Ionic Surfactant Molecules Research Articles

Molecular interaction of nonsteroidal anti‐inflammatory prodrug nepafenac with ionic surfactants

AbstractDrug‐surfactant interaction increases the solubility of poorly water‐soluble drugs and design better pharmaceutical formulations. The degree of interaction of nepafenac (NP), a nonsteroidal anti‐inflammatory prodrug was studied with ionic surfactant molecules such as cationic surfactant cetrytrimethyl ammonium bromide (CTAB) and anionic surfactant sodium dodecyl sulphate (SDS) in an aqueous medium at room temperature. NP made mixed micelles with CTAB and SDS. To investigate the influence of interactions, conductivity measurements, UV–visible spectroscopy, and fluorescence measurements were recorded. The quantification of NP–surfactant interactions was investigated using various mathematical models. The CMC values determined from conductivity measurements of pure surfactants were 0.96 mM for CTAB and 8.14 mM for SDS near to their literature values. At different mole fractions of NP in UV measurements, binding constants from lnKb were found 0.025 and 0.123 and number of NP molecules per micelles (n) 67, 46 for CTAB and SDS, respectively. The mixed micelles of NP–CTAB and NP–SDS revealed that CTAB has a strong interaction with NP than SDS. The Benesi–Hildebrand relationship, Stern–Volmer and Kawamura replica for the partition coefficient were used to confirm the findings. We are confident that the host–guest interaction mechanism can contribute to a better understanding of molecular recognition in the phospholipid membrane model.

Enhanced solubility of methyl ester sulfonates below their Krafft points in mixed micellar solutions

HypothesisMethyl ester sulfonates (MES) show limited water solubility at lower temperatures (Krafft point). One way to increase their solubility below their Krafft points is to incorporate them in anionic surfactant micelles. The electrostatic interactions between the ionic surfactant molecules and charged micelles play an important role for the degree of MES solubility. ExperimentsThe solubility and electrolytic conductivity for binary and ternary surfactant mixtures of MES with anionic sodium alpha olefin sulfonate (AOS) and sodium lauryl ether sulfate with two ethylene oxide groups (SLES-2EO) at 5 °C during long-term storage were measured. Phase diagrams were established; a general phase separation theoretical model for their explanation was developed and checked experimentally. FindingsThe binary and ternary phase diagrams for studied surfactant mixtures include phase domains: mixed micelles; micelles + crystallites; crystallites, and molecular solution. The proposed general phase separation model for ionic surfactant mixtures is convenient for construction of such complex phase diagrams and provides information on the concentrations of all components of the complex solution and on the micellar electrostatic potential. The obtained maximal MES mole fraction of transparent micellar solutions could be of interest to increase the range of applicability of MES–surfactants.

Simple formulas for prediction of the sizes of worm-like and globular micelles in symmetrical electrolyte solutions

The micro/macroscale properties of micellar solutions and the nanoscale characteristics of aggregates are controlled by a complex combination of the properties of the surfactant, electrolyte solution, and thermodynamic conditions. The science and practice of micellar solutions have relied predominantly on various experimental characterization methods to determine their physicochemical properties among which the size of micelles is the most crucial factor as other properties of the solutions are mainly influenced by it. In contrast to the extensive experimental work, so far only very few theoretical studies have been devoted to the prediction of phase behavior and properties of surfactant solutions. Our thorough review of the literature reveals that the vast majority of the previous theoretical models have been based on molecular thermodynamic approaches with different conditions for achievement of an equilibrium state during the micellization process. The lack of general models capable of predicting the contribution of each aggregation step to the free energy of micellization under different conditions is a critical factor that severely limits the practical application of such models. The main purpose of the present study is to circumvent this challenge by approaching the problem from a different theoretical point of view. In addition, it is aimed at increasing our understanding and capability for accurate prediction of the behavior and properties of micellar solutions. To do so, novel models are proposed to quantify adsorption of ionic surfactant molecules onto the surface of normal micelles in aqueous media based on the concepts of the electrical double layer (EDL) and surface charging. These models are developed in cylindrical and spherical configurations that are the most commonly observed structures of surfactant aggregates and validated by comparison with available experimental data. Also, the cylindrical model is used to obtain a criterion of the necessary condition for the formation and evolution of less frequently seen worm-like micelles. The results of this study allow academic researchers and industrial practitioners to calculate the size of micelles formed in aqueous solutions by ionic surfactants using a few simple parameters whose exact or estimated values are readily available.

