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Sugar-Based Surfactants: Adsorption and Micelle Formation of Sodium Methyl 2-Acylamido-2-deoxy-6-O-sulfo-D-glucopyranosides Reinaldo C. Bazito 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 December 5, 2001. In Final Form: April 5, 2002 Aggregation in aqueous solutions of sodium methyl 2-acylamido-2-deoxy-6-O-sulfo-D-glucopyranoside surfactants has been studied, where the acyl group is octanoyl, dodecanoyl, or hexadecanoyl. Critical micelle concentration, cmc, minimum area per surfactant at the air/solution interface, σ, degree of counterion dissociation, Rmic, and thermodynamic parameters (Gibbs free energy, enthalpy, and entropy) of adsorption and/or micellization were calculated from surface tension and conductance measurements. Static and quasi-elastic light scattering measurements were employed to obtain micellar weight-average molecular weight, aggregation number, Nagg, and hydrodynamic radius. Finally, the fluorescence spectrum of solubilized pyrene was employed for determination of micellar polarity. The similarities between these aminoglucosebased surfactants and simple anionic ones include decrease of cmc and of Rmic and increase of Nagg as a function of increasing the chain length of the hydrophobic group. Compared to simple anionic surfactants, the sugar-based counterparts showed more favorable free energies of adsorption and micellization, higher Nagg, and larger σ. These differences are explained on the basis of the attractive interactions between the voluminous sugar headgroups.
Introduction The increasing interest in studying carbohydrate-based surfactants is due to, inter alia, their preparation from renewable raw materials, ready biodegradability, mildness to the skin, and biocompatibility. An important structural feature of these surfactants is the sugar headgroup, a voluminous and relatively rigid moiety that can be functionalized by a myriad of reagents and synthetic schemes.1-5 The compound 2-amino-2-deoxy-D-glucopyranose (hereafter called “2-aminoglucose”) is widely present in nature, for example, as the building block of chitin, the second most abundant natural polymer after cellulose.6 This is an interesting starting material for the synthesis of sugarbased surfactants because it can be specifically functionalized, namely, at the amino or the hydroxyl group.7 Several anionic, cationic, and nonionic 2-aminoglucosebased surfactants are known.8-13 We have recently * To whom correspondence should be addressed. Fax: +55-113091-3874. E-mail:
[email protected]. (1) Egan, P. A. Chemtech 1989, 758-762. (2) Von Rybinski, W. Curr. Opin. Colloid Interface Sci. 1996, 1, 587597. (3) Von Rybinski, W.; Hill, K. Angew. Chem., Int. Ed. Engl. 1998, 37, 1328-1345. (4) Hill, K.; Rhode, O. Fat Sci. Technol. 1999, 101, 25-33. (5) Soderman, O.; Johansson, I. Curr. Opin. Colloid Interface Sci. 2000, 4, 391-401. (6) Peter, M. G. J. Macromol. Sci., Chem. 1995, 32, 629. (7) Foster, A. B.; Horton, D. Adv. Carbohydr. Chem. Biochem. 1959, 14, 213-281. (8) Boullanger, P.; Chevalier, Y.; Croizier, M. C.; Lafont, D.; Sancho, M. R. Carbohydr. Res. 1995, 278, 91-101. (9) Boullanger, P.; Chevalier, Y. Langmuir 1996, 12, 1771-1776. (10) Matsumura, S.; Kawamura, Y.; Yoshikawa, S.; Kawada, K.; Uchibori, T. J. Am. Oil Chem. Soc. 1993, 70, 17-22. (11) Kida, T.; Yurugi, K.; Takeda, T. J. Am. Oil Chem. Soc. 1995, 72, 773-780. (12) Molina, L.; Gerardin-Charbonnier, C.; Selve, C.; Stebe, M. J.; Maugras, M.; Infante, M. R.; Torres, J. L.; Manresa, M. A.; Vinardell, P. New J. Chem. 1997, 21, 1027-1035. (13) Fernandez-Bolanos, J.; Castilla, I. M.; Guzman, J. F. B. Carbohydr. Res. 1988, 173, 33-40.
