Competitive binding of counterions at the surface of mixed ionic/nonionic micelles: Application of the ion exchange formalism (original) (raw)
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Counterion Binding to Ionic Micelles: Effects of Counterion Specificity
Journal of Dispersion Science and Technology, 2001
The effects of counterion speci®city on the properties of sodium dodecyl sulfate micelles containing photochemically reactive solubilizates were studied by laser ash photolysis and light scattering, potentiometric, spectroscopic and microelectrophoretic measurements. The counterions investigated were an amphiphilic ion (cetyltrimethylammonium, CTA ) and two divalent ions (cupric, Cu 2 , and methylviologen, MV 2 ). Cu 2 and MV 2 showed lower effect than CTA in promoting changes of micelle size and electrostatic potential at the micelle/solution interface. This can be attributed to the complex interplay between electrostatic and hydrophobic interactions, which determine the average location of counterions, i.e., prevailing interfacial or intercalated binding to the micelle. Laser¯ash photolysis showed that Cu 2 enhanced the rate of decay of biphenyl triplet, while MV 2 did not show any effect. The differences between the quenching cations were attributed to the average location of MV 2 ions in the Stern layer further away from the micelle core than Cu 2 .
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2018
We report on combined experimental and theoretical investigations of the water/micelle interface of cationic, anionic, zwitterionic, and non-ionic surfactants using a new hydrophobic acid-base indicator 2,6dinitro-4-n-dodecylphenol. The indices of the so-called apparent ionization constant, app a pK , of the indicator fixed in the micellar pseudophase are determined by the spectrophotometric method. The data allows estimating the Stern layer's electrostatic potential of the ionic micelles . Molecular Dynamics modeling was used to locate the dye molecule and, in particular, its ionizing group OH Owithin the micelles of the studied surfactants. The comparison of the values estimated using 2,6-dinitro-4-ndodecylphenol with both our computer simulation and literature experimental results reveals obstacles in monitoring electrical interfacial potentials. In particular, the values of the surfactant micelles with alkylammonium groups determined via 2,6-dinitro-4-n-dodecylphenol are overestimated. The reason is specific interactions of the indicator anion with the surfactant head groups. For anionic surfactants, however, this indicator is quite suitable, which is confirmed by the location of HA and Aequilibrium forms in the pseudophase.
Journal of Colloid and Interface Science, 2012
as counterions, were determined by chemical trapping in micelles formed by two zwitterionic surfactants, namely N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (HPS) and hexadecylphosphorylcholine (HDPC) micelles. Appropriate standard curves for the chemical trapping method were obtained by measuring the product yields of chloride and bromide salts with 2,4,6trimethyl-benzenediazonium (BF 4) in the presence of low molecular analogs (N,N,N-trimethyl-propane sulfonate and methyl-phosphorylcholine) of the employed surfactants. The experimentally determined values for the local Br À (Cl À) concentrations were modeled by fully integrated non-linear Poisson Boltzmann equations. The best fits to all experimental data were obtained by considering that ions at the interface are not fixed at an adsorption site but are free to move in the interfacial plane. In addition, the calculation of ion distribution allowed the estimation of the degree of ion coverage by using standard chemical potential differences accounting for ion specificity.
Journal of Chemical & Engineering Data, 2010
The mixed micellar properties of two cationic gemini surfactants, alkanediyl-R,ω-bis(tetradecyldimethylammonium bromide) and alkanediyl-R,ω-bis(hexadecyldimethylammonium bromide), 14-s-14 and 16-s-16, in aqueous solution have been investigated by conductivity and fluorescence techniques. The conductivity method has been carried out to evaluate critical micelle concentration (c), degree of counterion binding (g), and other related parameters like ideal mixed critical micelle concentration (c*), micellar mole fraction (x), interaction parameter () from Rubingh's model, and x ideal from Motomura's model. Activity coefficients (f 1 and f 2) and Gibbs excess energy (G E) were also calculated. Fluorescence measurements were used to obtain the values of N agg and Stern-Volmer constant (K sv). The results suggest synergism in the system. The values are negative, and their magnitudes increase with increasing spacer chain lengths. x 1 > x 1 ideal values suggest that the contribution of the 14-s-14 component is greater as compared to that in the ideal state.
1996
A method for determining counterion exchange selectivities at the surface of ionic micelles from time-resolved fluorescence quenching measurements is developed. This method is employed to determine selectivity coefficients for thiosulfatelchloride exchange at the surface of cationic hexadecyltrimethylammonium chloride micelles (KTSlC, = 1.1 f 0.4) and for copper (II)/sodium exchange at the surface of anionic sodium dodecyl sulfate micelles C KC^, = 1.3 ?0.3). In both cases, the selectivity coefficients are found to be independent of detergent and added salt concentration.
