Article Cite This: Langmuir 2019, 35, 9867−9877
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Discerning the Structure Factor of Charged Micelles in Water and Supercooled Solvent by Contrast Variation X‑ray Scattering Santosh L. Gawali,†,‡ Mingming Zhang,§ Sugam Kumar,∥ Debes Ray,∥ Manidipa Basu,† Vinod K. Aswal,‡,∥ Dganit Danino,§ and Puthusserickal A. Hassan*,†,‡ †
Chemistry Division and ∥Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400 094, India § Faculty of Biotechnology and Food Engineering, TechnionIsrael Institute of Technology, Haifa 32000, Israel Downloaded via BUFFALO STATE on August 3, 2019 at 12:52:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: Sodium dodecyl sulfate (SDS) is a well-known anionic surfactant that forms micelles in various solvents including supercooled sugar−urea melt. Here, we explore the application of contrast variation smallangle X-ray scattering (SAXS) in discerning the structure and interactions of SDS micelles in aqueous solution and in a room-temperature supercooled solvent. The SAXS patterns can be analyzed in terms of a core−shell ellipsoid model. For aqueous SDS micelles, at low volume fractions, the features due to intermicellar interaction, S(q), in the SAXS pattern are poorly resolved because of the prominent contribution from shell scattering. Increasing the electron density of the solvent by the addition of the urea or fructose−urea mixture (at a weight ratio of 6:4) permits the systematic variation of shell scattering without influencing the structure drastically. For a 10% solution of SDS in water, the contribution from the shell can be completely masked by the addition of 40% urea or fructose−urea mixture. The fructose−urea mixture is a preferred additive as it can vary the scattering length density over a wide range and serves as a matrix to form supercooled micelles. The structural parameters of micelles in supercooled fructose−urea melt are obtained from contrast variation SAXS, small-angle neutron scattering, and high-resolution transmission electron microscopy.
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INTRODUCTION The spontaneous organization of amphiphiles in selected solvents to form nanoscale assemblies such as micelles, vesicles, lyotropic liquid crystals, etc. stems from an interplay of various noncovalent interactions. Such assemblies find applications in various fields including pharmaceutical, food, biomedical, cosmetic formulations, heterogeneous catalysis, etc.1−6 Environmental conditions such as electrolytes, pH, solvents, etc. influence the noncovalent interaction between amphiphiles and hence modulate the self-assembly process. Though water has been used extensively as the solvent for micellization, recently there have been efforts to understand micellization in other organic polar solvents, ionic liquids, deep eutectic solvents, etc.7−9 Our group has demonstrated the use of supercooled sugar−urea melt as a new solvent system for creating dynamically arrested micelles.10 Sugar−urea melts bear a similarity to natural deep eutectics as the ingredients are of natural origin.11,12 Moreover, the ability to form supercooled liquids at room temperature makes it attractive solvents with controlled viscosity and diffusion characteristics. The discovery of micelle formation in sugar−urea melt is significant as it enhances our understanding of the solvophobic effect in nonaqueous systems and its ability to sustain micelles over a wide temperature range, even at subzero degree celsius. Very © 2019 American Chemical Society
little information is available with respect to self-assembly in natural deep eutectics or supercooled solvents. To understand the molecular parameters that control the self-assembly process in such solvents, a description of the structure and interaction of surfactant aggregates in sugar−urea mixtures is crucial. To date, much effort has been made to estimate the structural morphology and interparticle interactions of ionic micellar systems using a variety of techniques including smallangle scattering, electron microscopy, NMR, fluorescence, and computational approaches.13−24 As far as micelles are concerned, one of the direct tools to understand the microstructure is the use of electron microscopy combined with cryogenic sample fixation. Validation of macromolecular assemblies in solutions by small-angle X-ray (SAXS)/neutron scattering (SANS) has become a key tool in surfactant research.25,26 The SAXS/SANS experiment involves determining the scattering profile of the colloidal suspension and its comparison with an appropriate theoretical model to get an insight into the microstructure and interaction. The scattering profile observed in small-angle scattering experiments involves Received: March 28, 2019 Revised: May 20, 2019 Published: July 4, 2019 9867
DOI: 10.1021/acs.langmuir.9b00912 Langmuir 2019, 35, 9867−9877
Article
Langmuir
obtained from SD Fine Chemicals, India. Deionized water from a Millipore-Milli-Q system (resistivity ≈ 18 MΩ cm) was used for preparation of aqueous samples. All chemicals were used as received. Sample Preparation. Aqueous SDS micelles were prepared at different surfactant concentrations (5, 10, 15, and 20 wt %) using deionized water and the solvent electron density is varied by the addition of different concentrations of urea and fructose individually up to their solubility limits. To study the effect of contrast variation over a wide range and probe structures in a supercooled matrix, the fructose−urea (6:4 w/w) mixture is used as the additive. To prepare water-free supercooled micelles, the fructose−urea melt is used as the solvent for micellization. For background subtraction in SAXS, the matrix is prepared in the same manner without adding any surfactant. In the case of SANS, the samples are prepared in D2O instead of water.
