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Langmuir 1999, 15, 6664-6670
Synthesis and Properties of N-Alkyl Amide Sulfates† Hiromoto Mizushima* Wakayama Research Laboratories, Kao Corporation, 1334 Minato, Wakayama 640, Japan
Takashi Matsuo and Naoki Satoh Tokyo Research Laboratories, Kao Corporation, 2-1-3 Bunka, Sumida-ku, Tokyo 131, Japan
Heinz Hoffmann and Dieter Graebner University of Bayreuth, 95447 Bayreuth, Germany Received December 31, 1998. In Final Form: May 27, 1999
The critical micelle concentration (CMC), micelle ionization degree (R), Krafft point (KP), and chemical stability in water of sodium and calcium salts of the title surfactants, CmH2m+1NHCO(CH2)nOSO3Me, abbreviated as m-n-Me (Me ) Na, 0.5Ca), have been investigated as a function of the numbers m and n of the hydrocarbon chain length and the spacer methylene chain length, respectively. For the m-n-Na series, the CMC was reduced with increasing both the spacer methylene chain length (n ) 1, 3, 5) and the hydrocarbon chain length (m ) 12, 14, 16). The value of the ratio of reduction in log CMC for m to n was found to be just 4:1, suggesting that the spacer methylene chain is less hydrophobic than in the alkyl chain. Inclusion of an amide linkage near the headgroup in alkyl sulfates leads to a significant increase in the micelle ionization degree and to an increase in Krafft temperature. Furthermore, we found a striking and interesting exception in the Krafft temperature for the sodium and calcium salt pair of m-1-Me homologues. The KP of the 12-1-0.5Ca surfactant was found to be below 0 °C and was lower than that of the corresponding sodium salt (25.6 °C). This phenomenon is believed to be the first observation that the KP of the divalent salt is lower than that of the monovalent salt having the same surface active ion.
Introduction In hard water, ordinary anionic surfactants are salted out and cannot be used without any sequestering agents, because the Krafft points of calcium salts of anionic surfactants are generally higher than ambient temperature. For example, the Krafft point of calcium dodecyl sulfate is about 50 °C.1,2 When one applies anionic surfactants, for example, in laundry or dish-washing detergents and/or personal-care products such as shampoos, precipitation of the calcium salts decreases the detergency or the foaming-ability. Much effort has been devoted to developing highly calcium-tolerant anionic surfactants.3-9 * To whom correspondence should be addressed at Biological Science Laboratories, Kao Corp., 2606 Akabane, Ichikaimachi, Haga, Tochigi 321-3497, Japan. Phone: +81-285-68-7577. Fax: +81-285-68-7571. E-mail: 134341@kastanet. kao. co. jp. † Abbreviations: CMC, critical micelle concentration; DSC, differential scanning calorimetry; IS-MS, ion-spray mass spectrometry; KP, Krafft point; MEAS, sulfated N-(2-hydroxyethyl)lauramide. (1) Hato, M.; Shinoda, K. J. Phys. Chem. 1973, 77, 378. (2) Hato, M.; Shinoda, K. Bull. Chem. Soc. Jpn. 1973, 46, 3889. (3) Shinoda, K.; Hirai, T. J. Phys. Chem. 1977, 81, 1842. (4) Tsujii, K.; Saito, N.; Takeuchi, T. J. Phys. Chem. 1980, 84, 2287. (5) Kooreman, P. A.; Engberts, J. B. F. N.; van Os, N. M. J. Surfactants Deterg. 1998, 1, 23. (6) Piasecki, A.; Sokolowski, A.; Burczyk, B.; Kotlewska, U. J. Am. Oil. Chem. Soc. 1997, 74, 33. (7) Zhu, Y.-P.; Rosen, M. J.; Morrall, S. W. J. Surfactants Deterg. 1998, 1, 1. (8) Schmidt, W. W.; Durante, D. R.; Gingell, R.; Harbell, J. W. J. Am. Oil. Chem. Soc. 1997, 74, 25. (9) Miyazawa, K.; Takahashi, T.; Kohashi, H.; Ishida, M.; Yoshino, S.; Kamogawa, H.; Sakai, H.; Abe, M. J. Jpn. Oil. Chem. Soc. 1998, 47, 11.
Previous investigators explored the effect of the incorporation of additional hydrophilic groups, such as oxyethylene chains1-4 and ester and/or amide linkages,9-15 into anionic surfactant molecules and found that such chemical modifications can generally improve the calcium stability. Especially, some amide group incorporated anionics, e.g., N-acylated amino acids7,9 or N-alkanoyl amide sulfates,11-15 exhibit many excellent properties, that is, low irritation to skin, high biodegradability in the environment, and compatibility with cationic surfactants, as well as more tolerance of water hardness than the similar conventional surfactants. These unique properties have made them desirable in personal-care or household products, cosmetics, and pharmaceuticals.7,9,15 Despite the practical importance of the amide-linked anionic surfactants, only limited information is available concerning their physicochemical properties in relation to their superior properties. Also, there has not been a systematic detailed study of the chemical structureproperty relationships for the amide-incorporated anionic surfactants.7,9,15 Moreover, only limited information is available concerning the effect of the inclusion of amide linkages into anionic surfactant molecules having divalent counterions. Prior to their industrial utilization, the Krafft (10) Hikota, T.; Morohara, K.; Meguro, K. Bull. Chem. Soc. Jpn. 1970, 43, 3913. (11) Desnuelle, P.; Micaelli, O. Bull. Soc. Chim. Fr. 1950, 671. (12) Weil, J. K.; Parris, N.; Stirton, A. J. J. Am. Oil. Chem. Soc. 1970, 47, 91. (13) Parris, N.; Weil, J. K.; Linfield, W. M. J. Am. Oil. Chem. Soc. 1972, 49, 649. (14) Weil, J. K. Anionic Surfactants Part I, Linfield, W. M., Ed.; Marcel Dekker: New York, 1976; pp 228-229. (15) Kuroiwa, S.; Fujimatsu, H.; Takagi, K.; Mukai, S. J. Jpn. Oil Chem. Soc. 1981, 40, 244.
