Thermodynamic study of polyethylene-glycols in micellar solutions water+sodium dodecylsulfate

Interactions of a series of polyethyleneglycols (PEG) in aqueous micellar solutions of sodium dodecylsulfate (SDS) were investigated through thermodynamic properties. Volumes, heat capacities and enthalpies of dilution were measured at 25°C. The resulting transfer properties of PEG are reflecting hydrophobic interactions which increase with the polymer length and the hydrophilic interactions occurring in the aqueous polar layer of the micelles. Typically heat capacities clearly evidence various structural changes taking place in the micellar solutions.     

Spectrophotometric study of interaction and solubilization of procaine hydrochloride in micellar systems

The interaction of Procaine hydrochloride (PC) with cationic, anionic and non-ionic surfactants; cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and triton X-100, were investigated. The effect of ionic and non-ionic micelles on solubilization of Procaine in aqueous micellar solution of SDS, CTAB and triton X-100 were studied at pH 6.8 and 29°C using absorption spectrophotometry. By using pseudo-phase model, the partition coefficient between the bulk water and micelles, K_x, was calculated. The results showed that the micelles of CTAB enhanced the solubility of Procaine higher than SDS micelles (K_x = 96 and 166 for SDS and CTAB micelles, respectively) but triton X-100 did not enhanced the solubility of drug because of weak interaction with Procaine. From the resulting binding constant for Procaine-ionic surfactants interactions (K_b = 175 and 128 for SDS and CTAB surfactants, respectively), it was concluded that both electrostatic and hydrophobic interactions affect the interaction of surfactants with cationic procaine. Electrostatic interactions have a great role in the binding and consequently distribution of Procaine in micelle/water phases. These interactions for anionic surfactant (SDS) are higher than for cationic surfactant (CTAB). Gibbs free energy of binding and distribution of procaine between the bulk water and studied surfactant micelles were calculated.      

Mixed micelles of polyethylene glycol (23) lauryl ether with ionic surfactants studied by proton 1D and 2D NMR

(1)H NMR chemical shift, spin-lattice relaxation time, spin-spin relaxation time, self-diffusion coefficient, and two-dimensional nuclear Overhauser enhancement (2D NOESY) measurements have been used to study the nonionic-ionic surfactant mixed micelles. Cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were used as the ionic surfactants and polyethylene glycol (23) lauryl ether (Brij-35) as the nonionic surfactant. The two systems are both with varying molar ratios of CTAB/Brij-35 (C/B) and SDS/Brij-35 (S/B) ranging from 0.5 to 2, respectively, at a constant concentration of 6 mM for Brij-35 in aqueous solutions. Results give information about the relative arrangement of the surfactant molecules in the mixed micelles. In the former system, the trimethyl groups attached to the polar heads of the CTAB molecules are located between the first oxy-ethylene groups next to the hydrophobic chains of Brij-35 molecules. These oxy-ethylene groups gradually move outward from the hydrophobic core of the mixed micelle with an increase in C/B in the mixed solution. In contrast to the case of the CTAB/Triton X-100 system, the long flexible hydrophilic poly oxy-ethylene chains, which are in the exterior part of the mixed micelles, remain coiled, but looser, surrounding the hydrophobic core. There is almost no variation in conformation of the hydrophilic chains of Brij-35 molecules in the mixed micelles of the SDS/Brij-35 system as the S/B increases. The hydrophobic chains of both CTAB and SDS are co-aggregated with Brij-35, respectively, in their mixed micellar cores.     

THE EFFECT OF MIXED MICELLES OF SURFACTANTS ON THE ALKALINE HYDROLYSIS OF ESTERS

The alkaline hydrolysis of aromatic and normal aliphatic acid esters have been studied at 25°C In mixed micellar solution of surfactants (Brij35—SDS, Brij35 — DTAB, Brij35—TTAB.Brij35-CTAB. Triton X—100 —CTAB, Triton X— 100 —SDS) with UV spectrophotometry and the method of thermokinetics The critical mice liar concentrations of the corresponding systems have also been measured with UV spectrophotometry in this paper. K_jnln micellar pseudo-phase has been calculated- The ratios of ki.to ki.for all mixed micellar systems Investigated are less than 1. The results indicate that these reactions are inhibited by the mixed micelles and the mixed micelles exhibit a greater effect of Inhibition on the reactions than those do In corresponding single ones. The reason for this is that the micropolarity of the compact Stern layer of mixed micelle is lower, and the micro viscidity Is higher. The Inhibition effect! also depend on the hydrophobic chain length of the surfactants and the hydrophobicity of the substrate.     

