Entropic hydrophobic effect

Hydrophobic effects arise from the exclusion of non-polar groups or molecules from aqueous solution. This situation is more energetically favourable because water molecules interact with themselves or with other polar groups or molecules preferentially. This phenomenon can be observed between dichloromethane and water which are immiscible. The organic solvent is forced away as the intersolvent interactions between the water molecules themselves are more favourable than the hole created by the dichloromethane. Hydrophobic interactions play an important role in some supramolecular chemistry, for example, the binding of organic molecules by cyclophanes and cyclodextrins in water (see Chapter 2, Sections 2.7.1 and 2.7.5, respectively). Hydrophobic effects can be split into two energetic components, namely an enthalpic hydrophobic effect and an entropic hydrophobic effect. 

Figure 1.23 Two organic molecules creating a hole within an aqueous phase, giving rise to the entropic hydrophobic effect - one hole is more stable than two.

Water-soluble globular proteins usually have an interior composed almost entirely of non polar, hydrophobic amino acids such as phenylalanine, tryptophan, valine and leucine witl polar and charged amino acids such as lysine and arginine located on the surface of thi molecule. This packing of hydrophobic residues is a consequence of the hydrophobic effeci which is the most important factor that contributes to protein stability. The molecula basis for the hydrophobic effect continues to be the subject of some debate but is general considered to be entropic in origin. Moreover, it is the entropy change of the solvent that i... 

In the case of the retro Diels-Alder reaction, the nature of the activated complex plays a key role. In the activation process of this transformation, the reaction centre undergoes changes, mainly in the electron distributions, that cause a lowering of the chemical potential of the surrounding water molecules. Most likely, the latter is a consequence of an increased interaction between the reaction centre and the water molecules. Since the enforced hydrophobic effect is entropic in origin, this implies that the orientational constraints of the water molecules in the hydrophobic hydration shell are relieved in the activation process. Hence, it almost seems as if in the activated complex, the hydrocarbon part of the reaction centre is involved in hydrogen bonding interactions. Note that the... 

This explanation for the entropy-dominated association of surfactant molecules is called the hydrophobic effect or, less precisely, hydrophobic bonding. Note that relatively little is said of any direct affinity between the associating species. It is more accurate to say that they are expelled from the water and —as far as the water is concerned —the effect is primarily entropic. The same hydrophobic effect is responsible for the adsorption behavior of amphi-pathic molecules and plays an important role in stabilizing a variety of other structures formed by surfactants in aqueous solutions. 

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. ... 

In the case of contact forces, the magnitude of the enthalpy term is relatively small. The entropic contribution, however, can be significant. When a nonpolar compound is in an aqueous solution, the water molecules form a highly ordered solvent shell around the nonpolar portions of the compound (Scheme 9.3). This phenomenon is called the hydrophobic effect. Once the compound buries itself into a binding site on a target, some solvating water molecules will... 

Earlier literature has used the term hydrophobic bond, but it is clear from the above discussion that no special hydrophobic force exists. Nonpolar groups self-associate in water because their dispersal throughout the solvent would be entropically unfavorable. Once they come together and water is largely excluded, enthalpically favorable interactions are possible, but these are just (for nonaromatic hydrocarbons) the normal weak London forces between any polarizable groups. There is no bonding that is specifically hydrophobic. The correct term is hydrophobic effect. 

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. 

If the solute is nonpolar, there is only weak van der Waals attraction with water, and water molecules arrange around the nonpolar solute such that they form the most extensive number of hydrogen bonds, with the ice clathrates (Part IV, Chap. 21) the extreme case. The ordering of water molecules is entropically unfavorable, since they lose orientational and translational freedom. This can be compensated for if the solvated solute molecules aggregate and the ordered water molecules are released from their surface into bulk water, a process which is entropically favorable and the main driving force for the hydrophobic effect [128 to 134]. 

Isothermal titration calorimetry (ITC) dilution experiments were used to measure association constants and thermodynamic parameters for the formation of dimers 15-15 (cf. Section 14.09.3.1) <20010L3221>. Aggregates 15-15 are highly associated at 298K and entropically driven. The change in heat capacity (ACp) for the formation of dimer 15-15 was determined by ITC measurements from 288 to 328 K yielding the negative value (ACp = — 185 6 cal mol-1 K 1). It was concluded that the dimerization process is driven by hydrophobic effect. 

Conversely, at lower temperatnres the hydrophobic effect entropically leads the adsorption of the solutes [12]. Actually the solvent strengths of aU mixtures change with temperature [13] and this influences selectivity. Also, the non-constancy of with changing temperature may also be due to the difference of the heat capacities of the analytes in the mobile and stationary phases according to the Kirkhoff equation... 

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