pubs.acs.org/Langmuir © 2009 American Chemical Society
Syntheses, Amphitropic Liquid Crystallinity, and Surface Activity of New Inositol-Based Amphiphiles Dirk Blunk,*,† Nils Bongartz,† Cosima Stubenrauch,‡ and Valeria G€artner‡ †
Universit€ at zu K€ oln, Institut f€ ur Organische Chemie, Greinstrasse 4, 50939 K€ oln, Germany, and ‡School of Chemical and Bioprocess Engineering, Centre for Synthesis and Chemical Biology (CSCB), SFI-Strategic Research Cluster in Solar Energy Conversion, University College Dublin, Belfield, Dublin 4 Ireland Received February 25, 2009. Revised Manuscript Received May 8, 2009
Carbohydrates are interesting starting materials for scientific and industrial syntheses as they allow a versatile chemistry. Moreover, they are of natural origin and environmentally benign. During the past few years, inositol, a rather “exotic” carbohydrate, and its derivatives have gained increasing attention. Here, we describe the syntheses of new regiochemically defined inositol monoethers and monoesters as well as regioisomeric inositol ester mixtures and investigate their amphitropic liquid crystallinity. Furthermore, first results on their surface activity in aqueous solutions are given and compared with classical sugar surfactants.
1. Introduction In this contribution, we discuss and compare some new inositol-based amphiphiles with respect to their syntheses, the amphitropic1 formation of liquid crystals, and their surface activity. Inositols (cyclohexane-1,2,3,4,5,6-hexols) belong to both the group of cyclitols and the family of carbohydrates, since they have the same molecular formula as conventional hexoses (C6H12O6) but a different molecular structure, that is, a different constitution.2 In a way, they are the homocyclic carbon analogue of pyranoses, which is why they are sometimes called “C-sugars”. The eight possible diastereomers of inositol are renewable primary natural products which differ only in their relative stereochemical configuration. Especially myo-inositol, which carries one axial and five equatorial hydroxyl functions, is a cheap and easily accessible compound. It is noteworthy that since the beginning of the eighties amphiphilic carbohydrates, cyclitol monoethers, and cyclitol monoesters have attracted increasing attention because of their thermotropic and lyotropic properties, because of their surface activity, and because of the fact that they are environmentally benign. Inositols have been used as hydrophilic headgroups in amphiphilic molecules, but up to now mainly the thermotropic liquid crystalline properties of the respective alkyl monoethers have been studied in *To whom correspondence should be addressed. Telephone: +49-221-470 5213. Fax: +49-221-470 3064. E-mail:
[email protected]. (1) Despite the common use of the term “amphotropic”, we decided to use “amphitropic” here, as proposed in the following: (a) Baron, M. Pure Appl. Chem. 2001, 73, 845–895. (b) Tschierske, C.; Pelzl, G.; Diele, S. Angew. Chem. 2004, 116, 6340-6368; translated by M. M€uller. (2) (a) Potter, B. V. L. Nat. Prod. Rep. 1990, 7, 1–24. (b) Billington, D. C. Chem. Soc. Rev. 1989, 18, 83–122. (3) Blunk, D.; Bierganns, P.; Bongartz, N.; Tessendorf, R.; Stubenrauch, C. New J. Chem. 2006, 30, 1705–1717. (4) Catanoiu, G.; G€artner, V.; Stubenrauch, C.; Blunk, D. Langmuir 2007, 23, 12802–12805. (5) Prade, H.; Miethchen, R.; Vill, V. J. Prakt. Chem. 1995, 337, 427–440. (6) Praefcke, K.; Blunk, D.; Hempel, J. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1994, 243, 323–352. (7) Praefcke, K.; Blunk, D. Liq. Cryst. 1993, 14, 1181–1187. (8) Praefcke, K.; Marquardt, P.; Kohne, B.; Stephan, W. J. Carbohydr. Chem. 1991, 10, 539–548. (9) Marquardt, P.; Praefcke, K.; Kohne, B.; Stephan, W. Chem. Ber. 1991, 124, 2265–2277. (10) Kohne, B.; Praefcke, K.; Stephan, W.; N€urnberg, P. Z. Naturforsch., B: Chem. Sci. 1985, 40B, 981–986.
