Composition Of Mixed Micelles Research Articles

Micellar Behaviour of Mixed Surfactant System of Linear Alkyl Benzene Sulfonate and Triton X-100 in Pure and Hard Water

The adsorption and the micellar behaviour of mixed surfactant system composed of Linear Alkyl Benzene Sulfonate-sodium salt (LABS) and Triton X-100 (TX-100) over the entire composition range have been tensiometrically studied in 0, 20, 50, 100, 150, and 200 ppm (CaCO 3 ) hard water. From the study, the surface properties like maximum surface excess concentration, (G max ) and the minimum surface area per molecule (A min ) have been evaluated. The micellization in the mixed systems have been analyzed in the light of Rubingh's Regular Solution model. The composition of the mixed micelle (x) and the interaction parameter (b) of the mixed systems both in pure and hard water have been computed. For all the systems studied, b was found to be negative. The effect of addition of Ca 2+ ions on the critical micelle concentration (cmc) of the surfactants was also analyzed in the light of the Corrin-Harkins equation. There was generally a deviation from the Corrin-Harkins behaviour indicating that the change in the cmc of the mixed systems cannot be explained on the basis of the counter ion binding effect only.

A small-angle neutron scattering study of sodium dodecyl sulfate-poly(propylene oxide) methacrylate mixed micelles

Mixed micelle of protonated or deuterated sodium dodecyl sulfate (SDS and SDSd25, respectively) and poly(propylene oxide) methacrylate (PPOMA) are studied by small-angle neutron scattering (SANS). In all the cases the scattering curves exhibit a peak whose position changes with the composition of the system. The main parameters which characterize mixed micelles, i.e., aggregation numbers of SDS and PPOMA, geometrical dimensions of the micelles and degree of ionisation are evaluated from the analysis of the SANS curves. The position q max of the correlation peak can be related to the average aggregation numbers of SDS–PPOMA and SDSd25–PPOMA mixed micelles. It is found that the aggregation number of SDS decreases upon increasing the weight ratio PPOMA/SDS (or SDSd25). The isotopic combination, which uses the “contrast effect” between the two micellar systems, has allowed us to determine the mixed micelle composition. Finally, the SANS curves were adjusted using the RMSA for the structure factor S ( q ) of charged spherical particles and the form factor P ( q ) of spherical core–shell particle. This analysis confirms the particular core–shell structure of the SDS–PPOMA mixed micelle, i.e., a SDS “core” micelle surrounded by the shell formed by PPOMA macromonomers. The structural parameters of mixed micelles obtained from the analysis of the SANS data are in good agreement with those determined previously by conductimetry and fluorescence studies.

Micellization and interaction of anionic and nonionic mixed surfactant systems in water

Interaction between binary surfactant mixtures containing anionic surfactants viz. sodium dodecyl sulphates (NaDS) and magnesium dodecyl sulphates (Mg(DS) 2) and a nonionic surfactants viz. dodecyl dodecapolyethylene glycol ether (C 12E 12) and dodecyl pentadecapolyethylene glycol ether (C 12E 15) in water at different mole fractions (0–1) were studied by surface tension, viscometry and dynamic light scattering (DLS) methods. 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 ( f 1 and f 2) of metal dodecyl sulphates (MDS)/C 12E m ( m = 12, 15) mixed surfactant systems were evaluated, which shows extent of ideality of individual surfactant in mixed system. The estimated interaction parameter indicates an overall attractive interaction in the mixed micelles, which is predominant for NaDS as compared to Mg(DS) 2. Counter ion valency has specific effect on the mixed micelles, as Mg(DS) 2 has less interaction with nonionic surfactants in comparison to NaDS due to strong condensation of counter ion. 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. DLS measurements and viscosity data reveals the synergism in mixed micelles, showing typical viscosity trends and linearity in sizes were observed.

Mixed micelle behavior of Pluronic L64 and Triton X-100 with conventional and dimeric cationic surfactants

The mixed micellar properties of a triblock copolymer, Pluronic L64, (EO) 13(PO) 30(EO) 13, and a nonionic surfactant, Triton X-100, in aqueous solution with conventional alkyl ammonium bromides and their dimeric homologues were investigated with the help of fluorescence and cloud point measurements. The composition of mixed micelles and the interaction parameter, β, evaluated from the critical micelle concentration (cmc) data for different mixtures using Rubingh's and Motomura's theories are discussed. It has been observed that the mixed micelle formation between monomeric/dimeric alkyl ammonium bromides and L64 was due to synergistic interactions which increase with the increase in hydrophobicity of the cationic component. On the other hand, synergistic mixing was observed in the mixed micelles of Triton X-100 and monomeric cationic surfactants, the magnitude of which decreases slightly with the increase in hydrophobicity of the cationic component. Antagonistic interactions were observed in the case of Triton X-100 and dimeric cationic surfactants.

