Viscoelastic micellar water/CTAB/NaNO(3) solutions: rheology, SANS and cryo-TEM analysis

Aqueous micellar solutions of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and sodium nitrate (NaNO(3)) were examined using steady and dynamic rheology, small-angle neutron scattering (SANS) and cryogenic-transmission electron microscopy (cryo-TEM). Upon addition of NaNO(3), the CTAB spherical micelles transform into long, flexible wormlike micelles, conveying viscoelastic properties to the solutions. The zero-shear viscosity (eta(0)) versus NaNO(3) concentration curve exhibits a well-defined maximum. Likewise, upon increase in temperature, the viscosity decreases. Dynamic rheological data of the entangled micellar solutions can be well described by the Maxwell model. Changes in the structural parameters of the micelles with addition of NaNO(3) were inferred from SANS measurements. The intensity of scattered neutrons at the low q region was found to increase with increasing NaNO(3) concentration. This suggests an increase in size of the micelles and/or decrease of intermicellar interactions with increasing salt concentration. Analysis of the SANS data using prolate ellipsoidal structure and Yukawa form of interaction potential between micelles indicates that addition of NaNO(3) leads to a decrease in the surface charge of the ellipsoidal micelles and consequently an increase in their length. The structural transition from spherical to entangled threadlike micelles, induced by the addition of NaNO(3) to CTAB micelles is further confirmed by cryo-TEM.     

Micellar Behavior of Polystyrene-Poly(Ethylene Oxide) Diblock Copolymers in Aqueous Media: Effect of Copolymer Composition,Temperature, Salt,and Surfactants

Formation and structure of micelles from two amphiphilic polystyrene-block-poly(ethylene oxide) (PS-PEO) diblock copolymers (PS mol.wt. 1000; PEO mol.wt. 3000 and 5000) were examined by surface tension, viscosity, steady state fluorescence, dynamic light scattering (DLS), small angle neutron scattering (SANS), and cryo-transmission electron microscopy (cryo-TEM). The critical micelle concentration (CMC) of the copolymers in aqueous solution was ca. 0.05%; micelle hydrodynamic diameter was 30–35 nm with a narrow size distribution. SANS studies show that the copolymers form ellipsoidal micelles with semi major axis ～23 nm and semi minor axis ～8 nm. No significant change in the structure was found with temperature and presence of salt. The copolymer micelles interaction with the ionic surfactants sodium dodecyl sulphate (SDS) and dodecyltrimethylammonium bromide (DTAB) was also examined by DLS and SANS.     

Tuning the structure of SDS micelles by substituted anilinium ions

The growth behavior of aggregates formed in aqueous solutions of the anionic surfactant sodium dodecyl sulfate (SDS) in the presence of the cationic hydrophobic salts o-toluidine hydrochloride (OTHC) and m-toluidine hydrochloride (MTHC) has been studied by dynamic light scattering (DLS) and small-angle neutron scattering (SANS) techniques. DLS studies indicate a progressive growth of SDS micelles with addition of less than equimolar concentrations of hydrophobic salts. A prolate ellipsoidal model is used to analyze the DLS data, which is further supported by SANS measurements. We explain the propensity for the strong growth of micelles in the presence of OTHC and MTHC by the high charge neutralization provided by these salts as the aromatic counterions are adsorbed on the surface of the micelles. When the substitution is at the meta position, i.e., for MTHC, micellar growth is favored at lower salt concentrations than for OTHC. The variation in growth behavior is explained in terms of the difference in the chemical environments of the substituents at the ortho and meta positions. Micellar parameters obtained from SANS data at elevated temperature also support enhanced growth of micelles in the presence of MTHC as compared to OTHC.     

Micellar characteristics of diblock polyacrylate-polyethylene oxide copolymers in aqueous media

The formation and structural features of micelles from low molecular weight diblock copolymers of poly(methylmethacrylate-b-ethylene oxide) PMMA-PEO (varying in total molecular weight) and poly(butylmethacrylate-b-ethylene oxide) PBMA-PEO in water, aqueous NaCl and urea solutions were examined by surface tension, dye spectral, cloud point, viscosity and small angle neutron scattering (SANS) measurements. The increasing concentrations of NaCl reduce the onset concentration of micellization and phase separation, while urea has reverse effect. The analysis of the SANS curves revealed the presence of prolate ellipsoidal micelles in diblock copolymers at various experimental concentrations and temperatures studied. The effect of temperature, NaCl and urea on the neutron scattering profiles are more or less the same which is well supported by viscosity and surface tension measurements. The diblock copolymers form spherical micelles of aggregation number in the range of 522-664. The micelles are very temperature stable.     

