Hardness hydrophobic effect

A conceptually complementary approach to describe hydrophobic effects has been introduced by Pratt and colleagues (78, 96). Their iifformation theory (IT) model is based on an application of Widom s potential distribution theorem (97) combined with the perception that the solvation free energy of a small hard sphere, which is essentially governed by the probability to find an empty sphere, can be expressed as a limit of the distribution of water molecules in a cavity of the size... 

Especially the weak interaction patterns like van der Waals forces, weak hydrogen bonds (i.e. those with bond energies less than about 10 kJ mol ) and n-n stackings are often discussed in the light of hydrophobic effects. Such effects are strongly system-dependent, they can hardly be understood in an ad hoc fashion, and are thus subject of constant debate (see Refs. [199, 200] for examples and Ref. [201] for a review). 

In the Pratt-Chandler theory, the distortion of the liquid structure and the work done when two neutral hard-sphere solutes are brought from infinity to a certain distance gives a measure of the hydrophobic effect. 

As already emphasized, theoretical development in the area of aqueous binary mixtures has been comparatively slow and to date no satisfactory molecular theory exists that can describe the complex physical chemistiy of a binaiy solution. The reason is the complexity of the intermolecular potential. While binary mixtures have often been studied by using a cell or lattice theory (as we discussed in the description of a polymer solution in the Hydrophobic effects chapter), even such a description is hard here because of the amphiphilic nature of the solute. It is really hard to develop a quantitative theory that includes the two different types of local heterogeneity at two sides of a given solute molecule. 

Nagai and coworkers reported a study of heterocoagulation driven by the hydrophobic effect of cationically charged hard poly[styrene-C( -(methacryloy-loxyphenyl-dimethylsulfonium methylsulfate)], or soft poly[styrene-co-(butyl acrylate)-co-(methacryloyloxyphenyl-dimethylsulfonium methylsulfate)] latex particles of ca. 220-240 nm in diameter onto neutral microspheres of crosslinked polystyrene (8.5[im in diameter) [37]. A separate study on the small cationic latex particles showed that their interface was hydrophobic, as the cationic surfactant cetyltrimethylammonium bromide (CTAB) adsorbed onto the surface, clearly driven by a hydrophobic effect [38]. The assembly of the cationic latex particles onto the larger microspheres was studied against increasing NaCl concentrations, which influenced the packing patterns from individually spaced to clusters (see Fig. 5). 

Fig. 5 SEM photographs of cationic polymer latex particles heterocoagulated onto the surface of crosslinked polystyrene microspheres driven by the hydrophobic effect, against increasing NaQ concentrations. Hard poly[styrene-co-(methacryloyloxyphenyl-dimethylsulfonium methylsul-fate)] particles at (a) 0.5, (b) 50, and (c) 200 mM of NaCl. Soft poly[styrene-co-(butyl acrylate)-co-(methacryloyloxyphenyldimethylsulfonium methylsulfate)] latex particles at (d) 0.5, (e) 50, and (f) 200 mM of NaCl...

In the previous section, we have seen that hard inorganic nanoparticles can adhere onto the surface of polymer latex particles via a stochastic process of collisions, which was referred to as heterocoagulation. Once deposited onto the surface of the latex particles, the strength of adhesion governed by attractive forces such as electrostatic attraction, the hydrophobic effect, and hydrogen bond interactions needs to outbalance repulsive forces and the entropy gain achieved when nanoparticles detach. This potential detachment of nanoparticles from the surface of the polymer latex particle is typically induced by the thermal energy of the system, k T (where is the Boltzmann constant and T is temperature). 

In the cavity-based model the hard core of water molecules is more important to the hydrophobic effect than H-bonding of water. The process of solvation is dissected into two components, the formation of a cavity in the water to accommodate the solute and the interaction of the solute with the water molecules. The creation of a cavity reduces the volume of the translational motion of the solvent particles. This causes an unfavorable entropic effect. The total entropy of cavity formation at constant pressure ... 

H(f)0 effect (see Sec. 4.12.3). Although there was no hard evidence for that conjecture, the hydrophobic effect was deemed to be the most important driving force in biochemical processes. For over half of a century, the fields of aqueous solutions, in general, and aqueous solutions of biomolecules, in particular, were dominated by HcpO effects. No other competing solvent-induced effects were recognized. 

The properties of derived surfactants are widely variable due to the multitude of possibly derived structures and to the flexibility in the hydrophilic-lipophilic balance adjustment. A striking feature of these surfactants is the fact that derivatives of dimethyl-siloxanes remain liquid, or at least keep an amorphous pasty consistency, up to high molecular weights also, they exhibit a rather low-temperature sensitivity. On the other hand, surfactants based on straight hydrocarbon chains and having similar solvophobic (or hydrophobic) effects are hard crystalline solids below their melting points, which are generally well above room temperature. 

The scaled particle model (SPM) was the first essentially molecular theory of hydrophobicities (see Scaled Particle Theory), It derived from an earlier scaled particle theory, a successful theoretical calculation of the thermodynamic properties of the hard sphere liquid. Pierotti then adapted the scaled particle theory to produce a solubility model for realistic liquids by a natural replacement of the hard sphere pressure with the measured pressure of the real solvent of interest. With attractive solute-solvent interactions treated perturba-tively this scaled particle model was remarkably successful. The SPM is the molecular theory of hydrophobicities most widely considered among biomolecular modelers. However, its success is somewhat fortuitous.For example, though the SPM predicts a reasonable value for the surface tension for the water liquid-vapor interface at room temperature, the predicted temperature dependence is wrong. Since entropies and temperature dependencies are special goals of theories of hydrophobic effects, this incorrect temperature dependence is important. 

Nylon-11. Nylon-11 [25035-04-5] made by the polycondensation of 11-aminoundecanoic acid [2432-99-7] was first prepared by Carothers in 1935 but was first produced commercially in 1955 in France under the trade name Kilsan (167) Kilsan is a registered trademark of Elf Atochem Company. The polymer is prepared in a continuous process using phosphoric or hypophosphoric acid as a catalyst under inert atmosphere at ambient pressure. The total extractable content is low (0.5%) compared to nylon-6 (168). The polymer is hydrophobic, with a low melt point (T = 190° C), and has excellent electrical insulating properties. The effect of formic acid on the swelling behavior of nylon-11 has been studied (169), and such a treatment is claimed to produce a hard elastic fiber (170). 

CMR represents the overall calculated molar refractivity. Its negative sign bring out a steric effect. It is interesting to note here that there is a high mutual correlation between ClogF and CMR (r= 0.966). Thus, it is very hard to predict for this data set if it is a negative hydrophobic or a polarizability effect. 

Gas diffusion in the nano-porous hydrophobic material under partial pressure gradient and at constant total pressure is theoretically and experimentally investigated. The dusty-gas model is used in which the porous media is presented as a system of hard spherical particles, uniformly distributed in the space. These particles are accepted as gas molecules with infinitely big mass. In the case of gas transport of two-component gas mixture (i = 1,2) the effective diffusion coefficient (Dj)eff of each of the... 

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