Letter pubs.acs.org/Langmuir
Mechanically Induced Gel Formation Jochem T. van Herpt, Marc C. A. Stuart, Wesley R. Browne,* and Ben L. Feringa* Center for Systems Chemistry, Stratingh Institute for Chemistry and Zernike Institute for Advanced Materials, Faculty of Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands S Supporting Information *
ABSTRACT: Mechanical triggering of gelation of an organic solution by a carbazole-based bisurea organogelator is described. Both the duration of the mechanical stimulation and the gelator concentration control the gelation process and the characteristics of the gel obtained.
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INTRODUCTION Stimuli-responsive supramolecular systems have seen remarkable growth in interest in recent years, not least in the field of low-molecular-weight gelators (LMWG)1 exhibiting intrinsic responsiveness, including pH,2 light,3 redox,4,5 and enzymatically6−9 driven switching of gel properties. A potentially highly important stimulus in terms of application is mechanical force (e.g., gelation caused by stirring or shaking). Although several examples have been reported in which the morphology and rheology of an aggregate are influenced by mechanical stimuli,10−15 mechanically triggered gel formation under ambient conditions is a field that has received relatively little attention to date.16−20 Gelation occurring after ultrasound treatment has been described.21−24 In these systems, the origin of gelation is not due to the mechanical aspects of ultrasound treatment but is attributed to either chemical modification of the sample or the fact that the amount of monomer in solution increases to above the critical gelation concentration (cgc) only after ultrasound treatment. Here we show that a transition from a clear solution to a gel can be triggered by a mechanical force. We report the remarkably selective gelation of DMSO and DMF by 1,1′-(9tetradecyl-9H-carbazole-3,6-diyl)bis(3-ethylurea) (1) upon mechanical induction under ambient conditions (Scheme 1).
Scheme 1. Structure of Gelator 1 and Its Gelating Behavior
agitation gave rise to more translucent gels. Notably, heating the sample yields the original solution, which is stable if left undisturbed but forms gels again upon further mechanical stimulation. Gelator 1 forms stable gels in DMSO upon shaking or stirring at concentrations above 20 mg/mL. In the concentration range of 10 to 20 mg/mL, gels could be obtained only by ultrasonication of the solution, whereas concentrations above 40 mg/mL yield gels spontaneously (Figure 1a). At concentrations of 1 between 10 and 40 mg/mL, heating the gels resulted in translucent solutions that, depending upon the temperature reached, will return to either a gel or a clear solution upon cooling to room temperature. This behavior is summarized in Figure 1b. This behavior is rationalized tentatively as follows. When the gel is heated to
