Enhanced Ion Mobility Research Articles

Computer simulation studies of the structure and dynamics of ions and non–polar solutes in water

The mobility of simple ions such as alkali–metal and halide ions at room temperature shows two anomalies. Firstly, there are maxima in mobilities as a function of ion size for both positive and negative ions and, secondly, the maximum for negative ions occurs at a larger ionic radius than the maximum for positive ions. Theoretical treatments of this problem are reviewed and it is concluded that a molecular treatment of the system is needed to understand the results. Computer simulation using the simple point charge model (SPC/E) for water reproduced the observations and is used to discuss the application of theories. In particular, the nature of the first solvation shell is correlated with ion mobility. Simulation reveals a further anomaly, namely that if the charge is removed from a large ion, then it moves more slowly. This is interpreted as the result of formation of a solvent cage around the hydrophobic solute. The changes in local structure resulting from changes in charge and size also affect the solvation thermodynamics. Simulations show that the solvation entropy has a double maximum when viewed as a function of charge. The local minimum near zero charge is interpreted as being due to hydrophobic order, and the maxima as the result of structure breaking. This double maximum in the entropy of solvation is a signature of the hydrophobic cage effect. Comparisons are made between ion mobilities in liquid water at ambient and supercritical conditions.

Enhanced Ion mobility in Aluminosilicate/Polysiloxane Network Polyelectrolytes

ABSTRACTA new series of polysiloxane-based single-ion conductors was prepared. These contain solvating oligoether sidechains and covalently linked aluminosilicate or alkoxy/siloxy-aluminate anions attached to the polysiloxane backbone. Of these two systems, the polymers containing aluminosilicate [(SiO)4Al]− anions show higher room temperature conductivities (10−6 S/cm) than those with alkoxy/siloxyaluminate [(SiO)2(CH2O)2A1]− anions (10−7 S/cm). The incorporation of longer covalent tethers between the alkoxy/siloxyaluminate anion and the polymer backbone results in enhanced room temperature conductivities at high ion loadings. Differential scanning calorimetry data provide a rationale for the high conductivity.

Electrochemical and Related Phenomena under Weightless Conditions: I . Flowing Electrolyte Thermocells

A flowing electrolyte thermogalvanic cell (Cu, ) has been studied under weightless conditions via drop‐tower experiments. Unexplained perturbations have been observed in the output potential in both the time‐dependent (emf build‐up) and quasi‐stationary state regions over a wide range of experimental conditions. Exhaustive control experiments as well as the general and over‐all reproducibility of perturbations per se in a large number of trials rule out oscillations in the hydraulic system as well as random transients as factors responsible for the observed phenomena. Stationary electrolyte systems including thermocells, concentration cells, and a Daniell cell, exhibited similar perturbations during the transition.The addition of urea was found to dampen the marked oscillations normally shown by a copper thermocell at the same output potential. The result is important in view of the known decreased ultrasonic attenuation and increased sound velocity caused by urea addition to water. At ambient temperatures the electrolytic conductivity of freshly prepared urea‐cupric sulfate solutions gradually increases and in subsequent drops the zero‐gravity induced oscillations reappear.Copper thermocells utilizing nonaqueous solvents (acetonitrile or absolute methanol) exhibit a voltage decrease (several millivolts) but the time/potential curve continues on its original course after impact. The potential of concentration cells (with linear and nonlinear gradients) increases (several millivolts in 0.7 sec), then decays to its initial value prior to impact; cells with linear gradients exhibit relatively larger changes. Inversion tests are also described. Daniell cells (1.09v at 20°C) show a similar behavior except that the voltage increase is about 40 mv.To understand better the influence of the transition on physico‐chemical processes, the electrochemical measurements were supplemented (with the appropriate controls) by a study of diffusional phenomena, heterogeneous and homogeneous chemical reactions. In all cases increased rates were observed and are attributed to enhanced ion mobility during weightlessness. On the basis of these experiments it must be concluded that an interaction exists between chemically reacting systems and gravitational fields.