Adsorption and aggregation properties of novel star-shaped gluconamide-type cationic surfactants in aqueous solution

The adsorption and aggregation behavior of novel star-shaped gluconamide-type cationic surfactants N-dodecyl-N,N-bis[(3-D-gluconylamido)propyl]-N-alkylammonium bromide (C_nDBGB, where n represents hydrocarbon chain lengths of 10, 12, and 14) has been studied on the basis of static/dynamic surface tension, conductivity, dynamic light scattering (DLS), and transmission electron microscopy (TEM) data. The static surface tension of the C_nDBGB aqueous solution measured at the critical micelle concentration (CMC) is observed to be significantly lower than that of the corresponding monomeric surfactants. The dynamic surface tension results indicate that adsorption process of above CMC is a mixed diffusion–kinetic adsorption mechanism. From the results of temperature dependent conductivity measurements, we could obtain the degree of counterion binding (β) and the thermodynamic parameters such as standard free energy (ΔG _mic ⁰ ), enthalpy (ΔH _mic ⁰ ), and entropy (ΔS _mic ⁰ ) of aggregation. With a combination of the DLS and TEM data, a size transformation of the micelles is suggested to occur with an increase in the concentration.     

Effect of electrolytes on surface tension and surface adsorption of 1-hexyl-3-methylimidazolium chloride ionic liquid in aqueous solution

Surface and bulk properties of 1-hexyl-3-methylimidazolium chloride [C₆mim][Cl] as an ionic liquid (IL) have been investigated by surface tension and electrical conductivity techniques at various temperatures. Results reveal that the ionic liquid behaves as surfactant-like and aggregates in aqueous solution. Critical aggregation concentration (cac) values obtained by conductivity and surface tension measurements are in good agreement with values found in the literature. A series of important and useful adsorption parameters including cac, surface excess concentration (Γ), and minimum surface area per molecule (A_min) at the air + water interface were estimated from surface tension in the presence and absence of different electrolytes. Obtained data show that the surface tension as well as the cac of [C₆mim][Cl] is reduced by electrolytes. Also, values of surface excess concentration (Γ) show that the IL ions in the presence of electrolyte have much larger affinity to adsorption at the surface and this affinity increased in aqueous electrolyte solution in the order of I^? > Br^? > Cl^? for counter ion of salts that was explained in terms of a larger repulsion of chloride anions from interface to the bromide and iodide anion as well as difference in their excess polarizability.     

Thermodynamic studies of the ionic liquid 1-hexyl-3-methylimidazolium chloride [C6mim][Cl] in polyethylene glycol aqueous solutions

Vapour pressure osmometery (VPO) measurements at T = 308.15 K for {[C₆mim][Cl] + water}, {[C₆mim][Cl] + (0.005, 0.0155, and 0.0263) mol · kg⁻¹ PEG2000 + water} and {[C₆mim][Cl] + (0.0017, 0.0052, and 0.0088) mol · kg⁻¹ PEG6000 + water} systems and isopiestic measurements at T = 298.15 K for {[C₆mim][Cl] + PEG2000 + water} and {[C₆mim][Cl] + PEG6000 + water} systems have been carried out. The VPO measurements were carried out at very low concentrations of PEG and from which the values of the water activities, osmotic coefficients, vapour pressure and activity coefficients were obtained. The data obtained from the VPO method show that over the whole concentration range of the ionic liquid (IL), the activity coefficients of [C₆mim][Cl] in the presence of PEG2000 are increased. Although, at high IL concentrations, the values of the activity coefficient of [C₆mim][Cl] in the presence of PEG6000 are also increased, however for low concentrations of IL the values of the activity coefficient of [C₆mim][Cl] in pure water are larger than those in aqueous PEG6000 solutions. For a known IL concentration, the values of water activity coefficient for the binary {[C₆mim][Cl] + water} system are larger than those for the ternary {[C₆mim][Cl] + PEG + water} systems and decrease by increasing the concentration of PEG or decreasing the molar mass of PEG. The constant water activity lines of the all ternary systems obtained from the isopiestic method show positive deviation from the linear isopiestic relation (Zdanovskii–Stokes–Robinson rule) derived using the semi-ideal hydration model. The results have been interpreted in terms of the solute–water and solute–solute interactions.     

