Molecular simulation of sodium butyl benzene sulfonate at air–water interface and in aqueous solution

Molecular mechanics(MM) calculations for interfacial behaviour of sodium n-butyl benzene sulfonate (NaNBBS), sodium iso-butyl benzene sulfonate (NaIBBS) and sodium tert-butyl benzene sulfonate (NaTBBS) show a significant effect of the butyl group geometry on the surface area occupied by these molecules at the air–water interface. NaNBBS, in comparison with NaIBBS and NaTBBS, shows a closer molecular packing at the interface. The simulation predicts minimum hydrotrope concentration of each hydrotrope to reach surface saturation and molecular surface area at the interface match with good accuracy. The shape, size and charge of the hydrotrope aggregates obtained by molecular dynamics simulation also match well with the results of small angle neutron scattering experiments on the same hydrotrope. The simulation shows non-regular and ellipsoidal hydrotropes aggregates with substantial charge on the surface. The aggregates are also more open structures as compared to surfactant micelles. The water accessible surface area of a NaNBBS aggregate was 25% lower in comparison to that of NaTBBS aggregate, indicating closer packing of NaNBBS molecules. The fractional charge on the NaNBBS aggregate decreases with the increase in the number of NaNBBS molecules in the aggregate indicating more counter-ion association.     

Transition from rigid to flexible structures in micelles of Undecylammonium chloride in aqueous solutions of NaCl

Small angle neutron scattering (SANS) data for Undecylammonium chloride (UAC) in heavy water in the presence of NaCl 0.0428 and 0.3422 M are consistent with the presence of elongated micelles. This micellar shape has been adopted to analyze viscosity data of UAC in water in the presence of NaCl. The results obtained from this last technique are consistent with the increase of the micelle aggregation number with increasing the surfactant concentration. Micelles change from prolate ellipsoidal shape to cylindrical and wormlike shapes by increasing the added NaCl concentration and surfactant concentrations. The differences between results for the micelle aggregation number calculated from viscosity, SANS and light scattering data have been attributed to the solvent effect on micelle formation as well as changes in the size, shape and flexibility of the micelle. Viscosity data provide qualitative information on the effect of the added NaCl concentration and surfactant concentration on the size, shape, flexibility of the micelles in diluted solutions.     

SANS and light scattering of nonylammonium chloride micelles in aqueous solutions of sodium chloride

Summary Aqueous solutions of nonylammonium chloride (NAC) containing sodium chloride were studied by using light scattering and small-angle neutron scattering (SANS) techniques. It was found that the aggregation number of NAC is independent of the surfactant concentration and increases smoothly with increasing the ionic strength. Micelles of NAC in D₂O with [NaCl]=0.3422 have prolate ellipsoidal shape and their average aggregation number is greater than that found in H₂O at the same ionic strength. A possible explanation of this difference may arise from a solvent effect. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Small-angle neutron scattering study of aggregate structures of multi-headed pyridinium surfactants in aqueous solution

The aggregate structures of a set of novel single-chain surfactants bearing one, two and three pyridinium headgroups have been studied using small-angle neutron scattering (SANS). It is found that the nature of aggregate structures of these cationic surfactants depend on the number of headgroups present in the surfactants. The single-headed pyridinium surfactant forms the lamellar structure, whereas surfactants with double and triple headgroups form micelles in water. The aggregates shrink in size with increase in the number of headgroups in the surfactants. The aggregation number (N) continually decreases and the fractional charge (α) increases with more number of headgroups on the surfactants. The semimajor axis (a) and semiminor axis (b = c) of the micelle also decrease with the increase in the number of headgroups in the surfactants. This indicates that hydrocarbon chains in such micelles prepared from multiheaded surfactants adopt bent conformation and no longer stay in extended conformation.     

Small-angle neutron scattering studies of nonionic surfactant: Effect of sugars

Micellar solution of nonionic surfactantn-dodecyloligo ethyleneoxide surfactant, decaoxyethylene monododecyl ether [CH₃(CH₂)₁₁(OCH₂CH₂)₁₀OH], C₁₂E₁₀ in D₂O solution have been analysed by small-angle neutron scattering (SANS) at different temperatures (30, 45 and 60° C) both in the presence and absence of sugars. The structural parameters like micelle shape and size, aggregation number and micellar density have been determined. It is found that the micellar structure significantly depends on the temperature and concentration of sugars. The micelles are found to be prolate ellipsoids at 30° C and the axial ratio of the micelle increases with the increase in temperature. The presence of lower concentration of sugar reduces the size of micelles and it grows at higher concentration of sugar. The structure of micelles is almost independent of the different types of sugars used.     

