J. Phys. Chem. 1982, 86, 1457-1461
1457
Polyion-Counterion Interactions in Aqueous Solutions Probed by Positron Annihilation Techniques. 1. Co2+-Polyanion Systems Raoul Zana,+ Sonla Millan,’ Jean-Charles Abbe,
’
and Gllles Duplatrd
Centre de Recherches sur les Macromo&ules, 67083 Strasbourg C&x, France, and Centre de Recherches Nuckires, Division de Chimie et Physique des Rayonnements, B.P.20 67037 Strasbourg C&x, France (Received: August 19, 1981; In Final Form: November 19, 1981)
The positronium (Ps)lifetime depends on its physicochemical environment. This property has been used to study Co2+-polyanioninteractions in aqueous solutions of polyelectrolytes. The polyanions investigated were polyphosphate (PP)and poly(styrenesu1fonate) (PSS),fully neutralized by tetramethylammonium (TMA) hydroxide, and an alternating copolymer of maleic acid and methyl vinyl ether (Gantrez)at neutralization degrees (by TMAOH), a,of 0, 0.5, and 0.95. The changes in the Ps lifetime on adding Co2+salts to the various polyelectrolyte solutions show the same features as those in the NMR water proton chemical shift and in the density determined in previous studies. It is shown that the Co2+ions bound to PP or to Gantrez (a= 0.95) with the loss of their first hydration shell are unreactive toward Ps,whereas those condensed without modification of the hydration shell essentially retain the same reactivity for Ps as in the pure aqueous solutions. These results suggest that Ps must penetrate into the first hydration shell of Co2+before reacting with this ion. They also demonstrate the usefulness of Ps lifetime spectroscopy for investigating polyanion-cation interactions.
Introduction A positron, or positive electron, emitted in a solution with an initial energy of a few hundred keV from a decaying nucleus, most generally nNa, progressively loses this energy through ionization and rotational and vibrational excitations. Once thermalized, it annihilates with an electron either spontaneously or after forming a quasiatom, the positronium, Ps, with emission of electromagnetic energy. Depending on the relative orientation of the spins of the constituting particles, Ps exists as a singlet, parapositronium (p-Ps), or as a triplet, orthopositronium (0-Ps).’ The mechanism of Ps formation is still a matter of discussion, and the two models most often invoked are the “hot Ps atomw2and the “spurn3models. However, the latter, which states that Ps is formed by the association of the positron with one of the electrons released a t the end of ita track (the spur), is most generally accepted. The relative probability of Ps formation depends on the properties of the solvent and on the presence of solutes which can modify the abundance5 of the reactive species in the spur (positron, electrons, radicals, ions, etc.) and thus inhibit or enhance Ps formation. Once formed, Ps can undergo chemical reactions with a solute, such as oxidation, complex formation, or spin conversion, this last process being restricted to paramagnetic solutes. These quenching reactions alter the apparent decay rate of the thermalized Ps, and the quenching reaction rate constant depends on the nature of the solute and also on the physicochemical environment. In particular, the rate constants are sensitive to the formation of ion pairs in solutions of electrolytes4* and Ps quenching reactions may be used to discriminate between contact and solvated ion pairs. This possibility has prompted us to apply the positron annihilation techniques (PATS)to the study of polyion-counterion interactions in aqueous solutions. I t is now generally accepted that there exist two types of bound counterions in polyelectrolyte solutions. The first comprises the site-bound counterions which have partly or completely lost their first solvation shell and are t Centre
d e Recherches sur lea Macromolecules.
* Centre de Recherches NuclBaires.
