Wormlike micelles in mixed amino acid-based anionic/nonionic surfactant systems

We present the formation of viscoelastic wormlike micelles in mixed amino acid-based anionic and nonionic surfactants in aqueous systems in the absence of salt. N-Dodecylglutamic acid (designated as LAD) has a higher Krafft temperature; however, on neutralization with alkaline amino acid l-lysine, it forms micelles and the solution behaves like a Newtonian fluid at 25 degrees C. Addition of tri(oxyethylene) monododecyl ether (C(12)EO(3)) and tri(oxyethylene) monotetradecyl ether (C(14)EO(3)) to the dilute aqueous solution of the LAD-lysine induces one-dimensional micellar growth. With increasing C(12)EO(3) or C(14)EO(3) concentration, the solution viscosity increases gradually, but after a certain concentration, the elongated micelles entangle forming a rigid network of wormlike micelles and the solution viscosity increases tremendously. Thus formed wormlike micelles show a viscoelastic character and follow the Maxwell model. Tri(oxyethylene) monohexadecyl ether (C(16)EO(3)), on the other hand, could not form wormlike micelles, although the solution viscosity increases too. The micelles become elongated; however, they do not appear to form a rigid network of wormlike micelles in the case of C(16)EO(3). Rheological measurements have shown that zero shear viscosity (eta(0)) increases with the C(12)EO(3) concentration gradually at first and then sharply, and finally decreases before phase separation. However, no such maximum in the eta(0) plot is observed with the C(14)EO(3). The eta(0) increases monotonously with the C(14)EO(3) concentration till phase separation. In studies of the effect of temperature on the wormlike micellar behavior it has been found that the eta(0) decays exponentially with temperature, following an Arrehenius behavior and at sufficiently higher temperatures the solutions follow a Newtonian behavior. The flow activation energy calculated from the slope of log eta(0) versus 1/T plot is very close to the value reported for typical wormlike micelles. Finally, we also present the effect of neutralization degree of lysine on the rheology and phase behavior. The formation of wormlike micelles is confirmed by the Maxwell model fit to the experimental rheological data and by Cole-Cole plots.     

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.     

BULK AND INTERFACIAL BEHAVIORS OF MIXED SYSTEM OF HEXADECYLPYRIDINIUM CHLORIDE AND Na+ 2-HYDROXY BENZOATE−

The micellization behavior of hexadecylpyridinium chloride usually called cetylpyridinium chloride (CPC) in presence of sodium salicylate ( NaSal) has been investigated. The surface tension, conductance and microcalorimetric methods have been employed to determine the critical micellar concentration (CMC) of CPC and NaSal mixed in different molar proportions. The interfacial area occupied by CPC in presence of NaSal, the free energy of adsorption and the energetics of micellizatton have been evaluated. The enthalpy of interaction of NaSal with CPC micelle has also been estimated from microcalorimetric measurements. The shear viscosity of the CPC-NaSal combination at equimolar proportion at different surfactant concentration and temperature, and also the shear viscosity of CPC-NaSal combination at different molarity ratios with a fixed surfactant concentration have been determined. The static and dynamic light scattering measurements of the CPC-NaSal system at different composition and in presence of NaCl have been reported. Worm-like micelles of concentration dependent dimension and intermicellar repulsive interaction have been envisaged.     

Viscoelastic micellar solutions in nonionic fluorinated surfactant systems

The formation and rheological behavior of a viscoelastic wormlike micellar solution in an aqueous solution of a nonionic fluorinated surfactant, perfluoroalkyl sulfonamide ethoxylate, of structure C8F17SO2N(C3H7)(CH2CH2O)10H was studied. Temperature-induced viscosity growth is observed even at low-surfactant concentration (approximately 1 wt %), and viscosity reaches the maximum at a temperature T(eta)-max. Upon successive increases in the temperature, the viscosity decreases, and ultimately a phase separation occurs. Small-angle X-ray scattering (SAXS) measurements confirm the presence of cylindrical aggregates at low temperature, which undergo continuous one-dimensional growth with increasing temperature, and ultimately, an indication of a slight lamellarlike structural pattern is observed, which probably comes from the formation of micellar joints or branching. Such changes in the microstructure result in a decrease in the viscosity and stress-relaxation time, while the network structure is retained; the trends in the evolution of shear modulus (Go) and relaxation time (tauR) with temperature are in agreement with this. With increased surfactant concentration, the temperature corresponding to the viscosity maximum (T eta-max) in the temperature-viscosity curve shifts to lower values, and the viscosity at temperatures below or around T eta-max increases sharply. A viscoelastic solution with Maxwellian-type dynamic rheological behavior at low-shear frequency is formed, which is typical of entangled wormlike micelles. Rheological parameters, eta(o) and Go, show scaling relationships with the surfactant concentrations with exponents slightly greater than the values predicted by the living-polymer model, but the exponent of tauR is in agreement with the theory. Dynamic light-scattering measurements indicate the presence of fast relaxation modes, associated with micelles, and medium and slow modes, associated with transient networks. The disappearance of the slow mode and the predominance of the medium mode as the temperature increases support the conclusions derived from SAXS and rheometry.     

