A systematic study of equilibrium structure, thermodynamics, and rheology of aqueous CTAB/NaNO(3) wormlike micelles

Wormlike micellar solutions formed by an anionic surfactant and a cationic surfactant with two head groups

Innovation in the molecular structure of surfactants is important for the preparation of soft materials with novel properties. In this study, we synthesized a cationic surfactant, N1,N1,N1,N1,N3,N3,N3-pentamethyl-N3-(3-stearamidopropyl)propane-1,3-diammonium bromide, hereafter referred to as C18-DQA. Unlike conventional cationic surfactants, C18-DQA contains two quaternary ammonium head groups and a long-saturated alkyl chain equal to a chain length of 21 carbon atoms. C18-DQA exhibits a low Krafft point of ∼0 °C and a water solubility >1000 mM at 25 °C. The critical micelle concentration (cmc) of C18-DQA was determined to be 0.59 mM using the Nile red method. C18-DQA was mixed with sodium laurate (SL) at different molar ratios to produce transparent solutions with excellent viscoelasticity over a wide concentration range. The 1 : 1.5 molar ratio C18-DQA/SL mixed solutions exhibited gel-like behavior for a total surfactant concentration of 2.88 wt% (75 mM). The solution with a total surfactant concentration of 300 mM (120 mM C18-DQA and 180 mM SL) achieved a maximum zero-shear viscosity (η0) of 4224 Pa s. Cryogenic transmission electron microscopy analysis revealed the formation of extremely long wormlike micelles, with a cross-sectional diameter of 5 nm and contour length >3 μm, in the mixed solutions. C18-DQA and SL molecules were drawn close by electrostatic attractions, leading to a suitable molecular geometry for the extensive growth of wormlike micelles. This work will act as an important reference for the future preparation of highly viscoelastic solutions by mixing cationic and anionic surfactants. The proposed system is also expected to have potential applications in cosmetic formulations, home care products, and oilfield fracturing fluids.

Stimuli-responsive viscosity modifiers

Stimuli responsive viscosity modifiers entail an important class of materials which allow for smart material formation utilizing various stimuli for switching such as pH, temperature, light and salinity. They have seen applications in the biomedical space including tissue engineering and drug delivery, wherein stimuli responsive hydrogels and polymeric vessels have been extensively applied. Applications have also been seen in other domains like the energy sector and automobile industry, in technologies such as enhanced oil recovery. The chemistry and microstructural arrangements of the aqueous morphologies of dissolved materials are usually sensitive to the aforementioned stimuli which subsequently results in rheological sensitivity as well. Herein, we overview different structures capable of viscosity modification as well as go over the rheological theory associated with classical systems studied in literature. A detailed analysis allows us to explore correlations between commonly discussed models such as molecular packing parameter, tube reptation and stress relaxation with structural and rheological changes. We then present five primary mechanisms corresponding to stimuli responsive viscosity modification: (i) packing parameter modification via functional group conditioning and (ii) via dynamic bond formation, (iii) mesh formation by interlinking of network nodes, (iv) viscosity modification by chain conformation changes and (v) viscosity modification by particle jamming. We also overview several recent examples from literature that employ the concepts discussed to create novel classes of intriguing stimuli responsive structures and their corresponding rheological properties. Furthermore, we also explore systems that are responsive to multiple stimuli which can provide enhanced functionality and versatility by providing multi-level and precise actuation. Such systems have been used for programmed site-specific drug delivery.

High Toughness Combined with High Strength in Oxide Ceramic Nanofibers

Traditional oxide ceramics are inherently brittle and highly sensitive to defects, making them vulnerable to failure under external stress. As such, endowing these materials with high strength and high toughness simultaneously is crucial to improve their performance in most safety-critical applications. Fibrillation of the ceramic materials and further refinement of the fiber diameter, as realized by electrospinning, are expected to achieve the transformation from brittleness to flexibility owing to the structural uniqueness. Currently, the synthesis of electrospun oxide ceramic nanofibers must rely on an organic polymer template to regulate the spinnability of the inorganic sol, whose thermal decomposition during ceramization will inevitably lead to pore defects, and seriously weaken the mechanical properties of the final nanofibers. Here, a self-templated electrospinning strategy is proposed for the formation of oxide ceramic nanofibers without adding any organic polymer template. An example is given to show that individual silica nanofibers have an ideally homogeneous, dense, and defect-free structure, with tensile strength as high as 1.41 GPa and toughness up to 34.29 MJ m-3 , both of which are far superior to the counterparts prepared by polymer-templated electrospinning. This work provides a new strategy to develop oxide ceramic materials that are strong and tough.

