Browsing by Author "Abhishek Kumar Gupta"

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Changes in dynamical behavior of ionic liquid in silica nano-pores
(Institute for Ionics, 2014) Manish Pratap Singh; Yogendra Lal Verma; Abhishek Kumar Gupta; Rajendra Kumar Singh; Suresh Chandra
Dielectric relaxation measurement has been carried out on an ionic liquid (1-butyl-3-methyl imidazolium hexafluorophosphate, [BMIM][PF6]) confined in nano-porous silica matrix. Two dielectric relaxation peaks have been observed in the confined ionic liquid (IL) while there is only one relaxation peak for bulk IL. Confinement results in layering of some IL molecules near the pore wall while other molecules, less affected by pore wall interaction, remain in the central core. The two relaxation peaks are assigned to the different dynamical behaviors of the central core and layered IL molecules. © 2013 Springer-Verlag Berlin Heidelberg.
Crystallization kinetics behavior of ionic liquid [EMIM][BF4] confined in mesoporous silica matrices
(Royal Society of Chemistry, 2014) Abhishek Kumar Gupta; Rajendra Kumar Singh; Suresh Chandra
The crystallization kinetics behavior of pure ionic liquid (IL) (1-ethyl-3-methylimidazolium tetrafluoro borate; EMIMBF4) as well as IL confined in mesoporous silica matrices (termed as ionogel) is the concern of our present study. The ionogels (IGs) were synthesized by a non-hydrolytic sol-gel process. DSC was employed to investigate the isothermal crystallization kinetics behavior of IL in bulk as well as in confinement. Isothermal crystallization temperatures were chosen a few °C above the onset crystallization temperature of IL in bulk and in confinement. Crystallization kinetic parameters such as relative crystallinity (α), crystallization half time (t1/2), crystallization rate constant (K) and Avrami exponents (n) have been determined by an isothermal technique using DSC. Crystallization kinetic parameters have been found to be dependent on the amount of IL and the crystallization isothermal temperatures. The experimental data based on the isothermal method show that confinement of the IL results in (i) delayed crystallization and (ii) reduced dimensionality of the crystallization kinetics from 3D (for bulk IL) to 1D. X-ray photoelectron spectroscopy (XPS) and transient plane source (TPS) studies have been used to explain the delayed crystallization. This journal is © the Partner Organisations 2014.
Low density ionogels obtained by rapid gellification of tetraethyl orthosilane assisted by ionic liquids
(2012) Abhishek Kumar Gupta; Manish Pratap Singh; Rajendra Kumar Singh; Suresh Chandra
A non-hydrolytic one pot sol-gel method has been used to synthesize mesoporous silica ionogels with the confined ionic liquid (IL) 1-ethyl 3-methyl imidazolium tetra fluoro-borate [EMIM][BF 4]. The precursor for obtaining the SiO 2 matrix was tetraethyl orthosilicate (TEOS) and formic acid was used as a catalyst. These ionogels have been characterized by density measurements, TEM, BET, DSC, TGA and FTIR. The incorporation of the ionic liquid [EMIM][BF 4] enhances the gellification rate which results in the ionogels having very low density (∼0.3 g cm -3). The low density has been explained on the basis of the creation of 'blind embedded pores' in the matrix (apart from open pores) due to very rapid gellification (∼1 min). Morphological studies provide experimental evidence for the presence of blind pores/voids inside the ionogel ingots. We have also shown that the IL entrapped in nanopores (∼7-8 nm pore size) of the SiO 2 matrix has different physical properties than the bulk IL viz. (a) the phase transition temperatures (T g, T c and T m) of the IL change upon confinement, (b) the thermal stability reduces upon confinement, and (c) the pore wall interaction with the IL results in changes in the C-H vibrations of the imidazolium ring and alkyl chain (the former increasing) which is also indicated in our DFT-calculation. © 2012 The Royal Society of Chemistry.
Preparation and characterisation of ionic liquid confined hybrid porous silica derived from ultrasonic assisted non-hydrolytic sol-gel process
(Elsevier, 2014) Yogendra Lal Verma; Abhishek Kumar Gupta; Rajendra Kumar Singh; Suresh Chandra
A series of porous silica matrices have been synthesized using ionic liquid (IL) 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]) by nonhydrolytic sol-gel method with and without ultrasonic irradiation during gelation. The properties of IL confined silica gel matrices so prepared have been studied using N2-sorption measurement (BET characterisation for determining pore parameters), pulse echo technique (for sound velocity and hence elastic modulus determination), DSC, TGA, FTIR, TEM, SEM and fluorescence techniques. From the N2-sorption measurement, it has been found that BET surface area increased (due to creation of pits on the surface caused by ultrasonic irradiation) while pore volume, average pore size and porosity for ultrasonicated samples decreased. Elastic modulus of the samples containing IL in silica matrices have been found to change with IL content. Glass transition temperature and thermal stability of the IL have been found to increase upon confinement in silica gel matrices. FTIR spectra (experimental as well as computed) show changes in vibrational bands of IL on confinement in pores of silica matrices. Particularly, for ultrasonic assisted samples, the bands related to the imidazolium ring, aliphatic chain and anion SCN of the IL are found to shift upon confinement. Optimised geometry of the IL in porous matrix shows interaction of these groups with the oxygen present on silica pore wall. Fluorescence spectra of samples containing IL shows a shift compared to the bulk IL. © 2014 Elsevier B.V. All rights reserved.
