Solute hydrophobic surface area

Micelles are formed in order to protect the hydrophobic regions of the amphiphilic surfactant from the aqueous solution. The surface area S occupied by the surfactant molecule can be determined by ... 

Polarity may be qualitatively defined as the ability of a solute to dissolve in a polar solvent, which results from interaction with surrounding molecules by dipolar, non-dispersive forces. By this definition, hydrocarbons are nonpolar because they possesses no permanent dipole moments, and the entire molecular surface must solely interact with its environment via dispersion forces. Thus methanol is more polar than octanol because the surface area of methanol that interacts only via dispersion forces (hydrophobic surface area) is much less than that of octanol. For liquids, increasing solute polarity generally causes an increase in water solubility. This is not necessarily true for solids because polarity... 

The water surrounding a hydrophobic group is ice-like (highly ordered) because of limitations imposed on its movement by the presence of the hydrophobic material. A decrease in hydrophobic surface area causes a decrease in order and hence an increase in entropy that helps lower the free energy of the solution. 

Figure 4,14. Diagram of the thermodynamic cycle used to explain retention in reversed-phase chromatography by solvophobic theory. Na = Avogadro number, AA = reduction of hydrophobic surface area due to the adsorption of the analyte onto the bonded ligand, y = surface tension, = energy correction parameter for the curvature of the cavity, V = molar volume, R = gas constant, T = temperature (K), Pq = atmospheric pressure, AGydw.s.i a complex function of the ionization potential and the Clausius-Moscotti functions of the solute and mobile phase. Subscripts i = ith component (solute or solvent), S = solute, L = bonded phase ligand, SL = solute-ligand complex, R = transfer of analyte from the mobile to the stationary phase (retention), CAV = cavity formation, VDW = van der Waals interactions, ES = electrostatic interactions.

One can see that this equation contains the composition of the stationary phase as well as the composition of the mobile phase together with a size-dependent factor, which describes the displacement of solvent molecules in the stationary phase by the solute. The latter explains the linear relationship observed experimentally between the logarithm of the retention factor and the hydrophobic surface area of analytes (1). Another experimental verification of this is the linear dependoioe of the logarithm of the retention factor on the number of methylene groups in a homologous series, which has been reported by many investigators ... 

One solution was given by dielectric analysis of myosin SI solution [7]. The study showed a 8% reduction of the hydrophobic hydration number of 1,400 per SI, which is proportional to the hydrophobic surface area [8, 9] of an SI molecule at the M.ADP.Pi state and hydrophobicity recovery at the M.ADP state. The surface of SI at the M.ADP.Pi state is less hydrophobic than that at the M.ADP state (Fig. 2). In addition a liquid chromatographic study [10] showed the surface hydrophobicity decrease of SI in the presence of ATP. Thus, AA>0 at the step from the M.ATP state to the M.ADP.Pi state was explained by dehydration of the protein, and AH is compensated by -TAS. The position of such a hydrophobicity change is considered to be the actin binding site as discussed previously [7]. 

There is then a sudden drop in the free energy as virtually all of the amphiphile molecules are incorporated into micelles, enabling their hydrophobic alkyl chains to be more or less completely shielded from the aqueous part of the phase. This leads to the familiar abrupt change in physical properties at the cmc. In chromonic systems there is also the aggregation of molecules in dilute solution before mesophase formation, but the pattern of association is different. The hydrophobic surfaces of the molecules cause them to aggregate in stacks like packs of cards. As these stacks grow, the fraction of the total hydrophobic surface area exposed to the aqueous part of the phase steadily falls, but there is no minimum free energy state, no cmc, and there is no structure directly comparable to the micelle [38]. 

Because of their relatively large hydrophobic surface area, apolipoproteins, in the absence of lipids, readily self-associate in aqueous solution (Stone and Reynolds, 1975 Vitello and Scanu, 1976). The rate of desorption of apolipoproteins from lipoprotein surfaces has not been studied systematically. Extensive studies of the reverse process, which is the assembly of lipid apoprotein complexes, have been conducted in considerable detail. The dynamic of lipid-protein interactions have been studied primarily with in vitro model systems. Analysis of the association of apolipoproteins with various phospholipid aggregates have provided important clues about the nature of the kinetically important steps in the transfer of apolipoproteins between lipoproteins (Pownall et al., 1977 1978a Massey et al., 1981a Mantulin et al., 1981). 

In this relationship. S is alkane solubility, A is the cavity surface area and a is the hydrophobic free energy per unit area. Extensive fitting of this equation [24] yields a value of 88 kJ mol A for the proportionality constant a. This value corresponds to an unfavourable free energy of about 3.6 kJ mol for the transfer of a CH2 group to aqueous solution. 

The nature of soliite-solnte and solute-solvent in teraction s is dependent on the solvent environment. Solvent influences the hydrogen-bon ding pattern, solute surface area, and hydrophilic and hydrophobic group exposures. 

The second step is to disperse the core material being encapsulated in the solution of shell material. The core material usually is a hydrophobic or water-knmiscible oil, although soHd powders have been encapsulated. A suitable emulsifier is used to aid formation of the dispersion or emulsion. In the case of oil core materials, the oil phase is typically reduced to a drop size of 1—3 p.m. Once a suitable dispersion or emulsion has been prepared, it is sprayed into a heated chamber. The small droplets produced have a high surface area and are rapidly converted by desolvation in the chamber to a fine powder. Residence time in the spray-drying chamber is 30 s or less. Inlet and outlet air temperatures are important process parameters as is relative humidity of the inlet air stream. 

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