Comparative Surface Activities of Di-and Trisaccharide Fatty Acid Esters

Langmuir 2002, 18, 667-673

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Comparative Surface Activities of Di- and Trisaccharide Fatty Acid Esters Manuel Ferrer,† Francisco Comelles,‡ Francisco J. Plou,*,† M. Angeles Cruces,† Gloria Fuentes,† Jose Luis Parra,‡ and Antonio Ballesteros† Departamento de Biocata´ lisis, Instituto de Cata´ lisis, C.S.I.C., Cantoblanco, 28049 Madrid, Spain and Departamento de Tensioactivos, Instituto de Investigaciones Quı´micas y Ambientales de Barcelona, C.S.I.C., Jorge Girona 18-26, 08034 Barcelona, Spain Received May 16, 2001. In Final Form: October 12, 2001 Sucrose, maltose, leucrose, maltotriose, and β-D-dodecylmaltoside fatty acid esters with acyl chains having 12 to 18 carbon atoms were enzymatically synthesized and their surface properties -critical micelle concentration, surface tension in water, and interfacial tension between water and xylene- were evaluated. The synthesized esters present critical micelle concentration (CMC) values in the range 2-250 µM with surface tension values ranging from 24.5 to 36.5 mN/m, and interfacial tension values from 1.0 to 9.4 mN/m depending on the compound. For the same acyl chain, CMC values were in the order maltotriose < leucrose < maltose < sucrose. The longest the fatty acid displays the lowest CMC, showing a decrease of one order of magnitude when the chain length was increased by six carbon atoms. β-D-dodecylmaltoside monoesters exhibited high efficiency as water-in-oil emulsifiers. All the newly synthesized di- and trisaccharide-based surfactants displayed better surface-active properties and higher solubility compared with similar monosaccharide esters.

Introduction Surface-active compounds synthesized from renewable resources, such as fatty acids and polyols, have increasing interest because of their advantages with regard to performance, health consumers, and environmental compatibility compared to petrochemicaly derived standard products.1,2 In particular, numerous attempts have been focused on the production of monoglycerides,3 sugar alcohol esters,4 n-alkyl polyglucosides,5-7 amino acid-based surfactants,8,9 and sugar fatty acid esters.10-13 Concerning properties of sugar esters, their surface tension-reducing capacity, penetration, dispersion, and detergent power are very outstanding.10 Sugar esters, which consist of a carbohydrate moiety as the hydrophilic group and a fatty acid as the lipophilic group, are very valuable as nonionic surfactants. By controlling the degree of esterification and the nature of fatty acid and sugar, it is possible to prepare sugar esters with a wide range of hydrophilic-hydrophobic balance (HLB). This is important because the HLB influences ultimate applications. Because sugar esters are nontoxic to organisms and biodegradable,2 these products are * To whom correspondence should be addressed: Francisco J. Plou, Departamento de Biocata´lisis, Instituto de Cata´lisis, C. S. I. C., Cantoblanco, 28049 Madrid, Spain. Phone: 34-91-5854869; Fax: 34-91-5854760. E-mail: [email protected]. † Department de Biocata ´ lisis, Instituto de Cata´lisis. ‡ Departmento de Tensioactivos, Instituto de Investigaciones Quı´micas. (1) Hill, K.; Rhode, O. Fett/Lipid 1999, 101, 25-33. (2) Baker, I. J. A.; Matthews, B.; Suares, H.; Krodkiewska, I.; Furlong, D. N.; Grieser, F.; Drummond, C. J. J. Surfact. Deterg. 2000, 3, 1-11. (3) Pastor, E.; Otero, C.; Ballesteros, A. Appl. Biochem. Biotech. 1995, 50, 251-263. (4) Ducret, A.; Giroux, A.; Trani, M.; Lortie, R. J. Am. Oil Chem. Soc. 1996, 73, 109-113. (5) Matsumura, S.; Imai, K.; Yoshikawa, S.; Kawada, K.; Uchibori, T. J. Am. Oil Chem. Soc. 1990, 67, 996-1001. (6) Balzer, D. Tenside Surf. Det. 1991, 28, 419-427. (7) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. Langmuir 2000, 16, 7359-7367.

