“Latent” Trialkylphosphine and Trialkylphosphine Oxide Organogelators Activated by Brønsted and Lewis Acids

Abstract

The weak bases tri-n-alkylphosphines (R3P, where R ) C14H29, C18H37) and phosphine oxides (R3PdO) have been reacted with the strong Lewis acid BF3 to form zwitterions (R3P + -OBF3or R3P + -BF3 -) and with Brønsted acids (pKa < 5) to form salts (R3P + -OH X -, where Xis several anion types, including chloride, p-dodecylbenzenesulfonate, and p-toluenesulfonate). The abilities of the zwitterions and some of the hydroxyphosphonium salts to gel a variety of organic liquids have been investigated and compared with those of the phosphine oxides (R3PdO, R ) C10H21, C14H29, C18H37). The gelating abilities of the zwitterions are similar to those of the phosphine oxides in that only liquids capable of donating protons or promoting ionic interactions could be gelled, and both are less efficient than the corresponding hydroxyphosphonium salts. Alcohols with short alkyl chains (<5 carbon atoms) were gelled preferentially by (C 18H37)3P + -OH X -(X -) chloride or p-toluenesulfonate), while those with longer alkyl chains were gelled better by the less polar p-dodecylbenzenesulfonate salt or (C18H37)3PdO. These observations suggest the participation of the solvent at the moment of network formation. Powder X-ray diffraction and temperature-dependent NMR measurements reveal that gel formation from the (C 18H37)3P + -OH Xsalts involves a competition among several concurrent processes, some of which are a consequence of salt dissociation during heating of sols. The one(s) that dominate determine the nature of the networks responsible for immobilization of the liquid components of the gels. Thus, some gel networks were comprised of (C 18H37)3PdO despite the fact that (C18H37)3P + -OH Clwas the added gelator and the same liquid was not gelled by the phosphine oxide alone! A hypothesis for the role of the acid in these gelation processes and models for molecular arrangements of the gelators in the neat solids and gel networks are presented.

A novel low-molecular-mass gelator with a redox active ferrocenyl group: Tuning gel formation by oxidation

“…The organogels from LMOGs not only are thermoreversible; some of them are even responsible to stimuli such light [13][14][15][16], sound [17], pH [15,18], host-guest interaction [19][20][21], charge transfer [22], complexation [22], oxidation/reduction [23], and even a combination of these [24,25]. To the best of our knowledge, light-and pH-responsive organogels are most commonly encountered in the supramolecular gels reported.…”

Supramolecular gels based on organic diacid monoamides of cholesteryl glycinate

“…The stimulus could be light [12][13][14][15][16][17], sound [18], pH [14,19], host-guest interactions [20][21][22], charge separation [23], complexation [23], or oxidative/reductive reactions [24,25], etc., and it can also be a combination of some of them. This distinct character of physical gels promises accessibility for designing and constructing sensors, actuators, and other molecular devices [26,27].…”

Insights into the mechanical properties of a silicone oil gel with a ‘latent’ gelator, 1-octadecylamine, and CO2 as an ‘activator’

“…This interest is motivated by the search for fundamental information on the basis for anisotropic self-assembly and recognition of the potential applications for such gels [ 4 – 6 ] – most can be reverted thermally to their solution (or sol) states – as models for several important biological aggregates (e.g., that are involved with neurodegenerative and other diseases, such as Alzheimer’s, mad cow disease, and sickle cell anemia) that are much more complex structurally. Previously, we described simple methods by which ‘latent’ LMOGs (i.e., molecules that do not aggregate into SAFINs efficiently, or at all, unless a chemical stimulant, such as an acid [ 7 ] or a triatomic gas [ 8 – 9 ], especially carbon dioxide [ 10 – 11 ], is added).…”