Study on partition equilibria of metal complexes in non-ionic micellar solutions from spectrophotometric data

The complexation equilibria for Zn(II)-8-quinolinol and Zn(II)-5,7-dichloro-2-methyl-8-quinolinol systems were studied spectrophotometrically in aqueous micellar solutions of the non-ionic surfactant Brij-35 in NaCl 0.1 M medium at 25 degrees C. The partition model, in which the different species involved in the equilibria can distribute themselves between aqueous and micellar pseudophases, was applied. Calculations were performed by means of the spdis program, developed specifically to handle multiwavelength spectrophotometric data in micellar systems. A factor analysis was applied to the spectrophotometric data in order to determine the number of species in equilibrium. A quantitative relationship was found between fluorescence intensity and the micellar solubilization of metal chelates.     

Utility of 1-octanol/octane mixed solvents for the solvent extraction of aluminum(III), gallium(III), and indium(III) with 8-quinolinol.

Using 1-octanol/octane mixed solvents, the extraction of aluminum(III), gallium(III) and indium(III) with 8-quinolinol was carried out at 25 degrees C. The formation constants of the respective metal(III) 8-quinolinolates in the aqueous phase and their partition constants between the mixed solvents and water were determined based on an analysis of the extraction equilibria. The relationship between the partition constants of 8-quinolinol and its complexes was analyzed by the regular solution theory. The molar volumes of aluminum(III), gallium(III) and indium(III) 8-quinolinolates, calculated from the present results, suggest that the electrostriction effect functions in complex forming. It has been found that octane/1-octanol mixed solvents were available not only for the extraction of metal ions, but also for determining the formation constants of these metal 8-quinolinolates in the aqueous phase and their partition constants.     

Solubilization of phenols in anionic polyelectrolyte gels with adsorbed cationic surfactant

Solubilization isotherms for various phenols in cetylpyridinium chloride (CPC)-polyelectrolyte gel aggregates have been determined in order to compare solubilization within these aggregates with that in free micelles and to examine the effects of gel chemistry and structure on solubilization. The isotherms describing solubilization are quite similar to those found for free surfactant in solution. Solutes that are more hydrophobic give rise to larger solubilization constants with trends similar to what is seen for hydrophobic effects in adsorption from aqueous solutions onto hydrophobic solids. The solubilization constants decrease as the fraction of solute in the aggregates increases, indicating that the solutes partition into the palisade region of the aggregates. Solubilization is found to be quite insensitive to changes in gel structure (cross-linker varying from 1% to 3%) and chemistry (poly(acrylic acid) versus poly(methacrylic acid) and neutralization from 50% to 100%). However, the switch from poly(acrylic acid) to poly(methacrylic acid) did give rise to a slight decrease in magnitude of the slope of the isotherm. The most significant factors appear to be the initial concentration of surfactant in solution and the ratio of surfactant solution to gel amount. A decrease in surfactant concentration (especially combined with an increase in solution volume) gives rise to a decrease in solubilization constants.     

Determination of submicrogram amounts of Mercury(II) with 5-(2'-carbomethoxyphenyl)azo-8-quinolinol in presence of anionic surfactant by derivative spectrophotometry

A new sensitive, rapid and selective method is proposed for the determination of mercury in submicrogram level. Mercury(II) forms an insoluble complex with 5-(2'-carbomethoxyphenyl)azo-8-quinolinol in aqueous medium at pH 4-4.8 which can be made soluble by the action of anionic surfactant. The dissolution of the complex results in an orange red colour which is stable for at least 20 hr. The molar absorptivity and Sandell's sensitivity are at 512 nm 8.00 x 10(4) l. mol(-1) . cm(-1) and 0.0025 mug/cm(2), respectively. Derivative spectrophotometry is used to achieve selectivity.     

Simultaneous cloud-point extraction of nine cations from water samples and their determination by flame atomic absorption spectrometry.

