Anal. Chem. 1983, 55,1072-1077
1872
Although background values due to contamination will limit the lower detection level, these levels can be easily monitored on the HID because of its sensitivity. Careful sample introduction procedures will also aid in limiting background problems, and Once a constant level is attained, determinations can readily be made on a routine basis with a high degree of precision. Registry No. Water, 7732-18-5; 1,3-butadiene,106-99-0;sulfur hexafluoride, 2551-62-4.
LITERATURE CITED Fisher, K. Agnew. Chem. 1935, 48, 394. Hachenberg, H. “Ifidustrial Gas Chromatographic Trace Analysis”; Heyden: London, 1973; pp 1933-1941. Egovlile, M. J.; DellaMonica, E. S. J . Chromatogr. 1981, 212, 121-125. AuBeau, R.; Champeix, L. et Mme.; Reiss, J. J. Chromatogr. 1984, 16, 7-21.
(5) Kaiser, R. Chromatographla 1989, 2, 453-461. (6) Knight, H. S.; Weis, F. T. Anal. Chem. 1982, 34, 749-751. (7) Latif, S.; Haken, J. K.; Wainwright, M. S. J. Chromatogr. 1983, 258, 233-237. (8) Musha, S.; Nishimura, T. Bunsskl Kagaku 1985, 14, 803. (9) Erley, D. S.Anal. Chem. 1957, 8 9 , 1564. (10) Hachenberg, H. “Industrial Gas Chromatographic Trace Analysis”; Heyden: London, 1973; pp 133-141. (11) Andrawes, F. F.; Brazell, R.; Gibson, E. K. Anal. Chem. 1980, 52. 891-896. (12) Andrawes, F . F.; Bayer, T.; Gibson, E. K. J . Chromatogr. 1981, 205, 4 19-424. (13) Andrawes, F. F.; Bayer, T.; Gibson, E. K. Anal. Chem. 1981, 53, 1544-1545. (14) Andrawes, F. F.; Gibson, E. K.; Bafus, D. A. Anal. Chem. 1980, 5 2 , 1377-1379. (15) Mindrup, R. J . Chromatogr. Scl. 1978, f 6 , 380-389. (16) Wilhite, W. F.; Hollis, 0. L. J . Chromatogr. Sci. 1988, 6 , 84-88. (17) Wang, W.; Ding, X.; Wu, X. J. Chromatogr. 1980, 199, 149-159.
RECEIVED for review May 2, 1983. Accepted July
1, 1983.
Coadsorption of Mixed Anionic and Cationic Surfactants in Reversed-Phase Liquid Chromatography Way-Yu Lin, Muoi Tang, John J. Stranahan, and Stanley N. Deming*
Department of Chemistry, University of Houston, Houston, Texas 77004
The combined effects of mixed sodium octanesulfonate and octylamine surfactants on the liquid chromatographic retention times of anillne, phenylethylamlne, benzenesulfonic acid, and chromotroplc acld are described by a thermodynamic model that assumes electrostatic interaction of charged solutes with anionic and catlonlc surfactants in the adsorbed phase.
The intentional addition of charged surfactants to aqueous mobile phases has proven to be a valuable factor in achieving improved separation of ionic mixtures in reversed-phase liquid chromatography (1-7‘). The separation of charged solutes can be enhanced either by increasing their retention times with surfactants of charge opposite to that of the solutes or by decreasing their retention times with surfactants of charge similar to that of the solutes (8, 9). To date, however, the use of mixed anionic and cationic surfactants does not appear to have been exploited in reversed-phase liquid chromatography. A mixture of anionic and cationic surfactants will exhibit marked deviations from ideal mixing of individual components in various physicochemical properties such as surface tension, conductivity, viscosity, solubilization, and solubility (10-12). Although anionic and cationic surfactants are not generally used together for applied purposes (13), studies of their coadsorption are of fundamental interest because of the enormous synergistic effects they have on surface and interfacial properties ( I O ) . In this paper, we present the results of an experimental study for measuring coadsorption and interaction effects between anionic and cationic surfactants using reversed-phase high-performance liquid chromatography. A retention model is derived that describes the retention behavior of different solutes in this mixed system.
