Nanoconfined imidazolium ionic liquids

Long-chain ionic liquids are attracting interest as alternatives to their short-chain analogues as electrolytes. At the same time, they are also considered as alternatives to mesogens and conventional surfactants as templates. Interestingly, the nanoconfinement of long-chain ionic liquids in a solid and porous matrix (to result in a macroscopically solid hybrid material) can in some cases result in improved properties. This is due to the different chemical and steric environment to which the ionic liquid is exposed in confinement. In this work, the effect of nanoconfinement on the mechanism of proton transport and the capacitive behaviour of the protic ionic liquid HC8imTFSI (1-octylimidazolium bis(trifluromethane-sulfonyl)imide) was studied. With this aim, proton conduction was investigated by broadband dielectric spectroscopy and pulsed-field gradient nuclear magnetic resonance spectroscopy, considering a mixture of imidazole and HC8imTFSI inside hydrophobised mesoporous silica. Results show that even inside narrow pores only ca 5 nm wide, the self-diffusion of the confined ionic liquid is unrestricted, due to weak interactions between the cations and the pore walls. By contrast, when pure HC8imTFSI was confined in the pores of hydrophilic silica, ca 3.5 nm wide, results from electrochemical impedance spectroscopy combined with equivalent circuit modelling indicated a high in-pore resistance. A further important finding is that at the in-pore protic ionic liquid/electrode interface an increased specific capacitance was measured, suggesting a higher charge density than for the interface between the electrode and the bulk protic ionic liquid. Another aspect that has been considered in this thesis, is the ability of long-chain ionic liquids to function as a templating agent. The templating mechanism, the limitations associated to the chemical structure and the resulting pore morphology have been investigated. Results have revealed that tetradecyl- and hexadecyl-methylimidazolium chloride are suitable soft-templates for the formation of vertically aligned, uniform, channel-like pores, running through the entire thickness of a silica film deposited with the electrochemically assisted self-assembly method. By contrast, dodecyl-methylimidazolium chloride as well as the protic analogue hexadecyl-imidazolium chloride do not show a templating function under the investigated synthetic conditions. In all cases studied, the mechanism of pore formation is critically discussed

Surface active alkyl-imidazolium ionic liquids studied as templates to form vertically oriented pores in silica thin films

In this study, thin films of mesoporous silica with vertically oriented and hexagonally ordered pores have been synthetized and investigated. A variety of alkyl-imidazolium ionic liquids have been used as the template, with the aim to understand how changing the chain length on the imidazolium cation from hexadecyl (C16) to dodecyl (C12) or the protic character of the imidazolium head affects the film formation and the order of the pores. In addition, different types of metal substrates have been used for the film deposition, i.e. indium titanium oxide (ITO) and gold. Complementary experimental methods, including GISAXS, Infrared spectroscopy, and cyclic voltammetry have been employed for a thorough investigation of the morphology and permeability of the deposited thin films

Charge transfer and electrical double layer of an amphiphilic protic ionic liquid in bulk and when confined in nanochannels

We report the behavior of the protic and surface active ionic liquid octylimidazolium bis(trifluoromethylsulfonyl)imide, [HC(8)Im][TFSI], in bulk and inside silica nanochannels, at the interface with the conductive substrate indium tin oxide (ITO) upon applied potential. The two distinct cases of the ionic liquid being in contact with a bare ITO substrate and an ITO substrate covered with a thin film of mesoporous silica containing vertically-aligned channel-like pores have been investigated. These correspond to the behavior of the bulk ionic liquid and the ionic liquid confined within nanochannels (approximately 3.5 nm wide and 65 nm long). Broadband dielectric spectroscopy (BDS) and electrochemical impedance spectroscopy (EIS) have been used as the experimental methods, while modelling with equivalent circuits has been applied to evaluate the experimental results. Thus, this study does not only show a functional ionic liquid/silica hybrid material, but also presents an in-depth electrochemical characterization revealing an enhanced specific capacitance at the confined-IL/ITO interface (similar to 16 mu F cm(-2)) as compared to the case of bulk IL/ITO (similar to 6 mu F cm(-2)). This suggests that local structure and ion ordering inside the nanochannels of silica are different from that of the bulk ionic liquid, favoring denser ionic packing and a higher specific capacitance at the metal interface

Transport Properties and Local Structure of an Imidazole/Protic Ionic Liquid Mixture Confined in the Mesopores of Hydrophobic Silica

