Lungmuir 1994,10, 2395-2398
2395
Coadsorption of 2-Naphthol and Cetylpyridinium Chloride at a Silicflater Interface in Relation with the Micellar Solubilization Effect V. Monticone, M. H. Mannebach, and C. Treiner* Laboratoire d%lectrochimie, URA CNRS 430, Universitk Pierre et Marie Curie, Bat.F., 4 Place Jussieu, Paris 75005, France Received February 24, 1994. In Final Form: May 4, 1994@ The coadsorption of 2-naphthol and a cationic surfactant, cetylpyridiniumchloride (CPyCl),at a silica/ water interfacehas been investigated at pH = 4.5 and pH = 6.5. In both cases, the adsorptionof 2-naphthol follows closely the surfactant adsorption. However, complete 2-naphthol desorption is observed as a consequence of the preferential solute solubilization at higher surfactant concentration when regular micelles are formed, without surfactant desorption. The partition coefficient of 2-naphthol between the surfactant layers and water is calculatedand compared to the micellar solubilizationdistribution constant as determined by two independent methods. The partition coefficient of 2-naphthol is shown to be systematically larger for the adsorption process than for the micellar solubilization. Thus, contrary to alcohols and dyes, 2-naphthol presents a good case of complete reversiblebehavior when coadsorbed with a cationic surfactant at a silicdwater interface.
Introduction Some hydrophobic solutes which do not adsorb at the interface between mineral oxide particles and water have been shown to be coadsorbed in the presence of surfactants. This phenomenon has been coined surface solubilization1 or adsolubilization.2 Most surfactants adsorb at the mineral oxiddwater interface provided the ionic strength and the pH of the system are carefully chosen. The precise structure of the adsorbate has been and still is, to some extent, the subject of much debate3 However, the general features of the evolution of these structureswith surfactant concentration in terms of surfactant monomers, small aggregates (hemimicelles or admicelles), and surfactant layers are generally accepted. It is not surprising then that hydrophobic solutes might penetrate such aggregative structures much as in the classical case of micellar solubilization. The solutes which have been investigated are not numerous. They include aliphatic and aromatic alcohols,4-8 some d y e ~ , drug ~ J ~molecules,'l and 2-naphthol.12 Few authors have attempted to describe the interaction of the solute with the adsorbed surfactant aggregates in terms of thermodynamic (distribution) constant^.^^^,^
* To whom all correspondence should be addressed.
Abstract published inAdvance ACS Abstracts, June 15,1994. (1)Nunn, C.C.; Schechter,R. 5.;Wade, W. H. J. Phys. Chem. 1982, 86,3271. (2)Scamehom,J.F.;Schechter,R. S.;Wade, W. H. J. Colloidlnterfbce iwa, 85,463. (3)(a) Yeskie, M. A; Harwell, J. H.; O'Fkar, E.A J . Phys. Chem. 1988,92,2346.6) Chandar,P.;Somaeundaran,P.;Turro,J. J . Colloid Interface Sci. 1987,117,31.(c) Harwell, J. H.; Yeskie, M. A. J . Phys. Chem. 1989,93,3372. (4)Lee, C.;Yeskie, M. A.; Harwell, J. H.; O'Rear, E. A. Lungmuir 1990,6,1758. (5)Esumi, K.;Shibayama, M.; Megum, K. Langmuir 1990,6,826. (6) Blokhus, A. M. Colloid Polym. Sci. 1990,268,679. (7)Backlund, S.;Sjoblom, J.; Matijevic, E. Colloid Surf.A 1993,79, 263. (8)Monticone, V.; Treiner, C. J . Colloid Interface Sci., in press. (9)Zhu, B-Y.;Zhao, X.; Gu, T. J . Chem. Soc., Faradny Trans. 1 1988, 11, 3951. (10)Esumi, IC; Sugimara, A.; Yamada, T.; Meguro, K. Colloid Surf. A 1992,62,249. (11)Janssen, J.;Treiner, C.;Vaution,C.;Puisieux, F.Int. J . P h a n . 1994,103,19. (12)Klumpp, E.; Heitmann,H.;Schwuger,M. J.CoZloidSurf.A 1993, 78,93.