0022-3654/82/2086-1457$01.25/0
in direct contact with one or more of the charged groups, or sites, of the polyion. Such polyion-counterion associations are analogous to contact ion pairs in solutions of usual electrolytes. The second type comprises the territorially bound counterions, which retain the integrality of their solvation shell in the binding process. The associations formed are analogous to solvated ion pairs (outersphere and outer-outer-sphere ion pairs) in solutions of nonpolymeric electrolytes. A number of methods have been used to investigate the binding of counterions to polyions.8 However, most of them do not allow the distinction between the two types of bound counterions. So far, only NMR,gJOdensity,*ll and ultrasonic absorption12J3measurements have proved to be useful for the study of site binding. The present work has therefore been initiated to assess the possibility of using PATS in the related fields of polyion-counterion interactions and metal ion complexation by polymeric neutral ligands. This first paper reports on the results for polyanion-Co2+ interactions. Met hod The measurements were performed by using the lifetime spectroscopy technique. The positron source consists of =Na nuclei introduced into the solution either in the form of a very high specific activity droplet or embedded in a (1)V. I. Goldanskii and V. P. Shantarovich in ‘Modern Physics in Chemistry”, E. Fluck and V. I. Goldanskii, Eds., Academic Press, New York, 1976,p 269. (2)A. Foglio Para and E. Lazzarini, J. Inorg. Nucl. Chem., 42,475 (1980). (3)See, e.g.: 0.E. Mogensen, J. Chem. Phys., 60,998 (1974);J.4. Abb6, G. DupEtre, A. G. Maddock, J. Talamoni, and A. Haessler, J. Inorg. Nucl. Chem., 43,2603 (1981). (4)A. G. Maddock, J.4. Abbe, G. Dupliitre, and A. Haessler, Chem. Phys., 26, 163 (1977). (5)G. Duplitre, L. M. Al-Shukri, and A. Haessler, J. R a d i o a d . Chem., 55, 199 (1980). (6)J.4. Abb6, G. Dupliitre, A. G. Maddock, and A. Haessler, Radiochem. Radioanal. Lett., 38,303 (1979). (7)G. Manning, Acc. Chem. Res., 12,443 (1979). (8)G. Manning, Ann. Rev. Phys. Chem., 23, 117 (1972). (9)P. Spegt, C.Tondre, G. Weill, and R. h a , Biophys. Chem., 1,55 (1973). (10)P. Spegt and G. Weill, Biophys. Chem., 4, 143 (1976). (11)C. Tondre and R. Zana, J. Phys. Chem., 76,3451 (1972). (12)R. Zana, C.Tondre, M. Rinaudo, and M. Milas, J. Chim. Phys. Phys.-Chim. Biol., 68,1258 (1971). (13)R. Zana and C. Tondre, Biophys. Chem., 1, 367 (1974).
0 1982 American Chemical Society
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The Journal of Physical Chemistry, Vol. 86, No. 8, 1982
thin glass f0il.14 The solutions were degassed by the usual freeze-thaw technique. A fast-slow coincidence circuit allows the measurement of the annihilation probability of the various positron-containing species formed in the solution, as a function of the time elapsed after the emission of the positron. The experimental decay curve usually includes three components with mean lifetimes ri and relative intensities Ij(i = 1,2,3) ascribed to p-Ps, e,+, and 0-Ps, respectively. In water, 710 = 0.12 ns, 72" = 0.4 ns, and r30 = 1.8 ns with Ilo= 9%, IZo = 63%, and I$ = 28% (superscript "0" refers to values for the pure solvent). The experimental setup and data processing have been described previ0us1y.l~ The results are presented in terms of the variations of the 0-Ps lifetime, r3,because the probability of observing quenching reactions is higher for 0-Ps, owing to its longer lifetime and higher yield. Also, the parameters related to this species are more easily accessible, because of the time resolution of the apparatus. Materials The measurements have been performed on three polyanions: polyphosphate (PP), poly(styrenesu1fonate) (PSS), and an alternating copolymer of maleic acid and methyl vinyl ether (Gantrez). The sample of PP, which had a polymerization degree of 420, was a gift from Professor U. P. Strauss (Rutgers University). The sample of PSS (purchased from Serva or K and K) had a molecular weight of about 70000. Gantrez was purchased in the form of an alternating copolymer of maleic anhydride and methyl vinyl ether. It was transformed into a polyanion and purified following the procedure previously described." Its polymerization degree was not known. Previous studies have shown that PSS is a polyion to which counterions are only territorially bound because of its low surface charge d e n ~ i t y . On ~ the other hand, a sizable fraction of divalent counterions (with respect to the total number of polyion charged groups) can be site bound on PP or on totally neutralized Gantrez.