THEORY Stranahan and Deming (14) have recently proposed a quantitative, thermodynamic model based on the assumption
that simple Langmuir adsorption of an added, charged surfactant at the stationary phase/mobile phase interface is one of the major factors governing the retention behavior of solutes in ion-interaction chromatography. Six basic assumptions were adopted from the work of Locke (15): either a porous microparticle or superficially porous silica support is used; surface hydroxyl groups on the silica support are all chemically bonded to an alkyl group; the organic layer is not cross-linked or polymerized; simple adsorption occurs which produces a monolayer; the eluent may contain one or more solvents, small samples are used to approximate infinite dilution. Locke’s model (15) can be stated
where the subscripts i, 1, and a correspond to the solute component I, the bulk liquid (mobile) phase, and the adsorbed (stationary) phase, respectively, Ki is the distribution coefficient of component I, Vo is the average molar volume, yi is the activity coefficient of component I, u is the interfacial tension of the system, uio is the interfacial tension of pure I in equilibrium with the solid adsorbent; si represents the area on the surface occupied by 1mol of I, R is the gas constant, and T is the temperature. In eq 1,the terms R, T , u p ,si, yil, Vlo, and Vao will be constant or approximately constant for a given component I and a given chromatographic system where only the concentration of surfactant modifiers is varied. We now consider the effects on Y~~of adding surfactants to the system. Adsorption of One Surfactant. We first consider the adsorption of a nonreacting surfactant on the surface of the stationary phase. The surfactant molecules may be thought of as adsorbing at so-called “vacant sites” (S) on the stationary phase surface. The adsorption process for an anionic surfactant A can be viewed as A+S*AS (2) The equilibrium constant KA is given by
0003-2700/83/0355-1872$01.50/0 Q 1983 American Chemical Societv
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
(3)
ct
=
cS
+ CA,ads + CC,ads
1873
(16)
where CA, Cs, and CAS are the volume or surf‘ace concentrations of anionic surfactant, vacant sites, and adsorbed anionic surfactant, respectively. A t equilibrium
Substituting eq 14 and 15 into eq 16 gives
CAS= KACACS (4) Similarly, for cationic surfactant C, the adsorption process can be expressed as
Finally, the surfacie mole fractions of adsorbed anionic surfactants XA and adlsorbed cationic surfactants xc can be expressed as
C+SGCS
ct = c s [ 1 + KACA + KCcC + 2KCACc]
(17)
(5)
The equilibrium constant, Kc is given by
K c := Ccs/CcCs
(6)
where Cc and Ccs are the volume or surface concentrations of cationic surfactant and adsorbed cationic surfactant, respectively. At equilibrium
ccs, = KCCCCS (7) Coadsorption of Anionic and Cationic Surfactants. The synergistic effect on surface pressure for mixed solutions of cationic and anionic surfactants has been studied extensively (10, 11, 16). For example, mixing dilute solutions of sodium dodecylsulfate-dodecyltrimethylammonium bromide a t a 1:l mixing ratio and a total concentration of 3.0 X mM causes the surface pressure to increase by more than 40 dyn cm-l compared to an 8 dyn cm-l increase for a 3.0 X mM concentration of dodlecyltrimethylammonium bromide alone (16). This is attributed mainly to the result of ‘‘strong additional adsorption of the double long-chain salt dodecyltrimethylammonium dodecylsulfate consequent on mixing” (16).
In our view, charged surfactants will be “coadsorbed” with the surfactant of opposite charge already on the surface. The coadsorption process can be written for anionic surfactants as
A t CS
~t
ACS
(9
Likewise, for cationic surfactants C t AS
F?