The cloud-point methodology was successfully employed for the preconcentration of heavy metal cations at trace levels from aqueous samples prior to flame atomic absorption spectrometry (FAAS). Cations were taken into a complex with 8-quinolinol in an aqueous non-ionic surfactant, Triton X-114, medium and concentrated in the surfactant rich phase by bringing the solution to the cloud-point temperature. The preconcentration of only 100 mL of the solution with 1% Triton X-114 and 10(-3) M 8-quinolinol at pH 7.0 gave a preconcentration factor higher than 100 for most cations. Under these conditions, the detection limits of the cloud-point extraction-FAAS system were 0.8 - 15 microg/L.     

Distribution of certain 8-quinolinols and their copper(II) chelates in a series of organic solvent--aqueous pairs

The partition coefficients of 8-quinolinol and the 2- and 4-methyl analogues and their copper(II) chelates between water and a number of common water-immiscible solvents are reported. An attempt is made to relate the values with the dielectric constant, solubility parameter and mutual solubility of the organic and aqueous phases.     

Micropartioning and solubilization enhancement of 1,2-bis(bis(4-chlorophenyl) methyl)diselane in mixed micelles of binary and ternary cationic-nonionic surfactant mixtures

The aqueous solubilization of the organoselenium compound viz., 1,2-bis(bis(4-chlorophenyl)methyl)diselane [(ClC(6)H(4))(2)CHSe](2) has been investigated experimentally in micellar solutions of two cationic (hexadecyltrimethylammonium bromide, CTAB, hexadecyltrimethylammonium chloride, CTAC) and one nonionic (polyoxyethylene(20)mono-n-hexadecyl ether, Brij 58) surfactants possessing the same hydrocarbon "tail" length and in their single as well as equimolar binary and ternary mixed states. Solubilization capacity determined with spectrophotometry and tensiometry has been quantified in terms of molar solubilization ratio and micelle-water partition coefficient. FTIR, UV-vis, fluorescence and zeta potential measurements have been utilized to ascertain the interaction of organochalcogen compound with surfactants. Equimolar cationic-nonionic surfactant combinations show better solubilization capacity than pure cationics or nonionics, whereas equimolar cationic-cationic-nonionic ternary surfactant systems exhibit intermediate solubilization efficiency between their single and binary counterparts. Locus of solubilization of [(ClC(6)H(4))(2)CHSe](2) in different micellar solutions was probed by UV-visible spectroscopy. The investigation has presented precious information for the preference of mixed surfactants for solubilizing water-insoluble compounds. Indeed the solubilization aptitude of these surfactants is not merely related to molar capacity. The results furnish adequate support to justify comprehensive exploration of the surfactant properties that influence solubilization.     

Alterations in phospholipid bilayers caused by sodium dodecyl sulphate/triton X-100 mixed systems

The mechanisms governing the subsolubilizing and solubilizing interaction of sodium dodecyl sulphate (SDS)/Triton X-100 mixtures and phosphatidylcholine liposomes were investigated. Permeability alterations were detected as a change in 5(6)-carboxy-fluorescein (CF) released from the interior of vesicles and bilayer solubilization as a decrease in the static light-scattered by liposome suspensions. Three parameters were described as the effective surfactant/lipid molar ratios (Re) at which the surfactant system a) resulted in 50% of CF release (Re _50%CF); b) saturated the liposomes (Re _SAT;c) led to a complete solubilization of these structures (Re _SOL). From these parameters the corresponding surfactant partition coefficientsK _50%CF,K _SAT andK _SOL were determined. The free surfactant concentrationsS _W were lower than the mixed surfactant CMCs at subsolubilizing level, whereas they remained similar to these values during saturation and solubilization of bilayers in all cases. Although theRe increased as the mole fraction of the SDS rose (X _SDS), theK parameters showed a maximum atX _SDS values of about 0.6, 0.4 and 0.2 forK _50%CF,K _SAT andK _SOL respectively. Thus, the higher the surfactant contribution in surfactant/lipid system, the lower theX _SDS at which a maximum bilayer/water partitioning of mixed surfactant systems added took place and, consequently, the lower the influence of the SDS in this maximum bilayer/water partitioning.Abbreviations PC Phosphatidylcholine - PIPES piperazine-1,4 bis (2-ethanesulphonic acid) - SDS sodium dodecyl sulphate - X _SDS mole fraction of sodium dodecyl sulphate in the mixed system - CF 5(6)-carboxyfluorescein - Re effective surfactant/lipid molar ratio - Re _50%CF effective surfactant/lipid molar ratio for 50% CF release - Re _SAT effective surfactant/lipid molar ratio for bilayer saturation - Re _SOL effective surfactant/lipid molar ratio for bilayer solubilization - S _W surfactant concentration in the aqueous medium - S _{W, 50%CF} surfactant concentration in the aqueous medium for 50% CF release - S _{W, SAT} surfactant concentration in the aqueous medium for bilayer saturation - S _{W, SOL} surfactant concentration in the aqueous medium for bilayer solubilization - S _B surfactant concentration in the bilayers - K bilayer/aqueous phase surfactant partition coefficient - K _50%CF bilayer/aqueous phase surfactant partition coefficient for 50% CF release - K _SAT bilayer/aqueous phase surfactant partition coefficient for bilayer saturation - K _SOL bilayer/aqueous phase surfactant partition coefficient for bilayer solubilization - PL phospholipid TLC-FID, thin-layer chromatography/flame ionization detection system - PI polydispersity index - CMC critical micellar concentration - r ² regression coefficient     

