Hydrophobic interactions thermodynamics

It can be seen from the figure that the electrostatic repulsive forces between the macrocations are overwhelmed, probably by hydrophobic attractive forces between their hydrophobic side groups. It should be noted that the complimentary base-base pairing is unimportant in the present case. If this is not the case, the mixtures of APVP and TPVP should show the largest hypochromicity. This, however, is not the case. The importance of the hydrophobic interactions between nucleic acid bases has been proposed by Ts o et al.I9 from thermodynamic parameters of various nucleic acid bases or nucleosides in aqueous media. 

Solvent effects also play an important role in the theory separating enthalpy and entropy into external and internal parts (134-136) or, in other terms, into reaction and hydration contributions (79). This treatment has been widely used (71, 73, 78, 137-141). The most general thermodynamic treatment of intermolecular interaction was given by Rudakov (6) for various states of matter and for solution enthalpy and entropy as well as for kinetics. A particular case is hydrophobic interaction (6, 89, 90). 

The most fundamental thermodynamic approach of Rudakov (6) applies to all condensed systems. The actual linear relationship is argued to exist between enthalpy (AH) and entropy (AS) of intermolecular interaction, as reflected in an approximately linear relationship between the total enthalpy and entropy. Special attention has been given to hydrophobic interaction (89, 90) in water solutions, which makes the isokinetic behavior more pronounced and markedly changes its slope. 

The qualitative discussion of solubility has focussed so far on the attractive forces in solute-solvent interactions. However, where water is concerned, it is also important to consider the forces of repulsion due to the so-called hydrophobic interactions that may arise in certain cases (Franks, 1975). These hydrophobic interactions may be explained in terms of thermodynamic concepts. 

If amphipathic molecules are mixed with water, three different types of lipid structure are possible the type of aggregate formed depends on the physicochemical conditions and the lipid species involved. The thermodynamic parameter involved is the hydrophobic interaction. 

The temperature stability of the complexes seems to be dependent on the molecular weight of the PEG chain, i.e., the larger the PEG the lower the temperature at which the complex dissociates. An important observation was that the complexation/decomplexation phenomenon was reversible by changing the temperature of the system. The positive values of the thermodynamic parameters as well as the experimental observations clearly indicate the important role of hydrophobic interactions in the stabilization of the PMAA/PEG complexes. Since PAA is considerably more hydrophilic than PMAA, hydrophobic interactions do not play an important role in stabilizing the PAA/PEG complexes. This is represented by the much... 

Experimental studies of the thermodynamic, spectroscopic and transport properties of mineral/water interfaces have been extensive, albeit conflicting at times (4-10). Ambiguous terms such as "hydration forces", "hydrophobic interactions", and "structured water" have arisen to describe interfacial properties which have been difficult to quantify and explain. A detailed statistical-mechanical description of the forces, energies and properties of water at mineral surfaces is clearly desirable. 

The arguments presented above lead to the conclusion that the adsorption of nonionic compounds such as halogenated hydrocarbons results primarily from "hydrophobic bonding" or, perhaps more appropriately, the hydrophobic interaction (7). The thermodynamic driving force for hydrophobic interactions is the increase in entropy resulting from the removal, or decrease, in the amount of hydration water surrounding an organic solute in water. Studies have shown that the adsorption of aliphatic amines onto clays (8)... 

The pseudothermodynamic analysis of solvent elfects in 1-PrOH-water mixtures over the whole composition range (shown in Figure 7.3) depicts a combination of thermodynamic transfer parameters for diene and dienophile with isobaric activation parameters that allows for a distinction between solvent elfects on reactants (initial state) and on the activated complex. The results clearly indicate that the aqueous rate accelerations are heavily dominated by initial-state solvation effects. It can be concluded that for Diels-Alder reactions in water the causes of the acceleration involve stabilization of the activated complex by enforced hydrophobic interactions and by hydrogen bonding to water (Table 7.1, Figure 7.4). °... 

They are of interest in studies on the thermodynamic behavior of nonionic compounds in aqueous media because they have the ability of forming hydrogen bonds with the solvent (through the two r/j-amide groups in the cyclic dipeptide ring), and give rise to hydrophobic interactions. ... 

HYDROPHOBIC INTERACTIONS. These bonding interactions arise from the tendency of nonpolar side chains of amino acids (or lipids) to reside in the interior, nonaqueous environment of a protein (or membrane/ micelle/vesicle). This process is accompanied by the release of tightly bound water molecules from these apolar side-chain moieties. The hydrophobic effect is thermodynamically driven by the increased disorder i.e., A5 > 0) of the system, thereby overcoming the unfavorable enthalpy change i.e., AH < 0) for water release from the apolar groups. 

Factors that influence the retentive powers and selectivity of such bonded phases include the surface concentrations of hydrodartenaceous ligates and free silanol groups. The thermodynamic aspectitm solute interactions with the hydrocarbonaceous ligates at the surface, which are hydrophobic interactions in the case of aqueous eluents, are discussed later in this chapter within the framework of the solvophobic theory. In practice, however, solute interactions with surface silanol which may be termed silanophilic interactions can also contribute ]to retention (71, 75, 93), particularly in the case of amino compounds. Consequently the retention mechanism may be different from that which would be ol served with an ideal nonpolar phase. Therefore, increasing attention is paid to the estimation of the concentration of accessible sianols and to their elimination from the surface of bonded phases. 

This indicates a lack of dynamic cohesion within the adducts i.e. the substrate has considerable freedom for reorientation within the receptor. The apparent reason for an absence of mechanical coupling is the nearly cylindrical symmetry of cucurbituril, which allows the guest an axis of rotational freedom when held within the cavity. Hence, the bound substrates show only a moderate increase in tc relative to that exhibited in solution. No relationship exists between values and the thermodynamic stability of the complexes as gauged by K (or K, cf. Tables 1 and 2). It must be concluded that the interior of cucurbituril is notably nonsticky . This reinforces previous conclusions that the thermodynamic affinity within adducts is chiefly governed by hydrophobic interactions affecting the solvated hydrocarbon components, plus electrostatic ion-dipole attractions between the carbonyls of the receptor and the ammonium cation of the ligands. 

Both of the tetraaza[3.3.3.3]paracyclophane (1) and tetraaza[n.l.n.l]paracyclo-phane (n = 6, 7, 8 cf. 2) rings have frequently been used as fundamental molecular skeletons for preparation of functionalized macrocyclic hosts [24-36]. Formation of three-dimensionally extended hydrophobic cavities was approached by introducing multiple hydrocarbon branches into the macrocyclic skeletons. Multiple hydrophobic chains thus placed in a macrocycle must be extended in the same direction and undergo mutual association to attain their optimal hydrophobic interactions in aqueous media due to thermodynamic reasons, while in nonaqueous media they presumably assume a free and separated configuration to minimize their mutual steric interactions. Consequently, such hydrophobic branches may provide a large hydrophobic cavity in aqueous media. 

---