Cationic Head Groups Research Articles

Role of Phospholipid Head Groups in Silver Nanoparticles Interaction with Membranes

In order to understand the biocompatibility and cell affinity of metal nanoparticles for biosensing and drug delivering applications and also for nanotoxicity aspects, we investigate the different charged state phospholipid head groups interacting with silver (Ag) nanoclusters. Binding interactions of Ag, which serve as simple catalytic models of Ag nanoparticles with phospholipids, was studied using density functional theory (DFT). Geometries of neutral, anionic, and cationic phopholipid head groups with silver nanoparticle were optimized using the DFT-B3LYP approach with combined basis sets. The combined basis set used here for phospholipid and silver clusters was represented by 6-311++G(d,p) and LANL2DZ, respectively. This work demonstrates that the interaction of silver clusters with phospholipid head group is governed by two major bonding factors: (a) the anchoring phosphotidyl O−Ag, and (b) the nonconventional N−H···Ag and C−H···Ag hydrogen bonds. Role of different charge state within the membrane lipids and metal affinity on their interaction with Ag nanpoarticle is very crucial and will be discussed here in detail. DFT based global and local chemical reactivity descriptor analysis and natural bond orbital analysis was performed to calculate reactivity, charge transfer, and Wiberg bond indices of the nano-bio complexes. Bader's “atoms in molecules” theory is used to determine the nature of interactions that exhibit both electrostatic and covalent characters.

Ionization Behavior of Amino Lipids for siRNA Delivery: Determination of Ionization Constants, SAR, and the Impact of Lipid pKa on Cationic Lipid−Biomembrane Interactions

Ionizable amino lipids are being pursued as an important class of materials for delivering small interfering RNA (siRNA) therapeutics, and research is being conducted to elucidate the structure-activity relationships (SAR) of these lipids. The pK(a) of cationic lipid headgroups is one of the critical physiochemical properties of interest due to the strong impact of lipid ionization on the assembly and performance of these lipids. This research focused on developing approaches that permit the rapid determination of the relevant pK(a) of the ionizable amino lipids. Two distinct approaches were investigated: (1) potentiometric titration of amino lipids dissolved in neutral surfactant micelles; and (2) pH-dependent partitioning of a fluorescent dye to cationic liposomes formulated from amino lipids. Using the approaches developed here, the pK(a) values of cationic lipids with distinct headgroups were measured and found to be significantly lower than calculated values. It was also found that lipid-lipid interaction has a strong impact on the pK(a) values of lipids. Lysis of model biomembranes by cationic lipids was used to evaluate the impact of lipid pK(a) on the interaction between cationic lipids and cell membranes. It was found that cationic lipid-biomembrane interaction depends strongly on lipid pK(a) and solution pH, and this interaction is much stronger when amino lipids are highly charged. The presence of an optimal pK(a) range of ionizable amino lipids for siRNA delivery was suggested based on these results. The pK(a) methods reported here can be used to support the SAR screen of cationic lipids for siRNA delivery, and the information revealed through studying the impact of pK(a) on the interaction between cationic lipids and cell membranes will contribute significantly to the design of more efficient siRNA delivery vehicles.

In situ synthesized Ag nanoparticle in self-assemblies of amino acid based amphiphilic hydrogelators: development of antibacterial soft nanocomposites

