Langmuir 1996,11, 1753-1759
1753
Coadsorption of Naphthalene Derivatives and Cationic Surfactants on Porous Silica in Aqueous Solutions V. Monticone and C. Treiner* Laboratoire djElectrochimie, UA CNRS 430, Universitt? Pierre et Marie Curie, 4 Place Jussieu, Bat. 74,Paris 75005,France Received November 7,1994. I n Final Form: February 8, 1995@ The coadsorption of cationic surfactants and several naphthalene derivatives on a porous silica, Sorbsil C30, has been investigated in aqueous solutions as a function of surfactant concentrationbelow and above the critical micelle concentration (cmc)at two pH values. 2-Naphthol and 2-(2-naphthyl)ethanolare not adsorbed onto the silica surface. Adsorptionof cetylpyridiniumchlorideor cetyltrimethylammoniumbromide on the silica induces a considerable coadsorption of the neutral molecules. The effect may be described by a partitioning process. Above the cmc, the solutes desorb from the surfactant structures as they are solubilized in the free micelles. Naphthalene, which is strongly adsorbed on the silica surface in the absence of surfactant, is also desorbed as micelles are formed. Comparisons of the micellar solubilization effect as determined from solute desorption or direct micellar solubilizationexperimentsdisplay differences that may be attributed to the retention of some solutes on the adsorbed surfactant structures (hemimicelles or admicelles) in the presence of free micelles. It is shown that for all solutes studied the free energy of coadsorption is larger than the free energy of micellar solubilization.
Introduction Surfactants in solution adsorb at solidlliquid interfaces below the critical micelle concentration (cmc). In the case of ionic surfactants and mineral oxides, it is generally admitted that a t very low concentration individual monomers adsorb a t localized ionic sites; a t higher concentration, lateral interactions induce the formation of various structures (hemimicelles or admicelles) that upon further surfactant concentration increase may, around the cmc, either collapse into mono or twodimensional layers or retain a patchwork-type aggregate structure. With either model, the surfactant structures may in aqueous solutions adsorb and thus concentrate scarcely soluble ions or molecules below the cmc. This effect seems to have been first described by Steigter et al. in the case of a dye with a cationic surfactant on g1ass.l Nunn et al. described essentially the same phenomenon also in the case of a dye with an anionic surfactant adsorbed on alumina dispersions.2 Koganovskii et al. discussed the same phenomenon more quantitatively for nonionic surfactants coadsorbed with phenolphthalein3 or naphthalene4on acetylene black. It was found that, a t a surfactant concentration such that hemimicelles are in equilibrium with regular micelles, the concentration of solubilized solute is identical in the adsorbed assemblies and in the regular micelles; the suggested interpretation model implied that surfactant and solute molecules compete for available surfaces on the solid. The proposed mechanism was that the solute molecules were transported to the solid surface as solubilized species in micelles. Somewhat later, Armstrong et al.5-7 introduced the micellar chromatographic technique, which is essentially
* To whom all correspondence should be addressed. Abstract published in Advance A C S Abstracts, April 15,1995. (1)Stigter D.;Williams R. J.;Mysels K. J. J. Phys. Chem. 1955,59, 330. (2)Nunn C.C.;Schechter R. S.; Wade W. H. J . Phys. Chem. 1982, 86,3271. (3)Koganovskii,A. M.; Klimenko, N. A.;Tryasorukova,A. A. Kolloidn. Zh. 1976,38,165. (4)Koganovskii, A. M.; Klimenko, N. A.; Tryasorukova,A. A. Kolloidn. Zh. 1974,36,861. (5)Armstrong, D. W.; Terril, R. Anal. Chem. 1979,51,2160. (6)Armstrong, D.W.; Stine, G. L. J . A m . Chem. SOC.1983,105,6220. (7) Armstrong, D.W.; Nome, F. Anal. Chem. 1981,53, 1662. @
based on the same phenomenon. A micellar solution containing the solute is passed over a column with a solid (stationary)phase. Two equilibrium are considered: on one hand, the solute is assumed distributed between this stationary phase (including the adsorbed surfactant) and water, and on the other hand, it is also distributed between the micellar pseudophase and water. The latter equilibrium corresponding to that described in classical micellar solubilization studies. In principle, two corresponding solute partition coefficientscould be detem~ined.