Liquid surface tension is an important parameter to characterize the gas-liquid contact area in gas absorption with direct implications for the overall absorber equipment size and the associated capital cost. Additionally, liquid surface tension is one of the main factors to determine the solvent foaming formation in gas absorption/desorption operations.
Figure 1: (left): A snap shot from simulations to show CO2 molecules in the gas phase, interface, and the PDMS solvent phase. (right): Simulated surface tension for the PDMS solvent before and after CO2 absorption. The experimental neat solvent surface is also shown for comparison.
To date, there are very few studies related to description at the molecular level of the effect of gas absorption on solvent surface tension. Furthermore, in order to control foaming formation it is very important to understand the relationship between solvent surface tension and gas solubility. In this work, we addressed both the issue of the gas absorption effects on solvent surface tension and the gas solubility-solvent surface tension relationship using atomistic scale simulation methods. For this purpose, an in-house molecular simulation tool was developed to calculate surface tension by using the test-area free energy perturbation method.
Simulations showed that upon CO2 absorption into a hydrophobic poly(dimethylsiloxane) (PDMS) solvent at 298 K and CO2 pressures between 2.5-20 bar, the solvent surface tension could appreciably decrease by 20-35% (Figure 1). The CO2 molecules exhibit significantly higher concentrations in the gas-liquid interface region compared with the solvent phase. It is expected that CO2 molecules in the interface and solvent regions will decrease the PDMS-PDMS interaction and results in a decrease in solvent surface tension upon CO2 absorption.
Additionally, simulations show that one must compromise between large (small) CO2 solubility and small (large) solvent surface tension. To obtain significantly large surface tension, the solvent-solvent interaction needs to be increased, which will lead to small volume expansion upon CO2 absorption at high CO2 pressure, and as a result smaller CO2 solubility may be obtained.
This work was recently published on the Journal of Physical Chemistry C (DOI: 10.1021/acs.jpcc.5b05806) coauthored by Wei Shi, Nicholas Siefert, Bryan Morreale. Click here to download the full paper.