The present work reports the development of a new class of antibacterial soft-nanocomposites by in situ synthesis of silver nanoparticle (AgNP) within the supramolecular self-assemblies of amino acid (tryptophan/tyrosine) based amphiphilic hydrogelators. Interestingly, the nanoparticle synthesis does not require the use of any external reducing/stabilizing agents. The nanocomposites were characterized by UV-vis spectra, transmission electron microscopy (TEM) images, X-ray diffraction spectroscopy (XRD) and thermo gravimetric analysis (TGA). Encouragingly, these soft nanocomposites showed excellent antibacterial activity against both Gram-positive and Gram-negative bacteria whereas the amphiphiles alone were lethal only toward Gram-positive bacteria. Judicious combination of bactericidal AgNP within the self-assemblies of inherently antibacterial amphiphilic gelators led to the development of soft nanocomposites effective against both type of bacteria. The head group charge and structure of the amphiphiles were altered to investigate their important role on the synthesis and stabilization of AgNP and also in modulating the antibacterial activity of the nanocomposites. The antibacterial activities of soft nanocomposites comprising amphiphiles with cationic head group were found to be more efficient than the anionic soft nanocomposites. Interestingly, these nanocomposites have shown considerable biocompatibility to mammalian cell, NIH3T3. Furthermore, the well-known tissue engineering scaffold, agar-gelatin film infused with these soft nanocomposites allowed normal growth of mammalian cells on its surface while being lethal toward both Gram-positive and Gram-negative bacteria.

Antimicrobial properties of ethoxyether-functionalized imidazolium salts

Amphiphilic ionic liquids and ionic liquid crystals, comprising a cationic imidazolium head group (represents as Im), a hydrophobic N-alkyl chain (CnH2n+1, denoted as Cn, n = 10, 12, 14, 16 and 18), and an amphiphilic N-2-(2-(2-ethoxy)ethoxy)ethanol (3OEG) functional group were synthesized (denoted as [Cn-Im-3OEG][Cl]). While the lipophilicity of [Cn-Im-3OEG][Cl] increases along with the alkyl carbon chain length, the water solubility of these compounds is enhanced with the 3OEG functionality. Compounds with lengths of n = 10, 12 and 14 are room temperature ionic liquids, whereas n = 16 and 18 show thermotropic SmA liquid crystal properties. Meanwhile, compounds of n = 14, 16 and 18 also exhibit lyotropic liquid crystal properties. The ionic and lyotropic liquid crystalline natures of these compounds lead to their superior antimicrobial activity against infectious pathogens. This is especially true for compounds with longer alkyl chains. Rupture of the bioenvelop appears to be operative in the process of antimicrobial activity.

Lipid chain geometry of C14 glycerol-based lipids: effect on lipoplex structure and transfection

The effects have been determined of a systematic alteration of the alkyl chain geometry of a C14 analogue of DOTMA on the detailed molecular architecture of the resulting cationic vesicles formed both in the absence and presence of 50 mol% DOPE, and of the lipoplexes prepared from these vesicles using either calf thymus or plasmid DNA. The C14 DOTMA analogues studied involved cis- or trans-double bonds at positions Δ9 or Δ11, and a compound (ALK) featuring an alkyne at position C9. For all of these analogues, examination by light scattering and neutron scattering, zeta potential measurement, and negative staining electron microscopy showed that there were no significant differences in the structures or charges of the vesicles or of the resulting lipoplexes, regardless of the nature of the DNA incorporated. Differences were observed, however, between the complexes formed by the various lipids when examining the extent of complexation and release by gel electrophoresis, where the E-lipids appeared to complex the DNA more efficiently than all other lipids tested. Moreover, the lipoplexes prepared from the E-lipids were the most effective in transfection of MDA-MB-231 breast cancer cells. As indicated through confocal microscopy studies, the E-lipids also showed a higher internalisation capacity and a more diffuse cellular distribution, possibly indicating a greater degree of endosomal escape and/or nuclear import. These observations suggest that the extent of complexation is the most important factor in determining the transfection efficiency of the complexes tested. At present it is unclear why the E-lipids were more effective at complexing DNA, although it is thought that the effective area per molecule occupied by the cationic lipid and DOPE head groups, and therefore the density of positive charges on the surface of the bilayer most closely matches the negative charge density of the DNA molecule. From a consideration of the geometry of the cationic lipids it is anticipated that the head groups of the E-lipids would occupy a smaller area per molecule than the ALK or Z-lipids.