Phase Behavior of Bis(Quaternary Ammonium Bromide)/Sodium Cholate/H2O System

Interactions in an oppositely charged surfactant mixture composed of a gemini surfactant (bis(quaternary ammonium bromide)) and a bile salt (sodium cholate) in water were studied at 30°C. A combination of techniques was used including surface tension, conductometry, light scattering, light microscopy, and microelectrophoretic measurements. A strong dependence of the phase behavior on the molar ratio and actual concentration of surfactants was found. The interplay between electrostatic effects, geometry of molecules, and dissimilar separation of the hydrophobic and hydrophilic moieties in the surfactants dictate the interaction mode and the microstructures formed. Instead of precipitation, in the equivalent mixtures formation of complexes, mixed micelles, vesicles, coacervates, and solid crystalline phases have been observed. The extent of interacting forces in mixed micelles formed in equivalent mixtures was evaluated by regular solution theory. A relatively high negative value of interaction parameter indicated a strong attractive interaction between surfactants. The compositions of both mixed micelles and mixed monolayer are found to be almost equimolar.

Complex formation in alkyldimethylamine oxide/sodium palmitate/water mixtures

The complex formation between nonionic alkyldimethylamine oxide (C nDMAO, n = 14 , 16, and 18) and sodium palmitate (NaPa) in the solid phase of C nDMAO/NaPa mixtures and the dependence of the interaction parameter β of the regular solution theory (RST) on the mixed micelle composition of C16DMAO/NaPa mixtures were investigated. The dissolution temperature showed a maximum at a NaPa mole fraction X Pa * of 0.3–0.4 for C16DMAO/NaPa and 0.2 for C18DMAO/NaPa. The compositions of the complexes suggested by X Pa * are C16DMAO: NaPa = 3:2 or 2:1 and C18DMAO:NaPa = 4:1. The composition X Pa * depended on the chain length of the amine oxides. The maximum was not observed in the case of the C14DMAO/NaPa/water system. In the range 0.7 ⩽ X Pa ⩽ 1.0 , dissolution temperature depression was observed with decreasing X Pa . The dissolution temperature depression was analyzed by taking into account the nonideal behavior in the mixed micelles and the counterion binding on the mixed micelle surface. The negative β values were obtained for all three mixed systems. It was shown that the counterion activity remained practically constant in the range of 0.7 ⩽ X Pa ⩽ 1.0 . The cmc values of C16DMAO/NaPa mixtures were determined by pyrene fluorescence measurement. For C16DMAO/NaPa mixtures, the dependence of the RST interaction parameter β on the mixed micelle composition X Pa was determined for a wide range ( 0.2 ⩽ X Pa ⩽ 0.9 ). In the range 0.2 ⩽ X Pa ⩽ 0.5 , the β values were obtained from an analysis of cmc based on the RST. In the range 0.7 ⩽ X Pa ⩽ 0.9 , the β values were obtained from an analysis of the dissolution temperature depression. From the analysis of the micelle composition dependence of the β values, a short-range attractive interaction between the headgroup of C16DMAO and palmitate anion is suggested.

Surface properties of cationic–nonionic mixed surfactant systems

Surface properties of the binary cationic–nonionic mixed systems (dodecyltrimethylammonium bromide + n-dodecyl-β-d-maltoside and dodecyltrimethylammonium bromide + n-dodecyl-β-d-glucoside) have been investigated by measuring the surface tension of aqueous solutions as a function of the total concentration and mole fraction of surfactants.The experimental results were analysed thermodynamically applying the Motamura theory, which allows for calculation of the composition of mixed monolayers and micelles as well as the excess free energy of adsorption and micelle formation, corresponding to mutual interactions between surfactants in mixed adsorbed film and micelle. Phase diagrams of adsorption and micelle formation and values of the excess Gibbs energy for both of the investigated systems were compared.The obtained results indicate that dodecyltrimethylammonium bromide mixes nonidealy with each of the investigated surfactants in both adsorbed film and micelle and proved that the size of hydrophilic group influences on the interactions between components in formed aggregates.

