Anomalous viscoelastic response of water-dimethyl sulfoxide solution and a molecular explanation of non-monotonic composition dependence of viscosity

Abstract

Amphiphilic molecules such as dimethyl sulfoxide (DMSO) and its aqueous binary mixtures exhibit pronounced nonideality in composition dependence of several static and dynamic properties. We carry out detailed molecular dynamics simulations to calculate various properties including viscosity of the mixture and combine the results with a mode coupling theory analysis to show that this nonideality can be attributed to local structures that are stable on a short time scale but transient on a long time scale to maintain the large scale homogeneity of the solution. Although the existence of such quasistable structures has been deciphered from spectroscopy, a detailed characterization does not exist. We calculate stress-stress autocorrelation functions (SACFs) of water-DMSO binary mixtures. We employ two different models of water, SPC/E and TIP4P/2005, to check the consistency of our results. Viscosity shows a pronounced nonmonotonic composition dependence. The calculated values are in good agreement with the experimental results. Fourier transform of SACF provides frequency-dependent viscosity. The frequency-dependent viscosity (that is, viscoelasticity) is also found to be strongly dependent on composition. Viscoelasticity exhibits sharp peaks due to intramolecular vibrational modes of DMSO, which are also seen in the density of states. We evaluate the wavenumber dependent dynamic structure factor and wavenumber dependent relaxation time. The latter also exhibits a sharp nonmonotonic composition dependence. The calculated dynamic structure factor is used in mode coupling theory expression of viscosity to obtain a semiquantitative understanding of anomalous composition dependence of viscosity. Both the self-diffusion coefficients and rotational correlation times of water and DMSO molecules exhibit nonmonotonic composition dependence.

Anomalous Concentration Dependence of Viscosity: Hidden Role of Cross-Correlations in Aqueous Electrolyte Solutions

“…First, we calculated the shear viscosity of the SPC/E and the TIP4P/2005 water models and obtained values of 0.73 and 0.86 mPa s, respectively. The results are in good agreement with the previous studies. ,− The shear viscosity obtained with the TIP4P/2005 water model shows less deviation from the experimental value (η exp. water = 0.89 mPa s) compared to that obtained with the SPC/E water model …”

Non-monotonic composition dependence of the breakdown of Stokes–Einstein relation for water in aqueous solutions of ethanol and 1-propanol: explanation using translational jump-diffusion approach

“…4,19 These properties are not only limited to the binary water/alcohol mixture but also apply to the aqueous mixtures of different solvents, such as dimethyl sulfoxide, 2-butoxyethanol, etc. 11,13,26,46–78 The origin of the above compositional anomalies is a matter of strong debate. The ‘iceberg’ model, originally proposed by Franks and Evans, 79 explains this qualitatively by assuming an ordering of the water molecules with stronger intermolecular H-bonding in the first solvation shell of the hydrophobic residues.…”

Sensing non-ideal microheterogeneity in binary mixtures of dimethyl sulfoxide and water

“…into the various physical properties of this mixture [2][3][4][5][6][7], viscosity-and density-related high-accuracy data are still missing for a wide temperature range [1]. In recent years theoretical investigations have sparked a growing interest in the anomalous behaviors of these binary mixtures at the microscopic level [8][9][10][11][12][13]. In this work, we exploit the optical tweezer technique to trap and manipulate polystyrene beads suspended in water-DMSO binary mixtures to study rheology at the microscopic level.…”