Molecular simulation of swelling and interlayer structure for organoclay in supercritical CO2

In this work, Monte Carlo simulations have been carried out to investigate the swelling stability and interlayer structures of alkylammonium-modified montmorillonite both in vacuum and in supercritical CO(2) (scCO(2)) fluid. In the vacuum (dry) condition, the stable spacing for this kind of organoclay was determined based on the energy minimum. In the stable spacing, the corresponding interlayer structure of dry organoclay is the monolayer arrangement with the intercalated surfactant chains lying parallel to the silicate surface. In scCO(2) fluid medium, the normal pressures within the organoclay gallery and the swelling free energy have been obtained from Gibbs ensemble Monte Carlo simulation. The mechanically and thermodynamically stable spacings of the organoclay have been determined. As compared with the case in vacuum, the simulation shows that the swelling of the organoclay is thermodynamically favorable in the environment of scCO(2) fluid. The interlayer structure and conformation have been used to analyze the mechanism of swelling. The headgroups of surfactant cations are distributed close to the clay surfaces. The presence of CO(2) molecules within the clay gallery can cause a specific steric arrangement of the long-chain alkylammonium cations.

Cytofectin amine head group modification and degree of liposome pegylation: factors influencing gene transfer.

The effectiveness of liposome-mediated gene transfer methods hinges, in part, on the nature of the interaction between the DNA cargo and the liposomes. Here we have examined the effect of quaternization of the cytofectin cationic head group on this interaction and the effect of concentration of the biocompatible, protective polymer polyethylene glycol2000 (PEG2000) on transfection activity. Thus 3β[N-(N’,N’-dimethylaminopropane)-carbamoyl] cholesterol (Chol-T) and 3β[N-(N’,N’,N’-trimethylammonium propane)-carbamoyl] cholesterol iodide (Chol-Q), differing only in the degree of head group methylation, have been formulated into liposomes with polyethylene glycol2000-distearoylphosphatidyl ethanolamine (DSPE PEG2000) and the neutral co-lipid dioleoylphosphatidylethanolamine (DOPE). Their DNA-binding characteristics have been determined and the gene transfer capabilities of resulting lipoplexes have been examined in HEK 293 human embryonic kidney cells. Quaternary ammonium Chol-Q liposomes were found to bind DNA more avidly than their tertiary amine Chol-T counterparts. The inclusion of PEG2000 in liposome formulations resulted in an increase in the optimal liposome-DNA binding ratio. Chol-T liposomes promoted transgene activity levels 5 times greater than those obtained with Chol-Q lipoplexes. Furthermore, a drop in transfection activity of only 17% was noted on increase of liposome pegylation from 2 to 5 mole percent. The study's findings suggest that strong association between cationic liposomes and DNA may lead to reduced levels of transfection activity as a result of poor release of nucleic acid after cellular uptake.

Binding Gibbs energy of ionic liquids to calf thymus DNA: a fluorescence spectroscopy study

The binding constants and binding Gibbs energies of [C(n)mim]Br (n = 4, 6, 8, 10, 12) and [C(4)mim][BF(4)] to calf thymus DNA-D1501 have been determined by the fluorescence measurements of the pyrene probe. It was found that values of the binding Gibbs energy decrease linearly with the increase of alkyl chain length of the ILs. Based on this observation, Gibbs energies for both the interaction between DNA and -CH(2)- of the ILs and the interaction between DNA and the cationic head group of the ILs have been reported. The result suggests that electrostatic interaction between DNA and the cationic head group of the ILs is predominant for the binding of the ILs with DNA although contribution from the non-electrostatic interaction between DNA and the alkyl chain of the ILs increases with increasing alkyl chain length of the ILs. The information obtained here may have application in the design of novel ILs-based DNA extraction/separation systems.

