Synthesis and Aggregation of Benzyl (2-acylaminoethyl

Publication Date (Web): December 12, 2002 ... RCO2H refers to decanoic, dodecanoic, tetradecanoic, and hexadecanoic acid, .... Langmuir 2004 20 (22), ...
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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 (+NH2CH2CH2N(CH3)2, toluene) f RCONHCH2CH2N(CH3)2 (1a-1d) (+PhCH2Cl, CH3CN) f RCONHCH2CH2N+(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 hydrophobic tail results in a decrease of the degree of the surfactant counterion dissociation, Rmic, and the critical micelle concentration, cmc, and an increase of the micellar aggregation number, Nagg.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(CH2)2N+(CH3)3X- and RCONH(CH2)2N+(CH3)2CH2* To whom correspondence should be addressed. Fax: +55-113091-3874. E-mail: [email protected]. (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. BunsenGes. Phys. Chem. 1995, 99, 1214. (c) Okano, L. T.; Quina, F.; El Seoud, O. A. Langmuir, 2000, 16, 3119.

C6H5X-.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 Nalkylamides, and polypeptides.9 Additionally, surfactants that carry the amide group and a (negative) charge, separated with a “spacer”, have some interesting interfacial 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(CH2)2N+(CH3)2CH2C6H5Cl-, where RCO ) C10, C10ABzCl; C12, C12ABzCl; C14, C14ABzCl; and C16, C16ABzCl; A and Bz stand for -NH(CH2)2N+(CH3)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, Rmic, and Nagg, as well as the Gibbs free energy of adsorption at the solution/air (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.

10.1021/la026286y CCC: $25.00 © 2003 American Chemical Society Published on Web 12/12/2002

RCONH(CH2)2N+(CH3)2CH2C6H5Cl- Surfactant Synthesis Scheme 1

interface, ∆G°ads, and/or the free energy of micellization, ∆G°mic. Contributions of the structural segments to the latter two quantities were calculated. For the present series, ∆G°ads and ∆G°mic are more favorable than those of typical cationic surfactants, e.g., alkyldimethylbenzylammonium chlorides, due to the above-mentioned Hbonding between surfactant molecules. This conclusion is supported by FTIR. Experimental Section Materials. Chemicals were purchased from Aldrich or Merck. Benzyl chloride and N,N-dimethylethylenediamine were fractionally distilled from CaH2. Other reagents were employed as received. Apparatus. Melting points were determined with IA 6304 apparatus (Electrothermal, London, UK). We used a Model GC 17A-2 gas chromatograph (Shimadzu, Kyoto, Japan), equipped with an FID detector and Supelcowax 10 capillary column (Supelco, Bellefonte, USA). The CG analysis conditions are given in the Supporting Information, vide infra. FTIR spectra were recorded with a Bruker Vector 22 spectrophotometer. 1H and 13C NMR spectra were recorded with Varian Innova-300 or Bruker DRX-500 spectrometers. Synthesis. Amidoamines, RCONH(CH2)2N(CH3)2. Chromatographically pure carboxylic acids were obtained by hydrolysis of their ethyl esters, as given in the Supporting Information. The following reaction was carried out under dry, oxygen-free nitrogen: to a stirred solution of the carboxylic acid (0.30 mol) in 150 mL of toluene was added, dropwise, 32.8 mL (0.30 mol) of N,N-dimethylethylenediamine. The mixture was heated at 60-70 °C for 2 h, the reaction flask was connected to a DeanStark trap, and the bath temperature was raised to 120-130 °C and kept at this temperature until complete removal of water occurred (ca. 16 h). Fresh toluene and 3 mL of the amine were added during this period, to compensate for losses by distillation. After solvent evaporation, product (1; Scheme 1) was purified either by fractional distillation, 1a and 1b, or by recrystallization from anhydrous acetone, 1c and 1d. Compound 1a, C9H19CONH(CH2)2N(CH3)2. bp 160-161 °C (1 mmHg); yield, 82%; yellowish solid. Anal. Calcd for C14H30N2O: C, 69.37; H, 12.47; N, 11.56. Found: C, 69.42; H, 12.55; N, 11.37. IR, 1H NMR, and 13C NMR analysis of this, and the other amidoamines (1b-1d) are given in the Supporting Information. Compound 1b, C11H23CONH(CH2)2N(CH3)2. bp 178-180 °C (1 mmHg); yield, 89%; yellowish solid; mp 4345 °C. Anal. Calcd for C16H34N2O: C, 71.06; H, 12.67; N, 10.36. Found: C, 71.27; H, 13.22; N, 10.22. Compound 1c, C13H27CONH(CH2)2N(CH3)2. Yield, 80%; white solid; mp 54-56 °C. Anal. Calcd for C18H38N2O: C, 72.43; H, 12.83; N, 9.38. Found: C, 72.75; H, 13.11; N, 9.39. Compound 1d, C15H31CONH(CH2)2N(CH3)2. Yield, 82%; white solid; mp 63-65 °C. Anal. Calcd for C20H42N2O: C, 73.56; H, 12.96; N, 8.58. Found: C, 73.42; H, 11.90; N, 8.75. Surfactants. The following reaction was carried out under dry, oxygen-free nitrogen: a mixture of 0.1 mol of compound 1 and 12.6 mL (0.11 mol) of benzyl chloride in 100 mL of anhydrous acetonitrile was refluxed for 8 h. The solvent and unreacted benzyl chloride were removed; the product was recrystallized from acetone and dried under reduced pressure. All surfactants were hygroscopic solids and were analyzed as (nonhygroscopic) tetraphenylborate (2a; Scheme 1) or iodides. Surfactant 2a, C10ABzCl. Yield, 50%; mp 55-57 °C. Anal. Calcd for C45H57N2OB: C, 82.80; H, 8.80; N, 4.29. Found: C, 82.64; H,

