Surfactant Counterion Research Articles

Surfactant Aggregates at a Metal Surface

Surfactant adsorption on metals has been used in the past to limit the activity of electrode surfaces and to stabilize colloidal clusters in solution, but the adsorption and aggregation morphology of these surfactants has not been known. We present direct images of ionic surfactant aggregates at a gold surface in aqueous solution using atomic force microscopy. These images are generally striped in appearance as viewed from the top, indicating linear aggregates (cylindrical or half-cylindrical) lying on the substrate plane. The orientation of these aggregates is controlled either by monatomic steps on the gold surface or by the gold lattice itself, depending on the surfactant counterion.

Micellar size of drag reducing cationic surfactants

Dynamic light scattering was employed to determine the effects of surfactant nature, concentration and counterion ratios, and shear on the hydrodynamic radii of micelles of commercial cationic surfactants which are powerful drag reducers in turbulent flow at high temperatures. Such surfactants are potentially useful for reducing pumping energy losses in district heating and cooling systems.

Micellar kinetics in aqueous acetonitrile. Part A. Sodium-hydrogen ion-exchange constant for 1-dodecanesulfonate surfactant. Part B. Substrate structural effects for micellar catalysis by perfluorooctanoic acid as reactive counterion surfactant

Micellar kinetics in aqueous acetonitrile. Part A. Sodium-hydrogen ion-exchange constant for 1-dodecanesulfonate surfactant. Part B. Substrate structural effects for micellar catalysis by perfluorooctanoic acid as reactive counterion surfactant

Micellar kinetics in aqueous acetonitrile. Part A. Sodium‐hydrogen ion‐exchange constant for 1‐dodecanesulfonate surfactant. Part B. Substrate structural effects for micellar catalysis by perfluorooctanoic acid as reactive counterion surfactant

The sodium-hydrogen ion exchange constant for the system sodium 1-dodecanesulfonate-hydrochloric acid in aqueous acetonitrile has been determined from the pseudo-phase ion exchange model for surfactant catalytic effects. The results indicate that the micellar system behaves similarly for the aqueous and the aqueous acetonitrile (2.106 M) solvent systems. The influence of substrate molecular structure on micellar catalysis by perfluorooctanoic acid of the hydrolysis of hydroxamic acids (R—CO—NHOH) in aqueous acetonitrile has been explored. Data for substrate structures of fifteen compounds with R=alkyl, aralkyl, alicyclylalkyl, phenylalkyl, alkyl-substituted phenylalkyl, and with chain branching at the α, β, and γ positions are compared. Relative binding constant values indicate that substrates with aromatic groups are less well solubilized in the perfluoro micellar environment than are substrates with saturated groups. There is now evidence for specific micellar effects on the reaction rate as well as general micellar catalysis. © 1997 John Wiley & Sons, Inc.

Effect of surfactant counterion on spectroscopic properties of water in oil microemulsions

The effect of replacing the Na + counterion of Aerosol OT by Cu 2+ on the hydration mechanism of AOT reverse micelles was studied by infrared spectroscopy. The IR properties of ternary systems formed by Cu(AOT) 2/[H 2O]/CCl 4 are reported and compared with the corresponding Na(AOT) system. The results show significant differences in interactions of Na + and Cu 2+ counterions with H 2O molecules and the SO 3 − headgroup of AOT.

Zeta Potential of Air Bubbles in Surfactant Solutions

A spinning tube electrophorometer was used to measure the ζ potential of air bubbles immersed in surfactant solutions. It was found that a suitable polymer coated on the inner surface of the tube prevents electroosmosis at pH values between 5 and 8, when anionic or nonionic surfactants are used. Generally speaking, ζ-potential variations principally reflect the kinetics and the saturation of the adsorption of surfactant molecules on the bubbles. Surfactant adsorption and counterion adsorption appear simultaneously, and as the former is substantially higher than the latter, it is not possible to distinguish each contribution. For a very pure nonionic surfactant and bubbles immersed in a solution with a very low counterion content, however, these contributions are sufficiently dissociated to produce a minimum ζ potential as a function of surfactant concentration.