~ In fact, only the second one may be obtained with a reasonable accuracy, but even then adsorption of a fraction of the solute on the stationary phase may introduce uncertainties of the determined partition coefficient. More recently, emphasis has been exercized on the first of the two above effects, i.e., the interaction between the solute and the surfactant structure a t the solidlliquid interface. This phenomenon has been coined adsolubi1izati0n.a~It specifically concerns solute molecules that are not adsorbed a t a solid interface but are solubilized in the surfactant aggregates which are present on the solid surface. A few investigations have been performed lately using mineral substrates such as alumina (in the presence of anionic surfactants), silica or clays (with cationic surfactants),or polystyrene latexes. The solutes were alcohols, both aliphaticgand aromatic,1°-12d ~ e s , l - ~ J ~ J ~ drug molecules,15and 2-naphthol.16J7 Although important results have been obtained concerning the effect of surfactant coverage on the increase (8) Harwell, J. H.; Hoskins, J. C.; Schechter, R. S.; Wade, W. H. Langmuir 1985,1, 251. (9)Lee, C.; Yeskie, M. A.; Harwell, J. H.; ORear, E. A. Langmuir 1990,6,1758. (10)Sjoblom, J.; Blockhus, A. M.; Sun, W. M.; Friberg, S. E. J . Colloid Interface Sci. 1990,140, 481. (11)Esumi, K.; Shibayama, M.; Meguro, K. Langmuir 1990,6, 826. (12)Monticone, V.; Treiner, C. J. Colloid Interface Sci. 1994,166, 394. (13)Zhu, B. Y.; Zhao, X.; Gu, T. J . Chem. SOC.Faraday Trans. 1 1988.84. 3951. _.__ ~ ~ (14)Esumi, IC;Sugimara,A.; Yamada, T.; Meguro, K. Colloids Surf. 1992,62,249. (15)Jansen, J.; Treiner, C.; Vaution, C.; Puisieux, P. Int. J. Pharm. 1994,103,19. (16)Klumpp, E.; Heitmann, H.; Schwuger, M. J. Colloids Surf. A 1993,78,93. (17)Monticone, V.; Mannebach, M. H.; Treiner, C. Langmuir 1994, 10,2395. I
- - I
0743-746319512411-1753$09.00/0 0 1995 American Chemical Society
Monticone and Treiner
1754 Langmuir, Vol. 11, No. 5, 1995 of neutral solute adsolubilization, these studies do not cover the entire surfactant concentration domain of interest. It has been recently shown1' i n the case of 2-naphthol adsolubilized with cetylpyridinium chloride (CPC) on a nonporous silica (Aerosil 200) that after reaching a maximum adsolubilization close to t h e cmc, 2-naphthol m a y be completely desorbed from t h e silicd water interface as more free micelles are formed. This type of behavior presents many aspects displayed by environmental systems where a particular pollutant is distributed between a soil, water, and a colloidal phase, t h e components of which being themselves partitioned ~J~ the between t h e colloid and the ~ o i l . ~ Likewise, situation where cationic surfactants are used in order to modify soil surfaces so that the migration of hydrophobic solutes may be retardedz0is related to the adsolubilization effect. Here, the solute partitioning under flow and batch experimental conditions were also compared. Thus, t h e adsolubilization phenomenon is at the cross-roads of many closely related research a r e a s that do not overlap. The purpose of the present investigation presents several aspects. (1)The conditions for using the distribution formalism for the description of t h e adsorption and desorption of solutes from t h e silicdwater interface will be outlined. The previous data obtained with 2-naphthol and CPC on a nonporous silica17 will be thus reanalyzed. (2)The adsolubilization effect as obtainedfor a phenol, an alcohol, a n d a hydrocarbon will be compared, namely, 2-naphthol with CPC and 2-(2-naphthyl)ethanol and naphthalene with cetyltrimethylammonium bromide (CTAB) on very pure samples of a porous silica will be described. It will be also of interest to investigate if t h e probable effect of porosity on the adsorption of surfactants has any specific consequence on the coadsorption phenomenon. (3) The solubilization and adsolubilization partition coefficients for 2-naphthol, 242-naphthyljethanol, and naphthalene as deduced from various experimental conditions and methods will be compared.
Materials and Methods A very pure (99.5 %) porous silica, Sorbsil C30 (BET surface equal to 700 m2 g-l; pore volume of 0.6 mL g-l), was used as received (Colpak from Rhone-Poulenc, France). The solutes origins were as follows: 2-(2-naphthyl)ethanol (NEtOH) (98% pure, Aldrich), 2-naphthol (NOH) (99% pure, Aldrich), and naphthalene (N)(99%pure, Sigma). The aqueous solubilities of these compounds in pure water at 25 "C were as follows: 0.00512 mol L-l for NOH, 0.00293 mol L-l for NEtOH, and 0.00020 mol L-l for N. Orange I1 (P-naphthobenzenesulfonic acid) was from Aldrich. The cationic surfactants cetylpyridinium chloride (CPC) and cetyltrimethylammonium bromide (CTAB)were from Sigma. The cmc, as determined from conductivity versus concentration plots at 25 i:0.02 "C (Wayne-Kerr,Model 6425 automatic conductivity bridge), agreed well with literature values. They were equal, and to 8.9 x mol L-l. The respectively, to 8.8 x conductivity cell (Philips PW 9550/60)had platinized electrodes; doubly distilled water was used. In the presence of 0.01 mol L-l NaCl or NaBr (Merck), the cmc of the two surfactants was respectively 2.3 x and 2.0 x mol L-l as determined from surface tension measurements (Kruss Model KlOT automatic tensiometer) at 25.00 f 0.02 "C. All adsorption experiments were performed in the presence of the two added sodium salts with common ion. The batch method used has been described before.12 Briefly summarized, 0.2 g of solid was equilibrated with 20 mL of aqueous solution for 12 h in a water thermostat at 25.0 0.1 "C. Some experiments
*
(18)Wang, L.; Govind, R. Enuiron. Sci. Technol. 1993, 27, 152. (19) Pankow, J. F.;McKenzie, S. W.Enuiron. Sci. Technol. 1991,25, 2046. (20)Wagner, J.;Chen,H.;Brownawell,B. J.; Westwall,J. C. Enuiron. Sci. Technol. 1994, 28, 231.