Metal nanowires grown in situ on polymeric fibres for electronic textiles

A key aspect of the use of conventional fabrics as smart textiles and wearable electronics is to incorporate a means of electrical conductivity into single polymer fibres. We present the transformation of thin polymer fibres and fabrics into conductive materials by in situ growth of a thin, optically transparent gold-silver nanowire (NW) mesh with a relatively low metal loading directly on the surface of polymer fibres. Demonstrating the method on poly(lactic-co-glycolic) acid and nylon microfibres, we show that the NW network morphology depends on the diameter of the polymer fibres, where at small diameters (1-2 μm), the NWs form a randomly oriented network, but for diameters above several micrometers, the NWs wrap around the fibres transversally. This phenomenon is associated with the stiffness of the surfactant templates used for the NW formation. The NW-decorated fibres exhibit a significant increase in conductivity. Moreover, single fibres can be stretched up to ∼15% before losing the electrical conductivity, while non-woven meshes could be stretched by about 25% before losing the conductivity. We believe that the approach demonstrated here can be extended to other polymeric fibres and that these flexible and transparent metal-coated polymer fibres could be useful for various smart electronic textile applications.

Scission energy and topology of micelles controlled by the molecular structure of additives

We employ coarse-grained (CG) molecular dynamics simulations (MD) to investigate the effects of the molecular structure of additives on the scission energy and morphology of charged micelles. Considering sodium dodecyl sulfate (SDS) as a representative charged surfactant and taking trimethylphenylammonium chloride (TMPAC) and octyltrimethylammonium bromide (OTAB) as oppositely charged additives, we show that the scission energy and topology of micelles vary significantly depending on the molecular structure of the hydrophobic part of the additives. The cyclic aromatic tail of the TMPAC disrupts the core structure of the SDS micelle and hence decreases the micelle scission energy, whereas the linear alkyl tail of the OTAB packs very well with the micelle core and increases the scission energy. Although both the additives have similar head structures, they lead to very different micelle morphologies because of the difference in the shape of their tail structures; ring-like or toroidal shaped micelles are formed in SDS/TMPAC solution whereas bicelle-like structures are formed in SDS/OTAB solution when the additive to surfactant ratio is higher than a certain value.

Complex supramolecular fiber formed by coordination-induced self-assembly of benzene-1,3,5-tricarboxamide (BTA)

HYPOTHESIS

In the quest for large but well-controlled supramolecular structures, the discotic benzene-1,3,5-tricarboxamide (BTA) has received quite some attention, because it can form hydrogen-bonded stacks that can be regarded as supramolecular polymers of which the single BTA molecule is the monomer. In this report, we consider a more complex BTA-based supramolecular polymer, namely one that is built up from supramolecular 'monomers'.

EXPERIMENTS

We design a tris-ligand L₃ consisting of a BTA core carrying three dipicolinic acid (DPA) groups. L₃ itself is too small to form polymers, but in the presence of appropriate metal ions, each L₃ can form three coordination bonds and so form (L₃)_n clusters that are large enough to stack successfully: at an appropriate metal dose, long and stable filaments with a cross-sectional diameter of 12 nm appear. We monitor the growth process by UV-vis spectroscopy and light scattering, and use small angle X-ray scattering (SAXS), TEM as well as molecular simulation to confirm the filamentous structure of the fibers and determine their dimensions.

FINDINGS

The formation and structure of the fiber are very similar for various transition metal ions, which enables introducing different functionalities, e.g., magnetic relaxivity, by proper choice of the metal ions. Hence, we obtain a doubly supramolecular polymer, connected axially by hydrogen bonds, and radially by coordination bonds. Not only does this realize a higher level of complexity, but it also allows to easily introduce and vary metal-derived functionalities.

Cryo-TEM and rheological study on shear-thickening wormlike micelles of zwitterionic/anionic (AHSB/SDS) surfactants

HYPOTHESIS

Shear-thickening micelles were mostly made of cationic surfactants, but shear-thickening was rarely reported in the zwitterionic/anionic surfactants. Since wormlike micelles were essential in shear-thickening systems, it should be common for the hybrid wormlike micelles formed by zwitterionic/anionic surfactants, and their fundamental features need to be clarified.

EXPERIMENTS

The micellization of zwitterionic surfactant homologies alkyl dimethyl amidopropyl hydroxyl sulfobetaine (AHSB) and sodium dodecyl sulfate (SDS) in brine was studied, and various environmental factors were considered systematically. Light scattering, rheology, zeta potential, ¹H NMR and cryo-TEM techniques were employed to characterize the AHSB/SDS wormlike micelles.

