Adsorption of aromatic counterions at the surfactant/water interface: a neutron reflectivity study of hydroxybenzoate and chlorobenzoate counterions at the hexadecyl trimethylammonium surfactant/water interface

Development of Cholinium-Based API Ionic Liquids with Enhanced Drug Solubility: Biological Evaluation and Interfacial Properties

We report an efficient sustainable two-step anion exchange synthetic procedure for the preparation of choline API ionic liquids (Cho-API-ILs) that contain active pharmaceutical ingredients (APIs) as anions combined with choline-based cations. We have evaluated the in vitro cytotoxicity for the synthesized compounds using three different cells lines, namely, HEK293 (normal kidney cell line), SW480, and HCT 116 (colon carcinoma cells). The solubility of APIs and Cho-API-ILs was evaluated in water/buffer solutions and was found higher for Cho-API-ILs. Further, we have investigated the antimicrobial potential of the pure APIs, ILs, and Cho-API-ILs against clinically relevant microorganisms, and the results demonstrated the promise of Cho-API-ILs as potent antimicrobial agents to treat bacterial infections. Moreover, the aggregation and adsorption properties of the Cho-API-ILs were observed by using a surface tension technique. The aggregation behavior of these Cho-API-ILs was further supported by conductivity and pyrene probe fluorescence. The thermodynamics of aggregation for Cho-API-ILs has been assessed from the temperature dependence of surface tension. The micellar size and their stability have been studied by dynamic light scattering, transmission electron microscopy, and zeta potential. Therefore, the duality in the nature of Cho-API-ILs has been explored with the upgradation of their physical, chemical, and biopharmaceutical properties, which enhance the opportunities for advances in pharmaceutical sciences.

Impact of Aromatic Counter-Ions Charge Delocalization on the Micellization Behavior of Surface-Active Ionic Liquids

The nature of counter-ions governs the micellar and structural characteristics of surface-active ionic liquids (SAILs). Especially, the introduction of aromatic counter-ions significantly increases their surface adsorption and induces the formation of various types of aggregates like prolate ellipsoidal micelles, rodlike micelles, vesicles, lamellars, etc. The present study reports the role of charge delocalization of two different aromatic counter-ions in the micellization behavior of their respective SAILs in aqueous medium. For this purpose, we have synthesized two SAILs, namely, 1-tetradecyl-3-methylimidzolium phenolate [C₁₄mim][PO] and 1-tetradecyl-3-methylimidzolium benzoate [C₁₄mim][BZ]. The O-atom of phenolate (PO^-) possesses negative charge, which is delocalized on its phenyl ring. Conversely, the negative charge of benzoate (BZ^-) is not delocalized on its phenyl ring. The more hydrophobic BZ^- counter-ion increases the hydrophobic interactions and reduces the electrostatic repulsions more efficiently as compared to PO^-, which results in a lower critical micelle concentration (cmc) of [C₁₄mim][BZ] than that of [C₁₄mim][PO]. Interfacial properties obtained by tensiometry reveal better surface activity and absorption efficiency of [C₁₄mim][BZ] as compared to [C₁₄mim][PO]. The increase of cmc and degree of counter-ion binding (β) with the rise of temperature for both SAILs has been observed by conductometry. The decrease in the polarity of pyrene microenvironment explains the higher compactness of [C₁₄mim][BZ] aggregates than that of [C₁₄mim][PO], observed by fluorimetry. The position of PO^- and BZ^- is in the stern and palisade layers of C₁₄mim⁺ aggregates, respectively, located by ¹H NMR. The existence of prolate ellipsoidal micelles for both SAILs has been established by small-angle neutron scattering measurements. Thus, the interfacial and bulk properties of [C₁₄mim][PO] lie somewhere in between those of the SAILs having perfect aromatic counter-ions, [C₁₄mim][BZ], and the SAILs having regular inorganic counter-ions like Cl^-, Br^-, etc.

