Anal. Chem. 1983, 55, 659-661 (3) Carter, S. P.; Frelser, H. Anal. Chem. 1979, 51, 1100. (4) Bankovskll, Yu. A,; Chera, L. M.; Ievin'sh, A. F.; Luksha, E. A. J . Gen. Chem. USSR (Engl. Trans/.) 1958, 28, 2310. (5) Bankovskll, Yu. A,; Chera, L. M.; Ievln'sh, A. F. Zh. Anal. Khlm. 1063, 78, 668. (6) Carter, S. P.; Freiser, H. Anal. Chem. 1980, 52, 511. (7) Bag, S. P.; Frelser, H. Anal. Chlm. Acta 1982, 734, 333. (8) Aklba, K.; Frelser, H. Sep. Scl. Techno/.1982, 77, 745. (9) McClellan, B. E.; Frelser, H. Anal. Chem. 1084, 36, 2262. (10) Oh, J S.; Frelser, H. Anal. Chem. 1987, 39, 295. (11) Ohashl, K.; Frelser, H. Anal. Chem. 1980, 52, 767. (12) Yamada, K.; Nakagawa, K.; Haraguchl, K.; Ito, S. Nippon Kagaku Kalshl 1975, 294. (13) Yamada, K.; Nakagawa, K.; Haraguchi, K.; Ito, S. Nippon Kagaku Kalshi 1975, 1431.
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RECEIVED for review May 6,1982. Resubmitted October 12, 1982. Accepted December 13,1982. This work was supported by the National Science Foundation.
Determination of Partition Coefficient of Polar Organic Solutes in OctanoVMicellar Solutions G. M. Janlnl" and S. A. Attar1 Department of Chemlstty, University of Kuwalt, Kuwalt
A shake-flask method is presented for the determination of the partition coefflclents involved in the distribution of polar solutes between octanol and aqueous micellar solutions. An equation is derived for the simultaneous determination of the partition coefficients of solutes between octanol and water and between micelles and water from simple measurements of the apparent partition coefficients of solutes in octanol/ aqueous micellar solutions at different surfactant concentrations. By application of basic chromatographic theory this treatment is readily extended to account for the partitioning behavior of solutes eluted with micellar mobile phases in reversed-phase liquid chromatography. The advantages and limitations of this method are discussed.
The partition coefficients of organic chemicals in octanol/water serve as an empirical measure of the hydrophobicity of these substances in biomedical and pharmacological systems (I). They have been used for the estimation of bioconcentration of organic pollutants in trout muscle (2),for correlations with basic solvent properties (3,4),and for determination of retention in reversed-phase high-pressure liquid chromatography (5). On the other hand, the partition coefficient of solutes in micelles/water is useful in the research areas of oil recovery (6) and micellar catalysis, enzymes, and biological membranes (7-9). It is well established that retention in reversed-phase liquid chromatography depends upon the partitioning characteristics of the solute between the aqueous mobile phase and the hydrophobic stationary phase. The addition of surfactants to the mobile phase affects the partitioning process significantly. Liquid chromatographic separations utilizing aqueous micellar mobile phase, whereby significant partitioning of solute occurs to discrete aggregates dissolved in the mobile phase, have resulted in a degree of selectivity not shared by conventional pure or mixed solvents (IO). As articulated by Armstrong and Nome, the partition coefficients involved include that of a solute between the stationary phase and water, between the stationary phase and the micelles, and between the micelles and water. A high-pressure liquid chromatography (HPLC) 0003-2700/83/0355-0659$01.50/0
method for the calculation of these partition coefficients has been outlined by the above authors (IO). In this work an analogous treatment of the partitioning of solutes between aqueous micellar solutions and octanol is presented. An equation is derived which accounts for all possible equilibria for such systems. Data are obtained by the shake-flask technique, where solute concentration in octanol and in the aqueous micellar phase is determined by HPLC. The method is tested by using sodium dodecyl sulfate (SDS) surfactant and dihydroxybenzene solutes.
THEORY The octanol/water partition coefficient, KoW,is defined as the ratio of molar concentration of solute (S)in octanol saturated with water, Cs0, to its concentration in water saturated with octanol, Csw, under equilibrium conditions.
The concentration of solute in the two phases will bear a constant ratio as long as their activity coefficients remain relatively constant. In many instances solute molecules may participate in different equilibria in the two phases, thus complicating the partition process. Here we consider the partition of solute between octanol and an aqueous micellar solution. The three equilibria involved are schematically represented in Figure 1. For such a system an apparent partition coefficient, Kapp, may be defined as
where m denotes number of moles, V denotes volume, and the numerator is the total concentration of solute in the aqueous phase (bulk water + micelles). Utilizing the definition of K M W = (mSM/VM)/(msw/ Vw), it could be easily shown by algebriac rearrangement that
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Table I. Partition Coefficients of Dihydroxybenzenes between Octanol and Aqueous Micellar Solutions as a Function of Micellar Surfactant Concentration
Organic l a y e r Aqueous layer
s w-\ K YW
Figure 1. A schematic representationof the equilibria Involved In the partition of solutes between octanol and micellar solutions (S = solute; 0 = octanol; W = water; M = micelles; KO, = Cso/Csw; K, = CsM/CSW, K, = CsO/CSM).
Here, one should first fractionate the left-hand side of eq 3 and then expand the fraction M s W / ( V + ~ Vw) in a power series neglecting second and higher terms, where it is assumed that (VM/Vw)