(Vapour + liquid) equilibria,volumetric and compressibility behaviour of binary and ternary aqueous solutions of 1-hexyl-3-methylimidazolium chloride,methyl potassium malonate,and ethyl potassium malonate

(Vapour + liquid) equilibrium data (water activity, vapour pressure, osmotic coefficient, and activity coefficient) of binary aqueous solutions of 1-hexyl-3-methylimidazolium chloride ([C₆mim][Cl]), methyl potassium malonate, and ethyl potassium malonate and ternary {[C₆mim][Cl] + methyl potassium malonate} and {[C₆mim][Cl] + ethyl potassium malonate} aqueous solutions were obtained through the isopiestic method at T = 298.15 K. These results reveal that the ionic liquid behaves as surfactant-like and aggregates in aqueous solutions at molality about 0.4 mol · kg⁻¹. The constant water activity lines of all the ternary systems investigated show small negative deviations from the linear isopiestic relation (Zdanovskii–Stokes–Robinson rule) derived using the semi-ideal hydration model. The density and speed of sound measurements were carried out on solutions of methyl potassium malonate and ethyl potassium malonate in water and of [C₆mim][Cl] in aqueous solutions of 0.25 mol · kg⁻¹ methyl potassium malonate and ethyl potassium malonate at T = (288.15 to 308.15) K at atmospheric pressure. From the experimental density and speed of sound data, the values of the apparent molar volume, apparent molar isentropic compressibility and excess molar volume were evaluated and from which the infinite dilution apparent molar volume and infinite dilution apparent molar isentropic compressibility were calculated at each temperature. Although, there are no clear differences between the values of the apparent molar volume of [C₆mim][Cl] in pure water and in methyl potassium malonate or ethyl potassium malonate aqueous solutions, however, the results show a positive transfer isentropic compressibility of [C₆mim][Cl] from pure water to the methyl potassium malonate or ethyl potassium malonate aqueous solutions. The results have been interpreted in terms of the solute–water and solute–solute interactions.     

Conductivity and fluorescence studies on the micellization properties of sodium cholate and sodium deoxycholate in aqueous medium at different temperatures: Effect of selected amino acids

In order to add to the existing knowledge of aqueous solution behavior of bile salts in presence of amino acids, the micellization properties of sodium cholate (NaC) (1 to 20) mmol · kg⁻¹, and sodium deoxycholate (NaDC) (0.5 to 10) mmol · kg⁻¹ in 0.1 mol · kg⁻¹ aqueous solution of glycine, leucine, methionine, and histidine have been investigated at different temperatures (293.15 to 318.15) K at intervals of T = 5 K by using conductivity and fluorescence probe studies. The critical micelle concentration (CMC) values have been determined and elucidated in terms of hydrophobicity as well as hydrophilicity of NaC and NaDC in aqueous solution of these additives. Thermodynamic parameters of micellization viz. standard Gibbs free energy (Δ_micG^o), standard enthalpy (Δ_micH^o), and standard entropy (Δ_micS^o) have also been calculated to extract information regarding the nature of micellization of bile salts in aqueous solutions. The (enthalpy + entropy) compensation plots have been interpreted to the contribution of chemical part towards micellization or stability of the micelle formed.     

Prediction of thermophysical properties of novel ionic liquids based on serine [Cnmim][Ser] (n = 3, 4) using semiempirical methods

Two novel ionic liquids based on serine [C_nmim][Ser] (n = 3, 4) were prepared by the neutralization method and their structures were confirmed by ¹H NMR spectroscopy and differential scanning calorimetry (DSC). The density, surface tension, and refractive index of the two ILs were measured from T = (298.15 to 338.15) K. Since these ILs [C_nmim][Ser] (n = 3, 4) could form strong hydrogen bonds with water, small amount of water in the ILs is difficult to removed by common methods. In order to eliminate the effect of trace of water, the standard addition method (SAM) was applied to these measurements. On the basis of the experimental data, the speed of sound (μ), thermal expansion coefficient (α), molecular volume (V_m), standard entropy (S⁰298), entropy of surface (S_a), energy of surface (E_a), parachor (P), molar polarization (R_m), and polarization coefficient (α_p) were calculated, and the relationship between each of these properties of [C_nmim][Ser] (n = 3, 4) and temperatures was discussed. According to the additivity, the average value of anionic parachor, P_−(ave), was 180.81 for [Ser]⁻. At the same time, the surface tension of these serine ionic liquids could be estimated from their parachor and refractive index. The estimated values of the surface tension and the corresponding experimental data were almost identical.     