Synthesis of 2-(2-nitrophenoxy)quinoxaline and its basic hydrolysis in aqueous solutions of non-reactive counter-ion surfactants with bulky head groups

The synthesis of the novel compound 2-(2-nitrophenoxy)quinoxaline ( 2) is described and its basic hydrolysis was studied in the presence of non-reactive counter-ion surfactants with different head group size. Micellar effects upon the reaction of OH − with ( 2) were analyzed by using a massaction-like equation.

Generalized water uptake modelling of water-in-oil microemulsions. New experimental results for Aerosol-ot-isooctane-water-salts systems

This work reports experimental results of the water uptake of Aerosol-OT (AOT) reverse micelles measured at several initial AOT concentrations, several concentrations of single and mixed salts in the aqueous phase, and several initial volume ratios of organic to aqueous phase. Due to the ion exchange between the cations in the aqueous phase and the sodium as surfactant counterion, the water uptake is strongly affected by the nature and concentration of the cations and it is insensitive to the nature of the anions. A model has been developed in terms of dimensionless groups using as input the initial conditions of the system. This model accurately represents the effect of the variables on the water uptake.

Partial phase mixing in solution-processed polyaniline-poly (methyl methacrylate) blends

Polyaniline protonated with camphor sulfonic acid is readily soluble in m-cresol. From appropriate solutions, blends of polyaniline with common insulating polymers can be cast as free-standing or ultra-thin films. Such blends exhibit non-standard conduction behaviour in that no conventional percolation threshold is observed, the conductivity rising smoothly from the insulating state with increasing polyaniline concentration. In this study, blends of polyaniline together with poly (methyl methacrylate) have been investigated by transmission electron microscopy to reveal the nature of the conductive pathways that are present at low filling fractions. The blends appear as a partially phase-separated structure with neither phase forming distinct domain boundaries. On removal of the insulating matrix, the polyaniline is observed to collapse into a complex network, in which spanning connections are present at all fractions above the conductive onset. Compatibilization of the polymers by the action of the surfactant counter ion and bound solvent is discussed as the means by which solubility and partial intimate mixing of the macromolecules are induced and from which conducting pathways arise.

Silver(I)-mediated separations by capillary zone electrophoresis and micellar electrokinetic chromatography: argentation electrophoresis.

The addition of Ag(I) to the run buffer in capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC), containing sodium dodecyl sulfate, applies the principles of argentation chromatography to electrophoretic separations and is termed "argentation electrophoresis". This technique is shown to provide a complementary method to CZE and MEKC for the separation of specific types of solutes that selectively complex with Ag(I). Baseline resolution in the CZE separation of nine sulfonamides is achieved by the addition of 50 mM silver nitrate to the run buffer. Retention mechanisms in MEKC separations can also be manipulated by the addition of Ag(I) to the micellar solution. Only slight resolution of a pair of sulfonamides was achieved under normal MEKC conditions. Upon the addition of Ag(I) to the mobile phase containing SDS micelles, baseline resolution of the compounds is shown. The retention order and resolution of five sulfonamides changed significantly when 25 mM Ag(I) was added to the SDS-containing buffer. The use of Ag(I) addition in MEKC is also applied to the separation of various other compounds that show selectivity toward Ag(I) complexation, including S-containing heterocycles and vitamin D compounds. The effect of the addition of Ag(I) on the elution range in MEKC is also investigated. A steady increase in the elution range is seen upon increasing the concentration of Ag(I). With a constant percentage of organic modifier (15%), the addition of higher concentrations of silver nitrate (25-50 mM) results in elution ranges greater than 12. The results using Ag(I) as buffer additives in MEKC are also compared to studies utilizing a mixed counterion surfactant of sodium/silver dodecyl sulfate.

