Interactions between cationic starch and anionic surfactants

The surface tensions and the phase equilibria of dilute aqueous cationic starch (CS)/surfactant systems were investigated. The degree of substitution of the CS varied from 0.014 to 0.772. The surfactants investigated were sodium dodecyl sulphate (SDS), potassium octanoate (KOct), potassium dodecanoate (KDod) and sodium oleate (NaOl). The concentrations of CS were 0.001, 0.01 and 0.1 w%.Critical association concentrations (cac) occur at surfactant concentrations well below the critical micelle concentrations of the surfactants, except for KOct, KDod and NaOl at the lowest CS concentrations investigated (0.001 w%). The surface tensions of CS/surfactant solutions decrease strongly already below the cac. This is attributed to the formation of surface active associates by ion condensation. Associative phase separation of gels formed by CS and surfactant takes place at extremely low concentrations when the surfactant/polymer charge ratio is somewhat larger than 1. The gel is higly viscous and contains 40–60% water, depending on the concentration of electrolyte, the surfactant hydrocarbon chain length and the nature of the polar head of the surfactant.The concentration at which the phase separation occurs decreases with increasing surfactant chain length and the concentration of simple electrolyte, factors that promote micelle formation. This indicates that the gels are formed by association of CS to surfactant micelles. When surfactant well in excess of charge equivalence is added, the gels dissolve because the CS/surfactant complexes acquire a high charge.     

Effects of concentration and temperature on viscoelastic properties of aqueous potassium oleate solutions

The rheological properties of semidilute aqueous solutions containing long cylindrical micelles of an anionic surfactant, potassium oleate, are studied. It is shown that, at surfactant concentrations above 1 wt %, the rheological properties of the solutions are adequately described in terms of the simple Maxwell model of a viscoelastic liquid characterized by a single relaxation time. Based on the analysis of normalized dependences of the loss modulus on the storage modulus, the characteristic times of the processes governing the rheological properties of the above systems, i.e., the average breaking time and reptation time of micelles, are estimated. It is found that the breaking time of micelles decreases and relaxation time increases with increasing surfactant concentration due to lengthening of micellar chains.     

Polymer–surfactant interaction for controlling the rheological properties of aqueous surfactant solutions

Controlling viscosity of aqueous surfactant solutions is very important for practical formulations. This can be done by having polymers interact with surfactants, thereby forming interconnected physical networks, where main ways of interaction are electrostatic and hydrophobic forces. Polymer–surfactant interactions are long established for viscosity control, but there are many ongoing activities. They are driven by wanting more biocompatible systems, which depend intricately on the choice of surfactant and polymer, and general predictions are not simple for such systems. Surfactants form spherical or wormlike micelles or vesicles. By choice of (co)polymers one can construct systems responsive to external parameters, like temperature or pH, for having tailored rheological properties. Here we describe recent developments with a focus on systems of low concentration, being interesting for applications. In summary, rheological control of polymer–surfactant systems is a versatile topic and a field of colloid science with high relevance for practical formulations.     

Controlling the melting of kinetically frozen poly(butyl acrylate-b-acrylic acid) micelles via addition of surfactant

