Structural and therapeutic properties of salicylic acid-solubilized Pluronic solutions and hydrogels.

Salicylic acid (SA) finds extensive applications in the treatment of rheumatic and skin diseases because of its analgesic, anti-inflammatory and exfoliating properties. As it is lipophilic in nature, there is a need for appropriate delivery systems to harness these properties for different applications. Herein, we examined the suitability of Pluronic P123/F127 micellar systems as delivery media by investigating the structural, flow and antimicrobial properties of P123/F127-SA solutions and hydrogels using DLS, SANS, rheological and zone inhibition measurement techniques. SA modulates the aggregation characteristics of these surfactant systems and brings about spherical-to-worm-like micelle-to-vesicular structural transitions in the hydrophobic Pluronic P123 system, a spherical-to-worm-like micellar transition in the mixed P123/F127 system and an onset of inter-micellar attraction in the hydrophilic Pluronic F127 system. SA-solubilized systems of both hydrophobic and hydrophilic Pluronics inhibit the growth of Gram-positive and Gram-negative bacteria with comparable MIC values. This suggests that the interaction of SA molecules with the bacterial cell membrane remains unobstructed upon encapsulation in Pluronic micelles. F127 hydrogel-based SA formulations with rheological properties suitable for topical applications and up to 15% SA loading were prepared. These will be useful SA ointments as F127 is an FDA-approved excipient for topical drug delivery applications. The results indicate that Pluronics remain effective as delivery agents for SA and exhibit interesting structural polymorphism upon its solubilization.

Fluorescent complex coacervates of agar and in situ formed zein nanoparticles: Role of electrostatic forces.

Herein, the complex coacervation between in situ formed spherical fluorescent zein nanoparticles and polyanion agar as function of mixing ratio (R=[Agar]/[Zein]) was investigated. This interaction yielded two distinguishable regions (at pH 5.4): Region I (R < 0.2), where fully charge neutralized soluble complexes with protein denaturation was noticed, and Region II (R > 0.2), where overcharged complexes were formed, with R = 0.2 defining the optimum binding. Small angle neutron scattering studies demonstrated that in the low-q region, nanoparticles formed the crosslink junctions and in the persistence regime of high-q region, the data captured the cross-sectional radius ( = 3.5 nm) for agar-zein complexes. The coacervates became more viscoelastic in salt-free samples because both the low frequency storage modulus and crosslink density were found to decrease with mixing ratio. Systematic decrease in storage modulus with ionic strength (0-0.01 M) implied screened Coulomb interaction was responsible for the observed coacervation. Further, we seek to find universality in complex coacervation of zein nanoparticle with biopolymers, and polysaccharides in particular.

Mixing ratio dependent complex coacervation versus bicontinuous gelation of pectin with in situ formed zein nanoparticles.

We report on the competitive phenomenon of complex coacervation versus bicontinuous gelation between pectin (P, a polyanionic carbohydrate, [P] = 0.01-2% (w/v)) and zein nanoparticles (Z, a hydrophobic protein and a weak polyampholyte, [Z] = 0.1 and 0.5% (w/v), in an ethanolic solution of effective concentration 4 and 27% (v/v)), which was studied below (pH ≈ 4), and above (pH ≈ 7.4) the pI (≈ 6.2) of zein at room temperature, 25 °C. The uniqueness of this study arises from the interaction protocol used, where the pectin used was in the extended polyelectrolyte (persistence length ≈ 10 nm) conformation while zein was used as a charged globular nanoparticle (size ≈ 80-120 nm) that was formed in situ. Their mixing ratio, r = [P] : [Z] (w/w), was varied from 0.02 to 4.0 (for [Z] = 0.5% (w/v)), and from 0.1 to 7.5 (for [Z] = 0.1% (w/v)) in the ionic strength range 10-4 to 10-2 M NaCl. Zeta potential data revealed that at pH ≈ 4, the complementary binding condition, r = 1 : 1 (equivalent to 1 : 5 molecule/nanoparticle) demarcated the coacervate from the gel region. The measured rigidity (G0, low frequency storage modulus) of these materials revealed the following: for r < 1, (low pectin content samples, coacervate region) the material had lower values of Gcoac0, whereas for r > 1, an excess of pectin facilitated gelation with Ggel0 ≫ Gcoac0. Above pI, surface patch binding caused associative interactions and complex coacervation though both biopolymers had similar net charge. The network density was used as a descriptor to distinguish between the coacervate and gel samples. Their microstructures were probed by small angle neutron scattering (SANS), and viscoelastic properties by rheology. Simple modeling shows that formation of the interpolymer complex was favored in higher protein containing samples. Mixing ratio dependent selective coacervation (a kinetic process) and bicontinuous gelation (a thermodynamic process) are rarely seen to coexist in biopolymer interactions.