Solubilities of ammonia in basic imidazolium ionic liquids

Solubilities of ammonia in four conventional imidazolium ionic liquids: [C_nmim][BF₄] (n = 2, 4, 6, 8) have been measured. Isothermally fixed temperatures are 293.15, 303.15, 313.15, 323.15 and 333.15 K; the pressure is from 0 to 1.0 MPa. High solubilities of ammonia are found, and it is also found that the solubilities of ammonia increase when the length of cations’ alkyl increases (the ILs have the same anion), that is: [C₈mim]⁺ > [C₆mim]⁺ > [C₄mim]⁺ > [C₂mim]⁺. The solubility data have been correlated by the Krichevisky–Kasarnovsky equation, and then Henry's constants and partial molar volumes of NH₃ at infinite dilution are obtained. The thermodynamic properties such as solution enthalpy (Δ_solH), solution Gibbs free energy (Δ_solG), solution entropy (Δ_solS), and solution heat capacity (Δ_solC_p) of these systems are obtained.     

Aggregation behavior of imidazolium-based chiral surfactant in aqueous solution

A novel imidazolium-based chiral surfactant with a Y-type hydrophobic chain, (S)-(+)-1-(2,3-bis(octanoyloxy)propyl)-3-methylimidazolium chloride ([Bopmim]Cl), was synthesized. The aggregation behavior of [Bopmim]Cl in aqueous solution was then investigated by surface tension, electrical conductivity, ¹H NMR, and fluorescence measurements. Compared with [C₁₂mim]Cl, the critical micelle concentration for [Bopmim]Cl is lower, indicating that the novel chiral surfactant has superior capacity to form micelles. A larger value of pC ₂₀, a greater minimum area per surfactant molecule (A _min), a smaller degree of counterion binding (β), and a looser aggregate are caused by the relatively larger Y-type hydrophobic chain of [Bopmim]Cl. Furthermore, analysis of the ¹H NMR spectra revealed that the introduced Y-type hydrophobic chain may prevent the hydrophobic group from forming an extended chain configuration and cause a changeover from trans to gauche conformations upon micellization. The micelles of the novel chiral surfactant may provide some potential applications in the stereochemical recognition of surfaces or of biological structures.     

Study on the enthalpy of solution and enthalpy of dilution for the ionic liquid [C3mim][Val] (1-propyl-3-methylimidazolium valine)

A new amino acid ionic liquid (AAIL) [C₃mim][Val] (1-propyl-3-methylimidazolium valine) was prepared by the neutralization method. Using the solution-reaction isoperibol calorimeter, molar solution enthalpies of the ionic liquid [C₃mim][Val] with known amounts of water and with different concentrations in molality were measured at T = 298.15 K. In terms of standard addition method (SAM) and Archer’s method, the standard molar enthalpy of solution for [C₃mim][Val] without water, ${\mathrm{\Delta }}_{\text{s}}{H}_{\text{m}}^{\circ }$ = (−55.7 ± 0.4) kJ · mol⁻¹, was obtained. The hydration enthalpy of the cation [C₃mim]⁺, ΔH₊ ([C₃mim]⁺) = −226 kJ · mol⁻¹, was estimated in terms of Glasser’s theory. Using the RD496-III heat conduction microcalorimeter, the molar enthalpies of dilution, Δ_DH_m(m_i → m_f), of aqueous [C₃mim][Val] with various values of molality were measured. The values of Δ_DH_m(m_i → m_f) were fitted to Pitzer’s ion-interaction model and the values of apparent relative molar enthalpy, ^φL, calculated using Pitzer’s ion-interaction model.     

Micellization behavior of the ionic liquid lauryl isoquinolinium bromide in aqueous solution

The micellization behavior of the ionic liquid lauryl isoquinolinium bromide ([C₁₂iQuin]Br) in aqueous solution has been assessed using surface tension, electrical conductivity, and ¹H nuclear magnetic resonance (NMR) measurements. The results reveal that the critical micelle concentration (CMC) and constant surfactant tension (γ _cac) are lower than that of butyl isoquinolinium bromide ([C₄iQuin]Br), octyl isoquinolinium bromide ([C₈iQuin]Br, and lauryl pyridinium bromide ([C₁₂Pyr]Br). ¹H NMR spectra show the evidence of paralleled π-stacking of adjacent isoquinoline rings. To elucidate the effect of the π–π interactions on the aggregation process, thermodynamic parameters such as the standard free energy, enthalpy, and entropy of aggregation have been discussed. These parameters are evaluated from the CMC with temperature by fitting these values to expressions derived from a micellization thermodynamic model. The enthalpy–entropy compensation phenomenon has been observed in the micellization process of [C₁₂iQuin]Br in water, and the presence of isoquinoline cations is responsible for the decrease in the ΔH _mic ^? , compared with [C₁₂Pyr]Br which has the same alkyl chain and counter-ion.     

