Somenath Panda

A Combined Experimental and Theoretical Approach to Understand the Structure and Properties of N-Methylpyrrolidone-Based Protic Ionic Liquids

Correlation of the structure and properties of ionic liquids (ILs) is essential for the development of optimized materials in the fields of gas capture and separation, battery electrolytes, and cellulose dissolution processes. In view of this, a detailed vibrational spectroscopic analysis and quantum-chemical calculations were performed to explore the interionic interactions in ILs based on the N-methylpyrrolidone cation and a carboxylate anion. FTIR and Raman spectroscopy were applied to identify the hydrogen-bonding interactions between ion pairs, in which redshifted vibrational modes were observed as a function of the anion chain length. This observation was verified by the bond lengthening and enhanced hydrogen-bonding energies, as manifested in the structure and natural bond orbital (NBO) calculations. Furthermore, conductivity was measured at different temperatures to envisage the effect of the alkyl chain on the mobility of ions in the ILs. Finally, rheological measurements were implemented to explain the flow behavior of these ILs, which revealed a decrease in shear viscosity with an increase in temperature, that is, a Newtonian trend over a range of shear rates. The observed trend in transport properties was supported by the ion-pair binding energy. Stronger interactions between the IL cations and anions led to a decrease in the number of free ions and lowered the conductivity. In these protic ILs, the intermolecular N-H⋅⋅⋅O and C-H⋅⋅⋅O interactions played an important role in governing their physicochemical properties.

Aggregation behaviour of biocompatible choline carboxylate ionic liquids and their interactions with biomolecules through experimental and theoretical investigations

Choline carboxylates are an important class of bio-derived ionic liquids (ILs) having potential applications in the pharmaceutical sector. Therefore, detailed knowledge of the physicochemical properties of this class of ILs and their mixtures with biomolecules is highly desired with a focus on sustainability and environmental impact. In view of this, a series of surface-active ILs (SAILs) based on choline carboxylates, viz. choline laurate ([Chl][Lau]) and choline palmitate ([Chl][Pal]), were synthesized. Various techniques such as surface tension measurements, conductivity measurements, dynamic light scattering (DLS), and rheology were employed to get detailed information about their self-assembly and viscoelastic behaviours in aqueous medium. A set of surface and thermodynamic parameters of these SAILs in water was determined, which revealed the superior surface activity of [Chl][Pal] compared to [Chl][Lau]. Ab initio and natural bond orbital (NBO) calculations manifested larger binding and hydrogen-bonding energies of [Chl][Lau]–water compared to [Chl][Pal]–water. Consequently, it is more difficult for [Chl][Lau] to self-aggregate in water. The steady-state and oscillatory rheological measurements demonstrated notable differences in solution viscosities in these systems. The size of the micellar aggregates was significantly affected by the hydrophobicity of carboxylate anions and concentration of SAILs. Finally, the interactions of these SAILs with bovine serum albumin (BSA) were systematically investigated in aqueous medium. The quenching effect of these SAILs on tryptophan moieties of BSA along with the information on the secondary structural conformation was monitored through fluorescence and circular dichroism techniques to understand the binding strength and mechanism. Understanding the aggregation behaviour and molecular level interactions of choline carboxylate SAILs with BSA is required to explore their potential applications in biotechnology.

Understanding Differential Interaction of Protic and Aprotic Ionic Liquids Inside Molecular Confinement

Considering the contemporary interests of water-free reverse micelles (RMs) in the field of organic reaction medium and potential drug delivery carrier, we synthesized two different classes of ionic liquids (ILs), protic N-methyl-2-pyrrolidonium hexanoate, [NMP][Hex], and aprotic choline hexanoate, [Chl][Hex], and subsequently incorporated them in a mixture of polyoxyethylene (20) sorbitan monooleate (Tween-80) and cyclohexane. In order to understand the differential nature of interinterionic interaction of two ILs, we performed DFT calculations on pure ILs to correlate with experimental results. The formation of IL-in-oil RMs was confirmed from phase behavior and DLS studies. Interestingly, [NMP][Hex]-based systems showed a larger monophasic region and droplet size along with higher shear viscosity compared to [Chl][Hex]-based systems. Stronger interaction between [NMP]+ and Tween-80 due to their protic nature might be the driving force for such observations which supported the resonance stabilization energy [E(2)] and charge population analysis by NBO calculation. Smaller E(2) values along with lesser NBO charges on atoms involved in H-bonding in pure [NMP][Hex] than [Chl][Hex] corroborated with the experimental observations. This primary hypothesis was further confirmed from FTIR and time-resolved fluorescence studies. These systems showed efficient thermal stability. Taking all of the results together, we anticipate that these RMs could be used as efficient delivery systems and for nanomaterial synthesis.