Aggregation Behavior Of Ionic Liquids Research Articles

Aggregation behavior of ionic liquid in molecular liquid medium: a dynamic light scattering study

ABSTRACT The aggregation behavior of a tri-n-octylmethylammonium bis(2-ethylhexyl)phosphate ([N1888][D2EHP]) in n-dodecane (n-DD) was investigated in detail by dynamic light scattering studies. The aggregate size was measured as a function of various parameters such as concentration of [N1888][D2EHP] in organic phase, concentration of nitric acid and Nd(III) in aqueous phase, and temperature of the system. The aggregate size increased with increase in the concentration of nitric acid and [N1888][D2EHP] in organic phase. The aggregate formation of [N1888][D2EHP] phase was compared with [N1888][NO3] and bis(2-ethylhexyl)phosphoric acid (D2EHPA) formed in organic phase upon nitric acid extraction. The results are reported in this paper.

Modulations in the aggregation behavior of ionic liquid 1-butyl-3-methylimidazolium octylsulfate in aqueous alcohol solutions

The aggregation behavior of a surface active ionic liquid (SAIL) 1-butyl-3-methylimidazolium octyl sulfate, [C4mim][C8OSO3], has been investigated with the addition of different alcohol additives 1,2-propanediol, 1-propanol, and 2-propanol at lower compositions (10%, 15% and 20%) in aqueous medium. The aggregation properties have been studied using surface tension and fluorescence techniques at 298.15K and using conductance at temperatures 288.15, 298.15, and 308.15K. The critical aggregation concentration (cac) of [C4mim][C8OSO3] is found to increase with the addition of alcohols. Various thermodynamic parameters of aggregation process viz. Gibbs free energy of aggregation (ΔGagg.o), standard enthalpy of aggregation (ΔHagg.o), and standard entropy of aggregation (ΔSagg.o) are calculated from the temperature dependent cac values. The negative values of (ΔGagg.o) suggest the spontaneous nature of aggregation process of [C4mim][C8OSO3] in presence of alcohols in aqueous medium. The large positive entropy values indicate that the process of aggregation is entropy driven in all investigated systems at studied temperature range. The cac was also measured from the variation in fluorescent vibronic bands of pyrene as probe (I376/I387) which is sensitive to the polarity of environment. A series of various surface active parameters such as surface tension at cac (γcac), surface pressure at the interface (Πcac), maximum surface excess concentration (Γmax), and minimum area occupied by the single SAIL molecule at air-solvent interface (Amin) are derived from surface tension measurements.

Study of salt effects on the aggregation behavior of ionic liquid 1-dodecyl-3-methylimidazolium bromide in aqueous solution

The effect of organic electrolytes Me4NBr, Et4NBr, Pr4NBr, Bu4NBr, Me4NCl and Me4NI on the aggregation behavior and thermodynamic properties of surface active ionic liquid (SAIL) 1-dodecyl-3-methylimidazolium bromide ([C12mim]Br) in aqueous solution was studied by volumetric, compressibility and conductometric measurements at different temperatures. The results show that all the electrolytes investigated effectively reduce critical micelle concentration and therefore have a salting-out effect on the aggregation of [C12mim]Br in aqueous solutions. It was also found that the salting-out-inducing anions are predominately responsible for the observed effect, while the cations have a very smaller effect on the salting-out strength. The ability of the anions to promote the aggregation of SAIL decreases in the order of I−>Br−>Cl−; however all the investigated cations Me4N+, Et4N+, Pr4N+ and Bu4N+ have a similar salting-out strength. Changes in the apparent molar volumes and isentropic compressibilities upon micellization were derived and the infinite dilution apparent molar properties of the monomer form of [C12mim]Br were determined. Furthermore the effect of electrolyte on the degree of anionic binding and the thermodynamic parameters of aggregation for [C12mim]Br in aqueous solutions were determined.

Effect of alkyl chain length and head group on surface active and aggregation behavior of ionic liquids in water

A series of surfactant-like ionic liquids (ILs), typically consisting of a long hydrocarbon tail and an ionic head group have been synthesized by a direct reaction of 1-methylimidazole and 1-choloroalkane, RCl, R=C10, C12, C14, C16 and C18, 1-methylpipridine or 1-methylpyrrolidine and 1-chlorooctadecane respectively. Surface activity and aggregation of these surfactants have been explored by surface tension and solution conductivity measurements. New results (critical aggregation concentrations (cac), and surface active parameters (at 298.15K), thermodynamic parameters of aggregation (at 298.15, 303.15 and 313.15K)) are reported. The increase in length of R decreased cac, minimum area/surfactant molecule at air/water interface and while the adsorption efficiency, surface excess concentrations, standard entropy of aggregation were increased indicating that the aggregation with in the temperature limits of present study is an entropy driven process. The analysis of the small angle neutron scattering (SANS) curves revealed that the aggregates are of oblate ellipsoidal shape and the aggregation numbers increased with the increase in the chain length (C10–C18) of alkyl branch and therefore it is suggested that the longer the alkyl chains, parallel would be their alignment in the core part of the aggregates. Comparison of surface active parameters for the three ILs with a common octadecyl chain but different cationic head groups revealed that the methylimidazolium moiety is more effective than methylpiperidine and methylpyrrolidine at air/water interface. Similarly, the number of molecules in an aggregate was found to be more, when the cationic head group is made up of π electron ring systems as compared to the one with point charge.

