Enthalpic interaction, hydrophobic

There are a number of different enthalpic interactions that can occur between polymer and packing, and in many cases multiple interactions can exist depending on the chemical structure of the polymer. Enthalpic interactions that are related to water-soluble polymers include ion exchange, ion inclusion, ion exclusion, hydrophobic interactions, and hydrogen bonding (12)- Other types of interactions commonly encountered in SEC, as well as in all other chromatographic separations, are dispersion (London) forces, dipole interactions (Keeson and Debye forces), and electron-donor-acceptor interactions (20). 

MR still is the chameleon [91] amongst the physicochemical parameters, despite its broad application in QSAR studies. It has been correlated with lipophilicity, molar volume, and steric bulk. Of course, due to its MW/q component it is indeed related to volume and size of a substituent. But two recent statements that MR is only based on these properties [91, 291] cannot be accepted. The refractive index-related correction term in MR accounts for the polarizability and thus for the size and the polarity of a certain group [158, 173, 286]. The larger the polar part of a molecule is, the larger its MR value will be. Even hydrophobic substituents have a weak enthalpic interaction due to dispersion forces, in addition to their entropic... 

The layered silicate nanoparticles are usually hydrophilic and their interactions with nonpolar polymers are not favorable. Thus, whereas hydrophilic polymers are likely to intercalate within Na-activated montmorillonite clays [24-29], hydrophobic polymers can lead to intercalated [23,30-32] or exfoliated [33] structures only with organophilized clays, i.e., with materials where the hydrated Na+ within the galleries has been replaced by proper cationic surfactants (e.g., alkylammonium) by a cation exchange reaction. The thermodynamics of intercalation or exfoliation have been discussed [34-37] in terms of both enthalpic and entropic contributions to the free energy. It has been recognized that the entropy loss because of chain confinement is compensated by the entropy gain associated with the increased conformational freedom of the surfactant tails as the interlayer distance increases with polymer intercalation [34,38], whereas the favorable enthalpic interactions are extremely critical in determining the nanocomposite structure [39]. 

Polymer Adsorption. The driving force for adsorption typically is the en-thalpic interaction between the interface and the polymer segments. Exceptions include aqueous solutions near hydrophobic substrates. In that case, the driving force may include the hydrophobic interaction between water and the surface. The enthalpic interactions may be of various forms such as hydrogen bonding, inter-facial tension, van der Waals attraction, polar interactions, and electrostatic attractions. This enthalpic interaction is offset by the loss in conformational entropy... 

Rheological characteristics of aqueous solutions are dictated by molecular structure, solvation, and by inter- and intrachain associations. In many cases, segmental interactions must be accoimted for in more rigorous terms than simple statistical encounters. Enthalpic interactions or entropically driven hydrophobic associations must be considered. 

The partitioning of free energy contributions in the explanation (and for design, the prediction) of binding constants is a subjective matter. Different workers choose different definitions, e.g. of hydrophobic binding, which may or may not include dispersion interaction, and different approaches to factorization of enthalpic and entropic components. 

Hydrophobic dehydration results from bonding of the protein s hydrophobic patches to the hydrophobic regions on the adsorbent. The enthalpic part of this interaction is small the entropy change is positive. Hydrophobic dehydration is relatively unimportant for hydrophilic surfaces and/or rigid hydrophilic proteins. 

Hydrophobic binding. The hydrophobic effect can have both enthalpic and entropic components, although the classical hydrophobic effect is entropic in origin (Section 1.9.1). Studies on the associations between planar aromatic molecules show an approximately linear relationship between the interaction energy and their mutual contact surface area with slope 64 dyn cm-1, very close to the macroscopic surface tension of water (72 dyn cm-1). Hence, in the absence of specific host or guest interactions with the solvent the hydrophobic effect can be calculated solely from the energy required to create a free surface of 1 A2 which amounts to 7.2 X 10 12 J or 0.43 kjA 2 mol. ... 

A further insight into the nature of the hydrophobic interaction is obtained by separating the free energy into enthalpic and entropic parts... 

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