Surfactant Ion Concentration Research Articles

Novel surfactant selective electrochemical sensors based on single walled carbon nanotubes

New poly(vinyl chloride) PVC membrane electrodes selective to the hexadecyltrimethylammonium ion (CTA +) and sodium dodecyl ion (SD −) have been constructed using a modified single walled carbon nanotube (SWCNT). This sensor was used as a sensing material and incorporated into the plasticized PVC-membrane. The membrane electrodes exhibited a Nernstian response (59.5 mV/decade) for SDS and a near-Nernstian response (57.2 mV/decade) for CTAB over a wide concentration range below their critical micelle concentrations (CMC). The electrodes showed a fast response time (t90% = 30 s) and could be used for 3 months without any divergence in potentials. The SD − ion selective electrode (ISE) can determine monomer units down to concentrations as low as 1.9 × 10 − 6 M and 5.2 × 10 − 6 M for CTA +. The effect of foreign anions along with primary ions on the performance of ion-selective electrode is investigated in terms of potentiometric selectivity coefficients, which were determined using the fixed primary method (FPM) at 5.0 × 10 − 5 M concentration of surfactant ions. The two proposed potentiometric sensors revealed acceptable selectivities for surfactant ions over a wide variety of other common inorganic and organic anions. The lifetime of the sensor is more than three months. This method for determining of anionic surfactants was found to be quite accurate when compared with classical methods.

Aqueous two-phase systems of cetyltrimethylammonium bromide and sodium dodecyl sulfonate mixtures without and with potassium chloride added

The aqueous two-phase systems (ATPS) formed in aqueous solutions of mixed anionic and cationic surfactants, cetyltrimethylammonium bromide (CTAB)/sodium dodecyl sulfonate (AS)/water mixtures, have been studied in detail at 313.15, 318.15 and 323.15 K. Two different cases, ATPS-A with AS in excess and ATPS-C with CTAB in excess are distinguished. Various other phase regions including liquid crystalline single-phase region are also presented. Temperature dependence indicates that the phase separation process is entropy driven. Typical tie-lines in ATPS on a three-dimensional pyramid schematic phase diagram are also presented based on composition analyses. The results confirm that the top phase is concentrated in surfactant ions, while the bottom phase has excess salt ions. Liquid crystal is formed in the top phase of ATPS-A, while the bottom phase of ATPS-C can be either birefringent or isotropic depending on the global surfactant ion concentration and the molar ratio of C 16H 33(CH 3) 3N + (CTA +) cation to C 12H 25SO 3 − (AS −) anion. Salt effect is also studied. Higher concentration of KCl leads to larger two-phase region. The microstructures of some aqueous two-phase systems are obtained by negative stained TEM method. Thread-like micelles, spherical, wormlike, rod-like and irregular aggregates are appeared at different conditions. Vesicles appear occasionally.

In-situ spectroscopic investigations of surfactantadsorption and water structure at the CaF2/aqueous solution interface

We report in-situ spectroscopic measurements of the structure and orientation of molecules in the interfacial region between the semi-soluble ionic solid CaF2 (fluorite) and an aqueous phase. This paper integrates a series of new experiments with earlier results to give a comprehensive understanding of this very dynamic interface. We employ the surface specific technique, vibrational sum-frequency spectroscopy (VSFS), to study the effect that alterations in the aqueous phase composition, such as pH, surfactant ion concentration, and ion composition have on the bonding interactions, ion exchange behavior, and electrical properties in the interfacial region. These studies demonstrate the complex nature of the interactions of semi-soluble solids with an aqueous phase and the complexity of the surfactant adsorption process. Fundamental studies such as these are essential for understanding the mechanisms involved in surfactant adsorption and interfacial charge reversal; information which is important for industrially relevant processes such as mineral ore flotation, waste processing and petroleum recovery.

