Surface tensions of imidazolium based ionic liquids: anion, cation, temperature and water effect

This work addresses the experimental measurements of the surface tension of eight imidazolium based ionic liquids (ILs) and their dependence with the temperature (288-353 K) and water content. The set of selected ionic liquids was chosen to provide a comprehensive study of the influence of the cation alkyl chain length, the number of cation substitutions and the anion on the properties under study. The influence of water content in the surface tension was studied for several ILs as a function of the temperature as well as a function of water mole fraction, for the most hydrophobic IL investigated, [omim][PF(6)], and one of the more hygroscopic IL, [bmim][PF(6)]. The surface thermodynamic functions such as surface entropy and enthalpy were derived from the temperature dependence of the surface tension values.     

Imidazolium-based ionic liquids formed with dicyanamide anion: influence of cationic structure on ionic conductivity

Ionic liquids composed of dicyanamide anion and various imidazolium-based cations were prepared, and the influence of structural variations such as substituting a hydrogen at 2-position and changing the sort of alkyl group at 1-position of imidazolium cations on their thermal behavior, density, solvatochromic effects, viscosity, ionic conductivity, and surface tension was characterized. The substitution of the 2-hydrogen for methyl group or N-methylimidazole decreases the fluidity and ionic conductivity, mainly caused by the increased cohesive energy associated with the increasing cation size. Chain branching at 1-position also gives rise to the pronounced depression of the fluidity and ionic conductivity, presumably as a consequence of the increased pi-pi interactions between imidazolium rings. We found that the surface tension of the present ionic liquids is in inverse proportion to the molar concentration, which can be originally rationalized on the basis of the hole theory.     

Preparation and characterization of amino or carboxyl-functionalized ionic liquids

New functionalized ionic liquids,1-carboxylmethyl-3-methylimimidazolium hexafluorophosphate or fluoborate and 1-ami- noethyl-3-methylimimidazolium hexafluorophosphate or fluoborate have been synthesized and investigated.The obtained amino or carboxyl-functionalized ionic liquids were all characterized by FT-IR,~1H NMR and MS(ESI)and their properties such as freezing point,viscosity,solubility,specific gravity,surface tension,and interracial tension were also determined.     

Surface tension measurements of imidazolium-based ionic liquids at liquid–vapor equilibrium

A series of high quality 1-alkyl-3-methylimidazolium-based ionic liquids are synthesized and used for studying their surface tension. The capillary rise method is used for measuring the surface tension of I⁻, Cl⁻, PF₆⁻, and BF₄⁻ salts in the temperature range 298–393 K. The capillary apparatus is evacuated and sealed under vacuum. The experimental results show that surface tension of these compounds depend systematically on temperature.     

Interface between ionic and nonionic liquids: theoretical study

We use a Flory-Huggins type approach to calculate the structure and the surface tension coefficient of the boundary between ionic and nonionic liquids. The mixture of ionic and nonionic liquids is treated as a "three-component" system including anions, cations, and neutral molecules. We show that if the affinities of the cations and the anions to the neutral molecules are different, the interface comprises an electric double layer. The presence of this layer (uncompensated electric field) stabilizes the interface: the field inhibits the ions segregation at the interface and increases the surface tension. On the other hand, the short-range volume interactions promote the segregation and decrease the surface tension. Furthermore, the surface tension coefficient can be negative, if the difference of the affinities is high enough. It implies a possibility of microphase separation of the system.     

