Electrochemical Behavior Of Fullerenes In Vesicular Solutions

In this paper, we mainly studied the electrochemical behavior of C₆₀ and carbon nanotubes in different self-assembly of surfactants by using the cyclic voltammetry. This paper includes three parts. In the fourth part, we explored to study the novel phase transitions in aqueous solutions of oppositely charged polyelectrolytes / surfactants mixtures.In the first section, we studied the electrochemical behavior of C₆₀ film and C₆₀/ lipid films in ionic liquids. C₆₀ is good acceptor of electrons, which can accept six electrons. RT-ILs are the advantageous electrolytes for the electrochemical study due to a high thermal stability, negligible vapor pressure, a high intrinsic conductivity and a large electrochemical window, making them attractive novel environmentally friendly solvents for enzyme catalyzed reactions, photoelectrochemical solar cells, and electrochemical devices. Firstly, we studied the electrochemical behavior of C₆₀ film in room-temperature ionic liquids. 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]). The cyclic voltammogram of C₆₀ films on glassy carbon (GC) electrode in [bmim][PF₆] shows two redox couples and an evident oxidation wave. The oxidation of C₆₀ h]as rarely been reported, because the oxidation of C₆₀ is occurred at more positive potential. So the papers mainly reported the reduction of C₆₀ In this part, we choosed the room-temperature ionic liquid, l-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF₆]) as the electrolyte and we can obtained C₆₀⁺ easily. It plays an important role in the synthesis and reaction of fullerene moleculars. In addition, we also studied the electrochemical behavior the C₆₀/ lipid films in ionic liquids. The studies of the electrochemical behavior of the C₆₀ / lipid films are focus on the cast film of C₆₀ / TTAL and C₆₀/ Tridodecylmethylammonium chloride (TDMACl) in ionic liquid. The cyclic voltammogram for cast films of C₆₀ / TDMACl in [bmim][PF₆] is different from aqueous solution (pH = 10) containing TDMACI as the supporting electrolyte. suggesting that electrolyte cations play important role for the generation of C₆₀ ar.ions. Upon reduction C₆₀, fullerene molecules are reduced to C₆₀, [bmim]⁺ cations diffuse into the film to balance the negative C₆₀ charges. There is a structural rearrangement of the film following the electron transfer. After reoxidation. [bmim]⁺ cations leave the films and the C₆₀ re-arranges to its initial structure. A typical salt-free zero-charged catanionic vesicular system was constructed from a cationic hydroxide-exchanged form [TTAOH, C₁₄N⁺(CH₃)₃OH^-] oftetradecyltrimethylammonium bromide (TTABr) mixing with lauric acid (LA). In this case, these counterions are H⁺ and OH^- which form water (TTAOH+LA→TTAL+H₂O). The vesicles of TTAL are used to enhance the solubility of C₆₀ Typical cyclic voltammogram of a C₆₀ / TTAL bilayer vesicle cast films on glassy carbon electrode in [bmim][PF₆] is shown in the paper. It occurred two consecutive electron-transfer reactions and we obtained C₆₀^2-. A novel suitable microenvironment offered for the facile electron-transfer reactions of fullerenes embedded in a vesicle phase of lipid films. Our finding would be applicable to a variety of fullerene derivatives and metallofullerenes.In the second part, we studied the electrochemical behavior of glassy carbon electrodes modified by multi-walled carbon nanotube / surfactant films in a phosphate buffer solution and ionic liquid, 1-n-butyl-3-methyIimidazolium hexafluorophosphate ([bmim][PF₆]). We choose three various surfactants with the same hydrophobic chains and different hydrophilic groups in our study. They are cationic surfactants: dodecyltrimethylammonium bromide (DTABr); anionic surfactants: sodium dodecyl sulfate (SDS); Zwitterionic surfactant: dodecyldimethylamine oxide, (C₁₂DMAO). In our study, typical reduction peaks of MWCNTs embedded in DTABr films on GC electrode are obtained in [bmim][PF₆]. Comparing the effects of SDS and C₁₂DMAO, no redox peaks were obtained for cast films of MWCNT / C₁₂DMAO but two small redox couples appeared for films of MWCNT / SDS. This represents a well-defined reversible two-consecutive electron-transfer process in contrast to the irreversible process observed for films of MWCNT / DTABr. Comparing the various surfactants with the same hydrophobic group, the zwitterionic C₁₂DMAO is better than anionic SDS and cationic DTABr in mediating the electron-transfer of MWCNTs in ionic liquid [bmim][PF₆]. Since the hydrophobic chains of cationic DTABr, anionic SDS, and zwitterionic C₁₂DMAO are the same, the hydrophilic groups must play an important role in the electrochemical behavior of MWCNTs in ionic liquid [bmim][PF₆]. In order to understand the influence of the hydrophobic chains of surfactants on the electron-transfer reaction of MWCNTs, we investigated the electrochemical behavior of cast films of MWCNT / TTABr and MWCNT / CTABr instead of MWCNT / DTABr on GC electrode in [bmim][PF₆]. Cationic DTABr, TTABr and CTABr have the same hydrophilic group but different hydrophobic chains. The peak current decreased as the hydrophobic chain length increased. This shows that the communication with the underlying electrode for DTABr is better than for TTABr and CTABr. Typical reduction peaks of MWCNTs embedded in TTABr and CTABr films on GC electrode are obtained in [bmim][PF₆] and they are irreversible. The phenomena