The surface tension decrease during adsorption of purified dilauroyl or dimyristoyl phosphatidylcholine (DLPC or DMPC) onto a water-air interface was studied via drop profile tensiometry, in order to gain insight into the mechanism of adsorption. These phospholipids are strongly hydrophobic and have low solubility limits in water, above which they separate as lamellar phases which may in turn be dispersed in the form of unilamellar vesicles. Thus, adsorption may involve transport to the interface of dissolved monomer, delivery by vesicles of phospholipid directly to the interface, or a mixed mechanism that depends on molecular exchange between vesicle and monomer. Within each pathway, bulk diffusion or the kinetics of adsorption onto the interface could be rate-controlling. Via dynamic surface tension experiments, we were able to gain insight into this adsorption process, which occurs slowly over timescales of 1000-10,000 seconds for DLPC and longer for DMPC. The adsorption rate for both phospholipids was found to be proportional to the vesicle concentration, indicating a clear mechanistic role for these aggregates: they either directly deliver phospholipid to the interface, or perhaps supply monomer to the solution sublayer next to the interface. Adsorption dynamics was shown to be independent of bulk convection, confirming that monomer diffusion alone is not controlling, and suggesting that the overall process may be limited by interfacial kinetic steps. The mechanistic process that takes place during the kinetically controlled adsorption step, however, remains unresolved. Finally, we have examined the effects of vesicle size, temperature, and chain length on dynamic surface tension as a means of developing an appropriate kinetic model. Success in formulating an accurate kinetic model for phospholipid vesicle adsorption is key for advancement in several areas, including food emulsification and foam formation, spray drying and pharmaceuticals.

Effects of phosphatidylcholine vesicles on adsorption kinetics at an air-water interface

253rd National Meeting (2017)

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