Crystal-to-Crystal Transition of Ultrasoft Colloids under Shear.

Ultrasoft colloids typically do not spontaneously crystallize, but rather vitrify, at high concentrations. Combining in situ rheo-small-angle-neutron-scattering experiments and numerical simulations we show that shear facilitates crystallization of colloidal star polymers in the vicinity of their glass transition. With increasing shear rate well beyond rheological yielding, a transition is found from an initial bcc-dominated structure to an fcc-dominated one. This crystal-to-crystal transition is not accompanied by intermediate melting but occurs via a sudden reorganization of the crystal structure. Our results provide a new avenue to tailor colloidal crystallization and the crystal-to-crystal transition at the molecular level by coupling softness and shear.

The effect of temperature, composition and alcohols on the microstructures of catanionic mixtures of sodium dodecylsulfate and cetyltrimethylammonium bromide in water.

The influence of mixing protocol, composition, temperature, ageing and added alcohols on the characteristics of the microstructures of sodium dodecylsulfate (SDS) + cetyltrimethylammonium bromide (CTAB) mixtures has been investigated in this paper. In this catanionic mixture (1 weight% total surfactant content) temperature induced microstructural transition occurs, which is (i) a micelle-to-vesicle transition (MVT) if αSDS (mole fraction of SDS) = 0.7, 0.8 or 0.9 and (ii) a vesicle-to-micelle transition (VMT) if αSDS = 0.1, 0.2 or 0.3. In the mixture of αSDS = 0.7, specific conductivity and dynamic light scattering measurements also support the occurrence of MVT. Transition electron microscopy and small angle neutron scattering measurements were also made to assess the characteristics of the microstructures. Alcohols added to the mixture of αSDS = 0.7 reduced the size of the vesicle, while only monohydric alcohols suppressed the temperature induced transition indicating that the number and location of -OH groups of the alcohols have a dramatic modulating influence on the structural transition occurring in catanionic mixtures. The influence of the alcohols is explained in terms of changes produced in the dielectric constant and hydrophobicity of the medium.

High hydrostatic pressure specifically affects molecular dynamics and shape of low-density lipoprotein particles.

Lipid composition of human low-density lipoprotein (LDL) and its physicochemical characteristics are relevant for proper functioning of lipid transport in the blood circulation. To explore dynamical and structural features of LDL particles with either a normal or a triglyceride-rich lipid composition we combined coherent and incoherent neutron scattering methods. The investigations were carried out under high hydrostatic pressure (HHP), which is a versatile tool to study the physicochemical behavior of biomolecules in solution at a molecular level. Within both neutron techniques we applied HHP to probe the shape and degree of freedom of the possible motions (within the time windows of 15 and 100 ps) and consequently the flexibility of LDL particles. We found that HHP does not change the types of motion in LDL, but influences the portion of motions participating. Contrary to our assumption that lipoprotein particles, like membranes, are highly sensitive to pressure we determined that LDL copes surprisingly well with high pressure conditions, although the lipid composition, particularly the triglyceride content of the particles, impacts the molecular dynamics and shape arrangement of LDL under pressure.

DNA ionogel: Structure and self-assembly.

