12660
J. Phys. Chem. B 2006, 110, 12660-12665
Nonionic Urea Surfactants: Formation of Inverse Hexagonal Lyotropic Liquid Crystalline Phases by Introducing Hydrocarbon Chain Unsaturation Darrell Wells,† Celesta Fong,*,† and Calum J. Drummond‡ CSIRO Molecular and Health Technologies, PriVate Bag 10, Clayton South, VIC, 3169, Australia, and CSIRO Molecular and Health Technologies, P.O. Box 184, North Ryde, NSW, 1670 Australia ReceiVed: December 20, 2005; In Final Form: March 27, 2006
The homo-interaction between urea moieties residing in close proximity to each other generally results in very strong intermolecular hydrogen bonding. The bifurcated hydrogen bonding exhibited by n-alkyl substituted ureas means that for those urea surfactants possessing medium and long hydrocarbon chain substituents the crystal to isotropic liquid melting point is high and the solubility in water is very low, compared to other similar chain length nonionic surfactants. In addition, saturated n-alkyl urea surfactants do not form lyotropic liquid crystalline phases in water. In this work the strong intermolecular hydrogen bonding of the urea headgroup has been ameliorated through the introduction of unsaturated hydrocarbon chains, viz., oleyl (cis-octadec-9enyl), linoleyl (cis, cis-octadec-9,12-dienyl), and linolenyl (cis, cis, cis-octadec-9,12,15-trienyl) with one, two, and three carbon double bonds, respectively. Unsaturation in the C18 urea surfactants lowers the melting point and promotes an inverse hexagonal phase, in oleyl urea-water and linoleyl urea-water systems, which is thermodynamically stable in excess water. As the degree of unsaturation is increased to three in linolenyl urea, there is a tendency for autoxidation/polymerization. The occurrence of an inverse hexagonal phase in the nonionic urea surfactant-water systems has been rationalized in terms of both local molecular and global self-assembled aggregate packing constraints.
Introduction Urea compounds can form linear hydrogen-bonding networks via a carbonyl group of one urea unit and two hydrogen atoms of a neighboring urea unit. This bifurcated hydrogen bonding can impose long-range rigidity, and supramolecular assemblies of urea and substituted urea molecules are well-known. As such, the urea moiety has played an important role in progressing the understanding of hydrogen bonding and hydrophobic interactions.1-21 In particular, the role of hydrogen bonds in condensed monolayers of long-chain alkyl ureas spread at the air-water interface has generated interest for more than 80 years.1-4,9-13 Specifically, polymorphism of n-octadecyl urea monolayers was first reported by Adam in 1922.1 Two temperature-dependent packing states were identified with switchable and reversible hydrogen-bond formation occurring with variation in temperature. More recently, Seki and co-workers14-19 have explored Langmuir monolayers and multilayers of substituted urea-azobenzene amphiphiles. The photoreactivity, film stability, and packing states of these surfactants were directly correlated to the degree of hydrogen substitution of the terminal nitrogen, which affected their ability to from bifurcated hydrogen bonds.14-19 Interestingly, Huo and co-workers20 have reported Langmuir monolayers with nontraditional architectures created from symmetrically disubstituted diacetylene urea molecules. Bifurcated hydrogen bonding was designed to be at the center of the molecule, which resulted in the unusual situation of hydrophobic tails being in contact with water.20 Wang et al.21 extended this work to make thermally stable nonlinear optical Langmuir-Blodgett films. * Author for correspondence. E-mail:
[email protected]. Tel: 613-95452608. Fax: 61-3-95452515. † Bag 10, Clayton South. ‡ P.O. Box 184, North Ryde.