CAS
(10)
where CAS stands for the coadsorbed pair of surfactants that was produced by this alternative mechanism. The corresponding adsorption equilibrium constant is
KCA:= CCAS/CCCAS
R T In
Yia
(aij
=
UijXj:
+ aikxk: + UimXm: +
+ a i k - ajk)XjaXka + (aij + a i m - ajm)XjaXma + (aik + a i m - akm)XkaXma (20)
where the a’s are the (energiesof interaction in the adsorbed phase between the paiirs of molecules indicated by subscripts and the x’s are the mole fractions of the indicated components in the adsorbed phase. If I, J, and K are charged molecules, then the interaction of these solute molecules with the neutral solvent is relatively much weaker than the strong interaction of the charged molecules with each other. Thus, we may assume ai, = ajm = a k m z 0. Further, for a system consisting primarily of M[, J, and K, xlais comparatively small, and xma= 1 - xj, Thus, without great error, eq 20 can be written as
(8)
where ACS stands for the coadsorbed pair of surfactants that was produced by this mechanism. The equilibrium constant KAC for anionic surfactants adsorbing at sites next to already adsorbed cationic surfactants is given as
KAC CACS / CACCS
Activity Coefficient in the Adsorbed Phase. For a quaternary system consisting of solvent molecules M and three kinds of solute molecules I, J, and K in equilibrium with the adsorbed phase, an activity coefficient for solute molecule I in the adsorbed phase (ria) can be defined to a first approximation following the development of Lucassen-Reynders (10)
(11)
Lucassen-Reyndem (10) has pointed out that many surfactant systems ”cannot be expected to comply with the rigid conditions of the molecular model leading to” eq 21; i.e., that (a.Jk ) l / * = 9 (aih)l/z.However, instead of omitting the ajk term, “for such systems the parameter ... is better left in ... as a phenomenological parameter to be assessed independently” (10). Retention Model for the System. Because of the already low interfacial tensions of the methanol-water eluents used in this study, the effect of the surfactants on the interfacial tension term of eq 1 can be neglected. The resulting eq 1can be expanded by substituting eq 21 for the interaction term to give
-
At equilibrium, the total concentration of the additional adsorbed surfactants CAC is equal to the sum of CAcsand Ccm, This may be obtained from eq 9 and 11 as
CAC:= CACS+ CCAS= KACCACCS+ KcACCCA~(12) Substituting eq 4 and 7 )into eq 12 gives C A C = KCACcCs (13) where K = KAcKc + Kc/,KA. The equations for the total concentrations of adsorbed anionic surfactants CA,& and adsorbed cationic surfactants can be expressed as
CA,ads =
CAS
CC,& = c:cs
Finally, letting I, J , and K represent the solute, anionic surfactant, and cationic surfactant, respectively, and substituting the capacity factor ki for the distribution coefficient Ki (ki is directly proportional to Ki), we obtain a model that can be used to describe the retention behavior of a solute in a reversed-phase ion-interaction liquid chromatographic system with mixed anionic and cationic surfactants
+ C A C = CS[KACA+ KCACC] (14)
+ CAC= Cs[KcCc + KCACC]
(15) The total concentration of adsorption sites on the adsorbed surface is given in this case by
where Poi is a collection of constant terms (including thLe proportionality constant p’o between ki and KJ, P A i is a measure of the interaction between A and I, is a measure
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
1874
Table I. Retention Times for Solutes
[CI, mM
[AI, mM
0
0 1
1.92 1.93 3.08 3.11
2 5 1
0 1 2
5 2
oa 1
2a 5 5
0 1
2 5
Table II. Estimated Parameters
observed retention time, min phenyl- benzene- chromosulfotropic ethylacid nate amine aniline
3.44 3.40 3.77 3.75 1.61 1.62 2.24 2.22 2.45 2.45 3.04 3.04 1.59 1.60 2.05 2.02 2.08 2.13 2.67 2.66 1.56 1.55 1.84 1.83 2.02 2.05 2.24 2.24
2.14 2.16 4.08 4.10 4.68 4.66 5.67 5.59 1.69 1.68 2.58 2.58 3.45 3.46 4.18 4.20 1.68 1.64 2.40 2.37 2.68 2.72 3.60 3.52 1.58 1.58 2.10 2.11 2.47 2.47 2.94 2.91
2.21 2.24 1.49 1.48 1.45 1.43 1.49 1.50 3.00 2.99 1.86 1.85 1.72 1.72 1.58 1.58 3.03 3.01 2.10 2.07 2.11 2.12 1.78 1.74 3.46 3.44 2.50 2.51 2.09 2.11
1.96 1.94
2.36 2.37 1.28 1.28 1.25 1.24 1.29 1.26 4.45 4.42 1.68 1.70 1.51 1.50 1.36 1.36 4.59 4.54 2.08 2.07 2.13 2.17 1.54 1.56 6.04 6.05 3.02 3.02 2.16 2.16 1.90 1.90
a Data were not included in model fitting because of a pH discrepancy in making up the eluents.