10.1021/la981777x CCC: $15.00 © 1999 American Chemical Society Published on Web 08/05/1999
Synthesis and Properties of N-Alkyl Amide Sulfates
temperatures or micellar properties of the divalent salts must be examined in relation to their calcium stabilities.1-4 In the present study, we report the synthesis and some physicochemical properties of a series of N-alkyl amide sulfates CmH2m+1NHCO(CH2)nOSO3Me (n ) 1, 3, 5; Me ) Na, 0.5Ca). Although their chemical structures are quite simple, to our best knowledge, there are no reports concerning these amide sulfates except our patented literature (USP 5599483). Relationships were discussed between the chemical structure and the physicochemical properties, such as Krafft point, critical micelle concentration (CMC), micelle ionization degree (R), and chemical stability in water. Among them, the Krafft point of calcium N-dodecyl glycolic amide sulfate (m ) 12, n ) 1, Me ) 0.5Ca) was found to be below 0 °C and is lower than that of the sodium salt (m ) 12, n ) 1, Me ) Na; 25.7 °C). The reason that the Krafft temperature of the divalent salt is lower than that corresponding monovalent salt is discussed. Experimental Section Materials. n-Dodecylamine, n-tetradecylamine, and n-hexadecylamine were purchased from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan). γ-Butyrolactone, -caprolactone, sodium hydroxide, calcium hydroxide, chlorosulfonic acid, tetrabutylammonium bromide, phosphoric acid, and other anhydrous solvents were obtained from Wako Pure Chemical Industries Ltd (Tokyo, Japan). Methyl glycolate was kindly provided from Ube Industries Ltd. (Tokyo, Japan). All reagents were of reagent grade and used as received. For measurements, distilled water of a conductivity κ ) 1.5 µS cm-1(HPLC grade) from Wako Pure Chemical Industries Ltd was used. 1H NMR spectra were recorded at 25 °C with a Bruker-AC200P 200 MHz spectrometer, using the chemical shifts of the solvents as internal standards. Infrared spectra were recorded on a Horiba (Kyoto, Japan) FT-710 Fourier transform infrared spectrometer as KBr pellets. The mass spectra were recorded on a PerkinElmer SCIEX API300 ion spray mass (IS-MS) spectrometer. Melting points were uncorrected. The purity of final compounds was checked by HPLC, with a Hitachi 655A-11 liquid chromatograph, in a column of LiChrosorb RP-18 (Cica-Merk) using a Shodex SE-51 RI detector. The mobile phase was a mixture of methanol/water (10/90, v/v) containing tetrabutylammonium bromide (0.02 M) and phosphoric acid (1.5 g/L). Electrical Conductivity. The electrical conductivity method was used to determine the critical micellization concentration and the approximate micelle ionization degree. The plots of the electrical conductivity (κ) against the surfactant concentration (C) showed a clear break point or a curved part connecting two linear parts. The CMC was taken as the concentration corresponding to the intersection of the extrapolations of the two linear parts. The micelle ionization degree was taken as the ratio of the values of dκ/dC above and below the CMC.16-20 A TOA CM-40V conductimeter (TOA Electronics Ltd., Tokyo, Japan) equipped with an electrode (cell constant 0.997 cm-1) was used. Differential Scanning Calorimetry (DSC). The DSC analyses were carried out on a DSC-100 instrument (Seiko Instrument & Electronics Ltd., Tokyo, Japan). The aqueous surfactant solution (30 mM, 20-30 mg) was put into a silver DSC capsule (Seiko). The capsule was sealed and placed in the DSC cell along with a vacant reference. The sample was cooled to 5-10 °C for at least 30 min and then heated to 80 °C from 10 °C at a rate of 1 °C/min. The Krafft point was determined by extrapolation to the baseline of the most rapid rise in the excess heat capacity curve as a function of temperature. The transition enthalpy (∆H) was determined by integrating the area under (16) Celik, M. S.; Somasundaran, P. J. Colloid Interface Sci. 1988, 122, 163 (17) Hikota, T.; Meguro, K. J. Am. Oil. Chem. Soc. 1970, 47, 197. (18) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (19) Binana-Limbele, W.; Zana, R.; Platone, E. J. Colloid Interface Sci. 1988, 124, 647. (20) Hoffman, H.; Ulbricht, W. Z. Phys. Chem. 1977, 106, 167.