perfect raw materials for personal care formulations14 and food emulsifiers.15 In addition, fatty acid esters of disaccharides show antitumoral and/or antibiotic activities.16-18 Vesicles containing nonionic surfactants such as sugar esters are able to encapsulate both hydrophilic and lipophilic drugs and protect them against acidic and enzymatic degradation in the gastro-intestinal tract.19,20 Several works have been carried out on physicochemical properties of esters of monosaccharides,4,11,21-23 sucrose,12,24-27 lactose, and gemini-type oligosaccharides.28 While, to our knowledge, other carbohydrate esters such as those from maltose, leucrose, and maltotriose have been (8) Beninaketti, H. S.; Mishra, B. K. In Design and Selection of Performance Surfactants; Karsa, D. R., Ed.; Sheffield Academic Press: Sheffield, 1999; Vol. 1, pp 1-49. (9) Holland, N. B.; Ruegsegger, M.; Marchant, R. E. Langmuir 1998, 14, 2790-2795. (10) Sarney, D. B.; Vulfson, E. V. Trends Biotechnol. 1995, 13, 164172. (11) Allen, D. K.; Tao, B. Y. J. Surf. Deterg. 1999, 2, 383-389. (12) Husband, F. A.; Sarney, D. B.; Barnard, M. J.; Wilde, P. J. Food Hydrocoll. 1998, 12, 237-244. (13) Ferrer, M.; Cruces, M. A.; Bernabe´, M.; Ballesteros, A.; Plou, F. J. Biotechnol. Bioeng. 1999, 65, 10-16. (14) Desai, N. B. Cosmet., & Toiletries 1995, 1, 55-59. (15) Nakamura, S. Oleochemicals 1997, 8, 866-874. (16) Okabe, S.; Suganuma, M.; Tada, Y.; Ochiai, Y.; Sueoka, E.; Kohya, H.; Shibata, A.; Takahashi, M.; Matsuzaki, T.; Fujiki, H. Jpn. J. Cancer Res. 1999, 90, 669-676. (17) Rentel, C. O.; Bouwstra, J. A.; Naisbett, B.; Junginger, H. E. Int. J. Pharm. 1999, 186, 161-167. (18) Van den Bergh, B. A. I.; Bouwstra, J. A.; Junginger, H. E.; Wertz, P. W. J. Control. Release 1999, 62, 367-379. (19) Van Hal, D. A.; Bouwstra, J. A.; van Rensen, A.; Jeremiasse, E.; de Vringer, T.; Junginger, H. E. J. Colloid Interface Sci. 1996, 178, 263-273. (20) Naoe, K.; Nishino, M.; Ohsa, T.; Kawagoe, M.; Imai, M. J. Chem. Technol. Biotechnol. 2000, 74, 221-226. (21) Jung, S.; Coulon, D.; Girardin, M.; Ghoul, M. J. Surfact. Deterg. 1998, 1, 53-57. (22) Scheckerman, C.; Schlotterbeck, A.; Schmidt, M.; Wray, V.; Lang, S. Enzyme Microb. Technol. 1995, 17, 157-162. (23) Havlıńova´, B.; Zemanovic, J.; Kosik, M.; Blazej, A. Tenside Det. 1978, 15, 119-121. (24) Abran, D.; Boucher, F.; Hamanaka, T.; Hiraki, K.; Kito, Y.; Koyama, K.; Leblanc, R. M.; Machida, H.; Munger, G.; Seidou, M.; Tessier, M. J. Colloid Interface Sci. 1989, 128, 230-236.

