1362
Anal. Chem. 1986. 58. 1362-1367
affecting efficiency in PLC are under study in the laboratory and will be the topic of a further study. CONCLUSION The model of Armstrong (1-3) for retention in PLC is well-suited to polar and apolar solutes but also to ionic solutes, provided they bind to micelles either by electrostatic interaction or by comicellization. On the other hand, the model fails if the stationary phase evolves by adsorption of surfactant (as in the case of naked silica-SDS (Figure 4). Ksw values give information about solute affinity for the stationary phases; in spite of the surfactant coverage, the polar nature of the bonded stationary phase is maintained. KMwvalues are independent of the nature of the stationary phase; the measured values are in agreement with those obtained using other techniques ( 4 ) . Registry No. SOBS, 28675-11-8; BTAB, 5350-41-4; CPC, 123-03-5;SDS, 151-21-3;CTAB, 57-09-0; toluene, 108-88-3;caffeine, 58-08-2; benzoic acid, 65-85-0; silica, 7631-86-9. LITERATURE CITED (1) Armstrong, D. W.; Terrili, R. Q . Anal. Chern. 1979, 51, 2160-2163. (2) Armstrong, D. W.; Henry, S. J. J . Llq. Chromatogr. 1980, 3 , 657-662.
(3) Armstrong, D. W.; Stine, G. Y. Anal. Chem. 1983, 55, 2317-2320. (4) Yarmchuck, P.; Weinberger, R.; Hirsch, R. F.; Cline Love, L. J. Anal. Chem. 1982, 54, 2233-2236. (5) Armstrong, D. W.; Nome, F. Anal. Chem. 1981, 53, 1662-1666. (6) Barford, R. A.; Sliwinski, B. J. Anal. Chem. 1984, 56, 1554-1556. (7) Dorsey, J. G.; De Etchegaray, M. T.: Landy, J. S. Anal. Chem. 1983, 55, 924-928. (8) Yarmchuck, P.; Weinberger, R.: Hirsch, R. F.; Cline Love, L. J. J . Chromatogr. 1984, 283, 47-60. (9) De Luccia, F. J.; Arunyanart, M.; Cline Love, L. J. Anal. Chem. 1985, 57, 1564-1568. (IO) Berthod, A.; Girard, I.; Gonnet, C. Anal. Chem. 1988, 58, 1356-1358. (11) Knox, J. H.; Hartwick, R. A. J . Chromafogr. 1981, 2 0 4 , 3-21. (12) Stranahan. J. J.: Deming, S.N. Anal. Chem. 1982, 54, 1540-1546. (13) Stranahan, J. J.; Deming, S. N. Anal. Chem. 1982, 54, 2251-2256. (14) Tang, M.; Deming, S. N. Anal. Chem. 1983, 55, 425-428. (15) Arunyanart, M.; Cline Love, L. J. Anal. Chem. 1984, 56, 1557-1561. (16) Berthod, A. J . Chim. f h y s . 1983, 80, 407-424. (17) Peiizzetti, E.; Pramauro, E. J . f h y s . Chem. 1984, 88, 990-996. (18) Kirkman, C. M.; ChaNat, S.B.; Elliot, W. G.; Stengie, T. R.; Uden, P. C. Pittsburgh, Conference, New Orleans, LA, 1985, paper 1008.
RECEIVED for review August 6, 1985. Accepted January 7 , 1986. This work was supported by the Centre National de la Recherche Scientifique UA 07 0435 and was presented a t the 9th International Symposium on Column Liquid Chromatography, Edinburgh, Scotland, July 1-5, 1985.