Influence of Tetraethoxylated Nonylphenol on the Properties of Dodecaethoxylated Nonylphenol Micelles in Aqueous Medium

Properties of the mixed micelles of tetraethoxylated nonylphenol and dodecaethoxylated nonylphenol (NP12) in water are studied by spectrophotometry, dynamic and static light scattering methods. Data on the composition, aggregation numbers (N), sizes (Rh), and hydration (nOE) of mixed and individual micelles of NP12 are obtained. Two NP12 concentration regions are disclosed where mixed micelles of various compositions but with the close values of N, Rh, and nOE are present. It is established that the sizes and aggregation numbers of mixed micelles are noticeably larger than those of individual micelles. Features of the mixing of two studied surfactants in a micellar state are analyzed in terms of the theory of phase separation.

Composition of mixed adsorption layers and micelles in solutions of sodium dodecyl sulfate and dodecyl acid diethanol amide

The surface tension isotherms of dodecyl acid diethanole amide (DADA), sodium dodecyl sulfate (SDS) and their mixtures at different molar ratios are measured by the Wilhelmy-plate method. The isotherms are analyzed by the theory of nonideal interactions in binary surfactant mixtures (NIBM) and by the nonideal multicomponent mixed micelle model (NMMM). The molecular interaction parameters (MIP) on the interface, βS, and in the micelles, βM, are determined by the NIBM method. By using the Ingram’s modification of this method, we calculate the compositions of the mixed adsorption layers and of the micelles, as functions of DADA and SDS concentrations both below and above the critical micelle concentration (CMC). It is shown that the DADA molecules prevail on the surface and in the micelles (>50%) at concentrations around CMC even when DADA presents a small fraction in the surfactant mixture (5 mol%). This result is explained by the much higher surface activity of DADA in comparison with SDS. At concentrations, which are about 1 order of magnitude above the CMC, the composition of the micelles approaches the composition of the total surfactant mixture. The composition of the surfactant adsorption layer also becomes much closer to the composition of the total surfactant mixture at concentrations well above the CMC. The results for the micelles composition at moderate and high molar fractions of DADA in the surfactant mixture (above 10 mol%), obtained by NMMM and by NIBM methods, agree well with each other. A discrepancy between the results obtained by these two models is found at low DADA fraction in the mixture, and this discrepancy is explained by the presence of traces of dodecanol (DOH) in the used SDS sample. © 2003 Elsevier B.V. All rights reserved.

Mixed micelles of homologous perfluoropolyether anionic surfactants in water

The mixtures of sodium and ammonium salts of three homologous perfluoropolyether carboxylic acids having Cl-terminated perfluoroalkyl group (Cl-PFPE) and differing in the average molecular weight (MW) were examined. The surfactants, namely n2, n3 and n4, have two, three and four PFPE units, respectively. Each surfactant was studied alone and in mixture with the other surfactants with the same counterion. 19 F NMR chemical shifts were measured for each surfactant and for the mixtures in different concentrations. For a given mixture the micelle composition, X i , can be determined from the observation of the chemical shifts of the micellar components. It was found that Cl-PFPE surfactant mixtures form in water mixed micelles which contain the surfactants in equilibrium with monomeric species. The analysis of 19 F NMR chemical shift variations allowed evaluating the partition of the various surfactants in the mixed aggregates as a function of the total concentration. Composition of mixed micelles resembles ideal mixing predictions particularly at high surfactant concentrations.

Intermolecular interactions in the binary systems of cationic and nonionic surfactants

The micellization of the mixtures of cationic (cetyltrimethylammonium bromide, cetylpyridinium bromide) and nonionic (Triton X-100) surfactants at various content of components in aqueous solutions, as well as the adsorption of surfactants at the air/solution interface were studied. The effect of negative deviation from the ideality was discovered during the formation of mixed micelles and adsorption layers. Various theoretical approaches to the quantitative description of mixed systems were compared within the framework of the model of pseudophase separation. The parameters of intermolecular interaction in mixed micelles and adsorption layers, excess free energies of micellization and adsorption, compositions of micelles and adsorption layers were calculated. It was shown that the use of various theoretical approaches for the analysis of interactions in surfactant mixtures leads to close numerical results. It was shown that the composition of mixed micelles and adsorption layers, as well as the parameters of intermolecular interaction and the excess free energy depend on the relative content and molecular structure of surfactants (mixture components).

