Zichen Jia

ETH Zurich, Zürich, ZH, Switzerland

Are you Zichen Jia?

Claim your profile

Publications (6)20.71 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The aggregation behavior of a soft, rubbery colloidal system with a relatively low glass transition temperature, T(g) approximately -20 degrees C, has been investigated. It is found that the average gyration and hydrodynamic radii, R(g) and R(h), measured by light scattering techniques, evolve in time in parallel, without exhibiting the crossover typical of rigid particle aggregation. Cryogenic scanning electron microscopy (cryo-SEM) images reveal sphere-like clusters, indicating that complete coalescence between particles occurs during aggregation. Since coalescence leads to a reduction in the total colloidal surface area, the surfactant adsorption equilibrium, and thus the colloidal stability, change in the course of aggregation. It is found that to simulate the observed kinetic behavior based on the population balance equations, it is necessary to assume that all the clusters are spherical and to account for variations in the colloidal stability of each aggregating particle pair with time. This indicates that, for the given system, the coalescence is very fast, i.e., its time scale is much smaller than that of the aggregation.
    Langmuir 07/2009; 25(17):9703-13. · 4.38 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A generalized model for colloidal stability has been validated against experimentally measured values of Fuchs stability ratio and critical coagulation concentration (ccc) for electrolytes with mono- or divalent cation, i.e., potassium chloride and magnesium chloride, respectively. Besides the classical DLVO theory, the generalized model accounts for the interplay between colloidal interactions and the association of cations with the particles surface charge groups. The model parameters are either obtained or estimated purely on the basis of independent information available in the literature. For the monovalent salt, the predictions agree well with literature experimental data, forecasting both the ccc values and stability ratios quantitatively. For the divalent salt the predictions for large values of the stability ratio tend to deviate from the experimental data produced in this work, but it is noted that the onset of stability, i.e., the ccc, and small stability ratios are correctly predicted. Moreover, a comparison of the above results with those neglecting the effect of counterion association with the particles surface charge groups indicates that the latter substantially overestimates stability ratios in the presence of high salt concentration in the case of the monovalent salt, and leads to unrealistic large values of the ccc for the divalent salt. Including the association of cations with the particles surface charge groups can explain the relatively low values of experimental ccc for divalent salts compared to the theoretical predictions by the classical DLVO theory neglecting ion association, which is apoint of interest in industrial coagulation processes.
    Langmuir 04/2009; 25(5):2696-702. · 4.38 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The colloidal stability, aggregation kinetics, and cluster structure of two styrene-acrylate copolymer latexes, stabilized with an aliphatic sulfonate and an aliphatic carboxylate surfactant, respectively, have been investigated experimentally in the temperature range between 283 and 323 K. The main objective of this study is to investigate the role of temperature and surfactant type on the aggregation kinetics and cluster structure. For this, the values of the Fuchs stability ratio and the time evolutions of the average radius of gyration, hydrodynamic radius, and structure factor of the clusters have been determined using static and dynamic light scattering techniques at different temperatures. It is found that although the two latexes exhibit a somewhat different dependence of the colloidal stability on temperature, all of the values of the average radius of gyration (or hydrodynamic radius) measured at different temperatures and surfactant types, which are plotted as a function of a properly defined dimensionless time, collapse to form a single master curve. Similarly, all of the measured average structure factors also collapse to form a single master curve when they are plotted as a function of the wavevector normalized using the average radius of gyration. These results indicate that, at least for the conditions investigated in this work, the aggregation mechanism and cluster structure are independent of temperature and surfactant type.
    Langmuir 10/2007; 23(20):10323-32. · 4.38 Impact Factor
  • Zichen Jia, Hua Wu, Massimo Morbidelli
    [Show abstract] [Hide abstract]
    ABSTRACT: In our previous work [Jia et al. J. Colloid Interface Sci. 2006, 302, 187], a generalized stability model has proposed, which accounts simultaneously for the interplays among three important physicochemical processes:  colloidal interactions, surfactant adsorption equilibrium, and association equilibria of surface charge groups with counterions at the particle−liquid interface. To further validate its reliability, in the present work, we apply the model to describe the stability of two polymer colloids that are stabilized not only by mobile ionic surfactants but also by fixed charges originated from polymer chain end groups. Moreover, two different surfactants, a carboxylate and a sulfonate, have been considered in order to investigate the effect of largely different protonation efficiencies. It is found that the generalized stability model can well describe the stability behavior of the two polymer colloids. Moreover, it is interesting to find out from the model predictions that the trends of surface charge variations with ionic strength, counterion type, and pH, and their effects on surfactant partitioning between the particle surface and liquid phase, can be substantially different, depending on the protonation efficiency of charge groups and the presence of fixed charges.
    Industrial & Engineering Chemistry Research - IND ENG CHEM RES. 06/2007; 46(16).
  • Zichen Jia, Hua Wu, Massimo Morbidelli
    [Show abstract] [Hide abstract]
    ABSTRACT: Thermal restructuring of fractal styrene-acrylate copolymer clusters dispersed in water has been investigated experimentally in the temperature range between 313 and 363 K. The particles constituting the clusters are of strawberry-like core-shell structure with a soft core and a rigid shell grafted on the core polymer chains. Due to the incomplete coverage of the core, the rather soft core may "flow out" through the open areas of the shell, leading to coalescence with the neighboring particles. The clusters were generated under diffusion-limited cluster aggregation conditions, and the restructuring kinetics was monitored by small-angle light scattering. Two sets of thermal restructuring experiments have been performed at various temperatures: (1) restructuring of growing clusters during aggregation and (2) restructuring of preformed clusters in the absence of aggregation. It is found that restructuring occurs only at temperature values above 323 K. In the absence of aggregation, restructuring leads to an increase of the fractal dimension and a decrease of the radius of gyration of the clusters. At sufficiently long times, both quantities reach a plateau value due to the presence of the grafted rigid shell, which constrains the coalescence of the soft core. A simple model, based on coalescence theory of liquid droplets and accounting for the incomplete coalescence and its dependence on temperature, has been developed to interpret the restructuring kinetics in the absence of aggregation. It is found that the proposed model can represent the measured experimental data well.
    Langmuir 06/2007; 23(10):5713-21. · 4.38 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A generalized model has been proposed to describe the stability of polymer colloids stabilized with ionic surfactants by accounting simultaneously for the interactions among three important physicochemical processes: colloidal interactions, surfactant adsorption equilibrium, and association equilibria of surface charge groups with counterions at the particle-liquid interface. A few Fuchs stability ratio values, determined experimentally for various salt types and concentrations through measurements of the doublet formation kinetics, are used to estimate the model parameters, such as the surfactant adsorption and counterion association parameters. With the estimated model parameters, the generalized model allows one to monitor the dynamics of surfactant partitioning between the particle surface and the disperse medium, to analyze the variation of surface charge density and potential as a function of the electrolyte type and concentration, and to predict the critical coagulant concentration for fast coagulation. Three fluorinated polymer colloids, stabilized by perfluoropolyether-based carboxylate surfactant, have been used to demonstrate the feasibility of the proposed colloidal stability model.
    Journal of Colloid and Interface Science 11/2006; 302(1):187-202. · 3.17 Impact Factor