of the interaction between C and I, and PACis a phenomenological parameter to be assessed independently (10).
EXPERIMENTAL SECTION A four-level, two-factor (42)factorial design (17, 18) was used to specify eluent compositions of 16 different mobile phases corresponding to all combinations of four concentrations of octylamine (0, 1.0, 2.0, and 5.0 mM) and four concentrations of sodium octanesulfonate (0, 1.0, 2.0, and 5.0 mM). The experimental order of evaluating the eluents was randomized to minimize the confounding of time trends with factor effects (19). Chromatographic System. The chromatographic system consisted of a Model 6000A solvent delivery system (Waters), a 5 cm X 3.9 mm i.d. Bondapak C18/Porasil (Waters) precolumn, a 30 cm X 3.9 mm i.d. pBondapak Cls main column (Waters), a Model U6K injector system (Waters), and a Model SP8200 ultraviolet (UV) detector (Spectra-Physics) operated at 254 nm. Precolumn and column temperatures were maintained at 25.0 & 0.1 "C by a Model FK constant temperature circulating bath (Haake). Mobile phase flow rate was maintained at 2.0 mL/min. The time equivalent of the void volume was determined by injecting 20 pL of water and measuring the time from injection to the first deviation from base line; to determined in this way was approximately 1.7 min. However, because some of the solutes eluted in considerably less than the system void volume, a value of to = 1.20 was used to calculate k values (8, 9). Additional Instrumentation. The analog UV detector output was recorded by a Model 281 strip-chart recorder (Soltec). Simultaneously, the signal from the detector was digitized by a Model ADC-12QZ analog to digital converter (Analog Devices) interfaced to a Model 9830A digital computer (Hewlett-Packard). Retention times were determined from digitized data. Models were fit by using a nonlinear sequential simplex algorithm (20) on a Model AS/9000N computer (National Advance Systems). The resulting data were transferred to the 9830A for plotting on
aniline poi
pci 0Ai
SSR
-0,476 -0.996 1.356
0.041
phenylethylamine
benzenesulfonate
chromotropic acid
-0.324 -0.805 1.775 0.117
-0.156 0.869 - 1.852 0.049
-0.019 11.589 --3.230 0.096
K , = 2.038 K, = 1.690 K = 0.359 PAC = 0.651
s Surface mole fraction of sodium octanesulfonate vs. concentrations of sodium octanesulfonate and octylamine. See text for discussion. Figure 1.
a Model 9862A plotter (Hewlett-Packard). Mobile Phases and Samples. Mobile phases were prepared by adding the specified amount of octylamine (Matheson Coleman and Bell) and/or 1-octanesulfonic acid sodium salt (Eastman Kodak) to 350 mL of HPLC grade methanol (Fisher) and 630 mL of distilled water in a 1-L volumetric flask. One molar HC104 (Fisher) was added dropwise until a glass electrode indicated pH 3.0. The volumetric flask was then brought to volume with distilled water. The solutes (Fisher and Eastman Kodak) were aniline (AN), phenylethylamine (PEA), benzenesulfonic acid (BSA), and chromotropic acid (CTA). Sample solutions were prepared by adding approximately 50 pmol of an individual solute to 10 mL of each specified mobile phase. Mobile phases and sample solutions were aspirated through 0.45-pm cellulose acetate filters (HAW04700; Millipore) and degassed in a Model ME4.6 ultrasonic bath (Mettler Electronics) before use.
