Cooperative hydration

Measured enthalpies of solution of tetra-n-butylammonium bromide (Bu NBr) in mixtures of water (W) with acetonitrile (ACN) and with ethylene carbonate (EC) are compared with those in mixtures of water with some other aprotic solvents which were reported earlier. The results can be fairly well described by an equation which can be derived either from a cooperative hydration model or from a chemical, pseudo equilibrium model. This equation is tested by varying systematically the nature of the solute and of the cosolvent. In order to describe the experimental results in W-ACN and in W-EC as such, it will be necessary to extend the equation with a term which comprises any specific (nonhydrophobic) interactions of the solute with the (inert) cosolvent. 

Recently (6) we found that the AH° of Bu4NBr in W-DMF, W-DMSO, and W-DMA mixtures can be described fairly well in terms of a cooperative hydration model (2,5) leading to the following equation ... 

Okada Y, Tanaka F (2005) Cooperative hydration, chain collapse, and flat LCST behavior in aqueous poly(N-isopropylacrylamide) solutions. Macromolecules 38 4465-4471... 

When there is strong attractive interaction between the attached R Bi molecules along the chain, they are adsorbed in contiguous sequences. These sequences may induce helical order on the main chain due to the steric hindrance of adjacent adsorbed molecules. The combinatorial factor changes to the number of different ways to select the specified sequences from the finite total length n. Such a correlated adsorption, or cooperative adsorption, was recently studied in detail in relation to helix formation on polymers by adsorption of chiral molecules [27,28] (Section 1.8). Cooperative hydration will be detailed in Section 6.5. 

Cooperative hydration in solutions of temperature-responsive polymers... 

Fig. 6.12 Sequential hydrogen bonds formed along the polymer chain due to cooperative hydration. The type of polymer-water associated complex is specified by the index j = (71,72, ), where is...

For cooperative hydration, the equilibrium constant is most generally written as... 

Fig. 10.15 Comparison of the phase diagrams of telechelic associating polymers (a) random hydration (cr = 1.0) for telechelic PEO, (b) cooperative hydration (a =0.3) for telechelic PNIPAM. Spinodal lines (solid lines) and sol-gel transition hnes (broken lines) are shown. The various curves correspond to polymers of different molecular weights. Other parameters are fixed at the values obtained from the single-chain study. (Reprinted with permission from Ref. [38].)...

For random hydration, the LCST and UCST merge at a molecular a weight between n = 50 and 100, and the phase separation region turns into an hourglass. For cooperative hydration, the LCST curves are very flat up to high polymer concentration. Molecular weight effect is weak. 

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