Additive Effects on Surfactant Adsorption and Ionic Solute Retention in Micellar Liquid Chromatography Alain Berthed,* Ines Girard, and Colette Gonnet
Laboratoire de Chimie Analytique 3, Universite‘ de Lyon 1, 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France
Twelve adsorptlon isotherms were determlned with two surfactants, two statlonary phases, and three moblle phases. The two surfactants were sodlum dodecyl sulfate and cetyltrlmethylammonlum bromlde. The two statlonary phases were ODS Hypersll, an octadecyl bonded slllca, and SAS Hypersll, a methyl bonded slllca. The three moblle phases were aqueous 0.1 M NaCl and 5 % v/v methanol In water. Above twice the crltlcal micellar concentratlon (cmc), the adsorbed amount of surfactant was almost constant, but the greatest Increase can reach 20%. I n all cases, It reached slmllar quantkles ((4-5) X IO-’ mol/m*). The physlcochemlcalparameters of the Ionic micelles are much more affected by 0.1 M NaCI than by 5 % v/v methanol. Yet, the “saltlng-out” effect compensated for the decrease of the surfactant cmc In the case of NaCl moblle phases. The retention of SIXsolutes of varlous polarles, apolar, polar, anlonlc, and catlonlc, has been studled. The Armstrong model for mlcellar llquld chromatography allowed for calculatlon of the partltlon coefflclents of the solutes (I) between the micelles and the bulk and (li) between the statlonary phase and the bulk phase. NaCl and methanol seem to have a greater Influence on these partltlon coefflclents than on the surfactant coverage of the statlonary phase.
Micellar liquid chromatography (MLC), or pseudo-phase liquid chromatography, as first described by Armstrong (1-3), 0003-2700/86/0356- 1362$01.50/0
uses surfactant solutions as mobile phases for reversed-phase liquid chromatography. The unique selectivity of such systems is due to the fact that a particular solute can partition to an aqueous micelle in one of three ways (1): (i) via electrostatic interactions in the micelle-Stern layer, (ii) via hydrophobic interactions in the hydrophobic core of the micelle, and (iii) via interactions midway between electrostatic and hydrophobic. The two important properties of surfactant molecules, as related to chromatography, are micelle formation and adsorption at interfaces. In MLC, the micelles play the role of the organic modifier, so their influence on retention has been intensively studied (3-7). At surfactant concentration above the critical micellar concentration (cmc), micelles are present and the amount of free surfadant is essentially constant (equal to cmc). This unique property of micellar solutions was used in micellar concentration gradient chromatography (8). The gradient can be run without altering the composition of the stationary phase, i.e., without changing the amount of surfactant adsorbed on the stationary phase (9). In a recent work (IO), we have confirmed that the adsorption isotherms of two ionic surfactants on four bonded silica showed a plateau beginning at a mobile-phase concentration equal to cmc. On the four bonded stationary phases, the adsorbed amount of surfactant reached similar quantities, regardless of the polarity of the bonding material. The aim of this work was to study the effect of two different additives in micellar liquid chromtography. Sodium chloride modifies the hydrophilic and ionic interactions, while methanol 0 1988 American Chemical Society
1383
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
Table I. Physicochemical Properties of the Studied Surfactants surfactant (mol wt) SDS (288.4)
CTAB (364.5)
medium
cmc: mol/L
r,* nm
water water-methanol ( 9 5 4 % v/v) water + NaCl (0.1 mol/L) water water-methanol (95-5% v/v) water + NaCl (0.1 mol/L)
8.2 x 10-3 8X 1.4 x 10-3 8X 9X 2 x 10-4
2.5 2.5 2.8 3.2 -2.5 >3.2
N,’
--
62 60 80 90
-10 >90
Pd
V,l L/mol
ref
0.65 0.6 0.91 0.84
Flgure 1. Adsorbed amount of ionic surfactants on two stationary phases in submicellar concentrations: open symbols, SAS Hypersil; full symbols, ODS Hypersil; (0 0)pure aqueous mobile phases, (A A) NaCl 0.1 moVL mobile phases, (m 0)methanol-water (5-95% v h ) .