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scarcely studied29 because of the lack of appropriate synthetic methods. Most of the reported works on functional properties of sugar esters use a mixture of isomers, while the functional properties of pure monoesters have been less studied.30 In recent publications, we described a simple and flexible enzymatic route for the preparation of a wide range of di- and trisaccharide-based surfactants.13,31,32 The synthesis relies on the use of solvent mixtures for the lipase-catalyzed regioselective transesterification of long fatty acid esters with carbohydrates. This paper describes the comparative surfactive properties -critical micelle concentration, surface tension in water, and interfacial tension between water and xyleneof a homologous series of acyl sugar esters. Sugar headgroup size was varied from disaccharides (sucrose, maltose, leucrose, and alkyl-maltosides) to a trisaccharide (maltotriose). Acyl chain length was varied between 12 (lauroyl) and 18 (stearoyl). Structures with two alkyl chains attached were also studied. The influence of the acyl group chain length and sugar moiety on surface properties was analyzed. Experimental Procedures Chemicals. Sucrose and dimethyl sulfoxide (DMSO) were supplied by Merck (Darmstadt, Germany). D-(+)glucose, maltose monohydrate, maltotriose, n-dodecyl-βD-maltoside, 2-methyl-2-butanol, and molecular sieves (3 Å, 8-12 mesh) were purchased from Sigma (St. Louis, MO). D-Leucrose and vinyl laurate were obtained from Fluka (Buchs, Switzerland). The vinyl esters of myristic, palmitic, and stearic acids were from Tokyo Kasei Organic Chemicals (Tokyo, Japan). All other reagents and solvents were of the highest available purity and used as purchased. Enzymatic Transesterification of Di- and Trisaccharides. Synthesis of 6-O-lauroylsucrose, 6-O-palmitoylsucrose, 6,1′-di-O-lauroylsucrose and 6,6′-di-O-lauroylsucrose was carried out according to a method previously developed in our laboratory.13 Synthesis of 6′O-lauroylmaltose, 6′-O-myristoylmaltose, 6′-O-palmitoylmaltose, 6′-O-stearoylmaltose, 6-O-lauroyl-leucrose, 6′′O-lauroylmaltotriose, 6′′-O-myristoylmaltotriose, 6′′-Opalmitoylmaltotriose, 6′′-O-stearoylmaltotriose, 6′-Olauroyl β-D-dodecylmaltoside, 6′-O-myristoyl β-D-dodecylmaltoside, 6′-O-palmitoyl β-D-dodecylmaltoside, and 6′-O-stearoyl β-D-dodecylmaltoside was performed using a similar procedure.31,32 In all cases, the enzymatic synthesis was performed by transesterification of the sugar with the corresponding vinyl ester in a medium constituted by two solvents. More specifically, the acylation was carried out in 2-methyl 2-butanol (tert-amyl alcohol) containing a low percentage (not higher than 20%) of dimethyl sulfoxide. The lipase from Thermomyces lanuginosus (formerly Humicola lanuginosa) immobilized on diatomaceous earth (Celite) was (25) Bazin, H. G.; Polat, T.; Linhardt, R. J. Carbohyd. Res. 1998, 309, 189-205. (26) Makino, S.; Ogimoto, S.; Koga, S. Agric. Biol. Chem. 1983, 47, 319-326. (27) Bolzinger-Thevenin, M. A.; Grossiord, J. L.; Poelman, M. C. Langmuir 1999, 15, 2307-2315. (28) Vulfson, E. In Novel Surfactants (Surfactant Science Series); Holmberg, K., Ed.; Marcel Dekker: New York, 1998; Vol. 74, pp 279300. (29) Kjellin, U. R. M.; Claesson, P. M.; Vulfson, E. N. Langmuir 2001, 17, 1941-1949. (30) Sarney, D. B.; Benhard, K. J.; MacManus, D. A.; Vulfson, E. V. J. Am. Oil Chem. Soc. 1996, 73, 1481-1487. (31) Ferrer, M.; Cruces, M. A.; Plou, F. J.; Ballesteros, A. Spanish Patent 9901020 1999. (32) Ferrer, M.; Cruces, M. A.; Plou, F. J.; Bernabe´, M.; Ballesteros, A. Tetrahedron 2000, 56, 4053-4061.