Pseudo-Laminarization of Mixed Microbubble Water and Complex Fluids in Capillary Flows

Symmetry in fluid flow is determined by external factors such as magnetic or electric fields. Owing to the electrical properties of fluids, liquids containing fine bubble or ionic surfactant molecules differ from the normal fluid flow. Therefore, the pressure drop and frictional coefficient in a pipe are measured and estimated for water alone, for mixed microbubble water (MB), and for complex fluids (anionic surfactant solution and polymer solution) in capillary flow with constant flow rate. For water alone, good agreement is obtained between the experimental and theoretical values for laminar Hagen–Poiseuille flow and the Blasius expression in turbulent flow. For the MB, pseudo-laminarization is suggested up to a Reynolds number of 4.2×103. Both the anionic surfactant solution and the polymer solution exhibit the same tendency as that of the MB. In explaining these behaviors, elasticity and electrical interaction are discussed, and electrical interaction on the capillary wall is strongly suggested to be a contributing factor. Additionally, surface tension is investigated and supports the discussion.

Effect of trace amounts of ionic surfactants on the zeta potential of DPPC liposomes

Surfactants are commonly found in today's world as an essential component of cleaning detergents, cosmetics and drug delivery systems. They can penetrate into lipid membranes, thus changing their properties. The aim of this paper is to compare the effect of addition of small amounts of cationic (DTAB) and anionic surfactants (SDS) with the same alkyl chain length on the zeta potential of DPPC liposomes with their influence on the corresponding DPPC monolayers. It was found that the addition of ionic surfactants with an initial concentration in the solution equal to 2.3, 4.5 and 9.1 μM to the liposome suspension changes their electrokinetic potential significantly. These changes increase with the increasing surfactant concentration and are greater for the anionic surfactant. This indicates the incorporation of surfactants into the structure of liposomes. Based on the analysis of π-area isotherms of DPPC monolayers it was proved that the ionic surfactant molecules are irreversibly integrated into the DPPC monolayer.

Organic superhydrophobic coatings with mechanical and chemical robustness

Abstract

Micellization of Ionic Liquid Surfactants Induced by Sodium Polystyrenesulfonate in Aqueous Solutions

The aggregation phenomena of two ionic liquid surfactants—1-decyl-3-methylimidazolium chloride (C10MeImCl) and 1-hexadecyl-3-methylimidazolium chloride (C16MeImCl) have been investigated in aqueous solutions in the presence of a polyelectrolyte, sodium polystyrenesulfonate (NaPSS), using electrical conductivity, surface tension, fluorescence and vapor pressure measurements. The counterion-condensation behavior of aqueous NaPSS has also been studied and approximately 62% of the counterions have been found to remain uncondensed in these solutions. The characteristic concentrations signifying different kinds of interactions, namely the critical aggregation concentration and the polymer saturation concentration, in the premicellar regime have been identified. Surface-active complexes with bound surfactant monomers onto the polyion backbone, non-surface active aggregates with small micellar units wrapped by polyion chains, and non-surface active micelles of ionic liquid surfactant monomers, were found to form in aqueous ionic liquid surfactant solutions in the presence of NaPSS. The surfactant concentration where polyelectrolyte–surfactant monomer complexes begin to form (cac), the concentration where the polyion chains get saturated with the small micellar units (psc), and the concentration where micelles of ionic liquid surfactant molecules begin to appear (cmc*) were identified in aqueous C16MeImCl solutions in the presence of NaPSS. Only two critical concentrations, namely the psc, and cmc* were, however, detected in aqueous C10MeImCl solutions in the presence of NaPSS. Effects of alkyl chain length of the ionic liquid surfactants, temperature, NaPSS concentration, and the charge parameter of NaPSS on the self-aggregation of the ionic liquid surfactants have been considered to elucidate the interactions in these mixed systems. The thermodynamics of micellization in these systems have also been studied and the spontaneity of the polyelectrolyte-induced micellization processes have been rationalized for the systems investigated, with the entropy terms superseding the enthalpy terms.

Nanorod morphology control of iron oxide nanoparticles induced by ionic surfactants and their solid – liquid interfacial adsorption

Morphology control is an important aspect of nanomaterials for their appropriate use in different applications. In this study, we report the morphology control of iron oxide nanoparticles (NPs) by using a variety of crystal growth controlling agents in a simple hydrothermal method. Sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) ionic surfactants proved to be excellent shape directing agents to produce fine magnetic nanorods whereas non-ionic surfactants and other bioactive molecules were unable to do so. Different techniques were applied to understand the mechanism of nanorod formation. It was attributed to strong solid – liquid interfacial adsorption of ionic surfactant molecules on {1 0 0}/{1 1 0} crystal planes of cubic lattice of growing nucleating centres to produce nanorod morphologies. XRD analysis clearly identified the magnetite nature of nanorods while XPS analysis helped in evaluating the amount and chemical composition of surface adsorbed molecules that were further related to the magnetic behavior. A compact and larger surface adsorption impeded the magnetization of nanorods and an optimum concentration of about 12 mM of SDS and CTAB was considered to be the best option to achieve both shape controlled synthesis as well as retaining appropriate magnetism.