Micellization of binary systems of a cationic gemini surfactant butanediyl-1,4-bis(dimethylcetylammonium bromide) (16-4-16) and cationic/nonionic hydrotropes (aniline-hydrochloride, 2-methylanilinehydrochloride, 4-methylanilinehydrochloride, hydroxybenzene, 1,3-benzenediol, benzene-1,2,3-triol) have been studied using a conductometric technique. The critical micelle concentrations (cmc) for different mixing mole fractions at different temperatures have been calculated. To explain and compare the results, theoretical models of Clint, Rubingh and Motomura have been used to obtain the ideal cmc, mixed micelle composition, interaction parameters (b m ), free energies of micellization, and activity coefficients. The mixtures show nonideal behavior and the interactions between the surfactants and the hydrotropes are synergistic in nature which is confirmed by high negative b m values and low values of the activity coefficients. Thermodynamic parameters were also obtained from the temperature dependence of the cmc values.
Studies of mixed micelle formation between cationic gemini and cationic conventional surfactants
Journal of Colloid and Interface Science, 2008
Mixed micellization of dimeric cationic surfactants tetramethylene-1,4-bis(hexadecyldimethylammonium bromide)(16-4-16), hexamethylene-1,6-bis(hexadecyldimethylammonium bromide) (16-6-16) with monomeric cationic surfactants hexadecyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPB), cetylpyridinium chloride (CPC), and tetradecyltrimethylammonium bromide (TTAB) have been studied by conductivity and steady-state fluorescence quenching techniques. The behavior of mixed systems, their compositions, and activities of the components have been analyzed in the light of Rubingh's regular solution theory. The results indicate synergism in the binary mixtures. Ideal and experimental critical micelle concentrations (i.e., cmc * and cmc) show nonideality, which is confirmed by β values and activity coefficients. The micelle aggregation numbers (N agg), evaluated using steady-state fluorescence quenching at a total concentration of 2 mM for CTAB/16-4-16 or 16-6-16 and 5 mM for TTAB/16-4-16 or 16-6-16 systems, indicate that the contribution of conventional surfactants was always more than that of the geminis. The micropolarity, dielectric constant and binding constants (K sv) of mixed systems have also been evaluated from the ratios of respective peak intensities (I 1 /I 3 or I 0 /I 1).
Langmuir, 2013
Specific ion effects in surfactant solutions affect the 14 properties of micelles. Dodecyltrimethylammonium chloride 15 (DTAC), bromide (DTAB), and methanesulfonate (DTAMS) 16 micelles are typically spherical, but some organic anions can induce 17 shape or phase transitions in DTA + micelles. Above a defined 18 concentration, sodium triflate (NaTf) induces a phase separation in 19 dodecyltrimethylammonium triflate (DTATf) micelles, a phenom-20 enon rarely observed in cationic micelles. This unexpected behavior 21 of the DTATf/NaTf system suggests that DTATf aggregates have 22 A dx.doi.org/10.1021/la304658e | Langmuir XXXX, XXX, XXX−XXX
Ion Specificity of Micelles Explained by Ionic Dispersion Forces
Langmuir, 2002
We consider the origin of the ion specificity found for the physical properties of micelles. Ions in solution have a polarizability different from that of the surrounding water. The excess polarizability is different for different ions and gives rise to ion specific dispersion forces toward, or away from, interfaces. We show that ionic dispersion forces have important influences on self-consistent potentials, ion distributions, and surface adsorption excess per headgroup on micelles.
Counterion condensation on mixed anionic/nonionic surfactant micelles: Bjerrum's limiting condition
Journal of Colloid and Interface Science, 1989
The degree of ion condensation of mixed micelles of sodium dodecylsulfate (SDS) with two nonionic dodecylpolyoxyethylene ethers with short (C12 E4) and long (Cl2 E23) polar chains was determined from potentiometric measurements using a sodium ion-selective electrode. It is shown, using the Bjerrum concept of ion-ion association, that a limiting micellar composition value X M can be defined above which ion condensation on a micellar surface cannot occur. For a given ionic surfactant component, this limiting value is only dependent on the radii of the mixed micelle. For the systems under investigation, X M is equal respectively to mole fractions of nonionics of 0.92 and 0.85, for the mixed SDS/C12 E4 and SDS/C12E23 systems, values which are compatible with the experimental results. It is concluded that the long oxyethylene chains do not introduce any steric effect upon the counterion condensation phenomenon which may be considered, in the case of mixed anionic/nonionic micellar systems, as the consequence of purely electrostatic forces.