Small angle neutron scattering studies on protein denaturation induced by different methods

Small angle neutron scattering (SANS) has been used to study conformational changes in protein bovine serum albumin (BSA) as induced by varying temperature and in the presence of protein denaturating agents urea and surfactant. BSA has prolate ellipsoidal shape and is found to be stable up to 60°C above which it denaturates and subsequently leads to aggregation. The protein solution exhibits a fractal structure at temperatures above 64°C, with fractal dimension increasing with temperature. BSA protein is found to unfold in the presence of urea at concentrations greater than 4 M and acquires a random coil Gaussian chain conformation. The conformation of the unfolded protein in the presence of surfactant has been determined directly using contrast variation SANS measurements by contrast matching surfactant molecules. The protein acquires a random coil Gaussian conformation on unfolding with its radius of gyration increasing with increase in surfactant concentration      

Spherical polyelectrolyte block copolymer micelles: Structural change in presence of monovalent salt

Spherical polyelectrolyte block copolymer micelles were investigated as a function of added NaCl salt concentration using Small-Angle Neutron Scattering (SANS) and Light Scattering (LS). The micelles are formed by the self-association of charged-neutral copolymers made of a long deuterated polyelectrolyte moiety (NaPSS_d)₂₅₁ and a short hydrophobic moiety (PEP)₅₂. In presence of salt, the core shape and the aggregation number of the micelles are not affected. The hydrodynamic radius of the micelle is found to be identical to the radius of the whole micelle deduced from neutron scattering and thus the hydrodynamic radius is a valid measure of the corona thickness. At the lowest salt concentrations investigated the thickness of the corona, R_s, remains essentially constant and a contraction is observed above an added-salt concentration c_s of 2×10^-2 M where this crossover concentration corresponds to the average ionic strength of the free counterions in the corona. The contraction takes place while maintaining a rod-like behavior of the chains at short scale and obeys to: R_s c_s^-0.18. The exponent 0.18 suggests an electrostatic persistence length proportional to the Debye screening length.     

Small angle neutron scattering study of mixed micelles of oppositely charged surfactants

Structures of mixed micelles of oppositely charged surfactants dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulphate (SDS) have been studied using small angle neutron scattering. The concentration of one of the components was kept fixed (0.3 M) and that of another varied in the range 0 to 0.1 M. The aggregation number and micellar size increase and fractional charge decreases dramatically with the addition of small amount of oppositely charged surfactant. The effect of addition of SDS on DTAB is significantly different from that of the addition of DTAB on SDS. The contrast variation SANS experiments using deuterated surfactant suggests the homogeneous mixing of two components in mixed micellar system.      

Form-factors for different aggregation models of micelles

Spherical micelles in ionic micellar solutions, often aggregate to form spherical, cylindrical or chain-like aggregates on addition of salt to the solution. It is known that the technique of small angle neutron scattering (SANS) can be used to distinguish spherical and cylindrical aggregates. To examine if SANS can be used to distinguish the latter two aggregation processes, we have calculated the angular distribution of scattered neutrons from 0.002 M CTAB solutions. These calculations show that aggregation of CTAB micelles results in large changes in SANS spectra. The shapes of SANS spectra are different for the above three types of aggregates, suggesting that technique of SANS can indeed be used to distinguish the three aggregation processes. The size of the aggregate can also be obtained from such studies.     

Small angle neutron scattering studies of mixed micelles of sodium cumene sulphonate with cetyl trimethylammonium bromide and sodium dodecyl sulphate

The aqueous solutions of sodium cumene sulphonate (NaCS) and its mixtures with each of cetyl trimethylammonium bromide (CTAB) and sodium dodecyl sulphate (SDS) are characterized by small angle neutron scattering (SANS). NaCS when added to CTAB solution leads to the formation of long rod-shaped micelles with a dramatic increase in the CTAB aggregation number. Its addition to SDS on the other hand results in the formation of smaller mixed micelles where part of SDS molecules in the micelle is replaced by NaCS molecules.      

Small-angle neutron scattering study on the cold rolled steel sheets

The effect of cold rolling on the small-angle neutron scattering (SANS) pattern was investigated for low-carbon steels. Several cold rolled steel samples with different reduction ratios and annealed samples after cold rolling were respectively measured by SANS. The cold rolled samples presented anisotropic two-dimensional (2D) SANS patterns. From the 2D SANS patterns two kinds of 1D pattern were calculated: one was for the Q⊥RD (rolling direction), the other for the Q//RD. The scatterer sizes calculated from the 1D patterns by using a model fitting increased with the reduction ratio, for the Q⊥RD section only. The annealed sample presented an isotropic SANS pattern. It can be concluded that the dislocations produced during the cold rolling process accumulated around the precipitates and then caused anisotropic 2D SANS patterns.     

