(543c) Alteration of Marangoni Flow in the Presence of a Pre-Deposited Insoluble Surfactant

Lungs produce pulmonary surfactant in the respiratory zone, but the distribution of these endogenous phospholipids in the upper airways is unknown and likely highly variable. Motivated by an interest in developing surfactant-aided, self-dispersing pulmonary drug carriers, here we investigate how the density of pre-existing insoluble phospholipid monolayers impacts Marangoni flows induced by exogenous surfactants on an aqueous subphase. A dipalmitoylphosphatidylcholine (DPPC) monolayer is pre-deposited at a predetermined lateral density on an aqueous subphase before depositing a two microliter drop of oleic acid as an exogenous, water-insoluble surfactant. Talc tracer particles monitor the extent of spreading and a Wilhelmy pin measures the surface tension before and after spreading. A trans-illumination absorbance method is used to measure the temporal evolution of the subphase surface deformation. This reveals the outward moving “Marangoni ridge”. For our choice of surfactants, oleic acid and DPPC, there is no mixing in the surface monolayer in the time of the spreading event.

As long as the surface pressure of the pre-deposited monolayer is less than that of the deposited surfactant monolayer, spreading proceeds until the surface pressures of the unmixed endogenous and exogenous surfactant monolayers are equal. Oleic acid spreads to form a monolayer, with residual oleic acid forming small lenses at the air/water interface. Since the equilibrium surface pressure of an oleic acid monolayer is fixed, the final surface pressure and thus the final area per DPPC molecule in the compressed endogenous monolayer is the same in every case. The final area per DPPC molecule is fixed at the area per molecule corresponding to the surface pressure of an oleic acid monolayer, independent of its starting surface concentration. The Marangoni flow and resulting surface deformation are altered by increasing the initial pre-deposited surfactant surface concentration. Unlike Marangoni flow with no pre-deposited surfactant, there is surface flow ahead of the Marangoni ridge as the pre-deposited monolayer is compressed. Due to the compression of the pre-deposited monolayer, the surface tension gradient now extends beyond the deposited monolayer into the pre-deposited monolayer. The change in the surface tension gradient causes normal and radial velocities to extend into the region occupied by the pre-deposited monolayer. This is in sharp contrast to surfactant spreading in the absence of pre-deposited surfactant, where no normal or radial velocity components precede the deposited exogenous surfactant front.