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Abbreviations: CTAB, cetyl trimethylammonium bromide; SDS, sodium dodecyl sulfate ..... 1991 and her master's degree in fine chemistry from Wuxi Institute of.
253

The Effects of Anionic and Cationic Surfactants on the Hydrolysis of Sodium Barbital Junhong Qian*, Shaohua Qian, and Rong Guo School of Chemistry and Chemical Engineering, Yangzhou University, 225002, P.R. China

ABSTRACT: Alkaline hydrolysis reactions of sodium barbital in micelles of sodium dodecyl sulfate (the anionic surfactant SDS), micelles of cetyl trimethylammonium bromide (the cationic surfactant CTAB), and mixed micelles of surfactant/nC5H11OH/H2O were studied by ultraviolet-visible spectrometry. The reaction rate and the activation energy of the hydrolysis of sodium barbital were calculated. The results showed that the rate of sodium barbital hydrolysis decreased with an increase in CTAB content, whereas it increased in the presence of SDS and n-C5H11OH. The different effects of CTAB and SDS on the hydrolysis of sodium barbital may be related to their interaction with sodium barbital. Paper no. S1475 in JSD 8, 253–256 (July 2005). KEY WORDS: bital.

CTAB, hydrolysis, micelles, SDS, sodium bar-

Surfactants are widely used as solubilizers, emulsifiers, and detergents in many industrial processes because of their unique physicochemical properties. The application of micellar systems or other organized molecular assemblies has been recognized for many years, and they have been exploited in many areas, such as the chemical and energy industries, in materials science, and in medicine (1–3). Microemulsions are systems consisting of a surfactant, co-surfactant (usually a medium-chain alcohol), water, and oil, and they are thermodynamically steady systems with isotropy and low viscosity (4,5). Microemulsions play an important role in biological, materials science, environmental, and other related fields (6,7). Recently, the effects of microemulsions on chemical reactions in many systems have been studied (8,9). Sodium barbital is an important hypnotic sedative; however, the drug is easily hydrolyzed and loses its efficacy in basic media (10,11). Reducing the hydrolytic action of sodium barbital to enhance its stability is a noticeable problem. The stability of the drug can efficiently be increased by using the associative structures of surfactants (12–14). However, few reports are available on the stability of sodium barbital in micelles. In the present paper, we studied the hydrolysis of sodium barbital in different media by ultraviolet-visible (UVvis) spectrometry. The results show that cetyl trimethylammonium bromide (CTAB) micelles enhance, whereas sodium *To whom correspondence should be addressed. E-mail: [email protected] Abbreviations: CTAB, cetyl trimethylammonium bromide; SDS, sodium dodecyl sulfate; UV-vis, ultraviolet-visible. COPYRIGHT © 2005 BY AOCS PRESS

dodecyl sulfate (SDS) micelles reduce, the stability of sodium barbital; the presence of small amounts of n-C5H11OH increases the hydrolysis of sodium barbital.

EXPERIMENTAL PROCEDURES Materials. SDS (Sigma, St. Louis, MO; 98%) was recrystallized twice in ethanol, and the surface tension of the SDS solution had no minimum around the critical micelle concentration. CTAB (Sigma; 99%), n-pentanol (Sigma; 99%), and sodium barbital (A.R., the chemical factory of East China Normal University) were used as received. The water used was twice distilled. Methods. (i) Measurement of the hydrolytic curve. Two interconverted tautomers exist in aqueous sodium barbital solutions (Eq. 1). Barbituric acid (a) has absorbs around 208 nm, and the tautomer monolactam (b) absorbs at both 208 and 258 nm (Fig. 1), but the hydrolysis products of sodium barbital and other reagents do not absorb around 258 nm. Tautomer b is the main form in basic media and tautomer a is the main form in acidic media; thus, the absorbance values (A) around 258 nm can be used to indicate quantitatively the change in sodium barbital concentration in basic media. Dynamic hydrolysis curves were obtained by plotting absorbance value A versus time t at different temperatures. The concentrations of sodium barbital and NaOH were 1 × 10−4 and 0.1 mol·L−1, respectively. UV-vis absorption spectra were recorded on a UV-2501 ultraviolet spectrometer (Shimadzu Co.).