Effect of temperature on the rheology of wormlike micelles in a mixed surfactant system

The formation and rheological behavior of a viscoelastic wormlike micellar solution in an aqueous solution of a mixed surfactant system of alkyl ethoxylate sulfate (AES), C(12)H(25)(OCH(2)CH(2))(3)OSO(-)(3)Na(+), and polyoxyethylene dodecyl ether, C(12)EO(3), and the unusual effect of temperature on the rheological behavior have been studied. Upon successive addition of C(12)EO(3) to the dilute micellar solution of AES, viscosity increases swiftly and reaches its peak where a viscoelastic solution with nearly Maxwellian behavior is formed. With the further addition of C(12)EO(3), viscosity decreases sharply, which is attributed to the formation of micellar joints. With increasing temperature, the extent of micellar growth increases and the viscosity maximum is achieved at a lower mixing fraction of C(12)EO(3), but the maximum viscosity attained by the system decreases. The evolution of relaxation time and network density of the viscoelastic network also suggests that with increasing temperature, enhanced micellar growth takes place, but an additional, faster relaxation mechanism becomes increasingly favorable at high concentrations of C(12)EO(3). These results can be explained in terms of the increase in free energy of hemispherical end-caps (end-cap energy) of the micelles with increasing temperature.     

A dually effective inorganic salt at inducing obvious viscoelastic behavior of both cationic and anionic surfactant solutions

Hydrazine nitrate (HN), an inorganic salt, was first found to have dual effects on inducing obvious viscoelasticity of both cationic and anionic surfactant solutions. It was interesting that the surfactant solutions exhibited characteristic wormlike micelle features with strong viscoelastic properties upon the addition of this inorganic salt. The rheological properties of the surfactant solutions have been measured and discussed. The apparent viscosity of the solutions showed a volcano change with an increase of the HN concentration. Correspondingly, the microstructures of the micelles in the solutions changed with the apparent viscosity. First, wormlike micelles began to form and grew with an increase of the HN concentration. Subsequently, the systems exhibited linear viscoelasticity with characteristics of a Maxwell fluid in the intermediate mass fraction range, which originated from a 3D entangled network of wormlike micelles. Finally, a transition from linear micelles to branched ones probably took place at higher HN contents. In addition, the origin of the dual effects brought by HN addition on inducing viscoelasticity in both cationic and anionic surfactant solutions was investigated.     

Rheological Properties of Anionic Surfactant Solutions in the Presence of Al3+ Counterion

Aqueous solutions of ionic surfactants with strongly binding counterions exhibit wormlike or network properties. The properties of anionic micelles of sodium dodecyltrioxyethylene sulfate (AES) in the presence of multivalent counterion Al³⁺ were investigated by dynamic rheological methods. The steady-shear viscosity and stress, the zero-shear viscosity, the complex viscosity, and the dynamic shear modulus have been determined as a function of the surfactant and salt concentrations. Some interesting and noticeable results have been obtained, which can express the micellar growth and structure. The formation of wormlike micelles or network structure in surfactant solutions becomes much easier with increasing surfactant and salt concentrations. The Cox-Merz rule and the Cole-Cole plot are not applicable perfectly to the systems studied. The nonlinear viscoelasticity and non-Newtonian behavior can be found in all solutions according to the comparison with the simple Maxwell model. The technique of freeze-fracture transmission electron microscopy (FF-TEM) was also applied to confirm the formation of these interesting structures.     

Cetylpyridinium chloride micelles as a selective fluorescence quenching solvent media for discriminating between alternant versus nonalternant polycyclic aromatic hydrocarbons

Fluorescence behavior of 41 polycyclic aromatic hydrocarbons (PAHs) dissolved in aqueous micellar cetyltrimethylammonium chloride (CTAC) solvent media and in five different cetyltrimethylammonium chloride + cetylpyridinium chloride (CPC) surfactant mixtures is reported. Experimental fluorescence measurements reveal that CPC is a selective fluorescence quenching agent for alternant PAHs. The cetylpyridinium ion effectively quenched emission intensities of the 21 alternant PAHs studied. Emission intensities of nonalternant PAHs, with a few noted exceptions, were unaffected by the presence of CPC in the mixed cationic surfactant micelles.     