DNA dissolved in ionic liquid (IL) solution (1-ethyl-3-methylimidazolium chloride, [C2mim][Cl]) showed a transition to the gel phase ([DNA] ≥ 1% (w/v)). The gelation time was 400 s for the 1% [IL] sample which reduced to 260 s for 5% [IL] concentration. Gelation times, obtained from the viscosity and ergodicity breaking from the dynamic structure factor data, were remarkably identical to each other. Correspondingly, the gelation temperature which was ∼60 °C increased to 67 °C with [IL] content. The small angle neutron scattering (SANS) structure factor profile revealed the presence of the following three distinct length scales: (a) mesh size, ξ ≈ 3 ± 0.5 nm for ionogels, and ≈0.73 ± 0.06 nm, for sol; (b) cross-sectional radius of DNA strand, Rc ≈ 1.6 ± 0.1 nm; and (c) the characteristic inter-cluster distance ≈33 ± 5 nm. Physical conformation of the DNA-IL complexes remained close to the Gaussian coil definition. It was observed that without IL, in the sol phase, the system was completely ergodic and did not gel, while on addition of IL a sudden transition to the non-ergodic (arrested) gel phase occurred. This was due to the formation of an amorphous network of DNA-IL complexes preceding gelation. In summary, it is shown that the DNA ionogels can be prepared with a tunable gel strength (27-70 Pa) and gelation temperature (60-67 °C). Further, the relaxation dynamics was found to be hierarchical in IL content of the gel, revealing considerable self-organization.

Internal structure and thermo-viscoelastic properties of agar ionogels.

Ionic liquids (IL) can alter the physical properties of agar hydrogels. Rheology studies show that gels with wide range of storage moduli (gel strength) G0 values ranging from 1 to 20 KPa could be made in imidazolium based IL solutions where the IL concentration may not exceed 5% (w/v). Gelation and gel melting temperatures (tgel and Tm) could be altered by as much as ≈ 10 °C. Small angle neutron scattering studies revealed the presence of fibre bundles of agar double helices having typical length of 120 nm that increased to ≈ 180 nm under favorable conditions. These structures gain flexibility from the cladding of the agar bundles by IL molecules which in turn caused partial charge neutralization of its surface. Raman spectroscopy revealed differential hydration of these bundles. It was found that IL molecules with longer alkyl chain (more hydrophobic) altered the gel homogeneity, and changed its thermal and mechanical properties significantly. Therefore, customization of agar hydrogels in green solvent medium (IL solutions) widens the scope of its application potential that may include sensing.

Electrolyte effect on the phase behavior of silica nanoparticles with lysozyme and bovine-serum-albumin proteins.

Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) studies have been carried out to investigate the effect of an electrolyte on the phase behavior of anionic silica nanoparticles with two globular proteins-cationic lysozyme [molecular weight (MW) 14.7 kDa] and anionic bovine serum albumin (MW 66.4 kDa). The results are compared with our earlier published work on similar systems without any electrolyte [I. Yadav, S. Kumar, V. K. Aswal, and J. Kohlbrecher, Phys. Rev. E 89, 032304 (2014)]. Both the nanoparticle-protein systems transform to two phase at lower concentration of protein in the presence of an electrolyte. The autocorrelation function in DLS suggests that the diffusion coefficient (D) of a nanoparticle-protein system decreases in approaching two phase with the increase in protein concentration. This variation in D can be attributed to increase in attractive interaction and/or overall increase in the size. Further, these two contributions (interaction and structure) are determined from the SANS data. The changes in the phase behavior of nanoparticle-protein systems in the presence of an electrolyte are explained in terms of modifications in both the repulsive and attractive components of interaction between nanoparticles. In a two-phase system individual silica nanoparticles coexist along with their fractal aggregates.

Structure and interaction in the polymer-dependent reentrant phase behavior of a charged nanoparticle solution.