[1]

(ii) Determination of the apparent hydrolysis rate of sodium barbital. The hydrolysis of sodium barbital in basic media is shown in Equation 2 (10):

[2]

One can see from Equation 2 that sodium barbital can be hydrolyzed and form ureide by a ring-opening reaction. It follows two steps: First, the hydroxide ion attacks the carbon JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 8, NO. 3 (JULY 2005)

254 J. QIAN ET AL.

tion. The activation energy of the hydrolysis reaction is illustrated in Equation 5: Ea = R

T1T2 k ln 1 T1 − T2 k2

[5]

where R is the gas constant; T1 and T2 are the absolute hydrolysis temperatures of sodium barbital; and k1 and k2 are the apparent reaction rate constants corresponding to T1 and T2, respectively.

RESULTS AND DISCUSSION

FIG. 1. Changes in the ultraviolet-visible spectra of sodium barbital with time (65°C). Times: 1, 0 h; 2, 1 h; 3, 2 h; 4, 4 h; 5, ∞.

atom of the amide to give diethyl carboxyl acetyl urea; second, the product of the first step further hydrolyzes to form 2-ethyl butyryl urea. The total hydrolysis rate is controlled by the first step (11). The hydrolysis of sodium barbital is a second-order reaction (11). When the concentration of the hydroxide ion is greatly excessive (i.e., when the sodium barbital content is 1 × 10−4 mol·L−1, whereas the OH− concentration is 0.1 mol·L−1), this reaction can be regarded as pseudo first-order. Using HA to represent sodium barbital, we can express the reaction rate as follows: −d[HA]/dt = k1[HA]

[3]

When other conditions are fixed (e.g., temperature, OH− concentration), there is a linear relationship between absorbance A and the concentration of sodium barbital. If the concentration of sodium barbital is replaced by its absorbance A, then the dynamic hydrolysis equation can be expressed as A − A∞ = k1t ln 0 At − A∞

Effect of SDS micelles on the hydrolysis of sodium barbital. The reaction rate constant of sodium barbital in SDS micelles at 70°C is shown in Table 1. As this table shows, the reaction rate constant increases with the SDS content, suggesting that SDS can promote the hydrolysis of sodium barbital. Sodium barbital has several polar groups with lone-pair electrons, such as the carbonyl and amino groups, so it has an intensive negative charge density. The hydrophilic group SO42− in the SDS molecule repels the negatively charged barbital, and it is difficult for sodium barbital to locate in the palisade of SDS micelles where the polar group links the hydrocarbon chain. Hence, SDS micelles can hardly protect sodium barbital from being hydrolyzed. On the other hand, H+ can be adsorbed at the surface of the SDS micelles, which increases the OH− concentration in the water continuous phase and then leads to an increase in the hydrolysis rate of sodium barbital. The number of H+ adsorbed on the SDS micellar surface increases with an increase in SDS concentration, thus enhancing the hydrolysis of sodium barbital. Table 1 shows that, in SDS/n-C5H11OH/H2O mixed-micellar system, the hydrolysis rate is also enhanced by the addition of n-C5H11OH. This result reveals that the addition of n-C5H11OH reduces the charge density of the SDS micellar surface and the number of H+ adsorbed on the micellar surface decreases, which increases the chance for OH− and sodium barbital molecules located on the SDS micellar surface to interact. Consequently, the hydrolysis rate increases. Effect of CTAB micelles on the hydrolysis of sodium barbital. The hydrolysis curves of sodium barbital in CTAB micelles at differing concentrations of CTAB are shown in Figure 2 at 65°C. Figure 3 gives the corresponding curves of ln(At/A0) vs. t (error < 7.53 × 10−5, R > 0.9961); Figure 4 shows the change in the apparent hydrolysis rate constant of sodium barbital with temperature; and Table 2 shows the apparent hydrolysis

[4]

where t is the time of the hydrolysis reaction; A0 and At are the absorbance of the reaction solution in the initial stage and at hydrolysis time t, respectively; A∞ is the absorbance of the final hydrolyzed product; and k1 is the reaction rate constant. One can see from Figure 1 that A∞ is close to 0. Thus, a linear relationship exists between ln(A0/At) and t, and k1 can then be obtained from the slope of the line. (iii) Determination of the activation energy of the hydrolysis reacJOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 8, NO. 3 (JULY 2005)

TABLE 1 Apparent Hydrolysis Rate of Sodium Barbital in Sodium Dodecyl Sulfate (SDS)/n-C5H11OH/H2O Mixed Micelles k/10−3 min n-C5H11OH (wt%)

H2O

SDS (0.1%)

SDS (0.5%)

SDS (1.0%)

0 0.5 1.0

1.85

1.89

1.92 1.98 2.07

1.96

255 EFFECTS OF SURFACTANTS ON THE HYDROLYSIS OF SB

FIG. 2. Hydrolysis curves of sodium barbital in cetyl trimethylammonium bromide (CTAB) micelles (65°C). CTAB content: 1, 0%; 2, 0.3%; 3, 0.6%; 4, 1.0%.