contributions from either intermicellar or intramicellar interferences, or a combination of both. As has been shown by several authors, SANS of ionic micelles exhibit characteristic features of both intramicellar and intermicellar interference effects.27−31 The structure of ionic micelles comprises a hydrocarbon core decorated with a palisade layer of hydrophilic head groups and associated counter ions. Though, the contrast variation technique is widely used in SANS, to highlight a single segment of a multipart structure, this was not employed in conventional micelles as the contribution from the shell is not observed because of the poor neutron scattering contrast between the shell and solvent. This simplifies the scattering form factor and hence the structure factor peak due to intermicellar interaction can be clearly captured by SANS. This was not the case with X-rays, and the SAXS pattern often contains significant scattering from the counter ion shell due to the high electron density of the shell. Thus, analysis of the SAXS pattern of charged micelles can be complex as compared to SANS. One approach to simplify the complex scattering pattern in SAXS is to use the X-ray contrast variation method in which the electron density of the solvent is systematically varied by adding suitable contrast agents. Recently, Grishaev at al. revealed the domain positions in a heavy atom-labeled protein−peptide complex using contrast-matched X-ray scattering.32 In a heavy atom-labeled protein, the contribution from the protein can be effectively masked by the addition of 65% sucrose. This enables precise estimation of the heavy atom distances in the protein at angstrom resolution using the SAXS data. Garcia-Diez et al. reported that the core−shell structure of carboxylated polystyrene nanoparticles can be resolved using contrast variation SAXS. They employed a solvent density gradient using aqueous sucrose solution for continuous contrast variation.33 Mykhaylyk et al. reported the structure of latex particles comprising polymethylmethacrylate (the core) and polyurethane (the shell) with the help of SAXS patterns in sucrose solutions.34 Though there have been attempts to use a contrast variation SAXS method to study the structure of colloidal particles in greater detail, still its application in micellar systems was limited. This is because of the possibility of structural alterations in micelles at high concentrations of additives. In this study, we intended to overcome this limitation by using a unique combination of fructose and urea that can nullify their individual effects and stabilize micelles even at high concentrations. We report a systematic investigation of the structure and intermicellar interactions in one of the widely used anionic surfactant sodium dodecyl sulfate (SDS) using contrast variation X-ray scattering. The scattering length density (SLD) of the solvent is varied over a wide range with minimum effect on structural parameters of micelles. A comparison has been made with results obtained from small-angle neutron scattering and high resolution transmission electron microscopy (HR-TEM). In particular, by contrast matching of the shell, the structure factor in aqueous micelles can be clearly captured using SAXS. In addition, we followed the systematic evolution of SDS micelle structure formed in pure water to the water-free supercooled matrix comprising a fructose−urea melt, using SAXS.