We have studied the melting of polymeric amphiphilic micelles induced by small-molecule surfactant and explained the results by experimental determination of the interfacial tension between the core of the micelles and the surfactant solutions. Poly(n-butyl acrylate-b-acrylic acid) (PBA-b-PAA) amphiphilic diblock copolymers form kinetically frozen micelles in aqueous solutions. Strong interactions with surfactants, either neutral or anionic [C12E6, C6E4, sodium dodecyl sulfate (SDS)], were revealed by critical micelle concentration (cmc) shifts in specific electrode and surface tension measurements. Since both polymer and surfactant are either neutral or bear negative charges, the attractive interactions are not due to electrostatic interactions. Light scattering, neutron scattering, and capillary electrophoresis experiments showed important structural changes in mixed PBA-b-PAA/surfactant systems. Kinetically frozen micelles of PBA-b-PAA, that are hardly perturbed by concentration, ionization, ionic strength, and temperature stresses, can be disintegrated by addition of small-molecule surfactants. The interfacial energy of the PBA in surfactant solutions was measured by drop shape analysis with h-PBA homopolymer drops immersed in small-molecule surfactant solutions. The PBA/water interfacial energy gammaPBA/H2O of 20 mN/m induces a high energy cost for the extraction of unimers from micelles so that PBA-b-PAA micelles are kinetically frozen. Small-molecule surfactants can reduce the interfacial energy gammaPBA/solution to 5 mN/m. This induces a shift of the micelle-unimer equilibrium toward unimers and leads, in some cases, to the apparent disintegration of PBA-b-PAA micelles. Before total disintegration, polymer/surfactant mixtures are dispersions of polydisperse mixed micelles. Based on core interfacial energy arguments, the disintegration of kinetically frozen polymeric micelles was interpreted by gradual fractionation of objects (polydisperse dispersion mechanism), whereas the disintegration of polymeric micelles in a thermodynamically stable state was interpreted by an exchange between a population of large polymer-rich micelles and a population of small surfactant-rich micelles (bidisperse dispersion mechanism). Finally, in our system and other systems from the literature, interfacial energy arguments could explain why the disintegration of polymer micelles is either partial or total as a function of the surfactant type and concentration and the hydrophobic block molar mass of the polymer.     

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.     

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.     

Self-assembled networks highly responsive to hydrocarbons

Rheological studies were performed with aqueous salt solutions of anionic surfactant potassium oleate and its mixtures with hydrophobically modified polyacrylamide. Semidilute solutions of the surfactant in the presence of salt (KCl) demonstrate viscoelastic properties due to the formation of a transient network of entangled wormlike micelles. These systems are highly responsive to hydrocarbons: the addition of n-heptane or n-dodecane reduces the viscosity of solutions by up to 4 to 5 orders of magnitude, thus inducing the transition of a gellike system to a fluid one. It is the transformation of cylindrical surfactant micelles into spherical ones upon absorption of hydrocarbon that disrupts the network. The addition of a small amount (0.5 wt %) of associating polymer leads to up to a 5000-fold increase in the zero-shear viscosity and enhances the susceptibility to hydrocarbons. SANS data show that independently of the presence of polymer the radius of wormlike micelles is roughly equal to the length of a surfactant molecule, whereas the radius of spheres formed upon the absorption of hydrocarbon is 2-2.5-fold higher. A possible structure of the spherical micelles is discussed.     

Self-aggregation of mixtures of oppositely charged polyelectrolytes and surfactants studied by rheology, dynamic light scattering and small-angle neutron scattering

In this study, the phase behavior, structure and properties of systems composed of the cationic, cellulose-based polycation JR 400 and the anionic surfactants sodium dodecylbenzenesulfonate (SDBS) or sodium dodecylethoxysulfate (SDES), mainly in the semidilute regime, were examined. This system shows the interesting feature of a very large viscosity increase by nearly 4 orders of magnitude as compared to the pure polymer solution already at very low concentrations of 1 wt%. By using rheology, dynamic light scattering (DLS), and small-angle neutron scattering (SANS), we are able to deduce systematic correlations between the molecular composition of the systems (characterized by the charge ratio Z=[+(polymer)]/[?(surfactant)]), their structural organization and the resulting macroscopic flow behavior. Mixtures in the semidilute regime with an excess of polycation charge form highly viscous network structures containing rodlike aggregates composed of surfactant and polyelectrolyte that are interconnected by the long JR 400 chains. Viscosity and storage modulus follow scaling laws as a function of surfactant concentration (η~c(s)(4); G(0)~c(s)(1.5)) and the very pronounced viscosity increase mainly arises from the strongly enhanced structural relaxation time of the systems. In contrast, mixtures with excess surfactant charges form solutions with viscosities even below those of the pure polymer solution. The combination of SANS, DLS, and rheology shows that the structural, dynamical, and rheological properties of these oppositely charged polyelectrolyte/surfactant systems can be controlled in a systematic fashion by appropriately choosing the systems composition.     

