The influence of n- hexanol on the morphology and composition of CTAB micelles

In situ small-angle X-ray scattering measurement at the Very Small Angle Neutron Scattering Instrument at the China Spallation Neutron Source

Small-angle X-ray and neutron scattering (SAXS and SANS) offer complementary insights into multi-scale and multiphase structures. Efforts have been made to integrate SAXS into SANS instruments, with only D22 at the Institut Laue-Langevin successfully implementing a SAXS setup; this was constrained to a horizontal geometry due to space limitations and high radiation noise. Here, we introduce an in situ vertical SAXS setup at beamline 14, Very Small Angle Neutron Scattering (VSANS) instrument, at the China Spallation Neutron Source. The compact vertical SAXS instrument without a beam stop, measuring 87 cm × 93 cm × 240 cm (W × L × H) and featuring a hoisting frame, can be easily installed in the VSANS sample room within 5 h. Utilizing a 50 mm-diameter neutron collimation guide, we can simultaneously detect X-rays from below and neutrons from behind, with the backboard of the SAXS chamber coated in a boron-aluminium alloy to reduce neutron background interference. Through testing with standard samples like deuterated and hydrogenated PEG, silver behenate, LaB6, and glass carbon, we demonstrate that concurrent neutron and X-ray measurements are successful.

Aqueous Binary Mixtures of Stearic Acid and Its Hydroxylated Counterpart 12-Hydroxystearic Acid: Fine Tuning of the Lamellar/Micelle Threshold Temperature Transition and of the Micelle Shape

This study examines the structures of soft surfactant-based biomaterials which can be tuned by temperature. More precisely, investigated here is the behavior of stearic acid (SA) and 12-hydroxystearic acid (12-HSA) aqueous mixtures as a function of temperature and the 12-HSA/SA molar ratio (R). Whatever R is, the system exhibits a morphological transition at a given threshold temperature, from multilamellar self-assemblies at low temperature to small micelles at high temperature, as shown by a combination of transmittance measurements, Wide Angle X-ray diffraction (WAXS), small angle neutron scattering (SANS), and differential scanning calorimetry (DSC) experiments. The precise determination of the threshold temperature, which ranges between 20 °C and 50 °C depending on R, allows for the construction of the whole phase diagram of the system as a function of R. At high temperature, the micelles that are formed are oblate for pure SA solutions (R = 0) and prolate for pure 12-HSA solutions (R = 1). In the case of mixtures, there is a progressive continuous transition from oblate to prolate shapes when increasing R, with micelles that are almost purely spherical for R = 0.33.

Small Angle X-Ray Scattering Data Analysis and Theoretical Modelling for the Size and Shape Characterization of Drug Delivery Systems Based on Vitamin E TPGS Micelles

We developed a simple two-dimensional/two-components theoretical model that describes the structure and functionality of a VitE-TPGS system of micelles assuming a hydrophobic inner core and an outer hydrated hydrophilic shell. We then conceptually applied the developed methodology to a simple system of VitE-TPGS micelles unloaded and loaded with an active pharmaceutical ingredient, eltrombopag, to verify if the model could reliably monitor the size change of the micelle upon loading. The fit of laboratory Small Angle X-Ray Scattering data against such model allows us to extract absolute values of the micelles size under a spherical shape hypothesis as well as the distribution within the system between components and level of hydration. The intensity scale of the SAXS experimental data needs to be normalized to a reference standard (pure water) to get absolute scattered intensities. The mathematical model which has been developed under a general hypothesis of ellipsoidal micelles, is applied to our experimental data under the simplified spherical assumption, which suitably fits our experimental data.

Wormlike micelles of CTAB with phenols and with the corresponding phenolate derivatives - When hydrophobicity and charge drive the coacervation

HYPOTHESIS

Hydrophobicity and the presence or absence of charge in phenol derivatives are relevant on the rheology and phase behavior when they are assembled with a cationic surfactant, forming wormlike micelles. The incorporation of phenols with a greater number of rings into the micellar palisade is entropically favored, but a solubilization limit or coacervation are two paths followed by the solutions, depending on the electrical nature of the aromatic co-solutes.