FIG. 4. Hydrolysis rate vs. temperature curve in CTAB micelles. CTAB content: 1, 0%; 2, 0.3%; 3, 0.6%; 4, 1.0%. For abbreviation see Figure 2.

activation energy in CTAB/n-C5H11OH/H2O mixed micelles. Figure 4 and Table 2 show that in CTAB micellar systems, the apparent hydrolysis rate decreases, whereas the apparent hydrolysis activation energy increases with an increase in CTAB content. This result indicates that CTAB can suppress the hydrolysis of sodium barbital. Many factors, including local concentration and electrostatic charge polarity, influence chemical reactions in the surfactant micelles (1). The effects of CTAB on the hydrolysis reaction can be generalized as follows. First, the hydrophilic group of the cationic surfactant CTAB, a quaternary ammonium salt, is an electron-deficient center. The micellar surface can absorb negatively charged OH−, which leads to the decreased concentration of OH− in the water continuous phase, so the hydrolysis rate of sodium barbital in the water continuous phase decreases. Second, some sodium barbital

molecules may be located in the palisade of CTAB micelles by the electrostatic attraction of CTAB, and these molecules have less chance to contact with OH−, so the hydrolysis rate decreases. Effect of CTAB/n-C5H11OH/H2O mixed micelles on the hydrolysis of sodium barbital. Figure 5 and Table 2 show that, in CTAB/n-C5H11OH/H2O mixed-micellar systems, the hydrolysis rate of sodium barbital decreases and the activation energy increases with an increase in CTAB content when the nC5H11OH content is fixed at 0.5 wt%. Figure 6 and Table 2 illustrate that the hydrolysis rate of sodium barbital decreases and the activation energy increases with an increase in n-C5H11OH content when the CTAB content is fixed at 0.6 wt%. Hence, CTAB and n-C5H11OH both inhibit the hydrolysis of sodium barbital in CTAB/nC5H11OH/H2O mixed-micellar systems. Conversely, Table 2 shows that the activation energies in CTAB/n-C5H11OH/H2O mixed-micellar systems are lower than that in CTAB micelles. This result reveals that the presence of small amounts of the co-surfactant n-C5H11OH is beneficial to the hydrolysis of sodium barbital. Three factors affect the hydrolysis rate of sodium barbital in the presence of n-C5H11OH. First, when n-C5H11OH is added to the micellar solution, it participates in the formation

TABLE 2 Apparent Activation Energy of the Hydrolysis of Sodium Barbital in Cetyl Trimethylammonium Bromidea (CTAB)/n-C5H11OH/H2O Mixed Micelles

FIG. 3. Curve of ln(At /A0) vs. time in CTAB micelles (65°C). CTAB content: 1, 0%; 2, 0.3%; 3, 0.6%; 4, 1.0%. For abbreviation see Figure 2.

Ea/kJ·mol−1 CTAB (0.3%) CTAB (0.6%) CTAB (1.0%)

n-C5H11OH (wt%)

H2O

0 0.1 0.2 0.5

85.12

87.86

88.07

82.38

84.16

85.62

88.53 83.12 85.63 87.53

a

Concentrations given in weight percentages. JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 8, NO. 3 (JULY 2005)

256 J. QIAN ET AL.

FIG. 5. Hydrolysis rate vs. temperature curve in CTAB mixed micelles at a fixed n-C5H11OH content of 0.6%. CTAB content: 1, 0%; 2, 0.2%; 3, 0.5%. For abbreviation see Figure 2.