FINDINGS

AHSB/SDS hybrid wormlike micelles were formed in a wide x_SDS region to endow them with apparent viscosities, in which the electrostatic and hydrophobic interactions between AHSB and SDS molecules were critical. AHSB with the longer tail, the higher c_AHSB and c_NaCl were advantageous to enhance the viscosity because of the longitudinal growth of wormlike micelles. The shear-thickening AHSB/SDS samples were commonly composed of unbranched wormlike micelles with various length, and the shear-induced alignment of wormlike micelles was the major cause as verified by cryo-TEM. Moreover, the quantitative relationships on the critical shear rate ɣ̇_c were established, and the activation energies were obtained from the temperature-dependent ɣ̇_c.

Surfactant desorption and scission free energies for cylindrical and spherical micelles from umbrella-sampling molecular dynamics simulations

HYPOTHESIS

The free energies associated with adsorption/desorption of individual surfactants from micelles and the fusion/scission of long micelles can be used to estimate the rate constants for micellar kinetics as functions of surfactant and salt concentration.

EXPERIMENTS

We compute the escape free energies △G_esc of surfactant from micelles and the scission free energies △G_sciss of long micelles from coarse-grained molecular dynamics simulations coupled with umbrella sampling, for micelles of both sodium dodecylsulfate (SDS) in sodium chloride (NaCl) and cetyltrimethylammonium chloride (CTAC) in sodium salicylate (NaSal).

FINDINGS

For spherical micelles, △G_esc values have maxima at certain aggregation numbers, and at salt-to-surfactant molar concentration ratios R near unity, consistent with experiments. For cylindrical micelles, SDS/NaCl shows a minimum, and CTAC/NaSal a maximum in △G_esc, both at R ~ 0.7, while △G_sciss of CTAC micelles also peaks at around R ~ 0.7 and that of SDS micelles increases monotonically with R. We explain the non-monotonic dependence of escape and scission free energies on R by a combination of electrostatic screening and the decrease of micelle radius with increasing R. Transitions from predominantly spherical to cylindrical micelles, and between adsorption/desorption and fusion/scission kinetics with changing salt concentration can be inferred from the free energies for CTAC/NaSal.

Where are those lipid nano rings?

Highly curved toroidal micelles with diameters as small as 100 nm have been successfully constructed by self-assembly of amphiphilic block copolymers. These structures may have potential applications in gene or drug delivery. Experimental observations suggest that toroidal micelles likely originate from spherical or disc-like micelles which are tricked into forming toroidal micelles upon external stimuli ('smart' materials). Since self-assembly of polymeric and lipid surfactants is guided by the same physical principles, we hypothesize that 'smart' lipid surfactants can be equivalently tricked into forming highly curved toroidal micelles that are tenfold smaller (≃10 nm diameter). Paradoxically, these 'nano rings' have never been observed. Using coarse-grained molecular dynamics (MD) simulations in conjunction with a state-of-the-art free energy calculation method (a string method), we illustrate how a thermo-responsive lipid surfactant is able to form toroidal micelles. These micelles originate from disc-like micelles that are spontaneously perforated upon heat shocking, thereby supporting a longstanding hypothesis on the possible origin of polymeric toroidal micelle phases observed in experiments. We illustrate that kinetically stable 'nano rings' are substantially shorter lived than their tenfold larger polymeric analogs. The estimated life-time (milliseconds) is in fact similar to the characteristic breaking time of the corresponding worm-like micelle. Finally, we resolve the characteristic finger print which 'nano rings' leave in time-resolved X-ray spectra and illustrate how the uptake of small DNA fragments may enhance their stability. Despite a shared kinetics of self-assembly, length scale dependent differences in the life-time of surfactant phases can occur when phases are kinetically rather than thermodynamically stable. This results in the apparent absence or presence of toroidal micelle phases on different length scales. Our theoretical work precisely illustrates that the universality of surfactants nevertheless remains conserved even at different length scales.

A new interpretation of the mechanism of wormlike micelle formation involving a cationic surfactant and salicylate

HYPOTHESIS

When tetradecyltrimethylammonium bromide, TTAB, is added to aqueous solution of sodium salicylate, NaSal, the threading of the aromatic anion into the micellar palisade leads to the formation of wormlike micelles. Based on the calorimetric titration of NaSal with TTAB, and on the lifetime of fluorescence of salicylate, we propose that the aggregation of the two components directly leads to the formation of wormlike micelles, without any pre-aggregation.

EXPERIMENTS

By using an isothermal titration calorimeter, aliquots of TTAB were added to a dilute solution of NaSal. The energy involved in each addition was then integrated and the variation of enthalpy was determined. In the same range of concentrations and molar ratios, the surface tensiometry and time-resolved emission spectroscopy experiments were performed.

