Volume 27, Issue 1-2

Proteins in solution form a number of condensed phases. Even omitting the amyloid structures formed after partial protein unfolding, these phases include crystals, polymers, and other solid aggregates, as well as dense liquids and gels. Some of these condensed phases underlie pathological conditions, while others play a crucial role in the biological function of the respective protein or are an essential part of its laboratory or industrial processing. In this review, we summarize the fundamentals and recent findings on the kinetics of nucleation of dense liquid droplets and crystals. We define the transition from nucleation to spinodal decomposition for these two phase transitions. We review the two-step mechanism of protein crystal nucleation, in which mesoscopic metastable protein clusters serve as precursors to the ordered crystal nuclei. The concepts and mechanisms reviewed here provide powerful tools for control of the nucleation process, by varying the solution thermodynamic parameters.

Polymeric nanocomposites consist of nanoparticles imbedded in organic polymers, comprising a new class of materials. The combination of conducting nanoparticles and conducting polymers, described as hybrid electrically conducting nanocomposites, is a new emerging field. These materials may exhibit unprecedented properties, attractive for both the industry and academia. There is a variety of applications for conductive nanocomposites, i.e., sensors, actuators, touch screens, etc. The combination of conductive polymers with conductive carbon nanotubes has already shown some synergistic properties. The goal of the present review is to summarize the literature reporting work done on combining polyaniline (PANI), an electrically conductive polymer, with carbon nanotubes (CNT) including the authors’ work.

Over the last decade, nonlinear phenomena in low-temperature fuel cells as well as high-temperature fuel cells have been reported in the open literature. Experimental and theoretical studies found multiple steady states as well as periodic oscillations. The present article gives an overview of publications on this subject. Instead of sorting the analyses according to the types of fuel cells, this work used the source of the nonlinearity for classification. In the first part of the contribution, a very simple prototype fuel cell model is introduced. The model helps to give a qualitative explanation of the majority of nonlinear effects reported in literature. It is further used to identify potential sources of nonlinear behavior in reaction kinetics, membrane properties, and mass transport mechanisms. A classification scheme that is based on types of negative differential resistance (NDR) and was originally introduced by K. Krischer in Modern Aspects of Electrochemistry (Vol. 32, p. 1, Plenum Press, 1999) for electrochemical systems is applied to fuel cells. The second part of the work classifies the findings from literature according to their NDR type. Instabilities resulting not from electrochemistry but from other mechanisms such as water formation and reactant starvation are also discussed.

Solid-liquid reactions are commonly encountered in industry as well as every day life. Vast amounts of the applied bulk and specialty chemicals are produced employing solid-liquid reactions. The knowledge of the kinetics is crucial for the design and development of these processes. Quantitative modeling of reactive solids behavior in liquids is a big challenge. The reaction mechanism can be a very complex one, often comprising several unknown elementary steps. The structure and structural changes of the solid material are also often difficult to determine. Kinetics and mass transfer effects are coupled; to determine the intrinsic kinetics, the experiments should be free from mass transfer limitations. External mass transfer resistance can be suppressed by creating strong enough turbulent effects around the solid particles, but internal mass transfer effects can be present for porous particles. Due to these facts, modeling of dissolution reactions is often simplified. Even though such simplifications are necessary, a more in-depth investigation of the related phenomena combined with quantitative studies and more thorough modeling brings new understanding and precision to process development and optimization. Several example cases of solid-liquid reactions were chosen for this work, based on their industrial relevance and varying nature. Moreover, different experimental cases were studied to illustrate a basis for the theoretical development. Two of the cases are organic reactions, while the rest are inorganic ones. The objective of the current review is to exemplify challenges related to studying solid-liquid reactions and how these challenges can be overcome. This is done through the nine example cases, which serve as good examples of the various difficulties often encountered. The published articles serve as a convenient route for finding more detailed information on the topic. Based on the literature evaluation and the experience gained in the present work, theoretical development was made based on physico-chemical fundamentals. The main advances of the current work are in the quantitative modeling of a solid phase during its reaction with a liquid. This includes the dynamic and flexible implementation of the surface morphology and particle size distribution, as well as the reaction mechanism into an overall kinetic model, in which the internal and external mass transfer phenomena are taken into account. This helps in model development and discrimination between rival models, as well as in the interpretation of curiosities in kinetic models which can be found in literature.