BINDING OF IONIC SURFACTANTS ON OPPOSITELY CHARGED POLYELECTROLYTES OBSERVED BY FLUORESCENCE METHODS

Our recent studies concerning the binding of ionic surfactants on oppositely charged polyelectrolytes observedwith fluorescence techniques are reviewed. The cationic surfactants cetyltrimethylammonium bromide (CTAB),dodecyltrimethylammonium chloride (DTAC), and nonionic surfactant octaethylene glycol monododecyl ether (C_(12)E_8) wereallowed to bind on anionic poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) and its pyrene and/or naphthalenelabeled copolymers. The relative excimer emission intensity I_E/I_M of a cationic probe l-pyrenemethylamine hydrochloride(PyMeA·HCl) and the non-radiative energy transfer (NRET) I_(Py)/I_(Np) of naphthalene to pyrene for labeled polyelectrolyteswere chosen to monitor the binding process and the conformation change of surfactant-bound polyelectrolytes. The 1:1aggregation of polyelectrolyte-CTAB with respect to the charge was found as long as the CTAB concentration was slightlyhigher than its critical aggregation concentration (CAC). The intermolecular NRET indicated that the CTAB-boundpolyelectrolytes aggregated together through the hydrophobic interaction between the CTAB tails. However, neither 1:1polyelectrolyte-DTAC aggregation nor intermolecular aggregation of DTAC-bound polyelectrolyte was observed owing to itsweaker hydrophobicity of 12 carbon atoms in the tail, which is shorter than that of CTAB. As known from the fluorescenceresults, nonionic surfactant C_(12)E_8 did not bind on the anionic polyelectrolytes, but the presence of PAMPS promoted themicelle formation for C_(12)E_8 at the CAC slightly below its critical micelle concentration (CMC). The solid complex of dansyllabeled AMPS copolymer-surfactant exhibited a decrease in local polarity with increasing charge density of thepolyelectrolyte or with alkane tail length of the surfactant. SAXS suggested a lamella structure for the AMPS copolymer-surfactant solid complexes with a long period of 3.87 nm for CTAB and 3.04 nm for DTAC, respectively.     

Micellization and Interaction Properties of Aqueous Solutions of Mixed Cationic and Nonionic Surfactants

The micellization process of binary surfactant mixtures containing cationic surfactants viz. dodecyl pyridinium halide (C₁₂PyX; X=Cl, Br, I), tetradecyl pyridium bromide (C₁₄PyBr), and hexadecyl pyridium halide (C₁₆PyX; X=Cl, Br) and a nonionic surfactants viz. dodecyl nonapolyethylene glycol ether (C₁₂E₉), dodecyl decapolyethylene glycol ether (C₁₂E₁₀), dodecyl dodecapolyethylene glycol ether (C₁₂E₁₂), and dodecyl pentadecapolyethylene glycol ether (C₁₂E₁₅) in water at different mole fractions (0–1) were studied by surface tension method. The composition of mixed micelles and the interaction parameter, β evaluated from the CMC data obtained by surface tension for different systems using Rubingh's theory were discussed. Activity coefficient (f₁ and f₂) of cationic surfactant (C_nPyBr)/C₁₂E_m (n=12, 14, 16 and m=10, 12, 15) mixed surfactant systems were evaluated, which shows extent of ideality of individual surfactant in mixed system. The stability factors for mixed micelles were also discussed by Maeda's approach, which was justified on the basis of steric factor due to difference in head group of nonionic surfactant.     

Interactions of Ionic Surfactants With PEO-PBO-PEO Triblock Copolymers in Aqueous Solutions

The interactions of triblock copolymers (TBP) with ionic surfactants were studied employing surface tensiometry, electrical conductivity, steady-state fluorescence (SSF), and dynamic light scattering (DLS) techniques. An increasing trend in the critical micelle concentration (CMC) of SDS/CTAB in the presence of triblock copolymers was observed especially at higher polymer to surfactant ratio. The delay in the CMC of surfactants was more pronounced in the presence of E₄₈B₁₀E₄₈ possibly due to its less hydrophobic nature. The negative values of free energy of micellization (ΔG_m) both in case of SDS and CTAB confirmed the spontaneity of the processes. The aggregation number (N_agg) and hydrodynamic radius (R_h) of polymer/surfactant mixed systems were determined by SSF and DLS. The suppression of the surfactant micelle size in the presence of TBP was confirmed by SSF and DLS studies.     