Flgure 2. Adsorbed amount of SDS from micellar mobile phases (a) on SAS Hypersli and (b) on ODS Hypersil. See Figure 1 caption. ii
curves obtained for both the surfactants on the two bonded phases resemble L-type curves, according to the Giles classification (17). The slope decreases with increasing surfactant concentration in the aqueous phase because vacant sites become more difficult to find with the progressive coverage of the surface. The surface adsorption onto the stationary phase could occur in at least two ways (18): (i) the hydrophobic one, the alkyl tail is adsorbed and the ionic head group would then be in contact with the polar solution or (ii) the silanophilic one, the ionic head group is adsorbed and the stationary phase becomes more hydrophobic. In the case of SAS Hypersil, a moderately polar stationary phase, surfactant adsorption occurs via mixed hydrophobic and silanophilic interactions (10). In the case of ODS Hypersil, the hydrophobic interactions are favored because, in the pure aqueous mobile phase, the hydrocarbon chains of the brushtype ODS phase are agglomerated (19,20). In this "collapsed state", silanophilic interactions are difficult. NaCZ Influence. Assuming that the adsorbed amount of surfactant is only dependent upon the free surfactant concentration, as the added NaCl decreases the cmc (Table I), it was expected to fiid an adsorbed amount of surfactant lower with NaCl than with pure aqueous mobile phases. However, the reverse was found (Figure 1). For both surfactants and on both stationary phases, the adsorbed amount of surfactant is enhanced by NaC1. This effect has been studied in detail by Bartha et al. (21). They have shown a linear increase of adsorbed amount of sodium butanesulfonate on ODS Hypersil at a constant mobile-phase concentration of the pairing ion with increasing sodium concentration. This "salting-out" effect lowers the ionic repulsions and enhances the hydrophobic interactions. The amount of adsorbed surfactant reaches a plateau for surfactant concentrations higher than cmc (Figure I), which confirms that surfactant adsorption is only dependent upon the free surfactant concentration. Methanol Influence. Low amounts of methanol have little effect on the free surfactant concentration (Table I). Nevertheless, the adsorbed amount of surfactant is decreased by 5% v/v methanol in the submicellar region (Figure 1). The decreasing is slight with ODS Hypersil but it reaches 40% with SAS Hypersil. These differences can be understood by taking into account the physicochemical structure of the bonded ODS layer. According to Scott and Simpson (22),the "collapsed state" of the ODS layer was destroyed when about 5% v/v methanol was present in the mobile phase. When the alkyl chains of the ODS bonded layer return again to the brush form, the specific surface is increased and/or silanols become
I
1-
............. ' .. ,:,. ..... ,,
Flgure 3. Adsorbed amount of CTAB from micellar mobile phases. See Figure 2 caption.
accessible. Methanol reduces the hydrophobic interactions and decreases the amount of surfactant adsorbed. These effects are exemplified by adsorption isotherms of both the surfactants on SAS Hypersil. In the case of ODS Hypersil, this decrease is partially compensated by the disappearance of the ''collapsed state". The setting upright of the alkyl chains allows the insertion of surfactant molecules. Adsorption Isotherms with Micellar Mobile Phases. Adsorption curves, obtained with micellar mobile phases, are shown in Figures 2 and 3. All the curves are of the H type (17);i.e., the amount of adsorbed surfactant increases rapidly and reaches a plateau for surfactant concentration higher than the cmc. The plateau is almost horizontal without any steps in the same concentration range as that indicated by Hung and Taylor (23). For SDS adsorption, a slight slope exists on SAS silica, which shows a