12802
Langmuir 2007, 23, 12802-12805
New Ethoxylated Inositol Surfactant Gabriela Catanoiu,† Valeria Ga¨rtner,‡ Cosima Stubenrauch,‡ and Dirk Blunk*,† UniVersita¨t zu Ko¨ln, Institut fu¨r Organische Chemie, Greinstrasse 4, 50939 Ko¨ln, Germany, and School of Chemical and Bioprocess Engineering, UniVersity College Dublin, Belfield, Dublin 4, Ireland ReceiVed August 22, 2007. In Final Form: October 16, 2007 Carbohydrates are an attractive class of starting materials for organic syntheses because they are of natural origin, environmentally friendly, and highly functionalized, in this way promoting a sustainable chemistry. A somewhat exotic but nevertheless readily available family of carbohydrates allowing a fascinating chemistry are inositols (cyclohexane1,2,3,4,5,6-hexols), which we currently use for the synthesis of new surfactants. In our previous work, we reported on the synthesis of a number of new regiochemically defined myo-inositol ethers and esters and studied their surface activity in aqueous solution as well as their ability to form thermotropic liquid crystals. It turned out that the hydrophilicity of the myo-inositol head group alone does not ensure sufficient water solubility of these surfactants. To improve the water solubility, we increased the inositol head group by the introduction of a tri(ethylene oxide) unit. The resulting surfactant is the first representative of a new class of inositol-based surfactants (CiEjIk) that combine the properties of classical sugar surfactants (CnGm) and oligo(ethylene oxide) alkyl ether surfactants (CiEj).
1. Introduction Sugar surfactants (CnGm) and oligo(ethylene oxide) alkyl ether surfactants (CiEj) are classic examples of nonionic surfactants that exhibit quite different behaviors.1 First, the physicochemical properties of aqueous solutions of sugar surfactants are less temperature-sensitive than those of oligo(ethylene oxide) alkyl ether solutions.2 Second, considering similar conditions and similar head group sizes (about three to four oxyethylene units are comparable to one glucose unit), it was found that the degree of hydration of oligo(ethylene oxide) surfactants is 1 order of magnitude higher than that of sugar-based surfactants.3 A third difference relates to their tendency to adsorb on hydrophilic silica from aqueous solution. Although CiEj surfactants adsorb strongly on silica, the sugar-based surfactant does not adsorb at all but stays in the aqueous phase.4,5 Last, classic sugar surfactants often possess a rigid head group such as a maltoside unit, and the CiEj surfactants have a very flexible hydrophilic part that can be considered to be a short polymer chain.6 The properties of both types of surfactants can be beneficial, depending on the application. In light of the different behavior of CnGm and CiEj surfactants, the chemical combination of both sugar and oligo(ethylene oxide) units in one molecule is exceedingly interesting. However, though ethoxylated sugar surfactants appear in some patents,7,8 they are studied surprisingly rarely.9-15 * To whom correspondence should be addressed. Tel: +49-2214705213. Fax: +49-221-4703064. E-mail:
[email protected]. † Universita ¨ t zu Ko¨ln. ‡ University College Dublin. (1) Angarska, J.; Stubenrauch, C.; Manev, E. Colloids Surf., A 2007, 309, 189-197. (2) Stubenrauch, C. Curr. Opin. Colloid Interface Sci. 2001, 6, 160-170. (3) Claesson, P. M.; Kjellin, M.; Rojas, O. J.; Stubenrauch, C. Phys. Chem. Chem. Phys. 2006, 8, 5501-5514. (4) Matsson, M. K.; Kronberg, B.; Claesson, P. M. Langmuir 2004, 20, 40514058. (5) Matsson, M. K.; Kronberg, B.; Claesson, P. M. Langmuir 2005, 21, 27662772. (6) Persson, C. M.; Kjellin, U. R. M.; Eriksson, J. C. Langmuir 2003, 19, 8152-8160. (7) Itano, M.; Kayane, S.; Ueno, K.; Mizushima, H. Anticaries Dentifrices Containing Alkylgalactosides. 2005-JP17863 2006035821, 20050928; 2006. (8) Kubota, H.; Ueno, K. Biofilm Inhibitory and RemoVal Agents Containing Glucosides. 2005-175331 2006347941, 20050615; 2006; Kubota, M.; Tsukuda, K. Antifogging Agents Containing Alkylglycosides for Glasses, Plastics, and Polymer Films. 92-138851 05331454, 19920529; 1993. (9) Millqvist-Fureby, A.; Gao, C. L.; Vulfson, E. N. Biotech. Bioeng. 1998, 59, 747-753.
