J. Phys. Chem. 1988, 92, 791-793
791
Evidence for the Essential Role of Hydrogen Bonding in Promoting Amphiphiiic Self-Assembly: Measurements in 3-Methylsydnone A. H. Beesley, D. Fennel1 Evans,* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455
and R. G. Laughlin Miami Valley Laboratories, The Procter & Gamble Company, Cincinnati, Ohio 45247 (Received: May 22, 1987; In Final Form: September 5. 1987)
Surface. tension and conductance measurements on tetradecylpyridinium nitrate and hexadecylpyridiniumbromide and diffusive interfacial transport experiments on dihexadecyldimethylammoniumacetate in 3-methylsydnone are reported. 3-Methylsydnone is aprotic, possesses a high dielectric constant (144 at 40 "C), and has a high cohesive energy density. Unlike hydrogen-bonding solvents of comparable polarity, no evidence for association colloid behavior is detected in 3-methylsydnone. These results confirm previous inferences that hydrogen bonding is a prerequisite for amphiphilic self-assembly.
Introduction
TABLE I: Properties of Polar Solvents'
The aggregation of amphiphiles to form micelles, vesicles, and bilayers has been established in water, hydrazine,'J ethylammonium formamide?.' and the glycol^.^ All of these solvents are hydrogen bonding, possess high dielectric constants, and have high cohesive energies. Many fundamental questions concerning aggregation remain unanswered including the nature of solvent-amphiphile interactions that drive self-assembly. In this paper, we describe experiments employing 3-methylsyndnone. This aprotic solvent has a dielectric constant of 144 and a cohesive energy density comparable to that of hydrazine or ethylammonium nitrate. It thus possesses two of the three characteristic properties associated with solvents that promote amphiphilic aggregation.
Experimental Section 3-Methylsydnone was synthesized following the procedure of Vasil'eva and Yashunshiis except a smaller excess of acetic anhydride was added to the N-nitroso-N-methylglycineand left for about 10 days. Excess acetic anhydride was removed with a rotary evaporator below 60 "C and then by vacuum distillation at the lowest temperature possible. High temperatures cause the sydnone to turn from dark yellow to brown. We experienced a minor fire during vacuum distillation (see warning in ref 9). The middle distillate cut was purified by repeated fractional freezing in an Erlenmeyer flask. The purest 3-methylsydnone had a very light yellow color and a melting point of about 36 "C. The infrared spectrum of our 3-methylsydnone in Figure 1 resembles those of N-(n-hexyl)- and N-(n-butyl)sydnones.'O Propylene carbonate (Aldrich) was used as received. Hexadecylpyridinium bromide was prepared by refluxing equimolar amounts of 1-bromohexadecane (Waltz and Bauer) and pyridine (MCB) in acetonitrile for 21/2days. The solvent was removed with a rotary evaporator. After washing with ether, the
(1) Ramadan, M.; Evans, D. F.; Lumry, R. J. Phys. Chem. 1983,87 4538. (2) Ramadan, M.; Evans, D. F.; Lumry, R.; Philion, S.J . Phys. Chem. 1985, 89, 3405. (3) Evans, D. F.;Yamauchi, A.; Roman, R.; Cassassa, E. Z . J . Colloid Interface Sci. 1982, 86, 89. (4) Evans, D. F.; Kaler, E. W.; Benton, W. J. J . Phys. Chem. 1983, 87, 533. ~~.
(5) Evans, D. F.; Yamauchi, A.; Wei, G. J.; Bloomfield, V. A. J . Phys. Chem. 1983, 87, 3531. (6) McDonald, C. J . Pharm. Pharamaco/. 1970, 22, 148. (7) Ray, A. Nature (London) 1971, 231, 313. (8) Vasil'eva, V. F. Yashunskii, V. G. J . Gen. Chem. USSR (Eng. Tram/.) 1962, 32, 2845. (9) Lemire, R. J.; Sears, P. G. J . Chem. Eng. Data 1977, 22, 316. (10) Fugger, J.; Tien, J. M.; Hunsberger, I. M. J . Am. Chem. SOC.1955, 77, 1843.
(-f/u)"'3
solvent 3-methylsydnone propylene carbonate water hydrazine ethylammonium nitrate formamide ethylene glycol
t
144 69 78.4
dyn/cm 14 9.3 27.5
51.7
21 13
109 37.7
15 12
77
dyn/cm 57
p,
D
7.3
41 72
66.4 46 58.2 47
1.85 1.86 3.73 2.28
"All physical properties at 25 OC except for 3-methylsydnone, which is at 40 "C. Data taken from Handbook of Chemistry and Physics, 51st ed; The Chemical Rubber Co.: Cleveland, OH, 1970, except for 3-methylsydnone; e, ref 9 and p, Schmid, G. H. J . Mol. Struct. 1970, 5, 236 for hydrazine, ref 1, and ethylammonium nitrate, ref 3.
hexadecylpyridinium bromide was recrystallized 6 times from acetone and dried in a vacuum oven at 25 "C. Surface tension measurements gave a small minimum at the cmc of 0.5 dyn/cm, indicating a trace amount of surface-active impurities. Dihexadecyldimethylammonium acetate was prepared by neutralization of dihexadecyldimethylammonium hydroxide. Surface tension was measured with a deNouy ring apparatus. The 3-methylsydnone-surfactantsolution was held in a jacketed flask (at 40 "C), through which warm water was circulated. The surface tension of pure 3-methyisydnone was measured, then aliquots of surfactantsydnone solution were added with a volumetric pipet, the solution was stirred, and surface tension was measured again. Electrical conductance was measured with a Jones-Dole Bridge using a Shedlovsky cell in a thermostatted oil bath. Aliquots of concentrated surfactant-sydnone solution were added from a weight buret. Solution conductances were corrected for solvent conductance. The relative solubility of several surfactants in 3-methylsydnone was determined. Tetradecylpyridinium nitrate and hexadecylpyridinium bromide were readily soluble at less than 40 "C. At 70 OC, an 11 wt % solution of tetradecyltrimethylammonium bromide can be prepared. Sodium dodecyl sulfate, sodium perfluorooctanoate, and distearoyllecithin were