Small angle neutron scattering study of sodium dodecyl sulfate micellar growth driven by addition of a hydrotropic salt

The structures of aggregates formed in aqueous solutions of an anionic surfactant, sodium dodecyl sulfate (SDS), with the addition of a cationic hydrotropic salt, p-toluidine hydrochloride (PTHC), have been investigated by small angle neutron scattering (SANS). The SANS spectra exhibit a pronounced peak at low salt concentration, indicating the presence of repulsive intermicellar interactions. Model-independent real space information about the structure is obtained from a generalized indirect Fourier transformation (GIFT) technique in combination with a suitable model for the interparticle structure factor. The interparticle interaction is captured using the rescaled mean spherical approximation (RMSA) closure relation and a Yukawa form of the interaction potential. Further quantification of the geometrical parameters of the micelles was achieved by a complete fit of the SANS data using a prolate ellipsoidal form factor and the RMSA structure factor. The present study shows that PTHC induces a decrease in the fractional charge of the micelles due to adsorption at the micellar surface and consequent growth of the SDS micelles from nearly globular to rodlike as the concentration of PTHC increases.     

Effect of phenol on the aggregation characteristics of an ethylene oxide-propylene oxide triblock copolymer P65 in aqueous solution

The effects of phenol on the micellization, micellar growth, and phase separation of a poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) amphiphilic copolymer (Pluronic P65: EO19 PO30 EO19) in aqueous solution have been studied by cloud point, viscosity, dynamic light scattering (DLS), differential scanning calorimetry (DSC), fluorescence spectroscopy, and small-angle neutron scattering (SANS). Various concentrations of P65 have been chosen to estimate the effect of phenol on different concentration regions of P65. Phenol interacts quite differently at low concentrations (0-2%) than at high concentrations (2-10%) of P65, as per the observation that phenol is more predominant at smaller concentrations of P65. A marked decrease in the cloud points of the P65 solutions is observed in presence of phenol. The critical micelle temperature (CMT) of P65 shows a synergistic effect of phenol on P65 aggregates. Micellar transitions, phase separation, and aggregation behaviours like micellization and micellar growth in the presence of phenol have been observed by combining viscometry, DLS, DSC, and CP. DLS shows that the effect of phenol is predominant at high temperatures. SANS shows a high increase in axial ratio and aggregation numbers in the presence of phenol at fixed concentrations of P65. Fluorescence data illustrate that addition of phenol makes micelles polar but at the same time its favours aggregation. Water-soluble phenol (present in low concentrations) forms aggregates with P65, which can be separated by cloud point extraction, making this study interesting for separation of phenol from the phenol-water system.     

Sodium chloride and ethanol induced sphere to rod transition of triblock copolymer micelles

Dynamic light scattering (DLS), small-angle neutron scattering (SANS), and viscosity studies have been carried out to examine the influence of NaCl and ethanol on the structure of triblock copolymer [(EO)20(PO)70(EO)20] (EO = ethylene oxide; PO = propylene oxide) micelles in aqueous medium. The studies show that while the pure triblock copolymer solutions do not show any significant growth of the micelles on approaching the cloud point, the presence of a small amount of ethanol (5-10%) induces a sphere to rod shape transition of micelles at high temperatures. Interestingly, this ethanol induced sphere to rod transition of micelles can be brought down to room temperature (25 degrees C) with the addition of NaCl. It is also found that NaCl alone cannot induce such sphere to rod transitions and excess ethanol suppresses them by increasing their transition temperature.     