Effects of additives on the viscoelastic responses of cationic gemini surfactant solutions

Several additives, including inorganic (NaCl) and organic salts (derivatives of benzoate), were added into aqueous solutions of a gemini cationic surfactant, 2-hydroxypropyl-1,3-bis (myristyldimethylammonium chloride) (abbreviated as 14-3(OH)-14(2Cl)). The mixed systems were investigated using rheological measurement, cryo-TEM and 1H NMR analysis. The results showed that addition of salts induced rich aggregate morphologies in the 14-3(OH)-14(2Cl)/salt systems. The influence of an inorganic salt on the viscoelasticity of 14-3(OH)-14(2Cl) solutions is much weaker than that of organic salts. Furthermore, the ability of three organic salts in enhancing the viscoelasticity of 14-3(OH)-14(2Cl) solutions is in the order sodium m-hydroxybenzoate > sodium o-hydroxybenzoate > sodium p-hydroxybenzoate. The different roles of these organic salt isomers arise from the different types of hydrogen bonding formed between 14-3(OH)-14(2Cl) and the organic counter ions.

Study of Cationic and Nonionic Mixed Micelles with NaBr and 3,5-Cl2C6H3CO2Na by Use of Probe Nucleophilic Reaction of Piperidine with Ionized Phenyl Salicylate

Behaviors of cationic and nonionic mixed micelles in the form of hexadecyltrimethylammonium bromide (HDABr) and hexadecyltrimethylammonium bromide-Polyethylene glycol hexadecyl ether (C₁₆E₂₀), in the presence of inert salts (NaBr and 3,5-dichlorosodium benzoate), by the use of reaction probe between Pp and ionized PhSH (Pp = piperidine and PhSH = phenyl salicylate), has been reported in this work. The values of R_X^Br (R_X^Br denotes ion exchange constants obtained in the presence of micelles of different structural features) or K_X^Br (K_X^Br denotes ion exchange constants obtained in the presence of micelles of the same structural features) for 3,5-Cl₂C₆H₃CO₂^- were almost the same at three different [HDABr]_T (0.006, 0.010 and 0.015 M). The average value of R_X^Br or K_X^Br determined, in the presence of pure HDABr micelles, using semi empirical kinetic (SEK) method appeared to be almost 2½-fold larger (R_X^Br or K_X^Br = 198) than that in the presence of mixed HDABr-C₁₆E₂₀ micelles (R_X^Br or K_X^Br = 78). Rheological measurements indicated the existence of wormlike/twisted micelles and vesicle at 0.015 M pure HDABr, various [3,5-Cl₂C₆H₃CO₂Na], and 25 and 35℃ whereas there were evidence of only spherical micelles in the presence of mixed HDABr-C₁₆E₂₀ ([HDABr]_T = 0.015 M and [C₁₆E₂₀]_T = 0.006 M) at both temperatures.

Influence of hydrotropic coions on the shape transitions of sodium dioctylsulfosuccinate aggregates in an aqueous medium

The slope change of the CH plot is a manifestation of the shape change of the ionic micelles. However, binding of the salicylate coion to ionic micelles is an exception to this inference.

The adsorption and self-assembly of mixtures of alkylbenzene sulfonate isomers and the role of divalent electrolyte

In this paper, the role of the different structural isomers of the anionic surfactant sodium para-dodecyl benzene sulfonate, LAS, on surface adsorption and solution self-assembly has been studied. Using a combination of neutron reflectivity, NR, and small angle neutron scattering, SANS, the effect of mixing an isomer with a short symmetric hydrocarbon chain with one which has an asymmetric hydrocarbon chain on both the equilibrium surface adsorption behavior and the solution microstructure of the mixtures, both in the presence and absence of a divalent cation (Ca(2+)), has been investigated. In the absence of electrolyte, the LAS isomer mixtures form small charged globular micelles throughout the composition range studied. The micelle aggregation number increases with the increase in the asymmetric isomer content, reflecting an increase in the packing efficiency within the micelle. The addition of calcium ions promotes the formation of planar aggregates, as multilamellar vesicles, but only when the symmetric LAS isomer is the major component of the mixture. At a surfactant concentration just above the critical micelle concentration, CMC, and in the absence of electrolyte, the variation in the surface composition is close to the solution composition. Regular solution theory, RST, calculations show that this variation is also close to what is expected for ideal mixing. The addition of Ca(2+) ions induces a different surface behavior, resulting in the formation of multilayer structures at the interface throughout the entire composition range.