To pursue this topic in more detail, we synthesized a surfactant, the head group of which consists of three oxyethylene units and an inositol group. The latter is an interesting alternative to conventional sugars such as glucose, galactose, and mannose because it has the same molecular formula as hexoses C6H12O6 but a different molecular constitution. Inositol is a homocyclic 6-C-ring sugar (C-sugar) (i.e., it lacks the anomeric center). As a result, it does not undergo ring-opening reactions even at low pH values. The family of inositols consists of eight diastereomers, one of which, myo-inositol (1), has the potential of becoming increasingly important16 because it is a cheap, easily accessible compound: it can be extracted from natural sources such as wheat germ, brewer’s yeast, nuts, and vegetables. In earlier work, we reported on the synthesis of a number of new regiochemically defined myo-inositol ethers and esters and studied their surface activity in aqueous solution16 as well as their ability to form thermotropic liquid crystals.17,18 In the following text, we present the synthesis of rac-1-O-(3,6,9-trioxa-henicosanyl)-myo-inositol (rac-5, C12E3I1) which is a representative of the above portrayed new class of ethoxylated inositol surfactants. Some selected properties of the new surfactant are compared with those of classic surfactants with similar structures, namely the surface activity and the phase behavior of both the single and binary water/surfactant systems. (10) Wilhelm, F.; Chatterjee, S. K.; Rattay, B.; Nuhn, P.; Benecke, R.; Ortwein, J. Liebigs Ann. 1995, 1673-1679. (11) Bendas, G.; Wilhelm, F.; Richter, W.; Nuhn, P. Eur. J. Pharm. Sci. 1996, 4, 211-222. (12) Haage, K.; Fiedler, H.; Esser, A. Ber. Bunsen-Ges. 1997, 101, 16901694. (13) Zimmermann, I.; Fo¨rster, G.; Rettig, W.; Wartewig, S. The Phase Behavior of Hexadecyl-R-D-hexaethyleneoxy-mannopyranoside; Spectroscopy of Biological Molecules; New Directions, 8th European Conference on the Spectroscopy of Biological Molecules, Enschede, The Netherlands, Aug 29-Sept 2, 1999; pp 353-354. (14) Czichocki, G.; Fiedler, H.; Haage, K.; Much, H.; Weidner, S. J. Chromatogr., A 2002, 943, 241-250. (15) Zimmermann, I.; Wolgast, S.; Chatterjee, S. K.; Fo¨rster, G.; Rettig, W.; Wartewig, S.; Nuhn, P. The Influence of Chain Length, Oligo-Oxyethylene Spacer Length, and Sugar Moiety on the Phase Behavior of n-Alkylglycopyranosides,; Arbeitstagung Flu¨ssigkristalle; 32. Topical Meeting on Liquid Crystals; Halle, Saale, Germany, Mar 24-26, 2004; p P29. (16) Blunk, D.; Bierganns, P.; Bongartz, N.; Tessendorf, R.; Stubenrauch, C. New J. Chem. 2006, 30, 1705-1717. (17) Praefcke, K.; Blunk, D. Liq. Cryst. 1993, 14, 1181-1187. (18) Praefcke, K.; Blunk, D.; Hempel, J. Mol. Cryst. Liq. Cryst. 1994, 243, 323-352.
