Hydrophobic effect entropy driven

Figure 5.5 Comparing liposome-water to octanol-water partition coefficients of a series of uncharged substituted benzylalkylamines [387]. The membrane partitioning of the smaller members of the series (n — 0 3) is thought to be dominated by electrostatic and H-bonding effects (enthalpy-driven), whereas the partitioning of the larger members is thought to be directed by hydrophobic forces (entropy-driven) [387]. [Avdeef, A., Curr. Topics Med. Chem., 1, 277-351 (2001). Reproduced with permission from Bentham Science Publishers, Ltd.]...

What distinguishes water from ordinary organic solvents and justifies the term hydrophobic interaction is the molecular origin of the effect, being entropy driven in pure water at room temperature and resulting primarily from the strong water-water interactions. 

Lipid bilayers are formed by self-assembly, driven by the hydrophobic effect. When lipid molecules come together in a bilayer, the entropy of the surrounding solvent molecules increases. 

Usually, dissolution of a small amount of one compound in a pure liquid is enthalpically unfavourable and driven by an increase in (mixing) entropy. At room temperature, the opposite is true for the dissolution of a small apolar compound in water. This unexpected behaviour is referred to as the hydrophobic effect [4]. Classically, this effect has been rationalised by ordered water structures around apolar compounds (entropy reduction) and the increase in number... 

Measurement of AH0 and AS0 showed that the former is small and positive and the second is large and positive. This implies that micelle formation is entropy driven and is described in terms of the hydrophobic effect (14). Then hydrophobic chains of the surfactant monomers tend to reduce their contact with water, whereby the latter form icebergs by hydrogen bonding. This results in reduction of the entropy of the whole system. Flowever, when the monomers associate to from micelles, these icebergs tend to melt (hydrogen bonds are broken), and this results in an increase in the entropy of the whole system. 

The tendency for hydrocarbon chains to become remote from the polar solvent, water, is known as the hydrophobic effect (Chap. 4). Hydrocarbons form no hydrogen bonds with water, and a hydrocarbon surrounded by water facilitates the formation of hydrogen bonds between the water molecules themselves. The bulk water is more structured than it is in the absence of the hydrocarbon i.e., it has lost entropy (Chap. 10) and is thus in a thermodynamically less favorable state. This state is obviated by the hydrocarbon being organized so that it is remote from water, thus rendering the water molecules near to it less ordered. Thus the hydrophobic effect is said to be entropically driven. 

Although there is overwhelming experimental evidence that the hydrophobic interaction is entropy-driven , this classical view is still a matter of debate [79a, 167, 227, 229-231, 343-347]. For example, it has been claimed that the major eontribution to the hydrophobic interaction between the methylene groups of n-alkanes is an enthalpic and not an entropic effect [230]. In other words, the poor solubility of non-... 

Temperature. The higher the temperature, the more and quicker the deposition [1,25,36,52,96,98], The optimal temperature to get almost complete deposition of the softener is 25 to 32°C [97], The effect has been assigned to a solubilization phenomenon [96], This proposal is paradoxical, as deposition and solubility are at first glance antagonistic. In contrast, CTAB adsorption onto cotton increases only slightly when the temperature rises [106], An alternative explanation is that DHTDMAC deposition involves entropy-driven phenomena, such as hydrophobic ejection and hydrophobic interaction. 

Figure 2.21 Binding of analyte molecules to hydrophobic interaction chromatography (HIC) media. Structured water covers the hydrophobic functional groups on the gel beads and hydrophobic surfaces of analyte molecules. This water is partially displaced when hydrophobic interactions take place between hydrophobic surfaces (see Hydrophobic effect, Chapter 1). Interactions are driven in part by entropy gains due to released water.

The physical nature of hydrophobic effects was previously considered to be entropic. Based on this hypothesis, it has often been claimed that the thermal stabilization of proteins in thermophiles may be correlated with an increase in the number of hydrophobic residues. A critical analysis proved the differences to be statistically insignificant the recent dramatic increase in sequence data from complete genomes of mesophilic and (hyper-) thermophilic bacteria and archaea clearly confirmed this finding (see below). Considering the real meaning of the word hydrophobic, it is clear that the aversion of nonpolar solutes to water becomes more ordinary and less entropy-driven at extreme temperatures, whereas in the mesophilic temperature regime the hydrophobic effect is indeed entropic. Maximum aversion arises at the temperature at which the ftee energy of transfer of nonpolar solutes into water shows its maximum. Under this condition, the entropy (i.e., the temperature derivative of AG) equals zero, so that the hydrophobic effect must be driven by enthalpic contributions, attributable to van der Waals forces in the core of the protein. ... 

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