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METHODS
Small-Angle X-ray Scattering. SAXS measurements were carried out at SAXSpace instrument (Anton Paar, Austria) which employs a line-collimated sealed tube X-ray source (Cu Kα) operated at 40 kV, 50 mA. The samples were placed in 1 mm quartz capillaries or paste cell and thermostated using a Peltier-controlled sample holder. The sample to detector distance was set to 305 mm. The scattering intensities were monitored in transmission geometry using a CCD detector (pixel size 24 μm), operated at −40 °C to reduce the thermal noise, to span q (momentum transfer) ranges from 0.01 to 0.65 Å−1. For each measurement, 400 frames were obtained at 5 s exposures and averaged. The 2D SAXS images were processed into 1D scattering profiles, and measured SAXS intensities were calibrated for transmission by normalizing with zero-q attenuated primary beam intensity. The scattering intensities were subtracted by dark counts, empty cells, and solvent scattering using standard protocols in SAXSquant software (Anton Paar). Small-Angle Neutron Scattering. SANS measurements were performed in the SANS beam line at the Dhruva Reactor, Bhabha Atomic Research Centre, Trombay, Mumbai, India. The mean wavelength (λ) of the neutron beam is 5.2 Å. The angular distribution of the scattered neutrons was recorded using a one-dimensional position-sensitive detector in the q range of 0.017−0.35 Å−1. The samples were measured in a quartz sample holder of 2 mm thickness. The measured SANS data were processed using standard protocols. Transmission Electron Microscopy. To prepare the samples for HR-TEM analysis, the solution vial was kept above the melting point, here above 80 °C, for 30−40 min. A Cu grid was dipped into the hot melt, excess solution was removed with a filter paper producing a thin liquid film, and the grid was then quickly quenched in liquid nitrogen. The samples were examined at room temperature at high-resolution using an FEI Talos F200C. The images were recorded on both a CETA 16 M camera and a Falcon 3 direct detector. Density Measurements. Density measurements of aqueous urea, fructose, and fructose−urea mixtures were made by using a digital density meter, model DSA 5000M (Anton Paar, Austria).
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ANALYSIS OF SAXS AND SANS DATA In general, two different approaches are used for the treatment of small-angle scattering data. First, a model independent “indirect Fourier transformation (IFT)” of the reciprocal space data to obtain the pair-distance distribution function (PDDF), p(r) from which information regarding the particle structure can be deduced.25 This interpretation is limited to nonperiodic structures at low concentrations, where the interparticle interactions are negligible. The value of Dmax obtained for a scattering pattern provides an approximate value of the maximum dimension of the particles. The information obtained from the PDDF can be used to develop a suitable model. Here, the SAXS data was analyzed by the model-free
EXPERIMENTAL SECTION
Materials. SDS (Molecular Biology grade) obtained from Sisco Research Labs Pvt. Ltd. India. Fructose and urea (all AR grade) were 9868
DOI: 10.1021/acs.langmuir.9b00912 Langmuir 2019, 35, 9867−9877
Article
Langmuir
where g(r) is their pair-correlation function. It may noted that for anisotropic particles as well as for polydisperse systems, the structure factor calculated by this approach should be considered as an effective structure factor S′(q) only, as polydispersity and particle anisotropy can influence the oscillations in the structure factor.36 Using the decoupling approximation, the apparent structure factor S′(q) is related to the true structure factor S(q) by the relation
IFT approach (GIFT algorithm of Anton Paar, Austria) to obtain the PDDF. The second approach consists of direct modeling of reciprocal space data using shape dependent models and the experimental scattering data is directly fitted to a mathematical model. These models can be used with both dilute and concentrated solutions, hence giving the possibility of evaluating both the size of the micelles and their intermicellar interactions, that is, structure factor. However this method requires the fitting of several parameters, therefore some priority information is required to obtain reliable results. Model fitting the SAXS and SANS data were done using the built-in model function core−shell ellipsoid form factor in the SasView 