The impact of multivalent counterions, Al3+, on the surface adsorption and self-assembly of the anionic surfactant alkyloxyethylene sulfate and anionic/nonionic surfactant mixtures

The impact of multivalent counterions, Al(3+), on the surface adsorption and self-assembly of the anionic surfactant sodium dodecyl dioxyethylene sulfate, SLES, and the anionic/nonionic surfactant mixtures of SLES and monododecyl dodecaethylene glycol, C(12)E(12), has been investigated using neutron reflectivity, NR, and small angle neutron scattering, SANS. The addition of relatively low concentrations of Al(3+) counterions induces a transition from a monolayer to well-defined surface bilayer, trilayer, and multilayer structures in the adsorption of SLES at the air-water interface. The addition of the nonionic cosurfactant, C(12)E(12), partially inhibits the evolution in the surface structure from monolayer to multilayer interfacial structures. This surface phase behavior is strongly dependent upon the surfactant concentration, solution composition, and concentration of Al(3+) counterions. In solution, the addition of relatively low concentrations of Al(3+) ions promotes significant micellar growth in SLES and SLES/C(12)E(12) mixtures. At the higher counterion concentrations, there is a transition to lamellar structures and ultimately precipitation. The presence of the C(12)E(12) nonionic cosurfactant partially suppresses the aggregate growth. The surface and solution behaviors can be explained in terms of the strong binding of the Al(3+) ions to the SLES headgroup to form surfactant-ion complexes (trimers). These results provide direct evidence of the role of the nonionic cosurfactant in manipulating both the surface and solution behavior. The larger EO(12) headgroup of the C(12)E(12) provides a steric hindrance which disrupts and ultimately prevents the formation of the surfactant-ion complexes. The results provide an important insight into how multivalent counterions can be used to manipulate both solution self-assembly and surface properties.

Surface and solution properties of anionic/nonionic surfactant mixtures of alkylbenzene sulfonate and triethyleneglycol decyl ether

The surface adsorption behavior and the solution microstructure of mixtures of the C(6) isomer of anionic surfactant sodium para-dodecyl benzene sulfonate, ABS, with nonionic surfactant monodecyl triethyleneglycol ether, C(10)E(3,) have been investigated using a combination of neutron reflectivity, NR, and small-angle neutron scattering, SANS. In solution, the mixing of C(10)E(3) and ABS results in the formation of small globular micelles over most of the composition range (100:0 to 20:80 ABS/C(10)E(3)). Planar aggregates (lamellar or unilamellar vesicles, ULV) are observed for solution compositions rich in the nonionic surfactant (>80 mol % nonionic). Prior to the transition to planar aggregates, the micelle aggregation number increases with increasing nonionic composition. The lamellar-phase region is preceded by a narrow range of composition over which mixtures of micelles and small unilamellar vesicles coexist. The variation in surface absorption behavior with solution composition shows a strong surface partitioning of the more surface-active component, C(10)E(3). This pronounced departure from ideal mixing is not readily explained by existing surfactant mixing theories. In the presence of Ca(2+) ions, a more complex evolution of solution phase behavior with solution composition is observed. The lamellar-phase region occurs over a broader range of solution compositions at the expense of the small-vesicle phase. The phase boundaries are shifted to lower nonionic compositions, and the extent to which the solution-phase diagrams are modified increases with increasing calcium ion concentration. The SANS data for the large planar aggregates are consistent with large polydisperse flexible unilamellar vesicles. In the presence of Ca(2+) ions, the surface adsorption patterns become more consistent with ideal mixing in the nonionic-rich region of the surface-phase diagram. However, in the ABS-rich regions the surface behavior is more complex because of the spontaneous formation of more complex surface microstructures (bilayers to multilayers). Both in water and in the presence of Ca(2+) ions the variations in the surface adsorption behavior and in the solution mesophase structure do not appear to be closely correlated.