Langmuir, Vol. 15, No. 20, 1999 6665 the curve from a plot of the excess heat capacity as a function of temperature. General Procedure for the Synthesis of N-Alkyl-2hydroxyethanamide (Ia-c). Glycolic amides of the long chain primary amines RNHCOCH2OH (R ) n-C12H25, n-C14H29, and n-C16H33) were typically prepared as follows: Methyl glycolate (1.14 mol) was added to the selected primary amine melted (1.14 mol) at 50-70 °C. The mixture was heated to 100 °C while stirring and purging with nitrogen. The reaction was completed by further agitation for 5 h at 100 °C under a nitrogen stream. After completion of the reaction, the crude products were recrystallized from a mixture of methanol and ethyl acetate (isolated yields: 70-90%). Spectral Characteristics for Ia-b. IR (KBr, ν/cm-1): 31003600 (NH, OH); 2916, 2850 (CH2); 1635 (CO-N amide I); 1550 (N-CdO amide II). 1H NMR (200 MHz, δ, CDCl3): 0.85 ppm (t, 3H, CH3); 1.2-1.4 ppm (m, CH2); 1.51 ppm (m, 2H, CH2CH2NH); 3.23 ppm (q, 2H, CH2NH); 3.98 ppm (s, 2H, COCH2OH); 5.06 ppm (br, 1H, OH); 6.98 ppm (br, 1H, NH). Mp: 82 °C (Ia); 87-88 °C (Ib); 91-92 °C (Ic). Synthesis of N-Dodecyl-4-hydroxybutanamide (II). γ-Butyrolactone (1.12 mol) was added dropwise to n-dodecylamine (1.12 mol) melted at 50 °C. The mixture was heated at 100 °C for 6 h with stirring under nitrogen stream. The crude product was dissolved in hexane at 60-65 °C and was then cooled to room temperature. The resulting precipitate (a white solid) was filtered out and dried in vacuo (isolated yield: 91.0%). IR (KBr, ν/cm-1): 3100-3600 (NH, OH); 2920, 2850 (CH2); 1633 cm-1 (CO-N amide I); 1543 (N-CdO amide II). 1H NMR (200 MHz, δ, CDCl3): 0.85 ppm (t, 3H, CH3); 1.2-1.4 ppm (m, 18H, CH2(CH2)9); 1.46 ppm (m, 2H, CH2CH2NH); 3.33 ppm (q, 2H, CH2NH); 2.31 ppm (t, 2H, COCH2(CH2)2OH); 1.83 ppm (m, 2H, COCH2CH2CH2OH); 3.63 ppm (t, 2H, CH2OH); 5.06 ppm (br, 1H, OH); 6.98 ppm (br, 1H, NH). Mp: 79-80 °C. Synthesis of N-Dodecyl-6-hydroxyhexanamide (III). -Caprolactone (1.09 mol) was added dropwise to n-dodecylamine (1.09 mol) melted at 50 °C. The mixture was heated at 120 °C for 4 h with stirring under nitrogen stream. The crude product was recrystallized from a mixture of methanol and ethyl acetate (isolated yield: 21.2%). IR (KBr, ν/cm-1): 3100-3600 cm-1 (NH, OH); 2920, 2850 (CH2); 1633 (CO-N amide I); 1541 (N-CdO amide II). 1H NMR (200 MHz, δ, CDCl3): 0.85 ppm (t, 3H, CH3); 1.1-1.8 ppm (m, 26H, CH2(CH2)10CH2NH, COCH2(CH2)3CH2OH); 2.33 ppm (t, 2H, COCH2(CH2)4OH); 2.81 ppm (s, 1H, OH); 3.20 ppm (q, 2H, CH2NH); 3.61 ppm (t, 2H, CH2OH); 5.97 ppm (br, 1H, NH). Mp: 80 °C. General Procedure for the Synthesis of Sodium N-Alkyl Amide Sulfates (m-n-Na). Sodium salts of N-alkyl amide sulfates were typically prepared as follows: the selected N-alkyl amide alkanol (0.121 mol) was suspended in hexane (500 mL) at room temperature with vigorous stirring. Chlorosulfonic acid (0.127 mol) was added dropwise to the slurry at room temperature. The mixture was then heated at 60 °C for 3 h to remove HCl vapor by introducing nitrogen. After completion of the sulfation reaction, the mixture was cooled to 30 °C. Sodium hydroxide (1.52 mol), water (200 mL), and ethanol (150 mL) were added and were stirred for 1 h at 30-60 °C until the precipitates dissolved completely. The aqueous layer was separated and held in the pH range 7-9 by the addition of a 5% phosphoric acid. The solution was concentrated under reduced pressure. The residual solid was purified by several recrystallizations from a mixture of 2-propanol and water or from water and then washed with acetone. The purities of the sodium salts (5 compounds) were checked by analytical HPLC and were found to be 97% or more. Sodium N-dodecyl-2-sulfooxyethanamide, 12-1-Na: Molecular mass ) 345.4; yield ) 63.3%; m/e (M + Na+) 368.0. Anal. Calcd for C14H28O5NSNa‚1/2H2O: C, 47.44; H, 8.24; N, 3.95. Found: C, 47.93; H, 8.28; N, 3.89. Sodium N-tetradecyl-2-sulfooxyethanamide, 14-1-Na: Molecular mass ) 373.5; yield ) 87.4%; m/e (M + Na+) 396.1. Anal. Calcd for C16H32O5NSNa‚1/3H2O: C, 50.64; H, 8.67; N, 3.69. Found: C, 50.55; H, 8.73; N, 3.67. Sodium N-hexadecyl-2-sulfooxyethanamide, 16-1-Na: Molecular mass ) 401.5; yield ) 58.3%; m/e (M + Na+) 424.1. Anal. Calcd for C18H36O5NSNa‚2/3H2O: C, 52.28; H, 9.09; N, 3.39. Found: C, 52.26; H, 9.10; N, 3.46. Sodium N-dodecyl-4-sulfooxybutanamide, 12-3-Na: Molecular mass ) 373.5; yield ) 37.4%; m/e (M + Na+)