Rheological behavior of viscoelastic wormlike micelles in mixed sodium dodecyl trioxyethylene sulfate–monolaurin aqueous system

Rheological behavior of viscoelastic wormlike micelles in an aqueous system of mixed sodium dodecyl trioxyethylene sulfate (SDES)–monolaurin (ML) is presented. Dilute aqueous solution of SDES has a high fluidity and follows Newtonian liquid-like behavior due to formation of small globular type of micellar structure. Addition of lipophilic nonionic cosurfactant ML to dilute or semidilute solution of SDES decreases the interfacial curvature of the aggregates favoring one dimensional micellar growth, and hence, viscosity increases. After a certain concentration of ML, the elongated micelles get entangled with each other leading to the formation of viscoelastic wormlike micelles. The viscoelastic solution follows Maxwell model of a single stress relaxation mode at low-frequency region. Further addition of ML decreases the viscosity of the solution due to formation of micellar joints in the network structure. The viscosity of the viscoelastic wormlike micelles decreases upon heating, and the system with poor viscoelastic character is observed at higher temperatures.     

Solubilization and Separation of Dicarboxylic Acids by Surfactant Micelles and Polyelectrolytes

Semi‐equilibrium dialysis (SED) and micellar enhanced ultra filtration (MEUF) methods are used to determine the extent of solubilization of water‐insoluble compounds by surfactant and polyelectrolyte. In this study, solubilization of ortho‐, meta‐ and para‐phthalic acids (OPA, MPA and TPA), 1,4‐ and 2,6‐naphthalene dicarboxylic acids (1,4‐NDCA and 2,6‐NDCA) into hexadecylpyridinium chloride (CPC), and the behavior of these acids to bind to the polyelectrolyte ionizable groups were investigated at 25 °C, using SED and MEUF methods. Polydimethyldiallylammonium chloride (PDMDAAC) is used in this study. It was found that the solubilization of organic acids decreases with increasing the solute mole fractions in micelles. Also, the best separation occurs at the lowest concentration of the phthalate ions and high concentrations of either CPC or PDMDAAC. The results support the idea of charge interaction between the anionic dicarboxylate groups and cationic surfactant or polyelectrolyte. The results also show that the presence of a second phenyl ring does not greatly affect the solubilization behavior of the acids.     

Phase behavior and microstructures of nonionic fluorocarbon surfactant in aqueous systems

The phase behavior and self-assembled structures of perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(CH2CH2O)20H (abbreviated as C8F 17EO20), a nonionic fluorocarbon surfactant in an aqueous system, has been investigated by the small-angle X-ray scattering (SAXS) technique. The C8F17EO20 forms micelles and different liquid crystal phases depending on the temperature and composition. The fluorocarbon micellar structure induced by temperature or composition change and added fluorocarbon cosurfactant has been systematically studied. The SAXS data were analyzed by the indirect Fourier transformation (IFT) and the generalized indirect Fourier transformation (GIFT) depending on the volume fraction of the surfactant and complemented by plausible model calculations. The C8F17EO20 forms spherical type micelles above critical micelle concentration (cmc) in the dilute region. The micelle tends to grow with temperature; however, the growth is not significant on changing temperature from 15-75 degrees C, which is attributed to the higher clouding temperature of the surfactant (>100 degrees C). On the other hand, the micellar structure (shape and size) is apparently unaffected by composition (1-25 wt %) at 25 degrees C. Nevertheless, addition of fluorocarbon cosurfactant of structure C8F17SO2N(C3H7)(CH2CH2O)H (abbreviated as C8F17EO1) to the semidilute solution of C8F17EO20 (25 wt %) favors micellar growth, which finally leads to the formation of viscoelastic wormlike micelles, as confirmed by rheometry and supported by SAXS. The onset sphere-to-wormlike transition in the structure of micelles in the C8F17EO20/water/C8F17EO1 system is due to the fact that the C8F17EO1 tends to go to the surfactant palisade layer so that the critical packing parameter increases due to a decrease in the effective cross-sectional area of the headgroup. As a result, spherical micelles grow into a cylinder, which after a certain concentration entangle to form a rigid network structure of wormlike micelles.     

