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Langmuir 2000, 16, 3214-3220
New Cationic Surfactants with Sulfonium Headgroups Bodo zu Putlitz, Hans-Peter Hentze, Katharina Landfester,* and Markus Antonietti Max Planck Institute for Colloids and Interfaces, Am Mu¨ hlenberg, 14424 Potsdam/Golm, Germany Received October 6, 1999. In Final Form: December 10, 1999 The synthesis of a new class of cationic surfactants with sulfonium headgroups is presented. The use of common and easily accessible reactants allows a broad variation of the molecular surfactant structures, i.e., the synthesis of not only linear homologues but also of bola- or star-shaped surfactants. Surface tension measurements were used to determine the critical micelle concentration (cmc) and the surface excess concentration Γ∞. The lyotropic phase behavior was investigated by polarized light optical microscopy. The influence of the hydrophobic chain length, the molecular structure of the headgroup, and the counterion on the surfactant properties is discussed.
Introduction Cationic surfactants are used for a number of stabilization problems and have found many technical applications. Because of the predominant negatively charged nature of natural colloids and surfaces, cationic surfactants form strong adsorption layers and hydrophobize the surfaces of these materials. Applications of cationic surfactants include softeners, cosmetic products, electro-dip coatings, and the stabilization of adhesive polymer latexes as well as in mining and paper manufacturing. Considering the widespread potential uses, it is worth noting that the headgroups of practically all known cationic surfactants are ammonium groups (prominent examples are cetyltrimethylammonium chloride and didodecyldimethylammonium bromide).1 This represents a serious restriction of cationic surfactants, because ammonium derivatives undergo Hofmann degradation, resulting in temperature sensitivity as well as the typical “amine odor” of cationic surfactants. In addition, amine derivatives are sensitive toward oxidation, which results in aging problems such as the yellowing of paper products or coatings based on these systems. In this paper, we therefore want to explore the potential of a different class of cationic surfactants based on sulfonium headgroups. The preparation of sulfonium salts is described in the literature.2,3 We expanded these reactions to synthesize surfactants. They are easy to make from simple, readily available chemicals by the ring opening of epoxy derivatives with thioethers, resulting in a large variety of new sulfonium surfactant structures. The synthesis route for the sulfonium surfactants is schematically described in Scheme 1. It is obvious that the characteristics of the sulfonium surfactants can be varied in different ways: (a) These surfactants can be modified by variation of the alkyl chain length R1 of the epoxide, which typically brings in the hydrophobic part of the surfactant. (b) These surfactants can be modified by variation of the alkyl chain lengths R2 and R3 as parts of the thioether. For industrial processes, this could be dimethyl sulfide or dodecylmethyl sulfide. For lab purposes, we recommend (1) Cationic Surfactants: Physical Chemistry; Rubingh, D. N., Holland, P. M., Eds.; New York, 1991. (2) German Patent 2,208,894. (3) Nagayama, M.; Okumura, O.; Yaguchi, K.; Mori, A. Bull. Chem. Soc. Jpn. 1974, 47, 2473.
