Nonpolar molecules dissolution

The dissolution of polar molecules in water is favored by dipole—dipole interactions. The solvation of the polar molecules stabilizes them in solution. Nonpolar molecules are soluble in water only with difficulty because the relatively high energy cost associated with dismpting and reforming the hydrogen-bonded water is unfavorable to the former occurring. 

The enthalpy of dissolution of gaseous nonpolar molecules into water is always negative at room temperature, and its absolute value is proportional to the accessible surface area of the solute molecule (Frank and Evans, 1945 Tanford, 1980 Dec and Gill, 1984, 1985a,b Olofsson et al., 1984). 

The enthalpy of dissolution of different liquid hydrocarbons in water provides additional information. The transfer of a nonpolar molecule from the pure liquid phase (1) to water (w) can be represented by two steps (1) transfer from the liquid phase to the gaseous phase, i.e., the vaporization, and (2) transfer from the gaseous phase into water (Fig. 8). Consequently, the enthalpy of transfer can be presented as... 

The heat capacity change upon aqueous dissolution of nonpolar molecules from the gaseous phase and from the liquid phase is positive and is proportional to the surface area of the solute molecule (Edsall, 1935 Gill and Wadso, 1976). 

The heat capacity change upon dissolution of nonpolar molecules from the gaseous phase decreases with increasing temperature. 

Based on a study of C60 fiillerene solubility [1], it has been found that Cf, fiillerene, being a nonpolar molecule, is insoluble in polar solvents, such as alcohol, acetone, tetrahydrofurane etc. Relying on the above, it has been concluded that the solvation mechanism of dissolving plays an insignificant part in the processes of fiillerene dissolution. 

Micelles are in dynamic equilibrium with their monomer surfactants. Two relaxation processes are related to this equilibrium, a fast one in the microsecond time domain associated with the exchange of individual monomers between the micelles and the bulk aqueous phase and a slower one on millisecond time-scale associated with the complete dissolution of the micelles into monomers [8], For example, the exit rate for the SDS anion from its micelle is about lO s, which is considered to be a diffusion-controlled process [8a]. Nonpolar molecules are usually attracted to the relatively hydrophobic inner core of micelles, whereas ionic reactants and products are either associated with the Stem and Gouy-Chapman layers or repelled from the micelles, depending on the sign of electrostatic interaction. For example, NMR studies show that nonpolar molecules such as benzene and naphthalene are... 

When a nonpolar molecule is surrounded by water, stronger than normal water-water interactions are formed around the solute molecule to compensate for the weaker interactions between solute and water. This results in an increasingly ordered arrangement of water molecules around the solute and thus a negative entropy of dissolution. The decrease in entropy is roughly proportional to the nonpolar surface area of the molecule. The association of two such nonpolar molecules in water reduces the total nonpolar surface area exposed to the solvent, thus reducing the amount of structured water, and therefore providing a favourable entropy of association. 

In spite of the high polarity of PA6, identification of additives was also feasible in formulations of PA6/additive dissolutions, although with decreased sensitivity. Hostavin N 20, Irganox B 1171, Tinuvin 320 and Tinuvin 350 can be determined in PA6 in technical concentrations, although the sensitivity is less than for nonpolar polymers, such as polyolefins. This was tentatively explained as follows. In a nonpolar polymer matrix, the electronically excited polar additive molecule can easily be desorbed. In the polar polyamide matrix, desorption of the additives is hindered by strong polar interactions (e.g. hydrogen bridges) between the excited analytes and the polymer matrix. This hinders selective desorption of the additives by laser irradiation. However, in a polymer/additive matrix-modified solution, evaporated to dryness, the interactions between the polar... 

The enhancement of kerosene dissolution occurs even at low humic acid content in the aqueous solution. In view of the fact that humic substances are relatively high molecular weight species containing nonpolar organic moieties, Chiou et al. (1986) assumed that a partition-like interaction between a solute of very low solubility in aqueous solution and a microscopic organic environment of dissolved humic molecules can explain solute solubility enhancement. 

The most reliable calorimetric data on the enthalpy of dissolution of various nonpolar gases, the noble gases and hydrocarbons, are collected in Table II. The very small gaseous molecules (helium and neon) were not included. The surface area of the considered molecules, As, have been calculated from the known spatial structure of the molecule and represented either in terms of A2 or the number of contacting water molecules, Nt, assuming that each water molecule occupies an area of about 9 A2 (Hermann, 1972). The direct correlation between the enthalpy of dissolution of a gas in water, A H, and the surface area of the solute molecule, At, or the number of water molecules contacting the solute molecule, Ns, is seen from column flve in Table II. 

If a solvent and solute are nonpolar, the interaction force between them is of the van der Waals type 1 kJ/mole) where as the distance between them becomes closer, the interaction energy increases rapidly to the twelfth power of the distance. The induced dipole-induced dipole interaction mainly accounts for the dissolution of nonpolar solutes in nonpolar solvents such as paraffin in petroleum benzin or wax in CC14. The net heat of solution becomes zero or very small for nonpolar systems when the solute and solvent molecules are of similar size and structure. In this case, the interaction energies of solute-solute, solvent-solvent, and solute-solvent are of the same magnitudes. Materials that can be soluble with zero heat of solution are often called ideal solutions. 

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