10.1021/la702596a CCC: $37.00 © 2007 American Chemical Society Published on Web 11/21/2007
Letters
Langmuir, Vol. 23, No. 26, 2007 12803 Table 1. Physicochemical Properties of Dodecyl Hexaethylene Oxide (C12E6), 1-O-(3,6,9-Trioxa-henicosanyl)-myo-inositol (C12E3I1), Dodecyl-β-D-maltoside (β-C12G2), and β-D-(3,6,9-Trioxa-henicosanyl)-glucoside (β-C12E3G1) cmc/mM σcmc/mN m-1 Γ∞ 10-6/mol m-2 Amin/nm2
surfactant C12E6 C12E3I1 (rac-5) β-C12G2 β-C12E3G1
0.09 0.14 0.16 0.19a
31.6 34.4 34.7 34a
3.1 3.1 3.8
0.53 0.53 0.44
a The cmc and σcmc values of β-C12E3G1 are estimated from graphs in ref 12.
Scheme 1. Synthesis of 1-O-(3,6,9-Trioxa-henicosanyl)-myo-inositol (rac-5, C12E3I1)
Figure 1. (a) Surface tension σ and (b) surface concentration Γ as a function of the surfactant concentration c of 1-O-(3,6,9-trioxahenicosanyl)-myo-inositol (rac-5, C12E3I1), β-dodecyl maltoside (β-C12G2), and dodecyl hexaethylene oxide (C12E6).
2. Synthesis The synthesis of rac-1-O-(3,6,9-trioxa-henicosanyl)-myo-inositol (rac-5, C12E3I1) starts with the natural product myo-inositol (1), which is converted to the appropriately protected tetra-benzyl intermediate rac-3 by well-known procedures (cf. Scheme 1).19 Tosylated ether 4 was synthesized by Williamson etherification of triethylene glycol (2) and subsequent tosylation. The regioselective unification of both building blocks (rac-3 and glycol ether 4) was carried out using dibutyltin oxide in the presence of cesium fluoride. After deprotection by hydrogenation, the desired rac-5 product was obtained in an overall yield of 22%. The synthesis details and spectroscopic characterizations will be published elsewhere. It should be mentioned that the synthetic sequence outlined here can also be performed in an enantioselective manner. However, the material under discussion in this letter is racemic. The thermal and phase behavior of the pure enantiomers might differ from those of the racemic material and will be part of a later study.
3. Surface Tension Figure 1a shows the surface tension σ as a function of the concentration c of rac-5. For the sake of comparison, the curve is plotted together with those of the corresponding sugar (β-C12G2) and oligo(ethylene oxide) (C12E6) surfactants. The surface tensions were measured as a function of the surfactant concentration by the Du Nou¨y ring method using a STA1 tensiometer from Sinterface Technologies. The surface tension increases with decreasing surfactant concentration until it levels off at the value of the surface tension of pure water (72.3 mN/m at 23 °C). Because the surface tension is very sensitive to slight impurities that are usually reflected in (19) Angyal, S. J.; Melrose, G. J. H. J. Chem. Soc. 1965, 6494-6955.
a minimum or a smooth leveling off at concentrations around the cmc, the σ-c curve serves as an indicator of the purity of a surfactant. As can be seen in Figure 1, the surface tension curve of the newly synthesized C12E3I1 (rac-5) has a sharp bend and no minimum at the cmc, which indicates that this surfactant is very pure. All σ-c curves are fitted with the Frumkin isotherm,20,21
(
σ ) σ0 + Γ∞RT ln 1 -
) ( )
Γ Γ + a′ Γ∞ Γ∞
2
(1)
where σ is the surface tension of the surfactant solution and σ0 is that of the solvent, Γ is the surface concentration, Γ∞ is the maximum surface concentration (saturation monolayer coverage), R is the gas constant, T is the temperature, and a′ is the interaction parameter. The cmc of C12E3I1 is very close to those of β-C12G2, β-C12E3G1,12,22 and C12E6, which is expected for nonionic surfactants with the same hydrophobic chain length (Table 1). (20) Frumkin, A. Z. Phys. Chem. 1925, 116, 466-84. (21) We evaluated all σ(c) curves with the Langmuir, the Frumkin, and the reorientation models. Moreover, we carried out a model-independent fitting of the curves with second-order polynomials. It turned out that the σ(c) curves of β-C12G2 and C12E3I1 are best described with the Frumkin model, whereas the σ(c) curve of C12E6 could be described with all models and a polynomial. The simple reason for this insensitivity of the C12E6 data is the fact that surface tension data at very low concentrations (i.e., at concentrations where the differences in the models are most visible) are not available and thus it is not clear which model would describe our data best. However, fitting the σ(c) curve of C12E6 with all three models as well as with the polynomial and comparing the resulting Γ(c) curves with those of β-C12G2 and C12E3I1 always lead to the same trend, namely, Γ∞(β-C12G2) > Γ∞(C12E6) g Γ∞(C12E3I1). (22) The exact values of the cmc and σcmc are not given in ref 12 but can be estimated from graphs published therein. Because of the unavoidable error introduced by this procedure, the values listed for C12E3G1 in Table 1 can only be considered as rough estimates.
