Electron diffraction studies of supersonic jets. IV. Conformational cooling of n‐butane

Expansions through small tapered nozzles (∼10−2 cm inlet diameter) have produced conformational cooling of gas phase n‐butane to estimated conformational temperatures as low as 180 K. Relaxation into the lower energy trans form was seen with neat butane and with addition of up to ∼30% helium or neon. Thin plate nozzles of comparable diameter do not seem to produce the same effects, presumably because the more rapid cooling they bring about is accompanied by many fewer collisions. Conformational analyses carefully checked for and took into account butane cluster scattering, which if present and ignored, artificially increases the apparent trans mole fraction. At higher concentrations of monatomic carrier gas the cluster scattering becomes strong enough to interfere seriously with the determination of conformational composition. Analysis of the present data and a reanalysis of earlier, conventional, gas electron diffraction data both gave the room temperature trans mole fraction as 64% (3σ=9%) in agreement with a recent spectroscopic inference of 68%.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70828/2/JCPSA6-78-3-1270-1.pd

Electron diffraction studies of supersonic jets. III. Clusters of n‐butane

Expansions of n‐butane from small nozzles were observed to generate large clusters, provided that enough helium or neon carrier gas accompanied the butane to remove the heat of condensation rapidly and to produce low temperatures. Although expansions of neat n‐butane led to substantial conformational cooling under some circumstances, clusters were never seen without carrier gas. Both thin‐plate and tapered glass nozzles yielded clusters. Diffraction patterns of the clusters are characteristic of a liquidlike structure perhaps as cold as 100 K. No evidence for inclusion of carrier gas atoms in the clusters was found. Cluster scattering signals are stronger and show less noise than those which could be expected from bulk liquid by x‐ray or neutron diffraction. Their interference terms are dominated by nearest neighbor intermolecular C...H contacts. Information about the distribution of these contacts has not yet been deduced in computer simulations of liquid butane but should soon become available for comparison.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70517/2/JCPSA6-78-3-1265-1.pd

Electron diffraction studies of supersonic jets. I. Apparatus and methods

We describe the apparatus and methods used to obtain electron diffraction patterns of low‐temperature species produced from molecules seeded into supersonic expansions of helium or neon carrier gases. Although systems designed for molecular beam or spectroscopic studies are unsuitable for electron diffraction, alternative arrangements were found that give diffraction patterns of good quality. Characteristics of gas jets issuing from different nozzle designs are discussed. Procedures are outlined for separating the desired signal from considerable background scattering by the carrier gas and to correct for broad gas density profiles in analyses of diffraction data.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70889/2/JCPSA6-78-1-236-1.pd

Electron diffraction studies of supersonic jets. II. Formation of benzene clusters

Expansions of benzene at 1–10 mol % in neon or helium with various nozzle types have produced clusters of benzene molecules. Preliminary comparisons with intensities from the bulk liquid and with model calculations based on solid state crystal structures suggest that on the average the clusters are probably considerably larger than a 13 molecule unit, and have a vibrational temperature on the order of 100–150 K. Unlike clusters of spherical or quasispherical molecules previously found to pack in nearly crystalline arrays, benzene molecules appear to be unable to organize into regular arrays in the time of our experiments. Local order in clusters is similar but not identical to that in the solid, resembling that expected for a supercooled liquid. The present approach, exploiting several advantages of electron diffraction over alternative methods, shows promise as a new means of studying liquids or glasses.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71235/2/JCPSA6-78-1-243-1.pd

The interfacial structure of polymeric surfactant stabilised air-in-water foams

Small-angle neutron scattering was used to probe the interfacial structure of nitrogen-in-water foams created using a series of tri-block polymeric surfactants of the poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (EOx–POy–EOx) range, from which the nature of the polymeric interface could be characterised. The data follow a pronounced Q−4 decay, along with a number of inflexions and weak but well-defined peaks. These characteristics were well-described by a model embodying paracrystalline stacks of adsorbed polymer layers, whose formation is induced by the presence of the air–water interface, adsorbed at the flat air–water (film lamellae) interface. A minimum of approximately five paracrystalline polymer layers of thickness of the order of 85–160 Å, interspersed with somewhat thicker (400 Å) films of continuous aqueous phase were found to best fit the data. The thickness of the layer (L) was shown to follow a relationship predicted by anchor block dominated polymer adsorption theories from non-selective solvents, L EO1PO1/3. The insight gained from these studies should permit a more rational design of polymeric stabilisers for hydrophilic polyurethane foams