+13 An interesting property of Gantrez is that its charge density can be continuously increased by increasing its neutralization degree, in contrast to PP and to PSS for which the monomeric units always bear a single charge, irrespective of the neutralization degree. Thus, Gantrez can in principle be used to determine the onset of site binding. The changes in the Ps lifetime were induced by adding Co2+salts (chloride or perchlorate) to the polyelectrolyte solutions partly or totally neutralized by NaOH or by tetramethylammonium (TMA) hydroxide (TMA+ is a rather large ion which can easily be displaced from the polyion sites by alkali metal ions or by divalent ions). Co2+ has been selected owing to its high reaction rate constant with Ps16and also because its interaction with the above three polyanions has been previously investigated.*13 Results and Discussion Before the polyanion-Co2+ systems were investigated, it appeared necessary to perform measurements in aqueous solutions of cobalt salts to determine the rate constant of the Co2+/Psreaction and to explore the influence of the temperature and of the solution viscosity as the presence of polyelectrolytes usually largely increases the latter. Co2+Reactivity in Aqueous Solutions. The variations of l/ r3 vs. CoC12concentration at various temperatures, (14)A. Haessler, J.4. Abb6, and G. Duplltre, Nucl. Zmtrum. Meth174,317 (1980). (15)A. G.Maddock, J.-C. Ab&, and A. Haessler, Chem. Phys., 17,343 (1976). (16)R. E.Green and R. E.Bell, Can. J.Phys., 36,1684 (1958).
ods,
Zana et al.
c 1 1 r---
0.4 0
aos
0.10
015
C(M
Flgure 1. Variations of l l r , with CoCI, concentration in water at various temperatures.
TABLE I: Co2+/PsReaction Rate Constant in Various Aqueous Solutions of Poly(oxyethy1ene glycol) CPOE,
wt %
T,K
0.2
294 3 13
0.4
333 353 294 31 3
k ' , ns-I mol-' dm'
vm, CP
1.5
1.84
1.8
2.1 2.5 1.5 2.0
1.21 27 20
shown in Figure 1, are linear. They can be described by eq 1, which is easily derived by solving the kinetic equal / r 3 = l/r30 k'C (1)
+
tions related to most Ps reactions, in terms of probabilities.' 73 and r? are the 0-Ps lifetimes at concentration C of solute and in the pure solvent, respectively, and k'is the quenching rate constant. The resulting values for this last parameter at 294,313,333, and 353 K are 1.7, 2.0,2.1, and 2.4 ns-l mol-l dm3, respectively. These values give a linear plot with T / q ( q = viscosity), but the intercept is not zero, which reveals that the reaction is not strictly diffusion controlled. This is confirmed by the activation energy, E(k3 = 0.05 eV, calculated on the basis of k'= k,' exp[-E(k?/kT] where k is the Boltzmann constant. E(k3 differs considerably from E(q) = 0.15 eV deduced from the temperature dependence of the viscosity. These results point to some peculiarity in the mechanism of the Co2+/Ps reaction. A similar conclusion may be drawn from the fact that l / r 3 is linear with C. This behavior is not expected on the basis of a pure conversion reaction' and may indicate some contribution of oxidation reactions to the Co2+/Psinteraction. Additional experiments involving other PATS are in progress in the laboratory to elucidate the detailed mechanism of this interaction. The influence of the viscosity on k 'has been investigated by using high molecular weight poly(oxyethy1eneglycol) (POE). This polymer is unreactive toward Ps, and its presence in the reaction medium greatly increases the macroscopic viscosity, qm, viz.,the viscosity measured by using conventional means. The k'values obtained for two polymer concentrations and at various temperatures are given in Table I, together with the viscosity vm measured with a flow viscosimeter. No simple correlation appears between vm and k', and the macroscopic viscosity does not very significantly influence the quenching rate constant. These data are consistent with previous works which have
The Journal of Physical Chemistry, Vol. 86, No. 8, 1982 1459
Polyion-Counterion Interactions
- ,
OI2
t
0 10.
UL
1
0
02
04
08
J
oa
r
0
1
2
3
4
I
I
r
Flgure 2. Variations of 1/r3 with r (and C&l2 concentration, upper scale) In a 0.114 M NaPP solution at (0)294, (A)313, and (0)333 K.