Effect of polymer on rheological behavior of heated solutions of potassium oleate cylindrical micelles

The rheological characteristics of aqueous solutions of potassium oleate cylindrical micelles and their mixtures with hydrophobized polyacrylamide are studied at different temperatures and polymer concentrations no higher than the concentration of overlapping of coils. It is shown that, at all temperatures, the viscosities of surfactant-polymer solutions appear to be noticeably higher than the viscosities of individual surfactant solutions; however, the presence of the polymer has no effect on the viscous flow activation energy.     

Thermogelling properties of triblock copolymers in the presence of hydrophilic fe(3)o(4) nanoparticles and surfactants

We investigate the supramolecular structure formed by thermogelation of a triblock polymer in the presence of nanoparticles and surfactant using rheometry and small-angle X-ray scattering (SAXS). The triblock copolymer, nanoparticle, and surfactant used in this study are poly(oxyethylene-oxypropylene-oxyethylene), Pluronic F108, Fe(3)O(4) nanoparticles, and sodium dodecyl surfactant, respectively. Addition of 1-5 wt % of Fe(3)O(4) nanoparticle, of average particle size ～10 nm, in a weak template of F108 (15 wt %) shows a decrease in the onset of gelation temperature and dramatic alteration in the viscoelastic moduli. The nanocomposite samples show a linear viscoelastic regime up to 5% strain. The SAXS measurement shows that the intermicellar spacing of the supramolecular structure of pure F108 is ～16.5 nm, and the supramolecular structure is destroyed when nanoparticles and surfactants are incorporated in it. Further, the addition of anionic surfactant to nanocomposites leads to a dramatic reduction in the viscoelastic properties due to strong electrostatic barrier imparted by the surfactant headgroup that prevents the formation of hexagonally ordered micelles. Our results show that the thermogelation is due to the clustering of nanoparticles into a fractal network rather than a close-packed F108 micelles, in agreement with the recent findings in Pluronic F127-laponite systems.     

SELF-ASSEMBLING AMPHIPHILIC POLYELECTROLYTES AND THEIR NANOSTRUCTURES

The self-assembling behavior of random copolymers of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS)and hydrophobic comonomers possessing dodecyl groups linked by various spacer bonds was discussed with a focus on theeffect of the spacer. The characterization of association behavior of such polymers in water using quasielastic light scattering,capillary electrophoresis, NMR relaxation, various fluorescence, and viscoelastic methods was described. These copolymersform a variety of self-assembled nanostructures depending on the type of the spacer. Random copolymers of AMPS and N-dodecylmethacrylamide show a strong preference for intrapolymer self-association even in concentrated aqueous solutionsforming single-macromolecular self-assemblies (unimolecular micelles). In contrast, random copolymers of AMPS anddodecyl methacrylate are prone to undergo interpolymer associations yielding multipolymer micelles. In random copolymersof AMPS and a methacrylate substituted a nonionic surfactant (HO(CH_2CH_2O)_(25)C_(12)H_(25)) (C_(12)E_(25)), dodecyl groups are muchless restricted by the polymer backbone because they are linked via a long, flexible hydrophilic spacer. Thus, the polymer-bound C_(12)E_(25) surfactant moieties form micelles similar to those formed by discrete surfactants, but they are bridged bypolymer chains forming a network structure.     

Structure and rheology of reverse micelles in dipentaerythrityl tri-(12-hydroxystearate)/oil systems

We report the formation of reverse rod-like micelles and their rheological properties in novel nonionic surfactant, dipentaerythrityl tri-(12-hydroxystearate) (designated as WO-6)/oil systems without external water addition. Small-angle X-ray scattering (SAXS) was used to investigate the structure of the micelles and their flow properties were studied by rheological measurements. We found that WO-6 spontaneously self-assembles into reverse micelles in a variety of organic solvents at ambient conditions, their structure depending on solvent molecular architecture, surfactant concentration, and temperature. Rod-like micelles with a maximum length of ca. 12 nm and a cross section diameter of ca. 2 nm were observed in cyclohexane. When cyclohexane was replaced with a linear chain octane, the length and the cross section diameter were simultaneously increased. With a further increase of hydrocarbon chain length of solvent oils from octane to hexadecane, the rod-like micelles grew axially, keeping the cross section diameter (ca. 3 nm) virtually constant. Increasing surfactant concentration also favored one-dimensional micellar growth. On the other hand, micelles shrunk with the rise of temperature, which is similar to a rod-to-sphere transition, and is essentially the opposite temperature dependence to that often observed in aqueous micellar systems. A structural picture drawn by SAXS is well supported by rheology; the relative (zero-shear) viscosity of the WO-6/oil systems was found to be markedly greater than that expected for a dispersion of spherical particles due to the elongated micellar structure, despite quantitative inconsistency with semi-empirically predicted values for rigid rod-like particles.     

