Abstract Neutron spin-echo techniques exploit Larmor precession of the neutron spin to encode either the scattering angle or energy. These are powerful means to extend the measurable momentum transfer ( Q ) and energy ( E ) range in neutron scattering measurements. Standard small-angle neutron scattering (SANS) instruments are sensitive in a range of $$\approx$$ ≈ 10–200 nm, whereas these techniques allow the study of structures in materials on length scales of tens of nm up to tens of $$\upmu$$ μ m. The Larmor instrument at ISIS is equipped to operate in spin-echo modulated SANS (SEMSANS) and spin-echo SANS (SESANS) mode. Two separate detectors were developed to cope with the performance demands set by these techniques. The first is a position sensitive ZnS:Ag/ 6 LiF scintillator-based detector coupled with wavelength shifting fibres that can be used for both SEMSANS and SESANS. A detector prototype using GS20 glass scintillator directly coupled to a multi-anode photomultiplier was developed as an alternative for SESANS measurements at higher incident neutron fluxes. The designs and results obtained with the two detectors are presented together with future improvements to both technologies. These, in addition to promising development routes, demonstrate the potential for utilising a WLSF ZnS:Ag/ 6 LiF scintillator detector and a pixelated GS20 detector for SEMSANS and SESANS applications.
Conductivity data for all cation-anion pairs [units given in the header for each column]. Small-angle neutron scattering (SANS) data (Q [1/Å], I(Q) [SAXS - arbitrary, SANS - 1/cm], error I(Q) [same units]) of PLMA48 as a 2 wt % solution in n-dodecande-d26.
Refractive index matched particles serve as essential model systems for colloid scientists, providing nearly hard spheres to explore structure and dynamics. The PMMA latexes typically used are often refractive index matched by dispersing them in a binary solvent mixture, but this can lead to undesirable changes, such as particle charging or swelling. To avoid this shortcoming, we have synthesized refractive index matched colloids using polymerization-induced self-assembly (PISA) rather than as polymer latexes. The crucial difference is that these diblock copolymer nanoparticles consist of a single core-forming polymer in a single non-ionizable solvent. The diblock copolymer chosen was poly(stearyl methacrylate)–poly(2,2,2-trifluoroethyl methacrylate) (PSMA–PTFEMA), which self-assembles to form PTFEMA core spheres in n-alkanes. By monitoring scattered light intensity, n-tetradecane was found to be the optimal solvent for matching the refractive index of such nanoparticles. As expected for PISA syntheses, the diameter of the colloids can be controlled by varying the PTFEMA degree of polymerization. Concentrated dispersions were prepared, and the diffusion of the PSMA–PTFEMA nanoparticles as a function of volume fraction was measured. These diblock copolymer nanoparticles are a promising new system of transparent spheres for future colloidal studies.
The formation of ions in nonpolar solvents (with relative permittivity εr of approximately 2) is more difficult than in polar liquids; however, these charged species play an important role in many applications, such as electrophoretic displays. The low relative permittivities of these solvents mean that charges have to be separated by large distances to be stable (approximately 28 nm or 40 times that in water). The inverse micelles formed by surfactants in these solvents provide an environment to stabilize ions and charges. Common surfactants used are sodium dioctylsulfosuccinate (Aerosol OT or AOT), polyisobutylene succinimide, sorbitan oleate, and zirconyl 2-ethyl hexanoate. The behavior of charged inverse micelles has been studied on both the bulk and on the microscopic scale and can be used to determine the motion of the micelles, their structure, and the nature of the electrostatic double layer. Colloidal particles are only weakly charged in the absence of surfactant, but in the presence of surfactants, many types, including polymers, metal oxides, carbon blacks, and pigments, have been observed to become positively or negatively charged. Several mechanisms have been proposed as the origin of surface charge, including acid–base reactions between the colloid and the inverse micelle, preferential adsorption of charged inverse micelles, or dissolution of surface species. While most studies vary only the concentration of surfactant, systematic variation of the particle surface chemistry or the surfactant structure have provided insight into the origin of charging in nonpolar liquids. By carefully varying system parameters and working to understand the interactions between surfactants and colloidal surfaces, further advances will be made leading to better understanding of the origin of charge and to the development of more effective surfactants.
