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.     

Formation and properties of reverse micellar cubic liquid crystals and derived emulsions

The structure of the reverse micellar cubic (I2) liquid crystal and the adjacent micellar phase in amphiphilic block copolymer/water/oil systems has been studied by small-angle X-ray scattering (SAXS), rheometry, and differential scanning calorimetry (DSC). Upon addition of water to the copolymer/oil mixture, spherical micelles are formed and grow in size until a disorder-order transition takes place, which is related to a sudden increase in the viscosity and shear modulus. The transition is driven by the packing of the spherical micelles into a Fd3m cubic lattice. The single-phase I2 liquid crystals show gel-like behavior and elastic moduli higher than 104 Pa, as determined by oscillatory measurements. Further addition of water induces phase separation, and it is found that reverse water-in-oil emulsions with high internal phase ratio and stabilized by I2 liquid crystals can be prepared in the two-phase region. Contrary to liquid-liquid emulsions, both the elastic modulus and the viscosity decrease with the fraction of dispersed water, due to a decrease in the crystalline fraction in the sample, although the reverse emulsions remain gel-like even at high volume fractions of the dispersed phase. A temperature induced order-disorder transition can be detected by calorimetry and rheometry. Upon heating the I2 liquid crystals, two thermal events associated with small enthalpy values were detected: one endothermic, related to the "melting" of the liquid crystal, and the other exothermic, attributed to phase separation. The melting of the liquid crystal is associated with a sudden drop in viscosity and shear moduli. Results are relevant for understanding the formation of cubic-phase-based reverse emulsions and for their application as templates for the synthesis of structured materials.     

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.     

Glycerol effects on the formation and rheology of cubic phase and related gel emulsion

We have investigated the effects of glycerol on the formation and rheological behavior of cubic phase (I(1)) and related O/I(1) gel emulsion in a water/C(12)EO(8)/dodecane system at 25 degrees C. The phase behavior of the water/C(12)EO(8)/dodecane system was studied by optical observation and structures of different liquid crystalline phases were identified by small-angle X-ray scattering (SAXS) techniques. Addition of dodecane (2 wt%) to aqueous solutions of C(12)EO(8) in a concentrated region (40 wt%) leads to the formation of the I(1) phase (which was absent without the addition of oil). The I(1) phase solubilized some amount of oil and at higher oil concentrations the I(1)+O phase was formed, allowing the preparation of O/I(1) gel emulsion. Rheological measurements have shown that the complex viscosity, |eta( *)|, of the I(1) phase is tremendously high ( approximately 10(7) Pas) and it increases with increasing oil concentration, attains a maximum value near the phase boundary, and then decreases drastically in the I(1)+O region. The increasing |eta( *)| or decreasing tandelta(G(')/G(')) can be ascribed with the interactions among the neighboring micelles. The decreasing trend of the |eta( *)| in the I(1)+O region is simply due to the low volume fraction of the I(1) phase. It has been shown that glycerol decreases the viscosity of the I(1) phase and related gel emulsion, which is due to the I(1)-hexagonal phase (H(1)) microstructural transition. Digital images show the physical appearance of the emulsion, which depends on the glycerol concentration changes from translucent to transparent.     

Cubic-Phase-Based Concentrated Emulsions

The effect of different types of added oil on the formation of a discontinuous micellar-type cubic phase was investigated in water-polyoxyethylene dodecyl ether (C(12)EO(25)) systems by phase study and small-angle X-ray scattering. The thermal stability of the cubic phase increases upon addition of oil, especially short-chain hydrocarbons. However, in the heptane system, the maximum melting temperature of the cubic phase is lower than that for decane due to the formation of a different liquid crystal phase. The effect of polyols on C(12)EO(25) cubic phases was also investigated. It was found that the thermal stability of the cubic phase decreases with polyol concentration. The destabilizing effect becomes large as the polyol molecule penetrates further into the surfactant palisade layer. Although the solubilization of oil in the cubic phase is very low, a large amount of excess oil can be incorporated and a transparent cubic-phase-based concentrated emulsion is formed. The transparency is attributed to the very small difference in the refractive indices between the cubic and excess-oil phases. Copyright 2000 Academic Press.     

