Micellization and adsorption of phospholipids and soybean oil onto hydrophilic and hydrophobic surfaces in nonaqueous media

Abstract

Micelle formation of phospholipids (PL) in bulk was investigated in nonaqueous media. Dye solubilization was used to determine the critical micelle concentration (CMC) of reverse micelles of phospholipids in a hexane–oil environment. The CMC, which represents the total PL monomer concentration in bulk, was also studied as a function of amount of water present in the system. The CMC was found to decrease with the addition of water. In addition, the sign of the surface charge of phospholipid micelles was determined in nonaqueous media using the phase analysis light scattering technique (PALS). Phospholipid reversed micelles appear to exhibit a negative charge even in nonaqueous media due possibly to a trace amount of water trapped inside the polar head center. These results are significant to the understanding of membrane separation processes since the main objective is to separate out micellar aggregates from hexane–oil mixtures. Two important factors which control the efficiency of membrane separations are: size exclusion and chemical interaction between membrane surfaces and bulk species. The micelle formation affects separation in a positive way since monomers are believed to pass through the membranes more easily than micelles. On the other hand adsorption of bulk species to membrane surfaces is frequently cited as the primary cause of fouling which results in pore plugging. Adsorption behavior of phospholipids and soybean oil onto acid (HF) treated silicon and polymer surfaces were analyzed by ex-situ FTIR/ATR and in-situ contact angle methods. In-situ contact angle measurements were found to be very efficient in detecting adsorbed oil and phospholipids on modified silicon and polymer surfaces. Results from this work reveal for the first time that hydrophobic surfaces (>50°) are more prone to phospholipid and soybean oil adsorption than hydrophilic surfaces in hexane–soybean oil media.