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Langmuir 1997, 13, 2480-2482
Aggregating Tendencies of Some Alkylsulfonates Ji-Liang Shi,* May Xiao-Wu Jiang, June Hong Zeng, and Xi-Kui Jiang Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China Received December 3, 1996. In Final Form: February 6, 1997X Aggregation of some sulfur-containing compounds has been studied. The aggregators studied are p-nitrophenyl esters of butylsulfonate (S-4), octylsulfonate (S-8), dodecylsulfonate (S-12), and hexadecylsulfonate (S-16). Their aggregating tendencies have been evaluated by measuring their critical aggregate concentrations (CAgCs) in dioxane-H2O binary mixtures of graded compositions. Our results indicate that the sulfonates, just like the carboxylates or phosphonates, can also be used as kinetic probes for evaluating CAgC values. If the above-mentioned three types of compounds bearing the same number of methylene groups in their chains are compared, then the order of increasing aggregating tendency appears to be phosphonate < sulfonate and carboxylate.
Simple aggregates (Ag’s) of electrically neutral organic molecules are formed by hydrophobic-lipophilic interactions (HLI).1,2 Organic molecules which tend to form simple Ag’s in solvents with solvent aggregating power (SAgP) are called aggregators (Agr’s).2,3 The aggregating tendencies of aggregators are generally evaluated by measuring their critical aggregate concentrations (CAgCs) in aqueous solutions or in aquiorgano binary mixtures with their composition designated by φ, the volume fraction of the organic component of the binary mixture. CAgC is the concentration of the Agr at the onset of aggregation. Under similar conditions, a smaller CAgC value signifies a greater aggregating tendency. In the past, CAgCs of many carboxylic ester Agr’s and some phosphonate and phosphinate Agr’s have been evaluated by plotting their hydrolytic rate constants (kob) against their initial concentrations ([Agr]i). The “break point” of such a log kob vs log[Agr]i plot, i.e., the narrow transitional concentration region between the horizontal and slanting lines (see, e.g., curve c in Figure 1; curve for S-8 in Figure 2 and curve for P-16 in Figure 3), represents the CAgC of that particular Agr under study. In recent years, the reliability and trustworthiness of this kinetic methodology have been firmly established time and again.1,4,5 However, all these previously studied simple Agr’s are either purely carbon compounds or some phosphorus-containing compounds, but aggregation behavior of sulfur-containing organic compounds has never been reported. In view of the importance of sulfurcontaining compounds in organic, bioorganic, and surfactant chemistry,6-8 it certainly would be of interest to study the aggregation behaviors of some sulfur compounds. Target compounds of the present study are new comX
Abstract published in Advance ACS Abstracts, April 1, 1997.
(1) Jiang, X. K. Acc. Chem. Res. 1988, 21, 362. (2) (a) Jiang, X. K. Pure Appl. Chem. 1994, 66, 1621 and pertinent references cited therein. (b) Tung, C. H.; Xu, C. B. Photochemistry and Photophysics; Rabek, J. F., Ed.; CRC Press: Boca Raton, FL, 1992; Vol. 4, Chapter 3. (c) Jiang, X. K. J. Chin. Chem. Soc. 1995, 42, 623. (d) Jiang, X. K.; Sun, S. X. Prog. Nat. Sci. 1995, 5, 527. (3) Jiang, X. K.; Ji, G. Z.; Zhang, J. T. Langmuir 1994, 10, 122. (4) (a) Zhang, J. T.; Nie, J.; Ji, G. Z.; Jiang, X. K. Langmuir 1994, 10, 2814. (b) Jiang, X. K.; Shi, J. L.; Chen, X. Langmuir 1996, 12, 3881. (5) (a) Jiang, X. K.; Ji, G. Z.; Tu, B.; Zhang, X. Y.; Shi, J. L.; Chen, X. J. Am. Chem. Soc. 1995, 117, 12679. (b) Zhang, X. Y.; Zhu, Y.; Ji, G. Z.; Jiang, X. K. Chin. Chem. Lett. 1996, 759. (6) Shigeru Oae, D. Se. Organic Sulfur Chemistry, Biochemical Aspect; CRC Press: Boca Raton, Ann Arber, London, Tokyo, 1992. (7) Doriano, C.; Gerald, E. G. National Sulfur Compounds, Novel Biochemical and Structural Aspect; Plenum Press: New York and London, 1980. (8) Freidlina, R. K.; Skorova, A. E. Organic Sulfur Chemistry; Pergamon Press: Oxford, 1980.