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detail.3-10 Furthermore, some regioisomeric mixtures of myoinositol monoesters with long side chains have been synthesized, and the surface activity of these compounds has been studied.11 It was only recently that the properties of a regiochemically pure inositol derivative in aqueous solution were studied. The combination of a polyethylene glycol and an inositol unit in the hydrophilic headgroup of this derivative led to very interesting features.4 Inositol-based amphiphiles are expected to have properties which are similar to those of sugar-based ones. The polyol structure of inositol makes it as suitable as conventional sugars to be used as a hydrophilic headgroup. However, in contrast to the latter, inositol cannot undergo mutarotation or ring-opening reactions, not even at low pH values, because an anomeric center is lacking. Thus inositol-based amphiphiles are chemically more resistant than the respective sugar surfactants but are expected to be as biodegradable as other carbohydrate derivatives due to the structural similarities. Compared to the properties of nonionic polyethylene glycol alkyl ethers, sugar-based surfactants are often superior because quite a few of their solution properties are relatively insensitive toward temperature changes.12-14 Moreover, they are regarded as physiologically mostly nontoxic and nonirritant.14-18 These properties open up a broad field of possible applications and thus an increased interest in the supramolecular characteristics of amphiphilic carbohydrate derivatives. 3,19-22 (11) Sohn, J.; Nam, K. Daehan Hwahak Hwoejee 1982, 26, 49–57. (12) Stubenrauch, C. Curr. Opin. Colloid Interface Sci. 2001, 6, 160–170. (13) Nonionic surfactants: Alkyl polyglucosides; Balzer, D., L€uders, H., Eds.; Marcel Dekker: New York, 2000. (14) Alkyl polyglycosides; Hill, K., Rybinski, W. v., Stoll, G., Eds.; VCH: Weinheim, 1997. (15) Pes, M. A.; Aramaki, K.; Nakamura, N.; Kunieda, H. J. Colloid Interface Sci. 1996, 178, 666–672. (16) Kameyama, K.; Takagi, T. J. Colloid Interface Sci. 1990, 137, 1–10. (17) J€onsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution, 2nd ed.; John Wiley & Sons: Chichester, 2002; p 24. (18) Matsumura, S.; Imai, K.; Yoshikawa, S.; Kawada, K.; Uchibori, T. J. Am. Oil Chem. Soc. 1990, 67, 996–1001. (19) Goodby, J. W.; G€ortz, V.; Cowling, S. J.; Mackenzie, G.; Martin, P.; Plusquellec, D.; Benvegnu, T.; Boullanger, P.; Lafont, D.; Queneau, Y.; Chambert, S.; Fitremann, J. Chem. Soc. Rev. 2007, 36, 1971–2032. (20) Vieira de Almeida, M.; Le Hyaric, M. Mini-Rev. Org. Chem. 2005, 2, 283– 297. (21) Vill, V.; Hashim, R. Curr. Opin. Colloid Interface Sci. 2002, 7, 395–409. (22) Blunk, D.; Praefcke, K.; Vill, V. In Handbook of Liquid Crystals; Demus, D., Goodby, J. W., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, 1998; Vol. 3, pp 305-340
Published on Web 06/11/2009
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However, it has to be kept in mind that all properties mentioned above depend not only on the hydrophilic but also on the hydrophobic part of the molecules. It is the delicate balance between the hydrophilic and the hydrophobic parts that ultimately determines the material’s features. This is an advantage rather than a drawback, as it allows designing and tuning of properties via the molecular structure. For example, the driving forces for the formation of liquid crystalline phases of amphiphilic carbohydrate derivatives are hydrogen bridges between the hydrophilic headgroups and van der Waals interactions between the hydrophobic parts. The combination of hydrophilic and hydrophobic effects leads to a microphase separation of the polar and the unpolar molecular regions. Cyclic and acyclic mono-, di-, or oligosaccharides can form thermotropic liquid crystal phases if they carry at least one suitably long side chain and if the headgroup contains enough free hydroxyl groups. Amphiphilic carbohydrates very often show amphitropic behavior; that is, they form both thermotropic and lyotropic liquid crystals, depending on the absence or presence of water. In both cases, the architecture and topology of the mesophases formed is determined by the volume and average area of the hydrophilic and the hydrophobic molecular parts and the curvature of their interfacial plane. Depending on the molecular constitution and shape of carbohydrate amphiphiles, smectic, columnar, or cubic mesophases are commonly observed.3,19,21,22 As the newly synthesized inositolbased surfactants have a single alkyl chain, they form a thermotropic smectic A (SmA) phase, as expected. To improve the knowledge on the properties of single-chain inositol-based surfactants, we synthesized a number of new regiochemically defined inositol monoesters rac-1 and monoethers rac-2 (Figure 1). To provide a practical and commercially attractive short synthetic route, we also tested a one-pot, one-step procedure to synthesize mixtures of inositol monoester regioisomers. The thermotropic liquid crystalline properties of the new compounds as well as their surface activity in aqueous solutions have been investigated.
