Protic Ionic Liquids for the Belousov–Zhabotinsky Reaction: Aspects of the BZ Reaction in Protic Ionic Liquids and Its Use for the Autonomous Coil–Globule Oscillation of a Linear Polymer

Abstract

Herein, we describe the physicochemical aspects of the Belousov-Zhabotinsky (BZ) reaction in hydrated protic ionic liquids (PILs) combined with different cations with a hydrogen sulfate ([HSO]) anion. PILs were prepared from the neutralization reactions of sulfuric acid with 13 different aliphatic amines. The amine structure was selected to investigate the effect of the number of active protons, alkyl chain length (hydrophilicity), and cationic linearity on the mechanism of the BZ reaction. The pK values of PILs were significantly higher (pK = 1.0-2.5) than those of inorganic acids (HSO = -3.0; HNO = -1.4), the conventional proton source in a BZ reaction. A periodic redox oscillation was observed in Ru(bpy) when appropriate amounts of BZ reaction substrates (NaBrO oxidant; malonic acid reductant) were added to the hydrated PILs. A long-lasting BZ oscillation was realized when hydrophilic cations (ammonium, ethylammonium, and dimethylethylammonium) were employed. Interestingly, a large ΔA (oscillation amplitude of absorbance for the scale of the oscillation stability observed after 5000 s from the initiation of the BZ reaction) was achieved for PILs possessing less than four carbon atoms in their cationic structure. The apparent BZ oscillation activation energy (E) in the hydrated PILs was estimated to be ∼40 kJ mol, which is 30 kJ mol less than that observed in a conventional system. The catalytic reaction in the BZ reaction subprocess suppresses the total activation energy of the reaction. To realize long-lasting self-oscillating polymeric materials acting under milder condition, we demonstrated an autonomous coil-globule polymer chain transition (BZ-driven) that directly converts chemical energy to mechanical motion in hydrated PILs without freely diffusing Ru(bpy) metal catalyst. Ethylammonium hydrogen sulfate ([ea-H][HSO]) is selected as the suitable proton source for the BZ reaction. A well-defined self-oscillating polymer that incorporated Ru(bpy) metal catalyst into the polymer backbone accompanied by good compatibility in hydrated [ea-H][HSO] is successfully prepared by ATRP, followed by postmodification of the metal catalyst. The rhythmic solubility changes of the polymer under milder conditions, realized by combination with PILs, will expand the potential applications of PILs to novel functional materials.

Chemomechanical Motion of a Self‐Oscillating Gel in a Protic Ionic Liquid

“…[14,15] Moreover,P ILs having high proton activity (comparable to as trongly acidic aqueous solution) can be developed as proton conducting media for non-humidified fuel cell membrane applications. [17,18] TheB Zr eaction proceeds with the slow oxidation of an organic substrate (for example malonic acid (MA)) by an oxidant (such as sodium bromate) in the presence of ametal catalyst (for example Ru(bpy) 3 )and protons.Importantly,the redox state of the metal catalyst changes periodically during the reaction. [17,18] TheB Zr eaction proceeds with the slow oxidation of an organic substrate (for example malonic acid (MA)) by an oxidant (such as sodium bromate) in the presence of ametal catalyst (for example Ru(bpy) 3 )and protons.Importantly,the redox state of the metal catalyst changes periodically during the reaction.…”

“…[18] When PILs having longer alkyl chains are used, hydrophobic interaction between the alkyl chain and Ru(bpy) 3 redox center seems to inhibit effective electron transfer,resulting in unstable oscillation. Our previous report revealed that stable redox oscillation of Ru(bpy) 3 during the BZ reaction could be achieved by using PILs possessing short alkyl chains in the cation structure.…”

“…So,when [maH + ][HSO 4 À ]isused as aproton source instead of HNO 3 ,the duration of process Aincreases and that of process Cd ecreases.I naprevious study,asimilar tendency was observed in unpolymerized Ru(bpy) 3 metal catalyst solution when diethylmethylethylammonium hydrogen sulfate was used as the proton source. [18] Thea bove results demonstrated for the first-time autonomous swelling-deswelling oscillation of polymer gels in ahydrated PIL as the BZ reaction medium. Also,t he shorter duration of process Cc an be explained by proton-accepting reaction in the right-hand side of sub-process C. When [maH + ][HSO 4 À ]i sm ixed with water, free neutral amine (methylamime,[ma]) is produced as aresult of proton release.…”

“…[16] Apart from these pioneering efforts,wehave reported for the first time that the Belousov-Zhabotinsky (BZ) reaction, aw ell-known oscillating chemical reaction, occurs in certain hydrated PILs without adding strong acids,such as HNO 3 and H 2 SO 4 that are conventionally used as ap roton source. [17,18] TheB Zr eaction proceeds with the slow oxidation of an organic substrate (for example malonic acid (MA)) by an oxidant (such as sodium bromate) in the presence of ametal catalyst (for example Ru(bpy) 3 )and protons.Importantly,the redox state of the metal catalyst changes periodically during the reaction. These spatiotemporal phenomena under nonequilibrium states are understood as aphysicochemical model of self-organization, analogous to many biological systems.In contrast to the conventional BZ reaction that requires strong acid, al ong-lasting and stable oscillation with ah igher frequency under milder condition can be successfully realized in hydrated PILs.…”

“…These spatiotemporal phenomena under nonequilibrium states are understood as aphysicochemical model of self-organization, analogous to many biological systems.In contrast to the conventional BZ reaction that requires strong acid, al ong-lasting and stable oscillation with ah igher frequency under milder condition can be successfully realized in hydrated PILs. [17,18] Our group has developed self-oscillating polymeric materials that convert energy from chemical oscillation of the BZ reaction into mechanical motion of the polymer network. [19,20] Atypical design of the self-oscillating polymer is astatistical copolymer of N-isopropylacrylamide (NIPAAm, aw ellknown polymer having al ower critical solution temperature (LCST) in aqueous solution) and aR u(bpy) 3 metal catalyst that is modified with ap olymerizable vinyl group (P(NIPAAm-co-Ru(bpy) 3 ).…”