Defining surface properties of fluorous bis(1H-1,2,3-triazol-4-ylmethyl) ethers
Graphical abstract
Surface tension measurements and atomic force microscopy are utilized to derive information about the way in which new, highly fluorinated (fluorous) bis(1H-1,2,3-triazolylmethyl) ethers self-assemble, and how this relates to their surface active properties. Morphology and surface activity are influenced by molecular composition and concentration.
Introduction
Fluorous surfactants are more efficient self-aggregators than non-fluorinated counterparts [1], [2], [3], [4], preferring to generate more stable, well-organized three-dimensional supramolecular assemblies, such as multi-layered structures [5], [6], micelles [7], [8], [9] and tubules [10], [11], [12]. Surfactant performance is often measured by determining the ability to lower interfacial surface tension, which can be further separated into efficiency and effectiveness [13]. Surfactant efficiency is determined by the surfactant concentration required to reduce surface tension by a given amount – a surfactant is more efficient if less is required – while effectiveness is the maximum surface tension reduction achieved at the surfactant critical aggregate concentration (CAC), with a more effective surfactant changing the surface tension to a greater extent [13]. Surface tension measurements have previously demonstrated that in non-fluorous surfactants, the surfactant efficiency increases as the hydrophobic chain length increases [14], [15]. Meanwhile, molecules with perfluoroalkyl chains generally exhibit greater surfactant efficiency than hydrocarbon chains [16], [17], [18]. It is argued that this arises because polyfluoroalkyl chains possess relatively low cohesive energy density and weak intermolecular interactions [16]. The length of perfluoroalkyl chains can also influence the architecture of self-assembled aggregates [19], [20]. As an example, a comparison of the nanostructures of hybrid surfactants C8F17C20H41 and C10F21C16H33, by atomic force microscopy (AFM), demonstrated that the compound with the longer perfluoroalkyl chain formed large crystalline-like dendritic structures, instead of the bilayer aggregation of the shorter perfluoroalkyl analogue [21].
Our group has previously evaluated a series of triazolyl and tetrazolyl-based fluorous surfactants by measuring their effect on surface tension of xylene solutions [22], [23]. The presence of the heterocyclic core was of particular interest because it provides potential for the molecules to form discotic self-assembled structures [24] through π–π interactions. The actual structures in the aggregates and any tertiary features, such as multilayers, or sheet or helical assemblies were not explored. More recently, attention has turned to bistriazoles [22], [25], [26], [27].
In this paper, a series of bistriazoles 1–8 (Fig. 1) with fluorous-lipophilic and fluorous-hydrophilic substituent combinations were synthesised through well-established copper-catalyzed azide–alkyne cycloaddition (CuAAC) reactions [22]. The surfactant properties of the bistriazole compounds 1–4 and 6–8 were evaluated through the use of surface tensiometry, with particular emphasis on the effect of the second triazole ring through comparisons with the previously reported data for monotriazole derivatives 9–14. AFM was used for the first time to examine the self-assembled aggregates derived from direct casting of selected examples of the hybrid fluorous bistriazoles in an effort to characterise the aggregates.
Section snippets
Materials
Fluorous triazoles 1–4 were available from previous studies [22]. Azide precursor 15 was synthesised from commercially available methoxydiethylenoxyethanol via mesylate 16 [28] following literature procedures [19]. Alcohol 21 was synthesised, using modified CuAAC conditions, from the azide 15 and propargyl alcohol in t-butanol. The dry alcohol 21 was treated under an inert atmosphere with sodium hydride, then propargyl bromide, to give alkyne 22. Triazoles 5–8 were synthesised using in situ
Surface tension measurements
Despite the expectation that decreases in surface tension would occur most rapidly with increasing concentrations of, and longer perfluoroalkyl chain length in, the surfactants this was not observed for fluorous-lipophilic compounds 1–4 (Fig. 3). Instead there was somewhat chaotic and unpredictable behaviour, with some crossover of the order of activity as concentrations increased. It was previously reported [23] that the directly related monotriazoles 9–11 exhibited a similar inconsistency
Conclusions
The data obtained demonstrate that bistriazolyl ethers can function as surfactants, and are capable of self-assembling to form a variety of three-dimensional structures. Surface tension measurements of the bistriazolyl ethers demonstrate that the additional triazole ring and/or presence of a hydrophilic substituent enhance the effectiveness of the compounds compared to the monotriazole derivatives. Microscopic examination of drop cast films of the bistriazolyl ethers showed a variety of
Acknowledgements
The authors acknowledge Dr Celine Heu and Mr Lev Lewis of the Biomedical Imaging Facility (BMIF) within the Mark Wainwright Analytical Centre at the University of New South Wales for AFM support and helpful advice. Assistance from Dr Alex Wu (Department of Chemistry, University of Melbourne) for arranging the collection of surface tension measurements is also greatly appreciated.
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Cited by (1)
Titration characteristics of fluorous 1,2,3-triazol-4-ylmethyl ethers, bis(1,2,3-triazol-4-ylmethyl) ethers and bis(1,2,3-triazol-4-ylmethyl) amines
2016, Journal of Fluorine ChemistryCitation Excerpt :It should be noted that in preliminary studies there was significant dependence of the stoichiometry on the concentration of the base, with the stoichiometry dropping to <1: 1 (acid: base site) as the concentration of the bistriazoles 4–10 was increased. This suggested some degree of aggregation at the higher concentrations, consistent with known surfactant properties of related compounds [36]. In order to avoid the impact of such, the final Job plots analysis above and subsequent analyses of equilibrium constants (see Sections 2.2 and 2.3) were carried out under conditions where no such aggregation would be expected.