An Investigation into Creep Cavity Development in 316H Stainless Steel

Creep-induced cavitation is an important failure mechanism in steel components operating at high temperature. Robust techniques are required to observe and quantify creep cavitation. In this paper, the use of two complementary analysis techniques: small-angle neutron scattering (SANS), and quantitative metallography, using scanning electron microscopy (SEM), is reported. The development of creep cavities that is accumulated under uniaxial load has been studied as a function of creep strain and life fraction, by carrying out interrupted tests on two sets of creep test specimens that are prepared from a Type-316H austenitic stainless steel reactor component. In order to examine the effects of pre-strain on creep damage formation, one set of specimens was subjected to a plastic pre-strain of 8%, and the other set had no pre-strain. Each set of specimens was subjected to different loading and temperature conditions, representative of those of current and future power plant operation. Cavities of up to 300 nm in size are quantified by using SANS, and their size distribution, as a function of determined creep strain. Cavitation increases significantly as creep strain increases throughout creep life. These results are confirmed by quantitative metallography analysis

Hydrophilic nanoparticles stabilising mesophase curvature at low concentration but disrupting mesophase order at higher concentrations

Silica nanoparticles form aggregates at mesophase domain boundaries, which may suppress or promote curvatures depending on the nanoparticle concentration.</p

Segregation versus interdigitation in highly dynamic polymer/surfactant layers

Many polymer/surfactant formulations involve a trapped kinetic state that provides some beneficial character to the formulation. However, the vast majority of studies on formulations focus on equilibrium states. Here, nanoscale structures present at dynamic interfaces in the form of air-in-water foams are explored, stabilised by mixtures of commonly used non-ionic, surface active block copolymers (Pluronic®) and small molecule ionic surfactants (sodium dodecylsulfate, SDS, and dodecyltrimethylammonium bromide, C12TAB). Transient foams formed from binary mixtures of these surfactants shows considerable changes in stability which correlate with the strength of the solution interaction which delineate the interfacial structures. Weak solution interactions reflective of distinct coexisting micellar structures in solution lead to segregated layers at the foam interface, whereas strong solution interactions lead to mixed structures both in bulk solution, forming interdigitated layers at the interface

The Solution Structures of Two Human IgG1 Antibodies Show Conformational Stability and Accommodate Their C1q and FcγR Ligands.

The human IgG1 antibody subclass shows distinct properties compared with the IgG2, IgG3, and IgG4 subclasses and is the most exploited subclass in therapeutic antibodies. It is the most abundant subclass, has a half-life as long as that of IgG2 and IgG4, binds the FcγR receptor, and activates complement. There is limited structural information on full-length human IgG1 because of the challenges of crystallization. To rectify this, we have studied the solution structures of two human IgG1 6a and 19a monoclonal antibodies in different buffers at different temperatures. Analytical ultracentrifugation showed that both antibodies were predominantly monomeric, with sedimentation coefficients s20,w (0) of 6.3-6.4 S. Only a minor dimer peak was observed, and the amount was not dependent on buffer conditions. Solution scattering showed that the x-ray radius of gyration Rg increased with salt concentration, whereas the neutron Rg values remained unchanged with temperature. The x-ray and neutron distance distribution curves P(r) revealed two peaks, M1 and M2, whose positions were unchanged in different buffers to indicate conformational stability. Constrained atomistic scattering modeling revealed predominantly asymmetric solution structures for both antibodies with extended hinge structures. Both structures were similar to the only known crystal structure of full-length human IgG1. The Fab conformations in both structures were suitably positioned to permit the Fc region to bind readily to its FcγR and C1q ligands without steric clashes, unlike human IgG4. Our molecular models for human IgG1 explain its immune activities, and we discuss its stability and function for therapeutic applications