Surface and Solution Properties of Cationic Gemini Surfactants with Primary Linear Alkanols

Using surface tension and fluorescence methods, the surface and solution properties of two cationic gemini surfactants {pentanediyl-1,5-bis(dimethylcetylammonium bromide) and hexanediyl-1,6-bis(dimethylcetylammonium bromide)} (referred to as 16-5-16 and 16-6-16) have been studied in the presence and absence of primary linear alkanols. Parameters studied include the critical micelle concentration (CMC), C ₂₀ (the surfactant concentration required to reduce the surface tension of the solvent by 20 mN·m^?1), Г _max (maximum surface excess), and A _min (minimum surface area per molecule). These parameters indicate mixed micelle formation and, therefore, surfactant-additive interaction parameters in mixed micelles and mixed monolayers, as well as activity coefficients, were calculated. A synergistic effect was observed in all instances and was found to be correlated with the chain length of the alkanols. The CMC values of 16-s-16 (s = 5, 6) decrease with increasing alkanol concentration and the extent of this effect follows the sequence: 1-octanol (C₈OH) > 1-heptanol (C₇OH) > hexan-1-ol (C₆OH) > 1-pentanol (C₅OH) > butanol (C₄OH). The micelle aggregation number (N _agg) of mixed micelles has been obtained using the steady state fluorescence quenching method. The micropolarity of gemini/alkanol systems has been evaluated from the ratio of intensity of peaks (I ₁/I ₃) of the pyrene fluorescence emission spectra. Results are interpreted on the basis of the structure of mixed micelles and monolayers.     

Micellar parameters of diblock copolymers and their interactions with ionic surfactants

The interactions of non-ionic amphiphilic diblock copolymer poly（oxyethylene/oxybutylene）(E₃₉B₁₈) with anionic surfactant sodium dodecyl sulphate（SDS） and cationic surfactant hexadecyltrimethylammonium bromide（CTAB） were studied by using various techniques such as surface tension,conductivity,steady-state fluorescence and dynamic light scattering.Surface tension measurements were used to determine the critical micelle concentration（CMC） and thereby the free energy of micellization(△G_mic),free energy of adsorption(△G_ads),surface excess concentration（Γ） and minimum area per molecule（A）.Conductivity measurements were used to determine the critical micelle concentration（CMC）,critical aggregation concentration（CAC）,polymer saturation point（PSP）,degree of ionization（α） and counter ion binding（β）. Dynamic light scattering experiments were performed to check the changes in physiochemical properties of the block copolymer micelles taken place due to the interactions of diblock copolymers with ionic surfactants.The ratio of the first and third vibronic peaks（I₁/I₃） indicated the polarity of the pyrene micro environment and was used for the detection of micelle as well as polymer-surfactant interactions.Aggregation number（N）,number of binding sites（n） and free energy of binding （△G_b） for pure surfactants as well as for polymer-surfactant mixed micellar systems were determined by the fluorescence quenching method.     

The time dependence of chitosan/nonionic surfactant solution viscosity

The measurement of the viscosity of semiconcentrated chitosan (0.08–0.14%) solutions in the system with octaethyleneglycolmonon-dodecylether (C₁₂E₈) was carried out using Cannon-Fenske capillary viscometer. The interaction was—as expected—very weak, vut when the time dependent hydrodynamic behaviour of the system was considered, the interaction has been established at particular surfactant concentrations. The most significant time dependence is shown in a form of sudden viscosity drop in a region close to and above CMC value of the surfactant, which implied existence of the interaction between chitosan and surfactant. At low surfactant concentrations viscosity values vere constant with increasing surfactant concentration, but solution also showed time dependent decrease in the viscosity which has been connected with well known time dependent viscosity of pure chitosan solution.The viscometry enabled monitoring of the extent of chitosan/surfactant association by establishing the viscosity decrease rate constant. The rate constant was derived from the first order constant of the quadratic polynomial curves used for the approximation of experimental values when these are presented in the form of viscosity-time profiles. This method showed the existence of critical surfactant concentration values (C ₁,C ₂ andC ₃). These values are closely connected with the proposed interaction model which is based on the assumption that spherical surfactant micelles are bound by chitosan molecule.On leave from Textile Engineering Dept., Faculty of Technology and Metallurgy, University of Belgrade, Yugoslavia     