RESULTS AND DISCUSSION Table I contains the observed replicated retention times for all solutes at each of the 16 different solvent compositions. Table I1 contains the best nonlinear least-squares estimates for the parameters of the model (eq 23) fitted to capacity factors for each solute. All parameters were estimated simultaneously by using a total of 112 experimental data points. All of the p's (except PAC)are unique for each solute and all of the K's are unique for each charged surfactant. The sum of squares of residuals (SSR) for each solute is also listed in Table 11. Figures 1 and 2 show pseudo-three-dimensional plots of eq 18 and 19 which predict the surface mole fractions XA and X C , respectively, of each adsorbed charged surfactant as a function of the concentrations of A (sodium octanesulfonate) and the concentrations of C (octylamine) added to the mobile phase. For the p H 3.0 aqueous mobile phase used in this study, sodium octanesulfonate is dissociated and exists as a negatively charged ion with a hydrophobic tail; octylamine is protonated
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983 -1
1875
Table 111. Surface Mole Fractions of Adsorbed Surfactants [CI, mM
[AI, mM
xc
XA
0
0 1 2 5 0 1 2 5 0 1 2 5 0 1 2 5
0 0 0 0 0.628 0.376 0.294 0.212 0.772 0.522 0.425 0.3 21 0.894 0.679 0.581 0.464
0 0.671 0.803 0.911 0 0.440 0.584 0.7 28 0 0.351 0.487 0.634 0 0.254 0.370 0.510
1
2 5-
Flgure 2. Surface mole fraction of octylamine vs. concentrations of
5
sodium octanesulfonate and octylamine. See text for dlscussion.
0 0.671 0.803 0.911 0.628 0.816 0.878 0.939 0.772 0.873 0.912 0,954 0.894 0.934 0.952 0.973
$-
Figure 3. Total surface mole fraction of sodium octanesulfonate and octylamine vs. concentrations of sodlum octanesulfonate and octylamine. See text for discussion.
and exists as a positively charged ion with a similar hydrophobic tail. In the presence of only one charged surfactant, a simple adsorption isotherm is olbtained. This effect is shown by the curve along the far left edge of the surface in Figure 1and by the curve along the near left edge of the surface in Figure 2. The curves rise rapidly at low concentrations (4mM) and increase less rapidly at higher concentrations. The competitive nature of the coadsorption processes in a mixed anionic and cationic surfactant system is evident in Figures 1and 2. At any given bulk liquid concentration of surfactant A (or C), the surface mole fraction of A (or C) actually decreases with increasing concentration of C (or A): because the total number of sites on the surface is constant, the more favorable adsorption of oppositely charged surfactants causes the displacement of some of the surfactant originally adsorbed. Figure 3 is a pseudo-three-dimensional plot of the surface mole fraction of total adsorbed surfactants (xA xc) as a function of the individual eluent concentrations of A and C. As expected, the response surface goes up very sharply at low concentrations of both surfactants and then increases much less rapidly at higher concentrations. Table I11 shows quantitative data for the predicted surface mole fractions of adsorbed anionic surfactant (xA), cationic surfactant (xc),and total surfactants (xA+ xc) at different combinations of surfactant concentrations. The synergestic effect of adsorption of a mixture of anionic and cationic surfactants in the chromatographic system can be seen from Table 111. For example, when [C] = 0 mM and [A] = 2 mM, XA = 0.803; when [C] =
+
Q
0
I
2
3
Ll
5
OCTYLRMINf/ MILLIMDLRA
Flgure 4. Effect of octylarnlne concentration on retention time of aniline at different levels of sodium octanesulfonate concentratlon (0,1, 2, and 5 mM). See text lor discussion.
