Jan Sefcik

Biocatalytic induction of supramolecular order

Supramolecular gels, which demonstrate tunable functionalities, have attracted much interest in a range of areas, including healthcare, environmental protection and energy-related technologies. Preparing these materials in a reliable manner is challenging, with an increased level of kinetic defects observed at higher self-assembly rates. Here, by combining biocatalysis and molecular self-assembly, we have shown the ability to more quickly access higher-ordered structures. By simply increasing enzyme concentration, supramolecular order expressed at molecular, nano- and micro-levels is dramatically enhanced, and, importantly, the gelator concentrations remain identical. Amphiphile molecules were prepared by attaching an aromatic moiety to a dipeptide backbone capped with a methyl ester. Their self-assembly was induced by an enzyme that hydrolysed the ester. Different enzyme concentrations altered the catalytic activity and size of the enzyme clusters, affecting their mobility. This allowed structurally diverse materials that represent local minima in the free energy landscape to be accessed based on a single gelator structure. Supramolecular gels show promise in diverse areas, including healthcare and energy technologies, owing to tunable properties that arise directly from the organization of their building blocks. Researchers have now been able to control this behaviour by combining enzymatic catalysis with molecular self-assembly. Although it seems counter-intuitive, gels that assembled faster showed fewer defects.

Effect of shear rate on aggregate size and morphology investigated under turbulent conditions in stirred tank

Aggregation and breakage of aggregates produced from fully destabilized polystyrene latex particles in turbulent flow was studied experimentally in both batch and continuous stirred tank. Detailed investigation of the initial kinetics showed that the collision efficiency, alpha, depends on the shear rate according to alpha proportional to G(-b), with a power law exponent, b, equal to 0.18. After steady state was reached the dynamic response of the system on a change in stirring speed and solid volume fraction was investigated. It was found that the steady-state values of two measured moments of the cluster mass distribution (CMD) are fully reversible upon a change in stirring speed. This indicates that although the moments of CMD at steady-state depend on the applied shear rate, the aggregate structure is independent of the shear rate in the given range of stirring speeds. This was proved by independent measurement of the fractal dimension, d(f), using image analysis which provided a d(f) equal to 2.62 +/- 0.18 independent of applied stirring speed. The critical aggregate size, below which breakage is negligible, determined by dilution experiments was consequently used to evaluate the aggregate cohesive force holding the aggregate together, which was found to be independent of the aggregate size and equal to 6.2 +/- 1.0 nN.

Aggregation kinetics of polymer colloids in reaction limited regime : experiments and simulations

The kinetics of reaction-limited cluster aggregation of fluorinated polymer colloids in a broad range of particle volume fractions has been investigated experimentally by measuring independently the Fuchs stability ratio W and the time evolution of both the average radius of gyration and the average hydrodynamic radius of the aggregates mass distribution. The W value is determined from the aggregation rate at the very initial stage of the aggregation, where the presence of triplets is negligible. The time evolutions of and are then simulated using the cluster mass distribution calculated from the population balance equations with various aggregation kernels proposed in the literature. It is found that, when the measured W value is used, the only kernels that can correctly simulate the experimental results are the product kernel and the one derived by Odriozola et al. (Europhys. Lett. 53 (2001) 797), with some proper tuning of the exponent in the kernel. For the particle volume fraction phi<1%, the obtained value for the exponent is 0.4 and independent of phi, while it tends to decrease for larger phi values, most likely indicating a significant effect of multi-body interactions on the aggregation kinetics.

Kinetic and thermodynamic issues in the early stages of sol-gel processes using silicon alkoxides

An understanding of the chemical processes that take place in the earliest stages of a sol-gel preparation can provide the potential to better control microstructural evolution of a catalyst. While the desired catalyst properties depend on specific details of a catalytic application, in general one wants at least to control textural and chemical homogeneity. Silica provides an excellent test system for the study of sol-gel processes starting from alkoxide precursors as it can exhibit a wide variety of structure and has been extensively studied. In this review the features of tetraethoxysilane (TEOS) polymerization as observed by Si-29-NMR spectroscopy are summarized. Trends in hydrolysis and condensation with increasing oligomer size are identified. The kinetics and equilibrium of these reactions, metastability and phase separation are reviewed. Finally we suggest a comprehensive reaction engineering picture of TEOS polymerization with special focus on the crossover between gelation and precipitation. Selected comments on other alkoxides, non-alkoxides, and on multicomponent formulations are also offered.