with N were equilibrated for 1week. After ultracentrifugation at 10 000 rpm for 45 min at 25 "C (Sigma 2K15),the supernatant was analyzed systematically for solute and surfactant on the same solutions. Cationicsurfactant analysis was performed with the Orange I1 method and chloroform extraction.21 AVarian TJV spectrophotometer (Cary 1E)was used. The pH ofthe solutions was measured using a combined glass electrode (Taccussel).Two pH values were chosen: 3.6 and 6.5. The latter value corresponds to a full bilayer coverage of the solid surface^.^^^^^ The former one corresponds to the spontaneous value of the aqueous Sorbsil C30 solutions in the presence of surfactant and added salt. Constant solute concentrations were used for the adsorption 3 x experiments. These were respectively equal to 4 x mol L-l for NOH, NEtOH, and N. These 10-4, and 1 x concentrations were chosen so that they would be of the same order of magnitude for the three solutes and would be below the solubility limit in pure water. Thus, dilute experimental conditions were fulfilled in all cases. As it will be shown below, a solute partition coefficient Pads between adsorbed surfactant aggregates and water can be calculated below the cmc. From adsorption experiments at equilibrium surfactant concentration in excess of the cmc, a partition coefficient Pdes between free micelles and water may also be derived. This coefficient tentatively describes the preferential solute solubilization in the free micelles, which are formed above the cmc in the presence of the coadsorbed molecules onto the solidsurfactant interface system. In order to be in a position to investigate the reliability of the latter P d e 5 values a d the difference, if any, between the energetics of solute coadsorption and micellar solubilization, two independent methods were used. The first one is the classical solubility method. The basic equation is simply
where St, S,, and Ct are the total solute solubility, the solubility in pure water, and the total surfactant concentration,respectively. The second one relates the variation of cmc to the solute concentration. A number of equations have been proposed in the l i t e r a t ~ r e .The ~ ~ following equation has been tested25 and may be preferred:
cmc' and cmc are the values in absence and in the presence of solute at a concentration mN; a is the degree of counterion association for the surfactant in pure water. The values of 0.27 and 0.15have been determined for CPC and CTAB, respectively.26 K s is the Setchenow constant. The value of k, may be estimated for NOH as -5.2 and for NEtOH as -3Az7 M, is the molar mass ofwater, andPmi,is the solute partition coefficientbetween micelle and water in the mole fraction scale. The advantage of the method based upon eq 2 over the solubility one is that subsaturation solute concentrations are employed. The highest solute concentration used was below half ofthe aqueous solubility limit. Thus, Pmi,may be considered as much closer to a true thermodynamic quantity than Ps0l. As the desorption experiments from which P d e s will be derived also correspond to subsaturation conditions, the comparison between Pdes and Pmic will be more meaningful. This method could not employed with N because the solubility limit in water was too low. The cmc was determined using conductivity measurements. (21) Few, A. V.; Ottewill, R. H. J . Colloid Sci. 1966, 11, 34. (22) Rennie, A. R.; Lee, E. M.;Simister, E. A,;Thomas,R. K. Langmuir 1990, 6, 1031.
(23) Cases, J. M.; Villieras, F. Langmuir 1992, 8,1251. (24) Treiner, C. In Solubilization in Surfactant Aggregates; Christian, S. D., Scamehorn,J. F., Eds.; Surfactant Science Series, Vol. 55; Marcel Dekker: New York, 1995; p 383. (25)Treiner, C.; Mannebach, M. H. J . Colloid Interface Sci. 1987, 118, 243. (26) Treiner, C.; Makayssi, A. Langmuir 1992, 8, 794. (27)Vesala, A.; Perkola, H.; Lonnberg, H. Finn. Chem. Lett. 1981, 3, 40.
Coadsorption of Naphthalene and Surfactants
Langmuir, Vol. 11, No. 5, 1995 1755
..
Ts(moVL) 0.75-
e
0.5-
0
-6
-5
-4
-3
-2
.
0.25
II
o
0
.
%
O
0
.
0
.
