A.N. North

Characterisation of adsorbed layers of a disordered coil protein on polystyrene latex

The combined use of small-angle X-ray scattering (SAXS) and photon correlation spectroscopy (PCS) to characterise adsorbed layers of β-casein at the solid/liquid interface is reported. The protein was adsorbed to polystyrene latex particles at room temperature, low ionic strength and neutral pH and adsorption densities assessed by a solution-depletion technique which showed a plateau in the adsorption. Results from the SAXS experiments were analysed to provide electron-density profiles. These were backed up with results from PCS which provided hydrodynamic thicknesses over the range of the adsorption isotherm. This information, together with calculated hydrophobicity and charge profiles for the protein, yielded a molecular model for the adsorbed layer. Although β-casein in solution has a largely randon coil conformation, it appears to adopt a much more compact form when it is adsorbed on polystyrene latex. Most of the protein lies close to the surface, leaving part of the chain extended into the aqueous phase. The most likely candidate for the extended chain is part of the highly charged sequence of 40 or so amino acids at the N terminus of the protein. The hydrodynamic thickness of the protein layers increases with adsorbed concentration of protein. The thicknesses reached are substantially greater than those predicted by theories of self-avoiding walks of the extended chain with volume exclusion interactions included and it is suggested that long-range electrostatic repulsive forces are involved.

Characterization of activated carbon fibers by small angle x-ray scattering

SAXS measurements have been made for two series of ACF prepared by CO 2 and steam activation.

Structure and dynamics of water-in-oil microemulsions stabilised by Aerosol-OT

Abstract Small-angle scattering techniques are used in conjunction with dynamic studies of collision processes to investigate the mechanism of droplet aggregation in AOT-stabilised water-in-oil microemulsions. The presence of ‘almost dry’ reversed micelles co-existing with the water droplets is postulated to explain the scattering and kinetics data. The equilibrium droplet distribution function is shown to be dependent on the dynamic processes occurring in the microemulsion.

Structural studies of microporous carbons by neutron diffraction

Neutron diffraction measurements have been made for a series of activated carbons of variable porosity. The results show that the structural features are unaffected by the degree of burn-off, despite a large variation in the small-angle scattering. Fourier transformation to a real-space distribution function gives well-defined peaks for the angle-averaged pair-correlation function. The first few peaks correspond to features within the lattice plane of graphite but there are deviations at larger values emphasising the defective nature of the microporous material.

Small-angle scattering studies of meso-scopic structures with synchrotron X-rays

Abstract The use of small-angle X-ray scattering techniques for the study of spatial inhomogeneities over the range 20 A to 2 μm is reviewed. The basic formalism for scattering by an inhomogeneous medium is developed with particular reference to liquid suspensions, porous solids and solid aggregates. The instrumentation available on the Synchrotron Radiation Source at the Daresbury Laboratory is briefly presented and the use of the Bonse-Hart method for studies at ultra-low scattering angles described. The extraction of structural information for a range of natural and synthetic materials is presented with particular reference to microemulsions, porous silicas, clays and composites. The complementarity of X-ray and neutron techniques is critically reviewed and prospects for future developments, particularly for the study of anisotropic systems, are discussed.

Small-angle x-ray scattering studies of heterogeneous systems using synchrotron radiation techniques

Abstract The high intensity X-ray beams from synchrotron radiation facilities can be used to study the structural characteristics of heterogeneous systems. The present paper reports the use of this technique for SAXS measurements on a particulate system which shows aggregation properties (microemulsion), a concentrated particulate system showing repulsive ordering, a colloidal suspension (platinum) and two porous solids (a silica gel and an oil-bearing shaly rock). The advantages in the use of synchrotron radiation for general SAXS studies concerning research topics in chemical physics and materials science are discussed in relation to other methods. Further developments are also considered for applications to new areas of investigation and the complementary nature of SAXS and SANS measurements is established.