Micelle Formation ofn-Decyltrimethylammonium Perfluorocarboxylates

Critical micelle concentrations (CMCs) of three ionic surfactants that containn-decyltrimethylammonium ion as a surfactant cation and bromide, perfluoroacetate, and perfluoropropionate anions as a counterion, respectively, were determined by electrical conductivity at several temperatures. The degrees of counterion binding to micelle were obtained at the same temperatures as the change in CMC with total counterion concentration. Molecular weights of micelles were determined by static light scattering. The change in monomeric surfactant ion concentration with total surfactant concentration was determined by membrane potential measurement. The mass-action model was applied to micelle formation to calculate the three micellization parameters: micellization constant, micelle aggregation number, and number of counterions per micelle. Thermodynamic parameters (ΔG0, ΔH0, ΔS0) of the micelle formations were calculated from the temperature dependence of these parameters. At lower temperatures micellization has been found to be entropy driven, whereas it is enthalpy driven at higher temperatures. Forn-decyltrimethylammonium perfluoropropionate, however, the tendency is quite different from those of other surfactants. The micellization parameters also made it possible to evaluate the concentration change of monomeric surfactant ion with total surfactant concentration. The evaluated concentration explains the results from the membrane potential measurement.

Changes of pH and counterion activity during the binding process of cationic surfactants to carboxylic polyions

The binding of a cationic surfactant, dodecylpyridinium ions (DP +), by an alternating copolymer of maleic acid with styrene (MASt) and an alternating copolymer of maleic acid with ethylene (MAE) and the pH change in the solutions of the copolymers during the process of cooperative binding have been investigated in the presence of a fixed concentration of NaCl by using potentiometric techniques based on a surfactant-ion-selective solid state membrane electrode and a pH electrode. In the cases of MASt at lower degrees of neutralization, α, and MAE, where the binding isotherms are single-step curves, the pH values increase at first, accompanied by a sharp rise in the binding isotherms, and then decrease in the leveling-off phase of the binding process. In the case of MASt at higher α values, however, the relationship of pH vs. the logarithm of the free surfactant ion concentration exhibits a characteristic double-peaked curve corresponding to the two-step binding isotherm. The change in sodium ion activity during the binding process of DP + ions has been also examined in salt-free aqueous solutions of poly(acrylic acid) (PAA) with use of an Na + ion-selective solid state membrane electrode. It was found that Na + ions are released in proportion to the concentration of bound surfactant ions and almost all the initially condensed Na + ions are released finally through the whole process of the surfactant ion binding.

The Interpretation of Surface Tension Data for Solutions of Ionic Surfactants in the Presence of Electrolyte

Abstract An improved method is presented for interpreting surface tension data on solutions of an ionic surfactant to which has been added an electrolyte whose co-ion has the same valence as the surfactant ion. Adequate account is taken of co-ion adsorption and of deviations from ideality of the solution. When a monolayer of surfactant ions is formed and the distributions of free surfactant ions and co-ions in the region adjacent to it are determined predominantly by electrostatic repulsion, it is shown that the separate contributions to the total Gibbs excess of surfactant ions from the monolayer and the adjacent region can be obtained. Finally some experimental data are re-examined and a simple explanation is given for the minimum reported to occur in co-ion adsorption as the surfactant ion concentration is increased at constant co-ion concentration.

Surfactant-selective electrodes : Part III. Evaluation of a dodecyl sulphate electrode in surfactant solutions containing polymers and a protein

An electrode, selective to the dodecyl sulphate anion, has been evaluated for use in surfactant solutions containing polymers and a protein. Results obtained with the electrode have been used directly to construct surfactant ion binding isotherms which show similar regions of change to those previously established in the literature by indirect methods. It is shown that when equilibrium dialysis measurements are used to construct surfactant anion binding isotherms, it is only in the presence of a swamping amount of common cation that this method gives a true indication of the unbound surfactant ion concentration.

Surface activity of sodium salts of alpha‐sulfo fatty esters: The air‐water interface

AbstractAdsorption at the air‐water interface of sodium hexyl α‐sulfopelargonate, sodium heptyl α‐sulfopelargonate, sodium methyl α‐sulfomyristate, and sodium methyl α‐sulfopalmitate has been investigated.The amount of surfactant adsorbed at the interface has been obtained by the application of a simple form of the Gibbs equation to surface tensions of aqueous solutions of varying surfactant ion concentration but at constant counterion (Na+) concentration. Surface tensions were determined by using the drop‐volume method. Counterion concentrations of 0.01 and 0.04N were used. Cross‐sectional areas for the surfactant anions are in the range 44 to 59Å2 (independent of concentration) and are consistent with close packing in which the carbon chains are perpendicular to the interface.For each surfactant, increase in counterion concentration from 0.01 to 0.04N results in a surface‐tension decrease of about 7.5 dyne/cm. This effect can be predicted by using bulk activity coefficients from the Debye‐Hückel theory.Dependence of surface tension on carbon chain length indicates that the carbon chains of the four compounds behave similarly at the air‐water interface.