Processing of the project:

Isabella Jung, M.Sc., Institute of Thermal Separation Processes

Supervision:

Prof. Dr.-Ing. Irina Smirnova, Prof. Dr.-Ing. habil. Dr. h.c. Stefan Heinrich

Objectives:

The aim of this work is the novel quantitative characterization of the surface structure of native and coated biopolymers using inverse supercritical fluid chromatography (SFC) to optimize loading and release processes of sensitive substances.

• Preparation and analysis of highly porous biopolymer aerogel particles in micrometer range.
• Modification of hydrophilic aerogel surface by post-processing via cold plasma polymerization.
• Experimental characterization of the biopolymer substrates by chromatographic and thermodynamic parameters using SFC.
• Determination of retention parameters of different polar substances from the food and pharam sectors on the biopolymer aerogels.

Figure 1: Sample preparation and chromatographic analysis in SFC.

• Model-based description of the interactions between aerogel-stationary-phase and organic substances using a semi-empirical sorption model (short: LSER model).
• Interpretation of the chemical contributions on the aerogel surface.
• Prediction of adsorption behavior of not yet tested organic substances on biopolymer aerogels.
• Simulation of adsorption behavior using  cellular automata model in combination with Lattice-Boltzmann theory.

Methods and working program:

• Production of mesoporous aerogel particles:

Figure 2: Production of aerogel substrates.

• Stationary phases from different biopolymer particles are obtained using different production processes: Gel particle production by spray- (two-substance nozzle) and wet-milling-processes (colloid mill).

Figure 3: Methods of microparticle production.

• The synthesis of biopolymer gels is followed by a one-step solvent exchange with ethanol (99.8 %). The alcohol particles are then subjected to supercritical drying at 120 bar and 60 °C in a high-pressure autoclave.

Figure 4: Schematic illustration of the plasma polymerization process

for surface modification.

• Subsequent cold plasma polymerization using C₄F₈ and PFAC-6/PFAC-8 mononers enables controllable hydrophobicity of the polar aerogel surface.

Characterization of the aerogel properties:

The hydrogel and aerogel particles produced are evaluated in detail in terms of their morphological and structural properties:

• Analysis of hydrogel particle size and shape (optical microscope).
• Determination of skeletal density of the powdered biopolymer aerogels (helium pycnometer).
• Determination of bulk density of the aerogel powder.
• Determination of specific surface area and pore properties (nitrogen-temperature adsorption).
• Size and shape analysis of the aerogel particles (Camsizer XT).
• Optische Analyse der Partikel- und Oberflächeneigenschaften (Rasterelektronenmikroskop).
• Optical analysis of particle and surface properties (scanning electron microscope).
• Experimental hydrophobicity determination of the plasma treated aerogels (contact angle measurement).
• Evaluation of the aerogel position in the chromatographic column after filling process (µ-CT measurements).

Characterization of the adsorption behavior:

• Determination of retention parameters by SFC and calculation of thermodynamic parameters (e.g. adsorption enthalpy).
• Semi-empirical LSER model: a multiple regression analysis was used to investigate correlations or dependencies between different variables such as retention factor and different LSER descriptors (solute descriptors) of the adsorption process, which can be applied to the strength and relevance of different interactions taking place.
• Cellular automata and Lattice-Boltzmann simulation: the model involves the separate simulation of two processes: Hydrodynamics of continuous collision detection between analyte-analyte and analyte-aerogel (sorption event). The simulation of hydrodynamics was performed using the Lattice-Boltzmann method (LBM); sorption was calculated using the original cellular automata model.

Figure 4: 2D-visualization of the modeled computational grid of a chromatographic aerogel-filled column and simulation of column flow of CO₂.

Results:

In this work, various biopolymer aerogels were produced in micrometer range. The focus of this work was on the development of controllable surface properties by changing the physical and structural properties before and after aerogel production. Biopolymers made from the polysaccharides alginate, cellulose, and chitosan, as well as the protein educts whey protein isolate (WPI) and potato protein isolate (PPI) were used, prepared and characterized using the methods presented. These methods allowed the preparation of aerogel microparticles with particle sizes ranging from 38-109 µm and with sphericity values in the range of 0.6-0.9. At the microstructural level, high specific surface areas in the range of 355-479 m²/g were obtained. Alginate aerogels exhibited the highest specific surface areas (455 m²/g) and mesopore volumes among polysaccharides.

