Hydration numbers temperature effects

The solvation numbers of ions such as Mg2+, Al3+, and Be2+ may be determined by low temperature PMR techniques as mentioned earlier. The solvation number for small spherical ions may be determined in certain circumstances using a titration technique suggested by Van Geet (15). It is based on the competition by water for the solvation sphere of sodium ions in tetrahydrofuran (THF) measured by Na shifts. The salt must contain a large anion, which is assumed to be unhydrated during the titration otherwise a sum of hydration numbers would be determined. The assumptions made by Van Geet are basically those of the present treatment. His apparent constant is for the reverse of the equilibrium of Equation 21 and can be identified as l/K[P]p, where [P]f is the free THF concentration, effectively constant in the early stages of the titration. 

Proton conductivity in PFSA membranes depends upon the polymer EW (nrrmber of charge carriers), and the hydration number (A, number of water molecrrles/srrlforric acid group), polymer stracture, membrane morphology and temperatirre, all of which affect proton mobility. The proton conductivity of some recent PFSA membrane materials is shown in Fig. 2.3-2 5. Figure 2.3 illustrates for 3MTM PFSA membranes the effect of polymer EW and hydration niunber on proton conductivity at 80 °C, Fig. 2.4 displays the variation of proton conductivity of Aquivion membranes with temperature and relative hirmidity, and Fig. 2.5 shows the conductivity of Nafion NR-211 at 30 °C, 50 °C and 80 °C over a range of relative humidity values. 

Hydration numbers at temperatures other than 25°C, up to 200°C, have been reported by Marcus [54, 83, 88] for some twenty common ions, obtained according to the electrostriction method and Equation 4.36. The values of increase appreciably as the temperature is raised and the compressive effect of the ionic electric field increases with the diminishing permittivity and structure of the water. 

In this chapter some problems connected with the utilization of subzero temperature differential scanning calorimetry (SZT-DSC) are discussed. Among them are the determination of hydration numbers of surfactants and organic compounds, the determination of the hydration shell thickness, the effect of alcohol on the distribution of water between free and bound states in nonionic surfactant-based systems, and some considerations regarding the problem of phase separation of such systems in subzero temperatures. The signihcance of SZT-DSC for some novel applications is also discussed. 

Potassium Phosphates. The K2O—P20 —H2O system parallels the sodium system in many respects. In addition to the three simple phosphate salts obtained by successive replacement of the protons of phosphoric acid by potassium ions, the system contains a number of crystalline hydrates and double salts (Table 7). Monopotassium phosphate (MKP), known only as the anhydrous salt, is the least soluble of the potassium orthophosphates. Monopotassium phosphate has been studied extensively owing to its piezoelectric and ferroelectric properties (see Ferroelectrics). At ordinary temperatures, KH2PO4 is so far above its Curie point as to give piezoelectric effects in which the emf is proportional to the distorting force. There is virtually no hysteresis. 

Figure 10 shows effects of the membrane thickness of DMPE LB films on the hydration behavior at three different temperatures. The hydration amount (W ) increased linearly with increasing the number of layers of LB films only around Tc, but not temperatures below and above Tc. This indicates that water molecules deeply penetrate into LB layers around Tc. The hydration rate (v<,) was very large and hardly depended on the membrane thickness around Tc. This means that water can penetrate from the top surface of the membrane, but not from the side part of LB films. 

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