Water molecules, hydrogen-bonded interactions among

Examples of three types of crystalline solids. Only part of the structure is shown in each case, (a) An atomic solid, (b) An ionic solid, (c) A molecular solid. The dotted lines show the hydrogen bonding interactions among the poiar water molecules. 

The hydrogen bond is a weaker bond between atoms than the covalent, ionic and metallic bonds. It occurs if a proton bridge can be formed between two atoms, e.g., in the interaction among water molecules in liquid water and ice and among many organic groups. Boranes are examples of molecules that have an intramolecular hydrogen bond. 

The effects of water content or RH on proton conductivity are described in the literature [45,49,50,61,69]. As shown earlier in Fig. 5.18, the proton conductivity of the Nafion 117 membrane increases with water content At low water content values, not all the acid sites in the membrane are dissociated and the interactions among water molecules via hydrogen bonding are low, leading to a low rate of proton transfer that results in relatively low conductivity. Our recent study [45] investigated the RH effect on the performance of a Nafion - 12 membrane-based PEM fuel cell at 120 °C with ambient backpressure. The AC impedance spectra at different RHs were recorded in situ, and the membrane resistances were simulated the membrane conductivities at 120 °C with different RHs were then calculated according to Eqn (5.1). As... 

FIGURE 51. Hydrogen bonding interactions among lattice water molecules, 0(15)-0(28) resulting in the formation of a (H20)i3 cluster. (Reprinted with permission from ref 106.)... 

FIGURE 14. (a) Hydrogen bonding interactions among the polymeric chains and (b) channels within the crystal structure of compound [Cu(p x)(H20)(7//-ade-KA )][Cu( j,-ox)( r-H20)(7//-ade-KA )] (10/3H2O . Crystallization water molecules have been omitted for clarity. 

Among the common amino acids, eleven have side chains that contain polar functional groups that can form hydrogen bonds, such as —OH, —NH2, and — CO2 H. These hydrophilic amino acids are commonly found on the outside of a protein, where their interactions with water molecules increase the solubility of the protein. The other nine amino acids have nonpolar hydrophobic side chains containing mostly carbon and hydrogen atoms. These amino acids are often tucked into the inside of a protein, away from the aqueous environment of the cell. 

Two characteristics of liquid water that are relevant to the present answer are the equation of state characteristics shown in Figs. 8.11 and 8.12. The isothermal compressibilities shown in Fig. 8.11 indicate that water is stiffer than organic solvents, and that the stiffness is only weakly temperature-dependent. We don t propose here a detailed explanation of that stiffness - it is due to intermolecular interactions among solvent molecules, hydrogen bonding in the case of liquid water (Debenedetti, 2003) - but the present empirical theory of hydrophobicity merely exploits those results. This stiffness is the principal determinant of the low solubility of inert gases in liquid water. In the simplest information models this stiffness, and its temperature dependence, is expressed by the experimental n (n — l))o, which is distinctive of liquid water. 

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