2. Syntheses 2.1. Regiochemically Defined Inositol Ethers and Esters. The well-established protecting group chemistry related to inositols mainly consists of selective acetalization, etherification, esterification, and the respective deprotection reactions.23-28 The dexterous application of protecting group strategies allows the synthesis of 1,4,5,6-tetra-O-benzyl-myo-inositol (rac-3),23 which can be selectively modified at the equatorial hydroxyl group despite the presence of the neighboring axial one (Scheme 1). Thus, it is possible to synthesize the 1-functionalized monoesters rac-4a-d by DMAP catalyzed esterification with different acid chlorides. The alternative Williamson etherification of rac-3 with alkyl bromides led to the monoethers rac-5. The etherification was carried out in a microwave oven which in some of our reactions remarkably increased the yield. In each case, cleavage of the benzyl ethers by hydrogenation resulted in the desired amphiphiles of type rac-1 or rac-2 in good overall yields. The synthetic procedures, the respective analytical data, and the instrumentation used are compiled in the Supporting Information. (23) Meyer zu Reckendorf, W. Chem. Ber. 1968, 101, 3652–3654. (24) Angyal, S. J.; Irving, G. C.; Rutherford, D.; Tate, M. E. J. Chem. Soc. 1965, 6662–6664. (25) Angyal, S. J.; Tate, M. E. J. Chem. Soc. 1965, 6949–6955. (26) Lee, H. W.; Kishi, Y. J. Org. Chem. 1985, 50, 4402–4404. (27) Shah, R. H.; Loewus, F. J. Labelled Compd. 1970, 6, 333–339. (28) Kohne, B.; Marquardt, P.; Praefcke, K.; Psaras, P.; Stephan, W.; Turgay, K. Chimia 1986, 40, 360–362.
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Figure 1. Structures of the investigated myo-inositol alkyl monoesters rac-1a-d and alkyl monoethers rac-2a,c,d6 (a: n=9, b: n=10, c: n=11, d: n=12).
2.2. Mixtures of Regioisomeric Inositol Esters. The synthesis of regioisomeric mixtures of inositol monoesters was carried out as a transesterification reaction of myo-inositol (6) with different methyl alkanoates using sodium methanolate as the base (Scheme 2) and steadily distilling off the formed methanol as azeotrope with benzene.11 The analysis of the crude products show that under the applied reaction conditions only traces of di-, tri-, or higher substituted esters are formed while the main part is unreacted myo-inositol. Since the polarity of the desired monoesters is sufficiently different from that of unsubstituted myoinositol as well as from that of any higher substituted derivatives, the monoesters could be separated by column chromatography from the other components of the crude products. In this way, the mixtures A-D were obtained in modest yields and purified but not separated into the regioisomers. The composition of the mixtures was investigated on the example of mixture B by integration of characteristic NMR resonances. According this analysis, all regioisomers form with an almost equal distribution; the details are given in the Supporting Information. Since the 1-substituted and the 4-substituted regioisomers 1 and 10, respectively, occur as two enantiomers, that is, with a double probability, they appear with approximately 2-fold excess. The synthetic procedure described in the Supporting Information is not completely optimized. Rather, it should be seen as a proof of concept that such inositol surfactants could be produced in a technical quality in a short reaction sequence without an expensive protecting group strategy.
