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Anal. Chem. iQ86, 58,1356-1358
Micellar Liquid Chromatography, Adsorption Isotherms of Two Ionic Surfactants on Five Stationary Phases Alain Berthod, I n e s Girard, and Colette Gonnet*
Laboratoire de Chimie Analytique 3 (Professeur M. Porthault), Universite' Claude Bernard, Lyon 1, 43 Boulevard d u 11 Novembre 1918, 69622 Villeurbanne Cedex, France
Adsorption Isotherms of sodium dodecyl sulfate (SDS) and cetyitrhethyiammonlum bromide (CTAB) were determined on flve statlonary phases of varlous polarities: three apolar slllca (ODS Hypersll, an octadecyl bonded silica; MOS Hypersll, an octyl bonded slllca; and SAS Hypersll, a methyl bonded slllca), and two polar slllca (CPS Hypersll, a cyanopropyl bonded silica, and Hypersll, the parent silica). Wlth submlceliar surfactant concentrations, adsorptlon isotherms were of the S type for both anlonlc SDS and cationic CTAB on the three less polar statlonary phases. With mlceliar mobile phases, H-type adsorption Isotherms were obtained. Above 2 times the micellar concentratlon (cmc) the adsorbed amount of surfactant was almost constant and reached similar quantitles on the alkyl-bonded slllca (4-5 X lod mol/m2) and was only 2.5 X lo-' mol/m2 of SDS and 3.5 X lo-' moi/m2 of CTAB on CPS Hypersll and 0.5 X lod mol/m2 of SDS and 2 X lod moi/m2 of CTAB on naked silica.
Ionic surfactants have been widely used as pairing ions in liquid chromatography at concentrations below the critical micellar concentration (cmc). Since 1979, several authors have studied the properties of micellar mobile phases, but it is Armstrong and Cline Love (1-7) who have worked most intensively on the subject. This type of chromatography has been named "pseudophase liquid Chromatography" (PLC) or micellar chromatography. The unique selectivity of such systems is due to the micellar core, which provides a hydrophobic site for interaction with apolar solutes in the pure aqueous phase (1-3), and due also to the combination of factors including the existence of adsorption, hydrophobic, and electrostatic effects. The micelles, therefore, play the role of the organic modifier. Primary studies were carried out in thin-layer chromatography (4-6). Further works, studying the dependence of retention on micellar concentration (1-3),selectivity (2, 7), and efficiency (8,9), have been published more recently. All these authors considered that, using micellar mobile phases, the amount of adsorbed surfactant is constant on a given surface, but i t greatly depends on the stationary-phase nature. The aim of the present work was to study the adsorption of two currently used ionic surfactants on stationary phases of various polarities in order to elucidate the role of the stationary phase in the retention mechanism of PLC. In another paper (lo), the retention of uncharged and ionic solutes was examined. EXPERIMENTAL SECTION Materials. Mobile phases were prepared with deionized and distilled water. Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were obtained from Merck (Darmstadt, GFR). SDS was biochemistry grade and CTAB, analytical grade. Characteristics of both surfactants are listed in Table I (11). Methanol and water, used in the preparation of mobile phases, were aspirated through 0.5-pm cellulose acetate filters (Millipore, Bedford, MA) and degassed in an ultrasonic bath. Micellar mobile phases were prepared by dissolving the appropriate amount of surfactant in water.
Table I. Characteristics of the Studied Surfactants surfactant SDS CTAB a
mol w t 288.4 364.5
cmc; mol/L 8.2 X 9X
cmc," 70 (g/lOO g)
r,b
V,'
nm
L/mol
0.236 0.033
2.5 3.2
0.246 0.364
Critical micellar concentration. *Micellar radius. Molar vol-
ume.
