Hydrophilic Hydration

Hydrophilic hydration signifies that a strong energetically favored direct interaction exists between dissolved polar or ionic particle and the surrounding water molecules by ion-dipole-, dipole-dipole-interactions and/or hydrogen bonds. 

It is possible to indicate by thermodynamic considerations 24,25,27 , by spectroscopic methods (IR28), Raman29 , NMR30,31 ), by dielectric 32 and viscosimetric measurements 26), that the mobility of water molecules in the hydration shell differs from the mobility in pure water, so justifying the classification of solutes in the water structure breaker and maker, as mentioned above. 

The order of cations and anions regarding these structure breaking and making properties is related to their position in the lyotropic or Hofmeister series, such as 

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Biological membranes present a barrier to the free transport of cations, as the hydrophilic, hydrated cations cannot cross the central hydrophobic region of the membrane which is formed by the hydrocarbon tails of the lipids in the bilayer. Specific mechanisms thus have to be provided for the transport of cations, which therefore allow for the introduction of controls. Such translocation processes may involve the active transport of cations against the concentration gradient with expenditure of energy via the hydrolysis of ATP. These ion pumps involve enzyme activity. Alternatively, facilitated diffusion may occur in which the cation is assisted to cross the hydrophobic barrier. Such diffusion will follow the concentration gradient until concentrations either side of... 

Hydrophobic effects are thus of practical interest. If we accept the goal of a simple, physical, molecularly valid explanation, then hydrophobic effects have also proved conceptually subtle. The reason is that hydrophobic phenomena are not tied directly to a simple dominating interaction as is the case for hydrophilic hydration of Na+, as an example. Instead hydrophobic effects are built up more collectively. In concert with this indirectness, hydrophobic effects are viewed as entropic interactions and exhibit counterintuitive temperature dependencies. An example is the cold denaturation of globular proteins. Though it is believed that hydrophobic effects stabilize compact protein structures and proteins denature when heated sufficiently, it now appears common for protein structures to unfold upon appropriate cooling. This entropic character of hydrophobic effects makes them more fascinating and more difficult. 

Scheme 4.13 a-Fluorination of carbonyl compounds reduces lipophilicity because stable hydrophilic hydrates (box) can be formed. 

In concentrated sugar solutions, the relative amount of such less ordered water increases. As previously noted (ref. 24), the same conclusion was found by the aid of the structural hydration interaction models put forward by Desnoyers et al. (ref. 56). As the sugar concentration increases the interactions of the hydrophilic hydration cospheres of the sugar (the Interaction model of type IV of reference (56)) will be enhanced producing the structure breaking effect. 

When Li+ ions are in the solution, water molecules are tightly bonded by Li+ ions—excluding the direct contact with Cl" ions—due to the strong local electric fields. When a large number of ions are in the solution, there is competition between ion-water and ion-ion interactions. The hydrophilic hydration of Li+ ions dominates therefore, driven by minimization of system energy, the coordination number between hydrated ions increases more significantly relative to the direct ion-ion contact. 

The residence times for the primary water shells around Li+ ions increase significantly with solution concentration, while only a moderate increase was observed for Rb+ ions and almost no noticeable change for Cs+ ions. This can also be explained in terms of different hydration shell structures around these ions. Strong hydrophilic hydration is daminating for small ions. 

The different adsorption behavior of the ODA surfactant molecules at the NaCl and KCl, surfaces demonstrates that the salt water structure makiug aud breaking characteristics have a significant effect on the adsorption state. As the ion size increases, the interfacial water molecules experience a transition from hydrophilic hydration (in the case of the NaCl system) to hydrophobic hydration (in the case of the KCl system). When interfacial water molecules are stable due to the presence of a structure-making salt such as NaCl, replacement of these water molecules with surfactant molecules is difficult. In contrast, at the structure-breaking KCl crystal surface, which naturally shows a weak hydrophobic character, the penetration of the ODA surfactant structure through the interfacial water layer and the further replacanent of interfadal water molecules is possible, and thus instantaneous adsorption is achieved. 

The effect of monovalent and divalent salts on the solubility of these hydrophobically associating polymers (HAPs) is similar to that of ionic surfactants. An increase in salt content decreases solubility. With increasing salinity, the hydrocarbon chains are forced into closer proximity to the point where the subtle balance between hydrophobic associative forces and hydrophilic hydration forces breaks down, and phase separation results. Divalent cations have a larger effect on decreasing polymer solubility than do alkaline earth or monovalent cations. This is particularly so when the polymer contains anionic functionality such as acrylate or sulfonate. Another interesting phenomena occurs in mixed salts with certain polymer compositions when the ratio of divalent to monovalent cation is varied. A window of solubility is observed similar to that found with anionic surfactant solutions. 

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