The adsorption of single polyacrylic acid (PAAc) molecules was investigated on stepped hydroxide-stabilized polar ZnO(0001)-Zn surfaces using atomic force microscope (AFM) topography and force distance spectroscopy. Stepped surfaces of ZnO(0001)-Zn were prepared by a wet chemical etching procedure and PAAc molecules were adsorbed from aqueous NaClO(4) solutions. AFM single molecule topography studies could be utilized to show that polyacrylic acid molecules specifically adsorb on the non-polar (10-10) step edge faces at low ionic strengths. The radius of gyration of the dissolved PAAc in aqueous solution was measured by means of static light scattering experiments yielding a radius of gyration of R(g)=136 nm at pH 7.4 in 50 mM NaClO(4)/NaOH solution, which is in good agreement with the size of the adsorbed PAAc molecules as measured using AFM. The obtained results could be rationalized in terms of binding-site configurations at step edges and the effect of the chemical environment on both local electric double layer charge and molecular conformation of the PAAc molecules. The point of zero charge of the ZnO(10-10) surface was measured with chemical force microscopy to be pH(PZC)=10.2 ± 0.2. The specific adsorption of polyacrylic acid at non-polar ZnO step-edges can be explained by coordinative bonds formed between the carboxylic acid group and the Zn-surface atoms. On the hydroxide stabilized polar surface only weak hydrogen bonds can be formed in addition to van-der-Waals forces. Thus a "diffusion and trapping" mechanism keeps the adsorbed PAAc molecules mobile on the ZnO(0001)-Zn surface terraces due to small interaction forces until they are trapped at the (10-10) step faces by stronger coordinative bonds from the carboxylic groups to zinc atoms located in the first atomic layer of the crystal structure.

A detailed light scattering investigation is presented on dilute solutions of long chain sodium polyacrylate in the presence of Cu 2þ ions under conditions which are close to the precipitation threshold of the respective Cu 2þ -PA chains. The results are compared with literature data ( Eur. Phys. J E 2001, 5, 117-126) from the corresponding system in the presence of Ca 2þ ions. In all cases the solvent is a 0.1 M NaCl solution in water. The PA coils shrink considerably with increasing Cu 2þ concentration as the conditions approach the precipitation threshold. Yet, the extent of shrinking can not be driven as far as for the respective Ca 2þ -PA system, where fully collapsed sphere-like polymers had been observed at the threshold. Analysis of the aggregation process with time-resolved static light scattering reveals loose coil-like aggregate structures for Cu 2þ -PA aggregates and compact sphere-like aggregates for Ca 2þ -PA in accordance with the limiting shape of the respective shrunken single chains. The onset of Ca 2þ or Cu 2þ induced aggregation of PA chains at the precipitation threshold borders an intramolecular coil shrinking process. The transition of shrinking into aggregation occurs more readily with Cu 2þ -PA as it does with Ca 2þ -PA.

The present work investigates the formation of well-defined heteroaggregates from a binary mixture of a red and a yellow azo-dyestuff in the presence of Mg(2+) ions. Combined static and dynamic light scattering together with laser induced liquid bead ion desorption mass spectrometry (LILBID-MS) has been applied to characterize the states of the pure red dye and the pure yellow dye as well as of their mixture in aqueous solution without Mg(2+). These experiments indicated that a structural reorganization on a molecular scale takes place as soon as the two dyes are combined. Solutions of the combined red and yellow dye contain micelle-like mixed entities with a size of a few tenths of nanometers. Upon the addition of Mg(2+), these micelles vanish in favor of elongated heteroaggregates, which grow by a stepwise addition of smaller building units. As unraveled by UV/vis spectroscopy, the heteroaggregates that are formed from the red and yellow azo dye in the presence of Mg(2+) obey a stoichiometric ratio of the two components of 1:1. A new multiangle scattering instrument allowed us for the first time to follow this aggregation process at the stoichiometric ratio by time-resolved combined static and dynamic light scattering, thereby providing further aspects of the worm-like nature of the growing heteroaggregates.

A binary dyestuff aggregate with a distinct stoichiometry is formed in dilute aqueous solution upon addition of Mg(2+) ions. The aggregation process was investigated with time-resolved multiangle static light scattering resulting in a sequence of static scattering curves. The scattering curves were analyzed with respect to the aggregation kinetics as well as the structure of the growing aggregates. The aggregation kinetics was based on the time evolution of the weight-averaged molar mass values extracted from the intercepts of the static scattering curves. A kinetic model that considers solely a nucleation step and monomer addition in its most simple form was developed in order to describe the evolution of time-dependent mass data. In addition, a kinetic model introduced by Lomakin et al. (Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 1125) for the description of β-amyloid aggregation was adapted to the same experimental data. Application of the two kinetic models offered significant information on the role of magnesium ions within the aggregation process and provided a deeper understanding of the aggregation mechanism. Correlation of the size parameters extracted from the initial slopes of the scattering curves with the respective mass data as well as direct fitting of the scattering curves with the wormlike chain model yield a consistent set of model parameters.

We report the polymer conformation and fullerene aggregation in a ternary system containing polystyrene, C 60 and toluene measured by small angle neutron, static and dynamic light scattering. We investigate polymer concentrations across the dilute and semidilute regime for five polymer molecular weights (M w =20k-1Mg/mol), and fullerene concentrations below and above its miscibility threshold in toluene.We find that the polymer radius of gyration (R poly g ), hydrodynamic radius (R h ) * To whom correspondence should be addressed † Centre for Plastic Electronics, Imperial College London, London, SW7 2AZ, United Kingdom ‡ Department of Chemical Engineering, Imperial College London, London, SW7 2AZ, United Kingdom ¶ Physikalische Chemie, Universität Paderborn, 33098 Paderborn, Germany § Institut Laue-Langevin, DS/LSS, F-38 042 Grenoble, France Materials Department, Imperial College London, London, SW7 2AZ, United Kingdom 1 and the mixture correlation length (ξ) remain unchanged upon addition of C 60 .The miscibility of C 60 in toluene, however, decreases upon addition of polystyrene forming aggregates with a time-dependent radius on the order of 100 nm, and this effect is amplified with increasing polymer M w . Our findings are relevant to the solution processing of organic photovoltaics, which generally require the effective solubilisation of fullerene derivatives and polymer pairs in this concentration range.