Table 11. Physicochemical Properties of the Studied Silica trade name Hypersil CPS Hypersil SAS Hypersil MOS Hypersil ODS Hypersil
bonded moiety unbonded cyanopropyl trimethyl octyl
octadecyl
S," %C, r,b m2/g CBET w / w wmol/m2 I50 115 104 129 105
130 41 18 24 24
0.3 4.2 2.6 7.0 8.5
4.5 4.1
OS(m2.g-') = specific surface area (determined by the BET method). r = Surface coverage. Five stationary phases from Shandon (Runcorn, Cheshvie, GB) have been used; they are spherical microparticles of 5 pm mean diameter. the four bonded phases are manufactured from the same parent silica (Hypersil) and possess a monolayer coverage of trimethylsilyl (SAS Hypersil), dimethyloctylsilyl (MOS Hypersil),octadecyl (ODS Hypersil),and cyanopropyl (CPS Hypersil) groups. Their physicochemical properties are listed in Table 11. Although the alkyl bonded surfaces are weakly energetic (low values of BET constant in Table 11), specific surface areas were determined by using a modified BET method (12, 13). As can be seen from Table 11, the value of the constant C decreased as the polarity of the stationary phase decreased. The elemental analysis of carbon (C%) was made by Service central d'analyse du CNRS (Solaize, France). From C% values (corrected for the C% value of naked hypersil), the surface concentration of the substituent can be estimated from
in which S is the specific surface area of the parent silica and n, and Mare, respectively, the carbon number and the molecular weight of the bonded moiety, provided that the bonded moiety is well-known. This is not the case for CPS silica and ODS silica: information from Hypersil supplier (Shandon) is incomplete, so the M value used in the above relationship (eq 1)is not known. Thus, these calculations are meaningful only for SAS Hypersil (trimethylsilyl groups) and MOS Hypersil (dimethyloctylsilyl groups). The calculated values of r closely approach the highest limiting concentration of a bonded monolayer (4.5 ,umol/m2) (14, 15).
Columns for isotherm measurements were 100 X 4.6 mm i.d. and were packed in the laboratory (16),using about 1g of each type of silica. The chromatographic apparatus consisted of two Chromatem 130 pumps (Touzart & Matignon, Paris), a thermostated bath (Lauda, Konigshofen, GFR), a Model 70-10 six-part Rheodyne injection valve, and a Waters R401 refractometric detector. Procedure. The adsorption isotherms for SDS and CTAB were determined at 30 "C by pumping the appropriate concen-
0 1986 American Chemical Soclety 0003-2700/86/035~-1356$01.50/0
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986 6.01
I
SDS SDS Cono.
Cmmole/l>
C T A B Cono.
Cmmol e / l >
I
Concentration
I
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I
Flgure 2.
Adsorbed amount of SDS from micellar mobile phases.
Figure 3.
C T A B C o n c e n t r a t i o n Adsorbed amount of CTAB from micellar mobile phases.
Figure 1. Adsorbed amount of ionic surfactants In submicellar concentrations on the five Stationary phases.
tra'tion of surfactant in the mobile phase through the column until the detector base l i e was constant with time. Methanol was used to desorb the surfactant until the detector response was again constant. The total eluent was collected. The amount of adsorbed surfactant was determined by selective titration (17) after evaporating the methanol under reduced pressure and was corrected for the dead volume of the system. The surfactant titration method being very precise, the main uncertainty occurred from the determination of the dead volume of the system (column + detector), Numerous attempts were made to minimize this uncertainty. The dead volume was finally obtained by injecting D20 in water solutions. In these conditions, the maximal experimental error was 8% for the dead volume values and, thus, for the determination of the amounts of adsorbed surfactant. Total desorption was checked by subtracting the total amount of surfactant passed through the column and the remaining amount in the mobile phase after adsorption. This difference was always very similar to the amount of methanol-desorbed surfactant. Data obtained by use of this procedure were rather similar to those obtained by using static determinations (18). The working concentrations of surfactants ranged from 0 up to 0.4 mol/L for SDS and from 0 up to 0.2 mol/L for CTAB. At the lowest concentrations of surfactants, adsorption equilibrium required large volumes of mobile phases; for example,with a 4 X lo4 mol/L CTAB solution, more than 1L of mobile phase had to be passed through the columns, Le., more than 17 h of elution time at a flow rate of 1 mL/min.