0743-7463/94/2410-2395$04.50/0
From the few systems investigated so far, it is not possible to derive general observationson the coadsorption phenomenon, but differences with micellar solubilization have already been noted;8 e.g., the possible interaction, however small, of the solutes with the solid surface is of paramount importance and may completely inhibit coadsorption. Hence, the parent classical situation where correlation of the adsorption constant of hydrophobic solutes with the partition coefficientin the octanol/water system, which have been extensively used as a general standard for the modeling of neutral compounds at soil/ water interfaces, may not apply easily here. In order to be able to compare quantitatively coadsorption and micellar solubilization effects,we shall report here the results of a complete case with 2-naphthol, a classical pollutant, as the hydrophobic solute, cetylpyridinium chloride as a cationic surfactant, and a very pure silica, Aerosil200, as the adsorbent. Coadsorption and micellar solubilization have been determined independently and will be compared at different pH values. Moreover, it will be shown that the coadsorbed aromatic pollutant may be completely desorbed from the silica surface in the presence of an excess of surfactant.
Materials and Results Aerosil200, a very pure silica, without pore, was a gift from Degussa-France. It was used as received. The BET surface was 200 f 15 m2g-l. Cetylpyridinium chloride (CPyCl) was from Sigma. The critical micelle concentration (cmc), as determined by surface tension measurements (Kruss KlOT) at 25 "C was 8.3 x mol*L-l, in excellent agreement with literature values. The Gibbs plot showed no sign of a minimum around the cmc. In the presence of 0.01 mol-L-' NaC1, the cmc dropped to 2.4 x mo1.L-l. All the adsorption experiments were performed in the presence ofNaC1. 2-Naphthol (99%pure from Aldrich) was used as received. Its aqueous solubility was determined at pH = 6.12 and found equal to 5.1 x mol-L-' in agreement with literature values. Doubly distilled water was passed over 0.2-pm filters. The experimental procedure was described before.s Briefly summarized, 2-naphthol and CgrCl were equilibrated in the presence of 1% silica for 12 h in a thermostated bath at 25 "C. The dispersion was cen0 1994 American Chemical Society
Monticone et al.
2396 Langmuir, Vol. 10, No. 7, 1994 Table 1. Characteristic Parameters for the Simultaneous Coadeorption of CPyCl and 2-Naphthol at a Silic-ater Interface in the Presence of 0.01 mol-L-' NaCl CbCl
CadmaX (mo1.L-1)
A (nm2/molecule) %-naphthol
.%admax(mobL-') xf (mo1-L-l)
1%
Pad
log Pln,,
DH= 4.5
DH= 6.5
4.40 x 10-3 0.76 3.17 x 8.3 10-5 2.94 2.37
8.05 x 10-3 0.41 3.7 x 3 x 10-5 3.19 2.79
trifugated (Sigma 2K15) at 10 000 rpm for 1 h at that same temperature. The supernatant concentrations of 2-naphthol and CPyCl were analyzed simultaneously by U V spectroscopy after adequate dilution respectively at 328 and 259 nm on the same samples (double-beam Cary 1E spectrophotometer). Total 2-naphthol concentration was maintained constant at 4 x mo1.L-l for all experiments while CPyCl concentration was varied from to 2 x mo1.L-'. In the absence of added surfactant, 2-naphthol does not adsorb on the chosen silica to any measurable degree down to the lowest concentrations attainable with the present UV instrument, i.e. around mo1.L-l with this solute. The adsorption experiments were performed at two pH values: 4.5 and 6.5. The first pH is the natural value of Aerosil 200 dispersed in water in the presence of salt, surfactant, and solute. The second pH value was imposed because it represented, for that surfactant and at the present solution ionic strength, the conditionfor maximum adsorption. It may be recalled that imposing the pH of the system, the surfactant adsorption is controlled but the dissociation of 2-naphthol is also depending upon the pH. However, the pK of this naphthalene derivative is equal to 9.51, thus at the two pH values investigated, 2-naphthol should be considered as essentially undissociated species. A partition coefficient for 2-naphthol between the adsorbed surfactant layers and water can be calculated at the maximum solute adsorption (corresponding to the plateau values of Figures 1 and 2) from the relation
__
max
where Xad" is the maximum adsorbed 2-naphthol concentration, xf the equilibrium (free) solute concentration at the plateau, and CadmaX is the corresponding maximum adsorbed CPyCl concentration. Two different P a d values are obtained a t the two pH values investigated (Table 1). The micellar partition coefficientwas determined using two completely different approaches. The first one uses a relation which relates the decrease of cmc of a surfactant with the addition of a neutral solute to the solute partition coefficient between the pseudomicellar phase and water. It has been s h o ~ d that ~ J ~the following equation is adequate
where cmco and cmc are respectively the cmc of the surfactant in pure water and in the presence of 2-naphthol at a molar concentration x . a is the degree of association of the surfactant counterions; a = 0.27 in the present (13) Treiner, C.; Mannebach, M.-H. J. Colloid Interface Sci. 1987, 118,243. (14) De Lisi, R.; Milioto, S.; Castagnolo,M.; Inglese, A. J.Solution Chem. 1987,16, 373.