FIG. 6. Hydrolysis rate vs. temperature curve in CTAB mixed micelles at a fixed n-C5H11OH content of 0.5%. CTAB content: 1, 0%; 2, 0.3%; 3, 0.6%; 4, 1.0%. For abbreviation see Figure 2.

of micelles. This makes the size of the micelles larger and increases the amount of sodium barbital located in the palisade of micelles, thus inhibiting hydrolysis. Second, the larger size of the mixed micelles increases water penetration into the micelle, thereby increasing the contact of sodium barbital with water and accelerating the hydrolysis rate. Third, the addition of n-C5H11OH reduces the charge density of the micellar surface and the number of OH− adsorbed on the CTAB micellar surface, which leads to an increased OH− concentration in the water continuous phase. Thus, it is easier for sodium barbital to hydrolyze in water. The results in Table 2 reveal that in the presence of small amounts of n-C5H11OH, the latter two effects play main roles, and the hydrolysis rate increases, whereas in the presence of larger amounts of n-C5H11OH, the first effect plays a main role, and the hydrolysis rate is suppressed.

8. Din, K.U., K. Hartain, and Z. Khan, Effect of Micelles on the Oxidation of Oxalic Acid by Chromium(VI) in the Presence and Absence of Manganese(II), Colloids Surf. A 193:1 (2001). 9. Lee, E.S., K. Na, and Y.H. Bae, Polymeric Micelle for Tumor pH and Folate-Mediated Targeting, J. Control. Release 91:103 (2003). 10. Peng, S.X., Pharmaceutical Chemistry, Chemical Industry Press, Peking, 2000. 11. Ji, W.S., and L.A. Li, Pharmaceutical Chemistry, Higher Education Press, Peking, 2001. 12. Qian, J.H., and R. Guo, Hydrolysis of Cephanone in Micelles with Different Charges, Colloid Polym. Sci. 282:979 (2004). 13. Qian, J.H., R. Guo, and X. Guo, The Hydrolysis of Penicillin-G Potassium Salt in the O/W Microemulsion with Different Charges, Colloids Surf., A 215:253 (2003). 14. Qian, J.H., R. Guo, and A.H. Zhou, Effect of Microemulsion Structures on the Hydrolysis of Acetylsalicylic Acid, J. Dispersion Sci. Technol. 22:441 (2001). [Received January 13, 2005; accepted May 26, 2005]

REFERENCES 1. Qian, J.H., X.H. Zhang, and R. Guo, Inhibition of Microemulsions of a CTAB/n-C5H11OH/H2O System on the Hydrolysis of Penicillin-G Potassium, Acta Phys. Chem. Sin. 16:80 (2000). 2. Trotta, M., Transformation on Indomethacin Release from Microemulsions, J. Control. Release 60 :399 (1999). 3. Zhong, J.F., The Application of Surfactants in Pharmaceutics, People’s Health Press, Peking, 1996, p. 472. 4. Shah, J.C., Y. Sadhale, and D.M. Chilakuri, Cubic Phase Gels as Drug Delivery Systems, Adv. Drug Delivery Rev. 47:229 (2001). 5. Guo, R., S.H. Qian, and J.H. Qian, Hydrotrope and Hydrotrope-Solubilization Action of Cephanone in a CTAB/nC5H11OH/H2O System, Colloid Polym. Sci. 283:15 (2004). 6. Husseini, G.A., G.D. Myrup, and W.G. Pitt, Factors Affecting Acoustically Triggered Release of Drugs from Polymeric Micelles, J. Control. Release 69:43 (2000). 7. Maeda, M., S. Moriuchi, A. Sano, and T. Yoshimine, New Drug Delivery System for Water-Soluble Drugs Using Silicone and Its Usefulness for Local Treatment: Application of GCV-Silicone to GCV/HSV-tk Gene Therapy for Brain Tumor, J. Control. Release 84:15 (2002).

JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 8, NO. 3 (JULY 2005)

Junhong Qian is an assistant professor in the School of Chemistry and Chemical Engineering at Yangzhou University. She received her B.S. degree in fine chemistry from Dalian University of Technology in 1991 and her master’s degree in fine chemistry from Wuxi Institute of Light Industry in 1993. She began her doctoral studies in September 2004 at East China University of Science & Technology. Her current research topic is the supramolecular chemistry of drugs. Shaohua Qian was born in 1978 in Suzhou, Jiangsu Province, P.R. China. She received her B.S. degree in chemistry from Yangzhou University in 2001 and her master’s degree in physical chemistry from Yangzhou University in 2004. Her research work focuses on the interaction between drugs and surfactants. Rong Guo is a professor in the School of Chemistry and Chemical Engineering at Yangzhou University. He is the president of Yangzhou University, chairman of the committee Colloid & Interface Chemistry Division Science and Technology. He received his Ph.D in physical chemistry from Clarkson University (Potsdam, NY). His current research topic is the physical chemistry of surfactants.