further adsorption of SDS in the presence of a very high micellar concentration (10). Recently such an adsorption has been observed by Hinze (24)with a nonionic surfactant on an ODS bonded silica. In our case, the further adsorption above the cmc was unaffected either by sodium chloride or by methanol. As the comments on the NaCl and methanol influence in submicellar mobile phases hold true in micellar mobile phases, the relative positions of the plateaus (Figures 2 and 3) can be explained. The adsorbed amount of SDS on ODS Hypersil is greater with NaCl than in pure water. The adsorbed amount of CTAB on ODS Hypersil with NaCl is identical with that of pure water (Figures 2b and 3b). The "salting-out" effect of NaCl is greater on SDS (common ion effect) than on CTAB
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
1365
Table 11. Slope and Intercept of the Plots of llk‘ vs. the Micellar Concentrationa CTAB in the following media water
solute
SDS in the following media water + NaCl water-methanol (0.1 mol/L) ( 9 5 4 % v/v,
watermethanol
water + NaCl (0.1 mol/L)
water
( 9 5 5 % v/v)
SAS Hypersil Stationary Phase toluene caffeine BTAB SOBS benzoic acid CPC
2.62 0.69 0.83 3.45
0.045 0.072 0.002 0.090
1.67 1.06 0.97 1.92
0.023 0.084
b
b
C
toluene caffeine BTAB SOBS benzoic acid CPC
0.012
3.65 1.18 2.40 4.20
0.70 0.27 0.012 0.122
b
b
b
b
C
C
C
C
C
0.32 1.53 0.28 1.50
0.006 0.165 0.0006 0.028
0.38 0.94 0.145 2.08
0.006 0.118 0.0011 0.021
0.37 0.925 0.114 1.73
0.0075 0.225 0.0005 0.045
b
b
b
b
b
b
C
C
C
C
C
C
0.007
1.20 3.32
0.012 0.235
0.96 2.25
0.0093 0.235
1.25 2.21
0.0175 0.330
b
b
b
b
b
b
c 0.88 1.10
C
C
C
C
C
0.0013
0.775
0.001
0.875
0.0045 0.0025
0.83 1.52
0.0011 0.002
0.71
0.005
5.0
0.40
0.65 6.0
0.0045 0.50
0.64 5.1
0.0075 0.73
b
b
b
b
b
b
c 1.71 1.60
C
C
C
C
C
0.0017 0.0015
1.38 1.43
0.0052
1.56 1.58
0.002 0.002
ODS Hypersil Stationary Phase
0.001
OThe regression coefficient, R, was greater than 0.98 in all cases. For each medium, slope is the left value and intercept is the right one. Nonbinding solute. C Tmuch ~ solute ~ retained. (only ionic strength effect). On SAS Hypersil, the “salting-out” effect seems to be less important, and ion exchange phenomena, enhanced by NaCl(25-27), can occur with surface silanols. The silanols have much greater affinity for CTA+ than for Na+ (28) and no affinity for anionic SDS. This produces an amount of adsorbed CTAB with NaCl greater than the one with pure water (Figure 3a) and the reverse for SDS adsorption on SAS silica (Figure 2a). Retention Study. At surfactant concentrations below the cmc, micelles do not exist, and, as demonstrated by Knox (291, Deming (30),and our previous work (311, the degree of retention was directly related to the surface charge arising from the adsorbed surfactant. With both the surfactants, the retention of neutral species (toluene and caffeine) slightly decreased when the amount of adsorbed surfactant was increased. When an anionic surfactant was adsorbed, the retention of negatively charged solutes (benzoate and SOBS) fell dramatically, whereas the retention of cationic solutes (BTAB and CPC) increased. The reverse occurred with cationic surfactant (31). The same kind of behavior was observed with pure aqueous mobile phases, 5-95% v/v methanol-water phases, and 0.1 mol/L NaCl phases on both stationary phases. Retention with Micellar Mobile Phases. Figures 4 and 5 present the capacity factor, log It’vs. the concentration of SDS and CTAB, respectively, for the ODS columns. Similar results were obtained with the SAS columns. As previously shown (31),benzoic acid and CPC had no affinity for SDS and CTAB, respectively. They act as nonbinding solutes (3). SOBS and CPC were so much retained by CTAB and SDS mobile phases, respectively, that the elution times were too long (more than 3 days at 1 mL/min) (31). The equation derived for the retention of binding solutes was (32)
1.