Effect of added poly(oxyethylene)dodecyl ether on the phase and rheological behavior of wormlike micelles in aqueous SDS solutions

Upon the addition of a short EO chain nonionic surfactant, poly(oxyethylene) dodecyl ether (C12EOn), to dilute micellar solution of sodium dodecyl sulfate (SDS) above a particular concentration, a sharp increase in viscosity occurs and a highly viscoelastic micellar solution is formed. The oscillatory-shear rheological behavior of the viscoselastic solutions can be described by the Maxwell model at low shear frequency and combined Maxwell-Rouse model at high shear frequency. This property is typical of wormlike micelles entangled to form a transient network. It is found that when C12EO4 in the mixed system is replaced by C12EO3 the micellar growth occurs more effectively. However, with the further decrease in EO chain length, phase separation occurs before a viscoelastic solution is formed. As a result, the maximum zero-shear viscosity is observed at an appropriate mixing fraction of surfactant in the SDS-C12EO3 system. We also investigated the micellar growth in the mixed surfactant systems by means of small-angle X-ray scattering (SAXS). It was found from the SAXS data that the one-dimensional growth of micelles was obtained in all the SDS-C12EOn (n=0-4) aqueous solutions. In a short EO chain C12EOn system, the micelles grow faster at a low mixing fraction of nonionic surfactant.     

Structural and rheological investigation of Fd3m inverse micellar cubic phases

In the present study we demonstrate that a bulk inverse micellar cubic phase of Fd3m structure can be obtained by adding a hydrophobic component, such as the food-grade limonene, to the binary system monolinolein/water in a well-defined composition. The Fd3m structure studied in this work had a very slow kinetics of formation, as a consequence of partitioning of water into two types of micelle populations with different sizes. The Fd3m structure formed at a ratio of limonene oil to total lipids of alpha = 0.4 is stable in the bulk up to a maximum hydration of 12.68 wt % water, beyond which it starts to coexist with dispersed water. At full hydration, by combining small-angle X-ray scattering and available topological models, the inverse micellar cubic phase of Fd3m structure was shown to be formed by 16 small micelles and 8 larger micelles per cubic lattice cell (Q227 group), with radii of the micellar polar cores ranging between 1 and 3 nm and 149-168 monolinolein molecules per micelle depending on the water content. The temperature dependence of the structural and rheological properties of the Fd3m mesophase was investigated using SAXS, rheology, and turbidimetry. It appeared that the Fd3m phase underwent crystallization below 18 degrees C and began melting in an inverse microemulsion (L2 phase) coexisting with water above 28.5 degrees C with complete melting obtained at 40-45 degrees C, as evidenced by SAXS and rheology. Macroscopic phase separation between the L2 phase and excess water was observed with time at higher temperatures. The investigation of the viscoelastic properties of the Fd3m inverse discrete micellar cubic phase revealed a rheological signature similar to that of the bicontinuous cubic phases Pn3m and Ia3d observed in homologous binary systems. However, the Fd3m phase presented a complex set of slower relaxation mechanisms leading to a shift by 1 order of magnitude of the dominant relaxation times and whole relaxation spectrum, as compared to those of inverse bicontinuous cubic phases. These findings have been tentatively explained by (i) the multiple relaxation of micelles upon deformation, (ii) the small hydration level of the Fd3m phase, and (iii) the low temperature at which this phase can be observed.     