The local structure and the molecular dynamics of an imidazole/protic ionic liquid mixture have been investigated while confined in only ca. 5 nm mesopores of silica particles. The walls of the silica pores were functionalized with trioctyl groups to ensure a hydrophobic character, and a series of hybrid materials with varying liquid-to-silica ratios were investigated. Results from vibrational spectroscopy (both Raman and infrared) indicate that the local ion-ion interactions as well as the nature of hydrogen bonds inside the nanopores are not significantly different from the case of the bulk liquid mixture. Nevertheless, the ionic conductivity decreases rapidly and monotonically with decreasing amount of liquid, while the self-diffusion coefficients measured by pulsed field gradient nuclear magnetic resonance (NMR) show a distinct dependence on composition. The population of molecules outside the particles seems to contribute with an enhanced diffusivity, while the molecules inside the mesopores diffuse at a rate comparable to that observed in the bulk liquid. In addition, when experimentally possible, we have measured higher diffusivities for the exchangeable -NH proton than for any other molecular species, which is indicative of a decoupled proton motion. Results from X-ray scattering, employed to elucidate the local molecular structure, reveal an additional feature characteristic of the nanoconfined state, which is associated with a real space distance of about 3.5 nm. This distance describes a specific molecular organization inside the mesopores and may reflect the formation of a monolayer of the octyl-imidazolium cations self-assembled at the hydrophobic silica surface. Such a local structure would favor the localization of charges, including the exchangeable protons. In addition, the analysis of molar conductivity suggests that a major problem with a low pore filling is the emergence of discontinuities throughout the liquid phase

Formulation of polyphthalaldehyde microcapsules for immediate UV-light triggered release

Triggered release from responsive drug reservoirs activated by remote stimuli is desired in a range of fields. Critical bottlenecks are cost-efficient formulation avenues applicable for industrial scale-up, viable triggers and immediate release rather than continuous release upon activation. UV-sensitive microcapsules based on self-immolating polymers in combination with thin shells and morphological weak spots should allow for immediate triggered release. Polyphthalaldehyde-based microcapsules were prepared using several variations of the internal phase separation route. In addition, a fluorescence microscopy method was developed to study both the microcapsule morphology and the triggered release in-situ. The microcapsule formation was driven by the surface activity of the stabilizer, effectively lowering the high polymer-water interfacial tension, which is in sharp contrast to conventional encapsulation systems. Contrary to previous findings, a core–shell morphology was obtained via slow emulsion-to-suspension transformation. Rapid transformation captured intermediate inverted core–shell structures. The capsules were highly sensitive to both acid- and UV-mediated triggers, leading to an unzipping and rupturing of the shell that released the core content. Poly(methacrylic acid)-stabilized microcapsules displayed immediate UV-triggered release provided by their stimuli-sensitive blueberry morphology. Both capsules in aqueous and dry environment started to lose their core content after less than one minute of UV light exposure

Local anisotropy in single crystals of zeotypes with the MFI framework structure evidenced by polarised Raman spectroscopy

Polarised Raman spectroscopy is used to characterise the local structure in single crystals of zeotypes, namely silicalite-1 and ZSM-5, which share the MFI framework structure. Attributes favourable for applying polarised Raman spectroscopy are the orthogonal axes of these single crystals and their size, i.e. 10 to 30 micrometers in all three directions. We show that the intensity of certain vibrational modes in silicalite-1 depends on the polarisation of the incident light, reflecting the anisotropic character of the molecular bonds contributing to these vibrations. Using these observations, and by estimating the depolarisation ratio (rho) and the pseudo-order factor (f), we propose a more accurate assignment of the Raman active modes. More precisely, Raman intensities peaked at 294, 360, 383 and 472 cm(-1) are attributed to bending modes in 10-, 6-, 5- and 4-membered rings, respectively. In the region of stretching modes, the vibration at 832 cm(-1) is assigned to Si-O-Si bonds shared between 5-membered rings, which have an orientation parallel to the a-axis of the crystal. By virtue of having a strongly polarised character, the modes at 472 and 832 cm(-1) can be used as orientational indicators. The proposed assignment is supported by the good agreement between experimental and simulated polar plots, where Raman intensities are plotted as a function of the polarisation angle of the incident light. Finally, upon partial substitution of Si atoms by Al, the crystalline structure is maintained and almost no spectroscopic changes are observed. The only significant difference is the increased width of most vibrational modes, which is consistent with the local lower symmetry. This is also seen in the angular dependence of selected vibrational modes that compared to the case of pure silicalite-1 appear less polarised. In the Raman spectrum of ZSM-5 a new feature at 974 cm(-1) is observed, which we attribute to Al-OH stretching. In the high frequency range, the O-H stretching modes are observed which arise from the Si-O(H)-Al Bronsted acid sites. The intensity of the characteristic mode at 3611 cm(-1) reveals an anisotropic character as well, which is in line with previous findings from solid state NMR that Al atoms distribute nonrandomly within the MFI framework structure

