Swelling and Force Generation of Absorbent Organic Materials

Paul Bonvallet, Department of Chemistry 

Swellable organically modified silica (SOMS) is a hybrid organic/inorganic media discovered at the College of Wooster. Sometimes called “swellable glass,” this hydrophobic material absorbs many times its mass in organic liquids like acetone (Figure 1). The reversible swelling behavior has applications ranging from personal care products to environmental remediation. SOMS has been commercialized by ABS Materials in Wooster and is sold under the trade name Osorb. However, the fundamental chemical and physical principles that govern its behavior remain poorly understood.

Figure 1. Composite photograph (a) and optical microscope images (b) of SOMS before and after being swollen with acetone.

  • Force Measurement Project. SOMS generates a mechanical force when it absorbs organic liquids and swells. We have an apparatus that measures this force, and we have made some interesting discoveries. Loading the device with more SOMS generates a greater swelling force (up to several hundred newtons), but linearly (Figure 2). We have developed a theoretical model that describes the differential force generated by a sample of SOMS as a function of its intrinsic modulus and the physical dimensions of the space that it occupies. Our future efforts will include investigating environmental factors that influence the swelling force (temperature, type of solvent) in addition to measuring the force generated by other swellable materials such as polyelectrolyte hydrogels.
Figure 2. Swelling force generated by SOMS as a function of the mass of material in the apparatus.

  • Chemical Formulation Project. The reversible swelling of SOMS is observed at the macroscopic scale, but ultimately comes from events at the molecular and meso-scale. We are seeking connections between the chemical structure of SOMS and its bulk properties such as absorption capacity and swelling factor. As part of our investigation, we will prepare certain chemical precursors using standard laboratory techniques (Figure 3). These precursors will be used to make derivatives of SOMS that may differ in swelling performance and physical / spectroscopic properties.
Figure 3. Synthetic plan for the preparation of a precursor to SOMS.

  • Spectroscopy Project. We can “load” SOMS by treating it with solutions of organic compounds in organic solvents. Evaporating the solvent leaves the solute behind, trapping it inside the SOMS matrix. Carbonyl-containing compounds exhibit differences in their vibrational (IR) spectrum when trapped inside SOMS. Interestingly, the carbonyl stretching frequency of numerous carboxylic acids, aldehydes, and ketones increases by ~15 cm−1 when the compound is trapped inside SOMS in comparison to the pure compound (Figure 4). Similarly, the emission wavelength of certain fluorescent materials changes inside SOMS. We are exploring the physical and chemical basis for these observations using IR and fluorescence spectroscopy.
Figure 4. Overlaid IR spectra of pure benzoic acid (blue), pure SOMS (black), and benzoic acid trapped inside a SOMS matrix (red).

Students will help to design their experiments in the areas described above. Knowledge of organic chemistry and its laboratory techniques is helpful, but not required. Curiosity, organization, and attention to detail are essential in these projects.