Interactions between Water-Soluble Polymers and Surfactants: Effect of the Polymer Hydrophobicity. 1. Hydrophilic Polyelectrolytes

The binding of DTAC (dodecyltrimethylammonium chloride) to two polyelectrolytes of differing hydrophobicity, the alternated copolymers poly(maleic acid-co-methyl vinyl ether) and poly(maleic acid-co-butyl vinyl ether) referred to as PS1 and PS4, respectively, has been investigated by potentiometry with a surfactant ion-specific electrode, by time-resolved fluorescence quenching, by fluorescence anisotropy, and by viscosimetry. The surfactant binding isotherms, the number of surfactants (N) making up polymer-bound aggregates, the lifetime of pyrene (τ2) in these aggregates, and their microviscosity (ηi) were thus obtained as a function of the surfactant concentration and copolymer neutralization degree α, which determines its electrical charge density. As observed in other studies, the binding is cooperative in the whole range of α for PS1 and for α ⩾ 0.5 for PS4. Aggregation numbers, pyrene lifetimes, and aggregate microviscosities were all found to be independent of the surfactant concentration but to depend strongly on α. N increased with α for PS1 but decreased for PS4. For PS1, τ2 and ηi increased when α decreased, i.e. with decreasing micelle size. The comparison of the τ2 values obtained with DTAC and dodecyltrimethylammonium bromide in the presence of PS1 revealed that the surfactant counterions are expelled from the surface of polymer-bound aggregates. This leads to a model where polymer chains tightly wrap around aggregates with their charged groups in contact with surfactant charged groups. The difference of behavior between PS1 and PS4 is attributed to the contribution of hydrophobic interactions between PS4 butyl side chains and surfactant alkyl chains. This contribution is small in the case of PS1, where binding is of essentially electrostatic nature. The high microviscosity of polymer-bound aggregates relative to free DTAC micelles is attributed to the electrostatic binding of surfactant ions to the polyelectrolyte and to the presence of the polymer main chain at the aggregate surface. The large values of the pyrene fluorescence lifetime are the result of the high microviscosity of the bound aggregates, which slows down the diffusive motion of reactants (probe and quencher, or any other reactant) in the aggregates and thus makes quenching (and other) processes much less efficient than in free micelles.

Effects of Volume Ratio and of Surfactant, Salt, and Alcohol Concentrations on the Ion Distribution of Dioctyldimethylammonium Chloride Reverse Micelles in Isooctane

The effects of volume ratio, of surfactant and alcohol concentrations, and of different ions on the ion distribution of a dioctyldimethylammonium chloride−decanol−isooctane reverse micellar phase in equilibrium with an aqueous phase were studied. The initial organic phase containing the surfactant and decanol, as cosurfactant, in isooctane was contacted with an aqueous phase containing electrolytes. Experimental data of the distribution of anions in this system have been measured at 23 °C. A mathematical model has been developed which accounts for the exchange of the surfactant counterion with the anions in the aqueous phase. This model uses one single parameter for each anion, which depends only on the nature of the anion. It accurately represents the distribution of anions between the two phases of the system. Dimensionless groups have been employed to combine the effect of several independent variables into a few groups.

Energy Transfer between Pyrene and Proflavine Solubilized in Polymer/Surfactant Complexes

Energy transfer from excited pyrene (donor) to proflavine (acceptor) was measured in sodium dextran sulfate/dodecyltrimethylammonium bromide (DxS/DTAB), DxS/dodecyltrimethylammonium chloride (DxS/DTAC), poly(vinyl sulfate)/DTAB (PVS/DTAB), and poly(styrenesulfonate)/DTAB (PSS/DTAB) mixed solutions by monitoring both the quenching of pyrene emission and the enhancement of proflavine emission as a function of acceptor concentration at 0.1, 0.2, 0.3, 0.4, and 0.5 mmol dm-3 surfactant. A remarkably high enhancement of energy transfer was found in DxS/DTAB, DxS/DTAC, and PVS/DTAB complexes, but the energy transfer efficiency was low in PSS/DTAB complexes. The enhanced energy transfer is attributed to the presence of polymer/surfactant (P/S) aggregates which solubilize the donor and acceptor in monomeric form. The high energy transfer efficiency in the DxS and PVS systems is due to the larger aggregate sizes in these systems, related to the higher degree of cooperativity observed in the surfactant binding isotherm. Smaller aggregate sizes in the PSS/DTAB complex lead to a lower energy transfer efficiency. ET efficiences are relatively independent of the surfactant counterion, bromide or chloride, indicating the complete exclusion of the counterion from the surface of the P/S aggregate.