An imidazolium ionic liquid as effective structure-directing agent for the fabrication of silica thin films with vertically aligned nanochannels

In parallel to the increasing variety of ionic liquids that show different kinds of nanometer-scale structuration in their pure and solved forms, there is a raising interest in exploring the possibility of using ionic liquids as soft-templates for the synthesis of mesoporous materials. We report the case of 1-hexadecyl-3-methylimidazolium chloride (C16MIMCl), a surface active ionic liquid (SAIL), here used as an excellent soft-template for the formation of vertically aligned, uniform mesochannels, with a well defined pore width of 2.5 nm in silica thin films deposited with the electrochemically assisted self-assembly (EASA) method. The obtained mesochannels run through the entire thickness of the films and after removal of the ionic liquid the emptied mesochannels ensure a thorough mass transport to the substrate, here monitored by the redox-active electrochemical probe Ru(II)/Ru(III) during cyclic voltammetry (CV). Moreover, the mechanism of pore formation is explained; unlike the mechanisms reported for short chain imidazolium ionic liquid silica templates, in the case of C16MIMCl the dominating so-called cooperative interaction is the electrostatic attraction between the C16MIM+ and the network-forming negatively charged silicate oligomers. Therefore, this study provides a better understanding of the templating behavior of long chain imidazolium ionic liquids and motivates further research on the synthesis of ionic liquid-based functional hybrid materials

Acidity as Descriptor for Methanol Desorption in B-, Ga- and Ti-MFI Zeotypes

The isomorphous substitution of Si with metals other than Al in zeotype frameworks allows for tuning the acidity of the zeotype and, therefore, to tailor the catalyst\u27s properties as a function of the desired catalytic reaction. In this study, B, Ga, and Ti are incorporated in the MFI framework of silicalite samples and the following series of increasing acidity is observed: Ti-silicalite < B-silicalite < Ga-silicalite. It is also observed that the lower the acidity of the sample, the easier the methanol desorption from the zeotype surface. In the target reaction, namely the direct conversion of methane to methanol, methanol extraction is affected by the zeotype acidity. Therefore, the results shown in this study contribute to a more enriched knowledge of this reaction

Thermally Responsive Amphiphilic Conetworks and Gels Based on Poly(N‑isopropylacrylamide) and Polyisobutylene

Novel amphiphilic conetworks (APCN) consisting of thermoresponsive poly(N-isoproplyacrylamide) (PNiPAAm) cross-linked by hydrophobic methacrylate-telechelic polyisobutylene (MA-PIB-MA) were successfully synthesized in a broad composition range. The resulting PNiPAAm-l-PIB conetworks (“l” stands for “linked by”) were obtained by radical copolymerization of NiPAAm with MA-PIB-MA in tetrahydrofuran, a cosolvent for all the components. Low amounts of extractables substantiated efficient network formation. The composition dependent two glass transition temperatures (Tg) by DSC analysis indicate microphase separation of the cross-linked components without mixed phases. It was found that the PNiPAAm-l-PIB conetworks are uniformly swellable in both water and n-hexane; i.e., these new materials behave either as hydrogels or as hydrophobic gels in aqueous or nonpolar media, respectively. The uniform swelling in both polar and nonpolar solutes indicates cocontinuous (bicontinuous) phase morphology. The equilibrium swelling degrees (R) depend on composition, that is, the higher the PIB content, the lower the R in water and the higher in n-hexane. The PNiPAAm phase keeps its thermoresponsive behavior in the conetworks as shown by significant decrease of the swelling degree in water between 20 and 35 °C. The lower critical solubility temperature (LCST) values determined by DSC are found to decrease from 34.1 °C (for the pure PNiPAAm homopolymer) to the range of 25–28 °C in the conetworks, and the extent of the LCST decrease is proportional with the PIB content. Deswelling-swelling, i.e., heating–cooling, cycle indicates insignificant hysteresis in these new thermoresponsive materials. This indicates that PNiPAAm-l-PIB conetworks with predetermined and thermoresponsive swelling behavior can be designed and utilized in several advanced applications on the basis of results obtained in the course of this study