RESULTS AND DISCUSSION Adsorption Isotherms in the Submicellar Region. Isotherms for the adsorption of SDS and CTAB from pure aqueous mobile phases are shown in Figure 1. The concentration, C, of adsorbed surfactant (pmol/m2) was calculated by using the surface areas of the various stationary phases (Table 11). I t can be seen that the curvature of adsorption isotherms for polar stationary phases is different from that of apolar bonded phases. Adsorption isotherms on cyanopropyl silica and naked silica are of the S type or cooperative adsorption (19): the slope at first increases along with surfactant concentration in the aqueous phases; this indicates that the number of sites capable of retaining a solute molecule increases. The curves obtained for both the surfactants on the alkyl bonded phases resemble L type, according to the Giles classification (19, 20). 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. It must be noted that SDS does not present any adsorption on naked silica in this concentration range. As expected, the amount of adsorbed surfactant decreases when the polarity of the stationary phase increases, with the
exception of short alkyl silica (SAS Hypersil). This confirms the assumptions that hydrophobic interactions are not the only ones responsible for surfactant adsorption. Indeed, the adsorption of an ionic surfactant onto the stationary phase could occur in at least two ways: (1)the hydrophobic tail is adsorbed and the ionic head group would then be in contact with the polar solution (this is the situation that is expected with the C18 bonded silica) or (2) if the ionic head group is strongly adsorbed, the stationary phase becomes more hydrophobic and could subsequently behave as a more hydrophobic surface. The second way is very likely the case of CTAB adsorbed on a silica surface. The existence of ionic interactions (ion exchange, hydrogen bonding) between cationic species and surface hydroxyl groups has been demonstrated (21,22), In the case of moderately polar stationary phases, such as SAS Hypersil, surfactant adsorption probably occurs via mixed hydrophobic and silanophilic interactions. Furthermore, it has been shown by Gilpin (23) and Girard (24) that, in the presence of water, the hydrocarbon moieties of long alkyl bonded phases can collapse on the surface, thus reducing the number of sites for hydrophobic interactions. This could explain the low surfactant adsorption on the C18 silica as compared to that on C1 bonded silica. Adsorption Isotherms with Micellar Mobile Phases. Adsorption curves obtained with micellar mobile phases are shown in Figures 2 and 3. With the exception of SDS on naked silica, all the curves are of the H type (19);i.e., the amount of adsorbed surfactant increases rapidly and reaches a plateau for surfactant concentration higher than the cmc. Two remarks should be made here: The first one is that the adsorption plateaus are, unexpectedly, very close to each other for C1, Cg,and Clg bonded phases. The second remark is that the maximum adsorption is obtained on SAS (C,) hypersil but
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986
not on the more hydrophobic CIEphase. In previous works (2, 7), it was considered that the amount of adsorbed surfactant was constant above the cmc value (because the free surfactant concentration remained constant) but was very different on stationary phases of various polarities, the concentration of adsorbed surfactant having to be higher on C18silica than on more polar C1 or CN silica. The decreasing order of adsorbed surfactant observed on the various stationary phases (Figures 2 and 3) is C1, CI8,CE,CN, and naked silica for both cationic and anionic surfactant. As stated above, the strong adsorption on C1 silica may be attributed to mixed polar and hydrophobic interactions; indeed, the slight difference observed between coverage values (Table 11)of C1 and CEbonded phases cannot explain the difference in the maximum amount of adsorbed surfactant on these two phases (3.7 pmol/m2 CTAB on CEand 5 pmol/m2 SDS on Cl). An original type of behavior was observed with SDS on naked silica (Figure 2): until a concentration of 20 times the cmc was reached, no adsorption was noticed. Then, a low adsorption of SDS occurred, resulting in an S-type curve (19, 20) with cooperative adsorption. Above the cmc value, the concentration of free surfactant remains constant, but the micelle concentration increases so that the micellar pseudophase volume increases and the aqueous-phase volume decreases. A large micelle concentration could initiate, on silica surface, a slight adsorption of SDS molecules via hydrogen bonding, and a further adsorption may occur via hydrophobic interactions. All the curves obtained above the cmc present a regular plateau region in the same concentration range as that indicated by Hung and Taylor (25),but no steps were observed. The plateau is almost horizontal for CTAB adsorption; for SDS adsorption, a slight slope exists (except for C18 silica) showing a further adsorption of SDS molecules in presence of very high micellar concentration.