Y
0 0
J
dk-k+Y+4
- : 0 . 0
SDS c o n c e n t r a t
1
on
i
0.0 0.00
SDS c o n c e n t r a t i o n
.os
.IO
.15
. 2
(mol /L>
Flgure 4. log k’vs. the SDS concentrationin the mobile phase on ODs
in which k’ is the capacity factor, V is the molar volume of the surfactant (Table I), and 4 is the phase ratio V,/V,, the stationary phase volume and the void volume, respectively. C, is the concentration of surfactant in the micellar form (i.e., total surfactant concentration minus cmc). KMw and Ksw are the solute partition coefficients between micelles and the bulk water and between the stationary phase and the bulk water, respectively. KMW and Ksw are dimensionless.
Hypersilstationary phase: (0)aqueous mobile phases, (A)NaClO.1 moi/L moblle phases, (W) methanol-water (5-95 % v/v).
The plot of l/It ’vs. the micellar concentration gave straight lines whose slopes and intercepts are listed in Table 11. It is then possible to obtain Ksw by using Ksw = l/($X intercept) (4) and KMw KMw = (slope/intercept)(l/V) 1 (5)
+
1366
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
Table 111. ICsw Valueso CTAB
SDS
water solute toluene caffeine BTAB SOBS benzoic acid CPC
water ODS SAS
+ NaCl
(0.1 mol/L)
ODS
watermethanol (95-5% v/v)
SAS
ODS
SAS
124 4.1 1800 21
13 3.1 70 7.0
155
46
33
19 12 420 9.4
7.8 840 77
11 140 45
b
b
b
b
b
C
C
C
C
C
140 5.6
1500
water water ODS SAS
(0.1
ODS
watermethanol
+ NaCl
mol/L) SAS
ODS
SAS
90 3.6
125 1.3
50
b
b
b
(95-5% v/v)
190 2.3
55
b
b
1.9 b
C
C
C
C
C
C
b
550
c
610
870 850
180 930
190 340
460 460
840 460
3.6
206
2.6
OConfidence limit 20%: ODs, ODS Hypersil; SAS, SAS Hypersil. bNonbindingsolute. C Tmuch ~ solute ~ retained. Table IV. KMwValuesn CTAB
SDS
water solute toluene caffeine
BTAB SOBS benzoic acid CPC
water ODS SAS
+ NaCl
(0.1 mol/L)
watermethanol
water
( 9 5 5 % v/v,
water
+ NaCl
(0.1 mol/L)
watermethanol ( 9 5 4 % v/v) ODS SAS
ODS
SAS
ODS
SAS
ODS
SAS
ODS
SAS
260
300
50 560 400
200 18 930 160
210
35
390 35
280 39
400 35
290 40
240 20
200 20
820 140
b
b
b
b
b
b
c
C
C
C
C
C
b
b C
2800 2900
1900 3000
730 3900
480
C
b c
2100 2200
2080 2100
240 38 1900 220
240 40 1700 160
b c
b
650 b
C
C
530
19
1000
Confidence limit 40%: left columns, ODS Hypersil; right columns, SAS Hypersil. KMwvalues are given for one surfactant molecule (32), the number, N , of surfactant molecules in a micelle is given in Table I. Nonbinding solute. Too much solute retained. Tables I11 and IV list the Ksw and KMWvalues for both surfactants and stationary phases. KSWvalues give information about the affinity of the solute for the surfactant-covered stationary phase. KMw values measure the solute affinity for micelles. KMW should be independent of the nature of the stationary phase in the same mobile phase. NaCl Influence. The main effect of NaCl is to decrease the electrostatic interactions. This effect can be observed with ionic solutes. NaCl affects the slopes and intercepts of ionic solutes much more than those of toluene and caffeine (Table 11). This produces minor variations of Ksw values of toluene and caffeine (Table 111),while the Kswvalue of SOBS in SDS micellar mobile phases increases from 33 in water to 77 with 0.1 mol/L NaCl on ODS Hypersil (from 9.4 to 45, respectively, on SAS Hypersil). SOBS has much more affinity for the stationary phases if NaCl is present (“salting-out” with common ion effect). The “salting-out” effect corresponds to a decreasing of