5
0.5
(I
I
I
'I a
**
0
,-n
0
LE95
W4OI
t
** 0.001
om
o.l
0. I
c , ~(mot.L.1)
Figure 1. Variation of 2-naphthol (0)(right coordinate) and CPyCl (0)(left coordinate) adsorption as a function of equilibrium surfactant concentration at pH = 4.5. Table 2. Variation of the Critical Micelle Concentration of CPyCl with 2-Naphthol Molarity in Water at 25 "C x , mo1.L-l
0
3.81 LOO 1.50 2.00 x
10-4 10-3 10-3 10-3
cmc, mo1.L-' 8.30 7.80 5.78 4.69 3.64
10-4 10-4 10-4 10-4 10-4
case.15 P (without subscript) is the micellar solubilization partition coefficient on the mole fraction basis, MI the molar mass of water, andk. the Setchenow constant which describes the interaction of the neutral solute and the surfactant below the cmc. The cmc determinations were performed using conductivitymeasurements (Wayne Kerr bridge, Model 6425) from conductivity versus surfactant concentration plots in pure water. It is known that the partition coefficient is hardly influenced by the presence of a small salt concentration such as that used in the present investigation (0.01 mol.L-' of NaC1). k, could be evaluated using the salting-in data obtained for naphthalene and pyridinium salts which were obtained by Vesala et a1.16 If the solubility of 2-naphthol and naphthalene in water are different, the relative solubility changes induced by the addition of the salt are much less sensitive to the effect of the presence of a polar group on the naphthalene moiety. One obtains k , = -5.2, a large value, indeed. From the cmc data of Table 2 one derives then a log Pvalue equal to 2.83 in the molal scale. Figure 3 illustrates the use of eq 2. Another approach can be suggested for the determination of the micellar solubilization constant. If one assumes that the decrease of 2-naphthol adsorption shown on Figures 1and 2 is due to the micellar solubilization effect (see the discussion below), then the concentration of solubilized solute xsol is simply: xsol= Xt-Xad-Xf where all quantities are known (Table 1);a partition Coefficient may then be calculated from the relationship
Note that xf, the concentration of free solute, is assumed identical whether adsorption or micellar solubilization occurs. Figure 4 presents the results obtained at the two pH values investigated. The ordinate is the ratio of (15)Makayssi, A.; Treiner, C. Langmuir 1992, 8,794. (16)Vesala, A.; Perkola, H.; hnnberg, H. Finn. Chem. Lett. 1981, 3, 40.
Langmuir, Vol. 10, No. 7, 1994 2397
Coadsorption of 2-Naphthol and Surfactant
The results obtained from all experiments are presented in Table 1.
I
0
IE45
O.W(l
0.001
0.01
I
0.1
C,,,(mol.L.')
Figure 2. Variation of 2-naphthol(O)(right coordinate) and CpyCl (0)(left coordinate) adsorption as a function of equilibrium surfactant concentration at pH = 6.5.
& , (mo1.L-1)
Figure 3. Determination o f P ~from c the variation of micellar solubilization as a function of micellized surfactant concentration (eq 3): 0, pH = 4.5; 0, pH = 6.5.