Conductivity studies of the molecular encapsulation of sodium perfluoroctanoate by?-cyclodextrin derivatives

The molecular encapsulation of sodium perfluoroctanoate (SPFO) by hydroxypropyl-β-cyclodextrin (HP-β-CD) or 2,6-di-O-methyl-β-cyclodextrin (DM-β-CD) has been analyzed by measuring the conductivity in solution of the ternary systems formed by CD + SPFO + H2O. The studies were carried out at 25 °C using a fully computerized electrical conductivity technique. The measurements were made as a function of CD concentration at various non-micellar concentrations of SPFO, and as a function of CD and SPFO concentrations with [CD]/[SPFO] constant at stoichiometric ratio. The inclusion complexes, HP-β-CD-SPFO and DM-β-CD-SPFO, were characterized through the stoichiometry, which has been found to be 1 : 1 in both cases, and the binding constants, which have been evaluated from the conductivity data with a model proposed by us considering the variation of the ionic molar conductivities with the concentration and the association of the surfactant counterion to the inclusion complex. The resultingK values indicate that the interaction between the CD cavity and the monomeric SPFO is strong and similar in both cases.

Investigation of surfactant alkyl chain length and counterion effects on the thermogelling EHEC system

Ethylhydroxyethylcellulose (EHEC) forms a reversible thermogelling system in water in the presence of ionic surfactants. In an effort to minimise the amount of ionic surfactant needed to produce a thermogelling system with timolol in salt form, we have examined in detail the system with respect to the alkyl chain length and counterion of the ionic surfactant. Two salts of timolol were also investigated, timolol (hydrogen)maleate and timolol chloride. It was found that in order to form a gel with a small amount of ionic surfactant, a number of criteria have to be fulfilled, e.g., (i) the ionic drug should typically be a co-ion to the ionic surfactant, (ii) the counterion of the drug and the ionic surfactant should preferably be inorganic and with a low polarisability and (iii) the ionic surfactant should have a low CMC but a Krafft temperature not higher than room temperature.

Effects of Surfactant Purity and Concentration, of Surfactant Counterion, and of Different Ions on the Water Uptake of Dioctyldimethyl Ammonium Salt-Decanol-Isooctane Reverse Micellar Systems

The effects of surfactant purity and concentration, of surfactant counterion, and of different ions on the water uptake of a dioctyldimethyl ammonium salt-decanol-isooctane reverse micellar phase in equilibrium with an aqueous phase were investigated. The initial organic phase containing the surfactant and decanol, as cosurfactant, in isooctane was contacted with an aqueous phase containing electrolytes. The water uptake at equilibrium was measured at several surfactant concentrations for different electrolytes with pure dioctyldimethyl ammonium chloride (DODMAC), dioctyldimethyl ammonium bromide (DODMAB), and their commercial forms. The purity of the surfactant had no effect on the water uptake per mole of pure surfactant. Changing the cation of a salt did not have any effect on water uptake while changing the anion altered the water uptake considerably. Systems containing mixed anions can show a maximum in the water uptake. The water uptake increased when either the charge number of the anion of the salt increased or the ionic radius of the anion decreased. The water uptake was a linear function of surfactant concentration at each salt concentration. A mathematical model was developed which accounts for the exchange of the surfactant counterion with the anions in the aqueous solution. The model accurately represented the water content in the organic phase both for single and multiple salt systems.