Synthesis and adhesion property of waterborne polyurethanes with different ionic group contents

In this study, a series of waterborne polyurethanes (WPUs) with different ionic group contents were synthesized by varying the amount of the internal emulsifier 2,2-dimethylolpropionic acid (DMPA). The effects of the ionic group content on the stability and adhesion behavior of WPUs were investigated in terms of particle size, viscosity, surface tension, interfacial tension, contact angle, and adhesion performance. It was found that stable WPUs could be obtained when the DMPA content was larger than 3.5 wt% (with respect to the total solid content). Adhesion performance of WPUs to substrates was mainly affected by the wetting of WPUs on the substrates and the molecular interactions between them. With the increase of DMPA content, the surface tension of WPUs as well as the interfacial tension and contact angle between WPUs and polyethylene terephthalate (PET) films increased. Moreover, the adhesion strength of the WPUs firstly increased and then gradually decreased. The type of debonding failure of all the synthesized WPUs on PET films was cohesion failure due to their lower cohesion strength, which was theoretically testified by the comparison between the adhesion energy and cohesion energy.     

Surface tension of 1-alkyl-3-methylimidazolium based ionic liquids with trifluoromethanesulfonate and tetrafluoroborate anion

The amount of available accurate experimental data on the surface tension of ionic liquids is still limited; in many cases the data are rare or even absent. In the present study, air-liquid interfacial tension data were determined experimentally for five 1-C_n-3-methylimidazolium based ionic liquids (n = 2, 4, and 6), three with trifluoromethanesulfonate and two with tetrafluoroborate anion, at atmospheric pressure in the temperature range from 268 to 356 K. The resultant surface tension data are average values of the measurements repeated many times at each set point temperature. The accuracy of the results, was confirmed by employing the Wilhelmy plate and the du Noüy ring methods in parallel, using the Krüss K100MK2 tensiometer. For the Wilhelmy plate data the combined standard uncertainty is estimated to be about 0.05 mN m⁻¹. The data obtained by du Noüy method show about up to seven times greater scatter than those obtained by the Wilhelmy plate method. To the 50 up to now published surface tension values for the five studied ionic liquids the present study adds further 175 data points. In contrast to that of n-alkanes, the surface tension of 1-alkyl-3-methylimidazolium based ionic liquids decreases and their surface entropy increases with the cation alkyl chain length.     

Molecular dynamics simulations of the surface tension of ionic liquids

We report molecular dynamics computer simulations of the surface tension and interfacial thickness of ionic liquid-vapor interfaces modeled with a soft core primitive model potential. We find that the surface tension shows an anomalous oscillatory behavior with interfacial area. This observation is discussed in terms of finite size effects introduced by the periodic boundary conditions employed in computer simulations. Otherwise we show that the thickness of the liquid-vapor interface increases with surface area as predicted by the capillary wave theory. Data on the surface tension of size-asymmetric ionic liquids are reported and compared with experimental data of molten salts. Our data suggest that the surface tensions of size-asymmetric ionic liquids do not follow a corresponding states law.     

Wetting study of imidazolium ionic liquids

In this work, we present a systematic contact angles study of a series of 1-alkyl, 3-methyl-imidazolium ionic liquids (ILs) on well-defined polar and nonpolar monolayer surfaces supported on Si wafers. The advancing and receding contact angles of ILs were used to determine the surface energy of the monolayer surfaces using Neumann's equation-of-state and Zisman's critical surface tension approaches. In parallel, the contact angles of conventional probe fluids (molecular liquids) including water, formamide, methylene iodide, ethylene glycol, and hexadecane were determined on the same surfaces. The results obtained showed a great deal of similarity in wetting behavior of ionic vs molecular probe fluids: the contact angles of both sets of liquids followed the same patterns in accord with the surface tension of the fluid. A good agreement was found between the surface energy determined by different sets of liquids.     

