Monomer Concentrations in Simple Micellar Systems from NMR Relaxation Times and C‐13 Chemical Shifts

The monomer concentrations for some simple micelle forming surfactants, octylammonium bromide (OTAB), sodium decyl sulphate, SDecS, and dodecyltrimethylammonium bromide (DTAB) were determined from the NMR‐paramagnetic relaxation enhancement experiment (the NMR‐PRE) on the H‐1 spin lattice relaxation times for the surfactant protons as a function of the surfactant concentration above and below the critical micelle concentration (cmc region). The NMR relaxation times, determined from the inversion recovery method, decrease steadily as the concentration of the surfactant is increased above the cmc value. A second, simple method is proposed whereby the p‐value of the surfactant in the micelle is determined from the two‐site model via extrapolation of the observed relaxation rate data to high concentrations. The p‐values and the calculated monomer concentrations are compared to each other and to those obtained using the mass‐action and phase separation models. In most cases, the monomer concentrations appear to remain constant at concentrations up to 2–3 times the cmc value, and then begin to decrease thereafter. These results are compared to the trends in monomer concentrations obtained from other NMR experiments in other surfactant systems.     

Volumetric properties of sodium perfluoroalkylcarboxylates in aqueous solutions at different temperatures

Densities and sound velocities of sodium perfluorohexanoate and sodium perfluorononanoate for different concentrations above and below the critical micelle concentration (cmc) have been obtained at different temperatures. Apparent molar volumes and compressibilities of the surfactants in the monomeric and micellar form have been estimated. The relevant results were plotted as a function of the temperature and the alkyl chain length by using previous data reported for sodium heptanoate and sodium octanoate. The expected linear behaviour in function of temperature and also alkyl chain length have been found. In order to analyze the influence of the substitution of the hydrogen by fluorine in the alkyl chain of the surfactant, the data were compared with the hydrogenated counterpart.     

A study on the protein concentration dependence of the thermodynamics of micellization

An investigation on the dependence of the thermodynamics of micellization of different surfactants such as sodium dodecyl sulfate (SDS), sodium octanoate (C8HONa), and sodium perfluorooctanoate (C8FONa) on the concentration of human serum albumin (HSA) has been realized. The critical micelle concentration (cmc) and ionisation degree of micellization, β, as a function of temperature (T), in solutions containing 0.125% and 0.250% in v/w of HSA, were estimated from conductivity data. From these results, the average number of surfactant monomers per protein molecule was calculated: higher values were found for C8HONa, the lowest value corresponded to SDS. For all the systems under study, electrostatic forces mainly drive the interaction between the surfactants and the proteins. Plots of cmc against temperature appear to follow the typical U-shaped curve with a minimum T_min. Thermodynamic functions of micellization were obtained by applying the theoretical models that best fit our experimental data, showing that the addition of HSA shows different patterns depending on the surfactant and thermodynamic quantity. Changes in the protein conformation due to the adsorption of surfactant molecules have been monitored by using UV-CD spectra. Greater changes in α-helical contents correspond with the concentrations over cmc, indicating that at low concentrations surfactants act as a structure stabilizer; meanwhile they act as a destabilizer at higher concentrations. C8HONa is the most effective reducing α-helical content, SDS is the less effective content.     

The aggregation of the sodium dodecyl sulfate –n - octanol–water system at low concentration

Micelle formation in sodium dodecyl sulfate (SDS)–n-octanol mixtures was studied by several techniques and the results were interpreted using regular solution theory for mixed-micelle formation. Octanol was considered as a nonionic surfactant. The composition of micelles at the critical micelle concentration (cmc) was computed together with the interaction parameter and the activity coefficient of the components of the micelles. The fluorescence quenching technique with pyrene was employed to obtain the SDS and octanol aggregation numbers at the cmc. The results were in agreement with similar studies on other alcohol–SDS systems. At the cmc spherical, almost fully ionized micelles formed, while at a higher concentration there was a transition to anisometric (probably rodlike) micelles which pushed sodium counterions into their Stern double layer. Mixed anisometric micelles were more ionized than pure SDS micelles. When the octanol:SDS total ratio exceeded 0.85:1, an emulsion of octanol appeared in equilibrium with the micelles. Received: 23 December 1998 Accepted in revised form: 3 March 1999     

基于光纤折射率传感原理的表面活性剂临界胶束浓度测定方法

提出并研究了一种新颖的基于光纤折射率传感原理的表面活性剂临界胶束浓度(cmc)测定方法.应用此方法测定有代表性的阴离子表面活性剂十二烷基硫酸钠(SDS)与阳离子表面活性剂十六烷基三甲基溴化铵(CTAB)在25℃下的cmc分别为8.05×10-3和9.11×10-4mol·L-1,与文献值比较,结果相当吻合.从而证实了本方法的准确性.进一步研究了各种条件对测量表面活性剂cmc的影响,结果表明温度和无机盐NaCl的加入对本方法测量的准确性影响小,证明了本方法对测试环境的要求不苛刻,适用性好.最后对本方法进行了重复性和稳定性测试,相对标准偏差(RSD)为0.17%,与预期符合,效果良好.     