S0743-7463(96)02069-0 CCC: $14.00
pounds, i.e., the p-nitrophenyl esters of butylsulfonate (S-4), octylsulfonate (S-8), dodecylsulfonate (S-12), and hexadecylsulfonate (S-16). They were prepared by reactions described by eq 1 and eq 2, and their aggregating tendencies have been compared with those of p-nitrophenyl esters of n-alkanoic acids (C-n) and O-hexadecyl O-4nitrophenyl methylphosphonate (P-16).
Pseudo-first-order hydrolytic rate constants (kob) of these p-nitrophenylsulfonates in alkaline solutions were measured in the dioxane (DX)-H2O system with graded φ values at 35 °C in order to evaluate the CAgCs of these Agr’s. Large alkyl chain-length effects on the rate of hydrolysis of the sulfonates have been observed. For the measurement of kob, two different buffers have been tested. Comparison of the aggregation behavior of the sulfonates with those of the carboxylates or phosphonates has been made by measurements of kobs and CAgCs performed in the same medium. There are two ways of comparing the sulfonates with the carboxylates: (1) Compare the probe molecules with the same number of carbon atoms in the chain, e.g., S-8 versus C-8. (2) Take the -COOAr group as a functional group just like the -SO2OAr group, and compare chains with the same number of methylene groups, e.g., S-8 versus C-9. In this paper, results corresponding to both of these two ways of comparison have been described. Experimental Section 1H
Apparatus. NMR spectra were obtained at 90 MHz on a Varian FX-90Q spectrometer with TMS as the internal standard. Chemical shifts are expressed in ppm (δ). UV-vis spectra were recorded on a Perkin-Elmer 559 spectrometer. (9) Proell, W. A.; Adams, C. E.; Shoemaker, B. H. Ind. Eng. Chem. 1948, 1129.
© 1997 American Chemical Society
Aggregating Tendencies Reagents and Substrates. All target compounds (S-n) were prepared in our laboratory and identified by elemental analysis and 1H NMR. They were purified by flash column chromatography on silica gel with petroleum ether-ethyl acetate as eluent. Preparation of the sulfonates S-n. A typical procedure for preparing the target compounds (S-n) is as follows: A mixture of sodium dodecylsulfonate and excess thionyl chloride in anhydrous benzene was stirred and refluxed for 3-4 h; excess thionyl chloride and benzene were removed under reduced pressure. A benzene solution of p-nitrophenol and pyridine was added and stirred and refluxed for another 2-3 h. The solution was washed with 5% NaHCO3 and saturated NaCl solution and dried by anhydrous Na2SO4, and the solvent was removed under reduced pressure. The product, S-12, was purified by flash column chromatography on silica gel with petroleum etherethyl acetate as eluent. p-Nitrophenyl Butylsulfonate (S-4). Yellowish liquid. 1HNMR (CDCl ): δ 0.8-2.3 (m, 7H), δ 3.3 (t, 2H), δ 7.1-8.3 3 (AA′BB′, 4H). UV-Vis absorption: λmax ) 268 nm, max ) 1.49 × 104 L‚mol-1‚cm-1 (CHCl3). Anal.: Calcd for C10H13SO5N: C, 46.34; H, 5.05; N, 5.40; S, 12.37%. Found: C, 45.94; H, 5.07; N, 5.41; S, 12.67. p-Nitrophenyl Octylsulfonate (S-8). Yellowish liquid. 