Neutron scattering studies SANS technique

There have been only a few neutron scattering studies that have examined the effects of additives on micellar properties but these do include some studies concerned with pharmaceutically relevant additives such as sugars or drugs. Despite limited work in this area, it should be noted that the technique of SANS is ideally suited to such studies, particularly when the surfactant and/or the additive are available in both hydrogenous and deuterated forms. 

The conformation of polymer chains in an ultra-thin film has been an attractive subject in the field of polymer physics. The chain conformation has been extensively discussed theoretically and experimentally [6-11] however, the experimental technique to study an ultra-thin film is limited because it is difficult to obtain a signal from a specimen due to the low sample volume. The conformation of polymer chains in an ultra-thin film has been examined by small angle neutron scattering (SANS), and contradictory results have been reported. With decreasing film thickness, the radius of gyration, Rg, parallel to the film plane increases when the thickness is less than the unperturbed chain dimension in the bulk state [12-14]. On the other hand, Jones et al. reported that a polystyrene chain in an ultra-thin film takes a Gaussian conformation with a similar in-plane Rg to that in the bulk state [15, 16]. 

Small-angle neutron scattering (SANS) can be applied to food systems to obtain information on intra- and inter-particle structure, on a length scale of typically 10-1000 A. The systems studied are usually disordered, and so only a limited number of parameters can be determined. Some model systems (e.g., certain microemulsions) are characterized by only a limited number of parameters, and so SANS can describe them fully without complementary techniques. Food systems, however, are often disordered, polydisperse and complex. For these systems, SANS is rarely used alone. Instead, it is used to study systems that have already been well characterized by other methods, viz., light scattering, electron microscopy, NMR, fluorescence, etc. SANS data can then be used to test alternative models, or to derive quantitative parameters for an existing qualitative model. 

The underutilization of neutron scattering is undoubtedly also owing to a lack of awareness as to what information the technique can provide. The aim of this entry is to address this deficiency. The review will start with a background and some introductory theory to the technique of SANS, as it is the neutron scattering technique most suited to drug delivery research. The entry will then describe some of the studies that have used this technique to understand the behavior and properties of drug delivery systems. 

When neutron scattering of a sample is combined with contrast variation, information can be obtained not only about the shape and size of the micelle but also about its detailed (internal) molecular architecture. Because of the unique level of information about micellar systems that can be obtained from SANS experiments, the technique is now an extremely well-established tool for investigating the shape, size, and, to a lesser extent, the internal structure of micellar aggregates with several hundreds of papers being published since the 1970s when micelles were the first colloidal systems to be studied using SANS. 

Small-angle neutron scattering (SANS) studies of polyradical 50 in THF-dg at 170 K (slightly above melting point of the solvent) revealed the molecular structure (Fig. 31), which was analogous to that obtained from conformational searches.94 Since SANS is a relatively low-resolution technique and the information content of the data is rather low, the structure of 50 might be best described as dumbbell-like with overall dimensions of 2 x 3 x 4nm. Also, this result confirms that polyradical 50 existed as a monomer in solution at low temperatures. 

Both small angle X-ray (SAXS) cind neutron scattering (SANS) are established techniques and their experimental application is similar. However, limitations on sample size, thickness and containment are much more restricted with X-rays because of absorption of radiation. One problem which can arise with neutrons is the subtraction of the flat incoherent contribution which can be quite large in the case of hydrogenous materials. This disadvantage can be partially offset by the possibility of using isotopic substitution. SANS is particularly powerful because the penetrating power of neutrons makes it possible to study material microstructure in the wet state. Instrumentally, both SAXS and SANS require a source of radiation, collimation system, sample containment and a detection system. 

In this section we address the question of accordance between coexistence conditions determined for polymer mixtures in the bulk and confined in thin bilayer films. Macroscopic samples with the size of ca. 1 mm are analyzed by Small Angle Neutron Scattering. It probes the compositional fluctuations away from binodal to yield the effective interaction parameter %SANs(bilayer films are studied by profiling techniques to yield concentrations < q and 2 at binodal. These are described by the composition dependent interaction parameter %(([>). In fact only the section of the relation %(([>) bounded by ( q and <(>2 is relevant as it describes the whole intrinsic profile of Fig. 2. 

---