Ferrer et al.

used as biocatalyst. Reactions were performed at 40 °C with magnetic stirring in the presence of molecular sieves. The products were isolated by column chromatography and/or solvent precipitation, and fully characterized by chromatography and spectroscopic techniques (HPLC, NMR, IR, HRMS). Determination of Surface Tension in Water. Surface tension of sugar ester solutions was measured according to the Wilhelmy plate method,33 with a Kru¨ss tensiometer (Processor tensiometer K-12, Hamburg, Germany), which determines directly the real tension values at the equilibrium, using a series of aqueous solutions at various concentrations of sucrose esters at 25 °C. The equilibrium time of the surface before the surface tension measurements was at least 1 h. The surfactant critical micelle concentrations (CMC) were determined graphically from the abrupt change in the slope of the surface tension values versus logarithm of surfactant concentration, expressed in µM. Determination of Interfacial Tension in a Biphasic System Water/Xylene. The interfacial tension was performed according to the du Nouÿ ring method34 with a Kru¨ss tensiometer (Processer Tensiometer K-12, Hamburg, Germany) which, through the corresponding automatic correction, enables us to obtain the real values of interfacial tension at 25 °C. The sugar-ester aqueous solutions were prepared at concentrations above the corresponding CMC, assuming that, above this value, the maximum interfacial adsorption capacity was reached. β-D-dodecyl-maltoside monoesters and sucrose diesters exhibit negligible solubility in water, therefore the interfacial tension was performed dissolving the corresponding esters in xylene and then measuring them against water. In this case, the sugar-ester solutions were prepared at different concentrations. Determination of the Area (A) Occupied by Adsorbed Molecule at the Saturated Water-Air Interface. The mean area occupied per molecule of adsorbed surfactant in the saturated water-air interface was calculated from the equation:

A ) 1016/NA × Γm where NA is the Avogadro’s number, Γm is the maximum concentration of surfactant molecules adsorbed in the saturated interface (moles/cm2), and the resulting A is expressed in squared Angstroms. The value of Γm can be determined by applying the Gibbs equation:

Γm ) -(dγ/d log c)/2.3030 n R T where (dγ/d log c) is the maximum slope of the linear plot of the graphical representation of surface tension against logarithm of surfactant concentration appearing immediately below the critical micelle concentration, R ) 8.31 J.mol-1.K-1, and T is the temperature in K. The value of n (the number of species into which the surfactant dissociates) is taken as one for nonionic surfactants. Results and Discussion Surface Tension and CMC of Synthesized Carbohydrate Esters. Carbohydrate fatty acid mono- and diesters are excellent oil-in-water (o/w) nonionic emulsi(33) International Standard ISO 304-1985. Surface active agents. Determination of surface tension by drawing up liquid films. (34) International Standard ISO 6889-1986. Surface active agents. Determination of interfacial tension by drawing up liquid films.

Di- and Trisaccharide Fatty Acid Esters

Langmuir, Vol. 18, No. 3, 2002 669

Scheme 1. Structures of Carbohydrate Fatty Acid Ester Surfactants Synthesized in This Work; n)10 (lauroyl), 12 (myristoyl), 14 (palmitoyl), and 16 (stearoyl).

fiers suitable for food, cosmetic, and pharmaceutical applications. For a surfactant to act as an emulsifier, it must show good surface activity and produce a low interfacial tension in the particular system in which it is to be used. This means that it must have a tendency to migrate to the interface, rather than to remain dissolved in either one of the bulk phases.35 In contrast with numerous works on properties of nonionic monosaccharide- and sucrose-based surfactants, few data is available characterizing other di- and trisaccharide esters. In this work, fatty acid (saturated C12C18) esters of sucrose, leucrose, maltose, maltotriose, and n-dodecylmaltosides mono- and diesters were synthesized and purified according to the protocols reported by our laboratory.13,31,32 In all cases, HPLC, NMR, and HRMS analyses showed a degree of purity over 99%. Structures of the surfactants studied are represented in Scheme 1. As a first indication of surfactant properties of synthesized sugar esters, the experimental determination of the surface tension vs concentration evidenced the capacity of such compounds to decrease the surface tension of water as well as to form micellar aggregates in aqueous media, typical properties of surfactant compounds. The minimum surfactant concentration needed to form micelles, the called critical micelle concentration (CMC), was determined graphically from the inflection appearing in the plots of surface tension vs logarithm of concentration. Table 1 summarizes the CMC values and the surface tension in water of these derivatives at 25 °C. All the new

Table 1. CMC and Surface Tension Values of Synthesized Sugar Fatty Acid Esters product

MW

CMC (µM)

surface tension at CMC (mN/m)