Mixed micelles of some metal dodecyl sulfates and triton X‐100 in aqueous media

AbstractTensiometric studies on several binary surfactant mixtures containing anionic surfactants, viz., metal (lithium, sodium, potassium, copper, cobalt, and magnesium) dodecyl sulfates and a nonionic surfactant (Triton X‐100) in water at different mole fractions (0–1) provide critical micelle concentration (CMC) values. The composition of mixed micelles and the interaction parameter, β, evaluated from the CMC data for different systems using Rubingh's theory, are discussed. Marked interaction is observed with monovalent dodecyl sulfates. The influence of counter‐ion valence on the formation of mixed micelles was investigated for anionic‐nonionic systems, and results indicated that mixed systems with bivalent counter‐ions in metal dodecyl sulfate resembled nonionic‐nonionic systems where weak/negligible interaction has been reported. Salt addition revealed the weakening of interaction in the mixed systems, which is attributed to the head group charge neutralization and the dehydration of the ethylene oxide units of the nonionic surfactants. A few cloud point and viscosity data are also reported.

Characterization of Mixed Micelles of SDS and a Sugar-Based Nonionic Surfactant as a Variable Reaction Medium

Time-resolved fluorescence quenching (TRFQ), electron paramagnetic resonance (EPR), and small-angle neutron scattering (SANS) are employed to characterize mixed micelles of sodium dodecyl sulfate (SDS) and the nonionic sugar-based surfactant dodecylmalono-bis-N-methylglucamide (DBNMG) as a reaction medium. Interpretation of the results from the three methods are constrained to fit the classical model of a hydrocarbon core surrounded by a polar shell. As measured by TRFQ, at 45 °C, the aggregation numbers increase from 48 for pure SDS to a maximum of 63 at 36 mole percent DBNMG and decrease again to 49 for pure DBNMG. The aggregation numbers of the pure DBNMG increase from 33 at 21° Ct o 49 at 45°C. SANS results of the pure DBNMG are interpreted by fixing the aggregation number at 49 (from TRFQ) and allowing the polar shell thickness to vary as a fitting parameter, yielding a value of 5.8 A at 45 °C. EPR, utilizing a hydrophobic spin probe, is used to measure the nonempirical polarity index, H(25 °C), of the polar shell. H(25 °C) is defined to be the ratio of molar concentration of OH dipoles in a solvent or solvent mixture to that in water at 25 °C; thus, both water and the sugar headgroups contribute. By fixing the volume in the polar shell inaccessible to water due to the presence of SDS (127 A 3 ) and DBNMG (580 A 3 ), theoretical values of H(25 °C) are computed, the result depending only on the number of OH bonds in DBNMG available to interact with the spin probe. A constant average value of 7.4 OH bonds out of a maximum possible number of 10 reproduces the measured values over the full range of mixed micelle compositions. At 45 °C, the microviscosity of the polar shell, as deduced from the rotational correlation time of the spin probe, varies from 2.79 ( 0.05 cP for pure SDS to 13.1 ( 0.2 cP for pure DBNMG departing only slightly from a linear dependence on the mole fraction of DBNMG. The uncertainties in the viscosity are the standard deviations in 10 measurements and therefore represent the uncertainty in the relative values of the viscosity. The viscosity decreases from 32 ( 4c P at 21°C to 13.1 ( 0.2 cP at 45 °C for pure DBNMG micelles. These viscosities are used to show that the quenching rate constant of pyrene by dimethyl benzophenone, measured by TRFQ, follows the Stokes-Einstein-Smolukhovsky equation with a quenching probability of P ) 0.5 whether the mixed micelle composition is changed at constant temperature or if the temperature is varied in pure DBNMG. The volume of the polar shell enters into the formulation because the effective concentration of the quencher depends on that volume; however, the quenching rates are not correlated with volume alone.