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Mizushima et al. Scheme 1
396.1. Anal. Calcd for C16H32O5NSNa: C, 51.46; H, 8.63; N, 3.78. Found: C, 51.18; H, 8.88; N, 3.58. Sodium N-dodecyl-6-sulfooxyhexanamide, 12-5-Na: Molecular mass ) 401.5; yield ) 68.2%; m/e (M+Na+) 424.1. Anal. Calcd for C18H36O5NSNa‚1/2H2O: C, 52.66; H, 9.08; N, 3.41. Found: C, 52.68; H, 9.12; N, 3.33. Spectral Characteristics for Sodium N-Alkyl Amide Sulfates (m-n-Na). m-1-Na (m ) 12, 14, 16). IR (KBr, ν/cm-1): 3332 (NH); 2920, 2850 (CH2); 1657 (CO-N amide I); 1549 cm-1 (N-CdO amide II); 1252, 1055, 1024 (OSO3-). 1H NMR (200 MHz, δ, DMSO-d6): 0.87 ppm (t, 3H, CH3); 1.1-1.5 ppm (m, CH2); 3.09 ppm (q, 2H, CH2NH); 4.15 ppm (s, 2H, COCH2OSO3Na); 7.65 ppm (br, 1H, NH). 12-3-Na. IR (KBr, ν/cm-1): 3330 (NH); 2921, 2850 (CH2); 1635 (CO-N amide I); 1527 (N-CdO amide II); 1267, 1207, 1072, 978 (OSO3-). 1H NMR (200 MHz, δ, D2O): 0.85 ppm (t, 3H, CH3); 1.2-1.4 ppm (m, 18H, CH3(CH2)9); 1.47 ppm (m, 2H, CH2CH2NH); 3.17 ppm (q, 2H, CH2NH); 2.34 (t, 2H, COCH2(CH2)2OSO3Na); 1.93 ppm (m, 2H, COCH2CH2CH2OSO3Na); 4.07 ppm (t, 2H, CH2OSO3Na). 12-5-Na. IR (KBr, ν/cm-1): 3330 (NH); 2922, 2850 (CH2); 1635 (CO-N amide I); 1529 cm-1 (N-CdO amide II); 1269, 1207, 1072, 972 (OSO3-). 1H NMR (200 MHz, δ, DMSO-d6): 0.89 ppm (t, 3H, CH3); 1.1-1.6 ppm (m, 26H, CH3(CH2)10, COCH2(CH2)3CH2OSO3Na); 2.08 (t, 2H, COCH2(CH2)4OSO3Na); 3.05 ppm (q, 2H, CH2NH); 3.71 (t, 2H, CH2OSO3Na); 7.75 ppm (br, 1H, NH). General Procedure for the Synthesis of Calcium N-Alkyl Amide Sulfates (m-n-0.5Ca). Calcium salts of N-alkyl amide sulfates were typically prepared as follows: The procedure of the sulfation reaction was exactly the same as those of the sodium salts as described above. After completion of the sulfation reaction of the selected N-alkyl amide alkanol (0.184 mol), the reaction mixture was cooled to 5 °C and added to a mixture of ice water (200 g) and 2-propanol (150 mL). The aqueous layer was separated and neutralized with calcium hydroxide (0.195 mol) at 5-10 °C. Excess calcium hydroxide was filtered off, and the resulting solution was evaporated in vacuo. The residue was crystallized twice from a mixture of 2-propanol and water or from water and finally crystallized from a mixture of ethanol and acetone. The purities of the calcium salts (5 compounds) were checked by analytical HPLC and were found to be 97% or more. Calcium N-dodecyl-2-sulfooxyethanamide, 12-1-0.5Ca: Molecular mass ) 342.5; yield ) 71.6%; m/e (2M + NH4+) 702.4. Anal. Calcd for C14H28O5NS‚1/2Ca: C, 49.10; H, 8.24; N, 4.09. Found: C, 48.69; H, 8.16; N, 3.99. Calcium N-tetradecyl-2-sulfooxyethanamide, 14-1-0.5Ca: Molecular Mass ) 370.5; yield ) 90.5%; m/e (2M + NH4+) 758.4. Anal. Calcd for C16H32O5NS‚1/2Ca: C, 51.87; H, 8.70; N, 3.78. Found: C, 51.59; H, 8.45; N, 3.58. Calcium N-hexadecyl-2-sulfooxyethanamide, 16-1-0.5Ca: Molecular mass ) 398.6; yield ) 88.6%; m/e (2M + NH4+) 814.5. Anal. Calcd for C18H36O5NS‚1/2Ca: C, 54.24; H, 9.10; N, 3.51. Found: C, 54.10; H, 8.88; N, 3.33. Calcium N-dodecyl-4-sulfooxybutanamide, 123-0.5Ca: Molecular Mass ) 370.5; yield ) 78.6%; m/e (2M + NH4+) 758.4. Anal. Calcd for C16H32O5NS‚1/2Ca: C, 51.87; H, 8.70; N, 3.58. Found: C, 51.62; H, 8.99; N, 3.78. Calcium N-dodecyl-6-sulfooxyhexanamide, 12-5-0.5Ca: Molecular mass ) 398.6; yield ) 80.5%; m/e (2M + NH4+) 814.5. Anal. Calcd for C18H36O5NS‚1/2Ca: C, 54.24; H, 9.10; N, 3.51. Found: C, 54.23; H, 9.07; N, 3.35. Spectral Characteristics for Sodium N-Alkyl Amide Sulfates (m-n-0.5Ca). Spectral characteristics (IR and 1H NMR spectra) of the calcium salts (5 compounds) were very similar to
those corresponding sodium salts. Especially, their 1H NMR spectra were identical with those corresponding sodium salts presented above. m-1-0.5Ca (m ) 12, 14, 16). IR (KBr, ν/cm-1): 3323 (NH); 2922, 2850 (CH2); 1653 (CO-N amide I); 1533 (N-CdO amide II); 1275, 1213, 1092, 1036 cm-1 (OSO3-). 12-3-0.5Ca. IR (KBr, ν/cm-1): 3340 (NH); 2920, 2850 (CH2); 1639 (CO-N amide I); 1527 (N-CdO amide II); 1244, 1223, 1107, 987 (OSO3-). 12-5-0.5Ca. IR (KBr, ν/cm-1): 3340 (NH); 2920, 2850 (CH2); 1637 (CO-N amide I); 1527 (N-CdO amide II); 1244, 1223, 1107, 982 (OSO3-).