3. Thermotropic Liquid Crystals The phase behavior of the inositol amphiphiles rac-1 and rac-2 was investigated by means of polarizing microscopy (POM) and differential scanning calorimetry (DSC) and compared with the literature known alkyl-β-D-glucosides (11) and alkyl-β-D-maltosides (12) shown in Figure 2. The specification of the equipment used for these measurements is given in the Supporting Information. The “unusual” melting behavior of long chain alkyl-β-D-glucosides as, for example, 11 was already recognized by Fischer and Helferich29 in 1911, but it was only 1938 when Noller and Rockwell30 interpreted it correctly as a phase transition into the liquid crystalline state of matter. A comprehensive investigation of the lyotropic and thermotropic properties of various glucosides and maltosides is given in refs 31 and 32. The results are summarized in Table 1. All inositol monoethers and monoesters investigated here form a smectic A (SmA) phase with melting temperatures of ∼120 to ∼130 C. Between the microscopic slides, the pentols rac-1 and rac-2 strongly tend to (29) Fischer, E.; Helferich, B. Ann. Chem. 1911, 383, 68–91. (30) Noller, C. R.; Rockwell, W. C. J. Am. Chem. Soc. 1938, 60, 2076–2077. (31) Boyd, B. J.; Drummond, C. J.; Krodkiewska, I.; Grieser, F. Langmuir 2000, 16, 7359–7367. (32) Laughlin, R. G. The Aqueous Phase Behaviour of Surfactants; Academic Press: London, San Diego, New York, Boston, Sydney, Tokyo, Toronto, 1996.
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Scheme 1. Preparation of Homologous Series of Regiochemically Defined Inositol Monoesters rac-1a-d and Monoethers rac-2a,c,d (a: n=9, b: n=10, c: n=11, d: n=12)a
a Each entry (a-d) in each series and the respective yields refer to a discrete synthesis of one homologue derivative with a distinct chain length (n given in brackets).
Scheme 2. Synthesis of myo-Inositol Ester Mixtures A-D
Figure 2. Molecular structures of alkyl-β-D-glucoside (11) and alkyl-β-D-maltoside (12).
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align homeotropically. In uncovered samples, in such areas, often stepped drops are formed as shown in Figure 3 typical for a SmA phase of amphiphiles. The type of mesophase was identified with POM by means of contact preparations for which the compound rac-2d was used as reference.6 Noteworthy is the fact that the melting enthalpies of the monoethers rac-2 are almost two to three times higher than the melting enthalpies of the corresponding monoesters rac-1. This might be due to differences in the intermolecular hydrogen bond networks resulting in distinct crystalline phases. In order to investigate this fact in detail, X-ray studies of the crystal structures are in progress. At the phase transition, ΔG=ΔH - TΔS=0 and Langmuir 2009, 25(14), 7872–7878
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Article Table 1. Phase Transition Data of myo-Inositol Monoesters rac-1a-d and Monoethers rac-2a,c,da
pentol
Cnc
Cr
T [C]b
ΔH [kJ mol-1]
ΔS [J mol-1 K-1]
rac-1a rac-1b rac-1c rac-1d
9 10 11 12
• • • •
128/126.8 138/138.1 131/131.6 132/133.8
8.1 14.0 11.3 14.8
20.3 34.0 27.9 36.4
T [C]
ΔH [kJ mol-1]
ΔS [J mol-1 K-1]
Iso
ΔSΣ [J mol-1 K-1]d
209/205.0 218/217.9 216/219.1 226/228.3
5.9 2.4 1.7 1.8
12.3 4.9 3.5 3.6
• • • •
32.6 38.9 31.4 40.0
2.6 1.6 1.8
5.3 3.2 3.6
• • •
70.2 63.8 83.5
M inositol esters SmA SmA SmA SmA
inositol ethers rac-2a rac-2c rac-2d
9 11 12
• • •
122/124.3 120/123.2 124/127.6
25.8 24.0 32.0
64.9 60.6 79.9
SmA SmA SmA
222/218.9 225/220.8 221/221.7
pyranosides 10 • 80/73.7 11.0 31.7 SmA 140/132.1 1.7 4.2 • 35.9 12 • 81/78.4 17.9 50.9 SmA 146/149.1 2.3 5.5 • 56.4 10 • 96-100/SmA 207-208/• 12 • 101/SmA 245/247.6 2.3 4.4 • a Data with respect to rac-2d are taken from ref 6; phase transition data of 11b,d and 12b, except ΔS, are cited from ref 31. b Phase transition temperatures measured by POM/DSC. All values are determined on heating. For DSC measurements, the temperature values correspond to the maximum heat flow (peak) of the phase transition applying a heating rate of 5 K/min. c Total number of carbon atoms in the side chain including the carboxyl group in case of the monoesters rac-1a-d. d ΔSΣ=ΔSCrfM+ΔSMfIso.
