Volumetric and Fluorescence Studies of Aqueous Solutions

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Volumetric and Fluorescence Studies of Aqueous Solutions Containing n-Octylamine, Cetyltrimethylammonium Bromide, and Salt Yongkui Yue,†,‡ Jianji Wang,*,† and Ming Dai*,‡ Department of Chemistry, Henan Normal University, Xinxiang, Henan 453002, P. R. China, and Research Laboratory of Solution Chemistry, Henan Institute of Chemistry, Zhengzhou, Henan 450003, P. R. China Received November 23, 1999. In Final Form: March 30, 2000 Densities of aqueous solutions containing n-octylamine, 0.0500 mol kg-1 cetyltrimethylammonium bromide (CTAB), and sodium chloride/potassium chloride have been determined and used to derive the standard partial molar volumes of n-octylamine at 298.15 K. The effect of the additives (sodium chloride, potassium chloride, and n-octylamine) on the polarities of the micelles have also been investigated by static fluorescence spectra using pyrene as a probe at 298.15 K. It has been observed that the postmicellar transitions of sphere-to-rodlike, as indicated by the amplified changes in the standard partial molar volumes of n-octylamine, are affected strongly by the addition of an electrolyte. The effect of sodium chloride on the postmicellar transition of CTAB is much weaker than potassium chloride. Polarity of the interior of the mixed micelle and therefore the solubilized site of pyrene vary with molality of the additives. This is considered to be caused by the difference in chemical structure and hydrophobic nature of the mixed micelle. The results of fluorescence spectra reveal the possible microscopic causes for the effect of additives on the postmicellar transition of CTAB.

Introduction Surfactants are widely used in a variety of industrial and commercial applications.1 The commercial surfactants are invariably surfactant mixtures, which are better in many ways than single surfactants.2,3 Studies on the physicochemical properties of surfactant mixtures have proved powerful for optimizing these properties by just changing the solution compositions. This has led to both theoretical and practical interests.4,5 Therefore, a thorough understanding of the physics and chemistry of such systems is highly desirable. It is well-known that cetyltrimethylammonium bromide (CTAB) undergoes a sphere-to-rodlike micellar transition in aqueous solution as its concentration increases.6-8 Like micellization, this transition occurs in a broad range of concentrations since it is actually a pseudophase transition. The concentration at which the micellar transition occurs seems to be technique dependent. A micellar transition of CTAB has been reported in the concentration range from 0.05 to 0.34 mol kg-1 by using viscosity, Rayleigh light scattering, 81Br NMR, SAXS, and other techniques.9-15 Micellar transitions can be promoted by the addition of additives, and the nature of the additives †

Henan Normal University. Henan Institute of Chemistry. * Corresponding author. E-mail: [email protected].

‡

(1) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley: New York, 1989. (2) Abe, M.; Ogino, K. Mixed Surfactant Systems; Surfactant Science Series No. 46; Marcel Dekker: New York, 1992. (3) Scamehorn, J. F. In Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; ACS Symposium Series 311; American Chemical Society, Washington, DC, 1986. (4) Thomas, H. G.; Lomakin, A.; Blankschtein, D.; Benedek, G. B. Langmuir 1997, 13, 209. (5) Rosen, M. J. J. Am. Oil Chem. Soc. 1989, 66, 1840. (6) Backluhnd, S.; Hoiland, H.; Kvammen, O. J.; Ljosland, E. Acta Chem. Scand. A 1982, 36, 698. (7) Reidblom, G.; Lindman, B.; Mandell, L. J. Colloid Interface Sci. 1973, 42, 400. (8) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964, 68, 3504.