Dynamic light scattering study of cetyltrimethylammonium bromide aqueous solutions

Cetyltrimethylammonium bromide (CTAB) aqueous solutions are studied by dynamic light scattering method in a wide concentration range covering the first and second critical micelle concentrations (CMC₁ and CMC₂, respectively). Nonmonotonic and ambiguous behavior of diffusion coefficients D with an increase in concentrations above CMC1 is revealed. An increase in the D values in the first decade of CTAB concentration above CMC1 agrees with known published data for aqueous solutions of ionic surfactants. It is shown that an increase in the ionic strength of solution with the addition of KBr leads to a decrease in the positive slope of the dependence of diffusion coefficients on CTAB concentration up to zero at 0.05 M KBr. Two relaxation processes corresponding to large and small D values are simultaneously observed in micellar solutions, beginning with 0.03 M CTAB concentration. The data obtained are compared with published data, as well as with the results of viscosity measurements. The performed analysis indicates that the observed relaxation processes are explained by the coexistence of spherical and nonspherical micelles. It is established that micelles acquire a cylindrical shape at CTAB concentrations ranging from 0.2 to 0.25 M. Hydrodynamic radii and lengths of micelles are calculated.     

Extraction kinetics and ultrafiltration removal of Nickel (II) by long chain alkoxypicolinic acids in cationic and mixed micelles

Micellar particles can solubilize lipophilic extractants similarly to the organic phase in classical biphasic extraction. This analogy is used here to investigate the kinetics of complex formation between Ni²⁺ ions and long chain 5-alkoxypicolinic acids (C_n-PIC, withn=12, 15, 18) solubilized in different types of micelles, namely cetyl trimethylammonium bromide (CTAB), hexaethyleneglycol-dodecylether (C₁₂EO₆) and CTAB/C₁₂EO₆ mixed micelles. In the case of CTAB micelles, the interaction between the carboxylic function of the extractant and the polar head of surfactant molecules was expected to decrease the rate of complex formation so as to make possible kinetic separation of mixtures of metal ions. The observed rate constants for complex formation at pH 4.5 or 7.0 are indeed much smaller in CTAB micelles than in C₁₂EO₆ or mixed micelles, but they still remain too high for the previous purpose, although the influence of the surfactant concentration demonstrates, as expected, a much stronger partitioning in the case of CTAB in comparison to C₁₂EO₆. On the other hand, it is shown that, once complex formation has occurred the removal of Ni²⁺ ions can be achieved using ultrafiltration. The yield of extraction increases withn, with the mole fraction of C₁₂EO₆, and with the ligand to metal ratio.Institut Nancéien de Chimie Moléculaire (I.N.C.M.)     

Study of Crown Ether Surfactants with Pyrene Fluorescence Probe

Both C₁₀H₂₁-18-crown-5 and C₁₀H₂₁-15-crown-5 were successfully synthesized and exhibited the distinctive characteristics of surfactants. Fluorescence of pyrene was used as a sensitive probe to study the micelle formation of the crown ether surfactant. The variation of the intensity ratio (I₁/I₃) of the first and third vibrational fluorescence bands of pyrene was employed to determine the critical micellar concentration (CMC). Both CMC and cloud points were found to depend on the kinds of cations and the ionic strength in solution. The quenching of pyrene fluorescence is also investigated for some cations under micelle and non-micelle circumstances.     