Assembly properties of Triton X-100/phosphatidylcholine aggregates during liposome solubilization

The assembly properties of the nonionic surfactant Triton X-100 and phosphatidylcholine (PC) aggregates during the overall solubilization process of PC liposome were investigated. Permeability alterations were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles and bilayer solubilization as a decrease in the static light scattered by liposome suspensions. A direct dependence was established between the bilayer/aqueous phase surfactant partition coefficients (K), the growth of vesicles and the leakage of entrapped CF in the initial interaction steps (surfactant to phospholipid molar ratioRe up to 0.2). These changes may be related to the increasing presence of surfactant molecules in the outer monolayer of vesicles. In theRe range 0.2–0.35 the coexistence of a low vesicle growth with a constant increase of CF release may be correlated with the decrease inK (increased rate of flip-flop of surfactant molecules). Furthermore, in theRe range between 0.64 and 2.0 (lytic levels) almost a linear dependence was detected between the composition of these aggregates (Re) and the decrease in both the surfactant-PC aggregate size and the static light scattered by the system. This dependence was not observed in the last solubilization steps (Re range 2.0–2.60) possibly due to the increased formation of mixed micelles in this interval. The fact that the free Triton X-100 concentration at sublytic and lytic levels showed respectively lower and similar values than its critical micelle concentration confirms that permeability alterations and solubilization were determined respectively by the action of surfactant monomer and by the formation of mixed micelles.Abbreviations PC phosphatidylcholine - PIPES piperazine-1,4 bis(2-ethanesulphonic acid) - T_X-100 Triton X-100 - CF 5(6)-carboxyflucrescein - Re enective surfactant/lipid molar ratio - Re _SAT effective surfactant/lipid molar ratio for bilayer saturation - Re _SOL enective surfactant/lipid molar ratio for bilayer solubilization - S _W surfanctant concentration in the aqueous medium - S _B surfactant concentration in the bilayers - S _T total surfactant concentration - K bilayer/aqueous phase surfactant partition coefficient - K _SAT bilayer/aqneous phase surfactant partition coefficient for bilayer saturation - K _SOL bilayer/aqueous phase surfactant partition coefficient for bilayer solubilization - PL phospholipid - TLC-FID thinlayer chromatography/flame ionization detection system - PI polydispersity index - CMC critical micellar concentration - r ² regression coefficient     

Apparent Molar Volume and Apparent Molar Adiabatic Compressibility Studies of 2,6-Di-tert-butyl-4-methylphenol and 2-tert-Butyl-4-methoxyphenol in Aqueous Micelle Solutions of Hexadecyltrimethylammonium Chloride as a Function of Surfactant Concentration and Temperature

