Phase boundary, hydrogen bonding

Stimulated by these observations, Odelius et al. [73] performed molecular dynamic (MD) simulations of water adsorption at the surface of muscovite mica. They found that at monolayer coverage, water forms a fully connected two-dimensional hydrogen-bonded network in epitaxy with the mica lattice, which is stable at room temperature. A model of the calculated structure is shown in Figure 26. The icelike monolayer (actually a warped molecular bilayer) corresponds to what we have called phase-I. The model is in line with the observed hexagonal shape of the boundaries between phase-I and phase-II. Another result of the MD simulations is that no free OH bonds stick out of the surface and that on average the dipole moment of the water molecules points downward toward the surface, giving a ferroelectric character to the water bilayer. 

It is noteworthy that benzyltriethylammonium chloride is a slightly better catalyst than the more lipophilic Aliquat or tetra-n-butylammonium salts (Table 5.2). These observations obviously point to a mechanism in which deprotonation of the amine is not a key catalysed step. As an extension of the known ability of quaternary ammonium halides to form complex ion-pairs with halogen acids in dichloromethane [8], it has been proposed that a hydrogen-bonded ion-pair is formed between the catalyst and the amine of the type [Q+X—H-NRAr] [5]. Subsequent alkylation of this ion-pair, followed by release of the cationic alkylated species, ArRR NH4, from the ion-pair and its deprotonation at the phase boundary is compatible with all of the observed facts. 

At the liquid-liquid interface between a hydrocarbon oil and water under mixing, the molecules encounter unbalanced attraction forces, pull inwardly, and contract as other molecules leave the interface for the interior of the bulk liquid. As a result, spherical droplets are formed. Customarily, the boundaries between a liquid and gas and between two liquids are the surface and the interface, respectively. The interfacial tension (or interfacial free energy) is defined as the work required to increase the interfacial area of one liquid phase over the other liquid phase isothermally and reversibly. Moving molecules away from the bulk to the surface or interfacial surface requires work (i.e., an increase in free energy). Water molecules and hydrocarbon oil molecules at the interface are attracted to the bulk water phase as a result of water-water interaction forces (i.e., van der Waals dispersion y and hydrogen bonding y ), to the bulk oil phase due to the oil-oil dispersion forces, y 1, and to the oil-water phase by oil-water interactions, y )W (i.e., dispersion forces). As mentioned in Chapter 3, the oil-water dispersion interactions are related to the geometric mean of the water-water and oil-oil dispersion interactions. The interfacial tension is written as ... 

AUhough the main alkylation reactions are thought to occur at or close to the Interface between the acid and hydrocarbon phases (7,15,16), the acid boundary layer at the interface seems more probable. This conclusion is based on the fact that other isoparaffins (including LE s, TMP s, OMH s, and HE s) appear to be much less reactive relative to hydride transfer steps as compared to Isobutane yet many of these Isoparaffins contain one or more tertiary carbon-hydrogen bonds. These heavy isoparaffins are, however, even less soluble in the-acid phase than isobutane. 

The infrared spectrum of Ice VI does not appear to have been studied but the large dielectric constant (about 193) and dielectric dispersion (Wilson et al. 1965) indicate that water molecules remain intact and orientationally disordered, at any rate at —30 °C, the temperature of the experiment. The non-equivalence of the hydrogen bonds makes it likely that proton ordering may occur at lower temperatures. The straightness of the VI-VIII phase boundary indicates that no such transition occurs above —80 °C but X-ray evidence (Kamb, 1965 a) suggests that ordering may have occurred by —196 °C. 

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