Two high‐resolution, general‐purpose, small‐angle neutron scattering instruments have been constructed at the National Institute of Standards and Technology's Center for Neutron Research. The instruments are 30 m long and utilize mechanical velocity selectors, pinhole collimation and high‐data‐rate two‐dimensional position‐sensitive neutron detectors. The incident wavelength, wavelength resolution and effective length of the instruments are independently variable, under computer control, and provide considerable flexibility in optimizing beam intensity and resolution. The measurement range of the instruments extends from 0.0015 to 0.6 Å−1 in scattering wavevector, corresponding to structure in materials from 10 Å to nearly 4000 Å. The design and characteristics of the instruments, and their modes of operation, are described, and data are presented which demonstrate their performance.

Specular reflection of neutrons has been used to characterize the structure of single lipid bilayers adsorbed to a planar silicon surface from aqueous solution. We used a novel experimental setup which significantly decreased the incoherent background scattering and allowed us to measure neutron reflectivities as low as 5 × 10-7. Thicknesses and neutron scattering length densities were determined by a fitting procedure using (i) randomly generated smooth functions represented by parametric B-splines and (ii) stepped functions based on the theoretical lipid composition. The size of lipid domains at the surface and the degree of surface coverage were determined by atomic force microscopy. Chain-protonated and -deuterated dipalmitoylphosphatidylcholine (DPPC) bilayers were investigated in 2H2O and a mixture of 2H2O and H2O which matches the scattering density of silicon. Also, one measurement on a distearoylphosphatidylcholine bilayer which has longer acyl chains was performed for comparison. The lipid adsorbs to the silicon surface as a continuous layer interrupted by irregularly shaped uncovered areas which are 100−500 Å in size. The surface coverage was estimated to be 70 ± 20%. The reflectivity measurements on DPPC at 60 °C show a silicon oxide layer with a thickness of the order of 4 Å, a rough silicon oxide/water layer between silicon oxide and lipid with a thickness between 2 and 8 Å, and a single lipid bilayer. Fitting resolved a central membrane layer with a thickness of 28 ± 2 Å which represents the lipid hydrocarbon chains. This layer is sandwiched between interface membrane layers of lipid head groups and water which are 11.5 ± 1 Å in thickness. The angstrom-scale thickness changes of the central membrane layer as a function of the phase state of the lipid and of the length of the hydrocarbon chains are easily detected.

The structural phases of magnetically alignable lipid mixtures were investigated as a function of temperature and lipid concentration using small-angle neutron scattering (SANS). Two systems were examined:  (a) an aqueous mixture of DMPC (dimyristoyl phosphatidylcholine) and DHPC (dihexanoyl phosphatidylcholine) lipids doped with Tm3+ ions resulting in the positive alignment of the system with the applied magnetic field and (b) the above aqueous Tm3+ doped lipid mixture containing a negatively charged lipid, DMPG (dimyristoylphosphatidylglycerol). For both systems, three different scattering patterns were observed corresponding to distinct structural phases at specific temperatures and lipid concentrations. At 45 °C and a lipid concentration of >0.05 g/mL, the high-viscosity liquid crystalline phase was found to be a perforated and possibly undulating lamellar phase consistent with NMR results. Upon dilution (<0.05 g/mL) at the same temperature (45 °C), the perforated lamellar phase transformed into a unilamellar vesicular phase, in which the bilayers may also be perforated. Below about 25 °C, the viscosity decreases considerably and the scattering data suggest that the lamellae present at higher temperatures break up into smaller entities characterized by the bicellar morphology proposed previously for the nondoped system. The structural dimensions of the vesicular and bicellar phases have been determined as a function of lipid concentrations from the SANS data. In the lamellar phase, the influence of Tm3+ ions and DMPG on bilayer structure (e.g., lamellar repeat spacing, bilayer rigidity, and magnetic alignment) were also investigated.

The structural phase behavior of phospholipid mixtures consisting of short-chain (dihexanoyl phosphatidylcholine) and long-chain lipids (dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylglycerol), with and without lanthanide ions was investigated by small-angle neutron scattering (SANS). SANS profiles were obtained from 10 degrees C to 55 degrees C using lipid concentrations ranging from 0.0025 g/ml to 0.25 g/ml. The results reveal a wealth of distinct morphologies, including lamellae, multi-lamellar vesicles, unilamellar vesicles, and bicellar disks.