Results and Discussion Synthesis. The synthetic sequence for the preparation of N-alkyl amide sulfates, CmH2m+1NHCO(CH2)nOSO3Me (abbreviated as m-n-Me), is presented in Scheme 1. In all cases, the chemical yields were excellent. In the amidation step, methyl glycolate and primary amines (n-C12H25NH2, n-C14H29NH2, and n-C16H33NH2) condensed quantitatively to afford the corresponding N-alkyl glycolic amide at relatively low temperature (100 °C) without any condensation reagent. Also, the reaction of n-dodecylamine and γ-butyrolactone was quantitative, and no undesirable byproducts were formed. In the reaction of n-dodecylamine with -caprolactone (chemical yield ) 78.9%), the desired product (III) further reacted with excess lactone to give a byproduct, n-C12H25NHCO(CH2)5OCO(CH2)5OH. The sulfation reactions of all amide alkanols (mp ) 80-92 °C) were achieved completely using an equimolar or slightly excess amount (1.00-1.05 equiv) of chlorosulfonic acid in hexane, although the reactions were heterogeneous. Neutralization with sodium or calcium hydroxide gave the corresponding sodium or calcium sulfates in excellent yield. Relatively satisfactory elemental analyses were obtained from these final products giving IS-mass, 1H NMR, and IR spectra consistent with the desired compounds. Also, the purities of all the final products were greater than 97% as measured by HPLC. Chemical Stability in Water. Prior to the industrial utilization of surfactants, it is essential to examine their chemical stability in water. Previous investigators11,12,14,15 reported that some sulfated alkanolamides, derived from long-chain fatty acids or esters, were excellent detergents and lime soap dispersing agents. However, the sulfated N-(2-hydroxyethyl)lauramide (n-C12H25CONHCH2CH2OSO3Na; MEAS), structurally related to the present N-alkyl amide sulfates, was found to be chemically unstable and easily hydrolyzed in water. Therefore, the chemical stability in water of the sodium N-dodecyl amide sulfates (12-1-Na, 12-3-Na, and 12-5-Na) was examined. For comparison, MEAS was synthesized by a method described in the literature.12 The chemical structure was identified by 1H NMR and IR measurements (data not shown). About 20 wt % aqueous solutions of individual surfactants were prepared. The solution pH was controlled
Synthesis and Properties of N-Alkyl Amide Sulfates
Langmuir, Vol. 15, No. 20, 1999 6667
Figure 1. Chemical stability of surfactants at 50 °C in the pH 7.0 buffer: 12-1-Na (filled circle), 12-3-Na (open triangle); 125-Na (open circle); MEAS (filled triangle). Scheme 2
Figure 2. Typical plots of conductivity vs concentration. The open circle is of 12-1-Na at 35 °C, and the closed circle 12-10.5Ca at 25 °C. The arrow indicates the critical micelle concentration. Table 1. Micellar Properties of Sodium Salts surfactant n-C12H25NHCOCH2OSO3Na
to be 7 by 50 mM phosphate buffer, and the surfactant solutions were kept at 50 °C. Before and after storage at 50 °C, the individual surfactant concentrations were determined by Epton titration, and the results are presented in Figure 1. For MEAS and 12-3-Na, the surfactant concentrations decreased markedly after 7 days at 50 °C, while the concentrations of 12-1-Na and 12-5-Na did not change for 20 days. The analyses by both the HPLC and 1H NMR measurements of the 12-3-Na solution, stored at 50 °C for 7 days, showed the formation of the parent amide alkanol, although dodecylamine was not detected (data not shown). Thus, the 12-3-Na compound was found to hydrolyze to the amide alkanol and hydrogen sulfate ion (HSO4-) in water. Desnuelle and Micaelli11 observed that MEAS showed considerably less stability to alkaline hydrolysis than sulfated 3-hydroxypropanamides and proposed that a fivemembered ring intermediate (the oxazoline ring) facilitated the hydrolysis of the sulfated ethanolamides (see Scheme 2). Also, the N-stearoyl-N-methylhydroxyethyl amide sulfate was found to hydrolyze more rapidly than MEAS to the corresponding amide alkanol. The most probable first intermediate in oxazoline formation is an oxazolium cation (the charge being shared between the carbonyl oxygen and the amide nitrogen as resonance structures show). This intermediate can be stabilized by splitting of the nitrogen-bound hydrogen to make the molecule uncharged. N-stearoyl-N-methylhydroxyethyl amide sulfate can also form an oxazoline ring but cannot be turned into a neutral molecule as methyl in contrast to hydrogen cannot be split off (Scheme 2). Thus, the primary product remains charged and therefore more reactive toward further hydrolysis. In the present study, MEAS and 12-3-Na were shown to hydrolyze easily in aqueous solution of pH 7. Also, the 12-3-Na molecule can form a similar charged first intermediate in a hydrolysis pathway, this time not an oxazolium compound but a similar five-membered ring
n-C14H29NHCOCH2OSO3Na n-C16H33NHCOCH2OSO3Na n-C12H25NHCO(CH2)3OSO3Na n-C12H25NHCO(CH2)5OSO3Na n-C12H25OSO3Na n-C12H25OCH2CH2OSO3Na a
CMC, mM
R
temp, °C
5.2 6.4 1.3 0.34 4.4 3.1 8.3a 4.8a
0.50 0.52 0.72 0.77 0.53 0.66 0.25a 0.32a
35 50 50 50 35 50 35 35
From ref 21. Table 2. Micellar Properties of Sodium Salts surfactant
CMC, mM
R
temp, °C
n-C12H25NHCOCH2OSO3-0.5Ca
1.4 1.8 2.4a 0.92a
0.088 0.10
25 55 55 25
n-C12H25OSO3-0.5Ca n-C12H25OCH2CH2OSO3-0.5Ca a
From ref 3.
