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Langmuir 1999, 15, 966-969
Influence of Urea on Sodium Decyl Sulfate Micellization by Kinetic and Solubility Studies Karen A. Berberich and Vincent C. Reinsborough* Department of Chemistry, Mount Allison University, Sackville, New Brunswick, Canada E4L 1G8 Received July 7, 1998. In Final Form: November 9, 1998 Stopped-flow and solubility studies were used to ascertain the effect 0-7 M urea had upon the micellization of anionic surfactants with sodium decyl sulfate (SDeS) as the surfactant of choice. The complexation of Ni2+ ion with the ligand trans-pyridine-2-azo-p-dimethylaniline (PADA) was the probe reaction for the stopped-flow investigation. The addition of urea increased the critical micelle concentration, and attenuated the rate enhancement ordinarily seen with this complexation in SDeS micellar solution. Urea interacted positively with water in the sense that the complexation rate doubled at 7 M urea with no surfactant present. In micellar solution, urea rendered the SDeS micelles far less accommodating to both Ni2+ ion and PADA which indicated that both the surface and core micellar regions were considerably urea modified especially beyond 4 M urea.
Introduction Ionic and polar additives serve as as a rule to decrease the critical micelle concentration (CMC) in aqueous surfactant systems. Thus, ever since Mukerjee and Ray1 first noted that urea in surfactant solution increased rather than decreased the CMC, there has been considerable interest in rationalizing this finding along with the other micellar modifications that urea has since been found to induce. Recent literature on the effects of urea on micellization is summarized by Abuin et al.2 and by Ruiz3 from which it is clear that a satisfactory explanation has not yet been realized. The principal investigative procedure adopted in the present work was micellar catalysis using the probe complexation reaction of Ni2+ with the organic ligand, trans-pyridine-2-azo-p-dimethylaniline or PADA. The kinetic parameters of this reaction as obtained through the Robinson version4,5 of the pseudophase model for micellar catalysis6,7 are sensitive to variations in micelle structure and dynamics.8-10 This technique seemed appropriate because debate revolves around whether urea interacts more significantly with the solvent (“indirect mechanism”) or with the micelles (“direct mechanism”) in bringing about the observed changes in properties in surfactant solution.2,3,11 Catalysis takes place within the micelle so, if urea does not partition into the micelle as postulated by the proponents of the indirect mechanism, then ideally there should be very little modification to the normal kinetic pattern of the probe reaction upon urea * To whom all correspondence should be addressed. (1) Mukerjee, P.; Ray, A.; J. Phys. Chem. 1963, 67, 190. (2) Abuin, E. A.; Lissi, E. A.; Aspe´e, A.; Gonzalez, F. D.; Varas, J. M. J. Colloid Interface Sci. 1997, 186, 332. (3) Ruiz, C. C. Mol. Phys. 1995, 86, 535. (4) James, A. D.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1978, 74 1978. (5) Fletcher, P. D. I.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1984, 80 1984. (6) Amado, S.; Garcı´a-Rı´o, L; Leis, J. R.; Rı´os, A. Langmuir 1997, 13, 687. (7) Bunton, C. A. J. Mol. Liq. 1997, 72, 231. (8) Connolly, T. J.; Reinsborough, V. C. Can. J. Chem. 1992, 70, 1581. (9) Favaro, Y. L.; Reinsborough, V. C. Can J. Chem. 1994, 72, 2443. (10) Drennan, C. E.; Hughes, R. J.; Reinsborough, V. C.; Soriyan, O. O. Can. J. Chem. 1998, 76, 152. (11) Ruiz, C. C.; Sa´nchez, F. G. J. Colloid Interface Sci. 1994, 165, 110.