A Chemical Theory for Ion Distribution Equilibria in Reverse Micellar Systems. New Experimental Data for Aerosol-OT-Isooctane-Water-Salt Systems

Aerosol-OT and some other surfactant molecules form reverse micelles in an organic phase in contact with an aqueous salt solution. Experimental data of the distribution of cations in Aerosol-OT-isooctane-water-salt systems have been measured at 23 o C for different chloride salts. The effect of different variables on the equilibrium ion distribution between the organic phase, containing reverse micelles, and an aqueous phase, containing different salts, has been formulated in terms of dimensionless groups. These variables are (i) the initial surfactant concentration, (ii) the nature and initial salt concentration of each salt, and (iii) the initial volume ratio of the two phases. The model distinguishes between different ions via the equilibrium constants of their ion exchange reactions with the surfactant counterion. It accurately predicts the distribution of single charge and multicharge ions between the two phases of the system. A model formulated in dimensionless form, with few independent dimensionless groups rather than many independent variables, has definite advantages for the treatment of complex systems. In this work, the predictions of the model are compared with experimental results and with the predictions of other phenomenological models published in the literature for reverse micellar systems

Transport across stand-alone conducting polypyrrole membranes containing dodecylsulfate counterions

The modification of the electrodynamic transport properties of conducting polypyrrole membranes by the use of dodecylsulfate anions during polymerization has been demonstrated. The use of small amounts of the surfactant counterion in combination with paratoluene sulfonate as the predominant anion during polymerization allows the production of conductive, mechanically stable materials with differing chemical properties. This, in turn, influences the transport characteristics and has been found to induce asymmetric transport character in the membrane.

Kinetics of complexation of metal ions by extractants solubilized in aqueous micellar solutions of cetyltrimethylammonium salts: Effect of the surfactant counterion

Several experiments have demonstrated that the complexation of metal ions by lipophilic extractants solubilized in cationic micelles does occur, although the electrostatic repulsions between the positively charged surface of the micelles and the metal ions are not favourable for the reaction to proceed. Owing to the very high concentration of anion X − which exists close to the micelle surface one may wonder about the possible contribution in this process of species of the type [MX 4] 2−. This problem is addressed here by comparing the kinetics of complexation of four metal extractant couples in different CTA +X − micelles where X − = Br −, Cl − or NO − 3. The existence of anionic metal species is assumed to be less probable with nitrate than with chloride or bromide. The results definitely show a different behaviour in the presence of nitrate when the extractant is known to be strongly partitioned in the micellar pseudophase, but no counterion effect is detected when less hydrophobic complexing agents are considered. These observations further substantiate the distinction previously made between interfacial complex formation reaction and bulk reaction, this being a function of the extractant hydrophobicity.

Superlocalization of the electronic wave functions in conductive polymer blends at concentrations near the percolation threshold

The ability to solution-process polyaniline (PANI) in the protonated conducting form through the use of surfactant counterions has enabled the fabrication of conductive polymer blends with a percolation threshold at a volume fraction near 1%. Electron micrographs of blends of PANI complexed with camphorsulfonic acid (CSA) in poly(methyl methacrylate) show a tenuous interconnected network; for concentration of PANI-CSA near the percolation threshold, the network is self-similar. Digital analysis of the micrographs shows that in thin two-dimensional ``slices`` the PANI-CSA networks are fractal, with the area (S) of the conducting network varying as S {alpha} r{sup D}, where D < 2 for concentrations below 4%. Near the threshold, they find D {approximately} 1.5, implying a fractal dimensionality in three dimensions of ca. 2.5. The electrical conductivity of these blends follows the Mott-Deutscher model for variable-range hopping on a fractal network, {sigma}(T) {approximately} exp[{minus}(T{sub 0}/T){sup {gamma}}]. They find that {gamma} increases from {gamma} = 1/4 in pure PANI-CSA (indicting variable-range hopping among exponentially localized states) to {gamma} {approximately} 2/3 as the PANI-CSA concentration is reduced to the percolation threshold, indicating variable-range hopping among superlocalized states on the fractal structure in the limit where the Coulomb interaction between the electron and the holemore » dominates the intersite hopping.« less