Freezing potential

We performed a molecular dynamics study of solutes at the basal ice/water interface. Our study of Na+ and Cl- ion behaviour at the basal ice/water interface aimed at shedding light on the microscopic picture of the Workman-Reynolds effect, which is claimed to be a massive charge separation and emergence of the so-called freezing potential during freezing of aqueous solutions. 

Phenomenological Description. Figure I shows a typical experimental setup for measuring freezing potentials and currents. The ice is grown on a platinum (or palladium) base or substratum that serves the double purpose of heat sink and electrical ground. A platinum electrode... 

Figure I. (a) Experimental arrangement for the measurement of freezing potentials (10 K, resistor not in circuit) and currents (10 K. resistor shunting the phases), V = electrometer C = recorder, (b) Electric analog of the system in the shunt case, Rb = interface barrier resistance = external shunt resistance Rj = ice resistance Ri = solution resistance Rm = ice metal interface resistance c = interface charge separation...

Figure 2 shows maximum freezing potentials as a function of ionic species and concentration. Figure 3 illustrates the freezing-rate and concentration dependence of the maximum potential. 

Figure 2. Freezing potential curves for different ionic species (a) and concentrations (b). Freezing rate was 5 microns a second, Ionic distributions for the 2,5 X lO M solutions (except NHjCl) are shown in Figure 10, Potential of NH.Cl solution is negative with respect to the ice, all others are positive...

Cobb (27) showed that the freezing potential is very sensitive to pH changes of the solution. He found that, in general, the maximum potential was obtained between pH values of 7 and 8. This is also true for the Group II solutes. 

It is conceivable that the presence of ionic impurities in the solution during ice growth provides an alternate or concurrent means of surface relaxation of fast growing ice crystals, as proposed by Workman and Reynolds or perhaps Fletchers oriented quasiliquid interface transition layer alone is the seat of the freezing potential. 

Nature of Interface Processes. At least three processes must be considered in the analysis of the freezing potential phenomenon. They... 

Concentration Dependence of Freezing Potential and Shunt Current. As has been pointed out, in Group I solutes the freezing potential curve in any experimental mn builds up to a maximum, after which it declines as the concentration on the liquid side of the phase boundary increases. The height of this maximum is itself a function of the initial solution concentration and shows a maximum value at an optimum concentration that depends on the particular solute. 

It appears plausible that the freezing potential maximum corresponds to an optimum combination of adsorbed charge density and interface resistance. The charge transfer through an external shunt shows a similar maximum size at an optimal mother solution concentration, which averages 10 times the concentration giving the optimum freezing potential. 

Differential Ion Incorporation. Differential ion incorporation may be due to different distribution coefficients of anions and cations or different diffusion coefficients or both. Furthermore, both coefficients are concentration-dependent. The presence of more than one solute species, electrically charged and which are incorporated at different rates, plus the concentration dependence of these distribution rates, is not taken into account by the analytical expressions discussed. As explained earlier in this paper, the macroscopic (or freezing-potential) electric... 

Application of the Tiller-Sekerka model to the internal field of the freezing potential leads to results both self-contradictory and in conflict with experimental evidence. The major diflBculty occurs when both selective incorporation of negative ions (Group I solutes) and of positive ions (Group II solutes) is considered, all other conditions being equal. 

Relatively small differences in diffusion coeflBcients may be paired with high freezing potentials, as in the examples just given. For HF solutions we have a large contrast between anionic and cationic diffusion coeflBcients (the ratio exceeds 1 8), yet the freezing potential is negligible within experimental accuracy. Thus the potential reversal in potassium and ammonium chlorides is probably best explained in terms of differences in distribution coeflBcients rather than of diffusion coeflBcients. 

Figure 16. Ionic distribution curves for 4.8 X lOr M KF solution frozen at a rate of 5 microns a second with different stirring speeds. The potassium-ion fraction decreases and the hydrogen-ion fraction increases as the stirring rate is increased. This may explain the reduction of the freezing potential by stirring (if the liquid phase is close to 0°C.)...

The smallness of the apparent distribution coeflBcients of ionic solutes in water suggests that all of them impose a considerable distortion on the ice lattice. Ionic distribution curves such as those discussed above show that there are differences in degree. Furthermore, the distortion imposed by a given ion depends also on its counterion. Thus, the ammonium ion in combination with the chloride ion is largely rejected, but in combination with the fluoride ion it is absorbed more readily than any other ion combination investigated. The freezing potential indicates that, even in NH4F, F" is somewhat more readily taken into the ice structure than NH4 ... 

Figure 26. Conductivity at —15°C. and highest value of the freezing potential as a function of concentration (melted ice, room temperature) for ice grown from dilute ammonium-fluoride solutions. Curve of HF ice for comparison (67). The ammonium-fluoride samples were prepared in pairs, with one sandwich electrode made from 5 X 10 M HF, and stored from two to 27 days at —20 C. before measurements. No effects of diffusion from sandwich electrodes into the samples were observed...

Development of electric potentials has been reported when (i) water containing an electrolyte as an impurity crystallizes [19], (ii) an electrolyte crystallizes from its supersaturated solution [20, 21], (iii) when an electrolyte dissolves in a solvent (water and non-aqueous solvents) [22] and (iv) molten electrolyte crystallizes [23]. With respect to (i) Workman and Reynolds suggested that preferential incorporation of ions in the ice lattice is an important factor responsible for the generation of freezing potentials. This view has been supported by other workers [24]. Alternatively, it has been suggested that these potentials might depend on dipole orientation, proton conductance and ionic entrapment in the ice lattice and ionic diffusion. It may be noted that freezing potentials are not developed when pure water is allowed to freeze. 

Bronshteyn, V. and A. Chernov, 1991 Freezing Potentials Arising on Solidification of Dilute Aqueous Solutions of Electrolytes. J. Cryst. Gr. 112, 129-145. 

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