An investigation of the volume transition in colloidal core−shell particles composed of a temperature-independent polystyrene core and a shell of thermosensitive cross-linked polymer chains by small-angle X-ray scattering (SAXS) is presented. The PS cores of the particles have a diameter of 80 nm whereas the shell composed of cross-linked poly(N-isopropylacrylamide) has a thickness of 32 nm in the swollen state at 25 °C and of 18 nm after shrinking by a continuous volume transition. The SAXS intensities measured at high scattering angles could be described by a Lorentz-type function at both states. This indicates the presence of liquidlike local concentration fluctuations of the gel which are still present in the shrunken state. The correlation lengths ξ measured in both states are of the order of a few nanometers (25 °C, ξ = 3.2 nm; 50 °C, ξ = 2.1 nm). The present analysis therefore shows that the core−shell microgels behave in a distinctively different manner than ordinary thermosensitive gels:  The cross-linked chains in the shell are bound to a solid boundary independent of temperature. The spatial constraint by this boundary decreases the maximum degree of swelling but also prevents a full collapse of the network above the volume transition.

We use small-angle x-ray scattering (SAXS) as a tool to study the binding of proteins to spherical polyelectrolyte brushes (SPB) in situ. The SPB consists of a solid core of approximately 100 nm diam onto which long polyelectrolyte chains [poly(styrene sulfonic acid, PSS) and poly(acrylic acid, PAA)] have been densely grafted. The proteins used in this investigation, Bovine Serum Albumine (BSA) and Bovine Pancreatic Ribonuclease A (RNase A), adsorb strongly to these SPB if the ionic strength is low despite their negative charge. Virtually no adsorption takes place at high ionic strength. SAXS demonstrates that both proteins are distributed within the brush. The findings reported here give further evidence that the strong adsorption of proteins to SPB is due to the "counterions release forces": The patches of positive charge on the surface of the proteins become multivalent counterions of the polyelectrolyte chains. Thus, a concomitant number of co- and counterions is thereby released and the entropy of the entire system is increased. The repulsive Coulombic interaction as well as the steric repulsion between the proteins and the brush layer are counterbalanced by this effect. The data discussed here demonstrate that the adsorption of proteins in SPB presents a new principle for the immobilization of proteins.

Time-resolved synchrotron small-angle X-ray scattering (SAXS) studies were performed to investigate the unseeded formation and growth of colloidal calcium carbonate particles. Equimolar aqueous solutions of CaCl2·2H2O and Na2CO3 were rapidly mixed in a stopped-flow apparatus, and SAXS data were recorded using an image-intensified CCD detector. It is shown that SAXS allows studying those processes in situ, with a very good time resolution. It can provide unsurpassed real-time information about the particle size, shape, polydispersity, inner structure, and density. In these studies, well-defined, spherical CaCO3 particles with colloidal dimensions up to ca. 270 nm and a remarkable uniformity in size could be observed. After a short nucleation period, the number density of the growing spheres remains constant. From the evaluation of the absolute scattering intensities, the particle mass density could be determined to be ca. 1.62 g/cm3, which is considerably lower than the density of the crystalline modifications. Our data thus point to the formation of colloidal, amorphous particles that are a precursor modification of the thermodynamically stable calcite. It was found that these particles are isolated and do not form larger aggregates. Upon lowering the concentration of the educts, particle formation and growth are considerably slowed and smaller particles are being formed.

The volume transition in thermosensitive colloidal core–shell particles is investigated by small-angle x-ray scattering (SAXS), small-angle Neutron scattering (SANS), and dynamic light scattering (DLS). The latex particles are dispersed in water and consist of a solid poly(styrene) core with a diameter of 100 nm. The thermosensitive shell is made up of poly(N-isopropylacrylamide) (PNIPA) chains crosslinked by 2.5 mol % N,N’-methylenbisacrylamide (BIS). Water is a good solvent for PNIPA at room temperature but becomes a poor solvent above 32 °C. The PNIPA network of the shell undergoes a volume transition at this temperature. As a result the diameter of the particle shrinks. The scattering intensities of the particles measured by SAXS and SANS as a function of temperature may be decomposed into a part deriving from the overall structure and a part originating from the fluctuations within the network. The analysis of the overall structure leads to the volume fraction of the swollen network at different temperatures. SANS in conjunction with contrast variation demonstrates that the network is confined in a well-defined shell. SAXS and SANS data therefore allow the phase diagram of the network in the shell of the particles to be derived, i.e., the average volume fraction of the network in the shell can be determined as a function of temperature. DLS corroborates this result but demonstrates that there is a small fraction of chains exceeding the outer radius derived from SAXS and SANS. The static intensity caused by the fluctuations of the network becomes the leading contribution at high scattering angles. SAXS data show that this part can be described by a Lorentzian both below and above the volume transition. The analysis demonstrates that critical fluctuations of the network around the transition temperature are fully suppressed. This finding is explained by the strong steric constraint of the network by its confinement within a shell of colloidal dimension. The swelling and shrinking can only take place along the radial direction and the chains are bound to the solid surface of the cores which remains constant during the transition.

The transformation of amorphous colloidal calcium carbonate into single microcrystals was observed in supersaturated solutions in situ. The observations are done by simultaneous time-resolved small-angle and wide-angle X-ray scattering experiments (TR-SAXS/WAXS) at a third generation synchrotron source. TR-SAXS/WAXS demonstrates that the particles generated by reaction of Ca 2+ and CO 3 2-ions are amorphous. The transformation of these amorphous CaCO 3 particles proceeds via dissolution and subsequent heterogeneous nucleation of the crystalline modification on the walls of the quartz capillary containing the reacting mixture. The crystalline modifications thus generated could be identified. No solid-solid transition is observed. TR-SAXS/WAXS is therefore well-suited to follow the mineralization from aqueous solution in great detail.The formation of calcium carbonate from supersaturated solutions has been studied for more than a century and it has received renewed interest, especially in the field of biomineralization. 1 The metastable forms of calcium carbonate can be investigated more precisely nowadays thanks to the improvement of the experimental techniques. 2 In particular amorphous calcium carbonate (ACC) has emerged as a precursor to the formation of more stable crystalline forms. 3 A topic of intense research activity is presently the detailed mechanism of phase transformation leading from the initial amorphous material to the final, thermodynamically stable, crystalline modification (calcite). Different mechanisms are being discussed, as, e.g., a solid/solid phase transition and a dissolution/crystallization process. This process also depends strongly on reagent concentrations and reaction conditions. 4 The transition from the amorphous state into a stable, crystalline modification is difficult to be studied in a timeresolved experiment. A number of experimental techniques have been applied to the study of the calcium carbonate formation, including optical, electron, and X-ray microscopies, dynamic light scattering, turbidimetry, and conductivity methods. 2,5 These methods, however, do not give the structural information as a function of time. On the other hand, the application of X-ray scattering methods is particularly well suited because it allows in-situ measurements of the size, morphology, and interactions of colloidal particles growing in solution. 6,7 Hence, scattering experiments probing directly the crystal structure of a given population of particles would be helpful to investigate the mineralization leading to a stable crystalline form.Recently, we studied the nucleation and growth of colloidal spheres made of calcium carbonate (CaCO 3 ) by using timeresolved small-angle X-ray scattering (TR-SAXS). 6 This method allows us to follow the growth of the particles from ∼20 ms after mixing the reagent solutions to their final size with a time resolution down to ∼0.1 s. In particular, we showed that the evolving particles consist of amorphous calcium carbonate. This could be deduced from the low m...