Interfacial water with special electron donor properties: Effect of water–surfactant interaction in confined reversed micellar environments and its influence on the coordination chemistry of a copper complex

In this work, we have investigated the behavior of the square-planar mixed-ligand transition-metal complex N, N, N′, N′-tetramethylethylenediamine copper (II) acetylacetonate tetraphenylborate, [Cu(acac)(tmen)][B(C 6H 5) 4], in anionic benzene/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT) reverse micelles (RMs) as well as in cationic benzene/benzyl- n-hexadecyl dimethylammonium chloride (BHDC) RMs with and without water, using absorption spectroscopy. In the absence of water, W 0 = [H 2O]/[surfactant] = 0, our results show that the Cu complex structures are not the same in both reversed micellar media. In the BHDC RMs, due to the Cl − counterion present in the system, the Cu complex structure that exists at the interface is the neutral [Cu(acac)(tmen)(Cl)]. In contrast, in AOT RMs the surfactant sulfonate anion cannot act as a ligand and therefore the complex structure is square planar [Cu(acac)(tmen)] +. When water is added to both reversed micellar systems, different situations are observed. At low water content, the water sequestrated by BHDC RMs is unable to act as electron donor because the oxygen nonbonding electron pairs are completely involved in the cationic BHDC polar head group solvation through an ion–dipole interaction. On the contrary, in AOT RMs the results suggest that since the water molecules solvate the sulfonate group through hydrogen bonding interactions, the bulk hydrogen bond network is destroyed at the interface and therefore, the nonbonding electron pairs are more available to interact with the Cu complex. Thus, the electron donor ability of water in AOT RMs is enhanced in comparison with the BHDC RMs and bulk water, giving a remarkable electron donor character to the AOT reversed micellar interfacial water.

Self-aggregation and antimicrobial activity of imidazolium and pyridinium based ionic liquids in aqueous solution

Two series of long-chain imidazolium and pyridinium based ionic liquids (1-alkyl-3-methylimidazolium and 1-alkylpyridinium bromides) were synthesised and the effect of the alkyl chain length and the nature of the cationic head group on micellization and antimicrobial activity of the ionic liquids (ILs) were investigated. Tensiometry, conductometry, spectrofluorimetry and PGSE-NMR were applied to study the self-aggregation of the amphiphilic ILs in aqueous solution. The ILs investigated displayed surface activity and the characteristic chain length dependence of the micellization process of surfactants. The antimicrobial activity was evaluated against Gram-negative and Gram-positive bacteria and fungi. ILs containing more than eight carbon atoms in the alkyl chain showed antimicrobial activity. Their efficiency as antimicrobial agents increased with the hydrophobicity of the amphiphilic cation being the C 14 homologous the most active compounds.

NMR study of the influence of pH on phenol sorption in cationic CTAB micellar solutions

Interactions between phenol and cationic cetyltrimethylammonium bromide (CTAB) micelles have been investigated by means of nuclear magnetic resonance spectroscopy. The combined use of 1H and NOESY techniques revealed that phenol has different preferred locations of interaction depending on the pH. At neutral pH (6.70) conditions, phenol molecules are preferentially located in the outer micelle region, at the micelle–water interface, while at more basic pH (9.94), the deprotonated phenol molecules (C 6H 5O −) are immersed into the palisade layer of the micelle. In addition, quantitative estimates of the solubilized fraction of phenol were obtained by using PFG-NMR. The results indicate that the phenol–CTAB interactions, although already present in neutral pH conditions, are largely favored in basic conditions as a consequence of the strong electrostatic interaction between the negatively charged phenolate ions and the positive charge of the cationic surfactant head group.