Thermodynamic modeling of precipitation behavior in mixed micellar systems

Results of thermodynamic modeling of solubility diagrams for various types of mixed micellar systems (nonionic surfactants with added alkanols, mixtures of two nonionics, two anionics, anionic plus semipolar surfactants) are summarized. Phase behavior over a wide range of surfactant concentrations is under study and the model description is aimed at prediction of the solubility diagrams in ternary systems on the basis of data for aqueous solutions of the pure surfactants. In this case the pseudo-phase separation approach is quite efficient. When the total surfactant concentration in mixed solutions far exceeds the CMC it is expedient to describe the dependence of the dissolution (precipitation) temperature on the relative content of two surfactants using the concentration variables of the micellar pseudo phase. It follows from thermodynamic equations that a substantial decrease of the precipitation temperature can be produced by additives forming mixed micelles with the basic surfactant, and the distribution coefficient between micelles and the aqueous surroundings is an important characteristics of the additive. In some cases (mixtures of nonionic surfactants, solutions of ionic surfactants without a salt background at concentrations substantially above the CMC) the composition of mixed micelles is very close to the overall surfactant based composition of the solution. That is why in the both cases the eutectic type curves ‘dissolution temperature versus surfactant-based mole fraction’ can be modeled in a similar manner which is simpler than calculations for ionic systems based on the solubility product approach. For mixtures with added salts the ion exchange between micelles and the aqueous surroundings should be taken into account. The specificity of systems whose phase behavior is affected by chemical reactions (protonation, complex formation) is discussed and a version of the pseudo-phase model is proposed for description of the solubility diagrams in aqueous sodium dodecylsulfate–alkyl amine oxide mixtures. Experimental results obtained by the author and collegues and data from literature serve to illustrate the model performance.

Interfacial and aggregation properties of some anionic/cationic surfactant binary systems II. Mixed micelle formation and surface tension reduction effectiveness

Mixed micelle formation and surface tension reduction effectiveness (γcmc) were investigated for the following systems: triethanolammonium dodecylpoly(oxyethylene)sulfate (TADPS, containing about two ethylene oxide units)/dodecyltrimethylammonium bromide, TADPS/hexadecyltrimethylammonium bromide and TADPS/hexadecylpyridinium chloride. For all these anionic/cationic systems, the mixed critical micelle concentration (cmc) values reflect a strong synergism in mixed micelle formation, with βM values ranging from −13.8 to −18.3. The mixed micelle composition is mixing-ratio dependent and, for equimolar mixtures, the mixed micelle is richer in the surfactant with the lower cmc. Precipitation is inhibited to a certain extent, thanks to the presence of ethylene oxide groups in the anionic species. The conditions for synergism in γcmc, differently expressed in the literature, can be derived from the surface tension equations established in our previous article. They can be conveniently described by a few characteristic constants: Γi∞ (saturated Gibbs excess), Ki (constant in the Szyszkowski equation), the cmc of the individual surfactants and the interaction parameters, βS and βM, of their mixtures. Excellent agreement between theoretically predicted and experimental results is obtained. With the increase in surfactant chain length, the βM values decrease faster than the βS ones and this can result in the loss of synergism in γcmc.

Interfacial compositions of cationic and mixed non-ionic micelles by chemical trapping: a new method for characterizing the properties of amphiphilic aggregates

A specially synthesized arenediazonium ion bound to amphiphilic aggregates decomposes spontaneously via rate determining loss of N 2 to give a highly reactive, unselective, aryl cation intermediate. This intermediate is trapped competitively by weakly basic nucleophiles in the interfacial region of aggregates such as micelles and other association colloids. Product yields, analyzed by HPLC with UV detection, are used to estimate, simultaneously, the interfacial concentrations of a number of different nucleophiles, including water, that are commonly found at the surfaces of biomembranes and in many commercial products. Two applications of the method are discussed. First, we show that the interfacial concentrations of X − ( X=Br, Cl) increase steadily with increasing cetyltrimethylammonium halide (CTA X) and tetramethylammonium halide (TMA X) concentrations and that the interfacial concentrations of these counterions increase continuously with their aqueous phase concentrations at a constant degree of micelle ionization. Interfacial Br − and Cl − concentrations also show marked increases at their respective sphere-to-rod transitions. This steady increase in interfacial counterion concentration with increasing aqueous counterion concentration contradicts a basic assumption of the pseudophase ion exchange (PIE) model of chemical reactivity in aggregates, i.e. that the total concentrations of ions at aggregate interfaces is constant and independent of the amphiphile and salt concentrations. The consequences for the PIE model are discussed. Second, the chemical trapping reaction is used to estimate: (a) distributions of terminal OH groups of non-ionic amphiphiles in mixed non-ionic micelles composed of amphiphiles with different lengths of oligoethylene oxide chains and (b) hydration numbers of the inner layers of interfacial region next to the hydrocarbon core in these mixed micelles. Terminal OH groups distributions are well fitted by a radial one-dimensional random walk model. The average hydration number for the inner layers at 40°C is about 3, in agreement with estimates from NMR water (D 2O) self-diffusion measurements and with the hydration number of 3 for aqueous solutions of polyethylene oxide. The results suggest that the hydration states of the ethylene oxide (EO) units near the micellar core are near their minimum value. Recent and potential applications of the chemical trapping method are briefly discussed.