Ultrasonic velocities and densities of aqueous solutions of cetyltrimethylammonium chloride have been measured at concentrations below 0.35 mol kg^-1 at 25, 35, and 45°C. Apparent molar volume and apparent molar adiabatic compressibility properties of the aqueous surfactant solutions were derived from these data. Apparent molar volumes and apparent molar adiabatic compressibilities of 2,6-di-tert-butyl-4-methylphenol and 2-tert-butyl-4-methoxyphenol dissolved in aqueous micellar solutions of cetyltrimethylammonium chloride were determined as a function of surfactant concentration and temperature. The results obtained for the binary and ternary systems are compared with those previously published for binary aqueous cetyltrimethylammonium bromide systems and for ternary systems of this surfactant containing the same additives. The degree of counterion dissociation from the micelles and the effect this has on the extent of hydration of the head group region of the micelle are shown to have an effect on the solubilization sites of hydrophobic-like additives in these micelles.     

Use of the semi-equilibrium dialysis method in studying the thermodynamics of solubilization of organic compounds in surfactant micelles. Systemn-hexadecylpyridinium chloride-phenol-water

The semi-equilibrium dialysis method has been used to infer solubilization equilibrium constants or, alternatively, activity coefficients of solutes solubilized into micelles of aqueous surfactant solutions. Methods are described for inferring the concentrationa of monomers of the organic solute and of the surfactant on both sides of the dialysis membrane, under conditions where the organic solute is in equilibrium with both the high-concentration (retentate) and low-concentration (permeate) solutions. By using a form of the Gibbs-Duhem equation, activity coefficients of both phenol (the solubilizate) and n-hexadecylpyridinium chloride (the surfactant) are obtained for aqueous solutions at 25°C throughout a wide range of relative compositions of surfactant and solubilizate within the micelle. The apparent solubilization constant, K=[solubilized phenol]/([monomeric phenol][micellar surfactant]), is found to decrease significantly as the mole fraction of phenol in the micelle increases.     

Effects on the micellar solubilization of organic compounds by surfactant micelles. I. Length of carboxylic side chains in aromatic acids

The semiequilibrium dialysis method has been used to determine solubilization equilibrium constants and activity coefficients of benzoic, phenylacetic, and hydrocinnamic acids (solubilizate) in micelles of the cationic surfactant hexadecylpyridinum chloride (cetylpyridinium chloride) in 0.1M HCl aqueous solutions. Methods described previously were employed to infer the concentrations of monomeric organic solute and surfactant on both sides of the dialysis cell. Values of the apparent solubilization constant K of the neutral acids have been correlated with mole fractions of the acid in the micelle X_A, where K=X_A/[monomeric acid]. The activity coefficients of both acid and surfactant were obtained, consistent with the Gibbs-Duhem equation. The solubilization constants of all three acids are nearly the same, indicating that there is no significant effect owing to the presence of one or more methylene groups between the carboxylate and the phenyl groups of benzoic acid. The solubilization constants also decrease appreciably, and the activity coefficients of the acids increase, as the mole fraction of the acid in the micelle increases.     

Solubilization of Cationic Hemicyanine Dyesin Anionic Surfactant Micelles: A Partitioning Study

Summary.  Cationic hemicyanine dyes of the 1-methyl-4-(4-(di-n-alkylamino)-styryl) pyridinium betain type (alkyl group=methyl to butyl) have been investigated by differential spectroscopy in order to ascertain their solubilization in aqueous micellar solution of sodium dodecylbenzene sulfonate (SDBS). The differential absorption spectra were recorded as a function of surfactant concentration at 25.0°C. Thermodynamic parameters were calculated from partition coefficient data. The results show that cationic hemicyanine dyes are solubilized at the micellar surface, indicating an electrostatic interaction between dyes and surfactant micelles. Received October 15, 1999. Accepted February 14, 2000     

Sorbent extraction in flow injection analysis and its application to enhancement of atomic spectrometry

The extraction of metals as their chelates from aqueous samples can be simplified, miniaturized and automated by flow injection/sorbent extraction techniques. The chelate is formed in the flow stream, sorbed on C-18 bonded silica and then eluted for transfer to the atomic absorption spectrometer. The proposed method is shown to be useful for the preconcentration of copper and lead by means of their chelation with diethyldithiocarbamate or 8-quinolinol; complexation with 1-(2- pyridylazo)-2-naphthol or 4-(2-pyridylazo)resorcinol is also possible. This approach enables organic reagents to be used directly, without immobilization, in “open” flow-injection systems for preconcentration or separation of analytes.     

