Water hydrogen-bond model

Various equations of state have been developed to treat association ia supercritical fluids. Two of the most often used are the statistical association fluid theory (SAET) (60,61) and the lattice fluid hydrogen bonding model (LEHB) (62). These models iaclude parameters that describe the enthalpy and entropy of association. The most detailed description of association ia supercritical water has been obtained usiag molecular dynamics and Monte Carlo computer simulations (63), but this requires much larger amounts of computer time (64—66). 

Poly(oxyethylene) (POE) (-OCH2CH2—) is an unusual polyether with practically uiilimited solubility in water, unlike other structurally related polymers. At elevated temperatures, however, the isotropic aqueous solution of POE separates into two phases. The mechanism of the water solubility of POE and the phase behavior has attracted much attention of many investigators. Various mechanistic models have in fact been proposed to account for these phenomena a water structure model, a hydrogen bond model, and a conformational model. ... 

In an attempt to explain the splitting of the optic modes, the two strength hydrogen bond model for ice Ih was proposed by Li and Ross [68]. In this model, a pair of hydrogen bonded water molecules have two different force constants according to their relative orientations. The values are 1.1 or 2.1... 

For kinetics studies, it was most convenient to measure the mole fraction (i.e., area fraction) of water or Hq in the gas phase. Fig. 3.3 shows the consumption of water and the production of Hq in the hydrolysis of Alq3 at 125°C. The data fit a rate law which shows a power-law dependence on the mole fraction (or partial pressure) of water in the gas phase 93 = P 20 where n 2-6. To explain this, a mechanistic model (Fig. 3.4) was developed considering the sorption and desorption of the volatile components. The first step of the reaction is the absorption of water into the solid and the formation of an intermediate where water hydrogen bonds with the electronegative oxygen atoms on the ligand. The reaction then follows where the water exchanges position with Hq and the products can be desorbed (or desolvated). 

In Table VII are the relative H chemical shifts of water in Nafion at several water contents. The experiments were conducted on specially prepared Nafion spheres in order to eliminate bulk susceptibility effects. These spheres behaved the same as the corresponding Nafion films and powders in limited 23Na NMR experiments. The H linewidths are sufficiently narrow to allow accurate measurement of the chemical shift. With decreasing water content, the resonance shifts upfield, suggesting the breakup of water-hydrogen bonding as for NaCl. The relative shift of pure water and water in saturated Nafion is not known at this time. The increased linewidth indicates decreased water mobility, as seen for the sodium ions. Additional experiments using model electrolytes and a chemical shift standard are warranted. 

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