EXPERIMENTS

The investigations were carried out with systems formed by a fixed concentration of hexadecyltrimethylammonium bromide (CTAB) and increasing concentrations of neutral phenols (1-naphthol, 2-naphthol, 2,3-dihydroxynaphthalene and R and S-binol) and with their corresponding phenolate derivatives. The monophasic limits of the systems were established, as well as their linear and non-linear rheology. The structural investigation of the coacervates formed with the phenolates were done using SAXS and Cryo-TEM.

FINDINGS

The zero-shear viscosity of the solutions reaches maxima values close to the solubility limit of the aromatics, which depends on the numbers of rings and hydroxyl groups (position and number). However, when the correspondent ionized phenols were investigated, beyond the maxima values for the zero-shear viscosity, liquid-liquid biphasic systems are formed, in which the upper phase contains a coacervate, associated with branched wormlike micelles. However, when the ratio between phenolate and CTAB is around 3:1 the coacervate evolves to a lamellar structure.

Influence of micellar size on the structure of surfactant-DNA complexes

We have studied the structure of complexes of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) with DNA as a function of surfactant to DNA base molar ratio (R) and salt concentration. Small-angle x-ray scattering data show the formation of nematic gels at lower and higher salt concentrations, irrespective of the value of R. Two crystalline phases are observed over intermediate salt concentrations; a square (S) phase for R>3 and a hexagonal (H_{S}) phase for lower R. Electron density maps of these phases show intercalated structures, with DTAB micelles sandwiched between long DNA strands. The composition of these complexes, estimated using elemental analysis, indicates that the micelles are not very long, and they occupy only about half of the interstitial volume between the DNA strands. This phase behavior is strikingly different from that of complexes of DNA with longer chain surfactants cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB), which show only a hexagonal (H) phase over similar ranges of R and salt concentration, the H_{S} structure observed in the present study being a sqrt[3]×sqrt[3] superlattice of the H structure. Madelung energies of the S and H structures, calculated from the electrostatic interaction between their cylindrical constituents, suggest that the former is preferred in DTAB-DNA complexes due to the smaller micellar radius of DTAB. The propensity of DTAB to form short micelles seems also to favor the H_{S} phase at lower R. These results illustrate the important role of micellar size in determining the structure of these two-dimensional macro-ion crystals.

Effect of Bromide on the Surfactant Stabilization Layer Density of Gold Nanorods

Some studies have speculated that the concentration of bromide ions plays a crucial role in the surfactant density surrounding gold nanorods (AuNR). Small-angle X-ray and neutron scattering (SAXS and SANS) experiments were conducted to analyze any influence the bromide ions might have on the stabilization layer and the aggregation behavior of the ligand CTAB molecules in general. The AuNR were immersed in solutions containing a fixed CTA⁺ concentration of 2 mM and varying bromide ion concentrations from 0 to 22 mM. A patchy AuNR stabilization shell at low bromide ion concentrations was found, contrary to previously published SANS studies on the AuNR stabilization shell. However, with increasing bromide ion concentration, the density of the stabilization shell increases asymptotically toward a closed/collapsed bilayer configuration. AuNR grown under similar conditions show higher anisotropy with larger bromide ion concentrations. Both results indicate that anisotropic growth strongly depends on a sufficiently dense stabilization layer established by high bromide ion concentrations.

A novel experimental approach for nanostructure analysis: simultaneous small-angle X-ray and neutron scattering

Exploiting small-angle X-ray and neutron scattering (SAXS/SANS) on the same sample volume at the same time provides complementary nanoscale structural information in two different contrast situations. Unlike an independent experimental approach, the truly combined SAXS/SANS experimental approach ensures the exactness of the probed samples, particularly for in situ studies. Here, an advanced portable SAXS system that is dimensionally suitable for installation in the D22 zone of ILL is introduced. The SAXS apparatus is based on a Rigaku switchable copper/molybdenum microfocus rotating-anode X-ray generator and a DECTRIS detector with a changeable sample-to-detector distance of up to 1.6 m in a vacuum chamber. A case study is presented to demonstrate the uniqueness of the newly established method. Temporal structural rearrangements of both the organic stabilizing agent and organically capped gold colloidal particles during gold nanoparticle growth are simultaneously probed, enabling the immediate acquisition of correlated structural information. The new nano-analytical method will open the way for real-time investigations of a wide range of innovative nanomaterials and will enable comprehensive in situ studies on biological systems. The potential development of a fully automated SAXS/SANS system with a common control environment and additional sample environments, permitting a continual and efficient operation of the system by ILL users, is also introduced.