Langmuir, Vol. 19, No. 2, 2003 239 8.69; N, 4.40. IR, 1H NMR, and 13C NMR analysis of this and the other surfactants (2b-2d) are given in the Supporting Information. Surfactant 2b, C12ABzCl. Yield, 78%; mp 63-65 °C; Anal. Calcd for C23H41N2OI: C, 56.55; H, 8.46; N, 5.73. Found: C, 56.43; H, 8.23; N, 5.89. Surfactant 2c, C14ABzCl. Yield, 88%; mp 74-76 °C. Anal. Calcd for C25H45N2OI: C, 58.13; H, 8.78; N, 5.42. Found: C, 58.31; H, 8.63; N, 5.30. Surfactant 2d, C16ABzCl. Yield, 91%; mp 78-80 °C. Anal. Calcd for C27H49N2OI: C, 59.55; H, 9.07; N, 5.14. Found: C, 59.69; H, 8.84; N, 5.16. Measurement of Surface Active Properties. Glass doubledistilled, deionized water was used throughout. Glassware was soaked in 0.001 M EDTA solution and thoroughly washed with water. Surface Tension. Solution surface tension was measured at 25 °C with a Lauda TE1C digital ring tensiometer, equipped with a thermostated solution compartment, and controlled with a home-developed software package. The tensiometer was programmed to repeat the measurement until the standard deviation among four successive readings was e0.10 mN m-1. Solution Conductivity. Conductivity measurements were recorded at 25 °C with a PC-interfaced Fisher Model Accumet 50 pHmeter/conductimeter, provided with a Schott Model Titronic T200 programmed buret, and controlled with a home-developed software package. Electromotive Force (emf) Measurements. The abovementioned pHmeter/conductimeter was employed for measuring emf, at 25 °C. A Corning double-junction Ag/AgCl reference electrode and IS-146Cl chloride selective electrode (Lazar Research Laboratories, Los Angeles) were employed. FTIR Measurements. The cmc of C10ABzCl in D2O was determined by FTIR, by employing the above-mentioned spectrophotometer, and a 0.015 mm path length ClearTran cell (WilmadGlass, Buena, USA). The spectrophotometer sample compartment was flushed with dry nitrogen, thermostated at 25 °C. The frequency of the amide I band was measured as a function of surfactant concentration at 1 cm-1 resolution. The background spectrum of pure D2O was subtracted from the spectrum of the sample.