plays an important role in this aspect. Generally, the micellar transition is achieved by the appropriate conditions of concentration, salinity, temperature, presence of the counterion, and so forth. Recently, Kumar et al.16-18 have shown that not only inorganic salts but also organic compounds such as n-alcohols, amines, and aromatic hydrocarbons are also potential candidates for such structural changes. It has been pointed out that thermodynamics give little direct information on the structure of the micellar species, but it is very sensitive to the interactions in these systems.19,20 So it can be used to detect the micellar transition, particularly at low concentrations of additives. The study of transfer functions of a hydrophobic solute from water to aqueous surfactant solutions is a good way to probe the postmicellar transitions in a system.21 An effort has been made to measure the changes in thermodynamic properties of hydrophobic probe in the presence (9) Roux-Desgranges, G.; Roux, A. H.; Grolier, J. P.; Viallard, A. J. Solution Chem. 1982, 11, 357. (10) Perron, G.; Delisi, R.; Davidson, I.; Genereux, S.; Desnoyers, J. E. J. Colloid Interface Sci. 1981, 79, 432. (11) DeLisi, R.; Milioto, S.; Castagnolo, M.; Inglese, A. J. Solution Chem. 1990, 19, 767. (12) DeLisi, R.; Milioto, S.; Triolo, R. J. Solution Chem. 1988, 17, 673. (13) Treiner, C.; Chattopadhyay, A. K.; Bury, R. J. Colloid Interface Sci. 1985, 104, 569. (14) Quirion, F.; Desnoyers, J. E. J. Colloid Interface Sci. 1986, 112, 565. (15) Ekwall, P.; Mandell, L.; Solyom, P. J. Colloid Interface Sci. 1971, 35, 519. (16) Kumar, S.; Kirti; Kumari, K.; Kabir-ud-Din J. Am. Oil Chem. Soc. 1995, 72, 817. (17) Kumar, S.; Kirti; Kabir-ud-Din J. Am. Oil Chem. Soc. 1994, 71, 763. (18) Kumar, S.; Aswal, V. K.; Singh, H. N.; Goyal, P. S.; Kabir-udDin Langmuir 1994, 10, 4069. (19) Desnoyers, J. E. J. Surf. Sci. Technol. 1989, 5, 289. (20) Christien, S. D.; Scamehorn, J. F. Solubilization in Surfactant Aggregates; Marcel Dekker: New York, 1995. (21) Perron, G.; DeLisi, R.; Davidson, I.; Genereux, S.; Desnoyers, J. E. J. Colloid Interface Sci. 1981, 79, 432.

10.1021/la991530f CCC: $19.00 © 2000 American Chemical Society Published on Web 06/28/2000

Solutions of n-Octylamine, CTAB, and Salt

and absence of additives. In particular, Desnoyers et al.22 and Roux-Desgranges and co-workers9 successfully used benzene and 1-butanol, respectively, to probe the postmicellar transition in aqueous sodium dodecyl sulfate (SDS) solutions. In general, the variation of the transfer functions for the thermodynamic probe in these systems are much more significant than those for each of the components in the corresponding binaries. The aim of the present work is to investigate the effect of additives (salinity and solubilized organic compound) on the CTAB micellar transition by density measurement using a new thermodynamic probe. The concentration of CTAB was kept constant at 0.0500 mol kg-1 (before the postmicellar transition). In this case, the system consists of sphere micelles. It presents a well-defined second cmc attributed to the sphere-rod transition in the absence of additives. Potassium chloride (KCl) was chosen as an additive because its addition only screens out Coulombic force between micelles.23 Sodium chloride (NaCl) was selected in order to investigate the difference in the effect of K+ and Na+ on the postmicellar transition. n-Octylamine (OA) was chosen as the solubilized compound and new thermodynamic hydrophobic probe because it is only slightly soluble in water but quite soluble in micellar solutions.24 Finally, the micropolarity of the site of solubilized pyrene in the solutions were determined by steady-state fluorescence spectra in order to reveal the possible microscopic causes for the micellar transition. Experimental Section Materials. CTAB (A. R., Beijing Chem. Reagent Co.) was twice recrystallized from the mixture of acetone and methanol, then washed with anhydrous dimethyl ether, and dried at 333 K under vacuum for at least 48 h before use. The purity was ensured by the absence of a minimum in the plot of surface tension versus logarithm of surfactant concentration near the cmc. NaCl and KCl (both G. R., Beijing Chemicals) were dried under vacuum for at least 48 h at 333 K, and no further purification was attempted. n-Octylamine (A. R., Fluka) was dehydrated over solid KOH for at least 24 h and purified by fractional distillation at reduced pressure. Pyrene (A. R., Sigma) was used directly as a fluorescence probe without further treatment. The deionized water was further purified by redistilling from an alkaline permanganate solution, and the conductivity was 1.0 × 10-6 S cm-1 at 298 K. Experimental Methods. All solutions were freshly prepared by mass. Density measurements were carried out with a vibrating-tube digital density meter (Anton Parr DMA 60/602, Austria) which has been described elsewhere.25,26 The temperature around the density meter cell was controlled by circulating water from a constant-temperature bath (Schott, Germany). A CT-1450 temperature controller and a CK-100 ultracryostat were employed to maintain the bath temperature to 298.15 ( 0.005 K. The density meter was calibrated with pure water and dry air from time to time at 298.15 K. The uncertainty in density was estimated to be (1.5 × 10-6 g cm-3. An appropriate amount of the stock solution of pyrene in methanol (G. R.) was weighed into a flask, and the solvent was evaporated by a gentle stream of dry nitrogen. A solution containing given molalities of CTAB and an electrolyte was added, and the concentration of pyrene was kept at 5.0 × 10-6 mol kg-1. The above solutions were mixed effectively for at least 30 min by using an ultrasonic washing machine (Wuxi, China). All the samples for fluorescence measurements were bubbled with high(22) Hetu, D.; Roux, A. H.; Desnoyers, J. E. J. Colliod Interface Sci. 1988, 122, 418. (23) Goyal, P. S.; Menon, S. V. G.; Dasannacharya, B. A.; Rajagopalan, V. Chem. Phys. Lett. 1993, 211, 559. (24) Yanashita, T.; Yano, H.; Harada, S.; Yasunaga, T. J. Phys. Chem. 1983, 87, 5482. (25) Pecker, P.; Tremblay, E,; Jolicoeur, C. J. Solution Chem. 1974, 3, 377. (26) Wang, J.; Yan, Z.; Liu, W.; Lu, J. Z. Phys. Chem. 1997, 199, 25.