Intermicellar Interactions and the Viscoelasticity of Surfactant Solutions: Complementary Use of SANS and SAXS.

In ionic surfactant micelles, basic interactions among distinct parts of surfactant monomers, their counterion, and additives are fundamental to tuning molecular self-assembly and enhancing viscoelasticity. Here, we investigate the addition of sodium salicylate (NaSal) to hexadecyltrimethylammonium chloride and bromide (CTAC and CTAB) and 1-hexadecylpyridinium chloride and bromide (CPyCl and CPyBr), which have distinct counterions and headgroup structures but the same hydrophobic tail. Different contrasts are obtained from small-angle neutron scattering (SANS), which probes differences between the nucleus of atoms, and X-rays SAXS, which probes differences in electron density. If combined, this contrast allows us to define specific intramicellar length scales and intermicellar interactions. SANS signals are sensitive to the contrast between the solvent (D2O) and the hydrocarbonic tails in the micellar core (hydrogen), and SAXS can access the inner structure of the polar shell because the headgroups, counterions, and penetrated salt have higher electron densities compared to the solvent and to the micellar core. The number density, intermicellar distances, aggregation number, and inter/intramicellar repulsions are discussed on the basis of the dependence of the structure factor and form factor on the micellar aggregate morphology. Therefore, we confirm that micellar growth can be tuned by variations in the flexibility and size of the the headgroup as well as the ionic dissociation rate of its counterion. Additionally, we show that the counterion binding is even more significant to the development of viscoelasticity than the headgroup structure of a surfactant molecule. This is a surprising finding, showing the importance of electrostatic charges in the self-assembly process of ionic surfactant molecules.

Enhanced Nanoparticle Removal Using Surfactants

ABSTRACTNanoparticles are used in chemical mechanical planarization for semiconductor manufacturing as well as in other precision manufacturing operations. Particles used in processing need to be removed from surfaces in order to enhance yields. Nanoparticles are difficult to remove from surfaces during cleaning due to the high van der Waals attractive forces between particles and surfaces relative to the low fluid drag forces that are used for typical removal methods. Ionic surfactant molecules can adsorb on particles and surfaces to create an electrostatic repulsion between particles and surfaces as well as provide a steric barrier to mitigate adsorption and adhesion. The effectiveness of the surfactant in enhancing particle removal is related to surfactant properties, and it can be correlated with and modeled relative to the critical micelle concentration of the surfactant. The general approach for modeling will be discussed, and the model will be compared with particle removal data.

Thermoreversible crystallization of charged colloids due to adsorption/desorption of ionic surfactants

We report that charged colloids exhibit thermoreversible crystallization via the adsorption of ionic surfactants onto particle surfaces. Due to the temperature dependence of the adsorption quantity, the colloids crystallized upon cooling and melted upon heating. To clarify the influences of surfactant adsorption on the crystallization, polystyrene (PS) particles dispersed in ethylene glycol (EG)/water mixtures were employed, enabling continuous tuning of the adsorption quantity by changing the EG concentration. The thermoreversible crystallization/melting behavior was found to be mainly attributable to changes in the ionic strength of the medium resulting from variation in the concentration of the non-adsorbed ionic surfactant molecules with temperature. We expect that the present findings will be useful for fine control of colloidal crystallization and the further study of colloidal crystallization in low permittivity media.

A model study of cooperative binding of ionic surfactants to oppositely charged flexible polyions

A novel statistical model for the cooperative binding of monomeric ligands to a linear lattice is developed to study the interaction of ionic surfactant molecules with flexible polyion chain in dilute solution. Electrostatic binding of a ligand to a site on the polyion and hydrophobic associations between the neighboring bound ligands are assumed to be stochastic processes. Ligand association separated by several lattice points within defined width is introduced for the flexible polyion. Model calculations by the Monte Carlo method are carried out to investigate the binding behavior. The hypothesis on the ligand association and its width on the chain are of importance in determining critical aggregation concentration and binding isotherm. The results are reasonable for the interpretations of several surfactant-flexible polyion binding experiments. The implications of the approach are presented and discussed.