12804 Langmuir, Vol. 23, No. 26, 2007
Letters
Table 2. Phase Transition Dataa of C12E3I1 and β-C12G2 amphiphile
Cr
C12E3I1 β-C12G2b
• •
M 81.0/80.1 (43.3) -/102 (-)
SmA SmA
iso 152.8/153.1 (0.5) -/245 (-)
• dec
a Temperatures in °C; polarizing microscopy/differential scanning calorimetry, PM/DSC; enthalpies (kJ mol-1) in brackets; heating rate, 5 K min-l; Cr, crystalline; M, thermotropic mesophase; SmA, smectic A; iso, isotropic liquid; dec, decomposition. bPhase-transition data of β-C12G2 cited from ref 23. See also ref 24.
However, the cmc values of C12E3I1 (rac-5) and β-C12G2 are much more similar than those of C12E3I1 and C12E6, which can be explained by similar monomeric solubilities. Also, the plateau value of the surface tension (i.e., the surface tension at concentrations above the cmc) of C12E3I1 (rac-5) is much more similiar to that of β-C12G2 than to the plateau value obtained for C12E6. The corresponding adsorption isotherms of the three surfactants are shown in Figure 1b, and the adsorption parameters are listed in Table 1. The isotherms were derived by differentiating the Frumkin fits and using the Gibbs equation. As can be seen, the surface concentration curve of C12E3I1 (rac-5) is very similar to that of hexaethylene oxide surfactant C12E6 and levels off in the same Γ∞ value. This is rather unexpected, recalling the observation that these two surfactants have different plateau values of the surface tension (Table 1). As a rule of thumb, one can say that the lower the plateau value the more densely packed is the monolayer. Thus, one would expect that maximum surface concentration Γ∞ of C12E6 to be larger than that of C12E3I1 (rac-5). However, this is not the case. A possible explanation could be that the E3I1 head group is as flexible as the hexaethylene oxide head group, thus needing as much surface area as the E6 unit. However, a monolayer consisting of E6 units can obviously pack more densely compared to one consisting of E3I1 units, leading to a lower surface tension at the cmc. It would be interesting to know the Amin value of the C12I1E3 surfactant (Scheme 2), the synthesis of which is currently underway.
4. Phase Behavior 4.1. Thermotropic Mesomorphism. rac-5 forms a thermotropic mesophase of smectic A type between =81 and =153 °C (cf. Table 2), which was investigated by polarizing microscopy (PM) and differential scanning calorimetry (DSC). Its phasetransition data are summarized in Table 2 together with the data of β-dodecyl maltoside. 4.2. Lyotropic Mesomorphism. The binary system waterC12E3I1 was studied by polarizing microscopy, and the resulting phase diagram is shown in Figure 2. Measurements with samples of known concentration at various temperatures were made only for concentrations less than ∼60 wt % surfactant. Sample preparation at higher concentrations was not possible because of the high viscosity of the solution. To obtain additional information about the phase diagram for concentrations higher than 60 wt %, we used the contact preparation technique. Thus, the concentrations to which the readings belong are only rough estimates. The temperature-dependent measurements revealed that the phase behavior is not temperature sensitive within the studied temperature range (23-70 °C). Consequently, drawing the vertical phase boundaries for that temperature range as seen in Figure 2 is justified. The phase diagram mainly consists of an isotropic phase (L1) up to a concentration of about 35 wt %, which is followed by a hexagonal phase (H1) from ∼40 to 57 wt % and a bicontinuous cubic phase (V1) from ∼80 to 85 wt
Figure 2. Phase diagram of the binary system water-C12E3I1 measured as a function of the temperature and the total surfactant concentration.
