Volume 93, Number 2, March-April 1988 Journal of Research of the National Bureau of Standards Partitioning of Alkylbenzenes and Aliphatic Alcohols between Hexadecane and Methanol- Water Mixtures Volume 93 Number 2 March-April 1988 Michele (Miller) Schantz Partition coefficients between n -hexade­ water in the aqueous phase. The mole­ cane and methanol-water mixtures are fraction-based activity coefficients calcu­ National Bureau of Standards reported and analyzed for a series of lated from the partition coefficients Gaithersburg, MD 20899, and alkylbenzenes and aliphatic alcohols. A compare favorably to those determined Georgetown University custom-designed flask which has a side by a headspace gas chromatographic arm attached near the bottom was used method. Finally. based upon the data Washington, DC 20057 for the measurements. The hexadecane contained herein, the retention mecha­ layer was sampled from the top of the nism in reversed phase liquid chro­ B. N. Barman flask, and the aqueous layer was sam­ matography appears to involve the pled through the side arm of the flask. stationary phase as more than just a pas­ Georgetown University Both phases were analyzed by an appro­ sive receptor. priate analytical technique. either gas or Washington, DC 20057, and liquid chromatography, to determine University of Utah concentrations. A lattice-model theory Key words: activity coefficients; alco­ Salt Lake City, UT 84112 suggests a correlation between the hex­ hols; alkylbenzenes; hexadeco.ne; adecane/methanol-water partition coeffi­ methanol-water mixtures; partition coef­ cients and the solute molar volume for ficients; reversed phase liquid chro­ and each solute group type and between the matography. hexadecane/methanol-water partition Daniel E. Martire coefficients and the volume fraction of Accepted: November 25, 1987 Georgetown University Washington, DC 20057 1. Introduction RPLC involves the distribution of a nonpolar or moderately polar solute between a polar mobile Partition coefficients have been measured or esti­ phase (the eluent) and a relatively nonpolar station­ mated for a large number of solutes in an octanol/ ary phase [3]. The stationary phase is made up of water system [I]. Recently, the partition silica gel particles to which n -alkyl chains (often, coefficients between n ~hexadecane and water were n -octadecyl) have been bound. The most common reported for a series of n -alkylbenzenes, n -alkanes, eluents are generally not pure solvents, but rather n -I-alkenes, n -I-bromoalkanes, and n -I-alcohols mixtures of an organic solvent (often, methanol or [2]. For this paper, the partitioning of n·alkylben­ acetonitrile) and water. When methanol and water zenes and I-alcohols between hexadecane and are mixed, the solvent properties change [4]. For methanol-water mixtures was studied in an effort to example, with an increasing percent of methanol in relate this partitioning to retention in reversed water, the viscosity first increases and then de­ phase liquid chromatography (RPLC). creases, reaching a maximum at approximately 161 Volume 93, Number 2, March-April 1988 Journal of Research of the National Bureau of Standards 35% (v/v) methanol in water. The dielectric con­ phase. Finally, the partition coefficients are used in stant decreases in an almost linear fashion, and the conjunction with chromatographic data to examine surface tension decreases more rapidly between 0 the retention mechanism in RPLC. and 20% (v/v) methanol in water than between 20 and 100% methanol in water. It has been argued that mobile phase interactions 2. Background Thermodynamics are of principal importance in the control of solute retention, with the nonpolar stationary phase act­ In this treatment, the solute is designated as com­ ing mainly as a passive solute acceptor [5]. Many ponent a, water as component b, methanol as com­ workers in the field (e.g., [6]-[8]) have measured ponent b', and hexadecane as component h. Hence, RPLC capacity factors (k ') as a function of the the partition coefficient of a solute between hex­ volume fraction of organic solvent in water. The adecane and methanol-water mixtures is denoted capacity factor (k ') is given by by Khl%w or Ka(hlb'+b)' In addition, the volume frac­ tion of water in the methanol-water mixtures is de­ k'=(t,-to)lto, (I) noted by lib, and that of methanol by lib" The partition coefficient between hexadecane and where t, is the retention time of the solute of inter­ methanol-water mixtures is then defined by est and to is the retention time of an "unretained solute. H The net retention volume, Vn, also has been correlated with the volume fraction of water in the mobile phase [9]. Most of these studies at­ where 'Y. is the volume-fraction-based infinite-dilu­ tribute the