Phase polymorphism by mixing of poly(oxyethylene)-poly(dimethylsiloxane) copolymer and nonionic surfactant in water

The phase behavior of the water/poly(oxyethylene)-poly(dimethylsiloxane) copolymer (Si25C3EO51.6)/pentaoxyethylene dodecyl ether (C12EO5) ternary system has been studied. Both the silicone copolymer and the surfactant have equal volumes of hydrophilic and lipophilic parts; i.e., these are balanced amphiphiles. Although only a lamellar phase is observed in water-Si25C3EO51.6 and water-C12EO5 binary systems, a variety of liquid crystalline phases, including normal micellar cubic (I1), hexagonal (H1), bicontinuous cubic (V1), lamellar (L(alpha)), reverse bicontinuous cubic (V2), and reverse hexagonal (H2), are observed in the copolymer-rich region of the ternary phase diagram. The small C12EO5 molecules dissolve at the hydrophobic interface in the thick bilayer of the Si25C3EO51.6 L(alpha) phase occupying a large area of the total interface of the aggregates and modulate the curvature of the aggregates. Hence a variety of self-assembled structures are observed. In contrast, Si25C3EO51.6 is not dissolved in the thin bilayer of the C12EO5 lamellar phase (L'(alpha)). Hence, the C12EO5 L'(alpha) phase coexists with copolymer-rich L(alpha) and H2 phases. Consequently, small surfactant molecules are dissolved in a large silicone copolymer aggregate to induce a change in layer curvature, but a large copolymer molecule is hard to incorporate with surfactant aggregates.     

Cubic phase prepared in an anionic/amphoteric surfactant/oleic acid/decane/water system and the relationship with the neighboring phase

The formation, properties, and structure of discontinuous cubic phase in the pseudo-ternary system consisting of N'-carboxyethyl N'-hydroxyethyl N-aminoethyl dodecylamide (imidazoriniumbetain), sodium and triethanol amine salt of polyoxyethylene (1.5 mol) myristyl ether sulfate, oleic acid, decane, and water at a constant surfactant/water ratio of 4/6 were studied by means of small-angle X-ray scattering, freeze-fracture transmission electron microscopy, static light scattering, and dynamic rheology to gain an insight in its origin and interrelation with neighboring phases. It was found that the cubic phase occupied a rather wide region in a constructed ternary phase diagram, including from 25 to 45% of decane. Its properties and structural parameters varied with changing the oil content. The decane addition caused the swelling of spherical micellar aggregates. This resulted in an increase of their diameter up to 35 nm, which was ca. nine times larger than that of the initial micelles, and micellar volume fraction (packing fraction) up to 72 vol. %, which was close to the theoretically possible value of 74 vol. % for the close-packed spherical particles. The cubic phase was surrounded by a micellar L1 phase from the water-rich side (separated by a short two-phase region), two-phase region (cubic + oil) from the oil-rich side, and a lamellar phase from the surfactant-rich side. A transition from the L1 phase to the cubic state at the packing fraction of 60 vol. % was caused by an increase in the packing density of micellar aggregates, occurring with the decane addition. When it reached 72 vol. %, the oil started forming a separated phase owing to the inability of micelles to dissolve it. The important observation is that the adjacent phase from the surfactant-rich side was a lamellar one made up of flat bilayers. The preliminary data showed that the lamellar phase coexisted with cylindrical micelles in the intermediate two-phase region separating the cubic and lamellar phases.     