.I
I
0
-I
Log Cs,eq
Figure 1. Coadsorption of NOH (right coordinate) ( 0 )and on Sorbsil C30 at pH = 3.6 (0.01 mol CPC (left coordinate)(0) L-l NaC1) as a function of free surfactant concentration. The
arrow indicates the cmc.
The solubilityexperiments were performed in the usual way: the excess soluteswith variable surfactant concentrationswere equilibrated for at least 48 h under stirring conditions at 25.0 =! 0.1 "C. Then, the solute was further equilibrated without stirring for 24 h; supernatant aliquots were withdrawn with a syringefrom the solutionsand analyzed spectrophotometrically. The concentration of added salt was too small (0.01 mol L-I) to have any effect on the values of Pmic or PSO1.
Results and Discussion Figure 1 presents simultaneously the coadsorption of the solute, NOH, and of the surfactant, CPC, on Sorbsil C30 as a function of the equilibrium (free) surfactant concentration a t the lower pH value of 3.6 in the presence of 0.01 mol L-l added salt. The right ordinate refers to the solute concentration and the left one refers to the surfactant concentration. Concentrations were expressed in mole per liter of solution instead of the more usual mole per gram of solid because ofthe attempted evaluation of the various effects in terms of solute partition coefficients. The surfactant adsorption isotherm is classical. A sigmoidal curve is obtained corresponding to three adsorption steps for cationic surfactants on this type of negatively charged material: adsorption of monomers, followed by the formation of small aggregates at higher concentrations; the small aggregates merge into layers (or bilayers) at concentrations close to the cmc. The solute adsorption curve follows the surfactant behavior up to the cmc. In the absence of surfactant, NOH is not adsorbed on silica; the surfactant structures adsorb the hydrophobic solute up to the cmc. At higher surfactant concentrations, free micelles are formed, and the solute desorbs from the surfactant structures as it is preferentially solubilized in the micelles. The driving force for this process is essentially the difference in the chemical potential induced by the difference in the concentrations of adsorbed and free surfactant ions above the cmc. In Figure 2, the NOH behavior is displayed a t the two pH values investigated: pH = 3.6 and 6.5. The results are presented as the fraction, z, of the total solute concentration that is adsorbed onto the surface as a function of free solute concentration in order to emphasize the strong adsorption induced by the surfactant structures. The pH increase promotes the adsorption of the cationic surfactant as expected. This effect, in turn, increases the coadsorption of the solute a s will be shown by the values of the adsorption partition coefficient values. Figures 3 and 4 present the results obtained for NEtOH with CTAB. The results are very similar to those described for NOH on CPC. The solute is not adsorbed on Sorbsil C30 in the absence of surfactant. The uptake of solute
T,(mol/L)
i
1
0.01
rsol(moW
0
0.0005 O 0 - t
0
0.005 .O
.
t
3
0.00025
-
-6
-7
-5
-4
-2
-3
-I
Log cs,eq
Figure 3. Coadsorption of NEtOH (right coordinate)( 0 )and on Sorbsil C30 at pH = 3.6 (0.01 mol CTAB (left coordinate)(0) L-l NaBr) as a function of free surfactant concentration. The
arrow indicates the cmc.
.
0
%oI 0
.
0.5-
-7
0
0
-6
-5
.
0
0
0
",
0
O
.
80 .
0
a .* -4
-3
-2
I
Log Cqeq
Figure 4. Fraction of coadsorbed NEtOH at two pH values as a function of free surfactant (CTAB)concentration. pH = 3.6 (0);pH = 6.5 (0).
follows closely the increasing concentration of adsorbed CTAB and decreases above the cmc as free micelles are formed (Figure 3). The increase of pH has the same effect on NEtOH as on NOH (Figure 4). The case of N coadsorbed with CTAB on Sorbsil C30 is different (Figure 5). Unlike the two naphthalene derivatives, this solute adsorbs strongly onto the silica surface in the absence of surfactant. Nevertheless, it desorbs from the solid surface a s the cmc is exceeded, just like NOH and NEtOH. In order to investigate the possibility of a strong interaction between the solute and the silica, which would interfere with the solute desorption, equilibrium experiments were performed over a period of 1 week instead of the usual 12 h. As shown in Figure 5, the same adsorption results were obtained within experimental error. Moreover there is no effect on pH on the coadsorption (or desorption) of this solute (not shown). We shall come back to this point below. In order to quantify these observations, the coadsorption effects were analyzed in terms ofpartition coefficients.AS
Monticone and Treiner
1756 Langmuir, Vol. 11, No. 5, 1995
Table 1. Characteristic Parameters for Coadsorption and Desorption of Neutral Molecules in the Presence of Cationic Surfactants at Silica Surfaces at 25 "C
T,(mol/L) U
e 0.0025
o
Pdes
solutes
I
0
B
0
B
0 -6
rb
-4
-5
-3
-2
NOH
Aerosil2000 CPC CTAB SorbsilC30 CPC NEtOH CTAB N CTAJ3
I -I
P,,,
S
silica
0
Cs,q
Figure 5. Adsorption of naphthalene (left coordinate) and CTAB (right coordinate)on Sorbsil C30 at pH = 3.6 (0.01mol L-' NaBr). (0) 24-h equilibration; ( 0 )1-week equilibration.