SIMULTANEOUS DETERMINATION OF MICELLAR DISSOCIATION CONCENTRATION AND CRITICAL MICELLAR CONCENTRATION OF BILE SALTS BY pH MEASUREMENTS

This paper demonstrates a method of using a pH meter to determine the micellar dissociation concentration (mdc) and the. critical micellar concentration (cmc) of eight bile salt samples: sodium cholate (NaC), sodium deoxycholate (NaDC), sodium glycocholate (NaGC), sodium glycodeoxycholate (NaGDC), sodium glyco-chenodeoxycholate (NaGCDC), sodium taurocholate (NaTC), sodium taurodeoxycholate (NaTDC) and sodium taurochenodeoxycholate (NaTCDC). The experiments were performed by diluting the bile salt solutions with freshly distilled water and following the pH changes with a hydrogen ion electrode at 25°C. One break appears in most of the pH-concentration plots of the bile salt solutions, signifying mdc. However, two breaks appear for NaC and NaDC samples, signifying mdc and cmc. The mdc and cmc values are in good agreement with values determined by surface tension and turbidity methods and with data reported in the literature. The method described in this paper is quick, simple and requiring no sample purification. It is the only method which can be used to determine mdc and cmc simultaneously.     

New fluorescence method for the determination of the critical micelle concentration by photosensitive monoazacryptand derivatives

A novel fluorescence method for the determination of the critical micelle concentration (cmc) is reported. The cmc values of nonionic and anionic surfactants were evaluated utilizing a photosensitive monoazacryptand-Ba2+ complex, whose fluorescence intensity is sensitively changed by environmental conditions based on the photoinduced electron transfer (PET) mechanism as a fluorescent probe (PET method). Based on a comparison of the cmc values obtained by the PET method versus those obtained by conventional fluorescence-based methods as well as the values reported in the literature, one can conclude that the PET method is useful for the cmc determination. In particular, the PET method was more effective for the cmc determination of nonionic surfactants with very low cmc values (< 10(-5) M) than any other fluorescence-based method. In the cases of anionic surfactants, the PET method revealed the formation of the premicellar aggregates comprised of surfactant molecules and fluorescent probes below the cmc. Moreover, the hydrophobicity around the monoazacryptand-Ba2+ complex incorporated into various nonionic surfactant micelles was evaluated by this PET method.     

Effects of Cs and Na ions on the interfacial properties of dodecyl sulfate solutions

The competitive binding of counterions to anionic dodecyl sulfate ions in aqueous solutions of cesium dodecyl sulfate (CsDS) and sodium dodecyl sulfate (SDS) mixtures, which significantly influences the critical micelle concentration (cmc) and surface (or interfacial) tension of surfactant solutions, was investigated. The cmc and degree of counterion binding were obtained through electrical conductivity measurements. The curve of cmc versus the mole fraction of CsDS in the surfactant mixture was simulated by Rubingh's equations, which enabled us to estimate the interaction parameter in micelles (W _R) based on the regular solution approximation. The curve-fitting exhibited a slightly negative value (W _R=−0.1), indicating that the mixing (SDS+CsDS) enhances micelle formation owing to a greater interaction between surfactant molecules and counterions than in pure systems (SDS). On going from SDS, SDS:CsDS(75:25), SDS:CsDS(50:50), SDS:CsDS(25:75) to CsDS, interfacial tension at the hexadecane/surfactant-solution interface showed a negative deviation from the mixing rule (interaction parameter in adsorbed film W _A=−0.38), indicating the replacement of Na⁺ bound to anionic dodecyl sulfate by Cs⁺ ions owing to the stronger interaction between the Cs⁺ and the dodecyl sulfate ions. Droplet sizes of emulsion formed with hexadecane and aqueous dodecyl sulfate solutions were investigated using the light scattering spectrophotometer. The higher binding capacity of Cs⁺, having a smaller hydrated ionic size than Na⁺, also resulted in a negative deviation in emulsion droplet size in mixed systems. Received: 10 May 2000/Accepted: 11 August 2000     

