Synthesis and Aggregation of Benzyl(2-acylaminoethyl)dimethylammonium Chloride Surfactants Susana Shimizu and Omar A. El Seoud* Instituto de Quı ´mica, Universidade de Sa ˜ o Paulo, C.P. 26077; 05513-970, Sa ˜ o Paulo, SP; Brazil Received July 23, 2002. In Final Form: October 9, 2002 The following scheme has been employed to synthesize the title cationic surfactants: RCO2H(+NH2- CH2CH2N(CH3)2, toluene) f RCONHCH2CH2N(CH3)2 (1a-1d)(+PhCH2Cl, CH3CN) f RCONHCH2- CH2N + (CH3)2CH2C6H5Cl - (2a-2d). RCO2H refers to decanoic, dodecanoic, tetradecanoic, and hexadecanoic acid, respectively. Aggregation of these surfactants in water has been studied at 25 °C by measuring solution conductivity, surface tension, and electromotive force and by using Fourier transform infrared spectroscopy (FTIR). Increasing the length of R resulted in an increase of the aggregation number and a decrease in minimum area/surfactant at the solution/air interface, critical micelle concentration, and degree of counterion dissociation. Gibbs free energies of adsorption at the solution/air interface and of micelle formation were calculated and compared to those of other cationic surfactants. Contributions to these free energies from methylene groups of the hydrophobic tail and surfactant headgroup were calculated. The former are similar to those of other cationic surfactants, whereas the latter are smaller, i.e., more negative. That is, transfer of the headgroup from bulk water to the interface and/or to the micelle is more favorable. This is attributed to intermolecular H-bonding of monomers at the solution/air interface and/or in the aggregate, via the amide group, in agreement with our FTIR data. Introduction Changes of the molecular structure of surfactants affect physicochemical properties and applications of their solutions, in both water and organic solvents. For aqueous micelles, increasing the length of the surfactant hydro- phobic tail results in a decrease of the degree of the surfactant counterion dissociation, R mic , and the critical micelle concentration, cmc, and an increase of the micellar aggregation number, N agg . 1-4 Cationic surfactants have been studied in detail because their structure can be tailored to the application of interest, e.g., by changing the counterion, the length of the hydrophobic tail, and the size of the headgroup. Previously, extensive work has been carried out on surfactants whose general structure is represented by RN + R′R′′R′′′X - , where X - ) halide ion; R ) octyl to octadecyl; and R′,R′′, and R′′′ generally represent identical alkyl groups, e.g., trimethyl. A number of studies have employed R′ and R′′ ) methyl and R′′′ ) alkyl, benzyl, or alkylphenyl group. 1,2,5,6 There are patents and a couple of publications on the synthesis and germicide activity of commercial RCONH- (CH 2 ) 2 N + (CH 3 ) 3 X - and RCONH(CH 2 ) 2 N + (CH 3 ) 2 CH 2 - C 6 H 5 X - . 7,8 There is no information, however, on the physicochemical properties of this series of surfactants where R represents a single hydrocarbon chain. These compounds carry an amide group; consequently, their monomers may, in principle, form direct or water-mediated intermolecular H-bonds, akin to those formed by N- alkylamides, and polypeptides. 9 Additionally, surfactants that carry the amide group and a (negative) charge, separated with a “spacer”, have some interesting inter- facial properties, due to the simultaneous presence of both moieties. 10 We were interested, therefore, in investigating how a similar structural feature (amide group and positive charge) bears on solution properties of the series studied. We report here on the synthesis of the following surfactants: RCONH(CH 2 ) 2 N + (CH 3 ) 2 CH 2 C 6 H 5 Cl - , where RCO ) C 10 ,C 10 ABzCl; C 12 ,C 12 ABzCl; C 14 ,C 14 ABzCl; and C 16 ,C 16 ABzCl; A and Bz stand for -NH(CH 2 ) 2 N + (CH 3 ) 2 and the benzyl group, respectively. Data of solution conductivity, surface tension, electromotive force, and Fourier transform infrared spectroscopy (FTIR) were employed to calculate cmc, R mic , and N agg , as well as the Gibbs free energy of adsorption at the solution/air * To whom correspondence should be addressed. Fax: +55-11- 3091-3874. E-mail: elseoud@iq.usp.br. (1) Attwood, D.; Florence, A. T. Surfactant Systems: Their Chemistry, Pharmacy, and Biology; Chapman and Hall: London, 1984. (2) (a) Rosen, M. J. Surfactants And Interfacial Phenomena; Wiley: New York, 1989; pp 33, 108. (b) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloid and Surface Chemistry, 3rd ed.; Marcel Dekker: New York, 1997; pp 297, 355. (3) (a) Bunton, C. A.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (b) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (c) Bunton, C. A. J. Mol. Liq. 1997, 72, 231. (4) Birdi, K. S. Handbook of Surface and Colloid Chemistry; CRC Press: Boca Raton, FL, 1997. (5) Zana, R. Colloids Surf. 1997, A27, 123, and references therein. (6) (a) Okano, L. T.; El Seoud, O. A.; Halstead, T. Colloid Polym. Sci. 1997, 138, 275; (b) El Seoud, O. A.; Bla ´ sko, A.; Bunton, C. A. Ber. Bunsen- Ges. Phys. Chem. 1995, 99, 1214. (c) Okano, L. T.; Quina, F.; El Seoud, O. A. Langmuir, 2000, 16, 3119. (7) (a) Lacko, I.; Mlynarcik, D.; Jadronova, M.; Karoskova, J. CS 237747, 1987; Chem. Abstr. 108, 186165. (b) Devinky, F.; Masarova, L.; Lacko, I. CS 240390, 1987; Chem. Abstr. 108, 221286. (c) Login, R. B.; Chaudhuri, R. K.; Tracy, D. J.; Helioff, M. W. U.S. Patent 4,837,013A, 1989; Chem. Abstr. 111, 201398. (d) Oshima, T. JP 01006210, 1989; Chem. Abstr. 111, 20620. (e) Oshima, T. JP 02160714, 1990; Chem. Abstr. 113, 217792. (f) Iwai, H.; Nomura, T.; Shiraiwa, T.; Mori, S.; Daiho, Y. JP 08206581, 1996; Chem. Abstr. 125, 250488. (g) Kaneda, E.; Ohtaki, H.; Oka, S. JP 54043731, 1994; Chem. Abstr. 91, 99908. (h) Bailey, A. V.; et al. U.S. Patent 473,477, 1974; Chem. Abstr. 84, 79590. (8) (a) Mod, R. R.; Magne, F. C.; Skau, E. L.; Sumrell, G. J. Med. Chem. 1971, 14, 558. (b) Bailey, A. V.; Sumrell, G.; Gurtner, T. E. J. Am. Oil Chem. Soc. 1974, 51, 515. (9) (a) Kollman, P. Chem. Rev. 1993, 93, 2395. (b) Ludwig, R. O.; Winter, R.; Weinhold, F.; Farrar, T. C. J. Phys. Chem. B 1998, 102, 9312. (c) Huelsekopf, M.; Ludwig, R. Magn. Reson. Chem. 2001, 39, 127. (10) Tsubone, K.; Rosen, M. J. J. Colloid Interface Sci. 2001, 244, 394. 238 Langmuir 2003, 19, 238-243 10.1021/la026286y CCC: $25.00 © 2003 American Chemical Society Published on Web 12/12/2002