Correlation between surface tension and void fraction in ionic liquids

An effort to systematize published and new data on the surface tension gamma of ionic liquids (ILs) is based on the hypothesis that the dimensionless surface tension parameter gamma V v (2/3)/ kT is a function of the void fraction x v = V v/ V m. The void volume V v is defined as the difference between the liquid volume V m occupied by an ion pair (known from cationic and anionic masses and liquid density measurements) and the sum V (+) + V (-) of the cationic and anionic volumes (known from crystal structures), while kT is the thermal energy. Our hypothesis that gamma V m (2/3)/ kT = G( x v) is initially based on cavity theory. It is then refined based on periodic lattice modeling, which reveals that the number N of voids per unit cell (hence the dimensionless surface tension) must depend on x v. Testing our hypothesis against data for the five ILs for which surface tension and density data are available over a wide range of temperatures collapses all of these data almost on a single curve G( x v), provided that slight (4%) self-consistent modifications are introduced on published crystallographic data for V (+) and V (-). An attempt to correlate the surface tension vs temperature data available for inorganic molten salts is similarly successful, but at the expense of larger shifts on the published ionic radii (8.8% for K; 3.3% for I). The collapsed G( x v) curves for ILs and inorganic salts do not overlap anywhere on x v space, and appear to be different from each other. The existence of a relation between gamma and x v is rationalized with a simple capillary model minimizing the energy. Our success in correlating surface tension to void fraction may apply also to other liquid properties.     

Surface adsorption and micelle formation of surface active ionic liquids in aqueous solution

Aqueous solutions of three kinds of surface active ionic liquids composed of the 1-alkyl-3-methylimidazolium cation have been investigated by means of surface tension and electrical conductivity measurements at room temperature (298 K). The surface tension measurements provided a series of parameters, including critical micelle concentration (cmc), surface tension at the cmc (gammacmc), adsorption efficiency (pC20), and effectiveness of surface tension reduction (Picmc). In addition, with application of the Gibbs adsorption isotherm, maximum surface excess concentration (Gammamax) and minimum surface area/molecule (Amin) at the air-water interface were estimated. The effect of sodium halides, NaCl, NaBr, and NaI, on the surface activity was also investigated. It was found that both the pC20 and the Picmc were rather larger than those reported for traditional ionic surfactants and the cmc values were somewhat lower than those for typical cationic surfactants, alkyltrimethylammonium bromides, and comparable to typical anionic surfactants, sodium alkyl sulfates. These results demonstrate that the surface activity of long-chained imidazolium IL is somewhat superior to that of conventional ionic surfactants.     

Influence of the anion on the surface tension of 1-ethyl-3-methylimidazolium-based ionic liquids

The air–liquid interfacial tensions of eight ionic liquids, from (298 to 343) K, are presented in this work. The studied ionic liquids are formed by the fixed 1-ethyl-3-methylimidazolium cation combined with the anions acetate, dicyanamide, dimethylphosphate, methylphosphonate, methanesulfonate, thiocyanate, tosylate, and trifluoromethanesulfonate. The selected ionic liquids allowed a comprehensive study through the influence of the anion nature on the surface tension and on their surface ordering. A slight dependence of the surface tension with the ionic liquid molar volume was identified. The surface thermodynamic functions are mainly controlled by the anion which constitutes a given ionic liquid. The hypothetical critical temperatures of all ionic liquids were estimated by means of the Eötvos and Guggenheim correlations and are presented.     

Aggregation behavior of a fluorinated surfactant in 1-butyl-3-methylimidazolium ionic liquids

The aggregation behavior of a fluorinated surfactant (FC-4) was studied by surface tension measurements in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4]) and hexafluorophosphate ([bmim][PF 6]) at various temperatures. A series of surface properties, including adsorption efficiency (p C 20), effectiveness of surface tension reduction (Pi CAC), maximum surface excess concentration (Gamma max) and minimum surface area/molecule (A(min)) at the air-water interface were estimated. By comparing the fluorinated surfactant with traditional surfactants, we deduced that the surface activity of the fluorinated surfactant in ILs was superior to the activity of other surfactants. From the CAC values and their temperature dependence, we estimated the thermodynamic parameters of aggregate formation. The thermodynamic parameters indicate that the aggregate of FC-4 in [bmim][BF 4] is a traditional micelle, while the aggregate of FC-4 in [bmim][PF 6] is nanodroplets composed of FC-4 molecules segregated from the solution phase. These results were further confirmed by (1)H NMR measurements.     