Flgwe 3. Variations of 117, with r in TMA-PSS solutions of concentrations of (A)0.067, (0)0.134, and (0)0.221 M. Insert: variation of k'(PSS) with C,,,. Temperature: 294 K.
TABLE 11: Influence of the Temperature on the Co2+/Ps Reaction Rate Constant, k ' , in Aqueous Solutions of Polyphosphate k ' , ns" T,K ro mo1-I dm3
TABLE 111: Influence of Various Parameters on the Co2'/Ps Reaction Rate Constant, k ' , in Aqueous Solutions of Poly(styrenesu1fonate) k ' , ns-1 CPSS, mol-' polymer Coz+salt mol dm-3 T , K dm3
294 31 3 333
0.42 0.30 0.31
1.2 2.6 2.6
shown that, in the presence of a polymer, the rate constants of diffusion-controlled reactions do not depend on the macroscopic viscosity but depend primarily on the microscopic viscosity, which is essentially that of the solvent."J* At the POE concentrations used, some slight change in the microviscosity cannot be excludedlg accounting for the small effects observed on k'. Polyelectrolyte Solutions. The data concerning the solutions of PP, PSS, and Gantrez are first presented; a general discussion follows in the last section. In the presentation of the results, the quantity r = 2C/CpE will be used, defined as the ratio of the concentration C of added Co2+expressed in equiv dm-3 to the polyelectrolyte concentration CpEin mol of monomer ~ l m - ~ . Owing to the s m d value of k' measured in pure water for the Co2+/Psreaction, measurable variations of r3 with C can only be expected if C and, therefore, CpE are higher than about 0.03 mol dmF3. Polyphosphate Solutions. The variations of 1/r3 with r (and C) in a 0.114 mol dm-3 solution of NaPP at three temperatures are shown in Figure 2. It appears that 1/r3 remains constant until r = ro N 0.3 - 0.4, depending on T,then increases linearly with r. The following analytical expression has thus been fitted to the experimental data: for r > ro = ~ / T ~ O k'(C - Co) (2) where = r°CpE is the Co2+concentration corresponding to r = rO. For r < rO, l / r 3 = 1/73. The various parameters deduced from the fitting are given in Table 11. At 294 K, k'is significantlylower than the value obtained in aqueous solutions, but not at the other two temperatures. The data thus indicate that, in the presence of NaPP, the Co2+ions are reactive toward Ps only above a critical ratio, rO. Similar qualitative and quantitative conclusions
+
(17) P.Turq, B.Brun, and M. Chemla, J. Chim. Phys. Phys.-Chim. Biol., 70,661 (1973). (18) H.Morawetz, "Macromoleculesin Solutions", Vol. XXI, H. Mark, C. S. Marvel, H. W. Melville, Eds., Wiley, New York, 1975, Chapter VI. (19) T. Malinski and Z. Zagorski, Polymer, 20, 433 (1979).
TMAPSS
CoCl, CoCl,
coc1, CoCl, Co(ClO,), NaPSS
coc1,
0.067 0.134 0.134 0.221 0.126 0.134
294 294 333 294 333 294
1.7 1.4 2.1 1.3 1.6 1.3
have been drawn by Spegt et aL9from the measurements of the water proton chemical shift by NMR in the same systems and also from density measurements. It seems reasonable to ascribe a common origin to these various observations. Poly(styrenesu1fonate) Solutions. The purpose of the experiments was to study the effects of the polyelectrolyte concentration CpE,of the temperature T, and of the nature of the Co2+salt on the quenching rate constants derived from the 1/r3 vs. r variations. Such variations are illustrated in Figure 3 for three polyelectrolyte concentrations. In all cases, l / increases ~ ~ linearly with r in the whole range of concentrations, in contrast to what was found in the PP solutions. Significantly enough, no peculiarity was found either in the previous measurements of the water proton chemical shift by NMR and of the solution density under very similar experimental conditions? Fitting eq 1 to the data leads to the values of k' listed in Table 111. An examination of the data leads to the following remarks. (i) The Ps quenching ability of Co2+decreases when CpE increases. However, k' is the same as in pure water in the less concentrated solution, CPss = 0.067 mol dm-3, at 294 K. (ii) At Cpss = 0.134 mol dm-3, k'increases with T,but while at 294 K its value is 18% less than in water, this difference amounts to only 4% at 333 K. This trend is similar to that observed in the POE solutions: the difference between the k'values in the presence and in the absence of polymer decreases when T increases. This, together with the preceding remark, suggests an effect of the viscosity on k ! (iii),k'is decreased by about 30% when the Co2+co-ion is C104- instead of C1-, at 333 K. (iv) k' appears to be independent of the nature of the neutralizing counterion, Na+ or TMA+. Gantrez Solutions. The variations of 1 / vs. ~ r ~(and C) in a 0.1 mol dm-3 Gantrez solution and for the degrees of
1480
The Journal of Physical Chemistty, Vol. 86,No. 8, 1982
..-.