Micellization and mixed micellization of cationic gemini (dimeric) surfactants and cationic conventional (monomeric) surfactants: Conductometric,dye solubilization,and surface tension studies

In the present study, we have investigated the self-association, mixed micellization, and thermodynamic studies of a cationic gemini (dimeric) surfactant, hexanediyl-1,6-bis(dimethylcetylammonium bromide (16-6-16)) and a cationic conventional (monomeric) surfactant, cetyltrimethylammonium bromide (CTAB). The critical micelle concentration (CMC) of pure (16-6-16 and CTAB) and mixed (16-6-16+CTAB) surfactants was measured by electrical conductivity, dye solubilization, and surface tension measurements. The surface properties (viz., C₂₀ (the surfactant concentration required to reduce the surface tension by 20 mN/m), Π_CMC (the surface pressure at the CMC), Γ_max (maximum surface excess concentration at the air/water interface), A_min (the minimum area per surfactant molecule at the air/water interface), etc.) of micellar (16-6-16 or CTAB) and mixed micellar (16-6-16+CTAB) surfactant systems were evaluated. The thermodynamic parameters of the micellar (16-6-16 and CTAB) and mixed micellar (16-6-16+CTAB) surfactant systems were also evaluated.     

Properties of aqueous solution of sodium glycocholate and a nonionic surfactant,and their mixtures

The micellar properties of aqueous binary mixed solutions of sodium glycocholate, NaGC, and octa-oxyethylene glycol mono-n-decyl ether, C₁₀E₈, have been studied on the basis of surface tensions, the mean aggregation number and the polarity of the interior of the micelles. The mean aggregation number, measured by steady state quenching method, decreased with the increase of the mole fraction of NaGC in the mixed system. The polarity of the interior, estimated by the ratio of first and third vibronic peak in a monomeric pyrene fluorescence emission spectrum, suggested that the hydrophobicity of intramicelles increased with the increase of the mole fraction of NaGC in the mixed system. These are considered to be caused by the differences in the chemical structure and the hydrophobic nature between NaGC and C₁₀E₈. The mean aggregation number and the polarity of the interior for each micelle near the CMC in lower total concentration of surfactants showed the tendency approaching those of pure micelle of the nonionic surfactant. This suggests that the ratio of NaGC in the initial micelles in the range of lower total concentration near the CMC is lower than that of the corresponding prepared mole fraction in the mixed system. This lower value was confirmed also from theoretical calculation of the ratio of NaGC at the CMC in the mixed micelle by regular solution treatment of Rubingh in the solution.     

Micellar effects of a triazole-based cationic gemini surfactant on the rate of a nucleophilic aromatic substitution reaction

The reaction between 2,4-dinitrochlorobenzene (DNCB) and hydroxide ion was studied spectrophotometrically at 25 °C in micelles of a triazole-based cationic gemini surfactant 18-triazole-18 or micelles of the conventional cationic surfactant CTAB. Both CTAB and 18-triazole-18 accelerated this nucleophilic aromatic substitution reaction. The binding constant of the substrate to the micelle, K _S, for 18-triazole-18 (K _S=335 M⁻¹) was found to be much larger than that for CTAB (85 M⁻¹) by fitting the kinetic results with pseudophase ion-exchange (PIE) model, which suggests that DNCB binds with gemini micelles more easily than it does with CTAB micelles. It was also found that 18-triazole-18 catalytic system was in accordance with PIE model at surfactant concentrations below ca. 0.5 mM, above which the increase of viscosity and the change of micelle size with increased surfactant concentration may remarkably influence the reaction. This was quite different from the reaction catalyzed by micelles of the conventional surfactant CTAB.     

Micellar Aggregation Behavior and Electrochemically Reversible Solubilization of a Redox-Active Nonionic Surfactant

For the purpose of studying the potential of a novel nonionic switchable surfactant, 11-ferrocenylundecyl polyoxyethylene ether (FPEG), applied to surfactant-enhanced remediation (SER), the surface properties and micelle solubilization behavior of FPEG were investigated with different inorganic salts. With the addition of inorganic salts (NaCl and CaCl₂), the critical micelle concentration (CMC) of FPEG dropped from 15 to 12 and 8 mg·L^?1, respectively, due to the salting-out effect on the alkyl chain. Thermodynamic parameters based on the CMCs indicated that micelle formation was an entropy-driven process. Dynamic light scattering measurements verified that these inorganic salts can decrease the hydrodynamic diameters (D _h) of the micelles. Solubilization experiments with three typical polycyclic aromatic hydrocarbons (PAHs) demonstrated that the system of FPEG with NaCl shows the highest solubilization ability, and the molar solubilization ratio and micelle–water partition coefficient (K _m ) values follow the order pyrene > phenanthrene > acenaphthene. After oxidation, PAHs can be released from the micelles through breaking up of the micelles, and the cumulative release efficiency of pyrene, phenanthrene and acenaphthene are 31.2, 42.8 and 44.6 %; the order of release efficiency is opposite to that of the reduced form for solubilization abilities. All the results suggest that the ferrocene-containing, redox-active surfactant FPEG has the potential to be recycled in SER technology through electrochemistry approaches.     

