Molecules polar versus nonpolar

It is worth mentioning that the equation for the data on y versus T, for polar (and associating) molecules like water and alcohols, when analyzed by Equation A.5, gives the magnitudes of ko and kl, which are significantly different from those found for nonpolar molecules such as alkanes, etc. This difference requires further analyses so that the relationship between y and Tc may be more completely understood (Birdi, 1997). 

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. 

Surfactants. Some compounds, like short-chain fatty acids, are amphiphilic or amphipathic that is, they have one part that has an affinity for the nonpolar media (the nonpolar hydrocarbon chain), and one part that has an affinity for polar media, that is, water (the polar group). The most energetically favorable orientation for these molecules is at surfaces or interfaces so that each part of the molecule can reside in the fluid for which it has the greatest affinity (Figure 4). These molecules that form oriented monolayers at interfaces show surface activity and are termed surfactants. As there will be a balance between adsorption and desorption (due to thermal motions), the interfacial condition requires some time to establish. Because of this time requirement, surface activity should be considered a dynamic phenomenon. This condition can be seen by measuring surface tension versus time for a freshly formed surface. 

Experimental data do not follow these equations very closely. This problem also arises in low-molecular-weight esterifications and is apparently due to the change in polarity of the medium as the ester formation proceeds. The catalytic efficiency of an acid is very sensitive to the polarizing forces in the surrounding medium the very process of esterification, however, involves the formation of a nonpolar molecule from two relatively polar reactants. This changes the situation and is reflected in a relation of the reaction rate versus time slightly different from the one given in the above formula. 

---