FINDINGS

A very characteristic calorimetric signal associated with wormlike micelle formation was obtained, being the enthalpy variation of this process, Δ_WLMH₂₉₈⁰ < 0. When 1.2 mmol L^-1 of NaSal is titrated with 11.0 mmol L^-1 of TTAB at 298.15 K, Δ_fH₂₉₈⁰ = -10.31 kJ per mol of injectant. By adding TTAB to NaSal solution, two fluorescence lifetimes of salicylate were observed solely after wormlike micelle being formed. The correspondent lifetime values of 4.0 ns and 7.2 ns are respectively associated with the free and associated species of salicylate. The new results demonstrated that wormlike micelles are the first aggregate formed when TTAB is added to salicylate. This aspect is relevant for understanding the mechanism of wormlike micelles formation.

Self-assembly of surface functionalized amphiphilic carbon dots: tuning in morphological manifestations

Despite the continuous surge of interest in supramolecular chemistry, the design and synthesis of building blocks to develop diverse examples of self-assemblies is still challenging. During the past decades, formation of self-assemblies such as micelles, vesicles, and gels with a fibril network using amphiphiles has been investigated at length. Considering the increasing applications of these self-aggregates across the scientific domain, it is crucial to adopt an alternative strategy for the preparation of self-aggregates using a new building block that has been applied in diverse domains. With this aim, surface functionalized carbon dots (CDs) with varying aliphatic/aromatic (cholesteryl, palmitoyl, naphthyl) substitutions linked with spacers such as ethylenediamine, p-phenylenediamine, 2,2'-(ethylenedioxy)bis(ethylamine) were developed. The surface passivated CDs formed self-assemblies in dimethylsulfoxide-water (DMSO-H₂O, 2 : 1, v/v). The roles of surface functionalities and spacer units in the formation of self-assemblies using the synthesized CDs were investigated by microscopic and spectroscopic studies. Progressive morphological transition was found from vesicle-to-fiber in DMSO-H₂O (2 : 1, v/v) which was dependent on surface passivating substitutions of the CDs from cholesteryl to naphthyl to palmitoyl. Together with the exclusive formation of self-assemblies using amphiphilic CDs, the present study enabled the tuning of self-organization behaviour of the CD by alteration of its surface functionality.

Stretch and Breakage of Wormlike Micelles under Uniaxial Strain: A Simulation Study and Comparison with Experimental Results

We use coarse-grained (CG) molecular dynamics simulations to determine the effect of uniaxial strain on the stress, scission stress, and scission energy of solutions of wormlike micelles of cetyltrimethylammonium chloride/sodium salicylate (NaSal). We find that the breaking stress, stretch modulus, and scission energy of the charged micelles are nonmonotonic functions of oppositely charged hydrotrope (NaSal) concentration. While the stretch modulus shows a peak at a value of surfactant-to-hydrotrope concentration ratio ( R) close to unity as expected due to neutralization of head-group charge at R = 1, the breaking stress and scission energy produce a peak at R < 1.0 because of thinning of the micelle diameter with increased R. The breaking stress from the simulations depends on the rate of deformation and roughly agrees with the experimental values of Rothstein ( J. Rheol. 2003 , 47 , 1227 ) after extrapolation to the much lower experimental rates. The method and results can be used to predict the effects of flow and mechanical stress on rates of micellar breakage, which is important in the rheology of wormlike micellar solutions.

Probing flow-induced nanostructure of complex fluids in arbitrary 2D flows using a fluidic four-roll mill (FFoRM)

Engineering flow processes to direct the microscopic structure of soft materials represents a growing area of materials research. In situ small-angle neutron scattering under flow (flow-SANS) is an attractive probe of fluid microstructure under simulated processing conditions, but current capabilities require many different sample environments to fully interrogate the deformations a fluid experiences in a realistic processing flow. Inspired by recent advances in microfluidics, we present a fluidic four-roll mill (FFoRM) capable of producing tunable 2D flow fields for in situ SANS measurements, that is intended to allow characterization of complex fluid nanostructure under arbitrary complex flows within a single sample environment. Computational fluid dynamics simulations are used to design a FFoRM that produces spatially homogeneous and sufficiently strong deformation fields. Particle tracking velocimetry experiments are then used to characterize the flows produced in the FFoRM for several classes of non-Newtonian fluids. Finally, a putative FFoRM-SANS workflow is demonstrated and validated through the characterization of flow-induced orientation in a semi-dilute cellulose nanocrystal dispersion under a range of 2D deformations. These novel experiments confirm that, for steady state straining flows at moderate strain rates, the nanocrystals orient along the principal strain-rate axis, in agreement with theories for rigid, rod-like Brownian particles in a homogeneous flow.