Role of reduced precursor and solvolytic reagent molar ratio on preparation and properties of ionogel
(Academic Press Inc., 2016) Abhishek Kumar Gupta; Yogendra Lal Verma; Manish Pratap Singh; Rajendra Kumar Singh
In the present study, ionogels have been synthesized by immobilizing IL (1-ethyl-3-methylimidazolium tetrafluoroborate) in silica gel matrices using non-aqueous route. In this process, tetraethyl orho-silane (TEOS) as a precursor to silicon dioxide and formic acid as a solvolytic gelating reagent in reduced molar ratio 1:4 were used. We find that reduced molar concentration of formic acid results the formation of ionogels having less number of closed pores (totally isolated from their neighbours), larger density and stable monolithic form. TEM and SEM measurements are used to visualize the morphology of sample and closed pores present in the sample. N2-sorption measurement is used to measure the pore parameters of the silica matrices which shows the mesoporous structure. DSC and TGA results show the change in phase transition temperature and thermal stability of IL upon confinement in silica matrices. Moreover, ionic conductivity of bulk and confined IL is measured using impedance spectroscopy and it has been found that it increases with increasing the temperature as well as concentration of IL in ionogels. Apart from these characterization techniques, ionogels have been characterized using FTIR and fluorescence spectroscopy which exhibit the change in vibrational frequencies and fluorescence behaviour of confined IL. © 2016 Elsevier Inc.
Studies on an ionic liquid confined in silica nanopores: Change in T g and evidence of organic-inorganic linkage at the pore wall surface
(2014) Abhishek Kumar Gupta; Yogendra Lal Verma; Rajendra Kumar Singh; Suresh Chandra
Ionic liquid (IL) 1-ethyl-3-methylimidazolium ethyl sulfate [EMIM][EtSO4] has been immobilized in silica matrix by using nonhydrolytic one-pot sol-gel method, with tetraethyl orthosilicate (TEOS) used as precursor and formic acid as a reagent. The glass transition temperature (Tg) of the confined IL was ∼20 C higher than the bulk IL. The thermal stability of IL also increased upon confinement. These changes are explained on the basis of interactions between the inorganic SiO2 pore wall surface and the organic cation [EMIM]+ and anion [EtSO 4]- of the ionic liquid. Quantum mechanical calculation based on DFT has been used to show that the oxygen of SiO2 interacts with the C-H of the imidazolium ring of [EMIM]+ cation and the surface Si atoms interact with S-O of the [EtSO4]- anion. The latter has led to Si-O-S linkages, which have been experimentally confirmed by FTIR studies. The silanol OH group is also likely to interact but less dominantly as compared to the earlier discussed interaction. These interactions have also led to changes in the fluorescence spectra of the confined IL. © 2013 American Chemical Society.
Studies on mesoporous silica ionogels prepared by sol-gel method at different gelation temperatures
(2013) Abhishek Kumar Gupta; Rajendra Kumar Singh; Suresh Chandra
Ionogels (IGs), a novel group of materials having an ionic liquid entrapped in a mesoporous silica matrix, have been synthesized by a non-hydrolytic sol-gel process at different gelation temperatures (30, 0, -10°C) using 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4] ionic liquid (IL), tetraethylortho-silicate (TEOS) as a metallic precursor and formic acid as a catalyst. The rate of gelation was lower at lower temperatures. Gelation temperature has also been found to affect (i) the densities of IGs, (ii) pore parameters of IGs like average pore diameter, pore surface area, pore volume and porosity, (iii) phase transition temperatures Tm, T c, and Tg, (iv) thermal stability of IL and (v) vibrational spectra. The porosity of IG ingots, prepared at lower gelation temperatures (0, -10°C) was higher. Unexpectedly, their densities were found to be higher than the IGs prepared at higher temperatures (say 30°C). Possibly, one of the reaction products of the sol-gel process (namely, ethyl formate) could not escape the matrix due to its slow evaporation at low temperatures which resulted in their higher densities. The presence of ethyl formate in the IGs was confirmed by FTIR. The variation of the other properties of the IGs, listed above, synthesized at different gelation temperatures is also discussed and explained in the paper. © 2013 The Royal Society of Chemistry.
Viscoelastic, surface, and volumetric properties of ionic liquids [BMIM][OcSO4], [BMIM][PF6], and [EMIM][MeSO3]
(American Chemical Society, 2014) Manish Pratap Singh; Satish Kumar Mandal; Yogendra Lal Verma; Abhishek Kumar Gupta; Rajendra Kumar Singh; Suresh Chandra
Thermophysical properties viz. surface tension, viscosity, density, and ultrasonic velocity of three ionic liquids 1-butyl-3-methyl imadazolium octyl sulfate [BMIM][OcSO4], 1-butyl-3-methyl imadazolium hexafluorophosphate [BMIM][PF6], and 1-ethyl-3-methyl imadazolium methanesulfonate [EMIM][MeSO3] have been measured in a wide temperature range. Experimental data so obtained have been used to calculate isentropic compressibility, isothermal expansion coefficient, surface entropy, surface enthalpy, and critical temperature (temperature where the distinction between liquid and gas phase vanishes and the surface tension tends to zero). Structure-property correlation for different ILs is also discussed. © 2014 American Chemical Society.