Determination of the second critical micelle concentration of benzyldimethyltridecylazanium chloride in aqueous solution by acoustic and conductometric measurements

Density, speed of sound, and conductivity of benzyldimethyltridecylazanium chloride as a cationic surfactant in aqueous solutions have been measured as a function of concentration at atmospheric pressure and at five temperatures (293.15, 298.15, 303.15, 308.15, and 313.15) K. Isentropic compressibility values have also been calculated from the experimental density and speed of sound results. The critical micelle concentration (cmc) values of investigated cationic surfactant were evaluated by using conductivity measurements. The speed of sound, isentropic compressibility and also the conductivity values of the solutions have been employed to determine the second critical micelle concentrations (2nd cmc). The temperature dependence of the speed of sound and isentropic compressibility is shown to be sensitive to the aggregation process. The 2nd cmc values of the surfactant obtained at different temperatures by conductivity, speeds of sound and isentropic compressibility data are in agreement with each other.     

Effect of terbium(III) chloride on the micellization properties of sodium decyl- and dodecyl-sulfate solutions

The effect of TbCl3 on the aggregation processes of the anionic surfactants sodium decyl sulfate (SDeS) and sodium dodecyl sulfate (SDS) has been investigated. Electrical conductivity data, combined with Tb(III) luminescence measurements suggest that the formation of micelles involving TbCl3 and SDS occurs at concentrations below the critical micelle concentration (cmc) of the pure surfactants; the formation of these mixed aggregates was also monitored by light scattering, which indicates that the addition of TbCl3 to surfactant concentration at values below the pure surfactant cmc results in a much greater light scattering than that found with pure sodium alkylsulfate surfactant micelles. This phenomenon is dependent upon the alkyl chain length of the surfactant. With Tb(III)/DS-, complexes are formed with a cation/anion binding ratio varying from 3 to 6, which depends upon the initial concentration of Tb(III). This suggests that the majority of the cation hydration water molecules can be exchanged by the anionic surfactant. When the carbon chain length decreases, interactions between surfactant and Tb(III) also decrease, alterations in conductivity and fluorescence data are not so significant and, consequently, no binding ratio can be detected even if existing. The surfactant micellization is dependent on the presence of electrolyte in solution with apparent cmc being lower than the corresponding cmc value of pure SDS.     

Properties of Surface and Micelle in a pH‐Mediated Ternary Surfactant Mixture in Salt Solution

We have investigated the mixing behavoir of a pH‐mediated ternary surfactant mixture at constant ratio of dodecyldimethylamine oxide (DDAO) and Triton X‐100 (9:1). From the equilibrium surface tension measurements at different pHs, the critical micelle concentration (cmc) data were obtained as functions of the pH. Values of the cmc and composition of the micelles were predicted using the regular solution approximation. To some extent, the experimental cmc values agree with the predicted cmc. The average degree of ionization of dodecyldimethylamine oxide in the mixed surfactant systems was estimated using potentiometric titrations. The surface electric potential of the micelles (Ψ_o) was determined using two methods, one by hydrogen ion titration and the other by the dissociation constants of an acid‐base indicator. In a high degree of ionization of DDAO in the micelles phase (a_m), Ψ_o estimated from acid‐base indicator is much higher than that from hydrogen ion titration. In the protonated dodecyldimethylamine oxide/TX‐100 binary surfactant system, Ψ_o estimated from hydrogen ion titration was as high as 89 mV. The micellar aggregation numbers evaluated by the steady‐state fluorescence probe method increase with pH except at pH=5.03. At pH=5.03, the maximum micelle aggregation number was observed.     

The effect of surfactant charge on micellization behavior

 The influence of the surfactant charge on micellization was studied on sodium n-dodecane-phosphonates, varying the charge of the surfactant ion from 0 to 2. The effects on the cmc, hydrolysis, micelle composition, and micelle ionization degree were studied. The increase of one e unit in the charge of the surfactant species reduces the cmc by about one order of magnitude. Received: 18 November 1996 Accepted: 5 March 1997     

A Viscometric and Conductometric Investigation of the Micellar and Solution Properties of Two-Headed Surfactant Systems, the Disodium 4-Alkyl-3-sulfonatosuccinates

