures at Subzero Temperatures Peter Westht Department of Chemistry, University of Copenhagen, 5 Universitetsparken, DK-2100 Copenhagen 0, Denmark Received: September 7 , 1993; In Final Form: December 15, 1993'
The specific volume, thermal expansivity, and specific heat capacity of liquid binary mixtures of water and dimethyl sulfoxide (DMSO) have been measured at temperatures ranging from approximately -25 OC to room temperature, and at DMSO concentrations from 0 to 85% (w/w). Apparent quantities (apparent molar volume, apparent thermal expansivity, and apparent molar heat capacity) of DMSO were evaluated. Around room temperature, the concentration dependence of these functions show a behavior characteristic for aqueous solutions of molecules with nonpolar moieties. However, at the lowest temperatures the concentration dependence of the apparent molar volume and heat capacity resembled what is observed for strongly hydrophilic nonelectrolytes. These observations suggest that the effect of (small amounts of) DMSO on connectivity and fluctuations of the three-dimensional hydrogen-bonding system of water depends strongly on temperature. Above approximately -15 OC DMSO promotes connectivity and introduces enhanced fluctuations of the H-bond network. At lower temperatures it inhibits fluctuations and possibly reduces connectivity of the water-water H-bonding system.
Introduction The importance of dimethyl sulfoxide (DMSO) within chemistry, biotechnology, and medicine' has motivated a number of investigations of the molecular properties of DMSO-water mixtures. Deviations from ideality of thermodynamicquantities such as heat of mixing,2g3heat capacity? volume? viscosity,5 and vapor pressure? as well as spectroscopic data?** have been interpreted in terms of interactions between DMSO and water. In concentrated aqueous solutions such interactions appear to be due primarily to dipoldipole forces and strong hydrogen bonds between the sulfoxidegroupand water molecules,9possibly leading to aggregates involvingtwo water molecules per DMSO molecule.5 Interpretation of experimental data for the water-rich concentration range appears to be more complex. Lindberg and coworkers inferred from thermodynamic, dielectric, and spectroscopic measurements that addition of small amounts of DMSO to water is accompanied by formation of highly polar aggregates and by a breakdown of the 'liquid lattice" ~fwater.~JO A similar conclusion was reached by Macdonald et al.11 More recent thermodynamicinvestigations?J* on the other hand, suggest that DMSO in dilute solution enhances the hydrogen-bond network of water due to so-called hydrophobic hydration of the methyl groups. Other experimental data299 support the hypothesis that small amounts of DMSO promote water structure. Lovelock and Bishop13 originally reported that addition of DMSO (ca. 10%w/w) to a suspension of biological cells provided a remarkable protection from the massive damages observed in response to freezing/thawing. This so-called cryoprotectiveeffect of DMSO has since then been documented for various biological systems, including cellular systems and solutions of macromolecules such as proteins and phospholipid bilayers.14J5 However, effects of DMSO on biological systems seem to be of a dual nature; DMSO can be toxic to cells and destabilize e.g. protein conformations at and above room temperature, but it promotes stability at subzero temperatures.15J6 In the present study the temperature and concentration dependences of the isobaric coefficient of thermal expansion (henceforth thermal expansivity), specific volume, and heat capacity of DMSO-water mixtures in the normal or supercooled liquid state are presented at temperatures ranging from room ~~
Carlsberg Foundation Fellow. Abstract published in Advance ACS Abstracts, February 15, 1994.
0022-3654/94/2098-3222$04.50/0
temperature down to ca. -25 OC. These measurements were undertaken for two reasons. One is that investigations of water and dilute aqueous solutions in the supercooled state may yield new insights into the hydrogen-bonded network of water.17 Another is that such measurements may contribute to a better understanding of cryoprotective effects of DMSO. Materials and Methods
Dimethyl sulfoxide from Fluka, Buchs (>99.5%), stored over molecular sieves, was used without further purification. Aqueuos solutionswere prepared by weighing using double-distilled water. Dilatometry/Volumetry. Thermal expansivity was measured by a simple visual method, in which the expansion of a "column" of a solution, enclosed in a capillary, was recorded (c.f. refs 18, 19). Fused silica capillaries (thermal expansivity 5 X le7K-l) from Polymicro Technologies, Phoenix, AZ, with a uniform bore of 50 fim were used. Approximately 19 cm of a capillary was flame-sealed in one end and filled with 14-15 cm of a degassed solution using a water jet. The capillary was then sealed in the other end and placed in an ethanol thermostat bath. The column length was measured by a microscopemountedon a mobile support equipped with an electronic calliper. Recordings were made at 2.5-5 OC temperature intervals during cooling from +10 to -30 "C (or the freezing temperature of the mixture detected as a large sudden volume change). The temperature of the ethanol bath was measured to the nearest 0.01 OC using a Physitemp BAT-10 thermometer. A polynomium [ I ( T ) ] was fitted to the length versus temperature data, and the thermal expansivity (a = 1/V dV/dT) of the mixtures was calculated as 4 T ) = 1 / m dl(T)ldT
The thermal expansion of the capillary was neglected. Specific volumes of DMSO-water mixtures at 10 OC were measured on an automated Anton Paar DMA 02 vibrating tube densimeter,described elsewhere.*O About 150data points in the 0-9595 (w/w) DMSO concentrationrange were recorded. These measurements were used together with the dilatometry data in order to obtain the specific volume of DMSO-water mixtures below 10 O C . Temperature cycling and repeated measurements on capillaries with given solutions indicated an overall accuracy of the specific volume at low temperature of 2 x 10-4cm3/g. 0 1994 American Chemical Society
Supercooled Dimethyl Sulfoxide Mixtures Calorimehy. Heat capacitieswere measured by a Perkin Elmer DSC-7/intracooler I1 differential scanning calorimeter system. The calorimeterwas temperature-calibratedwith n-decane (99.9% from Riedel De Haen, Seelze; melting at -29.66 "C) and gallium (99.9999%from Aldrich, Milwaukee;melting at 29.78 "C) under conditions where the so-called thermal lag is negligible (i.e. at low scan rate (