Results and Discussion Synthesis of Surfactants and NMR Spectra of Their Solutions. We synthesized amidoamines 1a-1d directly from carboxylic acids, instead of from the corresponding acyl chlorides,7,8 to avoid their possible contamination, e.g., by surface active RCONH(CH2)2N+H(CH3)2Cl-. In a trial run, 99% dodecanoic acid was recrystallized from ethanol and then converted into C12ABzCl as given in the Experimental Section (reactors not flushed with nitrogen). Although satisfactory IR and NMR results were obtained, a plot (not shown) of surface tension versus log [surfactant] indicated the presence of an impossible-to-remove surface active impurity. Thus the laborious synthetic route employed is necessary to obtain surface active pure products. Increasing [surfactant] from below to above its cmc (in D2O) resulted in two interesting 1H NMR spectral modifications: (i) the peak of H2-H4 split into two peaks, one at a higher field (i.e., close to TMS) corresponding to four methylene groups, and the other at a lower field, corresponding to (n - 8) methylene groups; (ii) the order of the aromatic ring protons is different, see Figure 1 for C14ABzCl. Both observations have been previously reported for alkyldimethylbenzylammonium chlorides and have been attributed to the interaction of the benzyl group with part of the methylene segments of the surfactant hydrophobic tail.11 Below the cmc, H13 (low field; see the (11) (a) Possidonio, S.; Siviero, F.; El Seoud, O. A. J. Phys. Org. Chem. 1999, 12, 325. (b) Ro´zycka-Roszak, B.; Cierpicki, T. J. Colloid Interface Sci. 1999, 218, 529.

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Figure 1. Dependence of 1H NMR spectrum of C14ABzCl on surfactant concentration: [surfactant] ) 4.7 × 10-4 mol L-1 (A, below cmc) and 5.0 × 10-3 mol L-1 (B, above cmc). See text for numbering of protons.

structure below for proton numbering) is separated from

the high field peaks of H11 (H11′) and H12 (H12′), whereas above the cmc the order is H11 (H11′, low field) and then H12 (H12′) and H13 (high field). This inversion of proton order is due to the upfield shift of H12 (H12′) and H13, because on micellization they are transferred from an aqueous pseudophase to a less polar environment, the micelle.11b Finally, we were unable to assign the individual 13 C NMR lines of C4, due to overlap of the 1H-13C crosspeaks, so that the spectrum could not be solved by the appropriate experiment (HMQC). This partial peak overlap, and difficulty of assignment, has been reported for other cationic surfactants.12 Properties of Aqueous Solution. The following discussion is organized in terms of the sequence of events that occur in the system, that is, adsorption of the surfactant at the solution/air interface followed by its aggregation in the form of aqueous micelles. Details of calculations of all quantities discussed in this work are given in the Supporting Information. Data for the series studied will be compared with those of alkyldimethylbenzylammonium chlorides R′BzCl and, where not available, alkyltrimethylammonium chlorides R′MeCl, where R′ ) decyl to hexadecyl. 1. Adsorption at the Solution/Air Interface. Solution surface tension at the cmc, γcmc, surfactant concentration required to decrease the surface tension of water by 20 mN m-1, C20, minimum area/surfactant molecule at the (12) (a) Ulmius, J.; Lindman, B.; Lindblom, G.; Drakenberg, T. J. Colloid Interface Sci. 1987, 65, 88. (b) Cabane, B. J. Phys. 1981, 42, 847. (c) McNeil, R.; Thomas, J. K. J. Colloid Interface Sci. 1981, 83, 57. (d) Stark, R. E.; Storrs, R. W.; Kasakevich, M. L. J. Phys. Chem. 1985, 89, 272. (e) Uzu, Y.; Saito, Y.; Yokoi, M. Bull. Chem. Soc. Jpn. 1989, 62, 1370.

Table 1. Comparison between Adsorption Properties of the Surfactants Studied and Other Cationic Surfactants, at 25 °Ca,b surfactant

γcmc, mN m-1

103(C20), mol L-1

C10ABzCl C12ABzCl C14ABzCl C16ABzCl C10MeClc C12MeClc C14MeClc C16MeClc

39.0 39.6 39.9 42.2 42.0 41.8 40.0 37.6

5.9 1.6 0.40 0.11 6.6 1.6 0.28 0.04

σ (nm2) 0.87 0.82 0.79 0.73 0.87 0.62 0.53 0.49 0.62e

∆G°ads (kJ mol-1) -33.3 -35.9 -38.9 -41.3 -19.6d -22.9d -26.4d -43.7d

a See the Supporting Information for details of calculations. b γ cmc, C20, σ, and ∆G°ads refer to solution surface tension at the cmc, surfactant concentration required to decrease the surface tension of water by 20 mN m-1, area/surfactant molecule at the interface, and Gibbs free energy of adsorption, respectively. c Results taken from ref 14a. d These results are for R′BzCl with the same hydrophobic tail as R′MeCl.14b e For C16BzCl, at 35 °C.6c