2 mM and [A] = 0 mM, xc = 0.772; and when [C] = 1mM and [A] = 1mM, xA xc = 0.816. These values indicate thlat the total amount of the mixed surfactants adsorbed on the surface is higher than the amount of either individual surfactant adsorbed on the surface at the same total bulk concentration. The estimated parameters KA and Kc are related to the free energy of adsorption of the charged anionic and cationic surfactants, respectively (13). The estimate of K Ais +2.038 L/mmol; this corresponds to a A G O of adsorption of approximately -6.89 kcal/mol for the octanesulfonate; R0da.kiewicz-Nowak (21) has reported a value of -7.04 kcal/mol for sodium octanesulfonate in aqueous solutions in a study based upon the drop-weight method. The estimate of Kc is +1.69 L/mmol; this corresponds to a A G O of adsorption of approximately -6.78 kcal/mol for the octylamine. Positively Charged Solutes. The pK, of aniline, a weak base, is 4.63 (22); at pH 3.0 used in this study, aniline exists almost completely in its protonated, positively charged conjugate acid form. Figure 4 plots the experimental data and the fitted model for aniline as a function of octylamine concentration at different eluent concentrations of octanesulfonate. As expected (8,9),in the absence of octanesulfonate ion in the mobile phase, the retention time of aniline decreases
+
1878
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
I
I
PHENYLETHYLRMINE
BENZENESULFDNIC RClD
I
I
m
1
0
I
1
,
I
2
3
Y
0
5
1
2
Y
5
q
5
3
OCTYLRMINEi MILLIMDLRR
OCTYLRMlNEi MILLIMDLRR
Flgure 5. Effect of octylamine Concentration on retention tlme of phenylethylamine at different levels of sodium octanesulfonate concentration (0, 1, 2, and 5 mM). See text for dlscusslon.
as the concentration of octylamine increases (see the bottom curve in Figure 4); a t a given eluent concentration of octylamine, increasing the eluent concentration of negatively charged octanesulfonate (the upper curves in Figure 4)causes the aniline ion to be more strongly attracted (or less strongly repelled). The parameter estimate Pa = -0.996 for aniline is related to the interaction energy of aniline and octylamine (10);the negative sign of Pci indicates a repulsive interaction energy between the same positive charges of solute and surfactant. The estimate of PAi = +1.356 is related to the interaction energy of aniline and octanesulfonate (10); the positive sign of PAiindicates an attractive interaction energy between the oppositely charged solute and surfactant. Phenylethylamine, pK, = 9.84 (23),is also expected to exist almost completely in its protonated positively charged form a t pH 3.0. The trends of retention time vs. surfactant concentration are thus the same for phenylethylamine as they were for aniline (see Figures 4 and 5 ) . Again, the negative sign of and the positive sign of PA^ (see Table 11) indicate the repulsive and attractive interaction energy between the solute and the surfactant. Negatively Charged Solutes. The pK, of benzenesulfonic acid, a moderately strong acid, is 0.70 (24);in pH 3.0 solution the solute exists primarily as a negatively charged ion. The plot of retention time vs. the concentration of octanesulfonate is shown in Figure 6. As expected (8,9),the general trend of the curves is opposite to that of the positively charged solutes AN and PEA. Similarly, chromotropic acid disodium salt (4,5-dihydroxy-2,7-naphthalenedisulfonicacid), also a moderately strong acid, is expected to form a negatively charged dianion in the mobile phase. Figure 7 plots the retention times and fitted model for CTA. The difference in retention behavior of CTA and BSA in Figures 6 and 7 can be explained on the basis of charge alone. The anionic benzenesulfonate is singly charged (-1),whereas the anion of chromotropic acid (a disulfonic acid) is doubly charged (-2). The charged surfactants exert either a greater attraction or a greater repulsion on the dianion of chromotropic acid than they do on benzenesulfonate. This is confirmed in the parameter estimates shown in Table 11, where Pci and indicate the interaction energy between solute and surfactant. The estimated values for chromotropic acid ( P a = 1.589 and PAi = -3.230)are almost twice the values for
CHRDMOTROPIC R C l D
-0
1
2
3
DCTYLRMlNEi MlLLlMDLRR
Figure 7. Effect of octylamine concentratlon on retention time of chromotropic acid at different levels of sodium octanesulfonate concentration (0,1, 2, and 5 mM). See text for discussion.