intermediate where the amide nitrogen is not part of the ring but nevertheless can take part in mesomeric structures spreading the positive charge and thus stabilize the molecule. Thus, the instability of 12-3-Na in water may be attributed to the formation of a five-membered ring intermediate. However, the newly synthesized amide sulfates, 12-1-Na and 12-5-Na, were apparently stable in water. The chain length (n) of the spacer, connecting the amide and sulfate groups, exerted an influence on the chemical stability of 12-n-Na surfactants in water. Critical Micellization Concentration. The electrical conductivity method was used to determine the critical micellization concentration (CMC) from the plots of the conductivity (κ) against the surfactant concentration (C). Typical plots of κ vs C for 12-1-Na (35 °C) and 12-10.5Ca (25 °C) are shown in Figure 2. Clear break points were observed at 5.2 mM for 12-1-Na and at 1.4 mM for 12-1-0.5Ca. The values of CMC, determined from the break points in the κ vs C plots, and of the micelle ionization degrees are summarized in Tables1 and 2. For the sake of a comparison, the data3,21 for n-dodecyl sulfate and n-dodecyloxyethyl sulfate are also presented. The inclusion of the amide group is seen to decrease the CMC. The CMC value of 12-1-Na (5.2 mM) at 35 °C is (21) Barry, B. W.; Wilson, R. Colloid Polym. Sci. 1978, 256, 251.
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Mizushima et al. Table 3. Values of Constants A and B and Free Energy Contribution per Methylene Group toward Micellization (Φ) of Surfactant Homologues type of surfactant
A
B
Φ, cal mol-1
temp, °C
CmH2m+1NHCOCH2OSO3Na C12H25NHCO(CH2)nOSO3Na CmH2m+1COO(CH2)3SO3Naa C9H12COO(CH2)nSO3Naa
1.63 -2.12 1.78 0.01
0.32 0.08 0.29 0.15
473 118 420 211
50 50 40 40
a
From ref 17.
The Gibbs energy of micellization (∆Gmic) can be written as
Figure 3. Plots of log CMC vs hydrocarbon chain length (m, n) of the 12-n-Na homologues (open circles) and the m-1-Na series (filled circles).
smaller than that of sodium dodecyl sulfate (8.3 mM), and is nearly equal to that of sodium dodecyloxyethyl sulfate (4.8 mM). The CMC depression by introducing oxyethylene chain has been explained as follows.3 Counterions are not only found on the surface of the micelle (i.e. at distances from the center of mass of the micelle greater than the corresponding distances of the sulfate headgroups) but also within the hydrated oxyethylene core in the micelle. This results in a decreased electrical potential at the micelle surface and in a decreased solubility of surfactants. This argument would apply to the current m-n-Me homologues, if we assume hydration of both the amide group and the spacer meythylene chain. Probably, counterions are distributed within a hydrophilic layer consisting of amide groups, spacer methylene chains, sulfate headgroups, and water which can be found around the hydrocarbon core of the micelle. Moreover, a linear relationship was found between the log CMC vs the hydrocarbon chain length (m) of the m-1Na series and also between the log CMC vs the spacer chain length (n) of the 12-n-Na homologues at 50 °C (see Figure 3). Because of its limited chemical stability at high temperature, the data for 12-3-Na was only measured at 35 °C. Empirically, the plots of the logarithm of CMC vs the hydrocarbon chain length are known to exhibit linear relationships with constant negative slopes for different kind of homologues of ionic surfactants with the same kind of counterion.21-23 The linear relationship obeys eq 1, where N is the
log CMC ) A - BN
(1)
hydrocarbon chain length. A and B are constants which depend on the hydrophilic group and the energy of transfer of a methylene group from the aqueous to the hydrocarbon phase, respectively.21 The value of B for the m-1-Na surfactants is 0.32 and is in good agreement with those of CmH2m+1OSO3Na (0.28)21 and of CmH2m+1OCH2CH2OCH2CH2OSO3Na homologues (0.29).18 However, for the 12-n-Na homologues, the constant B is only 0.08. Thus, the ratio of reduction in log CMC for m to n, is just 4:1. The spacer methylene chain is suggested to be less hydrophobic when n is 1-5. Also, the data for CMC as a function of chain length can conveniently be used to determine the contribution of -CH2- groups to micellization.16 (22) Klevens, H. B. J. Am. Oil. Chem. Soc. 1953, 30, 74. (23) Gu, T.; Sjoblom, J. Colloids Surf. 1992, 64, 39.
