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Articles Microcalorimetric Studies of the Interaction between DDAB and SDS and the Phase Behavior of the Mixture Guangyue Bai, Yujie Wang, Jinben Wang, Buxing Han, and Haike Yan* Center of Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received May 31, 2000. In Final Form: February 13, 2001 The interaction enthalpies between didodecyldimethylammonium bromide (DDAB) and sodium dodecyl sulfate (SDS) have been measured at 298.15 K using an isothermal titration microcalorimeter. From the variation of enthalpy with SDS concentration, the enthalpy of interaction between DDAB micelles and SDS, the enthalpy of formation of the DDA+SD-(cryst), the enthalpy of vesicle formation of DDAB-SDS, and the enthalpy of transition from vesicles to SDS-rich micelles have been determined to be -29.53, -125.8, 41.23, and 32.10 kJ/mol, respectively. The phase behavior of the DDAB-SDS aqueous mixture has also been investigated by turbidity measurements and transmission electron microscopy.
Introduction Since the observation of Kunitake et al.1 that the surfactant didodecyldimethylammonium bromide (DDAB) forms vesicles on ultrasonication, many investigators have studied surfactant vesicles, especially those spontaneously formed from surfactant mixtures.2-11 Any understanding of the mechanism of vesicle formation should be based at least in part on the energetics of the process. This in turn should be accessible through thermodynamic measurements, for which it is now well established12-16 that calorimetry is an excellent tool for surfactant aggregation. In this paper, we present results on the interaction enthalpies and phase behavior of the aqueous mixtures of DDAB and sodium dodecyl sulfate (SDS) obtained mainly by titration microcalorimetry. Vesicle formation in mixtures of SDS and DDAB has previously been * To whom correspondence should be addressed: Institute of Chemistry, the Chinese Academy of Sciences, Beijing, 100080, P. R. China. Tel: 86-010-62562821. E-mail:
[email protected]. (1) Kunitake, T.; Okahata, Y. J. Am. Chem. Soc. 1977, 9, 3860. (2) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (3) Safran, S. A.; Pincus, P.; Andelman, D. Science 1990, 248, 354. (4) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698. (5) Brasher, L. L.; Herrington, K. L.; Kaler, E. W. Langmuir 1995, 11, 4267. (6) Bergstro¨m, M.; Eriksson, J. C. Langmuir 1996, 12, 624. (7) Yuet, P. K.; Blankschtein, D. Langmuir 1996, 12, 3802. (8) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792. (9) Bernardes, A. T. Langmuir 1996, 12, 5763. (10) O’Connor, A. J.; Hatton, T. A.; Bose, A. Langmuir 1997, 13, 6931. (11) Villeneuve, M.; Kaneshina, S.; Imae, T.; Aratono, M. Langmuir 1999, 15, 2029. (12) Singh, S. K.; Nisson, S. J. Colloid Interface Sci. 1999, 213, 152. (13) Wang, Y. L.; Han, B. X.; Yan, H. K.; Kwak, J. C. T. Langmuir 1997, 13 (12), 3119. (14) Onori, G.; Santucci, A. J. Phys. Chem. B 1997, 101, 5224. (15) Van Os, N. M.; Daane, G. J.; Haandrikman, G. J. Colloid Interface Sci. 1991, 141 (1), 199. (16) McMahon, C. A.; Hawrylak, S.; Marangoni, D. G.; Palepu, R. Langmuir 1999, 15, 429.