solution/air interface, σ, and Gibbs free energy of adsorption, ∆G°ads, are listed in Table 1. The adsorption efficiency (C20) and efficacy (γcmc) are relevant to detergency and emulsification. Comparison of the data of RABzCl and R′MeCl (Table 1) shows that both quantities depend on the length of the surfactant hydrophobic tail, but only slightly on headgroup structure. In a recent article, Eastoe et al. have shown the effect of traces of alkaline earth metal ions on σ of anionic surfactants, and the advantage of determining their cmc (by surface tension) in the presence of EDTA.13 Although these impurities, if present, are not expected to perturb cationic micelles, we have soaked all glassware in EDTA solution. Determination of cmc of C12ABzCl in the presence of 6 × 10-5 mol L-1 EDTA ()0.01 × cmc) resulted, however, in a surface tension versus [surfactant] plot (not shown) (13) Eastoe, J.; Nave, S.; Downer, A.; Paul, A.; Rankin, A.; Tribe, K. Langmuir 2000, 16, 4511.

RCONH(CH2)2N+(CH3)2CH2C6H5Cl- Surfactant Synthesis

that is typical of an impure surfactant, probably because of inclusion of EDTA in the micelle. Calculated σ decreases as a function of increasing the length of R, due to concomitant closer packing of monomers in the micelle.2,4,5 These areas are larger, however, than those of other cationic surfactants with equivalent hydrophobic tails, namely RMeCl (R ) C10-C16) and C16BzCl. This indicates that the hydrated headgroup of RABzCl is larger than those of the other two series. One possibility is that the micellar interface lies behind the -N+(CH3)2CH2C6H5Cl- group, as will be discussed later. The Gibbs free energy of adsorption, ∆G°ads, is calculated from C20 and σ. It contains contributions from the transfer of surfactant segments from bulk water to the interface and bears, therefore, on the relative importance of surfactant hydrophilic and hydrophobic moieties to its adsorption. These contributions are due to the terminal CH3 group of the hydrophobic chain, ∆G°CH3; the methylene groups of the alkyl chain, (NCH2∆G°CH2), where N ) 8, 10, 12, and 14, respectively; and the headgroup, ∆G°headgroup, as given by2

∆G°ads ) ∆G°headgroup + ∆G°CH3 + NCH2∆G°CH2

Langmuir, Vol. 19, No. 2, 2003 241

Figure 2. Dependence of solution conductance (A) and surface tension (B) on surfactant concentration, at 25 °C.

(1)

Equation 1 predicts a linear correlation between ∆G°ads and NCH2, where the intercept includes a contribution from its terminal methyl group and headgroup. Since ∆G°CH3 is independent of the chain length of the surfactant, its contribution is constant in a homologous series; i.e., the intercept essentially reflects the effect of transfer of the headgroup from bulk solution to the solution/air interface.15 Application of eq 1 to the data reported in Table 1 gave a straight line (correlation coefficient r ) 0.9992). The present study and previous ones on R′BzCl gave the following results, respectively: -1.4 and -1.6 kJ/mol (∆G°CH2); -22.5 and -5.3 kJ/mol (∆G°headgroup + ∆G°CH3). The first result is expected since transfer of a CH2 group in the hydrophobic tail from bulk phase to the interface should be independent of headgroup structure. The reason for the more favorable free energy of transfer of the latter group relative to that of R′BzCl will be discussed later. 2. Aggregation: Critical Micelle Concentration, Degree of Micelle Dissociation, Thermodynamic Parameters of Micellization, and Aggregation Numbers. Plots of solution conductance and/or surface tension as a function of [surfactant] are shown in parts A and B of Figure 2. The corresponding plots of surfactant free counterion, [Cl-]f, and the IR frequency of amide I band versus [surfactant] are shown in parts A and B of Figure 3. Each graph consists of two straight lines intersecting at the cmc; these values are listed in Table 2, along with available data for the corresponding R′BzCl. Regarding this table, the following are relevant: (i) Results of the emf experiment were used to calculate three micellar parameters, namely, cmc, Rmic, and Nagg. The last two require measurements up to high surfactant concentrations, ca. 40 × cmc; high solution viscosity precluded carrying out the experiment with C10ABzCl. The surface tension plots show that the compounds studied are surface-active pure. (ii) These results can be employed to calculate Rmic, from which the Gibbs free energy of micelle formation, ∆G°mic, is obtained. Conductance results have been employed since data are available for all members of the series. Rmic may (14) (a) Rosen, M. J.; Li, F. Environ. Sci. Technol. 2002, 35, 954. (b) Blois, D. W.; Swarbrick, J. J. Colloid Interface Sci. 1971, 36, 227. (15) Tanford, C. The hydrophobic effect: Formation of micelles and biological membranes; Krieger: Malabar, FL, 1991.