∆Gmic ) -RT ln CMC
(2)
-RT ln CMC ) NΦ + ∆Gelec
(3)
where ∆Gelec is the electrostatic Gibbs energy of micellization and Φ is the Gibbs energy contribution per -CH2group toward micellization. From eqs 1 and 3,Φ can be written as
Φ ) 2.303 BRT
(4)
The value thus obtained for Gibbs energy contribution per -CH2- group toward micellization (Φ), together with the constants A and B, is listed in Table 3. The Φ value of 473 cal/mol for m-1-Na homologues is comparable to that (418-427 cal/mol) reported for sodium alkyl sulfates.16 On the other hand, Hikota and Meguro17 studied the effect of the position of the ester group on CMCs for ester-linked sulfonates, and they reported the relation of log CMC ) -0.147N + 0.011 at 40 °C for C9H19COO(CH2)nSO3Na homologues, where n ) 2-4. The B value for the spacer methylene chains connecting the ester and sulfonate group is 0.147 (see Table 3), which is about twice of that (0.08) for the current C12H25NHCO(CH2)nOSO3Na series, although there is no large difference in B value with the hydrocarbon chain between CmH2m+1NHCOCH2OSO3Na and CmH2m+1COO(CH2)3OSO3Na homologues (see Table 3). This result implies that the spacer chain in the amidelinked sulfates is less hydrophobic than that in the ester surfactants. An amide linkage, recognized as a strong hydrophilic group, was thus shown to decrease the hydrophobicity of the spacer methylene chain more strongly than an ester group, when it is incorporated into ionic surfactants. On the other hand, a drastic decrease in the CMC was observed for 12-1-0.5Ca in contrast to the corresponding sodium salt (see Tables 1 and 2). The CMC of the sodium salt is 5.2 mM at 35 °C, and that of the calcium counterpart is 1.4 mM at 25 °C. An increase in the counterion valency is always accompanied by a drastic decrease in the CMC. For example, the CMC of calcium dodecyl sulfate21 (2.4 mM at 55 °C) is much smaller than that of the sodium counterpart (8.3 mM at 35 °C). A similar trend was also seen in dodecyloxylethyl sulfates.3 Micelle Ionization Degree. The approximate micelle ionization degree (R) was estimated from the ratio of the values of dk/dC above and below the CMC21,23-25 (see Table 2). The micelle ionization degrees R, for the 12-n-Na homologues were found to be extremely larger (e.g., 0.50 for 12-1-Na at 35 °C) than that (0.25 at 35 °C) for sodium dodecyl sulfate.21 It must be recalled that for conventional single chain surfactants, with about 12 carbon atoms in the hydrocarbon chain, R is usually around 0.200.25.18,24-26 An increase in R is also seen for the (24) Satake, I.; Tahara, T.; Matsuura, R. Bull. Chem. Soc. Jpn. 1969, 42, 319.
Synthesis and Properties of N-Alkyl Amide Sulfates
Langmuir, Vol. 15, No. 20, 1999 6669
Table 4. Krafft Points and Enthalpies of Dissolution of Hydrated Solid Surfactants surfactants
Krafft point, °C
∆H, KJ mol-1
12-1-Na 12-1-0.5Ca 14-1-Na 14-1-0.5Ca 16-1-Na 16-1-0.5Ca 12-3-Na 12-3-0.5Ca 12-5-Na 12-5-0.5Ca C12H25OSO3Na C12H25OSO3-0.5Ca C12H25OCH2CH2OSO3Na C12H25OCH2CH2OSO3-0.5Ca
25.7 below 0 35.8 22.7 47.1 42.3 19.2 55.3 40.9 65.5 9a 50a 5a 15a
85.7
a
From ref 1.
a
63.4 19.5 43.3 23.4 46.7 64.1 49.5 68.8 50a 75a 39.3b 89.4b
From ref 4.
n-C12H25(OCH2CH2)nOSO3Na series 21 (0.25 for n ) 0, 0.32
for n ) 1, and 0.37 for n ) 2, at 35 °C), although the extent of the increase is smaller than that of the current amide sulfates. Lianos and Lang26 reported a large value for the micelle ionization degree (0.69 at 25 °C) found for sodium 4-dodecylbenzenesulfonate (4-DBS) micelles. The large value of R is due to the small aggregation number (n ) 32-35) at low surfactant concentrations. The consequence of this small aggregation number is that the area of the surfactant headgroup at the micelle surface is higher than in the case of conventional single chain surfactants of about the same length and, in turn, the charge density lower, which gives rise to the higher ionization of counterions. Furthermore for some dimeric or gemini surfactants having long spacer methylene chains, extremely large values of R were observed.27,28 For example, the R value of di-n-dodecyl R,ω-alkyl bis(phosphate) surfactant micelle27 is 0.84 when the spacer methylene chain length is 6. Apparently, the headgroup repulsion of the gemini surfactant molecules within the micelle is small, due to the existence of the long spacer, keeping the headgroups far apart from each other. Thus, for the present 12-n-Na homologues, the large values of R clearly suggest that the surface charge density of the 12-n-Na micelles is low. Also, a large reduction in R was observed going from Na to Ca counterions for 121-Me surfactants (see Table 2). This result clearly shows that the surface charge density of 12-1-0.5Ca micelles is higher than that of the sodium salt. Krafft Temperature. The Krafft temperatures of both the sodium and calcium salts of m-n-Me surfactants have been measured to examine their applicability in hard water. The Krafft temperatures and the transition enthalpies, determined by differential scanning calorimetry (DSC), are summarized in Table 4. Also, the typical DSC thermal profiles are shown in Figure 4. In each heating thermal profile, only one endothermic transition was observed. By direct visual observation of the same solution (30 mM), we confirmed that each endothermic peak is due to the melting of hydrated surfactant crystals. The surfactant with Krafft temperature listed as below 0 °C gave no precipitate when kept in ice water for 24 h, and there was only one endothermic transition attributed to ice melting at around 0 °C in the heating thermal profile from -20 to 80 °C. (25) Sasaki, T.; Hattori, M..; Sasaki, J.; Nukina, K. Bull. Chem. Soc. Jpn. 1975, 48, 1397. (26) Lianos, P.; Lang, J. J. Colloid Interface Sci. 1983, 96, 222. (27) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (28) Zana, R.; Levy, H. Langmuir 1997, 13, 402.
Figure 4. Typical DSC thermal profiles of surfactant aqueous solutions (30 mM). The upper curve is of 14-1-0.5Ca, and the bottom 14-1-Na surfactant.