Langmuir, Vol. 16, No. 15, 2000 6115 Table 1. Standard Partial Molar Volumes of n-Octylamine (OA) in Aqueous Solutions Containing CTAB and KCl/NaCl as a Function of Molality of the Electrolyte at 298.15 Ka OA + CTAB + KCl + water

OA + CTAB + NaCl + water

mKCl (mol kg-1)

o V2,φ (cm3 mol-1)

mNaCl (mol kg-1)

o V2,φ (cm3 mol-1)

0.0000 0.0100 0.0300 0.0500 0.0800 0.1000 0.1787 0.5000 1.0000

159.79 ( 0.02 159.50 ( 0.03 158.60 ( 0.05 157.40 ( 0.05 158.81 ( 0.05 159.92 ( 0.03 160.23 ( 0.03 160.61 ( 0.02 161.47 ( 0.05

0.0000 0.0100 0.0200 0.0500 0.1000 0.1500

159.79 ( 0.02 159.62 ( 0.07 158.92 ( 0.12 158.96 ( 0.13 159.52 ( 0.11 159.54 ( 0.12

a

The molality of CTAB was kept at 0.0500 mol kg-1.

purity hydrogen for about 20-30 min for the purpose of deoxygenation. The ratio of the first and the third vibronic peaks (I1/I3) for the fluorescence emission spectrum of pyrene was recorded on a fluorophotospectrometer (Shimadzu RF-540, Japan) equipped with a data recorder (Shimadzu DR-3, Japan). Excited and emission slit width were set to be 2 nm. The fluorescence intensities of pyrene (excitation wavelength at 339 nm yields the maximum fluorescence intensities) were obtained by monitoring the spectra from 350 to 450 nm. Throughout the fluorescence measurements, the temperature was controlled to be 298.15 ( 0.05 K by circulating water from a thermostat bath (Shimadzu TB-85, Japan).

Results and Discussion Apparent Molar Volumes. Densities of aqueous solutions containing n-octylamine, CTAB, and NaCl/KCl at 298.15 K are reported in the Supporting Information. The apparent molar volumes of n-octylamine are calculated from the solution densities by the following equation:

V2,φ ) M/F - 103(F - F0)/(m2FF0)

(1)

where M is the molar mass of n-octylamine, m2 is the molality of n-octylamine in aqueous solutions containing CTAB (0.0500 mol kg-1) and NaCl/KCl, and F and F0 are the densities of the solution (n-octylamine + 0.0500 mol kg-1 CTAB + NaCl/KCl +water) and the solvent (0.0500 mol kg-1 CTAB + NaCl/KCl + water) at 298.15 K, respectively. The calculated apparent molar volumes for n-octylamine as a function of the molalities of n-octylamine and KCl/NaCl are also included in the Supporting Information. The apparent molar volumes of n-octylamine in aqueous solutions containing CTAB and KCl are found to be well represented by the linear equation o + Svm2 V2,φ ) V2,φ

(2)

o is the apparent molar volume of n-octylamine where V2,φ at infinite dilution and Sv is the experimental slope. In o equals in value the standard partial molar fact, V2,φ volume of n-octylamine. Equation 2 was fitted to our calculated V2,φ data using a linear least-squares regression o values are listed in Table 1. analysis. Thus, obtained V2,φ However, the apparent molar volumes of n-octylamine in aqueous solutions containing CTAB and NaCl are found to be constant within the experimental uncertainty. In this case, the standard partial molar volumes have been calculated by taking an average of all the data points. The o V2,φ values obtained in this way are also included in Table

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Figure 1. Standard partial molar volumes of n-octylamine in aqueous solutions containing CTAB (0.0500 mol kg-1) and KCl as a function of the molality of KCl at 298.15 K.