Surface tension of four binary systems containing (1-ethyl-3-methyl imidazolium alkyl sulphate ionic liquid + water or + ethanol)

In this work, we present surface tension experimental measurements for eight binary systems containing water or ethanol and an ionic liquid (IL) of the 1-ethyl-3-methyl imidazolium alkyl sulphate family, being the alkyl chain of the anion: ethyl, butyl, hexyl and octyl. Measurements were performed at the temperature of 25.0 °C and atmospheric pressure. All four ILs are completely miscible with water and ethanol, but for a concentration range of the octyl sulphate IL aqueous system the mixture jellifies, and so it is not possible to measure its surface tension. These measurements allow us to study the influence of the anion size on the surface tension for the pure IL compounds, and the role of the two different solvents in the surface tension behaviour. Thus, we observe that it is completely different when mixed with water or with ethanol, as also happens in other mixtures with different ionic liquids. From the experimental data, we extract surface tension deviations using the most popular definition. The calculated deviations for the ethanol based system are fitted using the Redlich–Kister equation and a novel one previously reported by us. Furthermore, we have also calculated the reduced surface pressure for the aqueous mixtures, which is fitted with good agreement using a theoretical equation obtained from the Bahe–Varela pseudo-lattice model.     

The ionic parachor and predicting properties of amino acid ionic liquid homologues of 1-alkyl-3-methylimidazolium glycine

The amino acid ionic liquids(AAILs) [C3mim][Gly](1-propyl-3-methylimidazolium glycine) and [C4mim][Gly](1-butyl-3methylimidazolium glycine) have been prepared by the neutralization method and characterized by 1 H NMR spectroscopy and differential scanning calorimetry(DSC).The values of their density,surface tension and refractive index were measured at(298.15 ± 0.05) K.Since the AAILs can form strong hydrogen bonds with water,small amounts of water are difficult to remove from the AAILs by common methods.In order to eliminate the effect of the impurity water,the standard addition method(SAM) was applied to these measurements.A new concept which is called the ionic parachor has been put forward.The [C n mim] + cations were treated as a group of reference ions and the individual values of their ionic parachor were evaluated in terms of an extrathermodynamic assumption.Then,using the values of the ionic parachor of reference ions,the parachor,surface tension γ and refractive index n D of the ionic liquids investigated in this work were estimated.The estimated values correlate quite well with the corresponding experimental values.     

Molecular dynamics simulations of ionic liquid-vapour interfaces: effect of cation symmetry on structure at the interface

The influence of alkyl chain symmetry of the imidazolium cation on the structure and properties of the ionic liquid-vapour interface has been addressed through molecular dynamics simulations. The anion chosen is bis(trifluoromethylsulfonyl)imide (NTf(2)). Profiles of number densities, orientation of cations, charge density, electrostatic potential, and surface tension have been obtained. At the interface, both cations and anions were present, and the alkyl chains of the former preferred to orient out into the vapour phase. A large fraction of cations preferred to be oriented with their ring-normal parallel to the surface and alkyl chains perpendicular to it. These orientational preferences are reduced in ionic liquids with symmetric cations. Although the charge densities at the interface were largely negative, an additional small positive charge density has been observed for systems with longer alkyl chains. The electrostatic potential difference developed between the liquid and the vapour phases were positive and decreased with increasing length of the alkyl group. The calculated surface tension of the liquids also decreased with increasing alkyl chain length, in agreement with experiment. The surface tension of an ionic liquid with symmetric cation was marginally higher than that of one with an asymmetric, isomeric cation.     

