Octanol-water interface

In this overview, the direct measurement of the rate of a molecule crossing the octanol-water interface is presented as an appropriate extension to measurement of Pow, a thermodynamic property. In addition to reviewing previous work, we include results from a wide variety of chemical... 

It has been shown that the magnitude of the rate constant for crossing the octanol-water interface makes the energy barrier significantly larger than the diffusional barrier. It has also been shown that for compounds with log Pow less than =1.2, the overall rates are faster and the interfacial kinetics term more important. However, detailed development of a model would be needed to understand what the relative importance of diffusion and interfacial terms (such as cuticle or membrane permeation) are in vivo. No clear dependence of interfacial rate constants on log Pow was seen, but the initial emphasis of such a study should be on the intermediate... 

A similar imbalance of attractive forces exists at the interface between two immiscible liquids. Table 6.1 lists surface tensions of various liquids and also interfacial tensions at the liquid/water interface. The value of the interfacial tension is generally between those of the surface tensions of the two liquids involved, except where there is interaction between them. Table 6.1 includes several such examples. The interfacial tension at the octanol/water interface is considerably lower than the surface tension of octanol owing to hydrogen bonding between these two liquids. 

Chevrot, G., Schurhammer, R., and Wipff, G. 2007b. Synergistic effect of dicarbollide anions in liquid-liquid extraction A molecular dynamics study at the octanol-water interface. Phys. Chem. Chem. Phys. 9 1991-2003. 

The 1-octanol/water interface polarity was studied experimentally by Walker and coworkers using the molecular ruler idea discussed earlier. ... 

By modeling the substance behavior at the interface of two liquid phases, in particular, stationary and mobile phases in liquid chromatography, 1-octanol - water partition coefficients or partition coefficients in... 

To clarify the relation between potential oscillation and features of interface o/wl, potential oscillation at the interface was measured in the presence of inorganic and organic electrolytes in phase wl, and potential oscillations in the octanol membrane system were compared with interfacial potential between octanol and aqueous solutions of a two-phase octanol-water system with and without surfactant [27]. Figure 8 shows potential oscilla-... 

FIG. 9 Upper potential values, a.sds lower potential values, b.sds of the first oscillation at the interface between phases o and wl of the octanol membrane (A), interfacial potential of a two-phase octanol-water system in the absence of SDS, c.sds (B) and those in the presence of 10 mM SDS (in the case of inorganic electrolyte, 1 mM), d.sds (C)- TMACI tetramethylammonium chloride TEACI tetraethylammonium chloride TPACI tetrapropylammonium chloride TBACI tetra-butylammonium chloride AcNa sodium acetate PrNa sodium propionate, BuNa sodium n-butyrate VaNa sodium w-valerate. (Ref 27.)... 

In this model, one can argue that a peptide must have both an affinity for the interface (favorable n-octanol partition coefficient) and small desolvation energy (favorable A log PC) in order to efficiently cross a cell membrane. On the other hand, this model also predicts that a peptide with a large n-octanol/water partition coefficient and large desolvation energy, due to a significant number of polar groups, should adsorb and remain at the membrane interface. Both of these predicted events have been observed in the laboratory. 

Several studies have shown that sorption of various organic compounds on solid phases could be depicted as an accumulation at hydrophobic sites at the OM/water interface in a way similar to surface active agents. In addition Hansch s constants [19,199-201], derived from the partition distribution between 1-octanol and water, expressed this behavior better than other parameters. Excellent linear correlations between Koc and Kow were found for a variety of nonpolar organic compounds, including various pesticides, phenols, PCBs, PAHs, and halogenated alkenes and benzenes, and various soils and sediments that were investigated for sorption [19,76,80,199-201]. 

EPA Estimation Program Interface (EPI) Suite. Estimation for octanol-water partition... 

The partition coefficient or its logarithm serves as a quantitative measure of a compound s lipophilicity. For pharmaceuticals, water and 1-octanol are the two solvents used most frequently to measure lipophilicity. The two-phase octanol/water system closely models the cell membrane/interstitial fluid interface. While Lipinski s rules do not specify a minimum lipophilicity, a log P value of 5 or less prevents a drug from effectively hiding within membranes. 

The structure of the liquid- liquid interfadal layer depends on the difference in polarity between the two liquids (Kaeble, 1971). Asymmetric molecules of some liquids display a molecular orientation on the interface which is indicative of their structure. Thus, interfacial tension at the octane-water interlace is SO.S nm/m whereas at the octanol-water interne it is only 8.8 nm/m. Reduction of inter dal tension in the latter case points to the orientation of octanol hydroxyl groups toward water, in other words to the structure and polarity of the interfadal layer. Because of such an orientation, the stimulus for adsorption of other asymmetric molecules on the interface is decreased. A similar pattern is typical of the homologous series of lower attcy] acrylates at the interface with water the carbonyl groups of their asymmetrical molecules are oriented toward water this orientation is more eSective the higher the polarization of the carbonyl, i.e the smaller the alkyl. Interfadal tension decreases in the same order from 27.2 nm/m for hexyl acrylate (Yeliseyeva et at, 1978) to 8 nm/m for methyl acrylate (datum from our laboratory by A, Vasilenko). 

The extent of soiption of hydrophobic solutes at the solid-water interface depends on the organic carbon content of the sorbent. In other words, the organic material in the (porous) soils behaves like octanol and the hydrophobic material is absorbed into the organic matrix. 

Many examples exist of interfaces formed between two immiscible liquids. A well-known one is the interface between a long-chain hydrocarbon, for example, dodecane, and water, which is commonly known as the oil water interface. Dodecane and water are immiscible because the hydrocarbon phase is nonpolar. Liquid liquid interfaces are also formed between water and organic liquids with polar groups such as octanol and heptanoic acid, which also have rather long hydrocarbon chains. The polar liquid nitrobenzene, which has a relative permittivity of 35, is also immiscible with water. Another well-known system is the mercury polar liquid interface. This has been studied extensively, especially for aqueous electrolyte solutions. However, the mercury polar liquid interface is also an example of a metal solution interface which was considered in the previous section. The discussion here is limited to liquids with relative dielectric permittivities falling in the range 1-200, and systems which have poor conductivities as pure liquids. 

Because of its nonpolar and hydrophobic character, the mercury-water may serve as a good model interface for the adsorption study and determination of the organic substances that are adsorbed primarily because of hydrophobic expulsion. There is generally a proportionality of adsorbability (free energy of adsorption) found at the mercury electrode to a number of -CH2 groups in paraffinic hydrocarbon residues in nonpolar surfactants and a similar relation between the octanol water partition coefficient and chain length. This was recently also illustrated in the case of adsorption of aliphatic fatty acids (Ulrich ct al., 1988). 

---