6-O-lauroylsucrose 6-O-palmitoylsucrose 6′-O-lauroylmaltose 6′-O-myristoylmaltose 6′-O-palmitoylmaltose 6′-O-stearoylmaltose 6′′-O-lauroylmaltotriose 6′′-O-myristoylmaltotriose 6′′-O-palmitoylmaltotriose 6′′-O-stearoylmaltotriose 6-O-lauroyl-leucrose

524.2 580.2 524.2 552.2 580.2 608.2 686.2 714.4 742.5 770.5 524.2

250 28 240 37 6 32 52 28 13 2 135

31.5 35.3 34.7 35.0 32.5 32.5 24.5 36.5 35.0 35.5 30.0

synthesized O-acyl-carbohydrate derivatives displayed excellent surface-active properties. The CMC values determined for 6-O-acylsucroses are in the same range as those reported previously for pure sucrose esters (Table 2) and also for commercial sucroesters containing mainly monoesters.24,26,36-38 For the rest of synthesized carbohydrate esters, no data is reported in the bibliography except for a recent study on 6′-O-lauroylmaltose. However, we can observe from Table 1 that the presence of other di(35) Porter, M. R. Handbook of Surfactants; Chapman and Hall: New York, 1991. (36) Vlahov, I. R.; Vlahova, P. I.; Linhardt, R. J. J. Carbohydr. Chem. 1997, 16, 1-10. (37) Polat, T.; Bazin, H. G.; Linhardt, R. J. J. Carbohydr. Chem. 1997, 16, 1319-1325.

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Table 2. Literature Values of Critical Micelle Concentration (CMC) and Surface Tension of Pure Disaccharide Fatty Acid Monoesters carbohydrate

fatty acid

CMC (µM)

surface tension (mN/m)

reference

sucrose

C12 C12-C14 C12-C16 C12-C14 C12-C16 C12-C18 C12 C14-C16

350 400-91 531-17 150-91 210-4.1 71-3.3 330 43-11

n.r.a n.r. 30.9-33.3 n.r. 35.3-36.0 n.r. 36.5 38.6-39.5

12 25 36 37 38 25 29 38

sulfosucrose maltose lactose a

n.r. is the value not reported.

and trisaccharides instead of sucrose gave rise to improved surface activity. The 6′-O-acyl β-D-dodecylmaltosides and sucrose diesters were insufficiently soluble in water to determine their CMC values. In conclusion, the synthesized monoesters present CMC values in the range 2-250 µM, with surface tensions at CMC from 24.5 to 36.5 mN/m. The lowest CMC values were produced when a trisaccharide and a long fatty acid (C18) were linked. Effect of Acyl Chain Length. In general, the CMC of a surfactant in an aqueous medium decreases as the number of carbon atoms in the hydrophobic moiety increases.4,23,37 As expected, the CMC values decrease with longer acyl chain. When moving from monolaurate to monostearate the hydrophobic chain length increases, promoting micelle formation at lower concentrations, consequently, the CMC value decreases (e.g. from 52 to 2 µM in the case of maltotriose esters, see Figure 1). However, it is noteworthy that in the series of maltose esters, the palmitoyl-derivative showed lower CMC values than the corresponding stearate (Figure 1). An explanation for this anomalous phenomenon could be the possible coiling of the large hydrocarbon chain described for alkyl chain lengths higher than C16.47 This results in an apparent shorter hydrocarbon tail, with the consequent modification of the expected CMC value. However, further studies should be carried out to confirm this possibility. Some of the O-palmitoyl and O-stearoyl derivatives displayed exceptionally low CMC values, making these nonionic surfactants very attractive for further studies. The CMC values are about 1 order of magnitude lower when the chain length is increased in 6 carbon atoms. Similar effects have been described studying fatty acid esters of glucose,4,39 fructose,22 and ethyl-D-glucopyranosides.40 Effect of Carbohydrate Moiety. As shown in Table 1, the CMC values for the carbohydrates assayed follow (38) Garafalakis, G.; Murray, B. S.; Sarney, D. B. J. Colloids Interface Sci. 2000, 229, 391-398. (39) Otto, R. T.; Bornscheuer, U. T.; Syldatk, C.; Schmid, R. D. J. Biotechnol. 1998, 64, 231-237. (40) Bhattacharya, S.; Acharya, S.N. G. Langmuir 2000, 16, 87-97. (41) Bjo¨rkling, F.; Godtfredsen, S. E.; Kirk, O. J. Chem. Soc. Chem. Commun. 1989, 934-935. (42) Go´mez-Herrera, C.; Fernańdez-Bolanõs, J. M.; Iborra, N. B.; Riego, M. B. Grasas y Aceites 1987, 38, 116-119. (43) Go´mez-Herrera, C.; Fernańdez-Bolanõs, J. M.; Iglesias, F. Grasas y Aceites 1984, 35, 306-309. (44) Castro, M. J. L.; Kovensky, J.; Fernańdez-Cirelli, A. Proceedings of the Fifth World Surfactants Congress; Firenze: Italy, 2000; Vol. 1, pp 441-444. (45) Zhang, T.; Marchant, R. E. J. Colloids Interface Sci. 1996, 177, 419-426. (46) Wilk, K. A.; Syper, L.; Burzck, B.; Sokolowski, A.; Domagslska, B. W. J. Surf. Deterg. 2000, 3, 185-192. (47) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989.

