The aqueous cationic system sodium undecenoate-dodecyltrimethylammonium bromide at low concentration

The aqueous sodium undecenoate (SUD) –dodecyltrimethylammonium bromide (DTAB) catanionic system was studied at low concentration. The system did not precipitate, even at a 1:1 SUD:DTAB proportion, but showed the formation of a coacervate in a range of surfactant mixture compositions. Micelles have a preferential composition of 0.37 mole fraction of SUD. This behavior is attributed to the presence of the double bond at the distal extreme of the SUD molecule, which can form hydrogen bonds with water. Consequently, the –CH=CH₂ group is situated at the interface between the hydrocarbon micelle core and water, reducing the interfacial free energy. Structural computations demonstrate that the mentioned SUD proportion produces complete coverage of the micelle surface by the double bonds.     

Electrokinetic properties of mixed solutions of dodecyldimethylamine oxide and sodium dodecyl sulfate: specific adsorption effects of small ions

Electrophoretic light-scattering measurements and potentiometric titrations were carried out on aqueous mixtures of dodecyldimethylamine oxide and sodium dodecyl sulfate. The electrophoretic mobility and the surface charge density of the micelles were always negative, ranging from –2.5 to –3.1×10^–4 cm²V^–1s^–1, and –0.033 to –0.045 cm^–2, respectively, for all surfactant mixing ratios, indicating the specific adsorption of Cl^–, in addition to Na⁺, on micelles. The solution pH, as well as the aggregation number previously reported, displayed maxima at intermediate surfactant mole fractions, that is, the non-ideal behavior. The fractional adsorption of Na⁺ per surfactant molecule in the micelles increased gradually with mixing fraction up to 0.82 atX=[SDS]/([SDS]+[C₁₂DAO])=1, while that of Cl^– decreased from 0.25 atX=0 to zero atX>0.4.     

Aqueous sodium oleate–sodium dehydrocholate mixtures at low concentration

The aqueous mixed system sodium dehydrocholate (NaDHC)–sodium oleate (NaOL) was studied by several methods to determine the influence of the hydrophobic structure of both surfactants in the mixed micellization and the formation of the mixed monolayer adsorbed at the air–water interface. The molecular area at the critical micelle concentration in pure surfactant solutions suggests that the adsorbed oleate chain was folded to allow the double bond in the middle of the molecule to remain in contact with water, and that the NaDHC molecule was situated with its plane laying parallel to the water surface, allowing the three carbonyl groups in the hydrocarbon backbone to form hydrogen bonds with water. The interaction was repulsive at the surface, and in the mixed monolayer some molecules must move away the less hydrophilic groups from water (double bond of NaOL, carbonyl groups of NaDHC). The interaction in mixed micelles was strongly attractive, showing a preferential composition roughly equimolar. The hydrolysis in mixed micelles was augmented in comparison with pure surfactants systems, which could be explained by assuming the existence of a more hydrophobic mixed micelle core. The mixed micelle degree of ionization was below that of the pure micelles, thus indicating a high surface charge density.     

Effects of added dotab on thecmc and enthalpy of micelle formation at 298.2 K for CTAB(aq)

In a titration calorimetric study an aqueous solution held in a syringe and containing hexadecyltrimethylammonium bromide (CTAB; 15.4×10^–3 mol dm^–3) is injected in aliquots (5×l0^–3 dm³) into a sample cell containing initially water. Analysis of the data shows that thecmc equals 0.97×l0^–3 dm^–3 and the enthalpy of micelle formation equals –10.3 kJ mol^–1. When the solution in the syringe is replaced by a mixed surfactant solution, CTAB+dodecyltrimethylammonium bromide, at the same total concentration of surfactant, thecmc of CTAB decreases gradually with increasing mole fraction of DOTAB but the enthalpy of CTAB micelle formation is hardly affected. We conclude, therefore, that incorporation of DOTAB monomers into the CTAB micelles stabilizes entropically the CTAB micelles.We thank EPSRC for their support; the Commonwealth Scholarship Commission for an award to MCSS and the Royal Society for a grant awarded to PMC for the purchase of the Titration Microcalorimeter.     