Registry No. SDS, 151-21-3;CTAB, 57-09-0.
LITERATURE CITED Armstrong, D. W.; Nome, F. Anal. Chem. 1981, 53, 1662-1666. Yarmchuck, P.; Weinberger, R.; Hirsch, R. F.; Cline Love, L. J. Anal. Chem 1982, 54, 2233-2238. Arunyanart, M.; Cline Love, L. J. Anal. Chem. 1984, 56, 1557-1561. Armstrong, D.W. J. Llq. Chromatogr. 1980, 3 , 895-900. Armstrong, D. W.; Terrill, R. Q. Anal. Chem. 1979, 51, 2160-2163. Armstrong, D. W.; Henry, S. J. J. Liq. Chromatogr. 1980, 3 ,
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657-662. Armstrong, D. W.; Stine, G. Y. Anal. Chem. 1983, 55, 2317-2320. Dorsey, J. G.; De Etchegaray, M. T.; Landy, J. S. Anal. Chem. 1983, 55, 924-928. Yarmchuck, P.; Welnberger, R.; Hirsch, R. F.; Cline Love, L. J. J. Chromatogr. 1984, 283, 47-60. Berthod, A.; Girard, I.; Gonnet. C. Anal. Chem. 1986, 58,
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Berthod, A.; Georges, J. Nouv. J. Chim. 1985, 9 , 101-108. Serpinet, J.; Untz, g.; Gaget, C.; De Mourgues, L.; Perrin, M. J. Chlm. Phys. Phys. Chem. Blol. 1974, 71, 949-957. Pommier, 8.; Julllet, F.; Telchner, S. J. Bull. SOC.Chlm. Fr. 1972, 4,
1268-1 274. Erard, J. F.; Nagy, L.; Kovats, E. sz. Colloids Surf. 1984, 9 , 109-120. Gobet, F.; Kovats, E. sz. Adsorp. Scl. Technol. 1984, 9 , 77-89. Coq, B.; Gonnet, C.; Rocca, J. L. J. Chromatogr. 1975, 106,
249-262. AFNOR Standarts No. T73-320 and T73-258.
De Jong, A. W. J.; Kraak, J. C.; Poppe, H.; Nooltgedacht, F. J. Chromatogr. 1980, 193, 181-195. Giles, C. H. "Anionic Surfactants"; Lucassen-Reynders. E. H., Ed.; Marcel Dekker: New York, 1981; Voi. 1 1 , Chapter 4. Giles, C. H.;Smith, D.; Hultson, A. J. Collold Interface Sci. 1974, 47,
755-765. Bartha, A.; Vigh, 0. J . Chromatogr. 1983, 260, 337-345. Vigh, G.; Papp, E. J. Chromatogr. 1983, 259, 49-58. Gllpin, R. K. J. Chromatogr. Sci. 1984, 22, 371-377. Girard, I.; Gonnet, C. J. Llq. Chromatogr. 1985, 8 , 2035-2046. Hung, C. T.; Taylor, R. B. J. Chromatogr. 1981, 209, 175-190.
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 at the 9th International Symposium on Column Liquid Chromatography, Edinburgh, Scotland, July 1-5, 1985.