Pyrene solubilization capacity in octaethylene glycol monododecyl ether (C12E8) micelles

We used fluorescence quenching, vibronic band ratios and excimer fluorescence techniques to quantify the statistics of pyrene solubilization in nonionic octaethylene glycol monododecyl ether (C₁₂E₈) micelles. Using a two-phase model (aqueous and micellar pseudophases) to interpret fluorescence results, we found that all three of these experimental methods provide consistent information about pyrene partitioning between aqueous and micellar pseudophases. From dynamic quenching experiments we determined the pyrene partition coefficient and the average number of pyrene molecules solubilized per micelle over a range of surfactant concentrations. The pyrene partition coefficient increases with increasing surfactant concentration. We confirmed the partitioning results by excimer fluorescence measurements. Quenching results indicate that pyrene is accessible to Cu²⁺ quenchers even in the limit of high surfactant concentration where solubilized pyrene is in the infinite dilution limit in the micellar pseudophase. This suggests that solubilized pyrene resides in the micellar palisade layer. We determined the maximum number of pyrene solubilizates allowed per micelle (micellar solubilization capacity) by applying a three-phase model to fluorescence experiments conducted in the presence of solid phase pyrene. The estimated maximum capacity is 6 pyrene molecules per micelle. The three phase partitioning model successfully predicted the excimer fluorescence in the presence of solid pyrene.     

Spectrophotometric study of anionic azo-dye light yellow (X6G) interaction with surfactants and its micellar solubilization in cationic surfactant micelles

Solubilization and interaction of azo-dye light yellow (X6G) at/with cationic surfactants cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC) was investigated spectrophotometricaly. The effect of cationic micelles on solubilization of anionic azo dye in aqueous micellar solutions of cationic surfactants was studied at pH 7 and 25 degrees C. The binding of dye to micelles implied a bathochromic shift in dye absorption spectra that indicates dye-surfactant interaction. The results showed that the solubility of dye increased with increasing surfactant concentration, as a consequence of the association between the dye and the micelles. The binding constants, K(b), were obtained from experimental absorption spectra. By using pseudo-phase model, the partition coefficients between the bulk water and surfactant micelles, K(x), were calculated. Gibbs energies of binding and distribution of dye between the bulk water and surfactant micelles were estimated. The results show favorable solubilization of dye in CTAB micelles.     

Kinetics of complexation of nickel (II) and copper (II) with some azophenol derivatives in aqueous nonionic surfactant solution

The kinetics of formation of 11 complexes of nickel(II) and copper(II) ions with some azophenol derivatives in aqueous and micellar solution of a nonionic surfactant, Triton X-100, have been studied by a stopped-flow spectrophotometric method. Second order rate constants for the reactions were determined at 298 K and ionic strength 0.1 (NaClO₄) in aqueous solution. In the surfactant solution, the pseudo-first-order rate constants for the complexation reactions,k_obs, decreased with increasing the concentration of Triton X-100. This observation was explained by the assumption that the chelating reagents distribute between the micelle of the surfactant and bulk aqueous phase and rate-controlling reactions occur only in the bulk aqueous phase. On the basis of the relation betweenk_obs and the concentration of the surfactant, the partition constants of the reagents between micellar and aqueous phases were determined.     