Macroscopic and microscopic structural integrity in magnetic colloids—cationic micellar solution: Rheology, SANS and magneto-optical study

A stable mixture of two colloid system composed of double surfactant coated aqueous nanomagnetic fluid and aqueous micellar solution of cationic micelles of cetyletrymethyl ammonium bromide (CTABr) is prepared as a function of nanomagnetic fluid concentration. This mixed system is analyzed using three techniques such as zero field and field induced viscosity measurements, Small Angle Neutron Scattering technique and magneto-optical birefringence measurements. In field induced viscosity measurement it is observed that even 20% magnetic fluid concentration in CTABr aqueous solution shows 75% increase in viscosity compared to pure magnetic fluid. This suggests that in presence of CTABr micelles, a novel magneto rheological effect for low concentration of magnetic fluid is observed. From SANS measurements it is observed that aggregation number and a/b ratio increases with magnetic fluid concentration and magnetic birefringence reveals non-superimpose behavior of normalized field induced retardation. Results of these experiments are compared and indicate zero fields and field induced structural integrity between magnetic particles and soft micelles.     

Structure of hydrophobically and hydrogen-bonded complexes between amphiphilic copolymer and polyacid in water

We observed by SANS and NMR the structure of intermolecular complexes formed through hydrogen bonding and hydrophobic interactions between a polyacid and a neutral copolymer surfactant (PEO-PPO-PEO). The polyacid is perdeuterated and the contrast variation method enables us to measure separately the structure factor of each component in the complex and the cross structure factor as a function of the pH and the temperature. The evolution of NMR spectra and relaxation times with pH and temperature give a complementary insight into the inner structure of the aggregates. The interaction between the aggregates and the aggregation number of polyacid chains are governed by electrostatic effects; they depend on the polyacid charge and are insensitive to temperature. The number of copolymer chains which results from the cooperative action of hydrogen bonding and hydrophobic interactions is sensitive to both pH and temperature. The complexation preserves the micellar core-corona structure of the copolymer and shrinks the polyacid chains which adopt a compact structure. The non-dissociated polyacid sequences are bound to the PPO part of the copolymer forming the hydrophobic core of the complex, whereas the ionized polyacid sequences form with the PEO a stabilizing hydrated corona.Received: 27 April 2004, Published online: 3 August 2004PACS: 61.12.-q Neutron diffraction and scattering - 82.56.-b Nuclear magnetic resonance - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling - 82.70.-y Disperse systems; complex fluids     

Temperature and concentration dependence of SANS spectra of aqueous solutions of short-chain amphiphiles

The small-angle neutron scattering (SANS) of some aqueous solutions of short-chain amphiphiles (glycols, diglycols, diols) has been measured as a function of concentration and temperature. The analysis of the spectra in terms of the Teubner-Strey phenomelogical formula indicates that, on increasing the concentration of the amphiphile, the structure of all these systems evolves in a similar way, i.e. a transition from disordered structures toward correlated aggregates (microstructures). The transition is depressed by increasing the temperature.     

Solution properties of phenothiazine drug promazine hydrochloride with cationic hydrotropes in aqueous/electrolyte solution at different temperature

The current work deals with the mixed micellization phenomena of surface active promazine hydrochloride (PMZ) drug with cationic hydrotropes (para‐toluidine hydrochloride and ortho‐toluidine hydrochloride) in absence and occurrence of 50 mmol kg^?1 NaCl at five different temperature (293.15–313.15 K). PMZ is an amphiphilic phenothiazine drug and employed for the cure of mania and schizophrenia. Conductometry measurement was employed to gain a detailed picture of the interactions between drug and hydrotrope molecules. The experimental data were analyzed according to different mixing models within the outline of the pseudophase separation model. The evaluated values of critical micelle concentration (cmc) were found to be inferior than cmc^id values signifying attractive interactions involving the both components in the solutions. NaCl further reduces the cmc of pure amphiphiles and their mixed systems as a result of screening of the electrostatic repulsion between the polar head groups. The micellar mole fractions (X₁) of hydrotropes evaluated by various proposed models were constantly more than ideal values ( ) signifying high involvements of hydrotrope in mixed micelles. Activity coefficients ( and ) were always below one in all cases signifying synergism in mixed micelles. Thermodynamic parameters favor the process of micellization which is found to be entropy driven. The negative values of free energies of mixing demonstrated the stability of the mixed systems of drug and hydrotrope. Copyright © 2016 John Wiley & Sons, Ltd.     