Adsorption of the cationic surfactant cetyltrimethylammonium bromide to silica in the presence of sodium salicylate: surface excess and kinetics

The adsorption of cetyltrimethylammonium bromide (CTAB) to silica in the presence of sodium salicylate has been investigated using atomic force microscopy, optical reflectometry, and a quartz crystal microbalance. Salicylate is found to have a dramatic influence on surface adsorption in terms of the kinetics, surface excess, structure of adsorbed aggregates and the mechanical rigidity of the adsorbed film. This is consistent with the bulk solution behavior of more concentrated CTAB-salicylate solutions and reflects the higher local concentration induced by adsorption to the silica surface. Slow adsorption kinetics are found over a wide range of concentrations below the critical micelle concentration.

Equilibrium surface adsorption behavior in complex anionic/nonionic surfactant mixtures

Neutron reflectivity (NR) and small angle neutron scattering (SANS) have been used to investigate the equilibrium surface adsorption behavior and the solution microstructure of mixtures of the anionic surfactant sodium 6-dodecyl benzene-4 sulfonate (SDBS) with the nonionic surfactants monododecyl octaethylene glycol (C12EO8) and monododecyl triiscosaethylene glycol (C12EO23). In the SDBS/C12EO8 and SDBS/C12EO23 solutions, small globular mixed micelles are formed. However, the addition of Ca2+ ions to SDBS/C12EO8 results in a transition to a vesicle phase or a mixed vesicle/micellar phase for SDBS rich compositions. In contrast, this transition hardly exists for the SDBS/C12EO23 mixture, and occurs only in a narrow composition region which is rich in SDBS. The adsorption of the SDBS/C12EO8 mixture at the air-solution interface is in the form of a mixed monolayer, with a composition variation that is not consistent with ideal mixing. In water and in the presence of NaCl, the nonideality can be broadly accounted for by regular solution theory (RST). At solution compositions rich in SDBS, the addition of Ca2+ ions results in the formation of multilayer structures at the interface. The composition range over which multilayer formation exists depends upon the Ca2+ concentration added. In comparison, the addition of a simple monovalent electrolyte, NaCl, at the same ionic strength does not have the same impact upon the adsorption, and the surface structure remains as a monolayer. Correspondingly, in solution, the mixed surfactant aggregates remain as relatively small globular micelles. In the presence of Ca2+ counterions, the variation in surface composition with solution composition is not well described by RST over the entire composition range. Furthermore, the mixing behavior is not strongly correlated with variations in the solution microstructure, as observed in other related systems.

Origin of the sphere-to-rod transition in cationic micelles with aromatic counterions: specific ion hydration in the interfacial region matters

Sphere-to-rod transitions of cetyltrimethylammonium (CTA+) micelles with dichlorobenzoate counterions are remarkably substituent dependent. Simultaneous estimates of the interfacial molarities of H2O, MeOH, and Cl- and 2,6- and 3,5-dichlorobenzoate (2,6OBz and 3,5OBz) counterions were obtained by the chemical trapping method in mixed micelles of CTACl/CTA3,5OBz and CTACl/CTA2,6OBz without added salt. Increasing the CTA3,5OBz mole fraction produces a marked concurrent increase in interfacial 3,5OBz- and a decrease in interfacial H2O concentrations through the sphere-to-rod transition. No abrupt concentration changes are observed with increasing CTA2,6OBz mole fraction. Counterion-specific changes in the interfacial water concentration may be a major contributor to the delicate balance of forces governing micellar morphology.