Second CMC in surfactant micellar systems

Experimental reports of surfactant systems displaying a second critical micelle concentration (second CMC) have been surveyed. It turns out that surfactant micelles usually show a growth behavior with some typical features. (i) Micelles grow weakly at low surfactant concentrations but may switch to a much stronger growth behavior at higher concentrations. The second CMC is defined as the point of transition from weakly to strongly growing micelles. (ii) Micelles are found to be non-spherically shaped below the second CMC. (iii) At the second CMC micelles are found to be much smaller, with aggregation numbers typically 100–200, than expected for flexible micelles. (iv) Micelles of intermediate size are present in a narrow concentration regime close to the second CMC. (v) Micelles grow much stronger above the second CMC than expected from a sphere-to-rod transition. The conventional spherocylindrical micelle model predicts a smooth growth behavior that contradicts the appearance of a second CMC. Modifying the model by means of including swollen end caps neither account for the presence of micelles with intermediate size, nor the strong growth behavior above the second CMC. Taking into account micelle flexibility is not consistent with the rather low micelle aggregation numbers observed at the second CMC. On the other hand, a recently proposed alternative theoretical approach, the general micelle model, have been demonstrated to take into account basically all features that are typical of experimentally observed micellar growth behaviors.     

The Self-Association and Mixed Micellization of an Anionic Surfactant,Sodium Dodecyl Sulfate,and a Cationic Surfactant,Cetyltrimethylammonium Bromide: Conductometric,Dye Solubilization,and Surface Tension Studies

In the present study, we investigate the self-association and mixed micellization of an anionic surfactant, sodium dodecyl sulfate (SDS), and a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The critical micelle concentration (CMC) of SDS, CTAB, and mixed (SDS + CTAB) surfactants was measured by electrical conductivity, dye solubilization, and surface tension measurements. The surface properties (viz., C₂₀ (the surfactant concentration required to reduce the surface tension by 20 mN/m), Π_CMC (the surface pressure at the CMC), Γ_max (maximum surface excess concentration at the air/water interface), and A_min (the minimum area per surfactant molecule at the air/water interface)) of SDS, CTAB, and (SDS + CTAB) micellar/mixed micellar systems were evaluated. The thermodynamic parameters of the micellar (SDS and CTAB), and mixed micellar (SDS + CTAB) systems were evaluated.

A schematic representation of micelles and mixed micelles.     

Micellar Formation Study of Crown Ether Surfactants

Fluorescence probe and nuclear magnetic resonance (NMR) methods were employed to investigate the micellation of prepared crown ether surfactants, e.g. decyl 15‐crown‐5 and decyl 18‐crown‐6. Pyrene was employed as the fluorescence probe to evaluate the critical micellar concentration (CMC) of these surfactants in aqueous solutions while spin lattice relaxation times (T₁) and chemical shifts of H‐1 NMR were applied in non‐aqueous solutions. Decyl 15‐crown‐5 with lower CMC forms micelles much easier than decyl 18‐crown‐6 with higher CMC in aqueous solutions, whereas decyl 18‐crown‐6 forms micelles easier than decyl 15‐crown‐5 in nonaqueous solutions. Comparison of the CMC of crown ether surfactants and other polyoxyethylene surfactants such as decylhexaethylene glycol was made. Effects of salts and solvents on the micellar formation were also investigated. In general, additions of both alkali metal salts and polar organic solvents into the aqueous surfactant solutions increased in the CMC of these surfactants. The formation of micelles in organic solvents such as methanol and acetonitrile was successfully observed by the NMR method while it was difficult to study these surfactants in organic solutions by the pyrene fluorescence probe method. The NMR study revealed that the formation of micelles resulted in the decrease in all H‐1 spin lattice relaxation times (T₁) of hydrophobic groups, e.g. CH₃ and CH₂, and hydrophilic group OCH₂ of these surfactants. However, upon the micellar formation, the H‐1 chemical shifts (δ) of these surfactant hydrophobic groups were found to shift to downfield (increased δ) while the chemical shift of the hydrophilic group OCH₂ moved to up‐field. Comparison of the spin lattice relaxation time and H‐1 chemical shift methods was also made and discussed.     