0
0 02
0 04
006
ary
0 52 0
02
04
06
r
c
Flgure 4. Variations of 1/73 with r (and CoCi2 concentration, upper scale) in a 0.1 M TMA-Gantrez solution for neutralization degrees, a, of 0.0 (O),0.5 (0),and 0.95 (A),at 294 K.
TABLE IV: Influence of the Neutralization Degree, 01, on the Co7+/PsReaction Rate Constant in Aqueous Solutions of Gantrez k ’ , ns‘’ (Y ro mol-’ dm3 0 0.5 0.95
0 0 0.33
1.6 1.6 1.5
neutralization a = 0,0.5, and 0.95 are shown in Figure 4. While these variations are linear over the whole r range for a = 0 and 0.5 similarly to the PSS case, the curve for a = 0.95 resembles that for the PP solution, Co2+appearing to be reactive only at above a critical value, P, of r. Fitting eq 1to the data for a = 0 and 0.5 and eq 2 to the data for a = 0.95 leads to the parameters listed in Table IV. For a = 0 and 0.5, k’ is the same as in the aqueous CoClz solutions in the absence of polyelectrolyte, while it is lower for a = 0.95 when r > ro. Again, the results for a = 0.95 are in agreement with NMR and density data which also exhibit a break at r = 0.3 - 0.4.1° Discussion. Influence of the Chemical Environment of the Ion on Its Reactivity with Ps. One of the most striking features pointed out in the preceding sections is the definite analogy between our data and those obtained by using other techniques. In PP and Gantrez ( a = 0.95) solutions, the invariance of the water proton chemical shift up to r = P has been interpreted as resulting from the site binding of the metal ion with total loss of the first hydration shell. Density measurements support this interpretati~n.~ On this basis, the important conclusion may be inferred that site-bound, totally dehydrated, divalent ions are not reactive toward ps. Concerning the Co2+-PSS system, the NMR measurem e n t have ~ ~ shown that the first hydration shell of Co2+ is not modified by the interaction with PSS, because of the very weak surface charge density of the polyion which should be regarded as a charged cylinder rather than as a charged wire. This result has been confirmed by the dvolume changes associated with the divalent ion-PSS interaction compared to those observed with the divalent ion-PP system.2o Therefore, the fact that, in the presence of PSS at low concentration, k’ is very close to the rate constant in aqueous solutions is consistent with these findings. (20)V. P.S t r a w and Y. P. hung, J.Am. Chem. SOC.,87,1476(1965).