Small-angle neutron scattering (SANS) studies have been carried out to examine the evolution of interaction and structure in a nanoparticle (silica)-polymer (polyethylene glycol) system. The nanoparticle-polymer solution interestingly shows a reentrant phase behavior where the one-phase charged stabilized nanoparticles go through a two-phase system (nanoparticle aggregation) and back to one-phase as a function of polymer concentration. Such phase behavior arises because of the nonadsorption of polymer on nanoparticles and is governed by the interplay of polymer-induced attractive depletion with repulsive nanoparticle-nanoparticle electrostatic and polymer-polymer interactions in different polymer concentration regimes. At low polymer concentrations, the electrostatic repulsion dominates over the depletion attraction. However, the increase in polymer concentration enhances the depletion attraction to give rise to the nanoparticle aggregation in the two-phase system. Further, the polymer-polymer repulsion at high polymer concentrations is believed to be responsible for the reentrance to one-phase behavior. The SANS data in polymer contrast-matched conditions have been modeled by a two-Yukawa potential accounting for both repulsive and attractive parts of total interaction potential between nanoparticles. Both of these interactions (repulsive and attractive) are found to be long range. The magnitude and the range of the depletion interaction increase with the polymer concentration leading to nanoparticle clustering. At higher polymer concentrations, the increased polymer-polymer repulsion reduces the depletion interaction leading to reentrant phase behavior. The nanoparticle clusters in the two-phase system are characterized by the surface fractal with simple cubic packing of nanoparticles within the clusters. The effect of varying ionic strength and polymer size in tuning the interaction has also been examined.

Small-angle neutron scattering study of differences in phase behavior of silica nanoparticles in the presence of lysozyme and bovine serum albumin proteins.

The differences in phase behavior of anionic silica nanoparticles (88 Å) in the presence of two globular proteins [cationic lysozyme (molecular weight (MW) 14.7 kD) and anionic bovine serum albumin (BSA) (MW 66.4 kD)] have been studied by small-angle neutron scattering. The measurements were carried out on a fixed concentration (1 wt %) of Ludox silica nanoparticles with varying concentrations of proteins (0-5 wt %) at pH = 7. It is found that, despite having different natures (opposite charges), both proteins can render to the same kind of aggregation of silica nanoparticles. However, the concentration regions over which the aggregation is observed are widely different for the two proteins. Lysozyme with very small amounts (e.g., 0.01 wt %) leads to the aggregation of silica nanoparticles. On the other hand, silica nanoparticles coexist with BSA as independent entities at low protein concentrations and turn to aggregates at high protein concentrations (>1 wt %). In the case of lysozyme, the charge neutralization by the protein on the nanoparticles gives rise to the protein-mediated aggregation of the nanoparticles. The nanoparticle aggregates coexist with unaggregated nanoparticles at low protein concentrations, whereas, they coexist with a free protein at higher protein concentrations. For BSA, the nonadsorbing nature of the protein produces the depletion force that causes the aggregation of the nanoparticles at higher protein concentrations. The evolution of the interaction is modeled by the two Yukawa potential, taking account of both attractive and repulsive terms of the interaction in these systems. The nanoparticle aggregation is found to be governed by the short-range attraction for lysozyme and the long-range attraction for BSA. The aggregates are characterized by the diffusion limited aggregate type of mass fractal morphology.

Effect of ethylene glycol on the special counterion binding and microstructures of sodium dioctylsulfosuccinate micelles.

Sodium dioctylsulfosuccinate (AOT) micelle has a special counterion binding behavior (SCB), which refers to the abrupt twofold increase in the counterion binding constant (ß) at a critical concentration (c*) of added NaCl (in water c* ≈ 0.015 mol kg(-1)). In this paper, the SCB of AOT has been studied in a mixture of water and ethylene glycol (EG) by applying surface tension, fluorescence, and small angle neutron scattering (SANS) methods. The SCB exists in water + 10% (w/w) EG as well, but disappears when the EG% is ≥20. It has been found out that the SCB of AOT occurs in media having cohesive energy density values in the range of 2.3-2.75 J m(-3). SANS data indicate co-existence of vesicles and cylindrical micelles of AOT in water + 10% EG when the added NaCl concentration is greater than c* thereby revealing that change in the morphology of aggregated species is the probable cause for the SCB of AOT. From this study it has become clear that the Corrin-Harkins (CH) equation, commonly used for determining ß, can be applied only above a limiting concentration (ce(#)) of added electrolyte. In aqueous organic or pure organic polar solvents below ce(#) sharp deviation from the CH equation occurs with reversal of slope rendering this equation inapplicable for the determination of ß.