Influence of a hydrophobic diol on the micellar transitions of Pluronic P85 in aqueous solution

Micellization behavior of an amphiphilic ethylene oxide-propylene oxide-ethylene oxide tri-block copolymer Pluronic P85 [(EO)(26)(PO)(39)-(EO)(26)] in aqueous solution and in the presence of a hydrophobic C(14)diol (also known as Surfynol104) was examined by physico-chemical methods such as viscometry, cloud point (CP) and scattering techniques viz. dynamic light scattering (DLS) and small angle neutron scattering (SANS). The addition of diol decreases the cloud point and gelation temperature of aqueous Pluronic P85 copolymer solution. DLS and SANS measurements of the polymer in aqueous solution indicated micellar growth and sphere to rod transition in the presence of diol. Surfynol 104 is a sparingly water soluble diol surfactant with a solubility of approximately 0.1 wt%. However, up on addition to Pluronic solution, diol gets incorporated in the block copolymer micelles and leads to structural transition of the micelles. An increase in the temperature and the presence of added sodium chloride in the solution further enhances this effect. The addition of hydrophobic C(14)diol increases the hydrodynamic size and aggregation numbers of the micellar system. The micellar parameters for the copolymer in the presence of C(14)diol are reported at different temperatures and added sodium chloride concentrations.     

Determination of the rheological properties of aqueous solutions of branched PEO‐PPO‐PEO copolymers. Confrontation with small‐angle neutron scattering studies.

The physico‐chemical properties of Tetronic^® 908 in aqueous solution have been explored on a wide range of temperature and concentration. This system presents interesting rheological properties depending on the polymer concentration in solution, and temperature. For weight percentage p comprised between 5% and 20%, the viscosity of the solution passes through a maximum. Small angle neutron scattering experiments show that the increase in the viscosity is due to the progressive aggregation of the chains. For higher weight percentages, the viscosity of the system diverges and the mixture becomes “gel‐like”. SANS studies of the same solution indicate that the micelles are organized in a cubic structure. We have developed a model which suitably describes the SANS curves of the system in the region where micelles are formed. It allows the calculation of several parameters (volume fraction of the micelles, size, composition). The evolution of these parameters with temperature and polymer concentration is detailed. Comparisons with models of literature are discussed.     

Puroindoline-a, a lipid binding protein from common wheat, spontaneously forms prolate protein micelles in solution

The self-assembly in solution of puroindoline-a (Pin-a), an amphiphilic lipid binding protein from common wheat, was investigated by small angle neutron scattering, dynamic light scattering and size exclusion chromatography. Pin-a was found to form monodisperse prolate ellipsoidal micelles with a major axial radius of 112 ± 4.5 ? and minor axial radius of 40.4 ± 0.18 ?. These protein micelles were formed by the spontaneous self-assembly of 38 Pin-a molecules in solution and were stable over a wide pH range (3.5-11) and at elevated temperatures (20-65 °C). Pin-a micelles could be disrupted upon addition of the non-ionic surfactant dodecyl-β-maltoside, suggesting that the protein self-assembly is driven by hydrophobic forces, consisting of intermolecular interactions between Trp residues located within a well-defined Trp-rich domain of Pin-a.     

The aggregation behavior between anionic carboxymethylchitosan and cetyltrimethylammonium bromide: MesoDyn simulation and experiments

The aggregation behavior between carboxymethylchitosan (CMCHS) and cetyltrimethylammonium bromide (CTAB) is investigated by MesoDyn simulation and experimental techniques, for increasing CTAB concentrations. Mixed CMCHS/CTAB bulk aggregates are formed in the solution. Simulation results give the morphologies of aggregates clearly and illustrate the two stages for the formation of aggregates: the first stage is CTAB molecules aggregating on the CMCHS chain and the second stage is the equilibrium stage. A viscosity maximum and a hydrodynamic radius minimum at a certain CTAB concentration reveal the bridging structure of the polymer chains by the micelles. Transmission electron microscopy (TEM) images give the bridging structure clearly. At higher surfactant concentrations, light scattering and TEM show the existence of larger structures, whose size increases with CTAB concentration. According to the simulation and experimental results, the process of aggregate formation and aggregation mechanism are analyzed. Initially CMCHS and CTAB form network structure due to the bridge action of CTAB micelles, while the network structure disappears gradually and is replaced by ellipsoidal CMCHS/CTAB aggregate structure with CTAB concentration increasing.     

Rheo-SANS studies on shear-thickening/thinning in aqueous rodlike micellar solutions

Shear-induced thickening/thinning phenomena of aqueous rodlike micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium p-toluene sulfonate (NapTS) were investigated by means of simultaneous measurements of rheology and small-angle neutron scattering (SANS), the so-called Rheo-SANS. The aqueous CTAB/NapTS solutions were classified into five different categories dependent on their flow behavior and micellar structure. By increasing salt concentration and/or shear rates, the micelles underwent morphological transition from (i) spherical or short rodlike micelles to (ii) long rodlike micelles without entanglements, followed by (iii) those with entanglements. These transitions were recognized as changes in flow behavior from Newtonian to shear-thickening and shear-thinning flow, respectively. In the latter two cases, anisotropic SANS patterns appeared around these critical shear rates. The physical meaning of the anisotropic SANS patterns accompanied by shear-thickening flow behavior is discussed in conjunction with other shear-thickening systems.     