Zana et al. The results for the Co2+-Gantrez(a = 0 and 0.5) system resemble those with PSS, showing no effect comparable to those observed with PP and with Gantrez ( a = 0.95). Most likely, because of the very weak autodissociation of the carboxylic groups, the charge density of the polyion is insufficient to bring about site binding of the cation with total dehydration. A similar conclusion arises from ultrasound absorption measurements, as an excess ultrasonic absorption is only observed at above r = 0.4-0.5 with Gantrez solutions.21 Reactivity of the Condensed Ions with Ps. Our data, especially those concerning PSS and Gantrez ( a = 0 and 0.5), do not show any peculiar behavior of 1 / in~the~range of r values where the added Co2+ions are no longer condensed on the polyion. On the basis of the Manning the expected values of r at the saturation of the condensation effect are 0.83 for PSS and PP and 0.42 for Gantrez ( a = 0.5). Thus, in contrast to other properties related to the Co2+ions such as the EPR signal intensity and the nuclear relaxation time which show a break at r = 0.85 for PSS and for PP,23the rate constant for the reaction of the ions with Ps appears to be only very slightly affected, if at all, by the condensation. The quenching ability of the Co2+ions is the same whether these ions are free or involved in an ion pair in which the anion has only penetrated into the outer hydration shell of the cation (“outer-sphere complex”). Viscosity Effects. The influence of the viscosity on the reactivity of Co2+toward Ps is difficult to assess in the case of polymer solutions. In principle, information can be obtained by examining the results related either to modifications in the solution (concentration changes and/or effect of the nature of the polymer) or to temperature changes . For the PSS solutions, the ratio of the Co2+/Psreaction rate constants in the solution, k’(PSS), and in the aqueous solutions, k’ (H20),is close to unity at low PSS concentration but decreases when Cpss increases. The curve for the variations of k’(PSS) vs. CpSS has a sigmoidal shape (see insert in Figure 3), reminiscent of that reported by Malinski and Zag~rski’~ for the changes of the microviscosity with the polymer concentration. Examination of the various k’values for the polyelectrolyte solutions investigated is not very instructive, owing to the differences in the polyelectrolyte concentrations and molecular weights used, due to experimental constraints. More precise information may be expected from the measurement of k’ in solutions of an unreactive polymer as a function of its molecular weight and concentration since these two parameters should have distinct effects on the micro- and macroviscosity. The effect of temperature has been investigated for the PP and PSS solutions. In either case, k’ increases with T with a temperature coefficient, Ak ’/AT,higher than in the aqueous solutions; k ’(PP)becomes even higher than k’(H,O) at high T. A similar trend is observed with the POE solutions. Thus, the presence of a polymer seems to enhance the apparent Co2+/Psreaction rate constant with respect to the aqueous solutions, when T increases. Two explanations may be suggested for this unusual behavior: (i) The condensation of metal ions, resulting in associations similar to solvated ion pairs, may give higher lability to the solvation water molecules of the ions through an increase in the rate of exchange of these molecules; 24 this (21)C.Tondre and R. Zana, J. Phys. Chem., 75,3367 (1971). (22)G.S.Manning, J. Chem. Phys., 51, 924,934,3249 (1969). (23)P.C.Karenzi, B. Meurer, P. Spegt,and G. Weill, Biophys. Chem., 9,181 (1979).
J. Phys. Chem. 1082, 86, 1461-1465
might ease the access of the ion to Ps. However, the fact that k ' d w not change for r values much higher than those corresponding to the condensation limit makes this interpretation unlikely. (ii) When one considers that POE behaves similarly to the polyions, the changes in k' can arise from viscosity effects as yet unclear. The mechanism of the reaction between Ps and metal ions is not yet well understood. If part of the quenching is undoubtedly due to spin conversion, other reactions may compete with this process with varying contributions as the temperature is changed. It is possible that the polymer affects these various processes. More experiments are obviously needed for a better understanding of the mechanism of the reaction between Ps and Co2+ions.
Conclusion The present work demonstrates that the positron life(24)H. Diebler, M. Eigen, G. Ilgenfritz, G.Maass, and R. Winkler, Pure Appl. Chem., 20, 93 (1969).
1481
time spectroscopy can give information which is twofold, concerning the fields of Ps chemistry and of polyaniondivalent cation interactions. Regarding the Ps reactions, we have shown that divalent ions bound with loss of their first hydration shell are not reactive toward Ps. On the other hand, condensed ions which have retained their hydration shells have essentially the same quenching ability as in pure aqueous solutions. This would suggest that Ps must enter the first hydration shell of the divalent ion before reacting. For the polyion-cation interactions, our experiments have yielded results in qualitative and quantitative agreement with those obtained by using other techniques for the Co-PSS, Co-PP, and Co-Gantrez (a = 0.95) systems, thus pointing to the potential suitability of the positron annihilation technique in this field of research.