Micelle formation of a cationic surfactant in the presence of 1,n-alkanediol and the miscibility of alcohols in micelles

Micelle formation of dodecyltrimethylammonium bromide (DTAB) was examined in the presence of α,ω-alkanediols applying conductivity measurements. Octanediol and hexanediol promoted the formation of mixed micelles of DTAB and the alcohol, but butanediol interfered with micellization. Analysis of the critical micelle concentration (cmc) based on the lattice model for mixed solution with the Bragg–Williams approximation indicated an unfavorable interaction between alcohol and water and a favorable interaction between the alcohol and surfactant, with the exception of butanediol. The exchange energy between alcohol and water was 0.5kT higher for alkanediol (C_2n(OH)₂) than for the corresponding regular alcohol (C_nOH), which is believed to have resulted from the smaller mixing entropy for the alkanediol than for the corresponding regular alcohol. It was inferred from the analysis that the cmc increase for C₄(OH)₂ was caused by favorable interaction with water but unfavorable interaction with the micellar surfactant.     

Temperature effect on the viscoelastic properties of solutions of cylindrical mixed micelles of zwitterionic and anionic surfactants

The viscoelastic properties of semidilute mixed aqueous solutions of oleyl amidopropylbetaine and sodium dodecylbenzene sulfonate are studied in the temperature range of 20–40°C. It is shown that, at 20°C, the solution viscosity can be as high as 390 (Pa s), which is related to the formation of a network of entanglements of long cylindrical micelles of surfactants. It is revealed that, upon heating, the viscosity and relaxation time of the system decrease, while the contour length of cylindrical micelles decreases consider-ably. It is shown that this susceptibility of viscoelastic surfactant solutions to temperature is due to the low energy of break, which is much lower than the energy of covalent bonds.     

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 (C₁₂E₁₂) and dodecyl pentadecapolyethylene glycol ether (C₁₂E₁₅) 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 (f₁ and f₂) of metal dodecyl sulphates (MDS)/C₁₂E_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.     

The low concentration aggregation of sodium oleate–sodium linoleate aqueous mixtures

Sodium oleate (NaOL, C₁₈H₃₃O₂Na)–sodium linoleate (NaLin, C₁₈H₃₁O₂Na) mixtures were studied in the micellar and in the air/water interface states at 298.15 K. Three aggregation steps were found: a premicellar aggregation, the critical micelle concentration (CMC), and a structural change of micelles. Micelles, both at the CMC and at the structural change concentration, are richer in oleate than the overall mixture composition. Micelles are strongly non-ideal and the interaction is repulsive. The non-ideal behavior and the dependence of the micelle ionization degree with micelle composition are explained on the basis of the interaction of the π electrons of the surfactants’ chains with water at the hydrocarbon/water micellar interface. The air/solution adsorbed monolayer is also non-ideal, but the interaction is attractive and there is a preferential composition with a mole fraction of sodium oleate of about 0.7. The surface pseudophase behaves as if oleate were the solvent and linoleate a strongly soluble solute. This behavior and the dependence of the average area per adsorbed molecule were explained on the basis of the interaction of the double bonds with water. At the air/solution interface, the linoleate molecule area was similar to that of a heterogemini surfactant having a spacer with seven carbon atoms.     

Mixed micellar systems of octyl β,d-glucopyranoside with a nonionic surfactant and a water-soluble polymer

We have made a comparative study between the micellar regions of the octyl -d-glucoside (OG)–tetraethylene glycol monododecyl ether and the OG–poly(ethylene glycol) 20,000 systems by means of surface tension and viscosimetric measurements. The incorporation of the tetraethylene glycol monododecyl ether nonionic surfactant in the OG micelles decreases the critical micelle concentration, whereas the presence of polymer increases it. The nonionic surfactant mixture exhibits nonideal mixing behaviour. The data fit to Rubinghs treatment with a value of –5.1, which implies a modest attraction between both surfactants. The surfactant–poly(ethylene glycol) 20,000 system does not form mixed micelles. The incorporation of polymer increases the critical micelle concentration of the surfactant. The viscosity for the surfactant–polymer system is higher than that for the pure polymer, demonstrating a surfactant-induced structuring.     