reported the synthesis and some solution properties of two new aminoglucose-based series, namely, sodium methyl 2-acylamido-2-deoxy-6-O-sulfo-D-glucopyranosides (anionic)14 and methyl 2-acylamido-6-trimethylammonio2,6-dideoxy-D-glucopyranoside chlorides (cationic).15 We report here on the micellar properties of the former series, where the acyl groups are octanoyl (C8S), dodecanoyl (C12S), or hexadecanoyl (C16S). For each surfactant, the following quantities have been calculated from conductance measurements: critical micelle concentration, cmc, degree of micelle dissociation, Rmic, and the Gibbs free energy of micelle formation, ∆G°mic. On the basis of surface tension measurements, we have calculated cmc, minimum area/surfactant at the air/solution interface, σ, and Gibbs free energy for surfactant adsorption at the solution/air interface, ∆G°ads. Measurement of static and quasi-elastic light scattering, SLS and QELS, respectively, provided the micellar aggregation number, Nagg, and its hydrodynamic radius, RH, respectively. Our results underscore the similarities of and differences between the properties of the present surfactants and simple alkyl sulfates. The differences are attributed to attractive interactions between surfactant molecules in the micellar pseudophase. Experimental Section Materials. The reagents were purchased from Aldrich or Merck and were purified as described elsewhere.16 The surfactants were synthesized as shown in Scheme 1 and carefully purified by flash column chromatography.14 Prior to use, they were dried under reduced pressure over P2O5 to a constant weight. All aqueous solutions were prepared with all glass doubledistilled, deionized water. Solutions employed in the light (14) Bazito, R. C.; El Seoud, O. A. Carbohydr. Res. 2001, 332, 95102. (15) Bazito, R. C.; El Seoud, O. A. J. Surfactants Deterg. 2001, 4, 395-400. (16) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals; Pergamon Press: Oxford, 1988.
10.1021/la0117552 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/02/2002
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Table 1. Adsorption Properties at the Air-Solution Interface, Determined for Aqueous Solutions of the Studied Surfactants at 40 °C surfactant
γcmc (mN m-1)
σ (nm2)
104 × C20 (mol L-1)
∆G°ads (kJ mol-1)
C8S C12S C16S
35.3 34.7 33.2
0.94 0.90 0.92
39.6 3.27 0.162
-61.4 -73.7 -89.5
scattering measurements were filtered through cellulose acetate membranes, 0.22 mm (NaCl) or 1.0 mm (surfactant). Apparatus and Measurements. Conductivities were measured with a PC-interfaced Fisher Accumet 50 pH meter/ conductimeter provided with a Digimed model DM-C1 microconductivity cell, a Schott model Titronic T200 programmed buret, and a thermostated solution compartment. Aliquots of the surfactant stock solution were successively added to a known volume of water, and the conductivity of the solution was measured after thermal equilibration. Home-developed software was used for aliquot addition, data acquisition, and their subsequent treatment. Solution surface tension was measured with a Lauda TE1C automatic ring (du Nouy) tensiometer, equipped with a thermostated solution compartment, and controlled with homedeveloped software. Aliquots of the surfactant stock solution were successively added to a known volume of water, and the surface tension of the solution was measured after thermal equilibration. The tensiometer was programmed to repeat the measurements until the standard deviation between four successive readings was 0.999) whose slopes, ∆G°CH2, were -3.4 and -3.6 kJ mol-1 at 25 and 40 °C, respectively.14 These free energies of transfer of the methylene group from bulk solution to the micellar pseudophase are in the same range calculated for other surfactants, for example, alkyl sulfates (-3.4 kJ mol-1),26 ethoxylated alkyl sulfates (-3.7 kJ mol-1),20 and other ionic surfactants (-3.1 ( 0.3 kJ mol-1).22 The intercept, -12 kJ mol-1 for both 25 and 40 °C, is much more negative than those of other anionic surfactants, for example, alkyl sulfates (-2.8 kJ mol-1)26 and ethoxylated alkyl sulfates (0.1 kJ mol-1).20 This more favorable (∆G°headgroup + ∆G°CH3) is probably due to a combination of hydrophobic interactions of the surfactant molecules within the micellar aggregate (e.g., via headgroups), coupled to their H-bonding in the interfacial region, either directly (via the NHCO and OH groups) or via a water intermediary.14 It is instructive to analyze ∆G°mic in terms of enthalpy and entropy of micellization. Contributions of the surfactant moieties to ∆H°mic and ∆S°mic can be calculated by applying the same approach employed with ∆G°mic, that is,22,27
∆H°mic ) ∆H°headgroup + ∆H°CH3 + NCH2∆H°CH2 (3) ∆S°mic ) ∆S°headgroup + ∆S°CH3 + NCH2∆S°CH2 (4) (25) Moroi, Y. Micelles: Theoretical and applied aspects; Plenum Press: New York, 1992.