A new method of immobilizing ionic liquids into layered zirconium phosphates

In this work, a new method of a series of ion liquids (ILs) 1-alkyl-3-methylimidazolium chloride [Cnmim]Cl (n = 2, 4, 6, 8) intercalation into layered zirconium phosphates was investigated. It was found that theα-ZrP-2BA (i.e. pre-intercalated BA was arranged in a bilayer mode in the galleries ofα-ZrP) was a suitable host for intercalation ILs: ILs was inserted through exchanging pre-intercalated BA. And the intercalation orientation was investigated by use of X-ray diffraction (XRD) experiments and molecular modeling calculation.     

Relative hydrophobicity of equilibrium phases in biphasic systems (ionic liquid + water)

Partition coefficients for a series of dinitrophenylated (DNP) amino acids in biphasic systems composed of hydrophobic ionic liquids and water were experimentally determined. The ionic liquids used were three 1-alkyl-3-methylimidazolium tetrafluoroborates, [C_nmim][BF₄], with alkyl chain substituents hexyl, octyl, and decyl. The liquid–liquid phase diagram for the system ([C₁₀mim][BF₄] + water) was experimentally determined. DNP amino acids distribute preferentially to the IL-rich phase and ([C₁₀mim][BF₄] + water) was found to be the system with the lowest partition coefficients for the solutes studied. The experimental partition coefficients decrease as the size of the alkyl side chain in the ionic liquids increases. The free energy of transfer of a methylene group between phases was calculated through the partition coefficients, which provides a measure of the relative hydrophobicity of the equilibrium phases. It was found that the system ([C₁₀mim][BF₄] + water) presents a lower free energy (and thus a lower relative hydrophobicity) than the system ([C₈mim][BF₄] + water). In order to better understand this result, the micellar behavior of the three ionic liquids was studied. Electrical conductivities of several aqueous solutions of the ionic liquids were measured to determine the critical micelle concentration (CMC) and the degree of micelle ionization, α, of the three ionic liquids. From these two properties it was possible to obtain the free energy of micellization, ΔG_mic, for the ionic liquids.     

Complexation behavior of gelatin with amphiphilic drug imipramine hydrochloride as studied by conductimetry,surface tensiometry and circular dichroism studies

Herein we report our studies carried out on the interaction between IMP and gelatin in aqueous medium at 25 °C using conductimetry, surface tensiometry and circular dichroism (CD) techniques. Both surface tensiometry and conductimetry results indicate that the drug interacts with the gelatin in a surfactant-like manner, i.e., both critical aggregation (cac) and polymer saturation points (psp) were observed. The interaction starts with the formation of a highly surface-active complex as revealed by the lowering of surface tension on the addition of drug to the macromolecule. The decrease in cac on increasing gelatin concentration is an indication of the strong interaction between gelatin and IMP. However, at low concentration of gelatin the interaction was not much strong as exposed by surface tension study, i.e., the cac was not very clear (as with higher gelatin concentrations). As usual, the psp increased on increasing the gelatin concentration and was always higher than the critical micelle concentration of the drug in pure aqueous medium. Using CD measurements the influence of IMP on the secondary structure of gelatin in aqueous solutions was also investigated. CD studies (performed at very low drug concentrations) illustrated that the random coil content of gelatin increases with increasing drug concentration. Free energies of aggregation (ΔG_agg) and micellization (ΔG_mic) were computed with the help of degrees of micelle ionization obtained from the specific conductivity – [IMP] plots.     