11b 11d 12b 12d
Figure 3. (left and middle) Microscopy sample of rac-2a at 132.5 C in circular polarized light (uncovered, crossed polarizers with λ/4 plate) showing typical homeotropic alignment and stepped drops; (right) B^ atonnets appearing at the clearing temperature on cooling from the isotropic melt.
hence ΔS=ΔH/T. Note that the clearing enthalpies of both monoethers and monoesters are very similar and that their melting and clearing temperatures are in the same range. Together with the assumptions that the densities of the respective phases are alike and that the isotropic melts of these compounds are of comparable order, it follows that the crystal phases of the inositol ethers are more stable than those of their ester analogues. Finally, all mesophases cleared into the isotropic liquid at about 205-220 C, where also partial decomposition was observed. The clearing enthalpy is in the typical range for SmA phases of amphiphiles.33 The comparably high clearing enthalpy of 5.9 kJ mol-1 for rac-1a is compensated by a lower melting enthalpy in such a way that the total entropy change over all phase transitions, from the crystalline phase to the isotropic phase, is similar to the next odd homologue rac-1c. On the basis of these values, it can be concluded that the SmA phase of rac-1a is slightly more ordered than the SmA phase of the other inositol amphiphiles. Nevertheless, contact preparations clearly prove the similarity of the mesophases. Furthermore, for all amphiphiles rac-1a-d and rac-2a,c,d, a pronounced odd/even effect of the thermal values is observed. In comparison to the glucosides 11, the inositol ethers and -esters rac-1 and rac-2 show remarkably higher transition temperatures, (33) Vill, V.; von Minden, H. M.; Koch, M. H. J.; Seydel, U.; Brandenburg, K. Chem. Phys. Lipids 2000, 104, 75–91.
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while the clearing temperatures of the maltosides 12b and d are in the same range like those of the inositols. The transition enthalpies of the glucosides 11 are in the same range as those for the inositol ethers rac-1. All the discussed carbohydrate amphiphiles form a SmA mesophase as expected for single headgroup, single tail amphiphiles.19,22,31 On cooling, rac-1a-d and rac-2a,c,d solidify in a glassy state as it is often observed for carbohydrate amphiphiles. Bringing the organic glass phase of the monoester rac-1d in contact with water, one almost immediately observes a liquid crystalline phase with an unstructured texture even at room temperature. By POM, this has been identified as a water-swollen SmA phase, which, in turn, may be regarded as a lamellar phase (LR) with very high concentrations of the amphiphiles. This example again illustrates the smooth transition from a thermotropic SmA to the corresponding lyotropic LR phase in the case of surfactants32 which can be explained via the overall topology of the mesophase, remaining the same in the thermotropic and lyotropic cases. However, in contact preparations of the monoether rac-2d with water, this process does not occur or is at least strongly retarded. The glass phase of the monoether rac-2d remains unchanged under similar conditions over a long period of time. As a glass is a metastable nonequilibrium state, its behavior is often unpredictable. The observations discussed above can thus only be controlled by the “arrested” hydrogen bond network of the amphiphiles in the glass DOI: 10.1021/la900664r