Nonmonotonic Scission and Branching Free Energies as Functions of Hydrotrope Concentration for Charged Micelles

Using coarse-grained molecular dynamics simulations and an umbrella sampling method that uses local surfactant density as a reaction coordinate, we directly calculate, for the first time, both the scission and branching free energies of a model charged micelle [cationic cetyltrimethylammonium chloride (CTAC)] in the presence of inorganic and organic salts (hydrotropes). We find that while inorganic salt only weakly affects the micelle scission energy, organic hydrotropes produce a strong, nonmonotonic dependence of both scission energy and branching on salt concentration. The nonmonotonicity in scission energy is traced to a competition between electrostatic screening of the repulsions among the surfactant head groups and thinning of the micellar core, which result from attachment of the hydrotropes to the micelle surface. We are able to correlate the nonmonotonicity in the scission energy of CTAC micelles with the peak observed experimentally in viscosity versus hydrotrope concentration and the location of this peak in CTAC solutions.

Micelles Structure Development as a Strategy to Improve Smart Cancer Therapy

Micelles as colloidal suspension have attracted considerable attention due to their potential use for both cancer diagnosis and therapy. These structures have proven their ability to deliver poorly water-soluble anticancer drugs, improve drug stability, and have good penetration and site-specificity, leading to enhance therapeutic efficacy. Micelles are composed of hydrophobic and hydrophilic components assembled into nanosized spherical, ellipsoid, cylindrical, or unilamellar structures. For their simple formation, they are widely studied, either by using opposite polymers attachment consisting of two or more block copolymers, or by using fatty acid molecules that can modify themselves in a rounded shape. Recently, hybrid and responsive stimuli nanomicelles are formed either by integration with metal nanoparticles such as silver, gold, iron oxide nanoparticles inside micelles or by a combination of lipids and polymers into single composite. Herein, through this special issue, an updated overview of micelles development and their application for cancer therapy will be discussed.

Micro-mechanical, continuum-mechanical, and AFM-based descriptions of elasticity in open cylindrical micellar filaments

We present theoretical and experimental descriptions of the elasticity of cylindrical micellar filaments using micro-mechanical and continuum theories, and atomic force microscopy. Following our micro-mechanical elasticity model for micellar filaments [M. Asgari, Eur. Phys. J. E: Soft Matter Biol. Phys., 2015, 38(9), 1-16], the elastic bending energy of hemispherical end caps is found. The continuum description of the elastic bending energy of a cylindrical micellar filament is also derived using constrained Cosserat rod theory. While the continuum approach provides macroscopic description of the strain energy of the micellar filament, the micro-mechanical approach has a microscopic view of the filament, and provides expressions for kinetic variables based on a selected interaction potential between the molecules comprising the filament. Our model predicts the dependence of the elastic modulus of the micellar filaments on their diameter, which agrees with previous experimental observations. Atomic force microscopy is applied to estimate the elastic modulus of the filaments using force volume analysis. The obtained values of elastic modulus yield the persistence length of micellar filaments on the same order of the previously reported values. Consistent with previous studies, our results indicate that semi-flexible linear micelles have a relatively large local strain energy at their end points, which explains their tendency to fuse to minimize the number of end caps at relatively low total surfactant volume fractions. Also, the elastic modulus of micellar filaments was found to increase when the indentation frequency increases, a finding which agrees with previous rheological observations on the bulk shear modulus of micellar solutions.

Synthesis of Janus Au@periodic mesoporous organosilica (PMO) nanostructures with precisely controllable morphology: a seed-shape defined growth mechanism

Janus nanostructures that possess two or more distinct components and surface functions have attracted more and more attention. Here, we present a seed-shape defined growth mechanism for the preparation of anisotropic Janus nanostructures, in which the shape of periodic mesoporous organosilica (PMO) is determined by the shape of Au nanoparticles. Various shaped Au@PMO composite nanostructures, such as rods, spheres, and plates, are prepared based on this general growth mechanism. By adjusting the reaction parameters (temperature, surfactant), various shaped AuNR@PMO Janus nanostructures, including horsebean- and fingernail-like nanostructures, have been successfully prepared. We also demonstrate the potential applications of such composite nanostructures. As an example, the as-prepared rod-like Janus Au@PMO nanostructures show great performance in chemo-photothermal combination therapy because of the excellent photothermal effect of Au nanorods and the high surface area of PMO nanorods. This research may open a new direction to the controllable synthesis and practical application of dedicated nanostructures with desired properties.

Free Energy of Scission for Sodium Laureth-1-Sulfate Wormlike Micelles

Wormlike micelles (WLMs) are nanoscale, self-assembled components of many products from shampoos to fracking fluids due to their viscoelasticity. Their rheological behavior is largely governed by the contour length of the micelles and the concomitant propensity of the micelles to overlap and entangle. The large contour lengths, on the order of micrometers, is the result of a delicate balance between the scission enthalpy of the wormlike micelles on the one hand and entropic factors such as the mixing entropy of dispersion, the ordering of water molecules and counterions, and the mobility of branch points on the other hand. The structure and contour length of wormlike micelles assembled from sodium laureth-1-sulfate was determined at various temperatures using small-angle neutron scattering. The results allow the calculation of the enthalpy and entropy as well as the free energy of scission and are employed to critically evaluate the common methods to determine micellar scission energy from mean-field theory. Interesting behavior is observed when comparing branched and unbranched WLMs that may reflect on mechanistic differences in chain scission.