Effect of alkyl chain length and temperature on the thermodynamic properties of ionic liquids 1-alkyl-3-methylimidazolium bromide in aqueous and non-aqueous solutions at different temperatures

The alkyl chain length of 1-alkyl-3-methylimidazolium bromide ([Rmim][Br], R = propyl (C₃), hexyl (C₆), heptyl (C₇), and octyl (C₈)) was varied to prepare a series of room-temperature ionic liquids (RTILs), and experimental measurements of density and speed of sound at different temperatures ranging from (288.15 to 308.15) K for their aqueous and methanolic solutions in the dilute concentration region (0.01 to 0.30) mol · kg^?1 were taken. The values of the compressibilities, expansivity and apparent molar properties for [C_nmim][Br] in aqueous and methanolic solutions were determined at the investigated temperatures. The obtained apparent molar volumes and apparent molar isentropic compressibilities were fitted to the Redlich–Mayer and the Pitzer’s equations from which the corresponding infinite dilution molar properties were obtained. The values of the infinite dilution molar properties were used to obtain some information about solute–solvent and solute–solute interactions. The thermodynamic properties of investigated ionic liquids in aqueous solutions have been compared with those in methanolic solutions. Also, the comparison between thermodynamic properties of investigated solutions and those of electrolyte solutions, polymer solutions, cationic surfactant solutions and tetraalkylammonium salt solutions have been made.     

Surface active and aggregation behavior of methylimidazolium-based ionic liquids of type [Cnmim] [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.     

Modulation of the aggregation behaviour and physicochemical properties of 1-dodecyl-3-methylimidazolium bromide in aqueous solutions by β-CD

The modulation of aggregation behaviour of ionic liquids (ILs) in aqueous media is one of the important research topics. In the present work, aggregation behaviour of 1-dodecyl-3-methylimidazolium bromide ([C₁₂mim]Br) modulated by beta-cyclodextrin (β-CD) has been investigated by using conductivity, volume, fluorescence, dynamic light scattering and transmission electron microscopy techniques. The results suggested that the addition of β-CD significantly affects the aggregation of [C₁₂mim]Br in aqueous solutions. For example, the apparent critical micelle concentration increases with the increase of β-CD concentration; the average micellar size reduced with the increasing concentration of β-CD, and the process for micelle formation of [C₁₂mim]Br in aqueous β-CD solution is driven by entropy at lower temperature, while driven by enthalpy at higher temperature. It is expected that findings in this study would shed light on the potential applications of IL in supramolecular chemistry.     

Thermodynamic properties and polymorphism of 1-alkyl-3-methylimidazolium bis(triflamides)

Heat capacities in a range of temperatures of (5 to 370) K, enthalpies and temperatures of phase transitions for 1-ethyl-3-methylimidazolium bis(triflamide) ([C₂mim][NTf₂]) and 1-octyl-3-methylimidazolium bis(triflamide) ([C₈mim][NTf₂]) have been determined by adiabatic calorimetry. [C₂mim][NTf₂] has been found to form four crystalline phases with different fusion temperatures. Formation of the phases can be controlled by the temperature of annealing during crystallization. [C₈mim][NTf₂] forms three sequences of crystalline modifications, each including two polymorphs. Based on results of the measurements, thermodynamic functions for the compounds under study have been calculated.A heat-capacity anomaly near T = 230 K reported earlier for [C₄mim][NTf₂] and [C₆mim][NTf₂] have been found in some crystalline modifications of both the studied compounds. The position of the anomaly depends on the temperature of annealing of the crystals.     

Thermodynamic properties of 1-alkyl-3-methylimidazolium bromide ionic liquids

Heat capacities and enthalpies of phase transitions for a series of 1-alkyl-3-methylimidazolium bromide ionic liquids have been measured by adiabatic calorimetry. Thermodynamic properties of the compounds were calculated in the temperature range of (5 to 370) K. Water was found to have an additive contribution to the heat capacities of [C₄mim]Br in the liquid state above T_fus and in the solid state below 160 K at w(H₂O) ⩽ 5 · 10⁻³.     

Aggregation behavior of naphthalimide fluorescent surfactants in aqueous solution

A group of novel fluorescent surfactants, N-n-alkyl-4-(1-methylpiperazine)-1,8-naphthalimide iodine [C_nndi]I (n?=?8, 10, and 12), have been synthesized and their aggregation behavior in aqueous solution have been explored by surface tension, electric conductivity, hydrogen-1 NMR spectra, absorption, and fluorescence spectra. Compared with traditional cationic surfactants, the [C_nndi]I have a rather lower critical micelle concentration and higher surface activity. Absorption and fluorescence spectra were proved to be facile method to monitor directly the aggregation states of fluorescent surfactant molecules in solution and revealed clearly the formation of face-to-face stacked structure of the [C_nndi]I molecules driven by the π–π interactions. The micelle formation process for [C_nndi]I was demonstrated to be enthalpy-driven in the temperature range investigated. Possible aggregation process was given based on the experimental results. The combination of dye and surfactant provides a way for monitoring the formation process of micelle directly by fluorescence spectra.