Composition of mixed anionic/nonionic surfactant micelles

AbstractA proposed method of determining the composition of mixed micelles in equilibrium with monomer of known composition is described. The systems were sodium dodecyl sulfate‐polyoxyethylene 23 lauryl ether (Brij 35) in water and in 0.1 M sodium chloride solution at 25°C. This technique applies the Gibbs‐Duhem equation to the mixed micelle, which is treated as a pseudophase. This proposed methodology which needs only critical micelle concentration data as a function of monomer composition, is applied to an anionic/nonionic surfactant pair. The calculated monomer‐micelle equilibrium is found to be very similar to the much‐used regular solution for nonideal systems.

Mixed micelles of some anionic‐anionic, cationic‐cationic, and ionic‐nonionic surfactants in aqueous media

AbstractCritical micelle concentrations (CMC) were obtained from tensiometric studies on several binary surfactant mixtures (anionic‐anionic, cationic‐cationic, anionic‐nonionic, and cationic‐nonionic) in water at different mole fractions (0–1). The composition of mixed micelles and the interaction parameter β, evaluated from the CMC data for different systems using Rubingh's theory, are discussed. Marked interaction is observed for ionic‐nonionic systems, whereas it is weak in the case of similarly charged surfactants. The influence of counterion valence in the formation of mixed micelles was investigated, and results suggest that in similarly charged surfactant mixtures, the degree of counterion binding does have a major role in deciding the extent of interactions. Salt addition reveals a weakening of interactions in ionic‐nonionic systems, and this is attributed to head group charge neutralization and dehydration of the ethylene oxide units of the nonionic surfactants. Cloud point and viscosity data on these systems support the observation.

Arenediazonium Salts: New Probes of the Interfacial Compositions of Association Colloids. 5.1−4 Determination of Hydration Numbers and Radial Distributions of Terminal Hydroxyl Groups in Mixed Nonionic CmEn Micelles by Chemical Trapping

An aggregate-bound long tail arenediazonium ion, 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+, was used as a chemical trapping reagent to estimate the hydration state and terminal hydroxyl, OH, group distributions within the interfacial region of sets of binary mixed nonionic micelles of oligooxyethylene alkyl monoether, CmEn, surfactants: C10E4/C12E6, C10E4/C16E8, C10E5/C16E8, C12E6/C16E8, and C10E5/C12E5. 16-ArN2+ decomposes spontaneously to generate an aryl cation that is trapped by water and by the terminal OH groups of the two nonionic surfactants to give phenol and alkyl aryl ethers as products. Quantitative analyses of product yields by HPLC were used to estimate the hydration numbers and relative number densities of the terminal OH groups of the two surfactants in the two inner layers of the interfacial region adjacent to the micellar core. The thickness of one layer is set equal to the length of one ethylene oxide unit. The radial distributions of terminal OH groups and ethylene oxide units in the mixed micelles are described by using a simple radial one-dimensional random walk model without adjustable parameters. The calculated ratios of terminal OH groups in the two inner layers are in excellent agreement with those obtained from experimental product yields. The calculated hydration numbers of the inner two layers ranged between 1.8 and 3.5, depending upon oligooxyethylene chain length, mixed micelle composition, and temperature. The average hydration number of the inner two layers is about 3 at 18 °C.

Micellization of Mixed System of Bile Salt and Nonionic Surfactant: Sodium Deoxycholate and Decanoyl-N-methylglucamide

Micelle formation was studied for mixed surfactant systems of a nonionic surfactant, nonanoyl-N-methylglucamine (MEGA-10) with a bile salt, sodium deoxycholate (NaDC), in water at 30°C. NaDC and MEGA-10 are quite different from each other in terms not only of hydrophobic but also of hydrophilic groups. Surface tension of the aqueous solution (γ) at different mixing ratios was measured by the drop volume method, and critical micellization concentrations (CMCs) were determined from the γ vs CMC curves at discrete mole fractions of MEGA-10 (XMEGA) in the surfactant mixture. The CMC–XMEGAcurve showed a positive deviation from ideal mixing. The apparent interaction parameter (WR) upon micelle formation was evaluated by applying the Rubingh equations derived on the basis of the regular solution theory. The different theory developed by Motomuraet al.has been also applied. The analysis of CMC data indicated that the properties of mixed micelles are different between the ranges below and aboveXMEGA≅ 0.2 where an azeotrope exists and that the composition of mixed micelles is very similar to that of monomeric species in bulk solution.