Photoinduced reversible change of fluid viscosity

We report a reversible photoinduced fluid viscosity change. A small amount of a "photoswitchable" azobenzene-modified cationic surfactant (4-butylazobenzene-4'-(oxyethyl)trimethylammonium bromide, AZTMA) was added to a wormlike micellar solution of cetyltrimethylammonium bromide (CTAB) containing sodium salicylate (NaSal). The trans-AZTMA solution had a remarkably high viscosity as a result of the entangled network of wormlike micelles. UV light irradiation on the trans-AZTMA solution remarkably decreased the viscosity of the solution because the bulky structure of cis-AZTMA is likely to disrupt the network structure of wormlike micelles. This photoinduced viscosity change is perfectly reversible between the trans- and cis-AZTMA solutions.     

The effect of alkane tail length of CiE8 surfactants on transport to the silicone oil-water interface

The phase behavior and rheological properties of an anionic surfactant, bis(2-ethylhexyl) sulfosuccinate (AOT), mixed with a zwitterionic tetradecyldimethylamine oxide (C(14)DMAO) in aqueous solutions, were studied at different ratios, R=w(AOT)/(w(C(14)DMAO + w(AOT)). When R=1, the 6.0 wt% AOT solution is two-phase with dense vesicles as the lower phase. With an increase of C(14)DMAO fraction (decreasing R) at a total concentration of 6.0 wt%, the lower vesicle-phase (L(αv)-phase) extends to generate a single L(αv)-phase. Then the L(αv)-phase turns into a viscoelastic wormlike micellar phase and finally rod-like or spherical C(14)DMAO micelles. The wormlike micellar solutions (from R=0.3 to 0.2) are highly viscoelastic, indicating the formation of rigid network structures. The rheological properties of the viscoelastic solutions exhibit a typical Maxwell characteristic at low and intermediate oscillatory frequencies. A pronounced temperature effect on the wormlike micellar structures can be observed by rheological studies. With an increase in temperature, the samples become less structured due to shortening of the micelles. After introducing certain additives, e.g., octanol and divalent metal ions, a transition from wormlike micellar phases to birefringent L(αv)-phases was observed.     

Wormlike micelles mediated by polyelectrolyte

Aqueous solutions of the anionic surfactant potassium oleate (K-oleate) were studied using small-angle neutron scattering (SANS), steady-state rheology, and cryogenic transmission electron microscopy (cryo-TEM). The micellar structural changes induced by the addition of potassium chloride (KCl) and sodium polystyrenesulfonate (PSS) of different molecular weights were investigated. Upon addition of KCl, a transition from spherical to wormlike micelles was detected from the SANS data and confirmed by the cryo-TEM pictures. The rheological measurements revealed a strong dependence of the low-shear viscosity on the concentration of salt: a broad maximum in the viscosity curve was observed upon addition of KCl, characteristic of the growth of micelles into long worms, followed by branching. The addition of PSS to salt-free solutions of K-oleate had a significant effect on the scattering patterns, revealing partial growth of the spherical micelles into rodlike micelles. In contrast, in the presence of high salt concentrations, addition of PSS to solutions of wormlike micelles did not bring any noticeable modifications in the scattering. However, in the same salt conditions, a clear effect was observed on the low shear viscosity upon addition of PSS, which was found to depend significantly on molecular weight. This suggests a novel way of impacting the viscosity of solutions of wormlike micelles.     

2,4-二硝基氯苯碱性水解胶团催化的活化能

研究了阳离子表面活性剂氯化十六烷基吡啶(CPC)和十六烷基三甲基氯化铵(CTAC)胶团对2,4-二硝基氯苯(DNCB)碱性水解的催化作用和小分子极性有机物丁醇的加入对该反应的影响,计算了反应活化能.结果表明:(1)CPC和CTAC胶团对DNCB碱性水解都有明显的催化作用;(2)加入少量叔丁醇略有利于提高催化效果;(3)在CPC和CTAC胶团溶液中DNCB碱性水解反应的活化能约为49kJ/mol,比纯水中的91kJ/mol低得多,说明反应机制可能存在差异.     