1HNMR (CDCl ): δ 0.8-2.4 (m, 15H), δ 3.3 (t, 2H), δ 7.1-8.4 3 (AA′BB′, 4H). UV-vis absorption: λmax ) 268 nm, max ) 1.46 × 104 L‚mol-1‚cm-1 (CHCl3). Anal. Calcd for C14H21SO5N: C, 53.33; H, 6.71; N, 4.44; S, 10.17. Found: C, 53.63; H, 7.02; N, 4.39; S, 10.39. p-Nitrophenyl Dodecansulfonate (S-12). White solid, mp 51 °C. 1H NMR (CDCl3): δ 0.7-2.4 (m, 23H), δ 3.3 (t, 2H), δ 7.2-8.5 (AA′BB′, 4H). UV-vis absorption: λmax ) 270 nm, max ) 1.44 × 104 L‚mol-1‚cm-1 (CHCl3). Anal. Calcd for C18H29SO5N: C, 58.20; H, 7.87; N, 3.77; S, 8.63. Found: C, 57.76; H, 8.10; N, 3.75; S, 8.87. p-Nitrophenyl Hexadecansulfonate (S-16). White solid, mp 65 °C. 1H NMR (CDCl3): δ 0.6-2.3 (m, 31H), δ 3.3 (t, 2H), δ 7.1-8.4 (AA′BB′, 4H). UV-vis absorption: λmax ) 268 nm, max ) 1.83 × 104 L‚mol-1‚cm-1 (CHCl3). Anal. Calcd for C22H37SO5N: C, 61.79; H, 8.72; N, 3.28; S, 7.50. Found: C, 62.39; H, 9.01; N, 3.09; S, 7.83. Preparation of the Carboxylates C-n. Carboxylic esters C-n were prepared by previously reported procedures.10-12 All of them were identified by elemental analysis, UV, and 1H NMR.10-12 The esters C-8 and C-9 are liquids, while C-12 and C-13 are solids with melting points of 45.5-46.5 and 47.0-48.0 °C, respectively. Kinetics. Water was deionized, and dioxane (DX) was purified by a standard procedure.13 Kinetic measurements in DX-buffer systems with graded φ values were performed on a Perkin-Elmer Lambda 5 UV-vis spectrometer equipped with a thermostated cell holder at 35 °C by monitoring the formation of the p-nitrophenolate at 410 nm, as previously described.3,4a,5a,11,12,14a The experimental uncertainty for the kob values was within (5%. The aqueous buffer solutions used were as follows: buffer I, 0.01 M NaOH, 0.01 M NaHCO3, and 0.34 M NaCl (pH ) 11.90); buffer II, 0.064 M NaOH and 0.05 M KCl (pH ) 12.90). Buffer I was first used because it had been used in most of the previous CAgC measurements,1,4a,11,12 but it was found that the hydrolysis of the sulfonates was much too slow in this buffer, especially for S-12 and S-16. Buffer II was found to be convenient for measurements for all the sulfonates.
Results and Discussion The log values of hydrolytic rate constants of substrates in DX-H2O mixtures of graded φ values at various initial substrate concentrations were measured and plotted against the log of initial substrate concentrations ([Agr]i), (10) (a) Hui, Y. Z.; Wang, S. J.; Jiang, X. K. Huaxue Xuebao 1982, 40, 1148. (b) Hui, Y. Z.; Wang, S. J.; Jiang, X. K. J. Am. Chem. Soc. 1982, 104, 347. (11) Jiang, X. K.; Hui, Y. Z.; Fan, W. Q. J. Am. Chem. Soc. 1984, 106, 3839. (12) Jiang, X. K.; Li, X. Y.; Huang, B. Z. Proc.sIndian Acad. Sci., Chem. Sci. 1987, 98, 409; 1987, 98, 423. (13) Riddick, J. A. Organic Solvents, Vol. II, 939. (14) (a) Jiang, X. K.; Ji, G. Z.; Luo, G. L. Chin. J. Chem. 1991, 9, 448. (b) Jiang, X. K.; Ji, G. Z.; Luo, G. L. Chin. J. Chem. 1991, 9, 453.