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state in combination with the delicate hydrophilicity and hydophobicity balance of the ethers or esters, respectively. Note that contact preparations of water with crystals of rac-1d and rac-2d did not reveal any lyotropic mesophase formation during the measuring time of 60 min. Detailed phase studies of the respective binary systems are necessary to clarify the complex lyotropic behavior of the inositol esters of rac-1a-d. Unfortunately, due to the multistep syntheses of rac-1a-d the small amounts obtained were not sufficient for a detailed study of binary mixtures, yet. The thermal behavior of the inositol monoester mixtures A-D is complex and described, by way of example, in the following for mixture D. As could be expected for a mixture of compounds, DSC shows several distinct endothermal transitions at different temperatures during the first heating. According to the accompanying microscopic investigations during this first heating, thermomesomorphic behavior is observed from about 150 C on in the partly molten mixture. Thus, the various endothermal DSC transitions may be attributed to melting processes of different types of crystals obtained from the crystallization procedure out of the chromatography solvent (see experimental part in the Supporting Information for details). The highest melting temperature detected during the first heating is 212.8 C. Cooling the completely molten mixture D from the isotropic liquid, one observes recrystallization at about 170.3 C with a pronounced recrystallization enthalpy. This must be interpreted as the main part of the mixture which recrystallizes at once at this temperatue, probably in a type of mixed crystal of advantageous composition. A small part remains as molten liquid (with a different composition of the regioisomers) which then crystallizes at lower temperatures. Heating the crystallized sample, one observes these latter transitions in reverse order and thermomesomorphic behavior cannot be detected anymore.
Figure 4. Surface tension σ as a function of the surfactant concentration c for 1-O-dodecyl-myo-inositol (rac-2d), 1-O-dodecanoyl-myo-inositol (rac-1d), dodecyl-β-D-glucoside (11d),35 and dodecyl-β-D-maltoside (12d).36 Solid lines are Frumkin fits, while the dashed line is only a guide to the eye.
4. Surface Properties To compare the surface properties of the new amphiphilic inositol monoethers and monoesters with commonly used carbohydrate surfactants such as alkyl-β-D-glucosides (11) and alkylβ-D-maltosides (12) (Figure 2), we measured surface tension isotherms (σ(c)-curves) from which information on the adsorption layer can be extracted. The surface tensions were measured at room temperature by the Du No€uy ring method, using a STA1 tensiometer from Sinterface Technologies. Decyl-β-D-glucoside (11b) and decyl-β-D-maltoside (12b) were purchased from Glycon (Germany) and used as received. In the present study, we investigated the monoester rac-1b, the monoether rac-2d, and the monoester mixtures A-D, while the monoester rac-1d has been investigated in a previous study.3 For the sake of comparison, we also measured the σ(c)-curves of decyl-β-D-glucoside (11b) and decyl-β-D-maltoside (12b). All σ(c)curves are shown in Figures 4-6. The measured σ(c)-curves are fitted with the Frumkin isotherm,34 that is, 2 Γ Γ þ a0 σ ¼ σ 0 þ Γ¥ RT ln 1 Γ¥ Γ¥ 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 a0 is the interaction parameter. Fitting the experimental σ(c)-curves with (34) Frumkin, A. Z. Phys. Chem. 1925, 116, 466–84.
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Figure 5. Surface tension σ as a function of the surfactant concentration c for the four myo-inositol monoester mixtures A-D. All curves are fitted with Frumkin isotherms. See text for further details.
