Octanols, dielectric constants, dipole polarities

We have no measurements of micellar size, since the translation of micelle size into the number of monomers in the micelle is not a simple task and requires assumptions not easily experimentally tested. We are hopeful of extending experimentation in this direction in future research. Table II lists dielectric constants, dipole moments and effective polarities for methanol, 1- and 2-octanol, and water at 25°C. 

Table II. Dielectric Constants, Dipole Moments and Effective Polarities for Methanol, Octanols and Water at 25°C...

Comparison of dipole moments shows only small differences in polarity. From these data, it can be reasoned that micellization in methanol is feasible. Dielectric constants and effective polarities (dipole moment/molar volume) support this premise with more divergent values. It is noted that bis(2-ethyIhexyI) sodium sulfosuccinate forms micelles readily in water and 2-octanol which have the highest and lowest dielectric constants, respectively, but micelles are formed only at low concentrations in methanol whose dielectric constant is intermediate in value. 

Hydrophobicity, from the greek hydro water and phobia aversion, is a term referring to the way a molecule likes or does not like water. A compound with a high hydrophobicity will not be water soluble. It is ap-olar. Conversely, a compound with a low hydrophobicity is said to be hydrophilic or polar. It is likely to be water soluble. In between the two extremes, the hydrophobicity varies. A scale is needed. The problem is that the hydrophobicity, or the polarity of a compound, depends on several parameters such as the dipole moment, the dielectric constant, the polarizability, the proton donor or acceptor character, or even the boiling point to molecular mass ratio. Since the end of the nineteenth century, the octanol-water partition coefficient, P i, was used with success as a measure of hydrophobicity. The log is the convenient scale. Compounds with a positive log Po/w value are more and more hydrophobic or apolar as the value increases. Compounds with a negative log value are hydrophilic or polar [1]. 

A polarity ranking is not possible based only on dielectric constant and dipole moment because they do not take into account H-bonding thus, polarity series are often constructed empirically, using such factors as the solvent strength parameter obtained from the observed ability of various solvents to elute solutes from aluminum oxide absorbent. However, for environmental chemicals, a numerical index for polarity does not exist only the consequences of polarity, as reflected in measureable properties such as water solubility and octanol-water partition coefficient, are available for fate predictions. 

The influence of structure on molecular association in the alcohols was shown by Smyth (24), who determined the dielectric constant and molar polarization of 22 isomeric octanols in the pure state. Only general conclusions can be drawn from these results. As Smyth says, associations in which the dipoles reinforce each other, giving high P and e, seem to occur when the OH group is at the end of a long C chain and remote from a branch in the chain—i.e., when linear multimer formation is most favored. When the OH group is in the middle of the chain and there is also branching at that point as in 4-methyl-4-heptanol, P is approximately half the value for 1-octanol, as would be expected if cyclic multimers predominate. 

---