Water-soluble interpolyelectrolyte complexes of polyisobutylene-block-poly(methacrylic acid) micelles: formation and properties

We report on interpolyelectrolyte complexes (IPECs) formed by micelles of ionic amphiphilic diblock copolymers with polyisobutylene (PIB) and poly(sodium methacrylate) (PMANa) blocks interacting with quaternized poly(4-vinylpyridine) (P4VPQ). The interpolyelectrolyte complexation was followed by turbidimetry and small angle neutron scattering (SANS). The data obtained by means of a combination of SANS, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM) provide evidence on the core-shell-corona structure of the complex species with the shell assembled from fragments of electrostatically bound PMANa and quaternized P4VPQ fragments, original PIBx-b-PMAAy micelles apparently playing a lyophilizing part. The complex formation is followed by potentiometric titration as well. This process is initially kinetically controlled. In the second step larger aggregates rearrange in favor of smaller complexes with core-shell-corona structure, which are thermodynamically more stable. An increase in ionic strength of the solution results in dissociation of the complex species as proven by SANS and analytical ultracentrifugation (AUC). This process begins at the certain threshold ionic strength and proceeds via a salt-induced gradual release of chains of the cationic polyectrolyte from the complex species.     

Aqueous Mixtures of a Trisiloxane Surfactant and Oil Studied by SANS and NMR Self-diffusion: Effect of Temperature and Oil Concentration

Samples of a poly(ethylene oxide) trisiloxane surfactant, water, and decane have been investigated using pulsed field gradient NMR (PGSE NMR) and small-angle neutron scattering (SANS) to determine the solution structure. The surfactant/water weight ratio has been kept constant at a value of 3/2, with variation of the oil (decane) content. In the neutron scattering measurements the temperature was varied from 23 °C up to the phase separation limit for these systems. The combined NMR and SANS data show that on addition of decane, the system exists as a hexagonal phase of cylindrical decane-containing micelles at all temperatures investigated. The addition of decane changes significantly the values for the structure parameters in the system, inducing an increase in periodicity of 12–15 ?. By substitution of decane with its deuterated equivalent, decane-d₂₂, it was possible to obtain detailed information on the structural organization of the oil component in this ternary mixture.     

The structure of metallomicelles

The morphology of micelles formed by two novel metallosurfactants has been studied by small-angle neutron scattering (SANS) and small-angle-X-ray scattering (SAXS). The two surfactants both contain a dodecyl chain as the hydrophobic moiety, but differ in the structure of the head group. The surfactants are Cu(II) complexes of monopendant alcohol derivatives of a) the face-capping macrocycle 1,4,7-triazacyclanonane (tacn), and b) an analogue based upon the tetraazamacrocycle 1,4,7,10-tetraazacyclododecane. Here, neutron scattering has been used to study the overall size and shape of the surfactant micelles, in conjunction with X-ray scattering to locate the metal ions. For the 1,4,7,10-tetraazacyclododecane-based surfactant, oblate micelles are observed, which are smaller to the prolate micelles formed by the 1,4,7-triazacyclononane analogue. The X-ray scattering analysis shows that the metal ions are distributed throughout the polar head-group region, rather than at a well-defined radius; this is in good agreement with the SANS-derived dimensions of the micelle. Indeed, the same model for micelle morphology can be used to fit both the SANS and SAXS data.     

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.     