CO2 responsive wormlike micelles based on sodium oleate,potassium chloride and N,N-dimethylethanolamine

The anionic surfactant sodium oleate (NaOA) can self-assemble in aqueous solution in the presence of counter-ion inorganic salts to form wormlike micelles (WLMs), which exhibited viscoelastic behavior. In this paper, KCl was used to induce the formation of wormlike micelles with sodium oleate. In this process, we found that the addition of N, N-dimethylethanolamine (DMEA) can destroy the structure of WLMs leading significant decrease of viscosity. However, after introducing CO₂ into the ternary solution (KCl-NaOA-DMEA), the WLMs can be regenerated due to the electrostatic interaction between the protonated DMEA and the anionic surfactants. The addition of sodium hydroxide (NaOH) causes the electrostatic interaction between OA^- and DMEAH⁺ be destroyed, which results in the wormlike micelles becoming spherical micelles of lower viscosity. The transition of WLMs with high viscosity and low viscosity spherical micelles can be repeated several times by using CO₂ and NaOH.     

Viscoelastic micellar solutions in a mixed nonionic fluorinated surfactants system and the effect of oils

Formation and rheological behavior of viscoelastic wormlike micelles in aqueous solution of a mixed system of nonionic fluorinated surfactants, perfluoroalkyl sulfonamide ethoxylate, C8F17SO2N(C3H7)(CH2CH2O)nH (abbreviated as C8F17EOn) was studied. In the water-surfactant binary system C8F17EO20 forms an isotropic micellar solution over wide concentration range (>85 wt %) at 25 degrees C. With successive addition of C8F17EO1 to the aqueous C8F17EO20 solution, viscosity of the solution increases swiftly, and a viscoelastic solution is formed. The oscillatory rheological behavior of the viscoelastic solution can be described by Maxwell model at low-frequency region, which is typical of wormlike micelles. With further addition of C8F17EO1, the viscosity decreases after a maximum and phase separation occurs. Addition of a small amount of fluorinated oils to the wormlike micellar solution disrupts the network structure and decreases the viscosity sharply. It is found that polymeric oil, PFP (F-(C3F6O)nCF2CF2COOH), decreases the viscosity more effectively than the perfluorodecalin (PFD). The difference in the effect of oil on rheological properties is explained in terms of the solubilization site of the oils in the hydrophobic interior of the cylindrical aggregates, and their ability to induce rod-sphere transition.     

Rheological properties of wormlike micelles in sodium oleate solution induced by sodium ion

Aqueous solutions of anionic surfactant, sodium oleate (NaOA), have been studied by means of steady-state shear rheology and dynamic oscillatory technique. The micellar structure can be changed upon the addition of NaCl, Na₂CO₃ and NaCl/NaOH while NaOA concentration is maintained at 0.060 M. These systems except NaOA/NaCl show high viscosity and strong viscoelasticity. The hydroxide ion is very important for the formation of wormlike micelles. The anions of salts also have effect on the rheological properties of wormlike micelles. Three parameters: intersection frequency ω_i, plateau modulus G₀ and relaxation time τ are also discussed. The Maxwell model and Cole-Cole plot are applied to investigate the dynamic viscoelasticity of wormlike micelles. Variation in surfactant packing parameter R_P can be used to explain the change of rheology and microstructure of the micelles.     

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

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

Rheology of viscoelastic solutions of cationic surfactant. Effect of added associating polymer

Rheological studies were performed with aqueous salt solutions of viscoelastic cationic surfactant erucyl bis(hydroxyethyl)methylammonium chloride (EHAC) and its mixtures with hydrophobically modified polyacrylamide. The solutions of surfactant itself above the concentration of crossover of wormlike micelles exhibit two regions of rheological response. In the first region, they behave like polymer solutions in semidilute regime characterized by viscoelastic behavior with a spectrum of relaxation times. In the second region, unlike polymer solutions their relaxation after shear is dominated by a single relaxation time. Being composed of "living" micelles, the EHAC solutions easily lose their viscosity at the variation of the external conditions. For instance, heating from 20 to 60 degrees C reduces viscosity by up to 2 orders of magnitude, while added hydrocarbons induce a sudden drop of viscosity by 3-6 orders of magnitude. Polymer profoundly affects the rheological properties of EHAC solutions. The polymer/surfactant system demonstrates a 10,000-fold increase in viscosity as compared to pure-component solutions, the effect being more pronounced for polymer with less blocky distribution of hydrophobic units. A synergistic enhancement of viscosity was attributed to the formation of common network, in which some subchains are made up of elongated surfactant micelles, while others are composed of polymer. At cross-links the hydrophobic side groups of polymer anchor EHAC micelles. In contrast to surfactant itself, the polymer/surfactant system retains high viscosity at elevated temperature; at the same time it keeps a high responsiveness to hydrocarbon medium inherent to EHAC.