Excess free energy,enthalpy and entropy of surfactant-surfactant mixed micelle formation

Enthalpies of dilution and osmotic coefficients of sodium decylsulfate (NaDeS)-dodecyldimethylamine oxide (DDAO) mixtures in water were determined at 298 and 310 K, respectively. From the enthalpies of dilution, the apparent and then the partial molar relative enthalpies of the surfactant mixtures were calculated. From the osmotic coefficients, calculated at 298 K, the non-ideal free energies were derived. The latter were combined with the partial molar relative enthalpies to obtain the non-ideal entropies. From the apparent molar properties, using a previously reported approach, the excess thermodynamic properties for the surfactant-surfactant mixed micelle formation in water were evaluated as functions of the mixture composition at some total micellized concentration. In the whole range of the mixture composition, the excess free energy is negative, indicating that the mixed micelle formation is favoured with respect to that of pure micelles. This process is governed by the enthalpy and/or the entropy, depending on the mixture composition. The effect of the alkyl chain length was also studied by comparing the present results to those of the sodium dodecylsulfate-DDAO mixture.     

Nonideal mixing of dodecyltrimethylammonium halides and nonionic surfactant in adsorbed films and micelles

The ion–dipole interaction between dodecyltrimethylammonium cations and nonionic surfactant molecules in adsorbed films and micelles was investigated by concentrating on the difference in the degree of counterion binding by employing dodecyltrimethylammonium chloride (DTAC)–octyl methyl sulfoxide (OMS) and dodecyltrimethylammonium bromide (DTAB)–OMS mixtures. The phase diagrams of adsorption and micelle formation were constructed and then the nonideal mixing of different species of surfactants was demonstrated in terms of the excess Gibbs free energies of adsorption and micelle formation, and the surface excess areas. Furthermore the dependence of them on the counterion was clearly shown. All these results were found to support our previous view that the direct interaction between surfactant cation and the dipole of the hydrophilic part of a nonionic surfactant is essential in cationic-nonionic surfactant mixtures, i.e., the DTAC system with a lower counterion biding has more negative excess thermodynamic quantities than the DTAB system with a higher one.     

Variegated micelle surfaces: correlating the microstructure of mixed surfactant micelles with bulk solution properties

Electron paramagnetic resonance, viscosity, and small-angle neutron scattering (SANS) measurements have been used to study the interaction of mixed anionic/nonionic surfactant micelles with the polyampholytic protein gelatin. Sodium dodecyl sulfate (SDS) and the nonionic surfactant dodecylmalono-bis-N-methylglucamide (C12BNMG) were chosen as "interacting" and "noninteracting" surfactants, respectively; SDS micelles bind strongly to gelatin but C12BNMG micelles do not. Further, the two surfactants interact synergistically in the absence of the gelatin. The effects of total surfactant concentration and surfactant mole fraction have been investigated. Previous work (Griffiths et al. Langmuir 2000, 16 (26), 9983-9990) has shown that above a critical solution mole fraction, mixed micelles bind to gelatin. This critical mole fraction corresponds to a micelle surface that has no displaceable water (Griffiths et al. J. Phys. Chem. B 2001, 105 (31), 7465). On binding of the mixed micelle, the bulk solution viscosity increases, with the viscosity-surfactant concentration behavior being strongly dependent on the solution surfactant mole fraction. The viscosity at a stoichiometry of approximately one micelle per gelatin molecule observed in SDS-rich mixtures scales with the surface area of the micelle occupied by the interacting surfactant, SDS. Below the critical solution mole fraction, there is no significant increase in viscosity with increasing surfactant concentration. Further, the SANS behavior of the gelatin/mixed surfactant systems below the critical micelle mole fraction can be described as a simple summation of those arising from the separate gelatin and binary mixed surfactant micelles. By contrast, for systems above the critical micelle mole fraction, the SANS data cannot be described by such a simple approach. No signature from any unperturbed gelatin could be detected in the gelatin/mixed surfactant system. The gelatin scattering is very similar in form to the surfactant scattering, confirming the widely accepted picture that the polymer "wraps" around the micelle surface. The gelatin scattering in the presence of deuterated surfactants is insensitive to the micelle composition provided the composition is above the critical value, suggesting that the viscosity enhancement observed arises from the number and strength of the micelle-polymer contact points rather than the gelatin conformation per se.     