the order maltotriose < leucrose < maltose < sucrose. Figure 2 illustrates the variation of surface tension with the ester concentration for the different lauric acid esters. As can be observed, sucrose, maltose, leucrose, and maltotriose derivatives showed similar surfaceactivity profiles according to the surface tension reduction. Although the curve corresponding to 6′′-O-lauroylmaltotriose is significantly different to the rest of related derivatives (showing an apparent lower CMC), the existence of a minimum of surface tension seems to indicate the presence of some hydrophobic impurities (not detected by the analytical methods employed). The same phenomenon has been observed by other authors for similar compounds.29 The real CMC for the pure surfactant will be located at a higher concentration, close to that corresponding to the stabilized value of surface tension. This means that the real CMC value must be within the range obtained with the other derivatives. As regards to the value of area per molecule reported in Table 6, it must be considered with certain reservations because it was calculated from the experimental curve of surface tension/ log concentration. These results are in accordance with those obtained with monosaccharide esters, where the CMC was not significantly affected by the nature of sugar, probably because of the small size of the sugar group. However, the CMC value dropped when the acyl group chain length was increased.4,22,38 Comparison with Monosaccharide Fatty Acid Esters. Table 3 summarizes the reported CMC and surface tension values of different pure monosaccharide fatty acid monoesters. As shown, CMC values of the derivatives synthesized in this work are lower than those reported with related acyl monosaccharides. Some of the newly synthesized esters show the lowest CMC values reported in the literature for this type of surfactant (cf. Table 1 with Tables 2 and 3). Some of the di- and trisaccharide esters synthesized in this work exhibit significant advantages as surfactants compared with reported monosaccharide esters. First, the CMC values are significantly lower for the same fatty acid, reaching comparable values of surface tension. Second, the solubility in water of the di- and trisaccharide derivatives is notably higher than the corresponding monosaccharide esters, as a consequence of the increased hydrophilicity of the sugar headgroup. In fact, some of the results presented in Table 3 have been obtained using turbid solutions.38 The CMC values of our synthesized monoesters of diand trisaccharides are also lower than those corresponding to other well-known carbohydrate-based surfactants such as alkylpolyglucosides (>50 µM),5,6,8 n-alkyl glucamides (>80 µM), 8 or dimeric glucosides (>35 µM).43 Effect of the Degree of Acylation on CMC. To study the effect of incorporating two acyl chains in this class of surfactants, we synthesized two sucrose diesters with different acylation positions. As could be expected, the attachment of a second alkanoic chain to the sugar molecule changed notably the properties of the resulting compounds. It has been reported that sucrose diesters show lower surface activity than the monoesters, probably because the more open structure may pack less efficiently at the surface.12,40 In our study, the incorporation of a second lauryl chain led to a significant loss of solubility because the sucrose polar group is unable to compensate for the increase of hydrophobicity. This low solubility prevented us from collecting reliable CMC data for these compounds.