Salt-induced viscoelastic wormlike micelles formed in surface active ionic liquid aqueous solution

The growth and structure of the aqueous micellar solutions of a surface active ionic liquid, 1-hexadecyl-3-methylimidazolium bromide (C16mimBr), in the presence of an organic salt sodium tosylate (NaTos), were investigated by rheological measurements and freeze-fracture transmission electron microscopy at room temperature (298 K). As in some conventional ionic surfactant/salt aqueous systems, wormlike micelles and network structures could be formed in the C16mimBr/NaTos aqueous solutions, according to measurements of the zero-shear viscosity, the entanglement length, the average contour length, as well as application of the Cox-Merz empirical rule and Cole-Cole plots. FF-TEM images further confirmed that wormlike micelles were formed in these aqueous solutions. The wormlike micelles presented here would expand potential applications of ionic liquids in home care products, oilfield stimulation fluids, and nanobiotechnology.     

The behaviour of drag-reducing cationic surfactant solutions

The behaviour of two types of drag reducing surfactant solutions was studied in turbulent flows in pipes of different diameters. Our surfactant systems contained rod-like micelles; they consisted of equimolar mixtures ofn-tetradecyltrimethylammonium bromide,n-hexadecyltrimethylammonium bromide, and sodium salicylate. The structure of the turbulence was studied using a laser-Doppler anemometer in a 50 mm pipe. In the turbulent flow regime both surfactant solutions exhibited characteristic flow regimes. These flow regimes can be influenced by changing the amount of excess salt, the surfactant concentration, or the temperature. Shear viscosity measurements in laminar pipe and Couette flows show the occurrence of the so-called shear-induced state, where the viscosity increases and the surfactant solution becomes viscoelastic. The shape of the turbulent velocity profile depends on the flow regime. In the turbulent flow regime at low Reynolds numbers, velocity profiles similar to those observed for dilute polymer solutions are found, whereas at maximum drag reduction conditions more S-shaped profiles that show deviations from a logarithmic profile occur. An attempt is made to explain the drag reduction by rod-like micelles by combining the results of the rheological and the turbulence structure measurements.     

Effect of organic salts on micellar growth and structure studied by rheology

We report a rheological study on the effect of adding organic salts [sodium tosylate (NaTos) and benzoic acid potassium salt (BaPs)] on the micellar growth and structure of aqueous solutions of cethyltrimethylammonium chloride (CTAC) at a constant molar concentration ratio [salt]/[CTAC]. The rheological data show two well-defined domains of growth characterized by scaling laws for the surfactant concentration. The addition of NaTos leads to an unusual maximum in the viscosity-surfactant concentration curve. Before the maximum (domain 1), the analysis of the data (η₀, τ_R and G ₀) suggests the presence of branched micelles (connections). After the maximum (domain 2), however, the exponents of the scaling laws do not reflect either the relaxation of this branched structure or that of an entangled transient network structure. A faster mechanism of relaxation, not yet elucidated governs their dynamics. The exponents of the power laws in the presence of the BaPs are found, however, to be in accordance with the theory of equilibrium polymers. Received: 15 April 1998 Accepted in revised form: 20 October 1998     

Viscometric study of the sodium hyaluronate-sodium chloride-alkyl-(n)-ammonium surfactant system

