Molecular Mechanism Of Water Reorientation Dynamics In Dimethyl Sulfoxide Aqueous Mixtures

Perturbations of ionic,polar,hydrophobic and hydrophilic solutes on water hydrogen bonding network often lead to interesting and non-ideal hydration effects of significant interests in the fields such as energy,life sciences and catalysis.Aqueous mixtures of small amphiphilic molecules ubiquitously used in chemical and biochemical reactions are expected,when rationalizing the reaction mechanisms,to be near-ideal and homogeneous.On the other hand,non-ideal composition dependences are often observed for dynamics,thermodynamics and even chemical reaction kinetics in these mixtures,which have been speculated to originate from a structural microheterogeneity.Previous studies focusing on the thermodynamic non-idealities had difficulties in rationalizing this puzzling issue on a definitive molecular structural basis.Recently,the non-ideal dynamics of mixture was paid more attention with development of modern spectrum,but the microscopic mechanism is still not clear.We herein demonstrated theoretically that water reorientation in DMSO-water mixtuers,as a single molecular dynamical property,can serve as a good indicator for this purpose.Our simulations and a cyclic Markov chain model analysis indicates that the non-ideal,non-monotonic composition dependence of water reorientation in the aqueous mixture of a typical amphiphilic molecule DMSO is driven by a diffusive frame reorientation of water,which is a collective structural dynamics,instead of the hydrogen bond switching.A coarse graining analysis reveals that this diffusive component decelerates in the water-rich regime due to the strengthened hydrogen bonds.In the water-poor regime,at variance with the homogeneous picture,the water hydrogen bonding network is found to break into smaller aggregates,which leads to the acceleration of the diffusive reorientation.Calculations of the structural factor S(Q) further demonstrates that this structural microheterogeneity of aggregated hydrogen bonds is hard to be detected by neutron scattering duo to the cancellation effect of microscopic signals.Present results offered the new insights on how micro-segregation in solution structure leads to the realization of hydration non-ideality in general.