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Figure 1. Influence of the acyl chain length on the lowering of surface tension of water at 25 °C, for monoesters of maltose with lauric (C12), myristic (C14), palmitic (C16), and stearic acid (C18). Table 3. Literature Values of Critical Micelle Concentration (CMC) and Surface Tension of Pure Monosaccharide Fatty Acid Monoesters

carbohydrate glucose galactose fructose xylose ethyl-D-gluc opyranosides

Figure 2. Influence of the sugar headgroup of carbohydrate esters on the lowering of surface tension of water at 25 °C, for monolaurate esters of sucrose (1, ‚ ‚ ‚ ‚ ‚), leucrose (b, - - - - -), maltose (0, s) and maltotriose (O, s s s).

With respect to the position of acylation, when comparing the 6,1′-di-O-lauroylsucrose with the 6,6′-di-O-lauroylsucrose, the solutions of both compounds are turbid even at very low concentrations. Despite this turbidity, which makes it impossible to know the real amount of the compound dissolved, when comparing the lowest value of surface tension for the two compounds, the 6,1′-diester displayed 32.5 mN/m and the 6,6′-diester displayed 24.7

fatty acid

CMC (µM)

surface tension (mN/m)

C12 C8-C12 C8-C10 C14-C18:1 C10-C16 C18:1 C12-C14 C12-C16 C8-C18

365-298 2 × 104-150 1.8 × 104-500 150-20 2 × 103-30 76 200-40 41-15 2 × 103-8.3

27.6-28.2 27.3-30.5 32-43 31-43 27 31.6 24.8-32.8 28.9-41.0 31-44

reference 41 4 39 38 22 4 42 38 40

mN/m. This indicates that the spatial conformation of the two C12 chains for the latter compound led to a more effective adsorption at the water-air interface. Interfacial Tension of Sugar Esters. As it can be observed in Table 4, at low concentrations the synthesized derivatives are able to reduce the interfacial tension values between water and xylene, although this is conditioned by the chemical structure of the surfactant. The hydrophobic chain plays an important role in the interfacial tension value for sucrose esters, as well as the CMC and surface tension values; it is 4-fold lower for 6-Opalmitoylsucrose (1.0 mN/m) than 6-O-lauroylsucrose (3.8 mN/m). Given the lipophilic behavior of sucrose diesters and n-dodecyl-β-D-maltoside monoesters, the possibility to

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Table 4. Interfacial Tensions between Water Solutions of the Synthesized Sugar Fatty Acids and Xylene a

product

concentration (µM)

interfacial tension against xylene (mN/m)


> 250 > 28 > 240 > 37 >6 > 32 > 52 > 28 > 13 >2 > 135

3.8 1.0 1.8 3.3 2.5 3.4 9.4 2.6 3.1 n.d. b 2.5

a At defined concentrations above the corresponding CMC. b n.d. is the value not determined because of the breakdown of interphase.

Table 5. Interfacial Tension of Xylene Solutions of Sucrose Dilaurates and β-D-dodecylmaltoside Esters against Water

product

MW

6,6′-di-O-lauroylsucrose 706.9 6,1′-di-O-lauroylsucrose 706.9 6′-O-lauroyl β-Ddodecylmaltoside

692.5

6′-O-myristoyl β-Ddodecylmaltoside

720.5

6′-O-palmitoyl β-Ddodecylmaltoside

748.5

6′-O-stearoyl β-Ddodecylmaltoside

776.6

interfacial concentration tension against in xylene (µM) water (mN/m) n.s.a 849 38 4.1 × 103 413 165 16 2.9 × 103 234 23 2.7 × 103 267 107 11 2.7 × 103 269 108 11

n.s. 1.8 2.0 3.3 3.5 3.6 3.8 3.7 4.2 4.2 3.8 2.3 2.2 3.6 2.3 2.3 2.4 3.7

a n.s. This value not determined because of insolubility of the sample in xylene.