Thermochemistry of silicic acid deprotonation: Comparison of gas phase and solvated DFT calculations to experiment

Theoretical approaches to the thermochemistry of silicate anions have so far focused on gas-phase molecular orbital and density functional theory (DFT) calculations. These calculations predict that in the presence of hydroxide ligands the most stable singly charged anion of the silicic acid H4SiO4 is the five-coordinated anion H5SiO5−. However, experimental evidence from in situ nuclear magnetic resonance (NMR) experiments clearly shows that deprotonated silicic acid in alkaline aqueous solutions is four-coordinated, H3SiO4−. We compare gas-phase and solvated DFT calculations of monomeric anions of silicic acid in order to assess solvent effects on the thermochemistry of silicic acid deprotonation. We show that appropriate inclusion of solvation in quantum chemical calculations is critical for correct prediction of coordination and thermochemistry of silicate anions in aqueous solutions. Multiply charged anions of silicic acid are found to be electronically unstable in the gas phase and thus it is not possible to use thermodynamic cycles involving these species in thermodynamic calculations. However, a high dielectric constant solvent is sufficient to stabilize these anions, and solvated calculations can be used to directly compute their thermodynamic quantities. When we include the zero point energy (ZPE) and statistical mechanics contributions to the Gibbs free energy, we obtain accurate free energies for successive deprotonations of silicic acid in aqueous solutions. Although the pentacoordinate hydroxoanion of silicon is more stable in the gas phase than the four-coordinated one (by 18 and 5 kcal/mol in the self-consistent field (SCF) energy and the Gibbs free energy, respectively), it is less stable by 5 kcal/mol in the Gibbs free energy when hydration effects are appropriately accounted for. Solvated DFT calculations, validated here by their accurate description of silicate anions in aqueous solutions, should lead to more reliable predictions of important geochemical quantities, such as surface acidities and detailed reaction coordinates for dissolution of minerals.

Cooperative Self-Assembly of Peptide Gelators and Proteins

Molecular self-assembly provides a versatile route for the production of nanoscale materials for medical and technological applications. Herein, we demonstrate that the cooperative self-assembly of amphiphilic small molecules and proteins can have drastic effects on supramolecular nanostructuring of resulting materials. We report that mesoscale, fractal-like clusters of proteins form at concentrations that are orders of magnitude lower compared to those usually associated with molecular crowding at room temperature. These protein clusters have pronounced effects on the molecular self-assembly of aromatic peptide amphiphiles (fluorenylmethoxycarbonyl- dipeptides), resulting in a reversal of chiral organization and enhanced order through templating and binding. Moreover, the morphological and mechanical properties of the resultant nanostructured gels can be controlled by the cooperative self-assembly of peptides and protein fractal clusters, having implications for biomedical applications where proteins and peptides are both present. In addition, fundamental insights into cooperative interplay of molecular interactions and confinement by clusters of chiral macromolecules is relevant to gaining understanding of the molecular mechanisms of relevance to the origin of life and development of synthetic mimics of living systems.

Salt-induced control of supramolecular order in biocatalytic hydrogelation

Biocatalytic action and specific ion effects are both known to have dramatic effects on molecular self-assembly and hydrogelation. In this paper, we demonstrate that these effects are highly cooperative. Biocatalytic hydrogelation of Fmoc peptides in the presence of salts combines kinetic (through enzymatic catalysis) and thermodynamic (specific ion and protein templating) contributions when applied in combination. Spectroscopic data (obtained by fluorescence spectroscopy and circular dichroism) revealed that hydrophobic interactions are greatly affected, giving rise to differential chiral organization and supramolecular structure formation. The kinetic effects of catalytic action could be removed from the system by applying a heat/cool cycle, giving insight into the thermodynamic influence of both protein and salt on these systems and showing that the effects of catalysis, templating, and salts are cooperative. The variable molecular interactions are expressed as variable material properties, such as thermal stability and mechanical strength of the final gel-phase material. To gain more insight into the role of the enzyme, beyond catalysis, in the underlying mechanism, static light scattering is performed, which indicates the different mode of aggregation of the enzyme molecules in the presence of different salts in aqueous solution that may play a role to direct the assembly via templating. Overall, the results show that the combination of specific salts and enzymatic hydrogelation can give rise to complex self-assembly behaviors that may be exploited to tune hydrogel properties.