Assembly of Gold Nanoparticles on a Molecular Ultrathin Film: Tuning the Surface Plasmon Resonance

A number of methodologies for immobilizing metal nanoparticles in 2-dimensional aggregate structures on various substrates, some with concomitant tuning of the surface plasmon resonance (SPR), have been reported. Many of them involve special functionalization of the nanoparticles, multiple fabrication steps or lengthy procedures. The present study demonstrates that monolayer Langmuir-Blodgett (LB) film of a hemicyanine-based amphiphile with cationic headgroup is an easily fabricated platform for harnessing citrate-stabilized gold nanoparticles. It is shown that a single immersion step can be used to immobilize the nanoparticles uniformly on large area films and that systematic variation of the immersion time from 10 min to 6 h leads to controlled assembly of the particles and tuning of the SPR band over approximately 100 nm. A model for the structural reorganization in the LB film that facilitates the assembly of nanoparticles is presented and the advantages of the current methodology over earlier protocols are pointed out. The versatility of LB films in terms of the molecular level control of fabrication it enables and the variety of film structures that can be realized, point to the wide scope for future explorations, expanding upon the present observations.

Cationic Ester-Containing Gemini Surfactants: Physical−Chemical Properties

Three ester-containing cationic gemini surfactants, two with decanoyl chains and either a three-carbon or a six-carbon spacer unit and one with dodecanoyl chains and a three-carbon spacer, were synthesized and evaluated. A corresponding monomeric cationic ester surfactant was used for comparison. This type of amphiphile, a so-called esterquat, is known to undergo rapid hydrolysis above the critical micelle concentration because of micellar catalysis. The esterquat geminis of this work were found to be much more susceptible to hydrolysis than the esterquat monomer. This difference is believed to be caused by anchimeric assistance by the second cationic headgroup in the gemini amphiphiles. However, there is no correlation between the rate of chemical hydrolysis and the rate of biodegradation. The monomeric esterquat, which is the most stable in the chemical hydrolysis experiments, was the only surfactant that passed the test for "readily biodegradable". We also observed a considerable difference in the hydrolysis rate within the small series of gemini surfactants. The amphiphile with two decanoyl chains and a three-carbon spacer, N,N'-bis(2-(decanoyloxy)ethyl)-N,N,N',N'-tetramethyl-1,3-propanediammonium dibromide, had the fastest rate of hydrolysis. This surfactant also exhibited a considerably lower degree of micelle ionization than the other surfactants, which is believed to be due to the closer proximity of the charged groups on the micelle surface. A small distance between headgroups will give more pronounced neighboring group participation, accounting for the increased rate of hydrolysis. An interesting property of the surfactant that is the most susceptible to hydrolysis is that it gives rise to an extremly stable foam. We propose that the foam stability is a result of the partial hydrolysis of the surfactant generating sodium decanoate, an anionic surfactant, that forms a mixed film with the starting cationic gemini surfactant. It is known that mixed monolayers in which there is a strong attractive interaction between surfactant headgroups can lead to stable foams.

Novel Cationic Lipids Based on Malonic Acid Amides Backbone: Transfection Efficacy and Cell Toxicity Properties

Gene delivery using nonviral approaches has been extensively studied as a basic tool for intracellular gene transfer. Despite intensive research activity, the aim of creating a vector which meets all necessary demands has still not been reached. One possibility to solve the nonviral vector associated problem of low transfection efficacy is the development of new cationic amphiphiles. Therefore, the non-glycerol-based cationic lipids 1-9 have been synthesized and tested for in vitro gene delivery experiments. The backbone structure of the lipids consists of a malonic acid diamide with two long hydrophobic chains. The degree of saturation of the hydrophobic chains and the structure of the polar cationic headgroup were varied. The preparation follows an easy process and facilitates the trouble-free insertion of different alkyl chains. By studying in vitro gene delivery an increase of transfection efficacy was observed when using at least one unsaturated alkyl chain in the hydrophobic part and lysine or bis(2-aminoethyl)aminoethylamid as hydrophilic headgroup. This leads to cationic lipids exhibiting comparable or even higher transfection efficacies compared to the commercially available LipofectAmine and SuperFect.