Enthalpy and free energy of solubilization for 1-pentanol in anionic + cationic and anionic + nonionic surfactant mixtures

The standard enthalpy of solubilization as well as the partition coefficient of 1-Pentanol between micelles and water have been determined in the whole composition range of two binary surfactant systems from microcalorimetry experiments. The free energy results compare favorably with partition coefficients deduced from a head-space gas-chromatographic technique. It is shown that the surfactant mixing of sodium decylsulfate+decyltrimethylammonium bromide (system I) or sodium dodecylsulfate+dodecylpolyoxyethylene(23) (system II) is unfavorable to micellar solubilization. The solubilization of 1-pentanol is a highly endothermic process for system (I) and only slightly endothermic for system (II). The regular solution approximation predicts qualitatively the standard free energy as well as the standard enthalpy data. These results illustrate a discussion on the limits as well as the usefulness of the regular solution approach as applied to these three-component systems using available solubilization results from the literature for hydrophobic solutes in binary micellar surfactant solutions.     

On the interpretation of solubilization results obtained from semi-equilibrium dialysis experiments

The interpretation of intramicellar solubilization data obtained from semi-equilibrium dialysis (SED) experiments is described, and methods are presented for determining equilibrium constants for the solubilization of organic species by aqueous surfactant solutions as well as activity coefficients of both the organic solute and the surfactant within the micelle. The solubilization equilibrium constant of an organic solute in an aqueous micellar solution (K) is defined as the ratio of the mole fraction of organic solute in the micellar pseudophase (X) to the concentration of the unsolubilized monomeric organic solute in the aqueous phase (c ₀). Expressions compatible with the Gibbs-Duhem equation are used to represent the concentration dependence of activity coefficients of both the solubilizate and surfactant in the micellar pseudophase; the analysis leads to calculated values of the concentrations of free and intramicellar surfactant and organic solute in both compartments of the equilibrium dialysis cell. Solubilization equilibrium constants for many amphiphiles are well correlated by the simple expressionK=K ₀(1-BX)², whereB is an empirical constant andK ₀ is the limiting value ofK asX approaches 0.     

Experimental and theoretical investigation of the micellar-assisted solubilization of ibuprofen in aqueous media

Surfactants can be used to increase the solubility of poorly soluble drugs in water and to increase drug bioavailability. In this article, the aqueous solubilization of the nonsteroidal, antiinflammatory drug ibuprofen is studied experimentally and theoretically in micellar solutions of anionic (sodium dodecyl sulfate, SDS), cationic (dodecyltrimethylammonium bromide, DTAB), and nonionic (dodecyl octa(ethylene oxide), C12E8) surfactants possessing the same hydrocarbon "tail" length but differing in their hydrophilic headgroups. We find that, for these three surfactants, the aqueous solubility of ibuprofen increases linearly with increasing surfactant concentration. In particular, we observed a 16-fold increase in the solubility of ibuprofen relative to that in the aqueous buffer upon the addition of 80 mM DTAB and 80 mM C12E8 but only a 5.5-fold solubility increase upon the addition of 80 mM SDS. The highest value of the molar solubilization capacity (chi) was obtained for DTAB (chi = 0.97), followed by C12E8 (chi = 0.72) and finally by SDS (chi = 0.23). A recently developed computer simulation/molecular-thermodynamic modeling approach was extended to predict theoretically the solubilization behavior of the three ibuprofen/surfactant mixtures considered. In this modeling approach, molecular-dynamics (MD) simulations were used to identify which portions of ibuprofen are exposed to water (hydrated) in a micellar environment by simulating a single ibuprofen molecule at an oil/water interface (modeling the micelle core/water interface). On the basis of this input, molecular-thermodynamic modeling was then implemented to predict (i) the micellar composition as a function of surfactant concentration, (ii) the aqueous solubility of ibuprofen as a function of surfactant concentration, and (iii) the molar solubilization capacity (chi). Our theoretical results on the solubility of ibuprofen in aqueous SDS and C12E8 surfactant solutions are in good agreement with the experimental data. The ibuprofen solubility in aqueous DTAB solutions was somewhat overpredicted because of challenges associated with accurately modeling the strong electrostatic interactions between the anionic ibuprofen and the cationic DTAB. Our results indicate that computer simulations of ibuprofen at a flat oil/water interface can be used to obtain accurate information about the hydrated and the unhydrated portions of ibuprofen in a micellar environment. This information can then be used as input to a molecular-thermodynamic model of self-assembly to successfully predict the aqueous solubilization behavior of ibuprofen in the three surfactant systems studied.