Shape and Structure Formation of Mixed Nonionic–Anionic Surfactant Micelles

Aqueous solutions of a nonionic surfactant (either Tween20 or BrijL23) and an anionic surfactant (sodium dodecyl sulfate, SDS) are investigated, using small-angle neutron scattering (SANS). SANS spectra are analysed by using a core-shell model to describe the form factor of self-assembled surfactant micelles; the intermicellar interactions are modelled by using a hard-sphere Percus–Yevick (HS-PY) or a rescaled mean spherical approximation (RMSA) structure factor. Choosing these specific nonionic surfactants allows for comparison of the effect of branched (Tween20) and linear (BrijL23) surfactant headgroups, both constituted of poly-ethylene oxide (PEO) groups. The nonionic–anionic surfactant mixtures are studied at various concentrations up to highly concentrated samples (ϕ ≲ 0.45) and various mixing ratios, from pure nonionic to pure anionic surfactant solutions. The scattering data reveal the formation of mixed micelles already at concentrations below the critical micelle concentration of SDS. At higher volume fractions, excluded volume effects dominate the intermicellar structuring, even for charged micelles. In consequence, at high volume fractions, the intermicellar structuring is the same for charged and uncharged micelles. At all mixing ratios, almost spherical mixed micelles form. This offers the opportunity to create a system of colloidal particles with a variable surface charge. This excludes only roughly equimolar mixing ratios (X≈ 0.4–0.6) at which the micelles significantly increase in size and ellipticity due to specific sulfate–EO interactions.     

Solubilized states of water and formation of reversed micelles in polyoxyethylated nonylphenyl ethers in cyclohexane media

The limiting amounts of solubilization of water in the concentration range 0–200 mmol kg^–1 polyoxyethylene (6 and 10) nonylphenyl ethers (NP-6 and NP-10)/cyclohexane solutions were measured by the Karl-Fischer method at 25°C. Utilizing NMR, ESR, and near infrared spectroscopic techniques, the states of water in the solubilization region obtained were examined as a function of the molar ratio of water to surfactants at various surfactant concentrations. In NP-6 system, three solubilized states of water, i.e., water interacted directly with the oxyethylene moiety of surfactant, bound water next to the hydrated oxyethylene moiety, and bulk-like water were built up. However, in NP-10 system only directly interacted water was present. It was found that the directly interacted water is distributed between monomeric surfactants and reversed micelles, and others are distributed to swollen micelles and W/O microemulsions. In addition, the minimum amounts of water required to form reversed and swollen micelles were calculated.     

Adsorption and Micellization in Solutions of Dodecylpyridinium Bromide–Nonionic Surfactant Mixtures

Dependences of the surface tension of aqueous solutions of cationic (dodecylpyridinium bromide) and nonionic (Tween 80, Triton X-100) surfactants and their mixtures on total surfactant concentration and solution composition were studied. The values of critical micellization concentration (CMC) and excess free energy of adsorption were determined from tensiometric measurements. Based on Rubingh–Rosen model (approximation of the theory of regular solutions), the compositions of micelles and adsorption layers at the solution–air interface as well as parameters of interaction between the molecules of cationic and nonionic surfactants were calculated for the systems indicated above. It was established that, in the case of surfactant mixtures with considerable difference in the CMCs, the micelles of individual surfactant with lower CMC value are formed. The effect of negative deviation from the ideality during the adsorption of surfactants from mixed solutions at the solution–air interface was disclosed. It was shown that the interaction energy depends significantly on the composition of mixed systems.     

The aggregation ofn-dodecanephosphonic acid in water

Several techniques were employed to study the aggregation ofn-Dodecanephosphonic Acid (DPA) in water. At 22°C, the solubillity of DPA increases, probably due to the formation of small premicellar aggregates. The CMC is (5.4±2.4) ×10^–4 mol·dm^–3 and the solubility reaches the CMC value at 26°C. At 30°C and at a concentration of about 9×10^–3 mol·dm^–3, a lamellar mesophase appears. Both micelles and liquid crystal lamellae are almost uncharged. Their polar heads have strong hydrogen bonds between them. The ionization of DPA molecules in micelles and mesophase structures is strongly reduced in comparison with monomerically dissolved molecules.     