Structural variations induced by magnesium ions and temperature in the large ribosomal subunit from the extreme thermophile Sulfolobus solfataricus

Summary Small-angle neutron scattering (SANS) measurements on the large 50S ribosomal subunit of the extreme thermophile archaebacteriumSulfolobus solfataricus are presented. The structural effects induced by magnesium ions and temperature are investigated. Supramolecular aggregation was present in all the investigated conditions. Removal of magnesium ions and increase of temperature reduce the intensity of the scattering atQ=0. This reduction is discussed looking at the Fourier inversion of the scattering intensity, which includes both inter- and intra-particle contributions. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Anisotropy and temperature dependence of small angle neutron scattering from silicon-oxide precipitates in silicon

Czochralski grown silicon crystals contain interstitially dissolved oxygen which diffuses on heating to form precipitates of silica. We have examined these precipitates by small angle neutron scattering (SANS) in the Q-range 0.05 Å^–1<Q<0.4 Å^–1. The obtained SANS patterns reveal pronounced anisotropic intensity distributions which resemble the symmetry of the host crystal. The SANS spectra show an anisotropic central peak at Q<0.1 Å^–1 due to the single particle shape and a number of weak intensities for larger Q-values. These weak side maxima are considered correlation peaks or quasi-elastic interference peaks. They show, however, an unexpected and distinct temperature dependence: with decreasing temperature below values of 220 K their intensity is lost slowly, but reversibly. At T = 50 K only the central peak from the single-particle scattering remains unchanged. Upon heating, the correlation peaks regain their former value of intensity and Q-position without any evidence of thermal hysteresis.     

Structural investigation of viscoelastic micellar water/CTAB/NaNO<Subscript>3</Subscript> solutions

A highly viscoelastic worm-like micellar solution is formed in hexadecyltrimethylammonium bromide (CTAB) in the presence of sodium nitrate (NaNO₃). A gradual increase in micellar length with increasing NaNO₃ was assumed from the rheological measurements where the zero-shear viscosity (η ₀) versus NaNO₃ concentration curve exhibits a maximum. However, upon increase in temperature, the viscosity decreases. Changes in the structural parameters of the micelles with addition of NaNO₃ were inferred from small angle neutron scattering measurements (SANS). The intensity of scattered neutrons in the low q region was found to increase with increasing NaNO₃ concentration. This suggests an increase in the size of the micelles and/or decrease of intermicellar interaction with increasing salt concentration. Analysis of the SANS data using prolate ellipsoidal structure and Yukawa form of interaction potential between micelles indicate that addition of NaNO₃ leads to a decrease in the surface charge of the ellipsoidal micelles which induces micellar growth. Cryo-TEM measurements support the presence of thread-like micelles in CTAB and NaNO₃.      

Relating structure and flow of soft colloids

To relate the complex macroscopic flow of soft colloids to details of its microscopic equilibrium and non-equilibrium structure is still one big challenge in soft matter science. We investigated several well-defined colloidal model systems like star polymers or diblock copolymer micelles by linear/non-linear rheology, static/dynamic light scattering (SLS/DLS) and small angle neutron scattering (SANS). In addition, in-situ SANS experiments during shear (Rheo-SANS) revealed directly shear induced structural changes on a microscopic level. Varying the molecular architecture of the individual colloidal particle as well as particle-particle interactions and covering at the same time a broad concentration range from the very dilute to highly concentrated, glassy regime, we could separate contributions from intra- and inter-particle softness. Both can be precisely “tuned” by varying systematically the functionality, 6 ≤ f≤ 64, for star polymers or aggregation number, 30 ≤ N _agg ≤ 1000 for diblock copolymer micelles, as well as the degree of polymerization of the individual polymer arm 100 ≤ D _p ≤ 3000. In dilute solutions, the characteristic shear rate at which deformation of the soft colloid is observed can be related to the Zimm time of the polymeric corona. In concentrated solutions, we validated a generalized Stokes-Einstein approach to describe the increase in macroscopic viscosity and mesoscopic self diffusion coefficient on approaching the glassy regime. Both can be explained in terms of an ultra-soft interaction potential. Moreover, non-equilibrium structure factors are obtained by Rheo-SANS. All experimental results are in excellent quantitative agreement with recent theoretical predictions.     

Time-resolved nuclear spin-dependent small-angle neutron scattering from polarised proton domains in deuterated solutions

We have investigated the process of dynamic proton polarisation by means of time-resolved polarised small-angle neutron scattering (SANS) on frozen solutions of EHBA-Cr^V molecules in glycerol-water mixtures as a function of the concentration of EHBA-Cr^V and for different degrees of deuteration of the solvent. In the EHBA-Cr^V complex, the spins of the 20 protons which surround the paramagnetic Cr^V can be oriented using the method of dynamic nuclear polarisation (DNP), thereby offering the possibility to create locally a nuclear spin-dependent contrast for SANS. The time constants which describe the build-up of polarisation around the paramagnetic centre and the subsequent diffusion of polarisation in the solvent were determined by analysing the temporal evolution of the nuclear polarisation, which in turn was obtained by fitting a core-shell model to the time-dependent SANS curves. The results on the spin dynamics obtained using the scattering function of a core-shell could be independently confirmed by evaluating the integrated SANS intensity. A thermodynamic one-centre model is presented which is able to reproduce the observed dependence of the proton polarisation times on the proton concentration of the solvent.