Probing transport and microheterogeneous solvent structure in acetonitrile-water mixtures and reversed-phase chromatographic media by NMR quadrupole relaxation

Mixtures of CH3CN and H2O are the predominant solvent systems used in reversed-phase liquid chromatographic (RPLC) separations, as well as in a multitude of other applications. In addition, acetonitrile is the simplest model for an amphiphilic molecule possessing both organic and polar functional groups. Although many studies have focused on this solvent system, the general nature of the intermolecular interactions are not fully understood, and a microscopic description of the proposed microheterogeneity that exists is still not clearly established. In the present study, we measure the spin-lattice relaxation times (T-1) of N-14 to determine reorientational correlation times (tau(c)) of CH3CN-H2O solvent mixtures over the entire binary composition range and at temperatures ranging from 25.0 to 80.0 degreesC. At all compositions, the microscopic observable, T, is found to be directly proportional to the macroscopic solution viscosity when scaled for temperature (eta/T). This indicates that for a constant composition, this system's dynamics are well described by hydrodynamic theory on a microscopic level. These results suggest that under appropriate conditions, the measurement of changes in quadrupolar relaxation times is a reliable means of determining changes in solution viscosity. We stress the importance of this approach in systems not amenable to traditional viscosity measurements, such as those having species in interfacial regions. This approach is used to examine the changes in the interfacial solution viscosity of CH3CN-H2O mixtures in contact with a commercially available C-18-bonded stationary phase. The measurements indicate that CH3CN is motionally hindered at the stationary phase surface. The surface affected CH3CN has a larger dependence of tau(c) on temperature than the bulk CH3CN, indicating greater changes in the interfacial viscosity as a function of temperature. Additionally, the bulk relaxation data show direct correlations to existing models of proposed regions of structure for CH3CN-H2O mixtures. Using a microscopic hydrodynamic approach, we show that, quite unexpectedly, each of the experimentally determined parameters in the viscosity correlation plots change simultaneously, and we propose that these are indicative of changes in the distribution of species for this microheterogeneous liquid system. Although distinct regions for the onset of microheterogeneity have previously been proposed, within the framework of a microscopic hydrodynamic model and the recently proposed model of Reimers and Hall,(1) the present data support the existence of a microheterogeneous solvent structure that varies continuously over the full range of temperatures and compositions examined.