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Table 2. Critical Micelle Concentrations (cmc), Dissociation Degrees (rmic), and the Thermodynamic Parameters of Micellization (∆G°mic, ∆H°mic, and ∆S°mic) for the Surfactants Studied, at Various Temperatures 103 × cmc (mol L-1) surfactant
T (°C)
conductancea
C8S
25 40 20 25 30 40 40
24.4 24.9 1.65 1.69 1.71 1.72 0.117
C12S
C16S a
surface tension 20.0
1.58 0.0925
Rmic
∆G°mic (kJ mol-1)b
∆H°mic (kJ mol-1)
T∆S°mic (kJ mol-1)
0.33 0.33 0.24 0.24 0.24 0.24 0.17
-31.9 -33.5 -44.7 -45.3 -46.0 -47.5 -62.3
-2 -2 -7 -4 -1 0
30 32 37 42 45 48
The cmc data at 40 °C were taken from a previous work (ref 14). b Calculated from conductance data.
Table 3. Critical Micelle Concentration, Refractive Index Increments (dn/dc), Aggregation Numbers (Nagg), Static Second Virial Coefficients (B), Diffusion Coefficients (D0), Dynamic Virial Coefficients (Bdyn), Hydrodynamic Radii (RH), and Pyrene Polarity (I1/I3) Determined for 0.1 mol L-1 NaCl Aqueous Solutions of the Surfactants Studied, at 40 °C (C8S and C12S) and at 50 °C (C16S) C8S C12S C16S a
103 × cmca (mol L-1)
dn/dc (cm3 g-1)
Nagg
105 × Bstat (cm3 mol g-1)
18.6 1.40 0.146
0.128 0.126 0.127
109 125 794
1.2 38 0.27
108 × D0 (cm2 s-1) 77.9 10.9
Bdyn -7.7 -8.3
Rh (nm)
I1/I3
4.3 31.5
1.04 1.00 0.85
Determined by SLS measurements.
Application of eq 3 to our results gave ∆H°CH2 ) -0.5 and +0.5 kJ mol-1 and ∆H°headgroup + ∆H°CH3 ) +1 and -5 kJ mol-1 for aggregation at 25 and 40 °C, respectively. The corresponding values for eq 4, calculated as T∆S°mic, are as follows: slopes, 2.5 and 3.5 kJ mol-1, and intercepts, 17 and 13 kJ mol-1, for 25 and 40 °C, respectively. Table 2 shows the following: (i) At both temperatures, ∆H°CH2 and ∆S°CH2 are within the range observed for other surfactants.22,28 (ii) At both temperatures, T(∆S°headgroup + ∆S°CH3) are sizable (i.e., aggregation is entropy driven); their values are larger than those of other surfactants with a simple headgroup.24 (iii) As a function of increasing temperature, the transfer of the headgroup (and the terminal CH3) from bulk solution to the micellar pseudophase is accompanied by a decrease, both in enthalpy and entropy. Points ii and iii above and calculated σ are manifestations of the peculiar properties of the surfactant’s voluminous, hydrated,29 and relatively rigid headgroup. The surfactant also carries an amide group, capable of forming relatively strong H-bonds with water and with each other.30,31 Hydration and H-bonding of the amide group are responsible, in part, for the relatively large σ.19 Aggregation of sugar-based surfactants is accompanied by a sizable dehydration of the sugar moiety.29 This process should increase the entropy, due to the release of water molecules; this partially explains the favorable entropies calculated. Finally, the decrease in T(∆S°headgroup + ∆S°CH3) as a function of increasing T from 25 to 40 °C is due to the concomitant decrease in water structure and monomer hydration. 3. Aggregation Numbers, Hydrodynamic Radii, and Micellar Polarity. Table 3 shows the following properties for micellar C8S and C12S (at 40 °C) and C16S (at 50 °C): cmc determined by SLS, refractive index increments (dn/dc) of surfactant solutions, SLS-based Nagg, static second virial coefficients (Bstat), diffusion coefficients (26) Evans, H. C. J. Chem. Soc. 1956, 579-586. (27) Birdi, K. S. Handbook of Surface and Colloid Chemistry; CRC Press: Boca Raton, FL, 1997. (28) Muller, N. Langmuir 1993, 9, 96-100. (29) Pastor, O.; Junquera, E.; Aicart, E. Langmuir 1998, 14, 29502957. (30) Fuhrhop, J. H.; Schnieder, P.; Boekema, E.; Helfrich, W. J. Am. Chem. Soc. 1988, 110, 2861-2867. (31) Symons, M. C. R. J. Mol. Struct. 1993, 297, 133-140.