Paol 3.6
at PH 6.5
Pads
at PH
6.5
3.6
1710 570 900 1800 4800 10200 1810 1710 2170 2050 4500 7700 1650 1100 515 810 2340 2700 5200 1050 1050
a The lower pH value for Aerosil200 was 4.217 instead of 3.6 as for Sorbsil C30. All P values are expressed in the molar concentration basis. Wavelengths for the concentration determinations. NOH: 1 = 328 nm; NEtOH: 1 = 276 nm; N: 1 = 276 nm; CPC:
1= 259 nm.
The arrow indicates the cmc.
12
0
0.0025
0.005
0.0075
Cq,,
I
0.01
Ts(moliL)
Figure 6. Determination of the coadsorption partition coefficient ofNEtOH: CPC/Sorbsil C30 at two pH values: eq 3. pH = 6.5 ( 0 ) ;pH = 3.6 (0).
(mol/L)
Figure 7. Determinationof the desorptionpartition co&icient of NOH: CPC/Sorbsil C30 at two pH values: eq 4. pH = 6.5 (0);pH = 3.6 (0).
shown in Figures 1and 3, solute and surfactant adsorption are correlated. The following equation may then be expressed
ct - Cf
Pads
= CfCs,ads
(3)
where Ct, Cf, and Cs,ads are respectively the total solute concentration, the free solute concentration, and the corresponding adsorbed surfactant concentration. Equation 3 may only be applied in that region of the graphs where both surfactant and solute adsorption increase (below the cmc). Figure 6 shows the validity of this approach in the case of NEtOH a t the two pH values investigated, inasmuch a s straight lines are obtained in a reasonable concentration domain. The result of the calculations are displayed on the Table together with those concerning NOH and CPC. The previously published data for NOH and CPC coadsorbed on the nonporous silica Aerosil 200 (171, were recalculated using the present approach and are presented in Table 1. If the solute desorption a s defined above is entirely due to micellar solubilization, a partition coefficient may be written as
(4) where all variables have been defined.Of course eqs 3 and 4 rest upon the assumption that Cf, the free solute concentration, remains constant and equal to the minimum value attained as the solute adsorption is maximum. If the model is correct, Pdes should be equal to Pmic,as these two coefficients express the distribution process a t
-0
0.61
0.02
0.03
0.04
0.05
O.0G
C,,,qO"U
Figure 8. Desorption profile for the NEtOWCTAB/Sorbsil C30 system. pH = 6.5 (0);pH = 3.6 (0).
solute subsaturation concentrations, Le., dilute experimental conditions. Note that as the solute concentration is constant throughout the series of experiments for a single system, the relative decrease of desorption as the surfactant concentration increases must level off a s eventually, all solute molecules will be solubilized. The results of the above analysis are shown for NOH in Figure 7 where the relative solute desorption is plotted as a function of free surfactant concentration (above the cmc). The initial portion of the curves is linear at pH = 6.5 but not a t the lower pH value. The same observations could be made with the NOWCPC/Aerosil200 system.17 The results for NEtOH (Figure 8)display a very restricted linear concentration domain a t both pH values. A strong curvature is also observed for N (Figure 9). ThePdesvalues on Table 1 were evaluated by simulation of the desorption data with a quadratic equation when necessary and taking the initial slope. It should be pointed out that the adsorption and desorption analyses are performed on a linear concentration scale whereas, for reasons of presentation convenience, the raw experimental data on Figures 1-4 are presented in the logarithm scale.
Coadsorption of Naphthalene and Surfactants
Langmuir, Vol. 11, No. 5, 1995 1757
- 1
I
ts
I
Y"
0
0.01
0.03
0.02
C,,,q(mol/L)
Figure 9. Desorption profile for naphthalene for the CTAl3/ Sorbsil C30 system at pH = 3.6. ni
-.
Q
0.001 C,l(mol/L)
0.0005
0.0015
0.002
Figure 10. Determinationof the solubilitypartition coefficient of NOH (O), NEtOH (O), and naphthalene ( 0 ) in CTAB solution: eq 1.
I 2 2
d
d
20
0 0.01
0
0.02
0.03
C, - cmc (md/L)
Figure 11. Determination of the micellar partition coefficient for NOH with CTAB: eq 2.
0.75 3
Io I
I
0
0
m
O80
-7
-6
-5
-4
-3
-2
-1
Log C w q
Figure 12. Variation of the mole fraction of coadsorbed NOH as a function of the free equilibrium surfactant (CPC) concen-
tration.