Aggregation behavior of silicone surfactants in ethylammonium nitrate ionic liquid

The aggregation behavior of two silicone surfactants (monomeric and Gemini) was studied by surface tension measurements in a room temperature ionic liquid, ethylammonium nitrate (EAN), at various temperatures. A series of parameters, including critical micelle concentration (CMC), surface tension at the CMC (γ _CMC), adsorption efficiency (pC ₂₀), and effectiveness of surface tension reduction (Π _CMC), were obtained. By comparing the silicone surfactants with traditional surfactants, we deduced that the surface activity of the silicone surfactants in EAN was superior to the activity of other surfactants. In addition, from the CMC values and their temperature dependence, we estimated the thermodynamic parameters of the micelle formation, $ \Delta G_m^0 $ , $ \Delta H_m^0 $ , and $ \Delta S_m^0 $ . It was revealed that the micellization of the silicone surfactants is entropy driven at low temperature and enthalpy driven at high temperature. Isothermal titration calorimetry measurements were also carried out to study the micellization of Gemini silicone surfactant. ¹H NMR was performed to study the silicone surfactant micelle formation mechanism in EAN.     

Aggregation behavior of long-chain imidazolium ionic liquids in ethylammonium nitrate

The aggregation behavior of long-chain imidazolium ionic liquids, 1-alkyl-3-methyl-imidazolium bromide (C_nmimBr, n?=?12, 14, 16) was studied by surface tension measurements in a room temperature ionic liquid, ethylammonium nitrate (EAN), at various temperatures. A series of parameters including critical micelle concentration (cmc), surface tension at the cmc (γ _cmc), and the effectiveness of surface tension reduction (Π_cmc) were obtained. In addition, from the cmc values and their temperature dependence, we estimated the thermodynamic parameters of the micelle formation, $ \Delta G_{\rm{m}}^0 $ , $ \Delta H_{\rm{m}}^0 $ , and $ \Delta S_{\rm{m}}^0 $ . The contribution of enthalpy term to the micelle formation is superior to that of entropy term. ¹H NMR was performed to study the C_nmimBr micelle formation mechanism in EAN.     

Surface tension and refractive index of pure and water-saturated tetradecyltrihexylphosphonium-based ionic liquids

Experimental data on the surface tension and refractive index of tetradecyltrihexylphosphonium-based ionic liquids with bromide, chloride, decanoate, methanesulfonate, dicyanimide, bis(2,4,4-trimethylpentyl)phosphinate and bis(trifluoromethylsulfonyl)imide anions are reported. The data were obtained for pure and water saturated samples at temperatures from 283 K to 353 K and at atmospheric pressure. The refractive index of the investigated ionic liquids decreases with increasing the water content in the sample. On the other hand, no clearly dependence of the surface tension with the water content up to a weight fraction of 16% was found. The prediction of the refractive index for the studied ionic liquids was also accomplished by a group contribution method and new values for the cation and diverse anions were estimated and proposed. The studied ionic liquids show lower surface tension in comparison with imidazolium-, pyridinium- or pyrrolidinium-based ionic liquids with a similar anion; also they show higher surface entropy than cyclic nitrogen-based fluids which indicates a lower surface organization. The anion dependence of the surface tension and surface entropy for the investigated ionic liquids is weaker than that for short-chain imidazolium-based ionic liquids. Their critical temperatures evaluated from Eötvos and Guggenheim equations are also lower than those of N-heterocyclic ionic fluids.     

Peculiar surface behavior of some ionic liquids based on active pharmaceutical ingredients

The ionic liquids based on biologically active cations and anions, commonly designated by ionic liquids based on active pharmaceutical ingredients (ILs-APIs), are interesting compounds for use in pharmaceutical applications. Lidocaine docusate, ranitidine docusate, and didecyldimethylammonium ibuprofen are examples of promising ILs-APIs that were recently synthesized. They were submitted to biological testing and calorimetric measurements, but nothing is known about their surface properties. In this work, we measured the surface tension and the contact angles on both hydrophilic and hydrophobic surfaces in a temperature range as wide as possible. Based on the wettability data, the polarity fractions were estimated using the Fowkes theory. The peculiar surface behavior observed was tentatively attributed to the presence of mesophases.