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Synthesis and Properties of Nine New Polyhydroxylated Surfactants F. M. Menger,* K. K. Catlin, and X. Y. Chen Department of Chemistry, Emory University, Atlanta, Georgia 30322 Received July 28, 1995. In Final Form: October 25, 1995X The synthesis of nine new surfactants with multiple hydroxyls on the headgroup is described in detail. Comparisons of the cmc values with chemical structures lead to useful generalizations (e.g. polyhydroxylation aids micellization owing, probably, to hydrogen bonding among the headgroups in the Stern region). None of the compounds, unfortunately, is able to induce oil/water (o/w) microemulsions in the absence of an additional cosurfactant; simple and generally useful three-component o/w microemulsions remain a worthy goal. An X-ray analysis reveals how one of the nine surfactants, with a large headgroup and short tail, packs in the solid state.
Introduction The art of synthetic organic chemistry allows access to a host of new surfactants whose properties are, for all intents and purposes, unpredictable with our current state of knowledge. One might construct, for example, surfactants whose hydrocarbon chains are altered by the presence of heteroatomic substituents (e.g. -NO2 groups), rigidifying linkages (e.g. cyclopropyl units within the chains), or unusual branching groups (cyclohexyl or tertbutyl). Similarly, the headgroups can be endowed with diverse structural features (e.g. -N(C2F5)3 or -N(CH2CH2CH2CO-2)3). A litany of polar functionalities and metals can also be incorporated into the headgroups. The point here is that vast numbers of structural combinations, most of them as yet unexplored, provide rich opportunities to the surfactant chemist willing to carry out the preparative work. It is with this thought in mind that the research described herein originated. Actually, we had a more specific goal than simple testing of new surfactants, important though this might be. Our goal was related to the fact that microemulsions are typically prepared with the aid of a surfactant plus an alcoholic cosurfactant. The question thus arose as to whether a hydroxyl-bearing surfactant might be able to induce microemulsion formation in the absence of any cosurfactant. In effect, the surfactant and cosurfactant would be combined into a single molecule. This single molecule could, it was hoped, microemulsify water and oil. As it turned out, none of the nine new polyhydroxylated surfactants synthesized in our work functioned as planned. Yet the undertaking was still considered worthwhile because new synthetic strategies were developed and because the micellization of the new surfactants and conventional surfactants could be compared. The nine new polyhydroxylated surfactants are shown in Scheme 1. Two of the three sets have cationic charges, the other is neutral. Series 1 has two hydroxyls, whereas series 2 and 3 have 10 hydroxyls. Series 2 and 3 have amide bonds with and without hydrogen-bonding donor sites (i.e. a -NH group), and the chain lengths for series 1 and 2 vary from 8 to 16. Results and Discussion Known diol surfactants 4 and triol surfactants 5 were synthesized (for comparison purposes) according to the routes shown in Figure 1. Since previous preparations of these compounds have appeared mainly in the patent X Abstract published in Advance ACS Abstracts, February 15, 1996.
Figure 1. Synthesis of compounds 4 and 5. Chains have a total of 12 carbons. +
CH3(CH2)11 N (CH2CH2OH)2Cl–
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CH3(CH2)11 N (CH2CH2OH)3Cl–
CH3 4
5
literature or in rather obscure journals,1 we give the synthetic details in the experimental section. Figure 2 shows the synthesis of the series 1 surfactants. These materials were designed with the idea of extending the distance between the cationic nitrogen and the hydroxyls, (1) Weiland, B.; Haage, K. Abh. Akad. Wiss. DDR, Abt. Math., Naturwiss., Tech. 1986 (Pub. 1987), 595; Chem. Abstr. 1988, 108, 74772y. Haage, K.; Weiland, B.; Lenz, M. Ger. (East) DD Patent 219 478, 1985; Chem. Abstr. 1986, 104, 33758x. Limanov, V. E.; Ivanov, S. B.; Kruchenok, T. B.; Tsvirova, I. M. Khim.-Farm. Zh. 1984, 18, 703; Chem. Abstr. 1985, 102, 5637x. Jerzykiewicz, W.; Krasnodebski, Z.; Juzon, J.; Bekierz, G.; Szewczyk, H.; Atamanczuk, B.; Korek, A.; Naraiecki, B. Pol. PL Patent 112 243, 1981; Chem. Abstr. 1982, 96, 142252q. Bryden, D. W.; Connor, D. S. African Patent 76 03 959, 1977; Chem. Abstr. 1977, 87, 203418m. Limanov, V. E.; Epshtein, A. E.; Skvortsova, E. K.; Afef’eva, L. I.; Gleiberman, S. E.; Volkova, A. P. Pharm. Chem. J. (Engl. Transl.) 1976, 10, 55; Khim.-Farm. Zh. 1976, 10, 63. Dudzinski, Z. J. U.S. Patent 3 732 312, 1973; Chem. Abstr. 1973, 79, 4993e. Baird Chemical Industries, Inc. Br. Patent 1 130 905, 1968; Chem. Abstr. 1969, 70, 19567g. Yamamoto, T.; Sumida, S.; Takahashi, T. Kogyo Kagaku Zasshi 1966, 69, 2156; Chem. Abstr. 1967, 67, 7032f. Komori, S.; Sakakibara, S.; Fujiwara, A. Koˆ goyoˆ , Kagaku Zasshi 1957, 60, 908; Chem. Abstr. 1959, 53, 11216c.