The strong hydrogen bonding observed for insoluble monolayers of urea surfactants spread at the air-water interface and for films of urea-based surfactants deposited on substrates is also seen in the solid state as shown by a recent X-ray crystallographic study22 of n-alkyl ureas and by infrared spectroscopy studies of mono- and disubstituted ureas.5-8 Furthermore, plastic-like phases of the order-disorder type have been observed for several short to medium chain length n-alkyl ureas.22 These were similar to plastic phases observed for some long-chain fatty alcohols and fatty acids. The strong homo-association of the urea moiety clearly dominates the physicochemical properties of this class of compounds. Physical properties such as the melting point, solubility, and lyotropic behavior are heavily governed by such intermolecular forces. In particular, the melting point of n-alkyl ureas with alkyl chain length C4 to C22 are essentially independent of chain length.22-25 This contrasts with the melting-point behavior frequently observed for hydrocarbon compounds such as n-alkanes where the melting point increases with chain length.26 Moreover, until recently, lyotropic liquid crystalline phase formation had not been observed for nonionic substituted ureas. This is also a reflection of the dominance of the urea homo-interaction in many substituted urea systems. However, in a very recent communication,27 we have provided evidence of lyotropic mesophase formation in select urea surfactant-water mixtures. In the present work, we provide more details on how the strong intermolecular hydrogen bonding can be ameliorated by select modification of the hydrocarbon chain. The introduction of unsaturated hydrocarbon chains such as oleyl (cis-octadec9-enyl), linoleyl (cis, cis-octadec-9,12-dienyl), and linolenyl (cis, cis, cis-octadec-9,12,15-trienyl) with one, two, and three carbon double bonds, respectively, has been investigated. This strategy
10.1021/jp0574192 CCC: $33.50 Published 2006 by the American Chemical Society Published on Web 06/03/2006
Nonionic Urea Surfactants
J. Phys. Chem. B, Vol. 110, No. 25, 2006 12661 TABLE 1: Characteristic Infrared Frequencies of the C18 Urea Surfactants (cm-1) surfactant
environment
νNH
1-octadecyl urea solid KBr 3393, 3213 solid powder 3384, 3213 CHCl3a 3515, 3446, 3419 oleyl urea solid KBr 3395, 3210 solid powder 3394, 3212 CHCl3b 3515, 3445, 3421 CCl4c 3517, 3448, 3417 linoleyl urea solid KBr 3393, 3210 solid powder 3379, 3213 CHCl3d 3517, 3446, 3418, CCl4e 3526 linolenyl urea CHCl3f 3508, 3403
amide I amide II 1656 1655 1676 1655 1653 1676 1700 1656 1653 1675 1713 1678
1533 1534 1534 1531 1531 1533 1533 1534 1532 1540
3.19 × 10-3 M. b 3.22 × 10-3 M. c 4.34 × 10-3 M. d 3.24 × 10-3 M. e 4.37 × 10-3 M. f 4.37 × 10-3 M. a
Figure 1. Structural representations of the C18 urea-based surfactants with (from top to bottom) n-octadecyl urea, oleyl urea, linoleyl urea, and linolenyl urea.
was targeted at expanding the lateral packing and disrupting the order of the hydrocarbon chain region. The unsaturated hydrophobes are highly “kinked” because of their cis conformations and produce a relatively large chain volume with respect to the urea headgroup (Figure 1). In the case of oleyl urea and linoleyl urea, this approach succeeded in the promotion of urea surfactant self-assembly aggregates and, more specifically, the formation of an inverse hexagonal (HII) phase that was thermodynamically stable in excess water. This type of lyotropic phase behavior is relatively rare.
magnification). Each sample was cooled and heated through several cycles to accurately determine the phase behavior. Phase transitions were determined from the water penetration experiments. Water Penetration into Urea-Based Surfactants. Neat ureabased surfactant was melted between a microscope slide and cover slip and cooled to room temperature prior to addition of water at the edge of the cover slip. Capillary action drew the water between the two glass surfaces to surround the solidified material. The sample was heated at 2 °C/min or less in a Mettler FP82HT hot stage controlled by a FP90 central processor. The interaction of water with the solid urea-based surfactant was viewed using an Olympus IMT-2 microscope equipped with crossed polarizing lenses.
Experimental Section
Results and Discussion
General Syntheses of Saturated and Unsaturated UreaBased Surfactants. The urea-based surfactants were formed from the reaction of nitrourea with a long-chain amine using the method of Buck and Ferry.28 Nitrourea was synthesized by the method of Davis and Blanchard.29 The synthetic details are provided in the Supporting Information. Infrared Spectroscopy. Infrared Spectroscopy was performed using a BOMEM MB 101 from Extech Equipment Pty. Ltd. Spectra were recorded at room temperature in the 4000500 cm-1 range and were accumulated for 4 scans at a resolution of 4 cm-1. The neat material was examined as either pellets of compressed potassium bromide or a loose powder mixed with potassium bromide using the Harrick diffuse reflectance accessory. Samples were also examined in chloroform and carbon tetrachloride solution using a 0.1-mm-thick sodium chloride cell for which the background was taken into account. Differential Scanning Calorimetry (DSC). DSC was performed using a Mettler 3000 system at scan rates of either 2.5 or 0.1 °C/min. The temperature calibration ((0.3 °C) of the platinum thermometer was performed with ultrahigh purity n-hexane, water, and indium. The thermal calibration of the DSC instrument was performed by integration of the standard indium peak. The energies of the endotherms were calculated using the Mettler 3000 software. Binary Phase Behavior at Low Concentrations of UreaBased Surfactant. Surfactant urea-water mixtures were prepared by weighing an amount of solid into 12 mm screw cap tubes and adding water by weight. The heating rate was