benzenesulfonate (Pci = 0.869 and PAi = -1.852). Again, the positive and negative signs of the values indicate the attraction and repulsion between the solutes and the surfactants. The Parameter DAC. The interpretation of the parameter PAC is not straightforward chemically. Clearly, the positive sign of the estimated value of PACis opposite to the sign that would be expected if PACwere directly related to the interaction energy between anionic and cationic surfactants. Instead, it is probable that PACshould be treated as a phenomenological parameter (10) and be given a different interpretation. Mathematically, the parameter PACis an interaction term that is often included in empirical linear models to account for synergistic effects of two factors on each other (17). In the present case, rearranging eq 23 gives In hi = Poi
+ P C i x C + (PA1 + 0 A C X C ) X A
(24)
+ PACXA)XC
(25)
and
hi = P o i + P A i X A + ( P C i
Anal. Chern. 1983, 55, 1877-1881
An interpretation of eq 24 is that the slope of In lzi with respect to xAis (PAI P A C x C ) ; i.e., the "effect" of XA depends upon xC. Similarly, in eq 25 the slope of In k , with respect to xc PACxA);Le., the "effect" of xc depends upon xk is (Pa Substituting an estimated parameter value for, say, aniline 0 . 6 5 1 ~ ~for ) the xA effect, and (-0.996 + gives (1.356 0 . 6 5 1 ~ for ~ ) the xc2effect; similar effects are seen for the other solutes. In each case, the presence of one surfactant already adsorbed on the surface appears to cause a greater increase (or a lesser decrease) in In k , when the other surfactant becomes adsorbed. However, as seen in Figures l and 2, in. creased adsorption of one surfactant is usually accompanied by decreased adsorption of the other surfactant. Thus, there seems to be a type of "self regulatory feedback" associated with this parameter. The possibility exists that the net effect of the PAC parameter is zero and that it may be removed from the model. Elimination of the PACterm from the model expressed by eq 23 gives a total sum of squares of residuals only slightly larger than before (0.358 vs. 0.304). The values of the remaining parameter estimates change only slightly (35% in the worst case). Thus, whatever the real phenomenological interpretation of the parameter PAC, it does not appear to be a necessary parameter t o adequately model the systems described in this study. Registry No. Sodium octanesulfonate,5324-84-5;octylamine, 111-86-4;aniline, 62-53-3; phenylethylamine, 64-04-0;benzenesulfonic acid, 98-11-3; chromotropic acid, 148-25-4.
+
+
+
ian
(3) Knox, J. H.; Laird, G R. J . Chromatogr. 1976, 122, 17-34. (4) Knox, J. H.; Jurand, J. J . Chromatogr. 1976, 125,89-101. (5) Tomlinson, E.; Jeffeiries, T. M.; Riley, C. M. J . Chromatogr. 1970, 159,315-358. (6) Bidlingmeyer, B. A. J . Chromatogr. Sci. 1980, 18,525-539. (7) Hearn, M. T. W. I n "Advances in Chromatography"; Glddings, J. C., Grushka, E., Cazes, J., Eds.; Marcel Dekker: New York. 1980; Vod. 18, pp 59-100. (8) Bidlingmeyer, B. A.; Deming, S. N.; Price, W. P., Jr.; Sachok, B.; Petrusek, M. J . Chromatogr. 1979, 186, 419-434. (9) Kong, R. C.; Sachok, B.; Deming, S. N. J . Chromatogr. 1980, 1961, 307-316. (10) Lucassen-Reynders, E. H. I n "Progress in Surface and Membrane Science"; Cadenhead, D. A., Danielli, J. F., Eds.; Academic Pres!s: New York, 1976; Vol. 10, pp 253-360. (1 1) Corklll, J. M.; Goodman, J. F.; Harrold, S. P.; Tate, J. R. Trans. Farfitday SOC. 1967, 63, 247-256. (12) Nakamura, A.; Muramatsu, M. J . Colloid Interface Sci. 1977, 6;?, 165-171. (13) Rosen, M. J. "Surfactants and Interfacial Phenomena"; Wiley: New York, 1978. (14) Stranhan, J. J.; Denning, S. N. Anal. Chem. 1962, 5 4 , 2251-22513. (15) Locke, D. C. J . Chromatogr. Sci. 1974, 12, 433-437. (16) Lucassen-Reynders, E. H.; Lucassen, J.; Gibs, D. J . Colloid Interface Sci. 1981, 81, 150-157. (17) Box, G. E. P.; Hunter, W. G.; Hunter, J. S. "statistics for Experiments. An Introductlon to Design, Data Analysls, and Model Building"; Wiley: New York, 1978. (18) Natrella, M. G. "Explerimentel Statistics, National Bureau of Standards Handbook 91"; US. Govt. Printing Office: Washington, DC, 1963. (19) Mendenhall, W. "Introduction to Linear Models and the Design and Analysis of Experimlents"; Duxbury: Belmont, CA, 1968. (20) O'Neill, R. Appl. Statist. 