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Figure 3. Synthesis of compound 2.10
Figure 2. Synthesis of series 1.2-9 Scheme 1
Figure 4. Synthesis of compound 3.10 Table 1. Critical Micelle Concentrations for Polyhydroxylated Surfactantsa
a
thereby imparting more interfacial flexibility at the headgroup region. Two key factors of Figure 2 are worthy of note. (a) A Williamson ether synthesis permitted easy variation of the chain length via different alkyl iodides. (b) Reduction of a nitrile produced the primary amine which was subsequently quaternerized into a cationic nitrogen. Figures 3 and 4 give the synthesis of series 2 and 3, respectively. In both cases, attachment of two sugars, with five hydroxyls each, relied upon simple ester aminolyses by D-glucamine or N-methyl-D-glucamine in K2CO3/DMSO. With the nine polyhydroxylated surfactants in hand, we were ready to investigate their propensity to micellize. The critical micelle concentrations (cmcs) of nine hydroxylated surfactants are listed in Table 1. The following generalizations can be tentatively deduced from the data. (a) Comparison of 1a with 4 and 5 (all of which have roughly the same cmc despite the shorter chain length of 1a) shows that exposing the quaternary nitrogen in 1a
surfactant
chain length
cmc (M)
4 5 1a 1b 1c 2a 2d 3a 3b
12 12 8 12 16 8 16 16 16
1.4 × 10-2 1.2 × 10-2 1.3 × 10-2 2.8 × 10-3 3.8 × 10-4 1.4 × 10-2 5.9 × 10-6 1.1 × 10-3 2.8 × 10-4
Determined by tensiometry at 23 °C.
promotes micellization. (b) Similarly, comparison of 2a with 4 and 5 shows that polyhydroxylation aids in micellization owing, probably, to hydrogen bonding among the headgroups in the Stern region. (c) The much lower cmc for 2d relative to 3b (both of which have chain lengths of 16) suggests that the presence of charge inhibits micellization. This can be explained by the charge imparting greater solubility to the monomer and greater electrostatic repulsions within the Stern region. (d) The 3-fold lower cmc of 3b relative to 3a is not understood, but it may be related to the greater monomer solubility when an amide NH is present. One must always keep in mind that micellization is an equilibrium between monomer and micelle, and that structural factors can affect the cmc by perturbing either side (or, possibly, both sides) of the equilibrium. None of the surfactants in Table 1 were found capable of forming oil/water (o/w) microemulsions without the aid of an additional cosurfactant. There is no reason why, in
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Figure 5. X-ray structure of compound 1a.
principle, this should not be feasible. Certain o/w microemulsions, for example, are produced by AOT alone. Note that there are two models explaining the need for a cosurfactant. (a) The alkanol intrudes into the interfacial region, thereby affecting its curvature.2 (b) The alkanol lowers the interfacial tension and rigidity by adsorbing preferentially into a curved interfacial region.3 It should be possible to mimic the effect, whatever the model, using side chains attached directly to the surfactant itself. In fact, such a system would be entropically favored over conventional cosurfactants which become restricted in their mobility as they adsorb. Generally useful surfactant-cosurfactant “conjugates” remain, however, an elusive goal. As this paper was being prepared, there appeared a detailed study of the role of n-alkanol cosurfactants in preparing microemulsions with alkyl monoglucosides.4 With the aid of phase behavior data, the authors were able to deduce recipes for choosing the best alkanol for a given monoglucoside. Information of this sort will be valuable in designing surfactant-cosurfactant conjugates in the future. Surfactants tend to be waxy solids or powders, so we took the opportunity of obtaining an X-ray structure when 1a happened to provide X-ray quality crystals. The compound is interesting in that its headgroup is large and multifunctional, whereas its chain length is short (only eight carbons). As seen in Figure 5, the surfactant packs in the solid state with considerable interdigitation. No hydrogen bonding between the headgroups within a “bilayer” is evident. (2) Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601. (3) de Gennes, P.; Taupin, C. J. Phys. Chem. 1982, 86, 2294. (4) Kahlweit, M.; Busse, G.; Faulhaber, B. Langmuir 1995, 11, 3382.
Experimental Section Complete synthetic procedures including all analytical data (NMR, mass spectrometry, and elemental analyses) are given in a 10 page text (see supporting information). X-ray Structure. Compound 1a was recrystallized from acetonitrile to isolate a colorless needle of the dimensions 0.04 × 0.20 × 0.54 mm. This crystal was mounted on a quartz fiber such that the longest crystal dimension was parallel to the fiber axis, and the data were collected at room temperature. The unit cell parameters were determined on a Siemens P4 RA automated diffractometer using Cu KR radiation. A total of 31 reflections were machine centered and used in the least-squares refinement of the lattice parameters and orientation matrix. The unit cell parameters obtained were a ) 8.035(2) Å, b ) 8.255(2) Å, c ) 32.522(2) Å, V ) 2157(15) Å3, dcalcd ) 1.090 g cm-3, F(000) ) 784, and Z ) 4, and the space group was determined to be P212121. The intensity data were collected by the ω scan technique with a variable scan rate in ω of 5-60 deg min-1. Check reflections, which were monitored after every 100 scans, showed no significant change during the course of data collection. Lorentz and polarization corrections were made in the usual manner, and semiempirical absorption correction was also applied to the data. Of the 2361 reflections collected where 2.0° e 2θ e 113.5°, 1649 were found to be unique with 1 > 4σ(I). The structure was solved by direct methods using Siemens SHELXTL IRIS. Using anisotropic refinement of all non-hydrogen atoms except for those carbons which were disordered, the hydrogens were fixed into position and held isotropic. The final discrepancy index and weighted discrepancy index were R ) 7.10% and Rw ) 9.72%, respectively, where Rw ) ∑ω1/2(Fo - Fc)/∑ω1/2.
Acknowledgment. This work was supported by the Army Research Office. Supporting Information Available: Full X-ray data on surfactant 1a and synthetic procedures on all compounds (21 pages). Ordering information is given on any current masthead page. LA950634X