Hexagonal phase based gel-emulsion (O/H1 gel-emulsion): formation and rheology

The formation, stability, and rheological behavior of a hexagonal phase based gel-emulsion (O/H1 gel-emulsion) have been studied in water/C12EO8/hydrocarbon oil systems. A partial phase behavior study indicates that the oil nature has no effect on the phase sequences in the ternary phase diagram of water/C12EO8/oil systems but the domain size of the phases or the oil solubilization capacity considerably changes with oil nature. Excess oil is in equilibrium with the hexagonal phase (H1) in the ternary phase diagram in the H1+O region. The O/H1 gel-emulsion was prepared (formation) and kept at 25 degrees C to check stability. It has been found that the formation and stability of the O/H1 gel-emulsion depends on the oil nature. After 2 min observation (formation), the results show that short chain linear hydrocarbon oils (heptane, octane) are more apt to form a O/H1 gel-emulsion compared to long chain linear hydrocarbon oils (tetradecane, hexadecane), though the stability is not good enough in either system, that is, oil separates within 24 h. Nevertheless, the formation and stability of the O/H1 gel-emulsion is appreciably increased in squalane and liquid paraffin. It is surmised that the high transition temperature of the H1+O phase and the presence of a bicontinuous cubic phase (V1) might hamper the formation of a gel-emulsion. It has been pointed out that the solubilization of oil in the H1 phase could be related to emulsion stability. On the other hand, the oil nature has little or no effect on the formation and stability of a cubic phase based gel-emulsion (O/I1 gel-emulsion). From rheological measurements, it has found that the rheogram of the O/H1 gel-emulsion indicates gel-type structure and shows shear thinning behavior similar to the case of the O/I1 gel-emulsion. Rheological data infer that the O/I1 gel-emulsion is more viscous than the O/H1 gel-emulsion at room temperature but the O/H1 gel-emulsion shows consistency at elevated temperature.     

Wormlike micelles and microemulsions in aqueous mixtures of sucrose esters and nonionic cosurfactants

A study of the phase and rheological behavior of sucrose hexadecanoate (C16SE)/cosurfactant/water systems in the presence of solubilized oil, using complementary techniques such as dynamic light scattering and small angle X-ray scattering, is presented. Viscoelastic wormlike micellar solutions are found when a nonionic lipophilic cosurfactant is added to C16SE aqueous systems. Contrary to previous reports, the effect of oil solubilization on these wormlike micelles is not unique and depends on several factors. Linear alkyl chain oils that tend to solubilize in the micellar core have a disrupting effect, decreasing the relaxation time and the viscosity of the systems. This effect is larger as the molecular volume of oil increases and as the solubility of the cosurfactant in oil increases. On the other hand, oils that penetrate in the palisade layer, such as p-xylene, induce micellar growth and have a thickening effect at a given micellar composition. Thermodynamic considerations are used to explain the experimental results.     

Phase behavior of a mixture of poly(isoprene)-poly(oxyethylene) diblock copolymer and poly(oxyethylene) surfactant in water

The phase behavior of a mixture of poly(isoprene)-poly(oxyethylene) diblock copolymer (PI-PEO or C250EO70) and poly(oxyethylene) surfactant (C12EO3, C12EO5, C12EO6, C12EO7, and C12EO9) in water was investigated by phase study, small-angle X-ray scattering, and dynamic light scattering (DLS). The copolymer is not soluble in surfactant micellar cubic (I1), hexagonal (H1), and lamellar (Lalpha) liquid crystals, whereas an isotropic copolymer fluid phase coexists with these liquid crystals. Although the PI-PEO is relatively lipophilic, it increases the cloud temperatures of C12EO3-9 aqueous solutions at a relatively high PI-PEO content in the mixture. Most probably, in the copolymer-rich region, PI-PEO and C12EOn form a spherical composite micelle in which surfactant molecules are located at the interface and the PI chains form an oil pool inside. In the C12EO5/ and C12EO6/PI-PEO systems, one kind of micelles is produced in the wide range of mixing fraction, although macroscopic phase separation was observed within a few days after the sample preparation. On the other hand, small surfactant micelles coexist with copolymer giant micelles in C12EO7/ and C12EO9/PI-PEO aqueous solutions in the surfactant-rich region. The micellar shape and size are calculated using simple geometrical relations and compared with DLS data. Consequently, a large PI-PEO molecule is not soluble in surfactant bilayers (Lalpha phase), infinitely long rod micelles (H1 phase), and spherical micelles (I1 phase or hydrophilic spherical micelles) as a result of the packing constraint of the large PI chain. However, the copolymer is soluble in surfactant rod micelles (C12EO5 and C12EO6) because a rod-sphere transition of the surfactant micelles takes place and the long PI chains are incorporated inside the large spherical micelles.     

