Stability and molecular features of aqueous films

The behavior of foam films is significant in many areas such as mineral flotation, food processing, and oil recovery. The film stability between two bubbles in foams has been motivated by different research interests. This dissertation contributes to the understanding of the film stability based on several methods of evaluation. First, the structure of thin water films during the rupture process was investigated by a new approach, which combines molecular dynamics simulation (MDS) with image processing analysis. The procedure was developed to convert the MDS trajectories to readable 3D images. The films were studied at different thicknesses by MDS to determine the critical thickness at which film ruptures. The potential energy of each thickness during simulation time was analyzed. A new procedure involving the calculation of molecular porosity was developed. Results indicate a critical molecular porosity value of 49% for film rupture. Second, disjoining pressure (DP) in thin films was examined by using Matlab. The surface charge regulation approach was applied to develop a new model for computing of DP. The case of sodium dodecyl sulfate (SDS) adsorption from NaCl solution was studied. A model of film drainage rate was developed accordingly which showed a good agreement with experimental data from the literature. Finally, features of interfacial water at air-water interfaces of anionic SDS and cationic dodecyl amine hydrochloride (DDA) solutions were examined by combining sum frequency generation (SFG) vibrational spectroscopy measurements and MDS. The SFG spectra and MDS results revealed that interfacial waters for SDS solutions was highly-ordered compared with those for DDA solutions. SFG results revealed that interfacial waters at pH 9 in both SDS and DDA solutions showed less-ordered than at pH 7. Experiments on foam stability and foam weight were conducted under the same conditions. Results revealed that for SDS solutions, foams were more stable at pH 7 than at pH 9. The opposite was observed for DDA solutions, in which case stability decreased when pH increased. Results were explained based on the extent of surfactant hydration at interfaces which is thought to account for film stability as revealed from results on foam stability.