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.     

Self-assembly of nonionic surfactants into lyotropic liquid crystals in ethylammonium nitrate, a room-temperature ionic liquid

The stability of a variety of lyotropic liquid crystals formed by a number of polyoxyethylene nonionic surfactants in the room-temperature ionic liquid ethylammonium nitrate (EAN) is surveyed and reported. The pattern of self-assembly behaviour and mesophase formation is strikingly similar to that observed in water, even including the existence of a lower consolute boundary or cloud point. The only quantitative difference from water is that longer alkyl chains are necessary to drive the formation of liquid crystalline mesophases in EAN, suggesting that a rich pattern of "solvophobic" self-assembly should exist in this solvent.     

Structure of nonionic surfactant micelles in the ionic liquid ethylammonium nitrate

The structure of micelles formed by nonionic polyoxyethylene alkyl ether nonionic surfactants, C n E m , in the room-temperature ionic liquid, ethylammonium nitrate (EAN), has been determined by small-angle neutron scattering (SANS) as a function of alkyl and ethoxy chain length, concentration, and temperature. Micelles are found to form for all alkyl chains from dodecyl through to octadecyl. Dodecyl-chained surfactants have high critical micelle concentrations, around 1 wt%, and form weakly structured micelles. Surfactants with longer alkyl chains readily form micelles in EAN. The observed micelle structure changes systematically with alkyl and ethoxy chain length, in parallel with observations in aqueous solutions. Decreasing ethoxy chain length at constant alkyl chain length leads to a sphere to rod transition. These micelles also grow into rods with increasing temperature as their cloud point is approached in EAN.     

Aggregation behavior of long-chain ionic liquids in an ionic liquid

The aggregation behavior of long-chain ionic liquids 1-alkyl-3-methylimidazolium bromide (C(n)mimBr) in another ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)), was studied for the first time. Surface tension measurements revealed that aggregates are formed by C(n)mimBr, and freeze fracture transmission electron microscopy (FF-TEM) observations suggested the aggregates are spheres with a size much larger than traditional micelles. The sizes of the aggregates were further confirmed by dynamic light scattering (DLS) measurements.     

Ionic liquid nanotribology: mica-silica interactions in ethylammonium nitrate

Colloid probe atomic force microscopy has been used to study the nanotribological properties of the silica-ethylammonium nitrate (EAN)-mica system. Normal force curve measurements reveal a series of steps at separations that are consistent with the size of an EAN ion pair (0.5 nm) due to displacement of structured solvent layers as the two surfaces are brought together. At closest separations, two steps are measured with widths of 0.3 nm and 0.1 nm, which are too small to be due to an ion pair layer. The 0.3 nm step is attributed to a partial displacement of a silica-bound cation-rich layer, with residual cations being removed in the subsequent 0.1 nm step. Lateral force measurements reveal that the frictional response is dependent on the number of ion pair layers between the surfaces. At low forces, when there is more than a single layer of EAN between silica and mica, the lateral force increases relatively steeply with applied load, and is independent of the sliding speed. At intermediate forces, a single layer of cations in an intercalated bilayer structure is present between the surfaces. The friction coefficient (μ) increases logarithmically with sliding speed consistent with an activated, discontinuous sliding process. At high force, μ is small and once again, independent of sliding velocity. The adsorbed cation layer is bound primarily to mica and compressed by the high normal force. This robust layering with a well-defined sliding plane permits the colloid probe to slide easily over the mica surface.     

Aggregation behavior of p-n-alkylbenzamidinium chloride surfactants

The aggregation behavior of a novel class of surfactants, p-n-alkylbenzamidinium chlorides, has been investigated. The thermodynamics of aggregation of p-n-decylbenzamidinium chloride mixed with cationic and anionic cosurfactants has been studied using isothermal titration calorimetry. For mixtures of p-n-decylbenzamidinium chloride with n-alkyltrimethylammonium chlorides, the aggregation process is enthalpically more favorable than for the pure n-alkyltrimethylammonium chlorides, probably caused by diminished headgroup repulsion due to charge delocalization in the amidinium headgroup. A critical aggregation concentration between 3 and 4 mM has been extrapolated for p-n-decylbenzamidinium chloride at 40 degrees C, around two times lower than that of similar surfactants without charge delocalization in the headgroup and well comparable to that of similar surfactants with charge delocalization in the headgroup. In mixtures of p-n-decylbenzamidinium chloride with either sodium n-alkylsulfates or sodium dodecylbenzenesulfonate, evidence is found for the formation of bilayer aggregates by the pseudo-double-tailed catanionic surfactants composed of p-n-decylbenzamidinium and the anionic surfactant. These aggregates are solubilized to mixed micelles by excess free anionic surfactant at the measured critical aggregation concentration.     