Effect of Ethylene Glycol and Its Derivatives on the Aggregation Behavior of an Ionic Liquid 1-Butyl-3-methyl Imidazolium Octylsulfate in Aqueous Medium

The effect of ethylene glycol (EG) and its derivatives, ethylene glycol monomethyl ether (EGMME), or ethylene glycol dimethyl ether (EGDME), on the aggregation behavior of a surfactant-like ionic liquid (IL), 1-butyl-3-methyl imidazolium octylsulfate, [C(4)mim][C(8)OSO(3)], in aqueous solutions is investigated using conductivity, surface tension, fluorescence, (1)H NMR, and dynamic light scattering (DLS) measurements. Thermodynamic parameters such as Gibbs free energy (ΔG(m)°), standard enthalpy (ΔH(m)°), and standard entropy (ΔS(m)°) of aggregation are determined from the temperature dependence of conductivity. The interfacial properties of IL at the air/water interface in various mixed solvents are evaluated from surface tension measurements. Information about the local microenvironment and size of the aggregates is obtained from steady-state fluorescence using pyrene as a polarity probe and DLS measurements, respectively. (1)H NMR data has been employed to get detailed insight into the effect of organic additives on the IL aggregate structure and aggregation number. It has been observed that the addition of organic additives to water decreases the spontaneity of aggregation of IL.

Characteristics of aggregation in aqueous solutions of dialkylpyrrolidinium bromides

Three pyrrolidinium-based ionic liquids—N-dodecyl-N-methylpyrrolidinium bromide, N-butyl-N-octylpyrrolidinium bromide, and N-butyl-N-dodecylpyrrolodinium bromide—were synthesized and characterized by their decomposition temperatures (Td) measured by thermogravimetric analysis, and by their melting point (Tm), glass transition (Tg) and crystallization temperatures (Tcryst) determined by differential scanning calorimetry. Their self-aggregation properties in aqueous solution were studied and their behavior is compared with that of analogous conventional cationic surfactants, namely tetra-alkylammonium bromide salts. The critical micellar concentration, cmcs were obtained by isothermal titration calorimetry (ITC); which were further validated by measurements of interfacial tension, fluorescence and NMR spectroscopy. Enthalpies of micellization were measured at three different temperatures using ITC. The Taylor dispersion method and DOSY NMR were used to determine diffusion coefficients of the ionic liquid surfactants in aqueous solution at 298.15K. Several correlations between structural features of the surfactant species, such as the number and size of their alkyl chains, and the thermodynamic quantities of micellization—expressed by experimental values of cmc, counter-ion binding fraction, ΔmicG∘, ΔmicH∘, and ΔmicS∘—are established. We could interpret the different contributions of the two alkyl side chains to the aggregation properties in terms of the balance of interactions in homogeneous and micellar phases, contributing to understanding the aggregation behavior of ionic liquids in water and the parallel between these systems and traditional ionic surfactants.

Aggregation Behavior Modulation of 1-Dodecyl-3-methylimidazolium Bromide by Organic Solvents in Aqueous Solution

Material preparation in ionic liquids and environmental pollution control by ionic liquids are often closely dependent on the aggregation behavior of ionic liquids in solution. In the present work, conductivity, fluorescence probe, and dynamic light scattering techniques have been used to study the effect of organic solvents on the aggregation behavior of 1-dodecyl-3-methylimidazolium bromide in water. It was shown that the critical aggregation concentration (CAC), the ionization degree of the aggregates (α), and the standard Gibbs energy of aggregation (ΔG(m)°) of the ionic liquid increase, while its aggregation number (N(agg)) and aggregates' size decrease with increasing concentration of organic additives in water. These results have been discussed from the favorable interactions of alkyl chain of the ionic liquid with the mixed solvents. It is suggested that the solvophobic parameter, characterized quantitatively by Gibbs energy of transfer of hydrocarbon from gas into a given solvent, can be used to account for the effect of organic additives on the formation and growth of the ionic liquid aggregates in water. Aggregation behavior of ionic liquids in aqueous organic solutions can be modulated simply by the solvophobic parameters of hydrocarbon in the mixed solvents.