addition. PADA solubility measurements were required as ancillary to the kinetic analysis but also they independently provide another insight into micellar phenomena.12 The negatively charged micelles of anionic surfactants attract Ni2+ ions Coulombically and PADA molecules hydrophobically giving rise to rate enhancements of the Ni2+/PADA complexation reaction. Sodium decyl sulfate (SDeS) was the surfactant of choice in this work after preliminary results with several anionic surfactants showed that the catalytic effect was maximally and fully accommodated within the stopped-flow range by the SDeS system. Experimental Section A Cantech stopped-flow spectrometer was used to obtain rate coefficients of the Ni2+/PADA reaction in SDeS solutions under pseudo-first-order conditions (typically, [Ni2+] ) 5.00 × 10-3 M, [PADA] ) 1 × 10-5 M) at 25 °C.13,14 Solubilities of PADA in the SDeS system were obtained at the same temperature by sonicating the solutions for 1 h in contact with excess PADA and then keeping them covered in the dark for 24 h. Each solution was filtered by gravity and diluted appropriately to enable accurate absorbance measurements to be made at 472 nm, λmax for PADA. HPLC grade SDeS was obtained from Acros Organics and urea (99% purity) from Aldrich. All other chemicals were the best available commercially and not further treated.
Results and Discussion With no surfactant present, the value of kobs increased linearly in urea solutions until in 7 M urea it had approximately doubled (Figure 1). Now kobs ) kf[Ni2+] + kb where kf and kb are the forward and backward rates of Ni2+/PADA complexation, respectively. The value of kf in water is 1090 ( 50 dm3 mol-1 s-1, and no change could be detected in kb (0.10 ( 0.04 s-1) over the urea concentration range used. Thus, the increase in kobs is attributable to an increase in kf occasioned by the urea background. The addition of urea to an aqueous solution increases the (12) Connolly, T. J.; Reinsborough, V. C.; Xiang, X. Aust. J. Chem. 1992, 45, 769. (13) Reinsborough, V. C.; Robinson, B. H. J. Chem Soc., Faraday Trans. I 1979, 75, 2395. (14) Hicks, J. R.; Reinsborough, V. C. Can. J. Chem. 1984, 62, 990.
10.1021/la980834j CCC: $18.00 © 1999 American Chemical Society Published on Web 01/09/1999
Influence of Urea on Micellization
Langmuir, Vol. 15, No. 4, 1999 967 Table 1. Micellization Parameters for SDeS in Aqueous Urea Solutions
Figure 1. Comparison of observed rate coefficients of Ni2+/ PADA complexation reaction in urea solutions with [Ni2+] ) 0.50 mM (k-ratio is the ratio of the observed rate coefficient in aqueous urea solution to that in water).
Figure 2. Rate enhancements (R or k-ratio′ ) of the Ni2+/PADA complexation reaction in SDeS solutions with [Ni2+] ) 0.50 mM in urea solutions (k-ratio′ is the ratio of the observed rate coefficient in SDeS solution to that in water). Open circles denote no-urea solutions; ×’s, 2.0 M urea solutions; closed circles, 7.0 M urea solutions.
dielectric constant and a first explanation of the increase in kf might be in this direction. However, in another complexation reaction, the alkaline hydrolysis of Co(NH3)5Cl2+ in aqueous urea solutions, Calvaruso et al.15 noted a similar rate increase in the 0-7 M urea range. Since the reactants were oppositely charged in the latter case, a decrease in reaction rates rather than an increase would have been expected in this simplistic view. Clearly, the interaction between solvent, additive and solute in this situation will not admit to an easy explanation as attested to by the confusing picture that is found in the literature.2,3 Some possible interactions in the system under scrutiny can be eliminated: urea is not known to complex significantly with Ni2+(aq)16 and this was confirmed from Ni2+ ion spectra and urea even at 7 M did not within experimental error affect the solubility of the other reactant, PADA. Thus, given these two observations and the similarity of the rate increases with those found by Cavaruso et al.,15 the observed rate enhancement in surfactantless urea solutions was probably due to more specific urea/water interactions than permittivities would reveal. In SDeS solutions with urea as additive, the concentration dependence of the observed rate coefficients showed modified λ-curves typical of micellar catalysis.17 In Figure 2 are given the kobs curves for the no-urea, the 2 M urea and the 7 M urea SDeS systems (kinetic data were also obtained for the 5 and 6 M urea systems; 1 M urea showed no significant differences from the no-urea system). Urea served to attenuate the catalytic effect in dilute micellar solutions but in more concentrated micellar solutions may have even promoted it, especially in the more highly urea(15) Calvaruso, G.; Cavasino, F. P.; Sbriziolo, C.; Liveri, M. L. T. J. Chem. Soc., Faraday Trans. 1993, 89, 1373. (16) Sacconi, L.; Mani, F.; Bencini, A. In Comprehensive Coordination Chemistry: The Synthesis, Reactions and Applications of Coordination Compounds; Wilkinson, G., Ed.; Pergamon: Oxford, England, 1987; Vol. 5, p 71. (17) Reinsborough, V. C.; Robinson, B. H. J. Chem. Educ. 1981, 58, 586.