Organization of amphiphiles: XII. Evidence in favor of formation of hydrophobic complexes in aqueous solution

The effects of nonionic surfactants OP-10 and OP-30 (polyoxyethylated octyl phenols with 10 and 30 oxyethylene groups, respectively) in surfactant mixtures with ionic surfactants hexadecyltrimethylammonium bromide (CTAB) and sodium dodecyl sulphate (SDS) have been investigated by a conductometric method in conjunction with fluorescence, surface tension, zeta potential, and DLS measurements. The interactions are found to be antagonistic in nature for each of the systems; i.e., micellization of CTAB as well as SDS is hindered on addition of the nonionic surfactants. The antagonism is found to be more prominent in the presence of OP-10 compared to that of OP-30. Two types of mechanistic paths, path A operating below the critical micellar concentration and path B operating beyond the critical micellar concentration of nonionic surfactants, have been suggested. In path A, the retardation in micellization has been attributed to a decrease in monomeric concentration of the ionic surfactants from solution as a result of the formation of a hydrophobic complex between nonionic and ionic surfactants. In path B, the decrease in monomer concentration is due to the solubilization of the ionic surfactant in micelles of the nonionic surfactants in a 1:1 stoichiometric ratio. A theoretical treatment to the interaction in each ionic-nonionic pair yields a positive value of the interaction parameter supporting the concept of antagonism. The formation of the hydrophobic complex is supported by fluorescence and surface tension measurements. A schematic representation of the stabilization of these hydrophobic complexes has been suggested. The association of ionic surfactants by nonionic micelles is suggested by zeta potential and DLS studies.     

Thermodynamic properties of N-octyl- and N-dodecylnicotinamide chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl- and N-dodecylnicotinamide chlorides in water over an extended concentration region. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived as a function of the surfactant concentration. For both surfactants, plots of volumes, enthalpies and free energies vs. concentration are regular whereas those of heat capacities and entropies present anomalies at about 0.8 and 0.1m for the octyl and dodecyl compounds, respectively. Changes in the slope of a plot of osmotic coefficients times molality vs. molality were also observed at these same concentrations. These peculiarities are ascribed to micelle structural transitions. The nonideal free energies do not seem to depend on the alkyl chain length when they are plotted vs. m/C _cmc . Also, a plot of the nonideal free energy vs. logm/C _cmc is roughly independent of the nature of the surfactant because of the constant activity of surfactants in micellar solutions. Nonideal free energies, enthalpies and entropies have been calculated at 15 and 35°C. At each concentration the nonideal free energy is temperature independent as a result of a compensatory effect between enthalpy and entropy. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data suggest that the nicotinamide group possesses less hydrophilic character than the ammonium group.     

THE EFFECTS OF MIXED MICELLES OF SURFACTANTS ON POLYMERIZATION OF ACRYLAMIDE

The polymerization of acrylamide in mixed micellar solutions of surfactants, initiated by NaHSO₃ has been studied at 20 and 3Q° C with time variable method of thermokinetics for 1. 5-order reaction. The results indicate that the mixed micellar systems of cationic or anionic with zwitterionic surfactants (SLS/ CTAB, SLS/ TTAB, SLS/ SDS) and cationic with nonionic surfactants (Brij 357sol; CTAB, Bri-J35/TTAB, Brij35/ DTAB) have catalytic effect on the polymerization in the order, at 20° C. SLS/ SDS SLS/ TTAB SLS/ CTAB Brij35/ CTAB at 30° C SLS/ SDS SLS/ TTAB≈ / CTAB Bri-j35/ DTAB= sBrij35/ TTAB as Brij35/ CTAB, while Brij35/ SDS mixed micellar system has inhibition. These effects are attributed to the effect of the Stern layer of mixed micelles on the step of initiator (HSOT) to form free radical.     