working with thiodiethanole (R2 ) R3 ) C2H4OH), which does not smell and is easy to handle. (c) Because of the synthesis that starts from noncharged organic materials, the counterion X is usually an organic acid and can easily be modified. It can contain additional chemical functionalities (e.g., additional hydroxy groups) and can act as an assisting cosurfactant. Using the lines of epoxy chemistry, R1 does not necessarily have to be an alkyl chain but can also be an oligoethylene oxide, propylene oxide, or siloxane chain. The use of di- or multivalent expoxides leads directly to gemini surfactants or star-shaped surfactants (see Scheme 1b). The counterion modification is a nonclassical but very effective way to modify the surfactant properties, such as efficiency, effectiveness, and surfactant aggregation patterns. It was recently shown that the use of organic instead of inorganic counterions increases the area requirements of the surfactants due to counterion condensation on the interface, increasing the stabilization efficiency.4 Another effect of the counterion modification is that the packing parameter and related aggregate structures can be adjusted and regions of lyotropic mesophase formation can be manipulated. Experimental Section For the synthesis of sulfonium surfactants, (2-ethylhexyl)glycidyl ether, 1,2-octene oxide, 1,2-decane oxide, 1,2-dodecene oxide, 1,2-hexadecene oxide, 1,2-octadecene oxide (all Aldrich Co.), diepoxide end-functionalized polypropylene oxide with an average DP ) 14, triglycidyl tris(2-hydroxyethyl) isocyanurate (both BASF AG), Decanol Ex-411 (Nagase GmbH), bis(2hydroxyethyl) sulfide, diethyl sulfide, dimethyl sulfide, 2,2-bis(hydroxymethyl) propionic acid, acetic acid (all Aldrich Co.), and tetrahydrofuran (BASF AG) were used as received. Synthesis of Sulfonium Surfactants. In a typical reaction, 10 g of 1,2-dodecene oxide (0.0542 mol) were dissolved in 60 mL THF. The mixture was heated to the boiling point, and the stoichiometric amount of bis(2-hydroxyethyl) sulfide (6.52 g, 0.0542 mol) was added. The reaction vessel was kept under reflux for 1 h. After that time, the organic acid forming the counterion, e.g. 2,2-bis(hydroxymethyl) propionic acid (7.26 g, 0.0542 mol) and deionized water (1.66 g, 0.092 mol) were added. The mixture was stirred for another 3 h under reflux. The solvent was removed, and the product was dried under vacuum at room temperature. The purity of the product was (4) Antonietti, M.; Hentze, H. P. Adv. Mater. 1996, 8, 840.
10.1021/la991322n CCC: $19.00 © 2000 American Chemical Society Published on Web 02/04/2000
New Cationic Surfactants with Sulfonium Headgroups
Langmuir, Vol. 16, No. 7, 2000 3215 Scheme 1
a
b
controlled by 1H NMR and 13C NMR spectroscopy. The conversion and the reaction yield in all cases was almost quantitative (>98%), and side products could not be detected. Tensiometry. Surface tension measurements were performed using a K12 processor tensiometer (Fa. Kru¨ss), employing the DuNo¨uy ring method. The radius of the Pt-Ir ring (RI 12) was 9.545 mm with a wire radius of 0.185 mm. Each measurement was repeated 10 times, the obtained values were corrected according to Zuidema and Waters.5 To determine the cmc, the surface tension was measured as a function of increasing surfactant concentration. Therefore, a 1 g L-1 solution of the corresponding surfactant was stepwise added to 50 mL of deionized water. After each addition of the stock solution, the surfactant solution was stirred for 120 s before the surface tension was measured. All measurements were performed at 25 °C. Polarized Light Optical Microscopy. The lyotropic phase behavior of surfactant A18 was studied by polarized light optical microscopy (POM) using a Leica DM R optical microscope and a Linkam THM 600/S hotstage. To generate the binary phase diagram, aqueous surfactant solutions were prepared in 2-5 wt % interval. The samples were heated between two glass slides from room temperature (20 °C) up to 100 °C using constant heating rates from 2.0 to 0.1 °C min-1.
Results and Discussion Sulfonium surfactants with varying chain lengths, molecule architectures, and different counterions, namely, chloride, acetate (Ac), or the bulky bis(2-hydroxymethyl) propionate (DPA), were successfully synthesized. An overview of the different structures is given in Table 1. The hydrophobic chain is always a 2-hydroxy alkyl derivative with varying carbon numbers Cn from C8 to C18. In one case, a 2-hydroxypropyl-2-ethylhexyl ether was also used to introduce branching in the hydrophobic part of the surfactant. NMR characterization of the products was performed in D2O and CDCl3. Three regions in the spectra can easily be differentiated: the protons of CH3 (