are roughly consistent with the electrochemistry of MWCNT / DTABr, which indicates that the hydrophobic chains are not the primary factors influencing the electrochemistry of MWCNTs. It proves that the hydrophilic groups must play an important role in the electrochemical behavior of MWCNTs in ionic liquid [bmim][PF₆]. In order to determine the influence of the electrolyte on the electrochemical behavior of MWCNTs, we also studied the voltammograms of cast films of MWCNT / surfactant on a GC electrode in phosphate buffer solution (pH -6.86). Compared with the phosphate buffer electrolyte, great changes were observed for voltammograms of GC electrodes coated with MWCNT / DTABr, MWCNT / SDS, and MWCNT / C₁₂DMAO films in [bmim][PF₆]. This behavior may be attributed to large structural rearrangements of the films. Strong binding between. MWCNTs and [bmim]⁺ may change the structure of the modified films and lead to the change of electrochemical behavior. The changes in a buffer solution and an ionic liquid observed here should be significance for obtaining electrochemical behavior and structural rearrangements of the films of MWCNTs. This study provides further understanding of methods to disperse MWCNTs in various surfactants and their use in electrochemistry. MWCNT / surfactant film-modified electrodes will be applicable to catalysis of electrochemical reactions of biomolecules such as dopamine, epinephrine, uric acid and ascorbic acid.In the third part, we studied the electrochemical behavior of cationic tetradecyltrimethylammonium bromide (TTABr), anionic sodium dodecylsulfate (SDS), as well as cationic-anionic (catanionic) mixed surfactant self-assembled solutions at Pt wire electrode has been studied by cyclic voltammetry (CV). We determined self-assembled structures of vesicles by using Freeze-Fracture Transmission Electron Microscopy (FF-TEM). The high positive charges of the100 mmol·L^-1 TTABr micelles have much stronger electrostatic repulsion with H⁺. So it restrains the reduction of H⁺ and only cationic R(CH₃)₃N⁺ reduction occurred.The negative surface charges of 100 mmol·L^-1 SDS micelles, which has the electrostatic repulsion with OH^-. And anionic SDS micelles and cationic H⁺ ionsattract each other, which enrich H⁺ ions around the surface of anionic SDS micelles. The enrichment of H⁺ ions should have the higher cathodic current. We put forward the mechanism of anionic SDS and cationic TTABr micelle-phase solution near the electrode during the electrochemical reaction on the basis of the anionic SDS micelle model and cyclic voltammograms. For this vesicle-phase solution for the ratio of TTABr to SDS of 9.35:0.65 at the total surfactant concentration of 25 mmol·L^-1, the cationic TTABr surfactant is excessive. Surface membranes have the positive charges, which are electrostatic repulsion with H⁺. So it may restrain the reduction of H⁺ and only cationic R(CH₃)₃N⁺ could bereduced. This is consistent with the 100 mmol·L^-1 TTABr micelle solution. This experiment proves the mechanism of electrochemical reaction for vesicle-phase solutions with in the cationic surfactant excess is similar to cationic micelles. In addition, there is a pair of reversible reduction peaks at 0.7 to 0.6 V and 1.3 to 1.0 V, which may result from the reduction of positive charges TTABr / SDS vesicles. For this vesicle-phase solution for the ratio of TTABr to SDS of 1.25:8.75 at the total surfactant concentration of 25 mmol·L^-1, the anionic SDS surfactant is excessive. The excess of anionic SDS surfactant exist in the vesicle-phase membranes having the negative charges. Anionic SDS vesicles adsorb near the working electrode and takesup some space of OH^-. So only much fewer OH^- ions can be oxidated, whichleads to the decrease of the anodic peak current value. This experiment sufficiently proves the mechanism of electrochemical reaction at TTABr / SDS vesicles with the excess SDS is consistent with that of 100 mmol·L^-1 SDS micelle solutions. In addition, there is a pair of redox peaks at 0.90 to 0.75 V and 1.05 to 0.95 V, which may result from the redox of negative charges TTABr-SDS vesicles. Therefore cyclic voltammetry provides an easy and convenient route to distinguish different self-assemblies of surfactants in aqueous solution.In the fourth part, we present for the first time the novel phase transitions in aqueous solutions of oppositely charged polyelectrolytes poly(sodium 4-styrenesulfonate) / surfactants tetradecyltrimethylammonium bromide mixtures. With temperature increases, the novel phase transitions in this mixed solution from micelles to vesicles, then the coexistence of vesicles and aggregates having the morphology of melon seeds, and finally the precipitates induced by heating the samples. The mixed solutions undergo apparent transitions from transparent colorless solution to transparent sky-blue, opaque sky-blue, finally the opaque milk white. As reducing temperature to 25.0±0.1°C, the opaque milk white precipitates transforms to opaque sky-blue solution, then transparent sky-blue, finally transparent colorless, i.e., by reducing temperature, the precipitates can dissolve and transfer to precipitate-like aggregates with slightly flow birefringence, then flow birefringent La-phase, finally the clear Li-phase. Several heating-cooling cycles were performed on the same sample and the results obtained were reproducible, which indicates the reversibility of the phase transitions.