Effect of Competitive Surface Functionalization on Dual-Modality Fluorescence and Magnetic Resonance Imaging of Single-Walled Carbon Nanotubes.

It is well-known that ionic surfactant coated single-walled carbon nanotubes (SWNTs) possess higher near-infrared fluorescence (NIRF) quantum yield than nonionic polymer functionalized SWNTs. However, the influence of surface functionalization on the magnetic properties of SWNTs for T2-weighted magnetic resonance imaging (MRI) has not been reported. Here, we demonstrate that SWNTs functionalized by nonionic polymers display superior T2 relaxivity for MRI as compared to those coated by ionic surfactants. This difference may indicate that micelle structures formed by ionic surfactants are sufficiently tight to partially exclude water protons from the iron catalysts attached to the ends of SWNTs. On the basis of the different effects of the two types of suspension agents on NIRF and MRI of functionalized SWNTs, we further explore the competitive surface functionalization between ionic surfactants and nonionic polymers by stepwise replacing ionic surfactant molecules in a nanotube suspension with nonionic polymers. The superior NIRF of ionic surfactant coated SWNTs gradually quenches whereas no improvement on T2 relaxivity is observed during this replacement process. This result may indicate that nonionic polymers wrap around the outside of micelle structures to form small nanotube bundles rather than replacing ionic surfactants in the micelle structures to directly interact with the SWNT surface. Finally, we demonstrate the feasibility of dual-modality NIRF and MRI of nonionic polymer functionalized SWNTs in brain cells.

Potential for Measurement of Corrosion-Inhibitor-Micelle Presence as an Indicator of Optimum Dose

Summary This paper discusses the principle and proof of concept of a novel corrosion-management tool based on maintaining optimum corrosion-inhibitor dose in an inhibited environment. Film-forming corrosion inhibitors typically contain active ionic surfactant molecules, which adsorb on surfaces and form a protective barrier against corrosion. Upon saturation of surfaces by the inhibitor, the molecules are found in the aqueous phase as micelles or in the oil phase as reverse micelles. The proposed technique is based on the hypothesis that the presence of these micelles in the water phase could be used to indicate the total surface coverage and, therefore, the optimum dose. The technique has been applied in the laboratory simulating an actual inhibited multiphase production system. Fluorescent markers, which are extremely sensitive to micelle presence, have been developed and used during the laboratory testing. Results were compared with those from standard laboratory techniques, including interfacial tension (IFT) and particle-size analyses (diffraction), in order to determine the efficacy for micelle detection. Corrosion bubble tests were also used to determine any link to inhibitor efficiency. IFT results showed complex events occurring with increasing inhibitor concentration. The IFT vs. inhibitor concentration curve was nonclassical, probably because of the multicomponent nature of inhibitor formulations, but suggested micelle formation at approximately 150 ppm. Fluorescence analysis suggested that micelles formed starting at a 150-ppm dose. Particle-size analysis was consistent with micelle presence beyond this concentration. Separate experiments comparing fluorescence with corrosion rates demonstrated an apparent nonlinear variance with inhibitor concentration, supporting the hypothesis. The analytical comparisons supported the underlying principles of this micelle-detection technology. Development of a portable device is under way and will provide an important new tool for proactive corrosion management in the oil field as well as a useful laboratory qualification method.

Synergism of ionic liquid and surfactant molecules in the growth of LiFePO 4 nanorods and the electrochemical performances

In this work, the LiFePO 4 nanorods are synthesized in an ionic liquid (IL) in the presence of a surfactant. The samples are characterized by X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, electron diffraction and nitrogen adsorption. The as-obtained LiFePO 4 nanorods in the IL have the diameter of about 200 nm and the length of 1–2 μm; but the aggregated LiFePO 4 microparticles are obtained in the aqueous solution. The results show that a higher temperature and a longer duration are needed in the IL than in the aqueous solution for the formation of LiFePO 4; that both IL and the surfactant play the key roles in the formation of LiFePO 4 nanorods. Furthermore, their charge and discharge properties are investigated. The LiFePO 4 nanorods show a higher discharge capacity (150 at 1 C rate) and a higher rate capability than the aggregated particles (133 mAh g −1). The adopted IL approach may provide a “green” route to achieve excellent battery materials.

Cyclodextrin glucanotransferase (CGTase) catalyzed synthesis of dodecyl glucooligosides by transglycosylation using α-cyclodextrin or starch.