pH-Responsive Rheological Properties and Microstructure Transition in Mixture of Anionic Gemini/Cationic Monomeric Surfactants

Surfactant aggregates have long been considered as a tool to improve drug delivery and have been widely used in medical products. The pH-responsive aggregation behavior in anionic gemini surfactant 1,3-bis(N-dodecyl-N-propanesulfonate sodium)-propane (C₁₂C₃C₁₂(SO₃)₂) and its mixture with a cationic monomeric surfactant cetyltrimethylammonium bromide (CTAB) have been investigated. The spherical-to-wormlike micelle transition was successfully realized in C₁₂C₃C₁₂(SO₃)₂ through decreasing the pH, while the rheological properties were perfectly enhanced for the formation of wormlike micelles. Especially at 140 mM and pH 6.7, the mixture showed high viscoelasticity, and the maximum of the zero-shear viscosity reached 1530 Pa·s. Acting as a sulfobetaine zwitterionic gemini surfactant, the electrostatic attraction, the hydrogen bond and the short spacer of C₁₂C₃C₁₂(SO₃)₂ molecules were all responsible for the significant micellar growth. Upon adding CTAB, the similar transition could also be realized at a low pH, and the further transformation to branched micelles occurred by adjusting the total concentration. Although the mixtures did not approach the viscosity maximum appearing in the C₁₂C₃C₁₂(SO₃)₂ solution, CTAB addition is more favorable for viscosity enhancement in the wormlike-micelle region. The weakened charges of the headgroups in a catanionic mixed system minimizes the micellar spontaneous curvature and enhances the intermolecular hydrogen-bonding interaction between C₁₂C₃C₁₂(SO₃)₂, facilitating the formation of a viscous solution, which would greatly induce entanglement and even the fusion of wormlike micelles, thus resulting in branched microstructures and a decline of viscosity.     

ON THE ADSORPTION OF HYDROPHOBIC POLLUTANTS ON SURFACTANT/CLAY COMPLEXES: COMPARISON OF THE INFLUENCE OF A CATIONIC AND A NONIONIC SURFACTANT

The adsorption of the cationic surfactant dodecyl trimethyl ammonium bromide (DTAB) and of the nonionic surfactant dodecyl octaethylene glycol ether (C₁₂E₈) on four different layer silicates and their influence on the sorption processes of the fungizide biphenyl were studied. Unexpectedly, no great differences were found in comparing the adsorption of the two surfactants on the basis of physicochemical investigations, although the adsorption mechanism up to monolayer formation is fundamentally different (ion exchange and physisorption). Thus, the plateau values of the adsorption isotherms and the molar enthalpies of displacement Δ ₂₁h are of the same order of magnitude for both surfactants and the same basal spacing by intercalation is observed in the case of swelling clays. The isotherms of the hydrophobic contaminant biphenyl are of the linear Cl-type at all layer silicates and very low adsorption takes place approximately proportionally to the BET (N₂) surface area. If the surface is weakly hydrophobized by surfactants (c_surfactant<< critical;micelle concentration (CMC)), biphenyl adsorption is clearly increased. These processes can be adequately described using the distribution coefficients K and K_OC (Henry coefficient related to the organic carbon content). K_OC is hardly influenced by the type of layer silicate for DTA⁺-layer silicates, whereas the C₁₂E₈ layer silicate complexes generally show higher, but also different K_OC values. If the surfactant concentrations are above the CMC, solubilization and adsorption compete for the pollutant molecules, which leads to a significant decrease in biphenyl adsorption.