1. To the best of our knowledge, these data are reported for the first time. Figure 1 shows that the standard partial molar volumes of n-octylamine exhibit a break at about 0.05 mol kg-1 KCl in the quadruple systems, indicating occurrence of the micellar transition. This suggests that addition of KCl not only increases the ionic strength but also enhances the postmicellar transition. This observation is supported by the rapid growth of the aggregates of CTAB mixed micelles derived from the neutron scattering, light scattering, and heat capacity studies14,27 of CTAB in aqueous KCl solutions. On the basis of the studies of Desnoyers et al.,14 the volume changes of CTAB in aqueous solutions containing KCl are not significant within the experimental uncertainty. It is found, however, in this work that the changes of volume are amplified many times for the new thermodynamic probe n-octylamine. The postmicellar transition becomes more easily detected by changes of the standard partial molar volumes of n-octylamine forming mixed micelles. It can be seen from Table 1 that the variation of standard partial molar volumes of n-octylamine in aqueous solutions containing CTAB and NaCl is not significant within the experimental error. This indicates that the effect of NaCl on the micellar transition of CTAB is very weak. Considering the fact that the amplitude of the change for the standard partial molar volumes of n-octylamine in aqueous solutions containing CTAB and NaCl is much smaller than those in aqueous solutions containing CTAB and KCl, we believe that the effect of NaCl on the micellar transition of CTAB is much weaker than that of KCl. A similar result has been reported28 for tetradecyltrimethylammonium bromide + electrolyte + water systems from calorimetric measurements. n-Alkylamines are solubilized in CTAB micelles by electrostatic and hydrophobic effects with the amine group left on the surface of the micelle.23 There are at least two opposing factors responsible for the micellar transitions. One is the intercalation and solubilization of n-octylamine in the micelles, which decrease the intramicellar Coulombic repulsive forces and increase the hydrophobic forces among the monomers of the micelle and therefore favor the micelle transition. The other is due to the partial dissociation of n-octylamine into -NH3+ and -OH-, which may affect the electrostatic interactions with cationic (27) Perron, G.; Desrosiers, N.; Desnoyers, J. E. Can. J. Chem. 1976, 54, 2163. (28) Nguyen, D.; Bertrand, G. L. J. Colloid Interface Sci. 1992, 150, 143.

Figure 2. Variations of I1/I3 as a function of the molality of n-octylamine (OA) in aqueous solutions containing CTAB (0.0500 mol kg-1) and KCl at 298.15 K: 0, OA + CTAB + water; b, OA + CTAB + 0.0100 mol kg-1 KCl + water; 4, OA + CTAB + 0.0500 mol kg-1 KCl + water; ×, OA + CTAB + 0.1000 mol kg-1 KCl + water.

headgroup and thus hinder the micellar growth.29 Because the dissociation of n-octylamine is not significant in the systems investigated, the promotion effect of n-octylamine should be predominant for the micellar transition. The addition of KCl/NaCl induces the screening of the repulsive forces and increases the intermicellar interactions and thus enhances the postmicellar transitions of CTAB. Therefore, the changes of volumes in the micellar transition are the combined effect of n-octylamine and KCl/ NaCl. Polarity of the Solubilized Site. The solubilized site of different compounds within micellar systems can be correlated with the structural organization of aggregates. The relative fluorescence intensities of the vibronic bands in a monomeric pyrene fluorescence emission spectrum are known to be sensitive to the local polarity of the solubilized site of pyrene.30,31 In particular, the ratio of the first and the third vibronic peaks (I1/I3) has been shown to behave as a linear function of dielectric constant of solvents.32 Low values of I1/I3 indicate that the environment of the solubilized pyrene is apolar or hydrophobic as in hydrocarbon solvent. On the other hand, I1/I3 values in polar or hydrophilic solvents are generally high. The I1/I3 values obtained in this work for water, methanol, ethanol, and hexane are 1.70, 1.38, 1.15, and 0.60, respectively. They are in fairly good agreement with those reported in the literature.33 I1/I3 values above 1.8 suggest that most of pyrene is dispersed in aqueous phase; the opposite trend is found in hydrocarbon solvents. The values of I1/I3 are plotted in Figures 2 and 3 as a function of the molalities of n-octylamine and KCl/NaCl. It is evident that these values for the micelles are close to those for polar solvents such as ethanol. This indicates that CTAB micelles provide the solubilized pyrene with (29) Kabir-ud-Din; Kumar, S.; Kirti; Goyal, P. S. Langmuir 1996, 12, 1490. (30) Chen, M.; Gratzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1975, 97, 2052. (31) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (32) Glushkov, V.; Thaler, M. S.; Karp, C. D. Arch. Biochem. Biophys. 1981, 210, 33. (33) Ueno, M.; Kimoto, Y.; Ikeda, Y.; Momose, H.; Zana, R. J. Colloid Interface Sci. 1987, 117, 179.