Figure 3. Dependence of concentration of the free counterion [Cl-]f (A) and νC)O of amide I band (B) on [C16ABzCl] and [C10ABzCl], respectively, at 25 °C. In (A) the points are experimental and the line has been calculated as given in the Supporting Information.

be calculated by dividing the slopes of the straight lines above and below the cmc. Frahm’s method16a is a useful approximation when Nagg is not available; it results in Rmic higher than that calculated by the Evans method, because the conductivity of the micelle (a “macroion”) is not taken into account.16b ∆G°mic based on Rmic calculated by both methods is listed in Table 2. (iii) Calculation of Rmic according to the Evans equation requires knowledge of Nagg; this has been calculated from volumes of the monomer and micelle, respectively. The micellar interface may be located either at the amide group or at the quaternary ammonium ion of RCO-NH(CH2CH2)N+(CH3)2BzCl-. Although both assumptions were considered, the former Nagg values were employed since the latter ones (72, 96, 122, and 152 for RCO ) C10, C12, C14, and C16, respectively) were considered too high. We have also employed our emf data to calculate Nagg. As shown in the appropriate column of Table 2, there is an excellent agreement between theoretically calculated and experimental aggregation numbers. This corroborates our view regarding the location of the interface. (iv) Equation 2 for ∆G°mic is written analogously to eq 1:2

∆G°mic ) ∆G°headgroup + ∆G°CH3 + NCH2∆G°CH2

(2)

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Table 2. Dependence of Critical Micelle Concentrations (cmc), Degree of Micelle Dissociation (rmic), Micelle Aggregation Number (Nagg), and Gibbs Free Energy of Micelle Formation (∆G°mic) on Surfactant Structure, Measured by Different Techniques, at 25 °Ca Nagge

cmc surfactant

surface tension

conductance

emf

Rmic, conductanceb-d

emf

calcd

∆G°micf (kJ mol-1)

C10ABzCl C12ABzCl C14ABzCl C16ABzCl C10BzClh C12BzClh C14BzClh C16BzClh

25.0 5.8 1.3 0.27

24.0 5.9 1.5 0.40 38.0 8.8 2.0 0.49

g 5.8 1.5 0.39

0.28 (0.35) 0.23 (0.35) 0.22 (0.38) 0.19 (0.38) (0.47) (0.44) (0.48) (0.48)

g 58 84 110

43 61 82 107

-32.8 (-31.4) -40.1 (-37.3) -46.3 (-42.2) -53.0 (-47.5) (-27.7) (-33.7) (-38.6) (-43.8)

3.8 1.5 0.34 L-1

102h

All cmc values are in mol and were multiplied by Rmic was calculated by Evan’s method. c The figures in parentheses refer to Rmic calculated by Frahm’s method. d Rmic from emf measurements were: 0.23, 0.21, and 0.19 for C12ABzCl, C14ABzCl and C16ABzCl, respectively. e Calculated from emf measurements, or from the volumes of monomer and micelle; see Supporting Information for details. f The figures in parentheses refer to free energies of micellization based on R d High solution viscosity mic calculated by Frahm’s method. precluded performing this experiment. h Results for R′BzCl were taken from refs 6c, 17a, and 17c. a