the electrostatic repulsions by NaC1; but the addition of NaCl also decreases the electrostatic attractions. The Ksw value of BTAB in SDS mobile phases decreases from 1500 in water to 840 with added NaCl on ODS Hypersil (from 420 to 140 on SAS Hypersil). These observations are strengthened by the KMWvalues of the ionic solutes (Table IV). They are rather similar on both stationary phases as predicted by the theory. The mean KMW value of SOBS for both stationary phases increases from 190 in water to 525 with NaCl and SDS surfactant. In the same conditions, the mean KMw value of BTAB decreases from 1800 to 545, which correspond to a decrease of the electrostatic attractions by NaC1. These observations may explain why an ionic solute bearing the opposite charge of the ionic surfactant was less retained either when NaCl was present and with low micellar concentration (the “salting-out” effect prevailed over the micelle transport) or with no NaCl and high micellar concentration (the micelle transport prevailed over the stationary-phase retention). That is the case of BTAB (Figure 4) and benzoic
acid (Figure 5). In the case of the nonionic solutes studied, the influence of NaCl on the K m values is almost nil (Table IV) and rather weak on the KSWvalues (Table 111). Nevertheless, these weak KSWmodifications are the cause of the retention modifications shown by Figures 4 and 5 (toluene and caffeine). Methanol Influence. The main effect of methanol is to decrease the hydrophobic repulsions. In any case, this effect can be observed with all the studied binding solutes. Ksw and KMW values are decreased by 5% v/v methanol (Tables I11 and IV). This effect produces an unusual retention behavior: at low micellar concentrations, some solutes were less retained with methanolic phases than with aqueous phases. At high micellar concentrations, some solutes were less retained with aqueous phases due to micelle transport. Caffeine, in SDS micellar mobile phases, illustrated this effect (Figure 4). The retention of caffeine is decreased by methanol addition at low micellar concentrations and increased by methanol addition at high micellar concentrations. Table I11 shows that, if the values of the intercept in methanol phases were higher than the one in water, the values of the slope might be either smaller or higher in methanol phases than the one in water. Adsorption and Retention. Concerning the micelle-solute interaction, as expected, the state and the nature of the stationary phase had no influence upon the KMw partition coefficients (Table IV). Concerning the stationary phasesolute interactions, Table 111 shows that the bulk-phase composition was a more important factor than the amount of adsorbed surfactant. The adsorbed amount of CTAB on ODS Hypersil was similar in water and NaCl surfactant solutions (Figure 3), yet, the KSWvalues of the solutes were not similar in either solution (Table 111). The adsorbed amount of SDS was larger on ODS silica with NaCl and less on SAS silica (Figure 2))yet, the relative variations of the K ~ values w showed the same trend. As noticed in our recent paper (311, in spite of the surfactant coverage, the polar character of the underlying stationary phase remains. In any case, the KSW
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
1367
be the topic of a further study. LITERATURE CITED
1.
--..---
s
1.0
0 . 00
.01
.02
.os
o'nl -.