Finally, the P,,, values obtained from the cmc experiments (eq 2) for NOH and NEtOH with CTAB are presented (for NOH) in Figure 10 and in Table 1 together with the previously published value with CPC. The two values for NOH almost coincide. Figure 11 presents the results from the solubility experiments (eq 1) for NOH,
NEtOH, and N with CTAB; the partition coefficients calculated from the slopes of these curves are displayed in Table 1. Equations 1-4 are written in the molal basis. All P values are presented on the molar scale using the classical relation: P(m1 = P(,jVO, where v" is the partial polar volume of the cationic surfactant. The same values could be used for CPC and CTAB: v" = 0.351 L The various questions addressed in the present investigation may be now summarized as follow: To what extent eqs 3 and 4 may be suited to represent the data. Is there a solute coadsorption pH dependence as there is a pH dependence for cationic surfactant adsorption on oxide materials? Are there differencesbetween the coadsorption and the micellar solubilization phenomena? What are the implications of the specific adsorption behavior of naphthalene? Note that in the case of NOH, the surfactant used was CPC, while for the other two solutes, CTAB was employed. CPC was chosen for reasons of convenience as it was easier to analyze using spectrophotometry. However, CPC forms a t higher concentrations in the presence of NOH viscous solutions, which was not the case with NEtOH and N. This type of viscoelastic system has been extensively studied by several research groups29-32with CTAB or CPC and sodium salicylate and found also with some other aromatic compounds. It seemed in the present case that only the NOWCPC system displayed macroscopically this type of behavior. Although this secondary effect did not prevent the use of such a system for adsorption experiments, the microrheological properties of the NOW surfactant bilayer at the silica surface could lengthen the time allowed for thermodynamic equilibrium. Therefore, CTAB was preferred for the other compounds studied. It was first demonstrated that the isotherms of CPC and CTAB overlap. Furthermore, it could be shown as noted above that Pmicas deduced from eq 2 was identical for CPC and CTAB (see Table 1). Thus, the results obtained for NOH (with CPC) and for the other two solutes (with CTAB) could be compared on the same basis. (1) Desorption Phenomenon and Micellar Solubilization Effect. As pointed out above, the analysis of the so-called desorption data in terms of partition coefficients raises more questions than the coadsorption results; thus, in order to be in a position to compare the two processes on a sound basis, it was felt appropriate to discuss first the meaning of the desorption experiments. The results of Table 1 show that Pdes is independent of pH (a small difference is observed for the NOWCPC/ Aerosil 200 system, but it seems too isolated to be considered as significant). This is what would be expected if, as postulated, the desorption corresponds to the micellar solubilization of the solutes, an effect that should independent of pH (contrary to the coadsorption) a t least for pH < 7. In effect, the pK of NOH is equal to 9.6, which ensures in principle that the phenol is in undissociated species under the present experimental conditions. This conclusion is further substantiated by analysis of the results of NOH, the solute for which the maximum of various independent solubilization data is available. One observes that Pdes =Pmic. One recalls that P,,, is obtained from experiments a t subsaturations concentrations as is Pdes. This equality shows that the desorption is entirely ~~
~~
(28)De Lisi, R.;Milioto, S.; Triolo, R. J . Solution Chem. 1988,17, 673. (29)Wan. L. S.C . J . Pharm. Sci. 1966. 55. 1395. (30) Hoffmann, H.; Kalus, J.;Schwander, B. Ber. Bunsenges. Phys. Chem. 1987,91,99. (31)Hirata, H.; Sato, M.; Sakaiguchi,Y.; Katsabe, Y. Colloid Polym. Scz. 1988,266,862. (32)Kandori, K.;McGreevy, J. R.; Schechter, R. S. J . Phys. Chem. 1989,93,1506.
