Pressure dependence of hydrophobic

The simplicity and accuracy of such models for the hydration of small molecule solutes has been surprising, as well as extensively scrutinized (Pratt, 2002). In the context of biophysical applications, these models can be viewed as providing a basis for considering specific physical mechanisms that contribute to hydrophobicity in more complex systems. For example, a natural explanation of entropy convergence in the temperature dependence of hydrophobic hydration and the heat denaturation of proteins emerges from this model (Garde et al., 1996), as well as a mechanistic description of the pressure dependence of hydrophobic... 

Hummer, G., Garde, S., Garcia, A. E., Paulaitis, M. E., and Pratt, L. R. (1998b). The pressure dependence of hydrophobic interactions is consistent with the observed pressure denaturation of proteins. Proc. Natl. Acad. Sci. USA 95, 1552-1555. Hummer, G., Garde, S., Garcia, A. E., Pohorille, A., and Pratt, L. R. (1996). An information theory model of hydrophobic interactions. Proc. Natl. Acad. Sci. USA 93, 8951-8955. 

Hummer G, Garde S, Garca AE, Paulaitis ME, Pratt LR. The pressure dependence of hydrophobic interactions is consistent with the observed pressure denaturation of proteins. Proc. Natl. Acad. Sci. U.S.A. 1998 95 1552-1555. 

Ghosh T, Garca AE, Garde S. Enthalpy and entropy contributions to the pressure dependence of hydrophobic interactions. J. Chem. Phys. 2002 116 2480-2486. 

Pressure-induced denaturation of proteins and related problems are possibly linked to hydrophobicity. As a result, there has been considerable interest in studying the pressure dependence of hydrophobic interactions, highlighted by two studies. The first, by Rick, involved calculation of the pairwise... 

Stretching is expected to decrease volume without change in composition. This would be due to the increase in water of hydrophobic hydration, which yields a state of lesser volume. Under conditions of fixed volume, stretching is expected to yield a decrease in pressure. With the perspective that the inverse temperatiure transition occurs when AH, = T,AS, an adequate explanation of the pressure effect requires a separate determination of the pressure dependence of AH. 

Salts of fatty acids are classic objects of LB technique. Being placed at the air/water interface, these molecules arrange themselves in such a way that its hydrophilic part (COOH) penetrates water due to its electrostatic interactions with water molecnles, which can be considered electric dipoles. The hydrophobic part (aliphatic chain) orients itself to air, because it cannot penetrate water for entropy reasons. Therefore, if a few molecnles of snch type were placed at the water surface, they would form a two-dimensional system at the air/water interface. A compression isotherm of the stearic acid monolayer is presented in Figure 1. This curve shows the dependence of surface pressure upon area per molecnle, obtained at constant temperature. Usually, this dependence is called a rr-A isotherm. 

The salt effects of potassium bromide and a series office symmetrical tetraalkylammonium bromides on vapor-liquid equilibrium at constant pressure in various ethanol-water mixtures were determined. For these systems, the composition of the binary solvent was held constant while the dependence of the equilibrium vapor composition on salt concentration was investigated these studies were done at various fixed compositions of the mixed solvent. Good agreement with the equation of Furter and Johnson was observed for the salts exhibiting either mainly electrostrictive or mainly hydrophobic behavior however, the correlation was unsatisfactory in the case of the one salt (tetraethylammonium bromide) where these two types of solute-solvent interactions were in close competition. The transition from salting out of the ethanol to salting in, observed as the tetraalkylammonium salt series is ascended, was interpreted in terms of the solute-solvent interactions as related to physical properties of the system components, particularly solubilities and surface tensions. 

The most important example of liquid/liquid membrane contactors is membrane distillation, shown schematically in Figure 13.13. In this process, a warm, salt-containing solution is maintained on one side of the membrane and a cool pure distillate on the other. The hydrophobic microporous membrane is not wetted by either solution and forms a vapor gap between the two solutions. Because the solutions are at different temperatures, their vapor pressures are different as a result, water vapor flows across the membrane. The water vapor flux is proportional to the vapor pressure difference between the warm feed and the cold permeate. Because of the exponential rise in vapor pressure with temperature, the flux increases dramatically as the temperature difference across the membrane is increased. Dissolved salts in the feed solution decrease the vapor pressure driving force, but this effect is small unless the salt concentration is very high. Some typical results illustrating the dependence of flux on the temperature and vapor pressure difference across a membrane are shown in Figure 13.14. 