A family of two-headed surfactants, the disodium 4-alkyl-3-sulfonatosuccinates, has been prepared by reacting maleic anhydride with the appropriate chain-length alcohol and subsequent addition of sodium bisulfite to the corresponding monoester. The properties of the micelles formed by these compounds in aqueous solution (aggregation numbers, degrees of counterion binding, and the cmc values) have been investigated as a function of temperature and surfactant chain length using viscosity, density, and conductance measurements. The critical micelle concentrations (cmc's) and the aggregation numbers appear to indicate that, in agreement with the earlier literature on other two-headed surfactants systems, these amphiphiles have higher cmc and lower aggregation numbers when compared to single-headed surfactants of comparable chain length. In addition, viscosity B coefficients and the thermodynamic parameters of activation of viscous flow have been determined. These results are interpreted in terms of the structure-making or -breaking properties of the surfactant amphiphiles below the cmc region. Finally, the thermodynamic properties of micelle formation have been estimated from the dependence of the cmc on the absolute temperature according to the charged pseudo-phase separation model of micelle formation. All these results are discussed in terms of how the addition of the second charged surfactant headgroup alters the micellar and solution properties of two-headed surfactants vs. their single-headed counterparts.     

Estimation of degree of counterion binding and thermodynamic parameters of ionic surfactants from cloud point measurements by using triblock polymer as probe

Cloud point (C(P)) was measured for ternary mixtures of different ionic surfactants such as sodium dodecyl sulfate (SDS), dodecyltrimethylammonium bromide (DTAB), and dimethylene bis(dodecyldimethylammonium bromide) (12-2-12) plus triblock polymer (TBP) ((PEO)(2)(PPO)(15.5)(PEO)(2)) plus water, keeping the concentration of TBP constant and varying the surfactant concentration from pre- to postmicellar regions. These experiments were also performed in the presence of different fixed amounts of NaBr to evaluate the salt effect on the clouding behavior of these ternary mixtures. The C(P) value of TBP exhibits a drastic change at the cmc of each surfactant. The cmc values thus obtained both in the absence and in the presence of NaBr were used to evaluate counterion binding (beta) with the Corrin-Harkins method. beta values were also used to evaluate the thermodynamic parameters of these ionic surfactants. The results suggest that the beta values evaluated using this method, especially at low [TBP], are in good agreement with those reported in the literature.     

Conductometric study of the mixing behaviour of sodium benzoate and salicylate with an anionic surfactant sodium lauroylsarcosinate

Effects of two anionic hydrotropes – sodium benzoate (NaBz) and sodium salicylate (NaSal) – on the mixed-micelle formation with an amino-acid-based surfactant – sodium lauroylsarcosinate (SLS) – in water were investigated by the conductometric method. Specific conductivity was measured for SLS/NaBz/water and SLS/NaSal/water systems to determine the critical micelle concentration (cmc). Using the regular solution theory for non-ideal mixing, the pairwise interaction parameter, β₁₂, and micellar composition, χ, were estimated in the mixed micelle. The cmc values of the surfactant–hydrotropes mixtures were generally lower than those predicted from the ideal mixing theory. The β₁₂ values are generally negative for the two systems at all mole fractions with an average value of ?2.83 for the SLS/NaBz and ?3.31 for SLS/NaSal systems, respectively, indicative of a strong attractive interaction between the SLS/NaBz and SLS/NaSal mixed micelle. The calculated thermodynamic parameters of micellisation all indicated spontaneity in mixed-micelle formation for the systems studied.     

Surfactant Effect on Kinetic of Reaction of Some Sulphonamides with p‐Dimethylaminobenzaldehyde: Surfactant‐Modified Determination of Sulphonamides in Aqueous Solution

Kinetics of condensation reactions of six sulpha drugs (I‐VI) with p‐dimethylaminobenzaldehyde (DAB) in a weakly acidic EthOH/H₂O solution have been studied spectrophotometrically. The reaction was found to be first order with respect to DAB and zero order with respect to sulphonamide. The rate constants, activation energies, and other related thermodynamic functions have been determined. The effect of the presence of anionic surfactant sodium dodecyl sulphate (SDS) on the kinetics of this reaction in aqueous solution has been investigated. The observed rate constants increase with increasing the amount of SDS except for those of sodium sulphacetamide (VI). The surfactant molecules enhance the reaction rates (14–113 times) in concentrations less than critical micellar concentration (cmc). A developed spectrophotometric method for determining sulphonamides in aqueous solution by their reactions with an excess of DAB in the presence of SDS and HCl (pH = 2) at a wavelength of 447 nm has been introduced. Microgram amounts of sulphonamides can be estimated with accuracy better than ± 1.5% and reproducibility less than ± 0.064%. The results of application to sulphonamides in pure form indicate that the presented method is simple, sensitive, precise, accurate, and comparable to the colorimetric Bratton‐Marshall standard procedure. The effect of interferences and application of the presented method to two pharmaceutical preparations have been investigated.     