In addition, a novel post-modification method using cold plasma polymerization of octofluorocyclobutane monomers (C₄F₈) was developed to effect hydrophobization of the hydrophilic biopolymer structures (using alginate as an example) while maintaining the pore structure. Microscopic examination of the treated alginate aerogel particles showed a small reduction in specific surface area in the range (355-408 m²/g). It can be concluded that cold plasma polymerization is a successful post-treatment method for hydrophilic aerogels. The optimization in terms of stability against liquids opens another field of industrial applications for biopolymer aerogels outside the food sector (e.g. thermal insulation).

In reversed-phase operation, the SFC offered the possibility of characterizing the native and coated column materials of biopolymer aerogels (stationary phase) used by relating the resulting interaction pattern of the test analytes to the surface structure of the stationary phase. Prior to analysis in the instrument, the columns were filled with aerogel particles using an established packing process. An interpretation of the interactions taking place between mobile and stationary phase could then be made based on the determined experimental chromatographic retention parameters (e.g. dead time, retention time and factor) as well as calculated thermodynamic parameters such as adsorption enthalpy. With regard to the retention parameters determined, it can be summarized that in general test-substances with one or more carboxylic group (-COOH) exhibited medium to prolonged retention times. The combination of individually partly less polar groups like -COOH with a carbonyl (-CO) or a hydroxy group (-OH) could shift the retention behavior towards high retention times on alginate aerogel. For all the columns prepared, a maximum adsorption was observed at 55-60 °C, which is consistent with the maximum of the coefficient of thermal expansion as well as the density of CO₂ in the temperature range used.

The application of empirical linear solvation energy relations allowed the classification of type and strength of chemical interactions starting from the determined retention factor. By using these parameters, properties such as polarizability, dipolarity, ability to donate and accept hydrogen bonds (H-donor/H-acceptor), and molecular size or hydrophobicity could be included in the analysis of surface functionality between analytes and material. Comparison of system constants was demonstrated primarily on native and coated alginate aerogel particles: For native aerogel particles, strong interactions of the constants b (H-donor) and s (dipolarity) were observed. In comparison, all plasma-treated aerogels showed an increase in the value of system constant v, suggesting an increase in the carbon content due to the plasma treatment on the surface.

To verify the obtained findings, a simulation of the adsorption behavior of organic substances on an aerogel-filled chromatographic column was performed. A cellular automata (CA) model combined with the Lattice-Boltzmann method (LBM) was used to conduct the simulation. Using the LBM theory, a chromatographic column packed with aerogel particles could be simulated. By varying the parameters of column porosity, flow rate, adsorption- and desorption ratio, trends in adsorption behavior could be calculated in the model. For example, a decrease in porosity showed longer retention times in the column. This corresponds to a trend which is also observed in experimental measurements.

Literatur:

[1] Sun M., Ülker Z., Chen, Z.; Deeptanshu S.; Johannsen M., Erkey, C., Gurikov P. (2021): Development and validation of retention models in supercritical fluid chromatography for impregnation process design. In: Applied Sciences 11 (15): 7106 (2021-07-31), DOI:     10.15480/882.3714.

[2] Andlinger D., Schlemmer L., Jung I., Schroeter B., Smirnova I, Kulozik U. (2022): Hydro- and aerogelsfrom ethanolic potato and whey protein solutions: Influence of temperature and ethanol concentration on viscoelastic properties, protein interactions, and microstructure. In: Food Hydrocolloids(125), 107424, DOI: 10.1016/j.foodhyd.2021.107424.

[3] Roth M. (2004): Determination of thermodynamic properties by supercritical fluid chromatography. In: J Chromatogr A. 2004 May 28;1037(1-2):369-91, DOI: 10.1016/j.chroma.2003.10.126.

[4] Schroeter B., Jung I., Bauer K., Gurikov P., Smirnova I. (2021): Hydrophobic Modification of BiopolymerAerogels by Cold Plasma Coating. In: Polymers13(17), 300, DOI: 10.3390/polym13173000.