The role of the binding salt sodium salicylate in semidilute ionic cetylpyridinium chloride micellar solutions: a rheological and scattering study

The viscosity varies as a consequence of binding, first, and screening, later, action of the penetrating salt.

Viscoelasticity Enhancement of Surfactant Solutions Depends on Molecular Conformation: Influence of Surfactant Headgroup Structure and Its Counterion

During the anisotropic growth from globular to wormlike micelles, the basic interactions among distinct parts of the surfactant monomer, its counterion, and additives are fundamental to tune molecular self-assembly. We investigate the addition of sodium salicylate (NaSal) to hexadecyltrimethylammonium chloride and bromide (CTAC and CTAB), 1-hexadecylpyridinium chloride and bromide (CPyCl and CPyBr), and benzyldimethylhexadecylammonium chloride (BDMC), which have the same hydrophobic tail. Their potential to enhance viscoelasticity by anisotropic micellar growth upon salt addition was compared in terms of (i) the influence of the headgroup structure, and (ii) the influence of surfactant counterion type. Employing proton nuclear magnetic resonance ((1)H NMR), we focused on the molecular conformation of surfactant monomers in the core and polar shell regions of the micelles and their interactions with increasing concentration of NaSal. The viscoelastic response was investigated by rotational and oscillatory rheology. We show that micellar growth rates can be tuned by varying the flexibility and size of the surfactant headgroup as well as the dissociation degree of the surfactant counterion, which directly influences the strength of headgroup-counterion pairing. As a consequence, the morphological transitions depend directly on charge neutralization by electrostatic screening. For example, the amount of salt necessary to start the rodlike-to-wormlike micelle growth depends directly on the number of dissociated counterions in the polar shell.

Scattering Function for Branched Wormlike Chains

Wormlike or threadlike structures with local cylindrical geometry are abundantly found in nature and technical products. A thorough structural characterization in the bulk for a whole ensemble, however, is difficult. The inherent semiordered nature of the tortuous large-scale structure and especially the quantification of branching renders an assessment difficult. In the present work we introduce a hybrid function expressing the scattering intensities for X-rays, neutrons, or light in the small-angle regime for this system. The function is termed "hybrid" because it employs terms from different approaches. The large-scale structure is described via a Guinier term as well as a concomitant power-law expression in momentum transfer q taken from the so-called unified function. The local cylindrical shape, however, is taken into account through a form factor for cylinders from rigid-body modeling. In principle, the latter form factor can be replaced by an expression for any other regular body so that the new hybrid function is a versatile tool for studying hierarchical structures assembled from uniform subunits. The appropriateness and capability of the new function for cylindrical structures is exemplified using the example of a wormlike micellar system.

Modulation of collagen by addition of Hofmeister salts

Collagen can be modified by addition of chaotropic or kosmotropic salts of the reversed Hofmeister series. Hence, telopeptide-poor collagen type I was suspended in H2SO4 (pH 2) and 0.05-0.5 M KCl and KNO3 (chaotropes), as well as KI and KSCN (kosmotropes). Rheological parameters, including storage and loss modulus, intrinsic viscosity, and critical overlap concentration, were assessed and the microstructure was characterized by applying confocal laser scanning microscopy and scanning electron microscopy. The addition of up to 0.1 M KCl and 0.05 M KNO3 increased the intrinsic viscosity from 1.22 to 1.51 L/g without salt to a maximal value of 1.74 L/g and decreased the critical overlap concentration from 0.66 to 0.82 g/L to a minimal value of 0.57 g/L. Higher salt concentrations increased the collagen-collagen interactions due to ions withdrawing the water from the collagen molecules. Hence, 0.1 M KSCN delivered the largest structures with the highest structure factor, area value and the highest critical overlap concentration with 17.6 L/g. Overall, 0.5 M salt led to salting out, with chaotropes forming fine precipitates and kosmotropes leading to elastic three-dimensional networks. The study demonstrated that collagen entanglement and microstructure depend strongly on the ionic strength and type of salt.

Hydrogel formed by the co-assembly of sodium laurate and silica nanoparticles

Fibers formed by the co-assembly of sodium laurate and silica nanoparticles.