Aggregation behavior of a model ionic liquid surfactant in monosaccharide + water solutions

Alkylimidazolium salts are a very important class of ionic liquids (ILs). The ILs containing long alkyl chains are a kind of model surfactants. In this paper, the aggregation behavior of 1-decyl-3-methylimidazolium chloride ([C(10)mim]Cl) was investigated for the first time in aqueous monosaccharide (glucose, galactose, xylose and arabinose) solutions by conductivity, fluorescence, NMR and dynamic light scattering (DLS). Thus a series of physico-chemical parameters for the aggregation of [C(10)mim]Cl-the critical aggregation concentration (CAC), ionization degree of the aggregates (α), the standard Gibbs energy of aggregation (ΔG(m)(0)), and the aggregation number (N) were derived from the experimental data. The results show that addition of small amounts of monosaccharides in aqueous solution can cause a variation in aggregation properties of the IL. The CAC values decrease with increasing molality of monosaccharides. In particular for different kinds of monosaccharides, we found that the CAC values are in the order: glucose>galactose (hexoses), xylose>arabinose (pentoses); xylose>glucose (1e2e3e4e), arabinose>galactoses (1e2e3e4a). These trends may be attributed to the slight difference in the stereo-structure of monosaccharide molecules. Finally a mechanism for the interaction of these monosaccharides with [C(10)mim]Cl was proposed.     

Aggregation behavior of nitrophenoxy-tailed quaternary ammonium surfactants

Cationic surfactants N,N,N-trimethyl-10-(4-nitrophenoxy)decylammonium bromide (N10TAB) and N,N,N',N'-tetramethyl-N,N'-bis[10-(4-nitrophenoxy)decyl]-1,6-hexanediammonium dibromide (N10-6-10N), bearing aromatic nitrophenoxy groups in the ends of their hydrophobic chains, have been synthesized, and their self-assembling properties in aqueous solutions have been studied by conductivity, isothermal titration microcalorimetry, 1H NMR spectroscopy, and dynamic light scattering. Below the critical micelle concentration, N10-6-10N can form premicelles with 2 or 3 surfactant molecules. Beyond the critical micelle concentration, the two surfactants have strong self-aggregation ability and can form micelles of rather small size and with small aggregation numbers N, which are 30 +/- 3 for N10TAB and 20 +/- 2 for N10-6-10N, respectively. Also, the variations in 1H NMR signals at different surfactant concentrations provide the information on the environmental change of the surfactants upon their micellization progress. The most prominent phenomenon is the shielding effect of the aromatic groups over the protons in the aliphatic chains, implying that the nitrophenoxy groups partially insert into the micelles and face the several middle methylenes of the hydrophobic side chains.     

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.     

Inclusion complexes of beta-cyclodextrin with ionic liquid surfactants

The three kinds of ionic liquid (IL) surfactants, 1-dodecyl-3-methylimidazolium hexafluorophosphate (C(12)mimPF(6)), 1-tetradecyl-3-methylimidazolium hexafluorophosphate (C(14)mimPF(6)), and 1-hexadecyl-3-methylimidazolium hexafluorophosphate (C(16)mimPF(6)), were used to form the inclusion complexes (ICs) with beta-cyclodextrin (beta-CD). The surface tension measurements revealed that there were two kinds of inclusion formations, 1:1 and 1:2 (beta-CD/IL) stoichiometry for beta-CD-C(12)mimPF(6) and beta-CD-C(14)mimPF(6) ICs, and only 1:1 stoichiometry for beta-CD-C(16)mimPF(6) ICs. These inclusion compounds were further characterized by XRD, (13)C CP/MAS NMR, (1)H NMR, rotating frame nuclear Overhauser effect spectroscopy (ROESY), and thermogravimetry (TGA). The results showed that these ICs were fine crystalline powder. The host-guest system presented a channel-type structure, and each glucose unit of beta-CD was in a similar environment. It was suggested that hydrophobicity played a crucial role in supporting the formation of ICs. The decomposition temperature of these ICs was lower than those of their precursors. Furthermore, the possible inclusion structures were also speculated. These inclusion behaviors are likely to be used to recover ILs in the process of their preparation.     

Aggregation behavior and solubilization properties of 3-hydroxypiperidinium surfactants

Russian Chemical Bulletin - Systematic data on the aggregation behavior of novel 3-hydroxypiperidinium surfactants in aqueous solutions were obtained. The ability of the surfactants to solubilize...     