1758 Langmuir, Vol. 11, No. 5, 1995 due to the micellar solubilization effect and that the results correspond to infinite dilution conditions with respect to the solute. The value obtained from the solubility experiments P s o l is smaller than P m i c or P d e s . This result is again expected, as it has been shown in recent years using gas chromatog r a p h i P vapor pressure34 or fluorescence quenching measurement^^^ that solubility data for polar solutes in micellar systems lead to apparent partition coefficients, which incorporate activity coefficients effects. True partition coefficients should be larger than the apparent ones. The case of NEtOH is somewhat puzzling. The P d e s values may be considered as independent from the pH within experimental error, but these values are smaller than those obtained from the solubility experiments. This result is contrary to expectation following the above arguments for NOH. As noted before, Pdccould not be evaluated for this compound. This may mean that the desorption process is not complete in that case and that there is a resistance to desorption. Alternatively, it could mean that the desorption process is depending on the order of magnitude of the coadsorption. For NOH, at the cmc the solution is almost free of solute molecules (see Figure 2) whereas for NEtOH, the coadsorption is less than for NOH. While the solute concentration is almost identical to that of NOH, free solute molecules are in the solution as free micelles formed above the cmc. The same effect is observed with N. P d e s is much smaller than Psol, but the difference between these two values is now extremely large. Notably, for both N and NEtOH, a strong curvature is observed on the desorption curves 7 and 9. Thus, although NEtOH is not adsorbed on silica in the absence of surfactant but N is, both solutes present qualitatively the same desorption profiles. This behavior could be related to the degree of surfactant coverage that is dependent on the pH of the solution. A previous analysis of cationic surfactant isotherms on Aerosil 200 a t pH = 4.512 in terms of the GuggenheimFowler formalism had shown that, a t that pH, full surfactant coverage is not attained. It should be a t pH = 6.5. That this effect is not related to the present findings is demonstrated by the results obtained, namely, P d e s is independent of the solution pH. It may be appropriate to recall here a previous observation concerning a dye in a similar situation. It has been observed that a dye such as Yellow OB, which is not adsorbed on silica in the absence of a suitable cationic surfactant but is coadsorbed in the presence of a dioctadecyldimethylammonium, chloride, is completelyretained in the surfactant structure even after several washings with water. It was postulated that the dye was irreversibly fixed only when the surfactant is present.14 The extensive measurements performed on N and CTAB on Sorbsil C30, showing in particular that the desorption is not a function of time, cannot be interpreted along these lines. However, N and NEtOH may represent intermediate examples between completely irreversible systems and purely reversible ones. The latter case would be represented by NOH under the present experimental conditions. Thus, the mode of desorption of hydrophobic solutes may be a n interesting approach toward the elucidation of coadsorption mechanisms. (2) Coadsorptionand Micellar Solubilization. As noted before, the determination of P a d s constants is relatively straightforward. Figure 6 shows a n example (33) Treiner, C.; Khodja, A. A,; Fromon, M.; Chevalet, J. J.Solution Chem. 1989,18,217. (34) Christian, S. D.; Tucker, E. E.; Smith, G. A,; Bushong, D. S. J. Colloid Interface Sci. 1986, 113,439. (35) Abuin, E. B.; Lissi, E. A. J . Colloid Interface Sci. 1983,95,198.
Monticone and Treiner of such a determination for NOH. However, the description of the coadsorption as a single process may be a n oversimplification. It was interesting to calculate the mole fraction of coadsorbed solute with respect to adsorbed surfactant. Figure 12 presents the results obtained for NOH as a function of free CPC concentration. It shows that the solute uptake is extremely important for the first addition of surfactant. One recalls that in the absence of surfactant there is no measurable adsorption of that solute. The same behavior is observed for NEtOH. These observations may support the viewpointzz that even a t low surfactant concentrations very small aggregates are formed on the solid surface that may solubilize hydrophobic molecules. Evidently, the solute mole fraction decreases very rapidly as more surfactant is added (below the cmc) because the experiments are performed a t constant solute concentration. It could be therefore appropriate to consider the possibility of a distribution of P d e s values. This approach has been a d v ~ c a t e d However, .~ it seems that the linearity depicted by the results such as those of Figure 6 allows the oversimplification used. The comparison between P d e s (or P m i c ) and P a d s shows a remarkable feature. In all cases (whether with the relative ideal case of NOH) or NEtOH, the coadsorption partition coefficients are much larger than the micellar solubilization constants, whatever the pH value considered. This is of course also true for the corresponding standard free energies. This is an important conclusion drawn from thermodynamic experiments. It has been previously suggested from fluorescence decay measurements using pyrene as a probe36 that the surfactant structures adsorbed on solid oxides such as alumina induce a larger microviscosity than classical micelles. If general, the above suggestion concerningthe uptake of hydrophobic solutes could be of the same importance. The comparison between NOH and NEtOH raises further questions. For NOH, P a d s is larger a t pH = 6.5 than a t pH = 3.6. Such an effect is not observed for the aromatic alcohol. Here it seems that an additive phenomenon is involved. This cannot be the pH effect on the dissociation of NOH, as pointed out above. It cannot be the consequence of the well-known (andbadly understood) preferential interaction of aryl compounds with cationic micelles3' as such an effect is not displayed by NEtOH. Finally, it cannot be explained either by a hydrophobic effect as, taking the solubility results for the three solutes with CTAB micelles, P s o l increases in the order: NOH > NEtOH > N. It has been noted previouslyz5 in a comparison on partition coefficients of aromatic and aliphatic molecules in cationic and anionic micelles that a t a neutral pH phenol is more tightly bound to cationic micelles than the other aromatic compounds. This behavior may be related to the formation of viscoelastic gels with phenols and cationic surfactants, which occur a t larger concentrations of both surfactant and solute as noted above. Finally, comparison of the P a d s data for NOH with CPC on the nonporous Aerosil 20017 and the porous Sorbsil C30 (see Table 1) does not display any significant differences at the pH values investigated. The data a t our disposal do not permit us to elaborate further on this important issue. (3) Some Implications of Naphthalene Behavior. The case of N was also interesting because of its implications as a representative hydrocarbon molecule. The solute is strongly adsorbed on silica in the absence of (36)Chandar, P.; Somasundaran, P.;Turro, N. J. J.Colloid Interface Sci. 1987, 117, 31. (37) Levitz, P.; Van Damme, H.; Keravis, D. J. Phys. Chem. 1986, 88, 2228.