Molecular variations in aromatic cosolutes: critical role in the rheology of cationic wormlike micelles

Wormlike micelles formed by the addition to cetyltrimethylammonium bromide (CTAB) of a range of aromatic cosolutes with small molecular variations in their structure were systematically studied. Phenol and derivatives of benzoate and cinnamate were used, and the resulting mixtures were studied by oscillatory, steady-shear rheology, and the microstructure was probed by small-angle neutron scattering. The lengthening of the micelles and their entanglement result in remarkable viscoelastic properties, making rheology a useful tool to assess the effect of structural variations of the cosolutes on wormlike micelle formation. For a fixed concentration of CTAB and cosolute (200 mmol L(-1)), the relaxation time decreases in the following order: phenol > cinnamate> o-hydroxycinnamate > salicylate > o-methoxycinnamate > benzoate > o-methoxybenzoate. The variations in viscoelastic response are rationalized by using Mulliken population analysis to map out the electronic density of the cosolutes and quantify the barrier to rotation of specific groups on the aromatics. We find that the ability of the group attached to the aromatic ring to rotate is crucial in determining the packing of the cosolute at the micellar interface and thus critically impacts the micellar growth and, in turn, the rheological response. These results enable us for the first time to propose design rules for the self-assembly of the surfactants and cosolutes resulting in the formation of wormlike micelles with the cationic surfactant CTAB.

Wormlike micelles formed using Gemini surfactants with quaternary hydroxyethyl methylammonium headgroups

The aggregation behavior of Gemini surfactants with hydroxyl groups in their headgroups, butane-1,4-bis(hydroxyethyl methylalkylammonium) bromides hereafter abbreviated as m-4-m MEA (m = 12, 14, 16), has been investigated in aqueous solution. Each formed a viscous fluid in water at low concentration in the absence of a salt. In solutions of 14-4-14 MEA, the formation of highly viscoelastic wormlike micelles could be detected using steady and dynamic rheological measurements. The existence of these long column micelles has also been confirmed using cryo-transmission electron microscopy (cryo-TEM) and small angle X-ray scattering (SAXS). Compared with conventional bis(dimethylalkylammonium) bromide Gemini molecules with the same spacer (14-4-14), 14-4-14 MEA demonstrated a better ability to fabricate wormlike micelles because of the change in the headgroup structure. As for 16-4-16 MEA, which has longer alkyl chains, its aqueous solutions behave more like elastic gels at a concentration of 80 mmol L(−1). The unique viscoelastic behavior of m-4-m MEA in water can be attributed to the synergistic interactions of hydrophobic attraction and hydrogen bonding. The obtained results are believed to be an important addition to the effect that the headgroups of Gemini surfactants have on their aggregation behavior in dilute solutions.

Structural and rheological studies on growth of salt-free wormlike micelles formed by star-type trimeric surfactants

We investigated the growth mechanisms of wormlike micelles formed by star-type trimeric surfactant (3C(12)trisQ) with a hydrocarbon chain length of 12 in an aqueous solution. A 3C(n)trisQ molecule consists of three hydrocarbon chains and three hydrophilic groups connected by spacer chains, where n is the carbon number in the hydrocarbon chain. Our recent studies showed that the aggregates formed by 3C(12)trisQ exhibited sphere-to-rod transition and the growth of wormlike micelles in an aqueous solution in the absence of salt. We performed small-angle neutron scattering (SANS) and rheological measurements and investigated the aggregation behavior of 3C(12)trisQ with various surfactant volume fractions. All SANS profiles for the 3C(12)trisQ indicated peak-profiles in the q range of 0.02 Å(-1) < q < 0.05 Å(-1), where the magnitude of the scattering vector q is defined by q = 4π sin(θ/2)/λ (λ and θ represent the wavelength and scattering angle, respectively). These peaks were attributed to repulsive interparticle interactions between the micelles. The volume fraction dependence of the SANS peak-position was in agreement with the rheological behavior. These results suggest that 3C(12)trisQ shows sphere-to-rod transition and can produce wormlike micelles in the absence of salt. To determine the structural parameters quantitatively, model-fitting analysis was performed using a charged cylindrical or charged ellipsoidal particle scattering function. The radius, length, and number of water molecules per surfactant molecule (n(w)) inside the micelles were evaluated. The length increased and the n(w) value decreased with increasing φ, indicating that the growth of a wormlike micelle accompanies the extrusion of water from the micelle. The end-cap energies of star-type trimeric, gemini, and monomeric surfactants were evaluated from φ dependence of zero-shear viscosity. We found that wormlike micelles formed by 3C(12)trisQ exhibited a higher end-cap energy than gemini surfactant.