0
2.5
Figure 4. Determination of P from the variation of the surfactant cmc with 2-naphthol concentration (eq 2).
solubilized to free solute concentrations and the abcissa is the equilibrium surfactant concentration. The straight lines go through the origin at C,, equal zero, as expected. The more diluted surfactant concentrations (i.e. cor, just above the solute desorption threshresponding to C old (see below)) were used for these plots. The two lines enable the calculation of the partition coefficients at the two pH values.
Discussion Figures 1and 2 show the adsorption profiles for both 2-naphthol and CPyC1, where the compounds adsorption in mol-L-l units are plotted as a function of their free (equilibrium) concentration. At a concentration close to the cmc (inthe presence of 0.01mo1.L-l of NaCl), surfactant adsorption levels off, i.e., regular micelles begin to form. The 2-naphthol profile follows initially that of CPyC1: the solute adsorption rises initially with surfactant adsorption, then a plateau region is observed while empty micelles are formed. Finally 2-naphthol desorption occurs as micellar solubilization is preferred over coadsorption. As shown on the two figures, the surfactant does not desorb from the silica surface in the concentration range investigated and therefore the 2-naphthol desorption must be due to micellar solubilization. At higher surfactant concentration, 2-naphthol is, eventually, completely desorbed from the silica surface. It may be pointed out that at the higher pH value, the maximum solute adsorption is observed at a surfactant concentration somewhat below the cmc and that the plateau region is larger than at the lower pH. Both of these observations are further indications of a larger coadsorption phenomenon at pH = 6.5 than at 4.5. The 2-naphthol desorption upon micelle formation had been noted before on a clay/surfactant interface.12 However, complete desorption was not attained and the effect was not quantified. Alcohols in generalsp8J0do not seem to be desorbed from mineral oxide surfaces (silica or alumina) by micelle formation at surfactant concentrations up to 100 times the cmc. The quasi-irreversible adsorption of some dyes on silica and other surfaces is well-known.lJO The effect of pH needs also to be pointed out. Increasing the pH increases the surfactant adsorption. As the surface is negatively charged above pH = 3.2 (for the present silica),increasing the pH increases the number of adsorption sites for the cationic surfactant. Table 1shows the surface areaA (in nm2)occupied by a CF'yCl monomer at the silica interface, at the two pH values studied, as calculated using the classical relation
where N Ais Avogadro's number, m is the mass of silica in grams (0.20 g), V the volume of solution (20 mL), and S the BET surface (200 mLg-l). At pH = 4.5, the value found for A is close to that obtained from a Gibbs (vapor/ water) isotherm (0.71 nm2 versus 0.79 nm2 11) and therefore may be still compatible with a surfactant monolayer. At pH = 6.5,A = 0.41 nm2and hence abilayer has to be assumed. Recent studies support the view of a bilayer formationwith cationic surfactants at silidwater interfaces even at low surfactant coverage."J8 As the pH changes from 4.5 to 6.5, the increased CPyCl adsorption is not followed by a parallel increase for 2-naphthol. Up to 80% of the solute is already adsorbed a t pH = 4.5, an amount which increases to 93% at pH = 6.5. No attempt was made to optimize the solute adsorption, although it should clearly be possible by increasing the solution ionic strength or the pH. It may be pointed out that the 2-naphthol mole fraction in the surfactant layers, which can be easily calculated from the data of (17)~nnie,A.R.;Lee,E.M.;Sinister,E.A.;Thomas,R.K.Langmuir 1990,6, 1031. (18)Soderlind, E.;Stilbs, P.Langmuir 1993,9, 2024.