the Frumkin equation, one obtains the respective adsorption parameters, which are listed in Table 2. Note that only the adsorption parameters of the regiochemically defined compounds are given. An explanation of why the data of the mixtures are missing is given in section 4.2. 4.1. Regiochemically Defined Inositol Ethers and Esters. In Figure 4, the surface tension isotherms of 1-O-dodecyl-myoinositol (rac-2d) and 1-O-dodecanoyl-myo-inositol (rac-1d)3 are compared to those of dodecyl-β-D-glucoside (11d)35 and dodecylβ-D-maltoside (12d),36 respectively. The adsorption parameters are listed in Table 2. Looking at Figure 4, one sees that the σ(c)curves of rac-1d and rac-2d lie between those obtained for 11d and 12d. Unfortunately, a more detailed analysis is not possible, as the critical micelle concentration (cmc) values of the inositolbased amphiphiles could not be determined due to their low solubilities. Aqueous solutions of both myo-inositol surfactants went turbid at concentrations of 6 10-5 mol L-1 (rac-1d) and 210-5 mol L-1 (rac-2d). This shows that both compounds have a (35) Wydro, P.; Paluch, M. Colloids Surf., A 2004, 245, 75–79. (36) Buchavzov, N.; Stubenrauch, C. Langmuir 2007, 23, 5315–5323.
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Figure 6. (left) Surface tension σ as a function of the surfactant concentration c for the mixture B, decyl-β-D-glucoside (11b), and decyl-β-Dmaltoside (12b). Due to the small amount of mixture B the measurement of the complete surface tension curve was not possible. (right) Surface tension σ as a function of the surfactant concentration c for the mixture D, dodecyl-β-D-glucoside (11d),35 and dodecyl-β-D-maltoside (12d).36 Due to solubility problems of mixture D measurements of the complete surface tension curve were not possible. Solid lines are Frumkin fits, while the dashed line is only a guide to the eye. Table 2. Adsorption Parameters of 11b, 12b, 11d, 12d, rac-1d, and rac-2d
low solubility with the monoether being even less soluble than the corresponding monoester. The lower solubility of rac-2d is also reflected in the higher stability, that is, the higher melting temperature and enthalpy (Table 1), of its crystalline phase compared to rac-1d.37 Not being able to determine the cmc means that the values for the maximum surface concentration Γ¥ and the minimum headgroup area Amin given in Table 2 for rac-1d and rac-2d are only approximations. Despite the different solubilities, the surface activities of rac-1d and rac-2d seem to be similar. A further analysis is not possible with the surface tension data. It would be interesting to measure surface pressures in a Langmuir trough to get information on the packing and orientation of the two new amphiphiles in a surfactant monolayer. 4.2. Mixtures of Regioisomeric Inositol Esters. For economically attractive access to inositol surfactants, a one-pot, onestep synthesis was applied which leads to mixtures of all possible regioisomers. The four mixtures of monosubstituted inositol esters obtained in this way were studied with respect to their behavior in aqueous media. The surface tensions σ as a function of the surfactant concentration c for the mixtures A-D are shown in Figure 5. The surface tension curves agree with the general trends observed for homologous series of surfactants. They are shifted to lower concentrations with increasing hydrophobic chain length
of the surfactant molecules. This corresponds to Traube’s rule,38 which states that the surface activity of a homologous series of surfactants increases with increasing hydrophobic chain length. The curves have very similar slopes, especially those of mixture A and B on the one hand and those of mixture C and D on the other hand. These similarities indicate similar interfacial adsorption behavior. The determination of the cmc of the mixtures C and D was not possible because the surfactants were again not soluble enough. The cmc values for mixture A and B could not be determined either, as there was not enough of these surfactants to make a solution at concentrations higher than those seen in Figure 5. A calculation of maximum surface concentrations Γ¥ and minimum areas per molecule Amin was not carried out, as the inaccuracies would be too high and may lead to wrong conclusions. However, qualitative comparisons with the respective sugar-based surfactants are possible and will be made in the following. In Figure 6, the mixtures B and D are compared with the corresponding alkyl-β-D-glucosides (11) and alkyl-β-D-maltosides (12) of same chain length. Comparing the results obtained for mixture B with those of the corresponding sugar surfactants, namely, decyl-β-D-glucoside (11b) and decyl-β-D-maltoside (12b), one sees that the cmc values of all three amphiphiles are very similar. Similar cmc values for the three surfactants are expected, as they have the same hydrophobic chain length. The main difference is the higher σcmc value of 12b compared to the values of the two other surfactants indicating that the size of the myo-inositol unit is comparable to the glucoside group rather than to the maltoside group. Further support for the similarity between an inositol unit and a glucose unit is the slope of the respective σ(c)-curves: while the slope of the σ(c)-curve obtained for the mixture B is similar to that obtained for 11b, a different slope was obtained for the σ(c)curve of 12b. Similar slopes can be translated in similar headgroup areas! In the right panel of Figure 6, the σ(c)-curve of mixture D is shown once again and, for the sake of comparison, is plotted together with the results obtained for the corresponding sugar surfactants, namely, dodecyl-β-D-glucoside (11d)35 and dodecyl-
(37) We improved the solubility of inositol-based amphiphiles significantly by introducing ethylene oxide units into the hydrophilic headgroup which, in turn, led to a new and very promising class of surfactants.4
(38) Holmberg, K.; J€onsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons Ltd.: West Sussex, 2003; p 359.