%. The high concentration region above ∼80 wt % shows a lamellar phase (LR). In between these single-phase regions lie two-phase regions where both adjoining phases coexist. Above 35 wt %, all samples formed liquid-crystalline phases over the whole studied temperature range. In view of the fanlike texture seen between 40 and 57 wt % under crossed polarizers, this phase can clearly be identified as a hexagonal liquid crystal (H1), which is followed by the optically isotropically appearing bicontinuous cubic phase (V1). Both of these phases are much more viscous than the lamellar phase (LR), which follows in phase diagram at higher concentrations. In the following text, the phase diagram of the binary system water-C12E3I1 is compared with those of water-β-C12G224 and water-C12E6.25-27 The phase diagram of water-β-C12G2 is as temperatureinsensitive as that of water-C12E3I1. In the former, two liquidcrystalline phases are observed, namely, the hexagonal (H1) and the lamellar (LR) phases. In contrast to β-C12G2, the phase behavior of the binary system water-C12E6 is very temperature-sensitive. As is the case for C12E3I1, three liquid-crystalline phases are observed, namely, a hexagonal (H1), a bicontinuous cubic (V1), and a lamellar phase (LR). In addition, a miscibility gap is observed at higher temperatures and lower surfactant concentrations. The temperature insensitivity of aqueous solutions of the maltoside surfactant is due to the strong hydrogen bonds between the sugar units and water, which prevent significant dehydration of the head group with increasing temperature. However, the oxyethylene units can be easily dehydrated with increasing temperature because the interactions between water and oligo(ethylene oxide) are much weaker (mainly weak dipole-dipole interactions).3,28 Assuming that the hydration of the myo-inositol substructure is comparable to that of a glucoside head group, one expects “intermediate” behavior for the new C12E3I1 surfactant. This, however, is not the case. The presence of the three oxyethylene units obviously does not lead to temperature-dependent phase behavior, an observation that cannot yet be explained. As was the case for the surface tension, it will be very interesting to compare these results with those obtained for C12I1E3, where the (23) Marcus, M. A. Mol. Cryst. Liq. Cryst. 1986, 3, 85-89. (24) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. Langmuir 2000, 16, 7359-7367. (25) Clunie, J. S.; Goodman, J. F.; Symons, P. C. Trans. Faraday Soc. 1969, 65, 287-296. (26) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. I 1983, 79, 975-1000. (27) Strey, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 182-189. (28) Tyrode, E.; Johnson, C. M.; Kumpulainen, A.; Rutland, M. W.; Claesson, P. M.; J. Am. Chem. Soc. 2005, 127, 16848-16859.
Letters
Langmuir, Vol. 23, No. 26, 2007 12805
Scheme 2. Structure of 1-O-Dodecyl-4-O-(3,6-dioxa-8-hydroxy-octyl)-myo-inositol (6, C12I1E3)
“outer” part of the head group consists of the oligo(ethylene oxide) group (Scheme 2).
5. Summary and Outlook What is fundamentally not yet understood is the observation that the aqueous phase behavior of C12E3I1 is almost temperatureinsensitive despite its three ethylene oxide units. It might be the end-capping group of the molecule (i.e., the inositol in rac-5) that has a significant impact on the hydration properties or the very delicate hydrophilic/hydrophobic balance of the surfactant
that determines its supramolecular hydration characteristics. To answer this question, it will be interesting to study and compare the properties of a new surfactant with an inverse molecular arrangement of the oligo(ethylene oxide) and sugar units, as in 6 (Scheme 2), the synthesis of which is in progress. Furthermore, ongoing and planned molecular dynamic simulations of respective water/surfactant systems at the air/water surface as well as in the bulk phase may elucidate the supramolecular features of these interesting new surfactants and amphotropic materials. Acknowledgment. Funding by the European Community’s Marie Curie Research Training Network “Self-Organisation under Confinement (SOCON)”, contract number MRTN-CT-2004512331 and by the Seed Funding Scheme of the University College Dublin, is gratefully acknowledged. LA702596A