The interfacial properties of HM - PNIPAM, including the formation and the compression - expansion reversibility of the monolayers, at different subphase temperatures were more recently studied by using the Langmuir film balance technique [90], The stability and dynamic nature of the HM - PNIPAM monolayers were also further studied by the time - dependent surface pressure measurements. All results have suggested a compression - promoted temperature - and rate - dependent conformational rearrangement of the polymer on the water surface. Increasing the level of hydrophobic modifications progressively improved the monolayer compressibility and stability, and reduced the hysteresis. 

The particle size of rapidly polymerized minidroplets does not or does just weakly depend on the amount of the hydrophobe [34-36]. It was found that doubling the amount of hydrophobe does not decrease the radius by a factor of 2 (as expected from a zero effective pressure), it is just that the effective pressure (pressure difference) has to be the same in every droplet, a mechanism which in principle does not depend on the amount of hydrophobe [19]. However, a minimum molar ratio of the hydrophobe to the monomer of about 1 250 is required in order to build up a sufficient osmotic pressure in the droplets exceeding the influence of the first formed polymer chains. This also explains the fact that a small amount of high molecular weight polymer, e.g., polystyrene, can barely act as an osmotic stabilizing agent here stabilization can only be achieved for the time of polymerization [37,38]. 

Chemical reactions are often highly pressure-dependent. As a matter of fact, high pressure is an elegant way to perturb reversibly chemical equilibria and reactions. Another advantage of using the pressure parameter is that reactions are slowed or accelerated depending on the type of chemical interaction involved. For instance, pressure weakens electrostatic interactions, but stimulates some hydrophobic interactions, such as stacking between aromatic residues. Similarly to the activation enthalpy, obtained from... 

Figure 2 The distance dependence characterizing exclusion of small solutes from macromolecular surfaces follows the same exponential behavior as the hydration force between macromolecules. The extent of exclusion can be extracted from the dependence of forces on solute concentration. ITexcess is the effective osmotic pressure applied by the solute in the bulk solution on the macromolecular phase, and np is the maximal pressure from complete exclusion, riexcess/rio = 1 then corresponds to complete exclusion and n excess/Ho = 0 means no inclusion or exclusion. The distance dependent exclusion the polar polyols adonitol (A) and glycerol ( ) from hydrophobically modified hydroxypropyl cellulose (FIPC) and of the nonpolar alcohols i-propanol ( ) and methyl pentanediol (MPD) ( ) from spermidine +-DNA is shown. As in Fig. 1, interaxial spacings are converted to surface separations. The apparent exponential decay length varies between 3.5 and 4 A (solid lines indicate fits to the data).

An analysis of computer simulations of water at different pressures by Hummer et al. (110) suggested that hydrophobic contact pairs become increasingly destabilized with increasing pressure. The proposed scenario could explain the pressure denaturation of proteins as a swelling in terms of water molecules that enter the hydrophobic core by creating water-separated hydrophobic contacts. Additional support for the validity of Hummer s IT-model analysis has been achieved by pressure-dependent computer simulation studies of isolated pairs of hydrophobic particles, as well as rather concentrated solutions of hydrophobic particles (111, 112). Recently, the pressure-induced swelling of a polymer composed of apolar particles at low temperatures can be observed (113). 

The transport mechanisms through zeolite membranes depend on different variables such as operation conditions (especially temperature and pressure), membrane pore size distribution, characteristics of the pore surface of the zeohtic-channel network (hydrophilicity/hydrophobicity ratio), as well as the characteristics of the crystal boundaries and the characteristics of the permeating molecules (kinetic diameter, molecular weight, vapor pressure, heat of adsorption), and their interactions in the mixture. 

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