Co-solubilization of the Hydrophobic Drugs Carbamazepine and Nifedipine in Aqueous Nonionic Surfactant Media

Co-solubilization of the hydrophobic drugs Carbamezipine (CBZ) and Nifedipine (NFD) by micellar solutions at 25 °C, using two series of polyoxyethylene based nonionic surfactants, was measured and compared. The first series is composed of surfactants with a 12 carbon (C₁₂) hydrophobic chain while the second series had 16 carbon (C₁₆) hydrophobic chains. Experimental results were obtained for solubilization and co-solubilization of CBZ and NFD within the micelles at saturation and quantification was done in terms of the molar solubilization ratio and the micelle–water partition coefficient employing spectrophotometric and tensiometric techniques. The extent of micellar solubilization of CBZ is much greater than NFD. The C₁₂ series of surfactants exhibit higher solubilization capacities for CBZ than the C₁₆ series while the reverse is the case for NFD. Co-solubilization results showed competitive solubilization of the drugs. A synergistic effect on the solubilization of NFD was observed in the presence of CBZ in Brij30 and Brij56 surfactant systems while, in the remaining surfactants, the solubility of NFD was slightly reduced. Since the surfactants used in the present study are either nontoxic or have minimal toxicity, it is expected that they can be employed as drug delivery vehicles for co-administration of the two drugs in vivo. Both from industrial and research points of view, this paper reports a comprehensive study for co-solubilization of differently structured drugs in micellar media.     

Effects of addition of short-chain alcohol solvents on micellization and thermodynamic properties of anionic surfactants sodium dodecyl sulfate and sodium dodecyl sulfonate in aqueous solutions

Micellar and thermodynamic properties of anionic surfactants sodium dodecyl sulfate (SDS) and sodium dodecyl sulfonate (SDSn) in aqueous solutions of 5 wt% short-chain alcohols methanol, ethanol, and 1-butanol were investigated by experimental electrical conductivities, densities and sound velocities at 298.15 K. It was found that methanol behaves like a cosolvent and increases the critical micelle concentration (CMC) of both surfactants in aqueous solutions. However, the other investigated alkanols act as a cosurfactant and decrease the CMC by their presence. The values of the degree of counterion association on the micelles of both surfactants in aqueous methanol solution are same as those in pure water, and they decrease with increasing the alkyl chain length of alcohol. Furthermore, the values of the apparent molar volume and isentropic compressibility of the monomeric and micellar forms of the investigated surfactants were obtained from the experimental density and sound velocity data. It was found that the values of the apparent molar properties of both micellar and monomeric forms of the studied surfactants increase by increasing the alkyl chain of the alcohols.     

MICELLE FORMATION BY ANIONIC AND CATIONIC SURFACTANTS IN AQUEOUS 18-CROWN-6 ETHER,P-CYCLODEXTRIN,AND 1,10-PHENANTHROLINE SOLUTIONS AT DIFFERENT TEMPERATURES

The conductances of sodium perfluorooctanoate (SPFO), sodium dodecylsulphate (SDS), dodecyltrimethylammonium bromide (DTAB), and tetradecyltrimethylammonium bromide (TTAB) in 18-crown-6 ether + water (CR+W), p-cyclodextrin + water (CY+W), and 1,10-phenanthroIine + water (Phen+W) mixtures with fixed 4 mM of each additive were determined over the temperature range of 5-55 °C. The conductivity plots for all the surfactants showed single break from which the critical micellization concentration (cmc) and degree of micelle ionization (x) were computed. From the pre and the post micellar slopes of the conductivity curves, the equivalent conductivities of the monomeric (Aass) and the micellar states (Amjc), respectively, were calculated and discussed with respect to the surfactant-additive complexation. It was observed that the micelle formation of all the ionic surfactants irrespective of the nature of their head groups were delayed in CYC+W in comparison to that in CR+W and Phen+W systems over the temperature range studied. The micelle formation of SPFO and SDS in CR+W and Phen+W systems showed stabilization of the respective micelles due to the adsorption of Na⁺-CR and Na+-Phen complexes at the micelle solution interface in comparison to that of DTAB and TTAB.     

Calorimetric Study of Micelle Formation in Polyethylene Glycol Monooctyl Ether Solution

The micellar solutions of polyethylene glycol monooctyl ethers C8Ej (j = 3,4,5) were investigated by employing the high-precision isothermal titration microcalorimeter at seven temperatures from 10^° to 40^°C. From the thermodynamic analysis of the experimental results, the differential enthalpies of solution of monomer and micelle of C8Ej were obtained separately; they are negative and increase with increasing temperature and decrease with the number j. It was found that C8Ej molecules interact with water molecules more strongly in the monomeric than in micellar state. The enthalpy of micelle formation of C8Ej was positive and decreased with rising temperature, while they increased slightly with increasing j. By comparing the results of ethylene glycol oligomers (C0Ej) with those of C8Ej, it seems reasonable to suppose that the driving force of micelle formation of C8Ej is mainly the increment of entropy caused by dehydration of hydrophilic parts at higher temperatures, while the increment caused by dehydration of hydrophobic parts becomes dominant at lower temperatures in the micellization process.     