Hypothesis: Poloxamines are amphiphilic block copolymers that self-assemble forming polymeric micelles (PMs) and hydrogels. They have emerged as promising colloidal carriers for their potential in improving drug delivery and controlled release through their multi-responsive properties. Tetronic® 1307 (T1307) PMs and gels have been used herein to incorporate host–guest complexes of cyclodextrins (CDs) and miltefosine (MF), an amphiphilic, anti-parasitic drug effective against leishmaniasis. Experiments: The association of MF to αCD, βCD, and HPβCD and the topology of the complexes have been fully characterized by NMR spectroscopy. Then, the structure of the complex-loaded PMs and hydrogels investigated using diffusion nuclear magnetic resonance (DOSY), small angle neutron scattering (SANS), and dynamic light scattering (DLS). The antileishmanial activity of the constructs was evaluated against Leishmania major promastigotes and amastigotes, as well as their cytotoxicity in macrophages. Findings: All the CDs form highly stable inclusion complexes with MF in a 2CD:1MF stoichiometry that allow the existence of considerable proportions of complexed drug at high dilution, the HPβCD providing the highest stability and compatibility with the poloxamine. The complex incorporates preferentially into the hydrophilic shell of the PMs, inducing the elongation of the aggregates and the dehydration of the micellar core, formed mainly by the PPO blocks. At high concentrations and physiological temperature, the complex-loaded PMs pack in a BCC-type paracrystal network. The micellar formulations of the CD-complexed MF reduced the cytotoxicity of the drug, while improving its antileishmanial activity. This approach would improve the treatment, facilitating the administration of MF at lower concentrations and achieving relevant therapeutic effects, not only through the intravenous route, but also by topical formulations as injectable thermogels for the cure of cutaneous and mucocutaneous forms of the disease.
Sterically-stabilized poly(methyl methacrylate) (PMMA) latexes dispersed in nonpolar solvents are a classic, well-studied system in colloid science. This is because they can easily be synthesized with a narrow size distribution and because they interact essentially as hard spheres. These PMMA latexes can be charged using several methods (by adding surfactants, incorporating ionizable groups, or dispersing in autoionizable solvents), and due to the low relative permittivity of the solvents (εr ≈ 2 for alkanes to εr ≈ 8 for halogenated solvents), the charges have long-range interactions. The number of studies of these PMMA particles as charged species has increased over the past ten years, after few studies immediately following their discovery. A large number of variations in both the physical and chemical properties of the system (size, concentration, surfactant type, or solvent, as a few examples) have been studied by many groups. By considering the literature on these particles as a whole, it is possible to determine the variables that have an effect on the charge of particles. An understanding of the process of charge formation will add to understanding how to control charge in nonaqueous solvents as well as make it possible to develop improved technologically relevant applications for charged polymer nanoparticles.
Sodium dioctylsulfosuccinate (Aerosol OT or NaAOT) is a well-studied charging agent for model poly(methyl methacrylate) (PMMA) latexes dispersed in nonpolar alkane solvents. Despite this, few controlled variations have been made to the molecular structure. A series of counterion-exchanged analogs of NaAOT with other alkali metals (lithium, potassium, rubidium, and cesium) were prepared, and it was expected that this should influence the stabilization of charge on PMMA latexes and the properties of the inverse micelles.The electrophoretic mobilities of PMMA latexes were measured for all the counterion-exchanged AOT analogs, and these values were used to calculate the electrokinetic or ζ potentials. This enabled a comparison of the efficacy of the different surfactants as charging agents. Small-angle scattering measurements (using neutrons and X-rays) were performed to determine the structure of the inverse micelles, and electrical conductivity measurements were performed to determine the ionized fractions and Debye lengths.Sodium AOT is a much more effective charging agent than any of the other alkali metal AOTs. Despite this, the inverse micelle size and electrical conductivity of NaAOT are unremarkable. This shows a significant non-periodicity in the charging efficiency of these surfactants, and it emphasizes that charging particles in nonpolar solvents is a complex phenomenon.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