[urea] (M)
CMCa (mM)
CMCb (mM)
CMCc (mM)
KPADAd (M-1)
KNie (M-1)
Ve (M-1)
0 1.0 2.0 5.0 6.0 7.0
26
28 33 35 38 40 43
31 32
920 610 450 190 150 110
50
0.44
20 90 20 9
0.27 0.92 0.25 0.14
25 25 35 40
50
a Kinetic (error 10%). b PADA solubility (error 5%). c Surface tension.20 d Error 5%. e Error 15%.
Figure 3. Rate enhancements (R or k-ratio′ ) of the Ni2+/PADA complexation reaction in SOS solutions with [Ni2+] ) 0.50 mM in urea solutions. Open circles denote the no-urea solutions and the closed circles, the 5.0 M urea solutions.
doped systems. The apparent CMC indicated by the onset of rate enhancement was shifted to higher SDeS concentration with increasing urea addition (Table 1). The presence of 0.5 mM Ni2+ would be expected to depress the CMC of SDeS slightly.10 Figure 3 demonstrates that similar diminutions of the catalytic effect and increases in the CMC occurred in the sodium octyl sulfate (SOS) system with urea as background. In the pseudophase model of micellar cataylsis, which received its original formulation with Berezin,18 the complexation reaction is assumed to take place in both the bulk solution and the micellar “phase” with the partitioning tendencies of reactants and products between the two reaction media dictating in a conventional manner the resulting observed rate coefficients. Relatively crude assumptions, e.g., that the micelles do not change in size and the value of the CMC corresponds to the free surfactant concentration in micellar solution, do not prevent a successful quantitative correlation of multisourced experimental results in many surfactant systems including microemulsions.5 The simplified Robinson variant of this kinetic expression takes the form
1/R ) CV{1 + (CKPADA)-1}{1 + (CKNi)-1} where R is the ratio of the observed pseudo-first-order rate coefficients, with surfactant and without any surfactant present, i.e., skobs/okobs; C is the concentration of the micellized surfactant or (CT - CMC) where CT is the total surfactant concentration; V is the reaction volume per mole of micellar surfactant.8,19 KPADA and KNi are the partitioning constants for each of the reactants of the probe reaction distributing between the bulk solution and the micelles. KPADA is defined as (Ss - So)/CSo where S is the solubility of PADA and the subscripts correspond to the two different media, micellar surfactant solution and water, respectively. KNi is defined analogously, i.e., as (18) Martinek, K.; Yatsimirski, A. T.; Levashov, A. V.; Berezin, I. V. In Micellization, solubilization, and microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 489. (19) Fletcher, P. D. I.; Reinsborough, V. C. Can. J. Chem. 1981, 59, 1361.
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Figure 4. Solubility of PADA in SDeS solutions with various backgrounds of urea. Closed circles denote the no-urea values; crosses, 2.0 M urea; the open circles, 6.0 M urea. Data in the 1.0 M urea, 5.0 M urea, and 7.0 M urea systems were not displayed for the sake of clarity. The best straight line was drawn through the no-urea data points.