2398 Langmuir, Vol. 10, No. 7, 1994 Table 1, is very small. Hence the partition coefficients calculated from eq 1may be considered as limitingvalues, free from activity coefficient corrections, a condition which is also fulfilled when eq 2 is used. One of the purposes of this investigation was to compare the adsorption P a d and the micellar Pmicpartition coefficients. The results of Table 1show that P a d is systematically larger than Pmic.The value of P deduced from eq 2 is almost identical to that obtained from the desorption portions of Figures 1and 2, as deduced from eq 3 at pH = 6.5 (log P = 2.83 whereas log Pdc = 2.79). However, P a d seems to be dependent upon the pH, which is not the case for the pure micellar solubilization effect with CPyC1. The interpretation of the difference observed between the partition coefficients of the adsorbed and micellar solubilized molecules could be related to the structure of the surfactant aggregates. Micelles are small spherical bodies to which the Laplace pressure effect, as introduced by Mukerjeelg has often be invoked to explain the lower solubility of neutral molecules (nonpolar gases) in micelles when compared to liquid hydrocarbons. The Laplace pressure, which opposes the penetration of a hydrophobic solute, would be nil or negligible in the case of flat surfactant layers, hence the higher P a d values. Such an interpretation assumes the penetration of the surfactant structures (micelles or layers) by the solute molecule, a situation which cannot be easily proven. Furthermore, there are some indications that 2-naphthol interacts rather strongly with the surfactant ammonium headgroups. For example, the log Podvalue for the partition of a neutral solute between octanol and water is a useful index for modeling of the distribution of neutral compounds between organic mater (in soils) and water. Comparison of the log PWtvalues for naphthalene and 2-naphthol is interesting in that respect. They are close to each other, being equal respectively to 3.20 and 2.89. However, for the hexanelwater binary, the values are 3.38 and 0.26.21 Thus, penetration ofthe solute inside a hydrocarbon phase (such as a surfactant layer with the hydrocarbon chains pointing toward the bulk solution)cannot be invoked here; rather, these numbers indicate that the naphthalene derivative and octanol interact through hydrogen bonding. This point has been demonstrated before by microcalorimetric experiments for phenol in solvent phases.22 (19)Mukerjee, P.Kolloid 2.2.Polym. 1970,236,76. (20)Prapaitrakul, W.;King, A. D., Jr. J.Colloid Interface Sci. 1985, 106, 186. (21)Hansch, C.;Leo, A. In Substituent constants for correlation analysis in chemistry and biology;Wiley-Interscience: New York, 1979.
Monticone et al. Furthermore, it has been that log Poctmay be correlated to log Pmic values for neutral solutes in cationic and anionic micelles. One may infer then that 2-naphthol is solubilized in the regular micelles or adsorbed in the surfactant layers at the solidwater interface through the participation of the ammonium headgroups. This implies that these groups should be sterically available. The near equality of Pmic and P at the higher pH value, as deduced from the two approaches used, seems to indicate that a complete surfactant bilayer is indeed formed at pH = 6.5 but not at 4.5. As noted above, the maximum 2-naphthol adsorption occurs even below the surfactant cmc at pH = 6.5, but at the cmc at pH = 4.5. There could be an alternative explanation to the different P values obtained from adsorption and micellar experiments. The concentration of free 2-naphthol molecules is measured directly only in the former case. In the latter, this concentration is assumed constant whilexaddecreases andx,,l increases. This hypothesis is generally accepted, for example, in the case of equilibrium dialysis experiments. It is noteworthy that in the present cases xeqis very small and, therefore, weighs heavily on the numerical value of the partition coefficient. Nevertheless, in our opinion, the difference between the two xf values is too large to be considered as a consequence of such an artifact, and thus, the larger the consequence value ofPadwhen compared to P (orP ~is& of a real physical phenomenon.
Conclusion The present investigation has shown that, in the case of 2-naphthol and with a cationic surfactant such as CPyC1, solute adsorptioddesorption processes occur a t the silica/ water interface depending upon the surfactant concentration. The phenomenon is completely reversible contrary to the cases of alcohols, dyes, or most drug molecules investigated so far. Comparison between adsorption and solubilization partition coefficients shows the former coefficientsto be larger than the latter ones at the two pH values investigated. The coadsorption phenomenon seems to be favored by 2-naphthollammonium headgroup interactions. The difference of partition coefficients may therefore be related to the degree of availability of these headgroups for interaction which in turn should depend on the respective structures of the adsorbed and of the micellar aggregates. (22) Breslauer, K. J.; Witkowski, L.; Bulas, K. J . Phys. Chem. 1978, 82,675. (23)Treiner, C.J . Colloid Interface Sci. 1983,93,33.