Small-angle neutron scattering study of structure and interaction during salt-induced liquid-liquid phase transition in protein solutions.

The liquid-liquid phase transition (LLPT) in aqueous salt solutions of lysozyme protein has been studied by small-angle neutron scattering. Measurements have been carried out on fixed protein concentration with varying salt concentration approaching LLPT. The data are fitted considering protein interaction by the two Yukawa (2Y) potential which combines short-range attraction and long-range repulsion. We show that LLPT arises because of enhancement of non-DLVO (Derjaguin-Landau-Verwey-Overbeek) short-range attraction without any conformational structural change of the protein. The salt concentration required for LLPT as well as corresponding short-range attraction decreases significantly with increase in protein concentration.

Electric field control of the skyrmion lattice in Cu2OSeO3.

Small-angle neutron scattering has been employed to study the influence of applied electric (E-)fields on the skyrmion lattice in the chiral lattice magnetoelectric Cu(2)OSeO(3). Using an experimental geometry with the E-field parallel to the [111] axis, and the magnetic field parallel to the [11(-)0] axis, we demonstrate that the effect of applying an E-field is to controllably rotate the skyrmion lattice around the magnetic field axis. Our results are an important first demonstration for a microscopic coupling between applied E-fields and the skyrmions in an insulator, and show that the general emergent properties of skyrmions may be tailored according to the properties of the host system.

Small-angle neutron scattering study of temperature vs. salt dependence of clouding in charged micellar system.

Clouding is studied by small-angle neutron scattering (SANS) on a charged micellar system of sodium dodecyl sulphate (SDS) and tetrabutylammonium bromide (TBAB) with varying temperature and salt NaCl. We show that the clouding occurs as a result of increase in the attractive potential between the micelles mediated by the dehydrated TBA(+) counterions on increasing temperature and in the presence of salt. Both micelles and clusters coexist at cloud point temperature (CP) and beyond CP. The addition of salt can be used to obtain CP at room temperature (30° C). The relative effect of different salts on clouding has been found in the order CaCl(2) > MgSO(4) > Na(2)SO(4) > NaF > NaCl > KCl > CsCl > NaBr > NaNO(3). This order is explained on the basis of two important roles played by salt ions: i) counterion condensation that increases the size of the micelles and ii) dehydration of TBA(+) counterions by salt ions for bridging the micelles.

Structural properties of thermoresponsive poly(N-isopropylacrylamide)-poly(ethyleneglycol) microgels.

We present investigations of the structural properties of thermoresponsive poly(N-isopropylacrylamide) (PNiPAM) microgels dispersed in an aqueous solvent. In this particular work poly(ethyleneglycol) (PEG) units flanked with acrylate groups are employed as cross-linkers, providing an architecture designed to resist protein fouling. Dynamic light scattering (DLS), static light scattering (SLS), and small angle neutron scattering (SANS) are employed to study the microgels as a function of temperature over the range 10 °C ≤ T ≤ 40 °C. DLS and SLS measurements are simultaneously performed and, respectively, allow determination of the particle hydrodynamic radius, R(h), and radius of gyration, R(g), at each temperature. The thermal variation of these magnitudes reveals the microgel deswelling at the PNiPAM lower critical solution temperature (LCST). However, the hydrodynamic radius displays a second transition to larger radii at temperatures T ≤ 20 °C. This feature is atypical in standard PNiPAM microgels and suggests a structural reconfiguration within the polymer network at those temperatures. To better understand this behavior we perform neutron scattering measurements at different temperatures. In striking contrast to the scattering profile of soft sphere microgels, the SANS profiles for T ≤ LCST of our PNiPAM-PEG suspensions indicate that the particles exhibit structural properties characteristic of star polymer configurations. The star polymer radius of gyration and correlation length gradually decrease with increasing temperature despite maintenance of the star polymer configuration. At temperatures above the LCST, the scattered SANS intensity is typical of soft sphere systems.