Short haired wormlike micelles in mixed nonionic fluorocarbon surfactants

We have studied the rheological behavior of viscoelastic wormlike micellar solution in a mixed system of nonionic fluorinated surfactants, perfluoroalkyl sulfonamide ethoxylate, C(8)F(17)SO(2)N(C(3)H(7))(CH(2)CH(2)O)(n)H abbreviated as C(8)F(17)EO(n) (n=10 and 20). Above critical micelle concentration, the surfactant, C(8)F(17)EO(20) forms small spherical micelles in water and the viscosity of the solution remains constant regardless of the shear rate, i.e., the solutions exhibit Newtonian behavior. However, upon successive addition of the C(8)F(17)EO(10) the viscosity of the solution increases and at certain C(8)F(17)EO(10) concentration, shear-thinning behavior is observed indicating the formation wormlike micelles. Contrary to what is expected, there is a viscosity increase with the addition of the hydrophilic C(8)F(17)EO(20) to C(8)F(17)EO(10) aqueous solutions at certain temperature and concentration, which could be attributed to an increase in rigidity of the surfactant layer and to the shifting of micellar branching to higher temperatures. The oscillatory-shear rheological behavior of the viscoelastic solution can be described by Maxwell model at low frequency. Small-angle X-ray scattering (SAXS) measurements confirmed the formation of small spherical micellar aggregates in the dilute aqueous C(8)F(17)EO(20) solution. The SAXS data shows the one-dimensional growth on the micellar size with increase in the C(8)F(17)EO(10) concentration. Thus, the present SAXS data supports the rheological data.     

Rheological behavior of water-in-oil emulsions stabilized by hydrophobic bentonite particles

A study of the rheological behavior of water-in-oil emulsions stabilized by hydrophobic bentonite particles is described. Concentrated emulsions were prepared and diluted at constant particle concentration to investigate the effect of drop volume fraction on the viscosity and viscoelastic response of the emulsions. The influence of the structure of the hydrophobic clay particles in the oil has also been studied by using oils in which the clay swells to very different extents. Emulsions prepared from isopropyl myristate, in which the particles do not swell, are increasingly flocculated as the drop volume fraction increases and the viscosity of the emulsions increases accordingly. The concentrated emulsions are viscoelastic and the elastic storage and viscous loss moduli also increase with increasing drop volume fraction. Emulsions prepared from toluene, in which the clay particles swell to form tactoids, are highly structured due to the formation of an integrated network of clay tactoids and drops, and the moduli of the emulsions are significantly larger than those of the emulsions prepared from isopropyl myristate.     

Viscoelasticity and mass transfer in phenol–CTAB aqueous systems

The rheological and mass transport properties of phenol in micellar solutions of hexadecyltrimethylammonium bromide (CTAB) were studied by rheometry and spectrophotometry. The presence of phenol located between headgroups of the CTAB diminishes the repulsive forces between the cationic groups and induces a sharp increase in viscosity that is attributed to the one-dimensional micellar growth favoring the formation of worm-like micelles. It is found that the mass transfer of phenol between two immiscible phases is significantly retarded by the presence of CTAB. The transfer is particularly slow when the diffusion takes place from a surfactant solution phase to an organic phase. This behavior is attributed to the phenol–surfactant interaction that leads to micellar growth and viscoelastic behavior. However, at elevated temperature, viscosity decreases and mass transfer increases. This particular rheological behavior offers the possibility of regulating the mass transfer, which might be interesting for applications.     

Viscosity Variation During Evaporation of a Vegetable Oil Emulsion Stabilized by Tween 80R

The viscosity during evaporation was determined for emulsions in the system water, vegetable oil, a commercial surfactant, Tween 80^R, and the results related to the phases of the emulsion according to the phase diagram. The correlation between the viscosity and the fraction of liquid crystal in the emulsion was pronounced for the emulsions with the oil as the dispersed phase. For the emulsions with oil as the major phase, the effect was significantly less.     