Mixed Micellization Properties of Nonionic Fluorocarbon/Cationic Hydrocarbon Surfactants

AbstractThe physico‐chemical properties of mixed surfactant aqueous solutions at various proportions of dodecyltrimethyl ammonium bromide DTAB and undecafluoro n‐pentyl decaoxyethylene ether C5F11EO10 have been investigated at 25 °C using surface tension and conductivity measurements. The critical micelle concentration and superficial minimum tension have been experimentally estimated for different DTAB molar fractions. The micellar composition and mutual interaction parameters have been deduced on the basis the theoretical treatments proposed by Clint, Rubingh, and Motomura. The results show a significant synergism effect at about equimolarity of DTAB/C5F11EO10 system probably due to the efficient electrostatic self‐repulsion reduction between DTAB cationic head groups related to the presence of the nonionic surfactant in the mixed micelles.

The Characterization of Phospholipid Functional Group Probe Species on Respirable Silicon-Containing Dusts by Solid-State 13C and 31P Nuclear Magnetic Resonance Spectroscopy

Solid-state nuclear magnetic resonance (NMR) spectroscopic studies are reported for the interactions of probe molecules with respirable silicon-containing dusts as experimental evidence complementing computational studies reported by Snyder and Madura recently in J. Phys. Chem. B 112, 7095 (2008). The selected probe molecules represent the individual functional groups of a model lung surfactant dipalmitoylphosphatidyl choline (DPPC) deposited on a respirable silica and kaolin from water solution. (13)C and (31)P solid-state NMR spectroscopies were employed to detect chemical shift, line width, and chemical shift anisotropy, providing experimental evidence of mobility and relaxation changes describing the site and orientation of surface-associated species. NMR results confirm that only the phosphate and adjacent carbons are immobilized by surface hydroxyls on kaolin, while these and the carbons of the cationic head group are likewise immobilized by surface silanols on Miu-U-Sil 5. The phosphates in phosphoryl- and phosphatidyl-cholines were the primary interaction sites, with additional weak coordination with the trimethylammonium cation species. Covalent Al-O-P formation is not likely a factor in in vivo or in vitro toxicity mechanisms of respirable silicon-containing materials, but is rather the result of dehydration or demethoxylation reactions occurring over time or during heating or reduced pressure used in preparing materials for NMR spectroscopic study. Hydration is a critical factor in the formation and preparation for spectroscopic observation of coated dusts. Care must be taken to ensure that products formed and studied correspond to species formed in vivo under suitable concentration and hydration conditions.

Nanoconfined ionic liquids under electric fields

The effect of external electric fields (EEFs) on ionic liquid films confined within a nanogap has been investigated by measuring the film thickness with the thin film interferometry and calculating the effective viscosity. Experimental results indicated that the film thickness of ionic liquids could be increased obviously by the application of EEFs with strengths weaker than the electric interactions between cationic head groups and anions. The effect of EEFs on the confined ionic liquid film with a shorter alkyl side chain is more noticeable. It is thought that the charged anions and headgroups of the cations are structured near electrified walls to form ordered layers and short alkyl side chains at the interfaces are aligned along the EEF direction due to induced dipoles.

Binding Characteristics and Molecular Mechanism of Interaction between Ionic Liquid and DNA