changes in retention with a change in tion solute activity coefficient, with the convention volume fraction solely to mobile-phase effects. that 'Ya-?l as Oa-?l, where Oa is the volume fraction In the present study, solute partition coefficients of the solute. Note that eq (2) is analogous to equa­ have been determined between n -hexadecane and tions for the octanollwater and hexadecane/water methanol-water mixtures for a series of alkylben­ partition coefficients [II]. zenes and I-alcohols. In addition, the mutual solu­ From the Flory-Huggins theory, which is based bility between hexadecane and methanol has been on a random-mixing (Bragg-Williams) approxima­ investigated at 25 'C. Previous partition coefficient tion, the solute activity coefficients in water, measurements done in this laboratory have utilized methanol, and hexadecane are given by [12] a generator column attached to an extractor column, a short column packed with CI8 material In 'Y'(b) = [1-(r,lrb)] +r"x.b (3) [2]. Since the methanol contained in the aqueous phase would strip the solute from an extractor In 'Y.(b') = [1-(r,lrb.)] +r"x,b' (4) column, a sit-flask technique similar to that re­ ported by Polak and Lu [10] was used for the (5) present study. The flask had a side arm attached near the bottom which permitted sampling of the In these equations, r; is the total number of seg­ aqueous phase without contaminating the syringe ments in a molecule of component i which is pro­ with the hexadecane phase. The flask sat undis­ portional to Vi*, the molar volume of component i. turbed for 7 days at room temperature, 23 'c. Both Furthermore, Xij is the segmental interaction phases were analyzed by an appropriate analytical parameter. In the mixed solvent (b+b'), the solute technique, either gas or liquid chromatography. activity coefficient is given by [3] The partition coefficient is the ratio of the concen­ tration in the hexadecane phase to that in the aqueous phase. As for the hexadecane/water partition coeffi­ Substituting eqs (3) and (4) into eq (6) and coupling cient [2], a lattice-model theory suggests a correla­ that result with a substitution of eq (5) into eq (2), tion between the hexadecane/methanol-water one obtains partition coefficients and the solute molar volume for each solute group type. Using this lattice-model theory, an expression is obtained that relates the hexadecane/methanol-water partition coefficients + r.(X'b·-X.Jlib' + r,(X,b - X.h)9b - r"xb'blib,lib, to the volume fraction of water in the aqueous (7) 162 Volume 93, Number 2, March-April 1988 Journal of Research of the National Bureau of Standards By definition, the solute, component a, is com­ In K.(h/b+b,);::::r.[X13+ I/rh -I/rb'] posed of group type I, the methyl and methylene groups and group type 2, the substituent group. + r,,[X'3' - X 12 - X 13'] Water, component b, is composed of group type 3, +r.[X -X ,.+ I/rb,-I/rb]llb and methanol, component b', is composed of group 13 1 type 3'. Hexadecane, component h, is composed of + r",[X"-X,,,-X13 +X13,]lIb' (10) group type I, the methyl and methylene groups. Expanding eqs (3), (4), and (5) to include the group However, ra is proportional to Va*, the molar vol­ types and substituting the results into eq (7), the ume of the solute and r2a is proportional to V2a *, the hexadecane/methanol-water partition coefficient is molar volume of the solute functional group. The now given by partition coefficient can now be rewritten as + CV,. *[X" - X"' - X13 +X13 .]l1b, (II) where C is a constant. This equation predicts a lin­ (8) ear relationship between In Ka(hJb+b') and Va*, where the slope is C[X13+ I/rh-l/rb]+C[X13-X13+ I/rb' where ri. is the number of segments of group type i -l/rb]lIb and the intercept is CV2,*[X",-X12-X13'] in the solute molecule (r. = rl. + r,J and where Xii is now a group interaction parameter per unit seg­ + CV,. *[X" -X"' -X13 + X13·]lIb' Therefore, accord­ ing to eq (II), at a given lib the slope should be ment. independent of the solute functional group, Knowing the hexadecane/methanol-water parti­ whereas, the intercept should reflect the size and tion coefficient at a limited number of volume frac­ interactions of the functional group. Similar linear tions of water in methanol, Ob' it is convenient to be relationships between the octano1!water partition able to predict the partition coefficients at other coefficient, Ko/w, and Va *, as well as the hexade­ values of lib' Neglecting terms involving the cane/water partition coefficient, Kb/wl and Va>le, product IIbllb' (X ,;::::O) and noting that lib lib' = I, 33 + were discussed in a previous paper [2]. eq (8) becomes 3. Experimental A sit flask was used for the partition coefficient measurements. The solute was quantitatively added to the hexadecane phase.