5 research outputs found
Quasi-Immiscible Spreading of Aqueous Surfactant Solutions on Entangled Aqueous Polymer Solution Subphases
Motivated by the possibility of enhancing
aerosol drug delivery to mucus-obstructed lungs, the spreading of
a drop of aqueous surfactant solution on a physically entangled aqueous
polyÂ(acrylamide) solution subphase that mimics lung airway surface
liquid was investigated. Sodium dodecyl sulfate was used as the surfactant.
To visualize spreading of the drop and mimic the inclusion of a drug
substance, fluorescein, a hydrophilic and non-surface-active dye,
was added to the surfactant solution. The spreading progresses through
a series of events. Marangoni stresses initiate the convective spreading
of the drop. Simultaneously, surfactant escapes across the drop’s
contact line within a second of deposition and causes a change in
subphase surface tension outside the drop on the order of 1 mN/m.
Convective spreading of the drop ends within 2–3 s of drop
deposition, when a new interfacial tension balance is achieved. Surfactant
escape depletes the drop of surfactant, and the residual drop takes
the form of a static lens of nonzero contact angle. On longer time
scales, the surfactant dissolves into the subphase. The lens formed
by the water in the deposited drop persists for as long as 3 min after
the convective spreading process ends due to the long diffusional
time scales associated with the underlying entangled polymer solution.
The persistence of the lens suggests that the drop phase behaves as
if it were immiscible with the subphase during this time period. Whereas
surfactant escapes the spreading drop and advances on the subphase/vapor
interface, hydrophilic dye molecules in the drop do not escape but
remain with the drop throughout the convective spreading. The quasi-immiscible
nature of the spreading event suggests that the chemical properties
of the surfactant and subphase are much less important than their
physical properties, consistent with prior qualitative studies of
spreading of different types of surfactants on entangled polymer subphases:
the selection of surfactant for pulmonary delivery applications may
be limited only by physical and toxicological considerations. Further,
the escape of surfactant from individual drops may provide an additional
spreading mechanism in the lung, as hydrodynamic and/or surface pressure
repulsions may drive individual droplets apart after deposition