Bitumen-in-Water Emulsions: An Overview on Formation,Stability, and Rheological Properties

This paper describes the most relevant issues associated with the development of a technology; the formation of highly concentrated bitumen-in-water emulsions. Viscosity values for bitumen-in-water emulsions, containing between 70 and 85% (v/v) of bitumen, have been found to be several order of magnitude lower than the viscosity of the hydrocarbon itself. Thus, these emulsions, have potential applications in the processes of production, transportation, handling and commercialization of these extremely highly viscous hydrocarbons. The emulsions, the properties of which are discussed in this paper, were stabilized with mixtures of nonionic and natural surfactants (1,2) and formed using the HIPR (high internal phase ratio) technique (3). Information on the conditions required to produce emulsions with very narrow droplet diameter distributions is given. Results indicate that the mean droplet diameter, the droplet diameter distribution, and the bitumen volume fraction, significantly modify the rheological behavior. Emulsion stability was measured by following changes in the mean droplet diameter and in the rheological parameters with storage time.     

Wormlike micelles in mixed surfactant systems: effect of cosolvents

We have studied the structure and rheological behavior of viscoelastic wormlike micellar solutions in the mixed nonionic surfactants poly(oxyethylene) cholesteryl ether (ChEO15)-trioxyethylene monododecyl ether (C12EO3) and anionic sodium dodecyl sulfate (SDS)-C12EO3 using a series of glycerol/water and formamide/water mixed solvents. The obtained results are compared with those reported in pure water for the corresponding mixed surfactant systems. The zero-shear viscosity first sharply increases with C12EO3 addition and then decreases; i.e., there is a viscosity maximum. The intensity (viscosity) and position (C12EO3 fraction) of this maximum shift to lower values upon an increase in the ratio of glycerol in the glycerol/water mixed solvent, while the position of the maximum changes in an opposite way with increasing formamide. In the case of the SDS/C12EO3 system, zero-shear viscosity shows a decrease with an increase of temperature, but for the ChEO15/C12EO3 system, again, the zero-shear viscosity shows a maximum if plotted as a function of temperature, its position depending on the C12EO3 mixing fraction. In the studied nonionic systems, worm micelles seem to exist at low temperatures (down to 0 degrees C) and high glycerol concentrations (up to 50 wt %), which is interesting from the viewpoint of applications such as drag reduction fluids. Rheology results are supported by small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) measurements on nonionic systems, which indicate micellar elongation upon addition of glycerol or increasing temperature and shortening upon addition of formamide. The results can be interpreted in terms of changes in the surface curvature of aggregates and lyophobicity.     

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.     

Shear-induced topology changes in liquid crystals of the soybean lecithin/DDAB/water system

The viscoelastic behavior of the two different liquid crystalline lamellar phases and the liquid crystalline cubic phase of the mixed soybean lecithin/DDAB system in water was studied through rheology, with mechanical parameters studied as a function of composition. The swollen or diluted lamellar region is formed by vesicles, and its characteristic flow curve presents two-power law regions separated by a region where viscosity passes through a maximum. Yield stress and shear-dependent flow behavior were also observed. The microstructure suffers transformation under shear stress, and rheological response shifts from thixotropic to antithixotropic loops. Similar rheological behavior has been observed for samples in the collapsed or concentrated lamellar region, at the water-rich corner of the phase diagram. Vesicle formation may therefore occur by shearing the initial stacked and open bilayers. However, concentrated lamellar samples in the water-poor part of the phase diagram are less sensitive to shear effects and show plastic behavior and thixotropy. All lamellar samples manifest high elasticity. The dynamic responses of both lamellar topologies, i.e., vesicles and open bilayers, are comparable and exhibit an infinite relation time. The bicontinuous cubic, liquid crystalline phase is highly viscous. Its dynamic response cannot be modeled by a Maxwell model.