σcmc Γ¥ Amin a0 surfaccmc tant [10-3 M] [mN m-1] [10-6 mol m-2] [nm2] [mN m-1]c 11b 2.2 29.4 3.8 0.44 16.9 12b 2.0 36.4 3.3 0.51 2.5 a 0.17 29.0 5.0 0.33 11d 36 0.15 34.9 4.2 0.40 3.1 12d b b 4.9 0.34 24.3 rac-1d3 b b 4.3 0.39 21.3 rac-2d a Adsorption parameters are taken from ref 35 and not calculated from a Frumkin fit. b cmc and σcmc could not be determined due to limited solubilities. c Note that usually the dimensionless interaction parameter a of the Frumkin isotherm is given. It holds a0 =aΓ¥ RT.
Langmuir 2009, 25(14), 7872–7878
DOI: 10.1021/la900664r
7877
Article
β-D-maltoside (12d).36 Comparing the left and right panels of Figure 6, one sees completely different trends. We have seen that mixture B behaves similar to the corresponding glucoside 11b, while mixture D seems to behave similar to the corresponding maltoside 12d. According to the right panel of Figure 6, the slopes of the σ(c)-curves measured for the mixture D and the maltoside surfactant 12d are similar, which would mean that the headgroup areas of these two surfactants are similar. This, however, is not very likely except if the synthesis had led to a significant amount of “di-inositol” units. A final explanation can thus not be given without a more detailed analysis.
5. Conclusion We synthesized new myo-inositol monoesters and monoethers as well as myo-inositol monoester mixtures. The pure compounds display a thermotropic SmA phase in the temperature range of ∼120 C to ∼220 C. In contact with water, a lyotropic liquid crystalline lamellar phase (LR) was observed for 1-O-dodecanoylmyo-inositol (rac-1d) only. Measurements of the surface activity of the new amphiphiles revealed a modest solubility in water which partly prevented the determination of the cmc. However, the obtained surface tension curves and the extracted adsorption parameters are in very good agreement with those obtained for other nonionic carbohydrate-based surfactants, namely, the alkyl-β-D-glucosides and alkyl-β-D-maltosides. The investigations
7878 DOI: 10.1021/la900664r
Blunk et al.
of the liquid crystallinity and the surface activity showed that the water solubility of myo-inositol esters is slightly higher than that of the corresponding ethers. Our results show that the new inositol-based surfactants behave very similar to other carbohydrate derivatives with respect to their supramolecular properties while having a much higher stability. Therefore, we will continue designing and investigating new inositol-based surfactants in order to explore structureproperty relationships of this new class of amphiphiles. In our recent work, we synthesized and characterized partly fluorinated inositol-based surfactants39 which demonstrates the versatility of our synthetic approach. Acknowledgment. Part of the work was funded by the European Community’s Marie Curie Research Training Network “Self-Organisation under Confinement (SOCON)”, Contract Number MRTN-CT-2004-512331. We thank Dr. Sandeep Patil for measuring the surface tensions of β-decylmaltoside. Supporting Information Available: Details of the synthesis, purification, and characterization of all compounds. This material is available free of charge via the Internet at http:// pubs.acs.org. (39) Bongartz, N.; Patil, S.; Stubenrauch, C.; Blunk, D. Publication in preparation.
Langmuir 2009, 25(14), 7872–7878