Acknowledgment. S.M. thanks the French Foreign Ministry and the Mexican National Council of Science and Technology (CONACYT) for financial support.
Thermochemical Investigations of Gas-Liquid Chromatography. Partition Coefficients of Inert Solutes on Self-Associating Binary Solvent Mixtures William E. Acree, Jr. Department of Chemistry, University of Kansas, Lawrence, Kansas 66045 (Received:August 21, 1981; I n Final Form: Novembr 13, 1981)
A conventional nonelectrolyte solution model which has led to successful predictive equations for the partial molar excess properties of a solute in simple binary solvent systems is extended to include self-associatingsolvent components. An expression is developed and tested for its ability to describe gas-liquid partition coefficients in mixed solventsfrom measurements in the pure solvents. For n-hexane,n-heptane, and cyclohexane on blended mixtures of n-hexadecaneand n-octadecane with N,N-dibutyl-2-ethylhexylamide,the newly derived expression is found to describe the chromatographic data to within 2%.
Introduction During the past decade, gas-liquid chromatography has proven to be an accurate and convenient method for measuring vapor-liquid equilibria data for nonelectrolyte systems. This is particularly true in the low concentration regions (infinite dilution) normally inaccessible by conventional static vapor pressure techniques. The data generally obtained include the chromatographic partition coefficient KR and the Raoult's law activity coefficients of the volatile solute (component A) at infinite dilution 7 - A in the stationary liquid phase. Binary liquid mixtures have also been investigated by a large number of individuals. There are two main purp e s for this type of investigation. The first is the attempt to control the selectivity of a chromatographic column through the use of mixed liquid phases and the second is an attempt to measure equilibrium constants for weak association complexes, such as charge transfer and hydrogen-bonding complexes, between the solute and one of the solvent components. This later application of gasliquid chromatography has invoked a great deal of controversy, especially in regards to the appropriate thermodynamic description of mixed liquid phases.'-" (1)J. H. Purnell and J. M. Vargas de Andrade, J.Am. Chem. SOC.,97, 3585 (1975). 0022-3654/82/2086-1461$01.25/0
Purnell, Laub, and their co-~orkersl-~ have shown that solute retention behavior with binary liquid mixtures (B + C) can be described by a simple volume fraction average of the partition coefficients on the pure solvents (KORcB, and K'R(C)) (2)J. H.Purnell and J. M. Vargas de Andrade, J. Am. Chem. SOC.,97, 3590 (1975). (3)R.J. Laub and J. H. Purnell, J . Am. Chem. SOC.,98, 30 (1976). (4)R.J. Laub and J. H. Purnell, J . Am. Chem. SOC.,98, 35 (1976). (5)R.J. Laub, D. E. Martire, and J. H. Purnell, J. Chem. Soc., Faraday Trans. 2,213 (1978). (6)M. W. P. Harbison, R. J. Laub, D. E. Martire, J. H. Purnell, and P. S.Williams, J. Phys. Chem., 83, 1262 (1979). (7)C. L. Young, J . Chromatogr. Sci., 8, 103 (1970). (8) D.E.Martire, Anal. Chem., 48, 398 (1976). (9)H. L. Liao, D. E. Martire, and J. P. Sheridan, Anal. Chem., 45, 2087 (1973). (10)D.E.Martire, Anal. Chem., 46. 1712 (1974). (11)A. J. Ashworth and D. M.'HOoker, J. Chromatogr., 131, 399 (1977). (12)P.F. Tiley, J . Chromatogr., 179, 247 (1979). (13)R. W. Perry and P. F. Tiley, J. Chem. SOC.,Faraday Trans. I, 74, 1655 (1978). (14)s. D. Christian, E. E. Tucker, and A. Mitra, J. Chem. SOC., Faraday Trans. I, 73, 537 (1977). (15)E. E.Tucker and S. D. Christian, J. Am. Chem. Soc., 100,1418 (1978). (16)W.E.Acree, Jr., and G. L. Bertrand, J. Phys. Chem., 83, 2355 (1979). (17)C.-F. Chien, M. M. Kopecni, R. J. Laub, and C. A. Smith, J. Phys. Chem., 85, 1864 (1981).
0 1982 American Chemical Society
