Hydrophobic model

Fig. 4.8 compares data on the adsorption of lauric acid (C12) and caprylic acid (Cs) at a hydrophobic surface (mercury) as a function of the total bulk concentration for different pH-values. As is to be expected the molecular species becomes adsorbed at much lower concentrations than the carboxylate anions. The latter cannot penetrate into the adsorption layer without being accompanied by positively charged counterions (Na+). As was shown in Fig. 4.4, the adsorption data of pH = 4 can be plotted in the form of a Frumkin (FFG) equation. Fig. 4.9 compares the adsorption of fatty acids on a hydrophobic model surface (Hg) with that of the adsorption on Y-AI2O3. 

To demonstrate these differences, the experimental relationship between log P and log k across a range of eluent compositions was determined for each group of analytes. The results were then used to calculate the predicted logk in each case for a theoretical model compound with log P = 5 as a hydrophobic model, for log P = 1 as a hydrophilic model, and for log P = 3 as an intermediate model. 

Clearly, SPMD k s follow a hydrophobicity model, where k s fall with increasing... 

The ratio A Cp D)/A Cp(N D) should be a measure of the relative burial of hydrophobic surface areas in the transition and folded states, if the hydrophobic model is correct. The ratio is 0.51 for CI2, compared with a value of /3T of 0.6,27 which is a measure of the overall change in surface area (equation 18.9). 

Substances that undergo bioconcentration are hydrophobic and lipophilic, and therefore tend to undergo transfer from water media to fish lipid tissue. The simplest model of bioconcentration views the phenomenon on the basis of the physical properties of the contaminant and does not account for physiologic variables (such as variable blood flow) or metabolism of the substance. Such a simple model forms the basis of the hydrophobicity model of bioconcentration, in which bioconcentration is regarded from the viewpoint of a dynamic equilibrium between the substance dissolved in aqueous solution and the same substance dissolved in lipid tissue. 

Quantitatively, the hydrophobicity model of bioconcentration is viewed in the classical thermodynamic sense as an equilibrium between the uptake and elimination of substance X ... 

Evidence for the validity of the hydrophobicity model of bioconcentration is provided by correlations of it with the octanol-water partition coefficient, Km, using n-octanol as a surrogate for fish lipid tissue. The measurement of Kow consists of determining the concentration of a hydrophobic contaminant in water-immiscible n-oclanol relative to water with which it is in equilibrium. Typical Kow values range from 10 to 107, corresponding to BCF values of 1 to 106. Such Kow/BCF correlations have proven to be reasonably accurate when narrowly defined for a specified class of compounds, most commonly poorly metabolized organohalides. Major inconsistencies appear when attempts are made to extrapolate from one class of contaminants to another. 

Explain how bioconcentration relates to bioaccumulation. How does the hydrophobicity model pertain to bioconcentration What are the conditions under which bioconcentration is most applicable as a model ... 

Among hydrophobic model systems, one experimental investigation of particular interest concerns the structure of water contained in a carbon powder [20]. The structure of water has been determined by both x-ray and neutron diffraction, as a function of hydration, and from room temperature down to 77 K. In agreement with previous work [13-17,38,42,43], this study gave support to the existence of a region near the interface where the properties of water are markedly different from those of the bulk liquid. From x-ray measurements, which yield information about the oxygen-oxygen distribution function, it appears that, at the... 

When partially hydrated samples are cooled down to 77 K, no crystallization peak is detected by differential thermal analysis. The x-ray and neutrons show that an amorphous form is obtained and its structure is different from those of low-and high-density amorphous ices already known [5]. Samples with lower levels of hydration corresponding to one monolayer coverage of water molecules are under investigation. This phenomenon looks similar in both hydrophilic and hydrophobic model systems. However, in order to characterize more precisely the nature of the amorphous phase, the site-site partial correlation functions need to be experimentally obtained and compared with those deduced from molecular dynamic simulations. 

BodorLOGP Bodor hydrophobic model -> iipophiiicity descriptors... 

CASE approach - lipophilicity descriptors (O Klopman hydrophobic models)... 

Improvements of the Pohtzer hydrophobic model were later proposed using additional quantum-chemical descriptors derived from the molecular electrostatic potential, dipole moment, and ionization energies. These descriptors were searched for to give the best estimations of the cavity term, polarity/dipolarizability term, and hydrogen-bond parameters defined in -> linear solvation energy relationships [Haeberlein and Brinck, 1997]. 

This hydrophobic model is derived from an atom/fragment contribution method providing hydrophobic atomic and fragmental constants / measuring the hpophilic con-... 

Bodor hydrophobic model (. Bodor LOGP, BLOGP)... 

Meyer anchor sphere volume -> size descriptors Meyer-Richards similarity index -v similarity/diversity Meyer visual descriptor of globularHy -> shape descriptors Meylan-Howard hydrophobic model -> lipophilicity descriptors Mezey 3D shape analysis... 

VLOGP model Gombar hydrophobic model -> lipophilicity descriptors volume descriptors (V)... 

Bodor hydrophobic model = BLOGP —> lipophilicity descriptors... 

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