Coadsorption of Naphthalene and Surfactants surfactant. The same qualitative result had been obtained with benzene on Aerosil 200.12 The reasons for this behavior are unknown and outside the scope of this investigation. The results of Figure 5 show that there is no adsorption change of N on the solid surface as CTAB as initially added to the system. The desorption begins exactly a t the cmc.. This means that the formation of a n adsorbed bilayer of cationic surfactants ions has no influence of the energetics of N adsorption. This result may have some implication with the use of similar hydrocarbons such a s pyrene a s a means of probing the structure of an adsorbed surfactant structure using a dynamic fluorescence t e c h n i q ~ e . It ~ is ~ ,usually ~ ~ assumed that even if the probe molecule adsorbs onto the solid, the effect of the surfactant might be to desorb the probe from the surface into the surfactant bilayer. Hence, the probe would sit in a surfactant environment. At least in the present case with N there is no thermodynamic evidence of a change of solute environment as the surfactant is adsorbed on the silica surface until free micelles are formed (Figure 5). As already noted above, the desorption of the hydrocarbon does not follow a n initial regular (linear) pattern. If the molecule would desorb from the surfactant bilayer a s with NOH or, to a certain extent, NEtOH, a very high initial slope should have been observed in Figure 9 because the partition coefficient Psol is so large. The reason for the observed behavior is not clear, but it does not seem to be the consequence of a kinetic problem as indicated by the reproductibility of the adsorption experiments extending over a period of 1week but rather the consequence of a n energetic one related to the particular solute/solid interaction for this chemical system. One may consider that the concentration of N is small enough so that it occupies a very small portion of the solid surface. Its presence has no influence on the adsorption of the surfactant, and its desorption likewise is not dependent on the surfactant structure. An additional argument may be put forward in favor of the hypothesis of a strong adsorption of N on Sorbsil C30 preventing an easy desorption from the solid surface even in the presence of a n excess of micelles. The 1-octanol/ water binary has been successfully used as a model system for estimating the hydrophobicity of various nonpolar (38)Viaene, K.;Verbeek,A.;GeladB, E.;De Schryver,F.C.Langmuir 1986, 2,456.
Langmuir, Vol. 11, No. 5, 1995 1759 s o l ~ t e s . It~has ~ ~been ~ ~shown , ~ ~ beforez5that a correlation may be found between log P values for micellar solubilization in anionic and cationic surfactant systems and in the two-phase binary. The logPOctvalues for N and NOH are of the same order of magnitude being equal to 3.30 to 2.89, r e ~ p e c t i v e l y .In ~ ~so far as the coadsorption phenomenon may be regarded as a n adsolubilization effect, the above comparison suggests that the driving forces for adsolubilization are of the same order of magnitude for both solutes. Thus, the desorption differences observed for N and NOH must be due to the solid/liquid interaction of the former compound.
Conclusions 2-Naphthol and 2-(2)naphthyl)ethanol, solutes which are not adsorbed on silica dispersions in the absence of cationic surfactants, are uptaken by the surfactant structures of CPC or CTAB adsorbed on Sorbsil C30, a porous silica. Large solute/surfactant ratios are attained a t very low surfactant concentrations. This coadsorption phenomenon may be described by a single partitioning process. The free energy of adsorption is generally larger than that corresponding to the micellar solubilization effect. Above the equilibrium cmc, the solutes are desorbed from the surface and solubilized in the free micelles. However, the desorption process is less easily described by a simple distribution formalism because some solutes may be somewhat retained by the adsorbed surfactant structures. Naphthalene is adsorbed on silica in the absence of surfactant. Surface adsorption does not modify naphthalene adsorption until the surfactant cmc is reached; above that concentration, the hydrocarbon molecule desorbs from the silica surface. The adsorbed cationic surfactant ions may not be involved in this phenomenon. Finally, comparisons, in the case of NOH, of the coadsorption phenomenon in the presence of CPC on porous (Sorbsil C30) or nonporous (Aerosil 200) dispersions do not show any significant differences. LA940882D (39) Fujita, T.;Nishioka, T.;Nakajima, M. J.Med. Chem. 1977,20, 1071. (40)Valsaraj, K.T.; Thibodeaux, L. J. Sep. Sci. Technol. 1990,25,
369. (41)Hansch, C.; Leo, A. In Substituent constants in correlation analysis in chemistry and biology;Wiley-Interscience:New York, 1979.