103. b

where the terms ∆G°headgroup, ∆G°CH3, and ∆G°CH2 refer to contributions to ∆G°mic of the surfactant moieties, as discussed above for adsorption. Application of eq 2 to RABzCl gave a straight line (r ) 0.9994). The following results are for RABzCl and R′BzCl, respectively, and are based on Rmic, calculated by Frahm’s method: ∆G°CH2 ) -2.7 and -2.7 kJ mol-1; (∆G°headgroup + ∆G°CH3) ) -10.3 and -3.3 kJ/mol. Again, whereas ∆G°CH2 is similar for the two surfactant series, the free energy of transfer of the headgroup of RABzCl from bulk solution to the micelle is more favorable than that of R′BzCl. ∆G°CH2 and (∆G°headgroup + ∆G°CH3), based on Rmic, calculated by the Evans equation are -3.4 and -6.1 kJ/mol (r ) 0.9996), respectively. (v) The agreement between cmc and Rmic calculated from data of four different techniques is rewarding, considering that the IR experiment has been carried out in D2O. This is one of the few studies in which IR spectroscopy has been used to determine the cmc of surfactants,18 although IR and Raman spectroscopies have been fruitfully employed to study interactions (including H-bonding) within organized assemblies.19 (vi) The concentrations of [Cl-]f and free surfactant cation, [S+]f, can be calculated from emf data. A typical example is shown in Figure 4 for C12ABzCl. Above the cmc, [Cl-]f increases and [C12ABz+]f decreases, whereas the mean ionic molarity of the surfactant, i.e., ([Cl-]f[C12ABz+]f)0.5, increases slowly, in agreement with the mass-action model for micelle formation, as shown, e.g., for micellar sodium dodecyl sulfate.20 The more favorable free energies of adsorption and micellization of the headgroup of RABzCl can be explained by considering differences between polarities of bulk water and the micellar interfacial region (so-called “medium (16) (a) Frahm, J.; Diekmann, S.; Haase, A. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 566. (b) Evans, H. C. J. Chem. Soc. 1956, 579. (17) (a) Ledbetter, J. W., Jr.; Bowen, J. R. Anal. Chem. 1969, 41, 1347. (b) Rodriguez, J. R.; Czapkiewicz, J. Colloids Surf. A 1995, 101, 107. (c) Gonza´lez-Pe´rez, A.; Czapkiewicz, J.; Del Castillo, J. L.; Rodrı´guez, J. R. Colloids Surf. A 2001, 193, 129. (18) (a) Umemura, J.; Mantsch, H. H.; Cameron, D. G. J. Colloid Interface Sci. 1981, 83, 558. (b) Yang, P. W.; Mantsch, H. H. J. Colloid Interface Sci. 1986, 113, 218. (c) Etori, F.; Yamada, H.; Taga, Y.; Okabayashi, K.; Ohshima, H. K.; O’Connor, C. J. Vibr. Spectrosc. 1997, 14, 133. (19) (a) Lafrance, D.; Marion, D.; Pe´zolet, M. Biochemistry 1990, 29, 4592. (b) Nilsson, A.; Holmgren, A.; Lindblom, G. Chem. Phys. Lipids 1994, 69, 219. (c) Domingo, J. C.; Mora, M.; de Madariaga, M. A. Chem. Phys. Lipids 1994, 69, 229. (d) Domingo, J. C.; de Madariaga, M. A. Chem. Phys. Lipids 1996, 84, 147. (20) (a) Sasaki, T.; Hattori, M.; Sasaki, J.; Nukina, K. Bull. Chem. Soc. Jpn. 1975, 48, 1397. (b) Moroi, Y. J. Colloid Interface Sci. 1988, 122, 308.

Figure 4. Dependence of concentrations of the surfactant free counterion, [Cl-]f (b), free cation, [S+]f (f), and mean ionic molarity of the surfactant, i.e., ([Cl-]f × [S+]f)0.5 (2), on [C12ABzCl], at 25 °C.

effect”),21 and monomer intermolecular H-bonding. Consider first the transfer of the hydrophobic chain to the micellar interior, a process akin to its transfer from an aqueous solution to a hydrocarbon. NMR results have indicated that the monomers of several cationic surfactants in water have compact rather than extended shapes, or may exist as small aggregates.22 This should lead to a small medium effect on |∆G°CH2| since the hydrophobic tail of the monomer is not fully exposed to water. A similar argument can be used for adsorption, where the transfer is from solution to air. The noticeable difference between ∆G°CH3 + ∆G°headgroup of RABzCl and that of other cationic surfactants may be attributed to the favorable effect (on adsorption and/or micellization) of intermolecular Hbonding of surfactant monomers. This occurs via the amide group, akin to that of amides, polypeptides, and other surfactants that carry the amide headgroup.23 The preceding conclusion is corroborated by our IR results, as follows: at [surfactant] < cmc, the frequency of the amide I band is constant at 1628 cm-1, increasing to 1638 cm-1 (21) (a) Novaki, L. P,; El Seoud, O. A. Phys. Chem. Chem. Phys. 1999, 1, 1957. (b) Tada, E. B.; Novaki, L. P.; El Seoud, O. A. Langmuir 2001, 17, 652. (22) (a) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1989, 93, 1490. (b) Bazito, R. C.; El Seoud, O. A.; Barlow, G. K.; Halstead, T. K. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1933. (23) (a) Bazito, R. C.; El Seoud, O. A. Carbohyd. Res. 2001, 323, 95. (b) Bazito, R. C.; El Seoud, O. A. J. Surf. Det. 2001, 4, 395. (c) Bazito, R. C.; El Seoud, O. A. Langmuir 2002, 18, 4362.