Evidence for micellar structure in the gas phase

We have compared micelles, reverse micelles, and reverse micelles encapsulating myoglobin using electrospray mass spectrometry. To enable a direct comparison, the same surfactant (cetyltrimethylammonium bromide (CTAB)) was used in each case and micelle formation was controlled by manipulating the aqueous and organic phases. Tandem mass spectra of the resulting micelle preparations reveal differences in the ions that dissociate: those that dissociate from regular micelles have undergone >90% exchange of bromide ions from the headgroup with acetate ions from bulk solvent. By contrast, for reverse micelles, ions are detected without exchange of bromide ions from the headgroup, consistent with their protection in the core of the micellar structure. Tandem mass spectra of micelles and reverse micelles reveal polydispersed assemblies containing several hundred CTAB molecules, indicating the coalescence of the micellar systems to form large assemblies. For reverse micelles incorporating myoglobin, spectra are consistent with one holo myogolobin molecule in association with approximately 270 CTAB molecules. Overall, therefore, our results show that the solution-phase orientation of surfactants is preserved during electrospray and are consistent with interactions being maintained between surfactants and an encapsulated protein.     

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.     

Effect of ionic micellar medium on kinetics and mechanism of oxidation of bovine serum albumin: A pulse radiolysis study

Effect of protein–micelle interaction on bovine serum albumin (BSA) oxidation by trichloromethyl peroxyl radical (CCl₃O₂^·) in anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethyl ammonium bromide (CTAB) micellar media has been studied using nanosecond pulse radiolysis technique. Viscosity measurement and light scattering studies have suggested that SDS and CTAB micelles produce BSA–micelle aggregates of different sizes and polydispersity. Oxidation kinetics and transients have been affected both by anionic SDS and cationic CTAB micelles but in a different manner. Tryptophanyl-CCl₃O₂^· adduct radical to tyrosyl radical transformation in BSA has been observed in anionic SDS micelles but not in cationic CTAB micelles. Similar studies have also been done with tryptophan and tyrosine amino acids, which undergo oxidation in BSA. The study suggests that Coulombic and hydrophobic interactions between micelles and protein affect the structure of the protein to shield its functional amino acids, like tryptophan and tyrosine, to neutral oxidizing radical.     

Spectral studies of N-nonyl acridine orange in anionic, cationic and neutral surfactants

The spectroscopic and photophysical properties of N-nonyl acridine orange - a metachromatic dye useful as a mitochondrial probe in living cells - are reported in water and microheterogeneous media: anionic sodium dodecylsulfate (SDS), cationic cetyltrimethylammonium bromide (CTAB) and neutral octylophenylpolyoxyethylene ether (TX-100). The spectral changes of N-nonyl acridine orange were observed in the presence of varying amount of SDS, CTAB and TX-100 and indicated formation of a dye-surfactant complex. The spectral changes were also regarded to be caused by the incorporation of dye molecules to micelles. It was proved by calculated values K(b) and f in the following order: K(bTX-100)>K(bCTAB)>K(bSDS) and f(TX-100)>f(CTAB)>f(SDS). NAO binds to the micelle regardless the micellar charge. There are two types of interactions between NAO and micelles: hydrophobic and electrostatic. The hydrophobic interactions play a dominant role in binding of the dye to neutral TX-100. The unexpected fact of the binding NAO to cationic CTAB can be explained by a dominant role of hydrophobic interactions over electrostatic repulsion. Therefore, the affinity of NAO to CTAB is smaller than TX-100. Electrostatic interactions play an important role in binding of NAO to anionic micelles SDS. We observed a prolonged fluorescence lifetime after formation of the dye-surfactant complex tau(SDS)>tau(TX-100)>tau(CTAB)>tau(water), the dye being protected against water in this environment. TX-100 is found to stabilize the excited state of NAO which is more polar than the ground state. Spectroscopic and photophysical properties of NAO will be helpful for a better understanding of the nature of binding and distribution inside mammalian cells.     