(D0), dynamic virial coefficients (Bdyn), QELS-based micellar hydrodynamic radii (RH), and the micellar “polarity”, as determined from the intensities of the vibronic bands 1 and 3 (I1/I3) of solubilized pyrene. In LS measurements, use of NaCl was necessary in order to screen the electrostatic interactions between headgroups of the surfactant molecules. In the presence of NaCl, the Krafft temperature of C16S increased from 39 to 45 °C, this being the reason for carrying out its LS measurements at 50 °C. QELS results for micellar C8S are not reported in Table 3 because the corresponding acquisition times were prohibitively long. With regard to results of SLS and QELS, the following are relevant: (i) There is a good agreement between cmc’s determined by conductance and SLS measurements. (ii) The aggregation numbers of C8S and C12S are higher than those of other anionic surfactants with similar hydrophobic tails.22 Nagg of C16S is, however, very large, indicating the existence of nonspherical micelles (e.g., oblate ellipsoid),19 an arrangement that allows packing of a large number of molecules. (iii) In agreement with our previous discussion, the LS results reflect the characteristics of the surfactant structure. It seems plausible that neighboring molecules in the aggregate interact efficiently via hydrophobic interactions and H-bonding, for example, by the aminosugar moiety and/or the amide group; this leads to large aggregates. These interactions are enhanced by the presence of NaCl, due to attenuation of the electrostatic repulsion between the sulfate head-ions. Our conclusion agrees with the more favorable ∆G°mic, vide supra, and the highly negative Bdyn. Note that this coefficient is related to the interaction potential between aggregates; its values are 1.45 for noninteracting rigid spheres,32 1.45 when the interactions are repulsive.33 (iv) High Nagg and RH have already been reported for micelles of other nonionic sugar-based surfactants, as shown by the following examples (surfactant, technique, Nagg, RH, T (when available)): dodecyllactobionamide, LS, (32) Hou, M. J.; Kim, M.; Shah, D. O. J. Colloid Interface Sci. 1988, 123, 398-412. (33) Jada, A.; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387-395.