Viscosity and rheological properties of the system sodium hyaluronate (NaHA)-NaCl-alkyl-(n)-ammonium surfactant were studied. The system with monomeric (n=1) surfactant dodecyltrimethylammonium bromide DTAB separated into two phases below the 200 mM NaCl concentration. At high NaCl concentration (>0.3 M), the hyaluronate–surfactant interaction is screened. At low NaCl concentration (0.2 M), the hyaluronate–surfactant interaction appears at higher concentration of micelles. In the system with dimeric surfactant (n=2) alkanediyl-,ω-bis(dimethyldodecylammonium bromide) referred to as 12-s-12, the hyaluronate–surfactant interaction was observed at low surfactant concentration, opposite to the case of NaHA-NaCl-monomeric surfactant. In certain range of surfactant concentration, viscosity of the system can be fairly good described by the empirical Jones–Dole equation. However, the fit fails at high surfactant concentration. From the rheological measurements follows that NaHA aqueous solution shows non-Newtonian behaviour. When adding NaCl and shearing, viscosity increases as a result of affecting the ion atmosphere which surrounds micelles, by shearing. The same effect was observed in the system NaHA-NaCl-ammonium surfactant (both monomeric and dimeric). The hyaluronate–surfactant interaction is assumed at low shearing. The influence of surfactant structure on viscosity at the same NaCl concentration shows that the increasing value of the carbon number in spacer of dimeric surfactant increases viscosity and stretches the hyaluronate coils.     

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.     

New type flooding systems in enhanced oil recovery

Wormlike micelles, obtained in anionic surfactant sodium oleate (NaOA) solutions in the presence of sodium phosphate (Na₃PO₄), were studied using the steady and dynamic rheological methods. The laboratory simulation flooding experiments were used to investigate the effects of flooding for the wormlike micelles system. The results show that the oil recovery is 32.7%. This flooding system is a new type and has high activity with a low cost.     

三聚阴离子表面活性剂/阳离子添加剂混合体系的流变行为

表面活性剂的分子结构对蠕虫胶束的形成与性质有着重要影响。本文以十四酸和间苯三酚为起始原料,合成了一种三聚阴离子表面活性剂（2, 2', 2"-（苯基-1, 3, 5-三（氧））-三-十四酸钠,简写为Ph-TrisC₁₄Na）,并通过稳态和动态流变测试,研究了单组分的Ph-TrisC₁₄Na和Ph-TrisC₁₄Na/阳离子添加剂体系的粘弹性质。阳离子添加剂分别为正丁基三甲基溴化铵（C₄TAB）,正己基三甲基溴化铵（C₆TAB）和正辛基三甲基溴化铵（C₈TAB）。结果表明,依赖于独特的分子构型,Ph-TrisC₁₄Na分子自身即可形成蠕虫胶束,使溶液表现出明显的粘弹性。阳离子添加剂的加入可进一步优化Ph-TrisC₁₄Na的分子几何结构,促进蠕虫胶束更为快速地生长。随着阳离子添加剂疏水链长的增加,溶液的粘弹性显著增强,体系微结构对添加剂的敏感性也增加。对于50 mmol·L^-1的Ph-TrisC₁₄Na溶液来说,在C₈TAB与Ph-TrisC₁₄Na的摩尔比为0.5时,体系的零剪切粘度可达1535 Pa·s,蠕虫胶束的长度则达到4.0-7.5 μm。该体系体现出低聚表面活性剂在构筑表面活性剂粘弹溶液方面的优势,可拓展高粘弹性阴离子蠕虫胶束体系的研究范围。     

Monitoring the Transition from Spherical to Polymer‐like Surfactant Micelles Using Small‐Angle X‐Ray Scattering

Despite over a century of modern surfactant science, the kinetic pathways of morphological transitions in micellar systems are still not well understood. This is mainly as a result of the lack of sufficiently fast methods that can capture the structural changes of such transitions. Herein, a simple surfactant system consisting of sodium dodecyl sulfate (SDS) in aqueous NaCl solutions is investigated. Combining synchrotron radiation small‐angle X‐ray scattering (SAXS) with fast stopped‐flow mixing schemes allows monitoring the process where polymer‐like micelles are formed from globular micelles when the salt concentration is suddenly increased. The results show that “worm‐like” micelles are formed by fusion of globular micelles and short cylinders in a fashion that bears similarities to a step‐like polymerization process.