Activity coefficient organic molecules

A similar multiphase complication that should be kept in mind when discussing solutions at finite concentrations is possible micelle formation. It is well known that for many organic solutes in water, when the concentration exceeds a certain solute-dependent value, called the critical micelle concentration (cmc), the solute molecules are not distributed in a random uncorrelated way but rather aggregate into units (micelles) in which their distances of separation and orientations with respect to each other and to solvent molecules have strong correlations. Micelle formation, if it occurs, will clearly have a major effect on the apparent activity coefficient but the observation of the phenomenon requires more sophisticated analytical techniques than observation of, say, liquid-liquid phase separation. 

Another important aspect of the Marcus theory has also been systematically investigated with organic molecules, namely the quadratic, or at least the non-linear, character of the activation-driving force relationship for outer sphere electron transfer. In other words, does the transfer coefficient (symmetry factor) vary with the driving force, i.e. with the electrode... 

Generally, the expression f/fTe = y, xt = a, is referred to as the activity of the compound. That is, a, is a measure of how active a compound is in a given state (e.g., in aqueous solution) compared to its standard state (e.g., the pure organic liquid at the same T and p). Since yt relates a, , the apparent concentration of i, to the real concentration xt, it is only logical that one refers to yt as the activity coefficient. It must be emphasized here that the activity of a given compound in a given phase is a relative measure and is, therefore keyed to the reference state. The numerical value of Yi will therefore depend on the choice of reference state, since, as we have seen in Section 3.2, molecules of i in different reference states (i.e., liquid solutions) interact differently with their surroundings. 

In summary, we can conclude that the excess free energy of an organic compound in aqueous solution, and thus its activity coefficient, depends especially on (1) the size and the shape of the molecule, and (2) its H-donor and/or H-acceptor properties. 

Furthermore, for most compounds of interest to us, the octanol molecules present as cosolutes in the aqueous phase will have only a minor effect on the other organic compounds activity coefficients. Also, the activity coefficients of a series of apolar, monopolar, and bipolar compounds in wet versus dry octanol shows that, in most cases, Yu values changes by less than a factor of 2 to 3 when water is present in wet octanol (Dallas and Carr, 1992 Sherman et al., 1996 Komp and McLachlan, 1997a). Hence, as a first approximation, for nonpolar solvents, for w-octanol, and possibly for other solvents exhibiting polar groups, we may use Eq. 6-11 as a first approximation to estimate air- dry organic solvent partition constants for organic compounds as illustrated in Fig. 6.2. Conversely, experimental KM data may be used to estimate K,aw or Kitvi, if one or the other of these two constants is known. 

The UNIFAC (UNIQUAC functional group activity coefficient) method is an extension of the UNIQUAC (Universal quasi chemical) method, which has been used widely in chemical process engineering to describe partitioning in organic systems as occur in petroleum and chemical processing (Fredenslund et al., 1975,1977). It has been applied less frequently to aqueous systems. It expresses the activity coefficient as the sum of a "combinational" component, which quantifies the nature of the area "seen" by the solute molecule, and a "residual" component, which is deduced from group contributions. Arbuckle (1983,1986), Banerjee (1985), Banerjee and Howard (1988), and Campbell and Luthy (1985) have tested the applicability of the method to water solubility. 

While polar functional groups in molecule i (positive W values) lead to a decrease in the K value, the nonpolar organic molecule structure acts to increase the value due to its repulsive effect on the polar water (high activity coefficient value of n-alkanes). Table 4-6 contains W-W values for several structural characteristics. Using n-heptane as a reference substance (z = 7) one obtains from Eq. (4-100) at 25 °C ... 

More than a hundred years ago, Meyer and Overton made their seminal discovery on the correlation between oil/water partition coefficients and the narcotie potencies of small organic molecules (7,8). Ferguson extended this analysis by placing the relationship between depressant action and hydrophobicity in a thermodynamic context the relative saturation of the depressant in the biophase was a critical determinant of its narcotic potency (9). At this time, the success of the Hammett equation began to permeate structure-activity studies and hydrophobicity as a determinant was relegated to the background. In a landmark study, Hansch and his colleagues de-... 

C represents the equipotent concentration, k and m are constants for a particular system, and A is a physicochemical constant representative of phase distribution equilibria such as aqueous solubility, oil/water partition coefficient, and vapor pressure. In examining a large and diverse number of biological systems, Hansch and coworkers defined a relationship (Equation 1.62) that expressed biological activity as a function of physicochemical parameters (e.g., partition coefficients of organic molecules) (19). 

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