Effect of aging time and salt on the viscosity behaviour of a Gemini cationic surfactant

Rheological properties of micellar solutions of a cationic Gemini surfactant, 2-hydroxypropyl-1,3-bis (dodecyldimethylammonium chloride), are studied as a function of aging time and salt addition. The results show that the self-aggregating behaviour in solution changes as a factor of time, probably due to intermolecular hydrogen bonds. The viscosity of the solution undergoes a series of visible changes so that the solution changes from a flow state to highly viscoelastic state, and finally, to a transparent solid, with a corresponding 4–6-fold increase in zero shear state viscosity. Rheology and freeze fracture transmission electron microscopy (FF-TEM) measurements show rod-like micelles at the beginning, which then change to wormlike micelles, and eventually to a quasi-gel-like network. Addition of an inorganic salt (NaCl) induces salting out, while the addition of an organic salt (NaSal) promotes micellar growth. At a fixed NaSal-to-surfactant molar ratio of 3:5, all solutions show Maxwell fluid behaviour and maximum zero-shear-rate viscosity; these trends can be attributed to the formation of a network structure between the cationic ions of the surfactant and Sal^– as the surfactant concentration increases. Crystal analysis further confirms the presence of structures linked by intermolecular hydrogen bonds.     

Charge and Potential of the Diffuse Part of the Double Layer for Sodium Dodecyl Sulfate Micelles in NaCl Solutions

A method is proposed for calculating the ψ_d potential of the diffuse part of the electrical double layer in micelles on the basis of data concerning the binding of counterions by these micelles, the composition of the intermicellar medium, and the association of surface-active ions. The charges of sodium dodecyl sulfate micelles in aqueous solutions at various concentrations of the surfactant and background electrolyte are determined by the potentiometric method. The calculated ψ_d values turned out to be much lower than the values of the electrokinetic potentials, thus allowing the conclusion that the slipping plane corresponding to the electrophoretic motion of micelles with respect to the intermicellar medium is localized in the dense part of the double layer.__________Translated from Kolloidnyi Zhurnal, Vol. 67, No. 3, 2005, pp. 410–415.Original Russian Text Copyright © 2005 by Us’yarov.     

CHARACTERIZATION OF THE SUB-MICELLAR SPECIES IN MIXED SURFACTANT SOLUTIONS I: MIXTURES OF SODIUM LAURYL SULFATE AND AN ALKYL POLYETHOXY SULFATE

The formation of micelles and the composition and concentration of sub-micellar components of surfactant solutions was studied for sodium lauryl sulfate (SLS), alkyl polyethoxy (average of 7 EO) sulfate (AEOS 7EO) and mixtures of these two anionic surfactants both above and below the critical micelle concentrations (c.m.c). Radiolabeled surfactant was added to the solutions to follow the concentration and identity of the components. To separate micelles from sub-micellar components, an Amicon microfiltration apparatus containing either a 2, 5, or 30 thousand molecular weight cut off membrane was used. Results using a hydrophobic fluorescent probe demonstrated minimal disruption of the micelles during separation. Separations for single surfactants showed breaks in the concentration curves at the c.m.c. The observed sub-micellar composition and concentration were evidence of non-ideal behavior. Concentration dependence and surfactant selectivity in micellization (the “micro c.m.c.”phenomena) is described. This previously undocumented behavior reveals that the compositions of micelles and the sub-micellar phase for mixed surfactant systems do not reflect the composition of the original solution mixture.     

Spectrophotometric study of thiocyanato complexation of cobalt(II) and nickel(II) ions in micellar solutions of a nonionic surfactant triton X-100

Complexation of cobalt(II) and nickel(II) with thiocyanate ions has been studied by precise spectrophotometry in aqueous and micellar solutions of a nonionic surfactant Triton X-100 of varying concentrations (20–100 mmol-dm^–3). With regard to cobalt(II), the formation of [Co(NCS)]⁺, [Co(NCS)₂], and [Co(NCS)₄]^2– was established. The formation constant of [Co(NCS)₄]^2–, is increased with increasing concentration of the surfactant, suggesting that the [Co(NCS)₄]^2– complex is formed in micelles. In contrast, the formation constants of [Co(NCS)]⁺ and [Co(NCS)₂] are remained practically unchanged. On the other hand, with nickel(II), the formation of sole [Ni(NCS)]⁺ and [Ni(NCS)₂] was established in both aqueous and micellar solutions examined, their formation constants being also remained unchanged. Interestingly, no higher complex was confirmed in the nickel(II) system, unlike cobalt(II). The unusual affinity of the [Co(NCS)₄]^2– complex with micelles will be discussed from thermodynamic and structural points of view.