Berberich and Reinsborough
Figure 5. Solubility of PADA in aqueous urea solutions at constant micellar SDeS concentration (C). Open circles represent C ) 3 mM SDeS; closed circles, C ) 23 mM SDeS; ×’s, C ) 43 mM SDeS.
[Ni2+]s - [Ni2+]o/C[Ni2+]o. KPADA is obtained through PADA solubility measurements while KNi and V are most easily calculated as parameters of best fit of the kinetic data to the above equation. The values of KPADA, KNi, and V are listed in Table 1. The solubility itself of PADA in SDeS solutions with no urea showed little change until about 30 mM SDeS (Figure 4). Beyond this concentration it increased linearly until at 120 mM SDeS it was 102 times greater than its value in water (1.5 × 10-4 M PADA5). Urea in solution had the effect of pushing this solubility breakpoint, or CMC, to higher SDeS concentrations and the micellar uptake of PADA was less enhanced in urea solutions. In 6 M urea solutions, at 120 mM SDeS the solubility of PADA had increased only an order of magnitude above its pure water value as compared with 2 orders of magnitude when no urea was present (Figure 4). The CMC values for the SDeS system as detected by the break in the PADA solubility curves for 0-7 M urea backgrounds are given in Table 1 where they are compared with the stopped-flow results and surface tension data.20 The stopped-flow CMC results are undoubtedly too low because of the Ni2+ ion present in solution.10 The smoothly decreasing values of KPADA, obtained from the slopes of the linear plots of (Ss - So)/So against C for SDeS, are given in Table 1 (Ss is the PADA solubility in surfactant solution while So is its value without surfactant). Not surprisingly, the partition coefficients for PADA decreased as the urea concentration was increased reflecting the decrease in PADA solubility. This indicates that PADA was being less preferentially assumed into the micelles. It is also significant that So did not change within experimental error when the urea concentration was varied over the range 0-7 M urea, as has been stated earlier. Thus urea did not appear to be affecting the PADA solvation mechanism. This is significant because oftentimes urea interacts with molecular probes rendering interpretations problematic.2,21,22 As evidenced by the 5-fold drop in KNi values upon attaining urea concentrations of 7 M, the other reactant of the probe reaction, Ni2+ ion, was also less attracted to SDeS micelles as the urea background was increased (Table 1). This was not unexpected as urea is well-known to reduce the counterion association degree of micelles,2 which would result in a diminished micellar uptake of the oppositely charged Ni2+ ion. The dramatic drop in the surfactant reaction volume V that occurred over the same urea concentration range is less easily explained. The value of V of 0.44 dm3 mol-1
obtained in SDeS solutions without urea is the effective volume of the surfactant in the micelle,18 and is in agreement with an earlier determination.19 It should not be much different from the estimated molar volume of the surfactant (ca. 0.3 dm3 mol-1)23 since both reactants should be available for reaction in roughly 75% of the micelle.18 When salts are added to anionic surfactant solutions, the CMC is decreased, while micellar size and V are increased.14 The opposite tendencies are observed when urea is the additive.2 In the case of the urea systems, since the product CV defines the effective volume fraction of the solution that the probe reaction requires within the micelles, this means that the surface region of the micelles wherein Ni2+ ion has been adsorbed has become constricted (the molar volume of micellar sodium octyl sulfate is 0.18 dm3 mol-1 24 while V for SDeS in 7 M urea is only 0.14 dm3 mol-1). When micelles are reduced in size on shortening the length of the hydrocarbon tail, the CMC increases and the effective micellar molar volume expands as the micelles become less compact and delineated.18 The difference with urea in solution is that this increase in CMC and reduction in micellar size are apparently not accompanied by a loosening up of micelle structure. Das Gupta and Moulik25 have reported discontinuities in micellar properties in the vicinity of 4 M urea. The seemingly discordant values of KNi and V in the 5 M urea system (Table 1) may be a similar manifestation. An interesting graph in this regard is Figure 5 wherein the solubility of PADA is shown for various urea systems at constant concentration of micellized SDeS (C). For urea concentrations of 5-7 M urea, the solubility of PADA is the same for