Aggregation Number of Ionic Surfactants and its Application for Alkyltrimethyl Ammonium Bromides and Sodium Tetradecyl Sulfate by Potentiometric Technique

A potentiometric technique based on surfactant ion selective electrode has been used for various cationic and anionic surfactants. The data obtained contain m ₁ (surfactant monomer concentration); m ₂ (free counterion concentration) and α (degree of dissociation of micelle) were used for determination of aggregation number at and above cmc (critical micelle concentration). Data fitting show a relationship between aggregation number with such parameters. The correlation equation obtained shows that size of ionic micelle vary sharply after cmc. Also, the equation obtained shows size of micelle growth with increase in counterion concentration.     

Influence of β‐Cyclodextrin on the Mixed Micellization Process of Sodium Dodecyl Sulfate and Sodium Lauroyl Sarcosine and Formation of Inclusion Complexes

In this work, we have studied the influence of different concentrations of β‐Cyclodextrin (β‐CD) on the mixed micellization of anionic surfactants sodium dodecyl sulfate (SDS) and sodium lauroyl sarcosine (SLAS) at different SDS mole fractions (α_SDS). From conductivity data, the critical micellar concentration (cmc), the equivalent ionic conductivities of the monomeric species (Λ_m), the associated species (Λ_assc) and the micelle (Λ_mic), the degree of counterion dissociation (α) in the presence of β‐CD were evaluated from the slope of the conductivity versus concentration plots for the pure and binary mixtures. The apparent cmc of the surfactants vary linearly with the β‐CD concentrations. From the dependence of cmc of the surfactants on β‐CD concentration, we have deduced the association constant (K) of surfactant‐β‐CD inclusion complexes assuming 1∶1 stoichiometry. Theories of Clint, Regular solution, and Motomura's have been used for the evaluation of ideality or nonideality of the mixed system. Mixed micelles were found to be rich in SDS content at the cmc in the presence and the absence of β‐CD. The cmc values have been used to evaluate the transfer of standard free energy of micelles (ΔG⁰ _M,tr) from the aqueous medium to additive medium.     

A new method for simultaneous estimation of micellization parameters from conductometric data

A simple method for determination of the counterion binding parameter (alpha) and aggregation number (N) from conductivity data is proposed. The method is based on fitting the values of the first derivative of conductivity (kappa) versus total surfactant concentration (c(t)) function according to the equation derived from the mass action model (MAM) by using different conductivity models. Sodium dodecylsulphate (SDS) and dodecyltrimethylammonium bromide (DTAB) were chosen for validation of the proposed method. It was shown that the method gives a fairly accurate values for micellisation parameters of SDS (N=51-64, alpha=0.74-0.75) and DTAB (N=56-62, alpha=0.77-0.79), both in good agreement with the literature data. In addition, application of the proposed method does not require the value of the critical micelle concentration (cmc).     

Effect of ethanol on the interaction between poly(vinylpyrrolidone) and sodium dodecyl sulfate

The effect of ethanol on the interaction between the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic polymer poly(vinylpyrrolidone) (PVP) has been investigated using a range of techniques including surface tension, fluorescence, electron paramagnetic resonance (EPR), small-angle neutron scattering (SANS), and viscosity. Surface tension and fluorescence studies show that the critical micelle concentration (cmc) of the surfactant decreases to a minimum value around 15 wt % ethanol; that is, it follows the cosurfactant effect. However, in the presence of PVP, the onset of the interaction, denoted cmc(1), between the surfactant and the polymer is considerably less dependent on ethanol concentration. The saturation point, cmc(2), however, reflects the behavior of the cmc in that it decreases upon addition of ethanol. This results in a decrease in the amount of surfactant bound to the polymer [C(bound) = cmc(2) - cmc] at saturation. The viscosity of simple PVP solutions depends on ethanol concentration, but since SANS studies show that ethanol has no effect on the polymer conformation, the changes observed in the viscosity reflect the viscosity of the background solvent. There are significant increases in bulk viscosity when the surfactant is added, and these have been correlated with the polymer conformation extracted from an analysis of the SANS data and with the amount of polymer adsorbed at the micelle surface. Competition between ethanol and PVP to occupy the surfactant headgroup region exists; at low ethanol concentration, the PVP displaces the ethanol and the PVP/SDS complex resembles that formed in the absence of the ethanol. At higher ethanol contents, the polymer does not bind to the ethanol-rich micelle surface.