Surface active and aggregation behavior of methylimidazolium-based ionic liquids of type [C n mim] [X], n = 4, 6, 8 and [X] = Cl−, Br−, and I− in water

The surface active and aggregation behavior of ionic liquids of type [C n mim][X] (1-alkyl-3-methylimidazolium (mim) halides), where n = 4, 6, 8 and [X] = Cl−, Br− and I− was investigated by using three techniques: surface tension, 1H nuclear magnetic resonance (NMR) spectroscopy, small-angle neutron scattering (SANS). A series of parameters including critical aggregation concentrations (CAC), surface active parameters and thermodynamic parameters of aggregation were calculated. The 1H NMR chemical shifts and SANS measurements reveal no evidence of aggregates for the short-chain 1-butylmim halides in water and however small oblate ellipsoidal shaped aggregates are formed by ionic liquids with 1-hexyl and 1-octyl chains. Analysis of SANS data analysis at higher concentrations of [C8mim][Cl] showed that the microstructures consist of cubically packed molecules probably through π–π and hydrogen bond interactions.

Salt Effect on the Aggregation Behavior of 1-Decyl-3-methylimidazolium Bromide in Aqueous Solutions

Understanding of the specific salt effect on the aggregation behavior of ionic liquids (ILs) is relevant to multiple applications. In this work, the influence of a series of 15 salts on the aggregation behavior of [C(10)mim]Br in aqueous solutions has been investigated by conductivity, fluorescence, and dynamic light scattering. It was shown that NaCl, NaBr, NaI, CH(3)CO(2)Na, NaSCN, NaNO(3), NaBrO(3), NaClO(3), C(6)H(5)COONa, Na(2)CO(3), Na(2)SO(4), Na(2)C(4)H(4)O(6), and Na(3)CH(5)O(7) have salting-out effect, whereas FeBr(3) and AlBr(3) have salting-in effect on the aggregation of [C(10)mim]Br in aqueous solutions. The effect of anions of the added sodium salts on the critical aggregation concentration (CAC), degree of anionic binding (beta), and aggregation number (N(agg)) of the IL basically follows the Hofmeister series, and the CAC values decrease but the beta and N(agg) values increase with increasing concentration of the salts. Hydrophobicity of the anions is suggested to play important roles in the salt effect on the aggregation of [C(10)mim]Br in aqueous solutions. Furthermore, the IL aggregates were found to grow slowly as the increase of the salt concentrations under studied static conditions, and resulting in the increased aggregation number of the IL. These results are expected to be useful in the applications of ionic liquids.

Structural Effects of Anions and Cations on the Aggregation Behavior of Ionic Liquids in Aqueous Solutions

The formation of ionic liquids aggregates in aqueous solution is of great importance to the future applications of ionic liquids. In this work, aggregation behavior of 1-alkyl-3-methylimidazolium salts [C8mim]X (X = Cl, Br, [NO3], [CH3COO], [CF3COO], [CF3SO3], and [ClO4]), 1-octyl-4-methylpyridinium bromide (4m-[C8pyr]Br), and 1-methyl-1-octylpyrrolidinium ([C8mpyrr]Br) has been investigated in aqueous solutions by conductivity, volume, fluorescence, dynamic light scattering, and transmission electron microscopy. The critical aggregation concentration (CAC), ionization degree of the aggregates alpha, the standard Gibbs energy of aggregation deltaG(m)degrees, the average aggregation number N, the apparent molar volumes at critical aggregation concentration V(phi,CAC), the apparent molar volumes in aggregation phase V(phi)mic, and the change of the apparent molar volumes upon aggregation deltaV(phi,m), have been derived from the experimental data for these ionic liquids. It is found that both nature of the anions and ring type of the cations significantly affect the aggregation in aqueous solution. The anionic effect basically follows the Hofmeister series, and the ability of anionic hydration is predominant for the aggregation behavior of the ionic liquids. Hydrophobicity and steric hindrance of the cations as well as binding strength of the cations with the anions are suggested to play important roles in the aggregation of [C8mim]Br, 4m-[C8pyr]Br, and [C8mpyrr]Br. The investigated ILs were found to form spherical aggregates. Structures of anions and cations have very weak effects on the morphology, but they do affect the aggregate sizes.

Aggregation Behavior of Ionic Liquids in Aqueous Solutions: Effect of Alkyl Chain Length, Cations, and Anions

Self aggregation of the ionic liquids, 1-butyl-3-methylimidazolium chloride [C4mim][Cl], 3-methyl-1-octylimidazolium chloride [C8mim][Cl], 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim][BF4], N-butyl-3-methylpyridinium chloride [C4mpy][Cl], in aqueous solution has been investigated through 1H nuclear magnetic resonance (NMR) and steady-state fluorescence spectroscopy. Aggregation properties were determined by application of mass action theory to the concentration dependence of 1H NMR chemical shifts. Aggregation properties showed fairly good agreement with the previously reported results obtained from small angle neutron scattering, conductivity, and surface tension measurements. A detailed analysis of chemical shifts of water and various protons in ILs has been employed to probe the aggregate structure. Fluorescence spectroscopy provided important information about the critical aggregation concentration (cac) and the microenvironment of the aggregates. We could also observe a break point quite consistent with that of 1H NMR and fluorescence spectroscopy at cac from the concentration dependence of refractive index measurements. Standard free energies of aggregation DeltaGom of various ILs derived using the refractive index/concentration profiles were found comparable to those of classical ionic surfactants.