followed by measurement of particle size,17 by NMR,18,19 and by transmission electron microscopy, ζ potential measurement, glucose trapping, and surface tension,20 but there have been no calorimetric studies. Experimental Section Materials SDS purchased from Bethesda Research Laboratories (purity 98%) and DDAB from Aldrich Chemical Co. (Purity g 99.5%) were used as received. All the surfactant solutions (concentration > critical micelle concentration, cmc) were prepared using doubly distilled water (conductivity 1.2 × 10-6 S cm-1). Isothermal Titration Microcalorimetry. An improved LKB-2107 microcalorimeter with a 1 cm3 titration calorimeter was used, for which the instrumental and experimental procedures have been described in a previous paper.21 The calorimeter was calibrated electrically with a precision of (1% and tested by measuring the dilution enthalpy of a concentrated sucrose solution (0.985 mol/dm3), the value of which was -0.643 ( 0.015 kJ/mol, in agreement with the literature value of -0.653 kJ/mol at the final concentration (0.090 mol/dm3).22 For calorimetric experiments of dilution of DDAB or SDS, both sample cell and reference cell were loaded with 0.5 cm3 of pure water at the beginning of the experiment. Then, while stirred at 50 rpm, the concentrated solution of DDAB or SDS was injected to the sample cell using a 500 mm3 Hamilton syringe. The volume of each injection was 10-20 mm3 until the desired range of dilution has been covered. For the measurements on DDAB-SDS mixtures, the sample cell was loaded with 0.5 cm3 of 0.01 mol/dm3 DDAB solution. The titrant, 0.05 mol/dm3 SDS solution, was injected from the Hamilton syringe, the volume of each injection being 10 mm3 with a total of 30-40 injections per experiment. The interval between two injections was kept sufficiently long that (17) Marques, E.; Khan, A.; Miguel, M. Da G.; Lindman, B. J. Phys. Chem. B 1993, 97, 4729. (18) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M. Da G.; Lindman, B. J. Phys. Chem. B 1998, 102, 6746. (19) Marques, E. F.; Regev, O.; Khan, A.; Miguel, M. Da G.; Lindman, B. J. Phys. Chem. B 1999, 103, 8353. (20) Kondon, Y.; Uchiyama, H.; Yoshino, N.; Nishiyama, K.; Abe, M. Langmuir 1995, 11, 2380. (21) Bai, G. Y.; Wang, Y. J.; Wang, J. B.; Yang, G. Y.; Han, B. X.; Yan, H. K. Sci. China, Ser. B 2000, 43(6), 617. (22) Gucker, F. T., Jr.; Pickard, H. B.; Planck, R. W. J. Am. Chem. Soc. 1939, 61, 459.
10.1021/la000768x CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001
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Figure 2. Titration microcalorimetric curves from the addition of 0.05 mol/dm3 SDS to DDAB at 298.15 K: (a) the recorder plot of voltage (V)-time (t) of the addition of 0.05 mol/dm3 SDS to DDAB and (b) the variation of observed enthalpies of the titration of SDS into DDAB (O) and pure water (b) with concentration of SDS. removed using filter paper and then grid dried in a vacuum desiccator for 6 h before the TEM observation. Turbidity Measurements. The turbidity of the DDAB-SDS solutions was measured on a General TU-1201 UV-vis spectrophotometer at a wavelength of 400 nm in quartz sample cells 1 cm thick. The concentrations of the surfactant mixture solutions were the same as those used for the TEM experiments.
Results and Discussion
the signal could return to the baseline. Each complete experiment therefore took in excess of 16 h. A recorder plot of voltage (V)time (t), such as shown in Figure 2a, was obtained, and the observed enthalpy in kJ/mol was yielded from integration of each peak of the plot. All experiments were performed at 298.15 ( 0.02 K and were repeated at least three times. Vesicle Preparation. A solution 0.05 mol/dm3 in SDS was added slowly into a test tube containing 0.01 mol/dm3 DDAB solution with continuous slow shaking of the mixture. The mole fraction of SDS, XSDS (moles of SDS/total moles of DDAB and SDS) varied from 0.10 to 0.90 with an increment of 0.1. The aggregates of the mixture were observed by a transmission electron microscope (TEM) (Hitachi H800) using the negativestaining method.23 One drop of the mixed surfactant solution was spread on a 200-mesh copper grid coated with a carbon film, followed by the addition of one drop of the staining solution (1.5 wt % of uranyl acetate). The excess solution was then immediately
Calorimetric Measurements. The calorimetric measurements for dilution of concentrated SDS or DDAB solution are shown in Figure 1 where the observed enthalpies are plotted against DDAB or SDS concentrations. In the concentration range below the cmc, on dilution in the calorimeter any micelles of DDAB or SDS are dissociated into monomers and the monomer solution is further diluted, whereas above the cmc’s the solution remains micellar and the effect is only to dilute the micelles. The cmc values of DDAB or SDS aqueous solution can therefore be obtained from the break in each curve, whereas ∆Hmic can be obtained directly from the difference between the observed enthalpies of the two linear regions of the enthalpy dilution plot according to the method of Van Os et al.,15 as shown in Figure 1a,b. The values of cmc and ∆Hmic were respectively 0.31 ( 0.01 mmol/dm3 and -6.61 ( 0.13 kJ/mol for DDAB and 7.60 ( 0.15 mmol/dm3 and -0.26 ( 0.06 kJ/mol for SDS (8.4 ( 0.4 mmol/dm3 and -0.20 ( 0.05 kJ/mol in ref 24). The results for mixtures of DDAB and SDS are shown in Figure 2, where Figure 2a is the recorder plot of voltage
(23) Sava, L.; Aleksandra, P. J. Colloid Interface Sci. 1985, 103 (2), 586.