Aggregation behavior of aqueous solutions of ionic liquids

The aggregation behavior in aqueous solutions of three ionic liquids based on the 1-alkyl-3-methylimidazolium cation has been investigated by means of surface tension, conductivity, and small-angle neutron scattering (SANS) measurements. From analysis of the SANS data, models for the shapes and sizes of aggregates have been proposed: the short-chain 1-butyl-3-methylimidazolium tetrafluoroborate [C4mim] [BF4] system can be best modeled by treating it as a dispersion of polydisperse spherical aggregates that form above a critical aggregation concentration, whereas the 1-octyl-3-methylimidazolium iodide, [C8mim] [I], solutions can be modeled as a system of regularly sized near-spherical charged micelles that form above a critical micelle concentration. Solutions of 1-octyl-3-methylimidazolium chloride, [C8mim]-[Cl], display weak long-range ordering of possibly disklike particles culminating in the formation of structures with distinct long-range order at higher concentrations.     

Aggregation behavior of morpholinium surfactants in the presence of organic electrolytes

It was shown that the addition of organic electrolytes to aqueous solutions of morpholinium surfactants facilitates micellization, reduces the micellar surface potential, increases the hydrodynamic diameter of micelles and favors the formation of cylindrical micelles which behave as pseudoplastic fluids. At low shear rates, the viscosity is extremely large and changes insignificantly, and it decreases sharply with increase in shear rate. Significant decrease in viscosity upon the increase in shear deformations indicates the orientation of cylindrical micelles along the direction of the flow with the increase in shear rate.     

Aggregation behavior of hexadecyltrimethylammonium surfactants with various counterions in aqueous solution

Both thermodynamic and microenvironmental properties of the micelles for a series of cationic surfactants hexadecyltrimethylammonium (C16TAX) with different counterions, F-, Cl-, Br-, NO3-, and (1/2)SO4(2-), have been studied. Critical micelle concentration (CMC), degree of micelle ionization (alpha), and enthalpy of micellization (DeltaH(mic)) have been obtained by conductivity measurements and isothermal titration microcalorimetry. Both the CMC and the alpha increase in the order SO4(2-) < NO3- < Br- < Cl- < F-, consistent with a decrease in binding of counterion, except for the divalent anion sulfate. DeltaH(mic) becomes less negative through the sequence NO3- < Br- < Cl- < F- < SO4(2-), and even becomes positive for the divalent sulfate. The special behavior of sulfate is associated with both its divalency and its degree of dehydration. Gibbs free energies of micellization (DeltaG(mic)) and entropies of micellization (DeltaS(mic)) have been calculated from the values of DeltaH(mic), CMC, and alpha and can be rationalized in terms of the Hofmeister series. The variations in DeltaH(mic) and DeltaS(mic) have been compared with those for the corresponding series of gemini surfactants. Electron spin resonance has been used to assess the micropolarity and the microviscosity of the micelles. The results show that the microenvironment of the spin probe in the C16TAX surfactant micelles depends strongly on the binding of the counterion.     

Aqueous immiscibility of cholinium chloride ionic liquid and Triton surfactants

The immiscibility windows of aqueous solutions containing the ionic liquid cholinium chloride (N_1112OHCl) and the non-ionic surfactants Triton X-100 and Triton X-102 have been determined by the cloud point method at temperatures ranging from T = (298.15 to 333.15) K. The experimental values have been correlated by using two well-known equations. The tie-lines have been ascertained by means of density and refractive indices measurement, and the experimental data have been modeled by the Othmer–Tobias, Bancroft and Setschenow equations. The use of cholinium chloride involves greater demixing capacity than other imidazolium-based ionic liquids.     

Influence of phosphonium-based ionic liquid on the micellization behavior of surfactants system and potential application in paracetamol drug aggregation

The current study examines the effects of a phosphonium-based ionic liquid, trihexyltetra-decylphosphonium bis-(2,4,4-trimethyl pentyl)phosphinate [THTDPP], on the micellization properties of surfactants, namely sodium dodecyl sulfate (SDS) and Triton X-100 by using the stalagmometry, viscosity, colorimetric, and FTIR methods. The surface adsorption parameters, such as CMC, γ_CMC, Γ_max, A_min, π_CMC, and _pC₂₀, were determined using the stalagmometry method. The results show that with the addition of different weight percentages of [THTDPP], the CMC and γ_CMC values decreased considerably in the following order: water >0.5 wt% of IL > 0.7 wt% of IL > 1.0 wt% of IL. The A_min values decreased with an increase in the wt% of IL for Triton X-100, but for SDS, this value increased. The _pC₂₀ was observed to be greater in Triton X-100 compared to SDS. The ability of [THTDPP] to decrease the CMC was found to be greater in Triton X-100 compared to SDS. The relative viscosity was calculated, and the first observation was made at the pre-CMC stage, where the concentration of SDS+0.7 wt% IL was 4.0 mM. The second finding was made post-CMC at a concentration of 5.0 mM. Afterward, the relative viscosity graph grew slowly and gradually. The functional groups involved in the complexation between [THTDPP] and both surfactants were examined using FTIR spectroscopy. Additionally, the micellar solutions of surfactants + [THTDPP] were used to explore Paracetamol [PCM] aggregation. The findings from UV–vis spectroscopy show that Triton X-100 exhibits the highest binding affinity and has the most encouraging effect compared to SDS.     