constant C and is only slightly decreased when the urea concentration is increased. With less concentrated urea solutions, the solubility of PADA decreases markedly as urea is added and decreases severalfold when C is increased at constant urea concentration. This suggests that around 4 M urea the micelles undergo a transitional change. This could explain why in the more concentrated urea systems the micellar catalysis effect actually increases rather than decreases at high surfactant concentration (Figure 2). Beyond 4 M urea, a different type of micelle may exist but at this stage this cannot be verified without further measurements. Be that as it may, urea must be still substantially modifying the SDeS micelle. The rate of the probe reaction increases in bulk solution with urea as additive, 2-fold in 7 M urea, but this increase by itself cannot account for the kinetic data when SDeS micelles provide an ordinarily preferred alternate route for the reaction. The major reductions in both KNi and KPADA that occur on the addition of urea signify that the micelles are much less friendly environments for the reactants. Ni2+ ions are attracted to
(20) Sasaki, T.; Amano, H.; Suzuki, H. Bull. Chem. Soc. Jpn. 1978, 51, 1973. (21) Abu-Hamdiyyah, M.; Kumari, K. J. Phys. Chem. 1990, 94, 6445. (22) Gonzalez, M.; Vera, J.; Abuin, A. B.; Lissi, E. A. J. Colloid Interface Sci. 1984, 98, 152.
(23) Hicks, J. R.; Reinsborough, V. C. Aust. J. Chem. 1982, 35, 15. (24) Musbally, G. M.; Perron, G.; Desnoyers, J. E. J. Colloid Interface Sci. 1974, 48, 494. (25) Das Gupta, P. K., Moulik, S. P. Colloid Polymer Sci. 1989, 267, 246.
Influence of Urea on Micellization
the surface region of the micelle while the hydrophobic reactant PADA is probably situated near the surface region in the micellar core. The rapid exchange in microsecond time of PADA between bulk solution and micellar interior allows the probe reaction in its millisecond time range to proceed without interference into the surface region of the micelle. Thus, the finding that both reactants are much less attracted to the micelles in the presence of urea indicates that urea modifies significantly both core and micellar surface regions as well as having an influence in solvent structuring. Conclusions Urea promotes the Ni2+/PADA complexation reaction in water but when anionic micelles are present and the dominant reaction zone becomes the micellar pseudophase urea lessens the effectiveness of the micelles as “catalysts”. CMC values are increased on urea addition but the resultant micelles although smaller2,3 are less loosely organized than would be expected of a micelle of that size.2 Luminescent probe molecules are ejected to the bulk solution2 as the introduction of urea increases the local viscosity of the micellar interface and decreases its polarity.3 The findings of this work are consistent with these views. We find that urea does not appreciably interact with either of the reactants, so its effect on the complexation is either through modification of the water structure or of the micellar structure. The former must
Langmuir, Vol. 15, No. 4, 1999 969
occur to some extent as seen through the increase of reaction rate when no surfactant is present. However, urea addition also causes both PADA and Ni2+ ion to be less attracted to the micelle so that urea must be integrally modifying the micelle, with a radical change likely taking place in micelle structure around 4 M urea. The Ni2+/ PADA complexation reaction occurs in the micellar surface region and will be particularly sensitive to any structural changes induced by urea to this part of the micelle. Each of the two reactants find the urea-modified micelle surface much less accommodating, and the decrease in the reaction volume V reflects the micelle’s more compact structure. The puzzling increase in the rate enhancements at high surfactant and micellar concentrations that become more pronounced with further urea additions may be a consequence of increased reactivity outside the micelle in the direction seen when no surfactant is present (Figure 1). In terms of the indirect or direct mechanisms cited in the Introduction that have been proposed for urea interactions with micellar solutions, this study suggests both are operative, but the second leads to micellar modifications that have a dramatic effect on reaction rates. Acknowledgment. We wish to thank the Natural Sciences and Engineering Research Council of Canada for financial support and Mount Allison University for a Summer Research Scholarship to K.A.B. LA980834J