Langmuir, Vol. 13, No. 9, 1997 2481
Figure 1. Plots of log kob vs log[S-12]i in DX-H2O with graded φ values at 35 °C.
and for each target molecule and solvent system, a figure with curves corresponding to different φ values can be obtained. Whenever there is a break point in these curves, a CAgC value can be determined.4,5a Figure 1 shows the dependence of log kob on log [S-12]i at different φ values. These curves are typical: Curve e shows that at φ ) 0.40 there is only a monomeric region in which no aggregation of the target molecule (S-12) occurs. Curve b indicates that at φ ) 0.20 there is only an aggregated region in which kob decreases with increasing [Agr]i, i.e., the degree of aggregation increases with increasing [Agr]i. Curve c shows that at φ ) 0.30 there are three regions, i.e., a monomeric region, a transition region, and an aggregated region; thus the CAgC value of S-12 at φ ) 0.30 can be evaluated to be 1.19 × 10-5 M. These results also show that the occurrence of aggregation of S-12 depends on the φ value of the medium. This observation is very similar to the observation on the behaviors of the carboxylates12 or phosphonates.4b In other words, all our data indicate that the occurrence of aggregation depends on the substrate concentration [Agr]i and the solvent aggregation power (SAgP) of the medium, because within a certain range of the φ value, the SAgP of one specified solvent system is directly correlatable with the φ value.1,2,14b The CAgC values of S-8, S-12 and S-16 in the DX-H2O system in which the aqueous component is buffer II are listed in Table 1. Examination of Table 1 reveals the following observation: As expected, CAgC values increase with increasing φ values for S-8, S-12, and S-16 in the DX-H2O system. Figure 2 shows that there is a chain-length effect on the aggregation tendency of S-4, S-8, and S-12 in the φ ) 0.1 DX-H2O system. For S-4, there is only a monomeric region, for S-12, there is only an aggregated region, but for S-8, there are three regions, i.e., a monomeric region, a transition region, and an aggregated region. It shows that the aggregation tendencies of S-n depend on alkyl chain length. Figure 3 and Table 2 show that the aggregating tendency of S-16 is larger than that of P-16. It might be argued that it would not be totally legitimate to compare S-16 with P-16 because the latter contains an extra CH3 group. However, it is equally clear that the hydrophobic CH3 group (its hydrophobic fragmental constant f is equal to 0.7014b) should only give the P-16 molecule a greater tendency toward aggregation. Since the experimental
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Shi et al.
Table 1. The CAgC (×10-6 M) Values of S-n in the DX-H2O System (Buffer II) with Different O Values S-8 CAgC
S-12
S-16
φ ) 0.1
φ ) 0.2
φ ) 0.3
φ ) 0.35
φ ) 0.35
φ ) 0.4
22.3 ( 1.1
33.1 ( 1.6
11.9 ( 0.6
52.6 ( 2.6
4.29 ( 0.21
8.59 ( 0.43
Table 2. CAgC (×10-6 M) Values of S-n, C-n, and P-16 in DX-H2O Systems (Buffer I) of Different O Values φ ) 0.05 S-8
C-8
φ ) 0.1 C-9
S-8
C-8
φ ) 0.3 C-9
S-12
C-12
C-13
P-16a,b
S-16a
CAgC 9.12 ( 0.46 17.0 ( 0.8 10.0 ( 0.5 18.2 ( 0.9 36.0 ( 1.8 16.6 ( 0.6 10.7 ( 0.5 20.4 ( 1.0 12.6 ( 0.6 11.1 ( 0.2