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Langmuir, Vol. 19, No. 2, 2003 243

above the cmc, and then levels off at 1939 cm-1, at [surfactant] g 0.3 mol L-1, Figure 3B. The first frequency indicates that the monomer amide group is exposed to water,9c,19a,24a,c and that carbonyl group-water H-bonds are as strong as those in completely hydrated N-methylacetamide,24b,c a compound that is extensively employed to model H-bonding interactions in proteins.9,24 In aqueous binary mixtures whose polarity mimics that of interfacial water (e.g., aqueous acetonitrile and aqueous DMSO),21 the frequency of the amide I band of N-methylacetamide increases as a function of decreasing solvent polarity. This has been attributed to decreased hydration of the CO group, i.e., replacement of -CdO-water H-bonds by NHCO bonds.19a,24c In fact, the frequency of the surfactant amide I band above the cmc, 1639 cm-1, is very close to that of self-associated N-methylacetamide at 1640 cm-1.9,24 This indicates appreciable H-bonding between molecules of RABzCl at the solution/air interface, and/or in the aggregate, in agreement with calculated ∆G°CH3 + ∆G°headgroup. The relative strength of this H-bonding in the micelle can be judged by comparing ∆ν ()νC)O after cmc - νC)O before cmc) for RABzCl (10 cm-1) with that of the (strongly hydrated) carboxylate headgroup of sodium nonanoate or decanoate, e3 cm-1.18a Our IR results also indicate that the surfactant amide group is not localized in the (nonpolar) micellar interior, which would be the case, e.g., if the micellar interface were at the quaternary ammonium ion. This conclusion is arrived at by comparing νC)O of the amide I band of micellized C10ABzCl, 1640 cm-1, with that of N-methylacetamide in apolar solvents (e.g., hexane, 1697 cm-1) and polar aprotic solvents (1683

and 1667 cm-1 for THF and DMSO, respectively).24c Finally, the frequency plateau region of Figure 3B may indicate that the aggregate is undergoing a geometric change associated with a second cmc.2,17c Indeed, our conductance measurements, e.g., for C16ABzCl, show a second break at 1.3 × 10-3 mol L-1.

(24) (a) Miyazawa, T.; Shimanouchi, T.; Mizushima, S. J. Chem. Phys. 1958, 29, 611. (b) Chen, C. Y. S.; Swenson, C. A. J. Phys. Chem. 1969, 73, 2999. (c) Eaton, G.; Symons, M. C. R.; Rastogi, P. P. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3257. (d) Symons, M. C. R. J. Mol. Struct. 1993, 297, 133. (e) Schweitzer-Stenner, R.; Sieler, G.; Mirkin, N. G.; Krimm, S. J. Phys. Chem. A 1998, 102, 118.

Conclusions A homologous series of benzyl(2-acylaminoethyl)dimethylammonium chloride surfactants have been synthesized, and their aggregation in aqueous solution has been studied by conductivity, surface tension, measurement of solution emf (in H2O), and FTIR (in D2O). The free energies of adsorption at the solution/air interface and of micellization have been separated into contributions from the transfer of the discrete surfactant segments from bulk water to the interface and/or the micellar pseudophase, respectively. Both free energies of transfer of the headgroup of RABzCl are lower than those of other cationic surfactants, due to H-bonding between the surfactant amide groups. Acknowledgment. We thank the FAPESP for financial support and for a predoctoral fellowship to S.S., the CNPq for a research productivity fellowship to O.A.E., Paulo A. R. Pires and Reinaldo C. Bazito for their help with the calculations, and Profs. H. Chaimovich and I. M. Cuccovia for making the chloride ion electrode available to us. Supporting Information Available: Synthesis, spectroscopic characterization of all compounds, calculations of all quantities discussed in this paper, dependence of solution emf on log [surfactant], for C12ABzCl and C14ABzCl, respectively. This material is available free of charge via the Internet at http://pubs.acs.org. LA026286Y