(24) Johnson, I.; Olofsson, G.; Jo¨nsson, B. J. Chem. Soc., Faraday Trans. 1 1987, 83 (11), 3331.
Figure 1. Titration microcalorimetric curves of dilution of DDAB or SDS into pure water at 298.15 K: (a) dilution of DDAB with an initial concentration of 0.01 mol/dm3 and (b) dilution of SDS with an initial concentration of 0.05 mol/dm3.
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(V)-time (t) of the addition of 0.05 mol/dm3 SDS to DDAB and Figure 2b is variations of observed enthalpies of the titration of SDS into DDAB with SDS concentration together with the curve for SDS dilution from Figure 1b. The starting concentration and volume of DDAB were 0.01 mol/dm3 and 0.5 cm3 in the sample cell. The observed enthalpy (∆Hobs) consists of the interaction enthalpy of SDS and DDAB, the dilution enthalpy of SDS, and the dilution enthalpy of DDAB micelles. The diluent multiple of DDAB in each injection is very small and therefore contributes negligibly to the total observed enthalpy. The dilution enthalpy of SDS solution is of much smaller magnitude than the values of the total observed enthalpy of titration of SDS into DDAB. The latter therefore results mainly from the interaction enthalpy between SDS and DDAB. Accurate values of the interaction enthalpies were obtained by subtracting the dilution enthalpies of SDS from the total observed enthalpies at the corresponding concentration of SDS. When the concentration of SDS in the cell was lower than 0.00614 mol/dm3 ( 0.5 according to Marques et al.17 without V + L coexistence. Recently, Marques et al.18,19 discussed in detail the phase behavior of the same system using electron and light microscopy, NMR self-diffusion, ocular inspections, and turbidity measurements. The number of regions in our paper is less than in Marques’. The main reason would be that the titration microcalorimetry cannot identify some slow transition processes or the processes with very small enthalpy change. Conclusion
Figure 5. Ternary phase diagram of the DDAB/SDS/H2O system. The dotted lines denote lines of constant XSDS, and the points where they cross the experimental curve are points on the detectable phase and aggregate boundaries. L on the left indicates SDS-rich micelles, and L on the right indicates DDABrich micelles. P indicates the DDA+SD- precipitation, V indicates the vesicle, and V + L denotes coexistence of vesicles and SDS-rich micelles.
Phase Behavior of DDAB-SDS System. Though calorimetric measurements cannot be used directly to realize phase morphology, they do identify the phase boundaries unless the enthalpy change from one phase morphology to another is too small to be detected or the variational process is too slow to adapt to the titration calorimetry. By comparison of calorimetric results with electron micrographs and absorbance, it is then possible to establish the correct phase behavior. We can therefore deduce the ternary phase diagram of DDAB/SDS/H2O, and it is plotted in Figure 5. The apexes of the triangle correspond to H2O, 1 wt % SDS, and 1 wt % DDAB. The points where the dotted lines cross the experimental
We have studied the interaction of DDAB and SDS using an isothermal titration microcalorimeter. The enthalpy of interaction between DDAB micelles and SDS, the enthalpy of formation of the DDA+SDS- crystals, the enthalpy of vesicle formation of DDAB-SDS, and the enthalpy of the transition from vesicles to SDS-rich micelles have been determined to be -29.53, -125.8, 41.23, and 32.10 kJ/mol, respectively. By comparison of calorimetric results with turbidity measurements and transmission electron microscopy, the phase behavior of DDAB and SDS mixed solution was determined including a vesicle-micelle zone. Then, a ternary phase diagram of DDAB/SDS/H2O systems was deduced. Acknowledgment. We are grateful for financial support from the Royal Society, the Academia Sinica, and the National Natural Science Foundation of China (Grant No. 20073055, 29633020). Also, we thank Dr. R. K. Thomas, Oxford University, for his valuable discussion and help. LA000768X