Micellar effects on the electrochemistry of dopamine and its selective detection in the presence of ascorbic acid

The electrochemistry of dopamine (3-hydroxytyramine) was studied by cyclic voltammetry at a glassy carbon electrode in the presence of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) micelles at different pH. The anodic peak potential (E(pa)) and peak current (I(pa)) were found to be remarkably dependent on the charge and the concentration of the surfactant. The E(pa) and I(pa) change abruptly around the critical micellar concentration (CMC) of the surfactants and reach a plateau above the CMC. The E(pa) at the plateau shifts to more positive values in the cationic CTAB micellar solution, e.g. from 180 mV vs SCE in aqueous solution at pH 6.8 to 410 mV in CTAB micelle, whilst it shifts to less positive values in the anionic SDS micellar solution, e.g. 150 mV at pH 6.8. Therefore, the overlapped anodic peaks of dopamine and ascorbic acid in the mixture of the two compounds in aqueous solutions can be separated in CTAB micelles since the micelle shifts the E(pa) of ascorbic acid to less positive values. The two peaks are separated by ca. 400 mV at pH 6.8 in CTAB micelle, hence dopamine can be determined in the presence of 100 times excess of ascorbic acid. In SDS micelle and in the presence of ascorbic acid, the I(pa) of dopamine is greatly enhanced due to the catalytic oxidation of the latter that enables quantitative determination of both compounds.     

Thermodynamic properties of the aqueous solution of potassium salts of some 4-((alkylcarbonyl)amino)-2-hydroxybenzoic acids at 298 and 313 K

To understand the aggregation behavior of surface-active ligands with a salycilic polar head, we undertook a systematic study of some classes of anionic surfactants where the presence and the position of the -OH and the carboxylic group differ. This paper reports the dilution heats at 298 and 313 K of aqueous solutions of potassium 4-((alkylcarbonyl)amino)-2-hydroxybenzoate (KPAS-C(n) where n stands for the number of carbon atoms in the chain) in KOH at 0.1 m, measured as a function of concentration. From the experimental data, apparent and partial molar enthalpies vs concentration were obtained. By using a pseudo-phase-transition approach, the enthalpy changes upon micelle formation (DeltaH(m)) and assuming that in the restricted range of temperature examined heat capacities are constant, the heat capacity changes have been obtained. Micelle formation enthalpies are seen to be additive with a group contribution for the methylene group of -1.5+/-0.1 kJ mol(-1) per group at 298 K and -2.3+/-0.1 kJ mol(-1) per group at 313 K, comparable with that obtained for similar anionic compounds in the same experimental conditions and for N-alkylnicotinamide chlorides (cationic surfactants). The -CH(2)- group contribution to the micelle formation heat capacities is -53+/-1 J K(-1) mol(-1).     

A new model to study the phase transition from microstructures to nanostructures in ionic/ionic surfactants mixture

The phase behavior and aggregate structures of mixtures of the oppositely charged surfactants cetyltrimethyl ammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) are explored at high dilution by pulsed field gradient stimulated echo (PFG-STE) NMR. The aggregation numbers and hydrodynamic radii of vesicles and mixed micelles were determined by a combination of viscosity and self-diffusion coefficient measurements. The average size of the mixed micelles was larger than that of micelles containing uniformly charged head groups. Analysis of the variations of the self-diffusion coefficient and viscosity with changing concentration of CTAB or SDS in the cationic-rich and anionic-rich regions revealed a phase transition from vesicles to mixed micelles. Differences in the lengths of the CTAB and SDS hydrophobic chains stabilize vesicles relative to other microstructures (e.g., liquid crystalline and precipitate phase), and vesicles form spontaneously over a wide range of compositions in both cationic-rich and anionic-rich solutions. The results obtained from conductometry measurements confirmed this transition. Finally, according to the capacitor model, a new model was developed for estimating the surface potentials and electrostatic free energy (g(elec)). Then we investigated the variations of electrostatic and transfer free energy in phase transition between mixed micelle and vesicle.     

Interaction between biosurfactant surfactin and cationic surfactant cetyl trimethyl ammonium bromide in mixed micelle

An anionic/cationic mixed surfactant aqueous system of surfactin and cetyl trimethyl ammonium bromide (CTAB) at different molar ratios was studied by surface tension and fluorescence methods (pH 8.0). Various parameters that included critical micelle concentration (cmc), micellar composition (X ₁), and interaction parameter (β ^m) as well as thermodynamic properties of mixed micelles were determined. The β ^m was found to be negative and the mixed system was found to have much lower cmc than pure surfactant systems. There exits synergism between anionic surfactin and cationic CTAB surfactants. The degree of participation of surfactin in the formation of mixed micelle changes with mixing ratio of the two surfactants. The results of aggregation number, fluorescence anisotropy, and viscosity indicate that more packed and larger aggregates were formed from mixed surfactants than unmixed, and the mixed system may be able to form vesicle spontaneously at high molar fraction of surfactin.