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Table 4. Volumes for the Headgroup (Vheadgroup), the Hydrophobic Tail (Vtail), and the Surfactant Molecule (Vsurfactant) and Calculated Radii for an Equivalent Sphere (Rsphere) and for the Length of a Fully Extended Surfactant Molecule (Rsurfactant) (See Text for Details)
C8S C12S C16S
Vheadgroup (nm3)
Vtail (nm3)
Vsurfactant (nm3)
Rsphere (nm)
Rsurfactant (nm)
0.291
0.220 0.329 0.438
0.511 0.620 0.729
2.4 2.6 5.2
2.0 2.5 3.0
178, 7 nm, 40 °C;23 hexadecylmaltobionamide, LS, 32 573, 66 nm, 40 °C and 12 540, 44 nm, 50 °C;34 octanoyl-Nmethyl-glucamide (Mega-8), fluorescence, 88, 40 °C; 1-Ooctyl-β-D-glucopyranoside, fluorescence, 94, 40 °C; 1-Sthio-octyl-β-D-glucopyranoside, fluorescence, 114, 40 °C; 6-O-(N-heptylcarbamoyl)-methyl-R-D-glucopyranoside (Hecameg), fluorescence, 78, 40 °C.35 Nagg for dodecyl-βmaltoside is in the range of 100-120, and its RH is 3.5 nm.36 (v) Another argument in favor of nonspherical geometry of micelles of these surfactants, especially C16S, comes from the results of calculations reported in Table 4. Thus, the hydrodynamic radii obtained by QELS (Table 3) are greater than (a) the lengths of fully extended surfactant molecules (Rsurfactant) and (b) the radii (Rsphere) of hypothetical spherical micelles whose aggregation numbers are those calculated from our SLS measurements (Nagg, Table 3). That is, the micelles are either highly hydrated or not spherical. (vi) The ratio between the intensities of bands 1 and 3, I1/I3, is an empirical measurement of the polarity at the (average) solubilization site of dissolved pyrene.37,38 The micellar polarity measured by this probe is intermediate between that of 1-propanol (1.14) and 1-octanol (0.93) and decreases as a function of increasing the length of the surfactant hydrophobic chain, Table 3. This decrease in polarity has been explained by considering the effects of the surfactant chain length on the properties of the micelle. For example, a decrease of this length is accompanied with an increase of cmc, Rmic, and the thickness of its Stern (34) Denkinger, P.; Burchard, W.; Kunz, M. J. Phys. Chem. 1989, 93, 1428-1434. (35) Frindi, M.; Michels, B.; Zana, R. J. Phys. Chem. 1992, 96, 81378141. (36) Cecutti, C.; Focher, B.; Perly, B.; Zemb, T. Langmuir 1991, 7, 2580-2585. (37) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039-2044. (38) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984, 62, 2560-2565.
layer and a decrease of Nagg. As a result, the C8S micelles are smaller and less compact and their Stern layers are more “ionic” (due to higher Rmic) than C16S, in agreement with the observed order of polarity, Table 3.39-41 The relevant point, however, is that the I1/I3 ratios observed are in the same range calculated for other ionic (0.9-1.4) or sugar-based surfactants (1.0-1.25).35,37,42 That is, the hydrophobic core of the sugar-based micelles (the solubilization site of pyrene) does not seem to be more hydrated than those of other, simple surfactants. On the other hand, the interfacial area may not be different from that of, for example, ethylene oxide based nonionic surfactants due to dehydration of the carbohydrate moiety that occurs on micellization.29 That is, the data available are conveniently explained by nonspherical geometry, without invoking large differences between the water contents of sugarbased and other micelles. Conclusions The aggregation behavior of anionic sugar-based surfactants has been studied by conductivity, surface tension, LS, and fluorescence measurements. The surfactants showed cmc’s and adsorption properties similar to those of other ionic surfactants, but their Nagg and RH are higher, probably as a result of hydrophobic interactions and direct or water-mediated H-bonding between surfactant molecules in the aggregate. The hydrodynamic radii suggest the formation of nonspherical micelles. Acknowledgment. We thank the FAPESP Foundation for financial support and for a Ph.D. fellowship to R. C. Bazito. O. A. El Seoud thanks the CNPq for a research productivity fellowship. We thank P. A. R. Pires for his help with the LS measurements and Professor Frank H. Quina for making the spectrofluorimeter available to us. Supporting Information Available: Details of calculation of all quantities discussed in this paper, Debye plots, and plots of diffusion coefficients as a function of surfactant volume fraction. This material is available free of charge via the Internet at http://pubs.acs.org. LA0117552 (39) Novaki, L. P.; El Seoud, O. A. Phys. Chem. Chem. Phys. 1999, 1, 1957-1964. (40) Novaki, L. P.; El Seoud, O. A. Langmuir 2000, 16, 35-41. (41) Barr, S.; Jones, R. R. M.; Johnson, J. S. J. Phys. Chem. 1992, 96, 5611. (42) Kano, K.; Ishimura, T. J. Chem. Soc., Perkin Trans. 2 1995, 1655-1660.