Structural behaviour of mixed cationic surfactant micelles: a small-angle neutron scattering study

Self-assembly in mixtures of two single-chain cationic surfactants, with different tail lengths (CTAB and DTAB) as well as of a single-chain (DTAB) and a double-chain (DDAB) cationic surfactant, with identical tail lengths, have been investigated with small-angle neutron scattering (SANS) and rationalised in terms of bending elasticity properties. The growth behaviour of micelles with respect to surfactant composition appears completely different in the two surfactant mixtures. DTAB form small oblate spheroidal micelles in presence of [NaBr] = 0.1 M that transform into prolate spheroidal mixed CTAB/DTAB micelles upon adding moderate amounts of CTAB, so as to give a mole fraction y = 0.20 in solution. Most unexpectedly, upon further addition of CTAB the mixed CTAB/DTAB micelles grow with an almost equal rate in both length and width directions to form tablets. In contrast to this behaviour, mixed DDAB/DTAB micelles grow virtually exclusively in the length direction, in presence of [NaBr] = 0.1 M, to form elongated ellipsoidal (tablet-shaped) and subsequently long wormlike micelles as the fraction of DDAB in the micelles increases. Mixed DDAB/DTAB micelles grow to become as long as 2000 Å before an abrupt transition to large bilayer structures occurs. This means that the micelles are much longer at the micelle-to-bilayer transition as compared to the same mixture in absence of added salt. It is found that the point of transition from micelles to bilayers is significantly shifted towards higher fractions of aggregated DTAB as an appreciable amount of salt is added to DDAB/DTAB mixtures, indicating a considerable reduction of the spontaneous curvature with an increasing [NaBr]. By means of deducing the various bending elasticity constants from our experimental results, according to a novel approach by ours, we are able to conclude that the different growth behaviours appear as a consequence of a considerably lower bending rigidity, as well as higher saddle-splay constant, for DDAB/DTAB surfactant mixtures in presence of [NaBr] = 0.1 M, as compared to mixtures of CTAB/DTAB in [NaBr] = 0.1 M and DDAB/DTAB in absence of added salt.     

THE EFFECT OF BENZYL ALCOHOL ON THE MICELLAR PROPERTIES OF CTAB IN KBr SOLUTION

ABSTRACT

The effect of benzyl alcohol on the micellar size and shape of CTAB in KBr solution has been investigated by means of viscosity, LLS (laser light scattering), and NMR measurements. The surfactant CTAB (cetyltrimethylammonium bromide) and KBr content are kept constant at 0.01 mol.L^?1 and O.l^?1 mol.. The data from the various techniques are quantitatively in agreement. The viscosity of 0.01 mol.L^?1 CTAB/ 0.01mol.L^?1 KBr micellar system has a marked maximum at benzyl alcohol content 0.6%(v/v), where the size of rod micelles are largest. The results from 'H-NMR spectra of CTAB molecules show that before the maximum viscosity, benzyl alcohol is located in the interfacial region of CTAB micelles in the presence of KBr salt. If more benzyl alcohol is added, it starts solubilizing in the palisades of the micelles.     

Structural Transitions of CTAB Micelles in a Protic Ionic Liquid

Micellar solutions of hexadecyltrimethylammonium bromide (CTAB) in a protic ionic liquid, ethylammonium nitrate (EAN), are studied by shear rheology, polarizing optical microscopy (POM), conductivity measurements, and small angle neutron scattering (SANS). Three concentration regimes are examined: A dilute regime (with concentrations [CTAB] < 5 wt %) consisting of noninteracting spherical micelles, a semidilute regime (5 wt % ≤ [CTAB] ≤ 45 wt %) where micelles interact via electrostatic repulsions, and a concentrated regime (45 wt % < [CTAB] ≤ 62 wt %) where a reversible, temperature-dependent isotropic (L(1)) to hexatic (Hex) phase transition is observed. The L(1)-Hex transition, which has been predicted but not previously observed, is characterized by (1) a sharp increase in the shear viscosity, (2) the formation of focal conical birefringence textures (observed by POM), and (3) enhancement of the crystalline order, evidenced by the appearance of Bragg reflections in the SANS profiles. Ionic conductivity is not sensitive to the L(1)-Hex transition, which corroborates the absence of topological transitions.     

Hydrodynamic radius of polyethylene glycol in solution obtained by dynamic light scattering

In order to compare the size characterizations in poly(ethylene glycol) (PEG) obtained by dynamic light scattering (DLS) and small angle neutron scattering (SANS), DLS experiments were performed in various PEG solutions to ascertain the hydrodynamic radius. Data from the experiments were analyzed by using a method to eliminate effects of PEG aggregation on dynamic correlation functions. The results of the analysis were then compared to the radii of gyration reported from SANS experiments. The relation between the hydrodynamic radius, obtained by DLS, and the radius of gyration, obtained by SANS, in PEG in solution was found to be in agreement with a previously obtained relation for PEG, where the radius of gyration was found by static light scattering.