Characterization of pore size in activated carbons by small-angle x-ray scattering

Abstract Small-Angle X-Ray Scattering (SAXS) studies have been carried out over a wide range of Q-values (5 x 10 −3 -0.5 A −1 ) permitting analysis of structural features for a spatial regime of 0.5-200 nm. Experimental and analysis procedures are described and results presented for a series of activated carbons. It is found that the pore size distribution has a complex form that varies according to the degree of activation (burn-off). Additional measurements have been made by neutron diffraction. The results demonstrate the basic graphitic structure of the carbons but show that there is considerable distortion of lattice planes.

STRUCTURE OF COBALT AEROSOL-OT REVERSED MICELLES STUDIED BY SMALL-ANGLE SCATTERING METHODS

The surfactant–cyclohexane–water ternary phase behaviour and reversed micelle and water-in-oil (w/o) microemulsion structure of the cobalt (II) derivative of the anionic amphiphile Aerosol-OT (AOT)[Co(water)6](AOT)2 have been studied by polarising microscopy, small-angle neutron and X-ray scattering (SANS, SAXS). The surfactant forms an H2 reversed hexagonal phase on swelling with up to 25 wt.% cyclohexane. At higher concentrations of oil the fluid L2 reversed micellar phase is present, and a w/o phase forms up to w= 25.0 (w=[water]/[AOT–]). For w > 25.0 at 25 °C a Winsor II system separates cleanly i.e. a w/o droplet system at the ‘natural’ radius of the monolayer, co-existing with an essentially surfactant-free water phase. The SAXS l(Q) profiles show that major changes in aggregate shape occur as a function of w at constant surfactant concentration . At low surfactant concentration, [AOT–]= 0.075 mol dm–3, the w= 0 reversed micelles, formed from diluting the H2 phase, are small near-spherical aggregates. The scattering is consistent with cylindrical micelles at low w, 5–10, and spherical w/o droplets at the Winsor II boundary w= 25.0. The results are explained in terms of the influence of parent H2 and co-existing water phases on the aggregate shapes in the L2 phase. We have used the SANS contrast variation method to investigate the internal cross-section structure of the cylindrical, w= 5.0, reversed micelles. The results show that the radius of the polar core, r, is only slightly larger than the hydrated radius of the [Co(water)6]2+ counterion and that the surfactant shell thickness, δ, is essentially equal to the length of the AOT hydrocarbon chains. This suggests an open staggered ‘string of beads’ structure for the micelles, rather than a polar core that can be significantly swollen with water. This model gives us some insight into structure of the lyotropic H2 phase.

Determination of Anisotropic Features in Porous Materials by Small-Angle X-Ray Scattering

Abstract Some synthetic and natural materials exhibit anisotropic features due to the way that they have been fabricated. Small-angle scattering techniques with x-rays or neutrons may be used to probe this anisotropy by observation of the azimuthal variation in scattering intensity from an oriented sample. Results will be presented for a series of materials, in either fibre or disk form, which give distinctive patterns corresponding to ordered or disordered structures with spatial characteristics in the 10-2000A range. The possible extension of the method to other materials and further development of the technique is also considered.

The Investigation of Spatial Correlations in Liquid Phosphorus by Reverse Monte Carlo Calculations

Abstract The RMC method is used to model neutron diffraction results for liquid phosphorus. The adaptation of the basic routine for the treatment of molecular systems is described and the consequent changes in producing an initial configuration and achieving satisfactory convergence are critically examined. The programme is based on a structural unit of tetrahedral symmetry (P4) and random moves involving both translation and rotation of the molecule are introduced and selected on the usual χ2 probability criteria. The resultant configuration is found to have a relatively simple centre-centre correlation function resembling that of an atomic liquid or a close-packed disordered array of spheres. The relative orientation of molecules within the first neighbour shell has been investigated and is found to differ from that proposed earlier from a simple consideration of geometrical anisotrophy. The general use of RMC for molecular systems is discussed in relation to future work.