4.2.0 program (http://www.sasview.org/). The structure of SDS micelles can be very well represented by core−shell ellipsoids.35 The interparticle interaction is captured using a screened Coulomb interaction potential within the Hayter−Penfold rescaled mean spherical approximation (RMSA). Instrumental smearing was taken into account during data analysis using the measured beam profile. In small-angle scattering, the scattering intensity as a function of momentum transfer/scattering vector (q) for ellipsoidal particles can be given as Ä ÉÑ ÑÑ scale ÅÅÅÅ 1 2 ÑÑS(q) + background I(q) = | F ( q , r , α ) | d α Å i Å ÑÑ V ÅÅÇ 0 ÑÖ where scale is the proportionality constant that accounts for the volume fraction of the scattering objects, V is the volume of the individual scatterer. P(q) = ∫ 10|F(q,ri,α)|2dα is the form factor of the object and corresponds to the intraparticle contribution. Here, the form factor was averaged over all particle orientation because the particles are randomly oriented in the sample, and the integration is over the orientation variable α, equal to the cosine of the angle between scattering vector q, and the direction of the major axis of the ellipsoid. S(q) is the structure factor that includes the interparticle contribution to the scattering, and a flat background was added in the model to account for incoherent scattering and the electronic noise of the detector. F(q,ri,α) is the single particle scattering amplitude for the core−shell ellipsoid of the revolution model with semimajor axis rmaj and semiminor axis rmin and its analytical expression is given by
j
ρ=
[g (r ) − 1]r 2
sin(qr ) dr qr
∑ i
(2)
Zire VM
(7)
where, Zi is the atomic number of the ith of j atoms in a molecule of molecular volume VM, and re is the classical electron radius or Thomson scattering length (2.8179 × 10−15 m). For SDS micelles, the core is mainly composed of a dodecyl chain while the shell constitutes the sulfate head group with the associated solvent and counter ions. The SLD of the hydrocarbon core of SDS micelles is taken as that of undecane, considering that one −CH2 group of the dodecyl chain is attached to the sulphate head group. This provides a SLD value of 7.21 × 10−6 Å−2. For neutrons, the SLD of the core is calculated from the coherent scattering lengths of individual atoms and is obtained as −0.37 × 10−6 Å−2. For aqueous SDS micelles, it has been observed that the shell is composed of around 50% solvent. This degree of solvation is also taken into
Bessel function of the first kind. rmaj is the semimajor axis and rmin is the semiminor axis of the ellipsoid. α is the angle between the axis of the ellipsoid and vector q, while ρ represents the SLD of the core, shell, or solvent. The interparticle structure factor, S(q), is related to the total correlation function h(r) = g(r) − 1, as ∞
(5)
where Oi is an observed (measured) value, Ei is an expected (theoretical) value, Si is the standard deviation in the experimental data, and N is the number of data points. Here, the core of the ellipsoid comprises the hydrocarbon tail of the surfactant while the shell is composed of the hydrophilic part and the associated counter ions. The SLD of the core and the solvent is calculated from the atomic number and molar volume of the respective components and is used as a fixed parameter. The shell SLD is used as a variable, as it depends on the counter-ion binding and degree of solvation of the shell. The other parameters used as variables in the fit are: polar and equatorial radii of the core, shell thickness, and charge of the micelle. Background correction has also been applied. The core and solvent SLD for X-rays were estimated from the sample composition using the equation
where, Vcore = (4π/3)rmaj,crmin,c2, Vshell = (4π/3)rmaj,srmin,s2, ucore = q[rmaj,c2α2 + rmin,c2(1 − α2)]1/2, ushell = q[rmaj,s2α2 + rmin,s2(1 − α2)]1/2, j1(ui) = (sin u − u cos u)/u2, j1(ui) is the first order
∫0
β(q) = |⟨F(q)⟩|2 /⟨|F(q)|⟩2
where P(q) = ⟨|F(q)| ⟩ and β is a q-dependent factor between zero and one that accounts for the deviation of the true structure factor S(q) in the observed scattering spectrum from a polydisperse or nonspherical system of particles. From a detailed analysis of the effect of the axial ratio and polydispersity on the interparticle structure factor, it has been inferred that the contribution from anisotropy effects in the structure factor can be neglected, when qrmaj is less than 3.5 and the axial ratio for the ellipsoid core is about