Table 6. Area Per Molecule and pC20 Parameter for the Synthesized Sugar Fatty Acid Esters product

average area per molecule (Å2)

pC20


44 91 64 78 26 49 34a 75 77 21 40

4.5 6.1 4.8 5.9 5.8 5.5 5.3 5.8 6.9 6.3 4.8

a Approximated value, calculated from the experimental curve showing a minimum of surface tension (Figure 2).

dissolve these compounds in a hydrocarbon solvent was explored while keeping in mind their potential as w/o emulsifiers. Thus, the values of interfacial tensions dissolved in xylene against water were measured (Table 5). As can be observed, 6,1′-di-O-lauroylsucrose shows slightly lower interfacial tension than the rest of derivatives, although no significant differences were appreciated. The reduction of interfacial tension shows that 6,1′-sucrose diester and alkylmaltoside monoesters are excellent w/o

emulsifiers. The increase of the interfacial tension particles at lowest concentration may be the result of the loss of molecular aggregates to form reverse micelles. According to the interfacial tensions, it can be observed from Tables 4 and 5 that, in general, the values for the synthesized sucrose esters are similar or even lower than those corresponding to the equivalent commercial and reported products.5 This pattern provides a clear indication of the availability of these compounds in view of practical applications involving two immiscible phases, as emulsification or solubilization, because of their contribution to reduce the interfacial energy barrier. Area Occupied per Molecule (A) at the Saturated Water-Air Interface. With regard to the mean area occupied per molecule at the saturated water-air interface, Table 6 shows the corresponding values for the synthesized esters. The results can be analyzed from two distinct points of view; the influence of different saccharide structures for a given acyl chain length or, inversely, the effect caused by the increasing of the acyl group maintaining a fixed sugar group. With respect to the sugar structures, no influence has been reported for very similar saccharide groups such as glucose, mannose, and galactose,5 whereas in accordance with other authors45,46 our results evidenced the importance of the size of the saccharide segment. With respect to the influence of the alkyl chain length for a given sugar headgroup, the general behavior described for conventional surfactants47 is a decrease of the area when the alkyl chain increases because of a more closed-packed arrangement favored by the hydrophobic interactions between these chains. With respect to the sugar derivatives, this same pattern has been found in some cases,42,45 whereas in our case, a not so clear tendency has been reported.5,38,46,48 These dissimilar results evidence the complex phenomenon of interactions involved simultaneously in an aqueous solution of a saccharide derivative. A characteristic of mono- and disaccharides is their capacity to establish hydrogen bonds between the hydroxyl groups and the surrounding water molecules. The number of these hydrogen bonds depends on the number and position of the hydroxyl groups in the saccharide molecule and can be an important factor in view for the adoption of a given conformation. Besides the hydration of the polar headgroup, the hydrophobicity of the alkyl chains of a saccharide derivative plays a fundamental role in the adsorption on the water-air interface. Given the value of 20 Å2 for the cross-sectional area of an aliphatic chain oriented perpendicular to the interface, it is evident that the alkyl chains are tilted with respect to the interface. The progressive increase of the chains can promote changes in the area per molecule, in some cases growing up, either by a larger projection on the interface of the oriented chain or by a phenomenon of coiling. Whereas in other cases, the resulting area diminishes because of a more compact packing attained as a consequence of the above-mentioned hydrophobic bonds established between the acyl chains. It is interesting to note that when maltotriose is used as sugar headgroup the area occupied per molecule is lowest. Table 6 also includes the pC20 parameter, which corresponds to the negative logarithm of the molar concentration (called C20), needed to decrease the surface tension of the solvent by 20 mN/m (when the solvent is water, the C20 corresponds to 52 mN/m). It is an index of (48) Savelli, M. P.; Bault, P.; Douillet, O.; Gode´, P.; Goethals, G.; Martin, P.; Ronco, G.; Villa, P. Jorn. Com. Esp. Deterg. 1998, 28, 293303.


the efficiency of the interfacial adsorption. The higher the pC20 is, the more efficient the surfactant adsorption is. The pC20 values in Table 6 show that the most efficient surfactant is 6′′-O-palmitoylmaltotriose. Acknowledgment. We thank Prof. M. Bernabe´ (Instituto de Quı´mica Orgańica, CSIC, Madrid) and Dr. E.

Langmuir, Vol. 18, No. 3, 2002 673

Pastor for help with this work. We thank Ministerio de Ciencia y Tecnologıá for a research grant. This research has been supported by the Spanish CICYT (project BIO98-0793) and Comunidad de Madrid (project 07G/ 0042/2000). LA010727G

Comparative Surface Activities of Di-and Trisaccharide Fatty Acid Esters

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