The binding characteristics and molecular mechanism of the interaction between a typical ionic liquid (IL), 1-butyl-3-methylimidazolium chloride ([bmim]Cl), as a green solvent and DNA were investigated for the first time by conductivity measurements, fluorescence spectroscopy, dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), circular dichroism spectroscopy, (31)P nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy, isothermal titration calorimetry (ITC), and quantum chemical calculations. It was found that the critical aggregation concentration of [bmim]Cl is decreased in the presence of DNA, and the addition of [bmim]Cl induced a continuous fluorescence quenching of the intercalated probe ethidium bromide (EtBr), indicating that the interaction between the ionic liquid and DNA is sufficiently strong to exclude EtBr from DNA. DLS results show that [bmim]Cl can induce a coil-to-globule transition of DNA at a low IL concentration, which was confirmed by the cryo-TEM images of DNA-IL complexes. With [bmim]Cl added, the resulting globular DNA structures and the extended DNA coils are first compacted, and then grow in size. During the binding process, DNA maintains the B-form, but the base packing and helical structure of DNA are altered to a certain extent. The (31)P NMR and IR spectra indicate that the cationic headgroups of bmim(+) groups interact with the phosphate groups of DNA through electrostatic attraction, and the hydrocarbon chains of bmim(+) groups interact with the bases through strong hydrophobic association. ITC results reveal the interaction enthalpy between [bmim]Cl and DNA and show that the hydrophobic interaction between the hydrocarbon chains of [bmim]Cl and the bases of DNA provides the dominant driving force in the binding. On the basis of quantum chemical calculations, it can be inferred that at a low IL concentration, the cationic headgroups of [bmim]Cl would be localized within several angstroms of the DNA phosphates, whereas the hydrophobic chains would be arranged parallel to the DNA surface. When the IL concentration is above 0.06 mol/L, the cationic headgroups are near DNA phosphates, and the hydrocarbon chains are perpendicularly attached to the DNA surface.

Cleavage Energy of Alkylammonium-Modified Montmorillonite and Relation to Exfoliation in Nanocomposites: Influence of Cation Density, Head Group Structure, and Chain Length

Cohesion between layers of organically modified aluminosilicates creates a barrier to exfoliation in polymer matrices and can be thermodynamically described by the cleavage energy. While the measurement of cleavage energies at the nanometer scale is experimentally challenging, molecular simulation indicates a tunable range between 25 and 210 mJ/m2 upon variation of the cation exchange capacity (CECs of 91 meq/100 g and 143 meq/100 g), headgroup chemistry (primary and quaternary ammonium), and chain length (C2 to C22) in a series of 44 alkylammonium-modified montmorillonites. Designed organoclays of low cleavage energy can support exfoliation when the surface tension, which relates to reconstructed surfaces after cleavage, is comparatively higher. The difference between high and low cleavage energies is associated with Coulomb interactions depending on the position of the positively charged surfactant head groups in the interlayer with respect to the two clay layers at equilibrium separation and further wi...

Insights into the surface composition and enrichment effects of ionic liquids and ionic liquid mixtures

A systematic study of ionic liquid surfaces by angle resolved X-ray photoelectron spectroscopy (ARXPS) is presented. By reviewing recent and presenting new results for imidazolium-based ionic liquids (ILs), we discuss the impact of chemical differences on surface composition and on surface enrichment effects. (1) For the hydrophilic ethylene glycol (EG) functionalised ILs [Me(EG)MIm][Tf(2)N], [Et(EG)(2)MIm][Tf(2)N] and [Me(EG)(3)MIm][Tf(2)N], which vary in the number of ethylene glycol units (from 1 to 3), we find that the surface composition of the near-surface region is in excellent agreement with the bulk composition, which is attributed to attractive interactions between the oxygen atoms on the cation to the hydrogen atoms on the imidazolium ring. (2) For [C(n)C(1)Im][Tf(2)N] (where n = 1-16), i.e. ILs with an alkyl chain of increasing length, an enrichment of the aliphatic carbons is observed for longer chains (n > 2), at the expense of the polar cation head groups and the anions in the first molecular layer, both of which are located approximately at the same distance from the outer surface. (3) To study the influence of the anion on the surface enrichment, we investigated ten ILs [C(8)C(1)Im][X] with the same cation, but very different anions [X](-). In all cases, surface enrichment of the cation alkyl chains is found, with the degree of enrichment decreasing with increasing size of the anion, i.e., it is most pronounced for the smallest anions and least pronounced for the largest anions. (4) For the IL mixture [C(2)C(1)Im][Tf(2)N] and [C(12)C(1)Im][Tf(2)N] we find a homogeneous distribution in the outermost surface region with no specific enrichment of the [C(12)C(1)Im](+) cation.