Neutron optical beam splitter from holographically structured nanoparticle-polymer composites.

We report a breakthrough in the search for versatile diffractive elements for cold neutrons. Nanoparticles are spatially arranged by holographical means in a photopolymer. These grating structures show remarkably efficient diffraction of cold neutrons up to about 50% for effective thicknesses of only 200   µm. They open up a profound perspective for next generation neutron-optical devices with the capability to tune or modulate the neutron diffraction efficiency.

Osmotic shrinkage in star/linear polymer mixtures.

Multiarm star polymers were used as model grafted colloidal particles with long hairs, to study their size variation due to osmotic forces arising from added linear homopolymers of smaller size. This is the origin of the depletion phenomenon that has been exploited in the past as a means to melt soft colloidal glasses by adding linear chains and analyzed using dynamic light scattering experiments and an effective interactions analysis yielding the depletion potential. Shrinkage is a generic phenomenon for hairy particles, which affects macroscopic properties and state transitions at high concentrations. In this work we present a small-angle neutron scattering study of star/linear polymer mixtures with different size ratios (varying the linear polymer molar mass) and confirm the depletion picture, i.e., osmotic star shrinkage. Moreover, we find that as the linear/star polymer size ratio increases for the same effective linear volume fraction (c/c* with c* the overlapping concentration), the star shrinkage is reduced whereas the onset of shrinkage appears to take place at higher linear polymer volume fractions. A theoretical description of the force balance on a star polymer in solution, accounting for the classic Flory contributions, i.e. elastic and excluded volume, as well as the osmotic force due to the linear chains, accurately predicts the experimental findings of reduced star size as a function of linear polymer concentration. This is done in a parameter-free fashion, in which the size of the cavity created by the star, and from which the chains are excluded, is related to the radius of the former from first principles.

Slow dynamics, aging, and crystallization of multiarm star glasses.

Multiarm star polymers are model systems with tunable intermediate colloid to polymerlike character, exhibiting rich phase behavior, internal relaxations, and flow properties. An important puzzle for several years has been the lack of clear experimental proof of crystalline states despite strong theoretical predictions. We present unambiguous evidence via multispeckle dynamic light scattering (MSDLS) and small-angle neutron scattering (SANS) for such crystallization in a solvent of intermediate quality. An unexpected speed up of the short-time star diffusion observed in MSDLS was attributed by SANS to crystallization, via aging, of the multiam star glass. This delayed glass to crystal transition establishes a pathway for star crystallization that might be generic in colloidal glasses.

Small-angle neutron scattering study of protein unfolding and refolding.

Small-angle neutron scattering has been used to study protein unfolding and refolding in protein bovine serum albumin (BSA) due to perturbation in its native structure as induced by three different protein denaturating agents: urea, surfactant, and pressure. The BSA protein unfolds for urea concentrations greater than 4 M and is observed to be independent of the protein concentration. The addition of surfactant unfolds the protein by the formation of micellelike aggregates of surfactants along the unfolded polypeptide chains of the protein and depends on the ratio of surfactant to protein concentration. We make use of the dilution method to show the refolding of unfolded proteins in the presence of urea and surfactant. BSA does not show any protein unfolding up to the pressure of 450 MPa. The presence of urea and surfactant (for concentrations prior to inducing their own unfolding) has been used to examine pressure-induced unfolding of the protein at lower pressures. The protein unfolds at 200 MPa pressure in the presence of urea; however, no unfolding is observed with surfactant. The protein unfolding is shown to be reversible in all the above denaturating methods.