Although the incorporation of an oxyethylene chain into surfactant molecules depresses the Krafft temperature for both the sodium and calcium salts,3,21 the inclusion of the amide group and spacer methylene chain is seen to increase the Krafft point (KP), except for the m-1-0.5Ca homologues (see Table 4). The Krafft temperature is a reflection of the solubility of monomers in the presence of hydrated surfactant crystals.29 The addition of a hydrophilic group such as an oxyethylene chain or a hydroxyl group will increase the monomer solubility and depress the KP.21 The decrease in the CMC or in the solubility and the increased Krafft temperature by the incorporation of the amide group together with the spacer methylene chain should be attributed to stabilization in the hydrated solid state. Intermolecular hydrogen bonding between the amide groups seems to play an major role for stabilizing the hydrated crystals. The most striking and interesting feature of Table 4 is that the Krafft points of the m-1-0.5Ca series are lower than those for the corresponding sodium counterparts. The KP is affected by the type of counterions. If the counterion is strongly hydrated, such as Li+, the surfactants exhibit substantially lower KPs. On the other hand, the increase in valency of counterion is generally accompanied by an increase in the KP. The Krafft temperatures of bivalent salts of conventional anionic surfactants have always been found at higher temperatures than those of the corresponding univalent salts.1-4 The KP values of calcium alkyl sulfates are significantly depressed by introducing an oxyethylene chain between the hydrocarbon chain and the ionic group. However, the KP values of the calcium salts are still higher than those of the sodium salts3 (see Table 4). To our best knowledge, this is the first observation that the Krafft temperature of the univalent salt is higher than that of the bivalent counterparts having the same surface active ion. Moreover, linear relationships between the KP and the hydrocarbon chain length (m) were observed for both the sodium and the calcium salts of m-1-Me homologues, as shown in Figure 5, where the KP of 12-1-0.5Ca is plotted as 0 °C by estimation. Empirically, the plots of the KP vs the hydrocarbon chain length are known to exhibit linear relationships for different kinds of homologues of ionic surfactants with the same kind of counterion.23 The linear relationship obeys eq 5,
KP ) KcN - Ki
(5)
where N is the hydrocarbon chain length. Ki is a constant depending on the ionic headgroup, and Kc a constant mainly depending on the counterion valency. For many ionic surfactants, the Kc value for Ca salt is 11.0 and is twice that (5.5) of the corresponding sodium salts.23 The
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Langmuir, Vol. 15, No. 20, 1999
Mizushima et al.
The entropy value for 14-1-0.5Ca is 65.9 J mol-1 deg-1 and is smaller than that (205.2 J mol-1 deg-1) of 14-1-Na. The result strongly suggest that the lower Krafft point of m-1-0.5Ca homologues, compared to those for the corresponding sodium sulfates, would be attributed to the disordered hydrated crystalline state. To clarify this assumption, their hydrated crystalline structure must be examined. Concluding Remarks
Figure 5. Plots of Krafft point vs hydrocarbon chain length of m-1-Me surfactants. The open circle is of the sodium salts, and the filled circle the calcium salts.
calculated Kc value was found to be 5.4 for the sodium salts and 10.6 for the calcium salts; these are nearly equal to those of alkyl sulfates. Thus, the trends in the KP vs the hydrocarbon chain length (m) for the m-1-Me homologues were consistent with those for alkyl sulfates. Finally, we discuss the reason the Krafft temperatures of the calcium salts of m-1-Me series are lower than those for the corresponding sodium salts. In a comparison of the Krafft temperature and the phase transition enthalpy (∆H) for the sodium salts and those of the calcium salts, the higher Krafft temperature is always accompanied by the larger ∆H (see Table 4). The ∆H values of sodium alkyl sulfates are larger than those for the corresponding calcium salts, and the Krafft points of the sodium salts are lower than those of the calcium salts. Hydration effects might be the major factor determining the enthalpy value. The m-n-Na surfactants might be hydrated strongly, compared to those for the corresponding calcium salts. However, the ∆H values of the m-1-Na homologues with higher Krafft points are smaller than those for the corresponding calcium salts. This result cannot be explained only by hydration effects. Therefore, the KP of these surfactants seems to be mainly reflected by the hydrated crystalline state, e.g., the packing.23,29 The entropy of dissolution (∆S/J mol-1 deg-1) for 14-1-Me surfactants was calculated from the equation ∆S ) ∆H/T, where T is the Krafft point.
Chemical structure-physicochemical property relationships were studied for a new series of N-alkyl amide sulfates abbreviated as m-n-Me, where m is the hydrocarbon chain length, n is the spacer methylene chain connecting the amide and sulfate groups, and Me is the counterion. The inclusion of an amide group in sodium alky sulfates results in a decrease in the CMC value and in a marked increase in the micelle ionization degree. Two linear relationships were found between the log CMC vs m for the m-1-Na series and between the log CMC vs n for the 12-n-Na homologues. From the comparison with the effect of the methylene chain length on CMC and Gibbs energy contribution toward micellization, part of the spacer methylene chain was suggested to behave like a hydrophilic group. A large depression in the Krafft temperature was found for the m-1-0.5Ca surfactants, in contrast to the calcium salts of alkyl sulfates having the same hydrocarbon chain length. Moreover, the Krafft point of the 12-1-0.5Ca compound was found to be below 0 °C and is lower than that of the corresponding sodium salts (25.6 °C). The 121-0.5Ca amide sulfate has a lower CMC (1.4 mM at 25 °C) than that of the sodium counterpart (5.2 mM at 35 °C). These features seem to resemble to dimeric or gemini surfactants, which have a smaller CMC and lower Krafft temperature than the corresponding monomeric surfactants.27, 28, 30 The 12-1-Na surfactant was shown to be soluble in hard water. This unique surfactant shows some excellent properties including high foaming ability even in hard water, as well as complete calcium stability, and has been utilized by the authors as a shampoo detergent since 1996. LA981777X (29) Davey, T. W.; Ducker, W. A.; Hayman, A. R.; Simpson, J. Langmuir 1998, 14, 3210. (30) Rosen, M. J. Chemtech 1993, 30.