Shape and size of highly concentrated micelles in CTAB/NaSal solutions by Small Angle Neutron Scattering (SANS)

Highly concentrated micelles in CTAB/NaSal solutions with a fixed salt/surfactant ratio of 0.6 have been studied using Small Angle Neutron Scattering (SANS) as a function of temperature and concentration. A worm-like chain model analysis of the SANS data using a combination of a cylindrical form factors for the polydisperse micellar length, circular cross-sectional radius with Gaussian polydispersity, and the structure factor based on a random phase approximation (RPA) suggests that these micelle solutions have a worm-like micellar structure that is independent of the concentration and temperature. The size of the micelle decreases monotonically with increasing temperature and increases with concentration. These observations indicate that large micelles are formed at low temperature and begin to break up to form smaller micelles with increasing temperature.

Applications of isothermal titration calorimetry in pure and applied research--survey of the literature from 2010

Isothermal titration calorimetry (ITC) is a biophysical technique for measuring the formation and dissociation of molecular complexes and has become an invaluable tool in many branches of science from cell biology to food chemistry. By measuring the heat absorbed or released during bond formation, ITC provides accurate, rapid, and label-free measurement of the thermodynamics of molecular interactions. In this review, we survey the recent literature reporting the use of ITC and have highlighted a number of interesting studies that provide a flavour of the diverse systems to which ITC can be applied. These include measurements of protein-protein and protein-membrane interactions required for macromolecular assembly, analysis of enzyme kinetics, experimental validation of molecular dynamics simulations, and even in manufacturing applications such as food science. Some highlights include studies of the biological complex formed by Staphylococcus aureus enterotoxin C3 and the murine T-cell receptor, the mechanism of membrane association of the Parkinson's disease-associated protein α-synuclein, and the role of non-specific tannin-protein interactions in the quality of different beverages. Recent developments in automation are overcoming limitations on throughput imposed by previous manual procedures and promise to greatly extend usefulness of ITC in the future. We also attempt to impart some practical advice for getting the most out of ITC data for those researchers less familiar with the method.

Stagnation point flow of wormlike micellar solutions in a microfluidic cross-slot device: effects of surfactant concentration and ionic environment

We employ the techniques of microparticle image velocimetry and full-field birefringence microscopy combined with mechanical measurements of the pressure drop to perform a detailed characterization of the extensional rheology and elastic flow instabilities observed for a range of wormlike micellar solutions flowing through a microfluidic cross-slot device. As the flow rate through the device is increased, the flow first bifurcates from a steady symmetric to a steady asymmetric configuration characterized by a birefringent strand of highly aligned micellar chains oriented along the shear-free centerline of the flow field. At higher flow rates the flow becomes three dimensional and time dependent and is characterized by aperiodic spatiotemporal fluctuations of the birefringent strand. The extensional properties and critical conditions for the onset of flow instabilities in the fluids are highly dependent on the fluid formulation (surfactant concentration and ionic strength) and the resulting changes in the linear viscoelasticity and nonlinear shear rheology of the fluids. By combining the measurements of critical conditions for the flow transitions with the viscometric material properties and the degree of shear-thinning characterizing each test fluid, it is possible to construct a stability diagram for viscoelastic flow of complex fluids in the cross-slot geometry.

Structure and dynamics of poly(oxyethylene) cholesteryl ether wormlike micelles: rheometry, SAXS, and cryo-TEM studies

In this article, we provide direct evidence for 1-D micellar growth and the formation of a network structure in an aqueous system of poly(oxyethylene) cholesteryl ether (ChEO(20)) and lauryl diethanolamide (L-02) by rheometry, small-angle X-ray scattering (SAXS), and cryo-transmission electron microscopy (cryo-TEM). The ChEO(20) self-assembles into spheroid micelles above the critical micelle concentration and undergoes a 1-D microstructural transition upon the incorporation of L-02, which because of its lipophilic nature tends to be solubilized into the micellar palisade layer and reduces the micellar curvature. The elongated micelles entangle with each other, forming network structures of wormlike micelles, and the system shows viscoelastic properties, which could be described by the Maxwell model. A peak observed in the zero-shear viscosity (η(0)) versus L-02 concentration curve shifted toward higher L-02 concentrations and the value of maximum viscosity (η(0 max)) increased with the increasing ChEO(20) mixing fraction with water. We observed that η(0 max) increased by 2 to 4 orders of magnitude as a function of the ChEO(20) concentration. The Maxwell relaxation time (τ(R)) shows a maximum value at a concentration corresponding to η(0 max) (i.e., τ(R) increases with L-02 concentration and then decreases after attaining a maximum value, whereas the plateau modulus (G(0)) shows monotonous growth). These observations demonstrate microstructural transitions in two different modes: L-02 first induces 1-D micellar growth and as a result the viscosity increases, and finally after the system attains its maximum viscosity, L-02 causes branching in the network structures. The microstructure transitions are confirmed by SAXS and cryo-TEM techniques.