1971, 20, 338-345. (21) Rodakiewicz-Nowak, J. J . Collold Interface Sci. 1982, 85,586-591. (22) Weast, R. C., Ed. "CRC Handbook of Chemistry and Physics", 56th ed.; CRC Press: Cloveland, OH, 1975; p D-147. (23) Rappoport, Z.,Ed. "CRC Handbook of Tables for Organic Compound Identification", 3rd ed.; CRC Press: Cleveland, OH, 1967; p 438. (24) Weast, R. C., Ed. "CRC Handbook of Chemlstry and Physics", 56th ed.; CRC Press: Clieveland, OH, 1975; p D-150.
LITERATURE CITED (1) Wittmer, D. P.; Nuessle, N. 0.; Haney, W. G., Jr. Anal. Chem. 197!i, 4 7 , 1422-1423. (2) Sood, S. P.; Nuessle, N. 0.; Haney, W. G., Jr. Anal. Chem. 1876, 4 8 , 796-798.
RECEIVED for review July 26, 1982. Resubmitted March '7, 1983. Accepted June 16,1983. This work was supported iin part by a grant from1 Chevron Research Co.
Liquid Chromiat ography with Rapid ScanninCJ EIect roc hemicaI Detection at Carbon Electrodes W. Lowry Caudill, Andrew G. Ewing, Scott Jones, and R. Mark Wightman* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Rapld scannlng electroclhemical detectlon has been Investlgated with channel-type! electrochemical flow cells whlch utllize glassy carbon and carbon flbers as working electrode materlals for use with high-performance llquld chromatography or flew injection analysis. Normal pulse voltammetry, back-step corrected nornnal pulse vottammetry, and stalrcase voltammetry were investigated for their utllity with rapid scanning electrochemlcal detectlon. Staircase voltammsgrams, which have beeln subtracted from the background slgnals, were found to give the most useful results. The vottammograms, which tare acquired In flowing streams, can facllltate ldentiflcatlon in sample unknowns. The chromatographic detectlon llmlts are approximately 3 orders of magnitude hlgher than those obtained with amperometric detectlon. However, the technique of HPLC with rapid scanning electrochemical detectlon provides a more complete Identlflcatlon than HPLC with the amperometrlc (dc), pulse, or dual electrode detectlon modes.
High-performance liquid chromatography with electro0003-2700/83/0355-1877$01 .B0/0
chemical detection (LCEC) has become a widely used technique for trace organic analysis. Amperometric (i.e., dc applied potential) is the most common detection mode because it offers high sensitivity (femtomole), requires relatively simple instrumentation, and provides considerable selectivity through judicious selection of the applied potential (I, 2 ) . Improved potential selectivity can be obtained with the use of dual electrodes operated amperometrically ( 3 ) . In the flow cell, the two working electrodes can be placed in parallel, in series, or geometrically opposed in the flow cell and usually are operated at different potentials (4). The increase in selectivity obtained with this approach arises because the differences in the electrochemical properties of various compounds can be exploited (4). Another mode of detection which has been investigated involves potential pulse techniques ( 2 , 5 , 6 ) .Pulse techniques are attractive because the detector response is relatively insensitive to flow rate a t short pulse times (is), because pulse techniques can also be used for increased selectivity and because pulse methods can be used for electrode cleaning (7). However, the use of the pulse mode has not been widespread because of reported higher detection limits which have been attributed to interference by electrode charging currents. 0 1983 American Chemical Society