Takes Two to Tango: Choreography of the Coadsorption of CTAB and Hexanol at the Oil-Water Interface

Mixed surfactant systems at the oil-water interface play a vital role in applications ranging widely from drug delivery to oil-spill remediation. Synergistic mixtures are superior emulsifiers and more effective at modifying surface tension than either component alone. Mixtures of surfactants with dissimilar polar head groups are of particular interest because of the additional degree of control they offer. The interplay of hydrophobic and electrostatic effects in these systems is not well understood, in part because of the difficulty in examining their behavior at the buried oil-water interface where they reside. Here, surface-specific vibrational sum frequency spectroscopy is utilized in combination with surface tensiometry and computational methods to probe the cooperative molecular interactions between a cationic surfactant cetyltrimethylammonium bromide (CTAB) and a nonionic alcohol (1-hexanol) that induce the two initially reluctant surfactants to coadsorb synergistically at the interface. A careful deuteration study of CTAB reveals that hexanol cooperates with CTAB such that both molecules preferentially orient at the interface for sufficiently large enough concentrations of hexanol. This work's methodology is unique and serves as a guide for future explorations of macroscopic properties in these complex systems. Results from this work also provide valuable insights into how interfacial ordering impacts surface tensiometry measurements for nonionic surfactants.

Concentration dependent morphology and composition of n-alcohol modified cetyltrimethylammonium bromide micelles

Cetyltrimethylammonium bromide (CTAB) is one of the most commonly used surfactants in nanoparticle synthesis and stabilization. Usually, CTAB is used in high concentrations besides co-surfactants leading to well defined products but the complex mesoscopic CTAB structures stay mostly unknown. N-alcohols for instance are widely used co-surfactants which modify the properties of native CTAB dispersions. In this paper we report about a detailed structure analysis of n-alcohol modified CTAB micelles. In particular, n-pentanol and n-hexanol exhibit a significantly different influence on the size, shape and composition of CTAB micelles. Using a combination of small-angle x-ray spectroscopy (SAXS) and neutron scattering spectroscopy (SANS), we applied a method for a complete structural characterization of such micelles. The incorporation of n-pentanol into CTAB micelles generally does not influence the morphology but enhances the number of micelles due to the volume of the added alcohol. N-hexanol, however, leads to an elongation of the micelles as a function of its concentration. It was found by extended contrast variation measurements that this difference is caused by a different distribution of the alcohols between the micellar core and shell. N-pentanol molecules are generally located at the core-shell interface of the CTAB micelles with not only the head group but also two additional methylene bridging groups located in the micellar shell. This leads to an increase of its effective head group volume. In comparison, in n-hexanol modified micelles the whole alkyl chain is located within the micellar core. The detailed structure for n-alcohol modified CTAB micelles is described herein for the first time. The knowledge of the structural details found is indispensable for an in-depth understanding of CTAB-n-alcohol-water interfaces in general which is relevant for the synthesis of many functional nanostructures like mesoporous silica and gold or silver nanoparticles.

Study of Intermolecular Interactions of CTAB with Amino Acids at Different Temperatures: A Multi Technique Approach

The densities, ρ, viscosities, η and specific conductivities κ, of (0.0002, 0.0004, 0.0006 and 0.0008 m) CTAB in 0.1 m aqueous valine, leucine and isoleucine were measured at different temperatures. The measured data were used to calculate various useful thermodynamic parameters. A complete characterization of any mixture can be performed by means of these thermodynamic properties. The apparent molar volume, ϕv , partial molar volume, ϕ v 0 $\phi _v^0$ and partial molar isobaric expansibilities, ϕ E 0 , $\phi _E^0,$ were calculated using density data. The viscosity data were analyzed using Jones–Dole equation to obtain viscosity coefficients, A- and B-, free energy of activation per mole of solvent, Δμ 1°∗, and solute, Δμ 2°∗, enthalpy, ΔH ∗ and entropy, ΔS ∗ of activation of viscous flow. Measuring the changes in these properties has been found to be an excellent qualitative and quantitative way to obtain information regarding the molecular structure and intermolecular interactions occurring in these mixtures. Various structure-making/breaking ability of solute (cetyltrimethylammonium bromide) in presence of aqueous amino acid solutions were discussed. In addition, fluorescence study using pyrene as a photophysical probe has been carried out, the results of which support the conclusions obtained from other techniques.