Solutions of n-Octylamine, CTAB, and Salt

Figure 3. Variations of I1/I3 as a function of the molality of n-octylamine (OA) in aqueous solutions containing CTAB (0.0500 mol kg-1) and NaCl at 298.15 K: 0, OA + CTAB + water; b, OA + CTAB + 0.0100 mol kg-1 NaCl + water; 4, OA + CTAB + 0.0500 mol kg-1 NaCl + water; ×, OA + CTAB + 0.1000 mol kg-1 NaCl + water.

a microenvironment that is nearly as apolar as hydrocarbon solvents. For the micelles of ordinary surfactants, the solubilized site of aromatic hydrocarbons such as pyrene is the palisade layers of micelle owing to their slight surface activity.34 Judging from our I1/I3 values, the solubilized site of pyrene in the CTAB micelles also seems to be the palisade layers. On the other hand, I1/I3 values for the mixed micelles decrease with increasing concentration of n-octylamine at any given molalities of the electrolyte, suggesting that the solubilized site of pyrene moves from the palisade layers to the interior of the micelle. Such changes indicate that the hydrophobic structure of the mixed micelles has been changed in the present case. It can also be seen from Figures 2 and 3 that values of I1/I3 at given molalities of n-octylamine decrease with increasing concentration of the electrolyte. This indicates the increased hydrophobic nature in the micelle and the changed solubilized site of pyrene as the concentration of the electrolyte increases. It seems that the variations in (34) Cardinal, J. R.; Mukerjee, P. J. Phys. Chem. 1978, 82, 1614; J. Phys. Chem. 1978, 82, 1620.

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polarity of the mixed micelles are due to the transition of the solubilized site of pyrene from palisade layers to inside of the micelles. Therefore, the decreased I1/I3 values observed suggest that chemical structure of the mixed micelle varies with the molalities of KCl or NaCl. From the above observation, it can be deduced that the solubilized pyrene is efficiently shielded from the aqueous phase in the case of the mixed micelles in CTAB + n-octylamine + KCl/NaCl systems. Because NaCl/KCl are known to be absorbed at the micellar surface, the repulsion of the inter-headgroups decreases,31 and the mixed micelle becomes more tight in the presence of the electrolyte. Therefore, the microenvironment of the solubilized pyrene turns more hydrophobic with increasing concentration of an electrolyte in the system investigated. In the concentration range of electrolyte from 0 to 0.05 mol kg-1, the shape of the mixed micelles appears to be nearly spherical, and the solubilized site of pyrene is the palisade layers of mixed micelle. It seems to form an outer shell with the -NH2 group facing the aqueous phase, and the micellar inside becomes more hydrophobic. Therefore, pyrene is considered to be incorporated inside the micelles together with the alkyl chain of n-octylamine. The penetration water between the palisade layers was pushed out, resulting in the decreased values of I1/I3. Conclusions 1. The postmicellar transitions of sphere-to-rodlike were affected strongly by the addition of the electrolyte and organic additive. The effect of KCl on the postmicellar transition of CTAB is much stronger than NaCl. 2. n-Octylamine, KCl, and NaCl promote the micellar transition. n-Octylamine could be used as a novel thermodynamic probe for detecting the micellar transition of CTAB. 3. CTAB micelles provide the solubilized pyrene with a microenvironment that is nearly as apolar as hydrocarbon solvents. The micropolarities of the mixed micelles decrease with increasing molalities of n-octylamine and KCl/NaCl. Acknowledgment. The authors are grateful to the Natural Science Foundation of Henan Province for financial support and to the referees for their valuable suggestions. Supporting Information Available: Densities and apparent molar volumes for n-octylamine in aqueous solutions containing CTAB and KCl/NaCl. This material is available free of charge via the Internet at http://pubs.acs.org. LA991530F