Lyotropic liquid-crystalline phases formed by Pluronic P123 in ethylammonium nitrate

Aggregation of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymer, Pluronic P123, is promoted in a room temperature ionic liquid, ethylammonium nitrate (EAN). A series of lyotropic mesophases including normal micellar cubic (I1), normal hexagonal (H1), lamellar (Lalpha), and reverse bicontinuous cubic (V2) are identified at 25 degrees C by using polarized optical microscopy and small-angle X-ray scattering techniques. Such self-assembly behavior of P123 in EAN is similar to those observed in H2O or 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMim(+)][PF6(-)]) systems except for the presence of the V2 phase in EAN and the absence of the I 1 phase in [BMim(+)][PF6(-)]. This suggests that the ionic solvent of EAN plays similar roles as H2O and [BMim(+)][PF6(-)] during the aggregation process and solvates the PEO blocks through hydrogen-bond interaction. Furthermore, the hydrogen bonds are considered to form between the ethylammonium cations and oxygen atoms of the PEO blocks as confirmed by Fourier transform infrared spectra of P123-EAN assemblies. This deduction is also consistent with the results from differential scanning calorimetry and thermogravimetric analysis. The additional V2 phase appearing in the P123-EAN system is attributed to the higher affinity for the relatively hydrophobic PPO blocks to EAN than to water, which might reduce the effective area of the solvophilic headgroup and increase the volume of the solvophobic part. The obtained results may help us to better understand the self-assembly process for amphiphilic block copolymers in protic solvents.     

Aggregation and thermodynamic properties of ionic liquid-type gemini imidazolium surfactants with different spacer length

The aggregation behavior and thermodynamic properties of micellization for the ionic liquid-type gemini imidazolium surfactants with different spacer length ([C₁₂–s–C₁₂im]Br₂, s = 2, 4, 6) have been investigated by means of surface tension, electrical conductivity, dynamic light scattering and fluorescence measurements. The values of cmc, γ _cmc, Γ _max, A _min, π _cmc, pc₂₀ and cmc/pc₂₀ suggest that the shorter the spacer, the higher the surface activity of [C₁₂–s–C₁₂im]Br₂ is. The cmc and γ _cmc values are decreased significantly in the presence of sodium halides, and the values decrease in the order NaCl < NaBr < NaI. The thermodynamic parameters of micellization (, , ) indicate that the micellization of [C₁₂–2–C₁₂im]Br₂ and [C₁₂–4–C₁₂im]Br₂ is entropy-driven, whereas aggregation of [C₁₂–6–C₁₂im]Br₂ is enthalpy-driven at lower temperature but entropy-driven at higher temperature. Finally, the fluorescence measurements show that the micropolarity of micelles increases but the aggregation numbers decrease with increasing the spacer length of [C₁₂–s–C₁₂im]Br₂.     

Aggregation of ionic surfactants to block copolymer assemblies: a simple fluorescence spectral study

A simple and elegant method based on steady-state fluorescence spectral measurement is demonstrated to study the interaction mechanism of copolymers and ionic surfactants with a suitable selection of fluorescent probe and also its general applicability in studying other systems. Three different concentration regions have been indicated from the changes in full width at half-maximum of the emission spectra and fluorescence intensity of coumarin 153 with the molar ratio of ionic surfactant to triblock copolymer (n). At low n values, copolymer-surfactant complexes are basically copolymer-rich micelles with few surfactant molecules, and at very high n values, copolymer-rich micelles are destroyed and surfactant-rich micelles with free copolymer monomers are formed. It has been observed that, in the intermediate surfactant concentration region, the transformation of a dominantly copolymer-rich complex to a mainly surfactant-rich complex can be either gradual incorporation of surfactants into the copolymer-rich micelles with freeing of copolymer units until surfactant-rich micelles are formed (type I) or simultaneous buildup of surfactant-rich micelles together with the destruction of copolymer-rich micelles (type II). The interaction mechanism for nonionic copolymers (P123 and F127) with ionic surfactants (SDS and CTAC) is mainly type II, but at higher copolymer concentrations interaction via the type I mechanism also operates. However, it is dominantly the type I mechanism that operates for common nonionic (TX100) and ionic surfactants.