Prediction of crystallization diagrams for synthesis of zeolites

Zeolites comprise a rich class of microporous crystalline aluminosilicates: typically crystallized in nearly pure form from alkaline aluminosilicate aqueous solutions in batch reactors. It is well known that zeolite synthesis outcomes are sensitively dependent on the total batch composition, temperature, time and other initial and boundary conditions of the reaction system. Effects of the total batch composition on the nature of zeolite products are usually presented in the form of crystallization diagrams, but previous modeling efforts have not been able to explain structure of such diagrams. We calculate theoretical crystallization diagrams for synthesis of zeolites A and X under the assumption of pseudoequilibrium between the: two zeolite phases and a homogeneous solution. This approach allows us to analyze solubility effects separately from others, such as nucleation and competitive kinetics of crystal growth. First, we identify a solubility product of zeolite X using a thermodynamic-solution model that accurately represents speciation in zeolite mother liquors. As the silicon-to-aluminum ratio of zeolite X approaches the limiting value of I its solubility product approaches that of zeolite A. Then we calculate a theoretical crystallization diagram for zeolites A and X in the Na2O-SiO2-Al2O2H3O-H2O system. The results are ina very good agreement with experimental observations. This suggests that selective nucleation is not necessary to produce a pure zeolite product and that solubility considerations alone can explain observed crystallization diagrams in this particular case. Solubility based calculations using the present solution model can be used for steady-state design of reactors for synthesis of zeolites A and X when nucleation of desired phases is assured. A though we cannot yet conclusively discriminate between selective nucleation, growth kinetics and solubility to explain the product selectivity in particular zeolite producing systems, we present a modeling framework capable of such discrimination.

Gelation Mechanism of Resorcinol-Formaldehyde Gels Investigated by Dynamic Light Scattering

Xerogels and porous materials for specific applications such as catalyst supports, CO2 capture, pollutant adsorption, and selective membrane design require fine control of pore structure, which in turn requires improved understanding of the chemistry and physics of growth, aggregation, and gelation processes governing nanostructure formation in these materials. We used time-resolved dynamic light scattering to study the formation of resorcinol-formaldehyde gels through a sol-gel process in the presence of Group I metal carbonates. We showed that an underlying nanoscale phase transition (independent of carbonate concentration or metal type) controls the size of primary clusters during the preaggregation phase; while the amount of carbonate determines the number concentration of clusters and, hence, the size to which clusters grow before filling space to form the gel. This novel physical insight, based on a close relationship between cluster size at the onset of gelation and average pore size in the final xerogel results in a well-defined master curve, directly linking final gel properties to process conditions, facilitating the rational design of porous gels with properties specifically tuned for particular applications. Interestingly, although results for lithium, sodium, and potassium carbonate fall on the same master curve, cesium carbonate gels have significantly larger average pore size and cluster size at gelation, providing an extended range of tunable pore size for further adsorption applications.

Mechanism and kinetics of nanostructure evolution during early stages of resorcinol-formaldehyde polymerisation.

Resorcinol and formaldehyde react in aqueous solutions to form nanoporous organic gels well suited for a wide range of applications from supercapacitors and batteries to adsorbents and catalyst supports. In this work, we investigated the mechanism and kinetics of formation of primary clusters in the early stages of formation of resorcinol-formaldehyde gels in the presence of dissolved sodium carbonate. Dynamic Light Scattering measurements showed that size of freely diffusing primary clusters was independent of both reactant and carbonate concentrations at a given temperature, reaching the mean hydrodynamic radius of several nanometres before further changes were observed. However, more primary clusters formed at higher carbonate concentrations, and cluster numbers were steadily increasing over time. Our results indicate that the size of primary clusters appears to be thermodynamically controlled, where a solubility/miscibility limit is reached due to formation of certain reaction intermediates resulting in approximately monodisperse primary clusters, most likely liquid-like, similar to formation of micelles or spontaneous nanoemulsions. Primary clusters eventually form a particulate network through subsequent aggregation and/or coalescence and further polymerisation, leading to nanoscale morphologies of resulting wet gels. Analogous formation mechanisms have been previously proposed for several polymerisation and sol-gel systems, including monodisperse silica, organosilicates and zeolites.