Dodecyl glucooligosides, a class of interesting non ionic surfactant molecules were synthesized by cyclodextrin glucanotransferase from Bacillus macerans using either α-cyclodextrin (α-CD) or soluble starch as glycosyl donor and dodecyl β-d-glucoside (C12G1) or dodecyl β-d-maltoside (C12G2) as acceptor substrates. The primary coupling products obtained in the respective reactions were identified as dodecyl glucoheptaoside and dodecyl maltooctaoside by mass spectrometry. Higher yields of coupling products were obtained using α-CD as donor, while more dispoportionation occurred with starch. Nearly 78% conversion of the acceptor substrate C12G1 into dodecyl glucooligosides could be achieved at 132μg/ml of CGTase in 20min, while 93% of C12G2 could be transformed into products at 17.6μg/ml of enzyme in 120min using soluble starch as donor substrate. For applications requiring pure compounds like C12G7, synthesis using α-CD is advantageous. However, for applications in which a mixture of elongated alkyl glycosides is needed, reactions employing starch are clearly competitive.

The effect of sodium lauryl sulphate, cetrimide and polysorbate 20 surfactants on complex coacervate volume and droplet size.

The effects of sodium lauryl sulphate (SLS), cetrimide and polysorbate 20 surfactants at concentrations below, at and above their critical micelle concentration (CMC) on the complex coacervation of varying concentrations of gelatin and acacia have been described. The overall effect of increasing concentration of SLS was to reduce the weight of coacervate formed. The addition of increasing concentrations of cetrimide produced an increase in the weight of coacervate. The two lowest concentrations of polysorbate 20 produced an increase in coacervate weight while the highest concentration, above the CMC, reduced the coacervate weight. These effects have been explained in terms of shielding of electrostatic attractions between gelatin and acacia polyions by adsorption of ionic and non-ionic surfactant molecules onto the polyions. The addition of surfactants influenced the size distribution of the coacervate droplets that were produced. It is believed that the reduction in interfacial tension by the aggregation of surfactant molecules at the coacervate-equilibrium liquid interface permitted the formation of smaller coacervate droplets.

Effect of liquid paraffin on the stability of aqueous foam in the presence and absence of electrolytes

The foam stability of dodecyl diphenyl ether disulfonate solution with liquid paraffin droplets in the presence and absence of electrolytes was evaluated, and the stabilization mechanisms were deduced. The foam film is stabilized when the monovalent and divalent counterion concentration is lower than a critical value. However, the foam stability declined with the addition of trivalent counterions. There are two mechanisms we have speculated. Firstly, the counterions reduce the repulsive interaction between the polar groups of surfactants adsorbed at the air/water and oil/water interfaces in the pseudo-emulsion film. Secondly, comparing with the monovalent counterions, the multivalent counterions are not only able to interconnect head groups of adjacent ionic surfactant molecules which are adsorbed at the air/water or oil/water interface, but also to attract the couples of neighboring surfactant head groups adsorbed at the air/water and oil/water interfaces. The attractive interaction between both the interfaces promotes the emulsified droplets piercing the air/water interface.

Mesoporous Silica Materials Synthesized via Sol-Gel Methods Modified with Ionic Liquid and Surfactant Molecules

Mesoporous silica materials were synthesized via a sol-gel method employing a room temperature ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4]) as a new solvent medium and further modified with surfactant (hexadecyl-trimethyl-ammonium bromide, CTAB) as a pore templating material. The synthesized samples were characterized by the transmission electron microscopy, X-ray diffraction, and N2 adsorption-desorption techniques. The results indicated that the mesoporous silica synthesized by using [bmim][BF4] and CTAB as mixed templates showed better mesostructural order and smaller pore size, compared with mesoporous silica materials synthesized by using single [bmim][BF4] as template under the same conditions. This indicates that the presence of surfactant can affect the microstructures of silica prepared by the present synthesis method.

Probe three-dimensional structure of soft organized surfactants at solid–liquid interface with an AFM in MAC Mode and contact mode

Atomic force microscopy provides a unique direct-visualization tool to study the three-dimensional structure of adsorbed surfactants on solid surfaces. Ionic surfactant molecules spontaneously adsorbed onto hydrophilic surfaces from aqueous solution above the critical micelle concentration (cmc) have been imaged using an atomic force microscope in magnetic ac mode (MAC Mode) and contact mode. It was found that the soft organized surfactants were highly compressible and therefore showed a wide range of corrugations depending on imaging forces. When using gentle MAC Mode, corrugations of the organized surfactants around half of the estimated height of the proposed surfactant aggregate cylinders on mica have been stably observed. Traditional contact mode operating in the pre-contact double-layer electrostatic interaction region, however, showed significantly reduced height of the organized molecules.