05. 0 0
.01
C T A B concentrat 1 on
.01
.D2
.09
.Ol
0.00
C T--_ A B c-~ oncentrat
1
on
.02
.03
1 '
I. 0. 0 OD
(1) Armstrong, D. W.; Terrill, R. Q. Anal. Chem. 1979, 51, 2160-2163. (2) Armstrong, D. W.; Henry, S. J. J. Liq. Chromatogr. 1980, 3, 657-662. (3) Armstrong, D.W. Sep. Purif. Methods 1985, 14, 213-304. (4) Yarmchuck, P.; Weinberger, R.; Hirsh, R. F.; Cllne Love, L. J. Anal. Chem. 1982, 54, 2233-2238. (5) Arunyanart, M.; Cline Love, L. J. Anal. Chem. 1984, 56, 1557-1561. (6) Cline Love, L. J.; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1984, 56, 1132A-1148A. (7) Pramauro, E.; Saini, G.; Pelizzetti, E. Anal. Chim. Acta 1984, 166, 233-239. (8) Landy, L. S.; Dorsey, J. G. J. Chromatogr. Sci. 1984, 22, 68-70. (9) Dorsey, J. G.; Khaledy, M. G.; Landy, J. S.; Lin, J. L. J. Chromatogr. 1984, 316, 183-191. (10) Berthod, A.; Glrard, I.;Gonnet, C. Anal. Chem. 1986, 5 8 , 1356- 1358. (11) Berthcd, A.; Georges, J. Nouv. J. Chim. 1985, 9 , 101-108. (12) Berthod, A. J. Chim. Phys. 1983, 8 0 , 407-421. (13) Kresheck, G. C. I n Water, a Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1975; Vol. 4, pp 95-167. (14) Magld, L. I n Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; pp 427-453. (15) Almgren, M.; Swarup, S. I n Surfactant in Solutlons; Mittal, K. L.; Lindman, B., Eds.; Plenum: New York, 1984; pp 613-625. (16) Ionescu, L. G.; Romanesco, L. S.; Nome, F. I n Surfactant in Solutions; Mittal, K. L.; Lindrnan, B., Eds.; Plenum: New York, 1984; pp 789-803. (17) Giles, C. H. I n Anionic Surfactants; Lucassen-Reynders, E. H., Ed.; Dekker: New York, 1981; Vol. 11, Chapter 4. (18) Nahum, A.; Horvath, Cs. J. Chromatogr. 1981, 203, 53-63. (19) Gilpin, R. K. J. Chromatogr. Sci. 1984, 22, 371-377. (20) Girard, I.; Gonnet, C. J. Liq. Chromatogr. 1985, 8 , 2035-2046. (21) Bartha, A.; Billlet, H. A. H.; De Galan, L.; Vigh, G. J. Chromatogr. 1984, 291, 91-102. (22) Scott, R. P.; Simpson, C. F. J. Chromatogr. 1980, 197, 11-20. (23) Hung, C. T.; Taylor, R. B. J. Chromatogr. 1981, 209, 175-190. (24) Borgerding, M. F.; Hinze, W. L. Anal. Chem. 1985, 57, 2183-2190. (25) Unger, K. K. J. Chromatogr. Lib. 1979, 16, 130-138. (28) Van der Houven, 0. A. G. J.; Sorel, R. H. A.; Hulshoff, A,; Teeuwsen, J.; Indemans, A. W. M. J. Chromatogr. 1961, 209, 393-404. (27) Tramposch, W. G.; Weber, S. G. Anal. Chem. 1984, 56,2567-2571. (28) Papp, E.; Vlgh, G. J. Chromatogr. 1983, 282, 59-70. (29) Knox, J. H.; Hartwlck, R. A. J. Chromatogr. 1981, 204, 3-21. (30) Tang, M.; Deming, S. N. Anal. Chem. 1983, 55, 425-428. (31) Berthod, A.; Girard, I . ; Gonnet, C. Anal. Chem. 1988, 5 8 , 1359-1362. (32) Armstrong, D. W.; Nome, F. Anal. Chem. 1981, 53, 1662-1666. (33) Dorsey, J. G.; De Echegaray, M. T.; Landy, J. S. Anal. Chem. 1983, 55, 924-928. (34) Landy, J. S.; Dorsey, J. G. Anal. Chim. Acta, in press.
.02
.03
Cmol / L >
Figure 5. log k'vs. the CTAB concentration on ODS Hypersil. See Figure 4 caption. values were lower on SAS Hypersil than on ODS Hypersil except for caffeine, a polar solute. An important problem of MLC is the poor efficiency. Dorsey (33,341 has shown that the addition of 3% v/v propanol in surfactant mobile phases enhanced the efficiency. The addition of NaCl or methanol modifies the efficiency, The study of these effects is in progress in our laboratory and will
RECEIVED for review December 12,1985. Accepted February 21, 1986. This work was supported by the Centre National de la Recherche Scientifique UA 0435.
