(33c) Breathing New Life Into Classical Colloid Science

Lung surfactant is a mixture of lipids and specific proteins that forms monolayer films at the air-liquid interface in the alveoli that reduce surface tension, allowing easy breathing. Surfactant deficiency results in serious health problems, including Acute respiratory distress syndrome, which affects about 150,000 in the US each year with a 35% mortality rate. ARDS causes capillaries to leak, resulting in higher than normal concentrations of blood proteins in the alveolar fluids. Many of these proteins are surface-active ? for example, albumin acts like a simple surfactant, lowering the surface tension by about 20 mN/m at milligram concentrations. Lung surfactant adsorption is strongly inhibited by the competitive adsorption of these proteins to the interface. This reduction in lung surfactant adsorption leads to an increased surface tension in the lung and an increased work of breathing, which is likely an important factor in the development and severity of ARDS.

The factors influencing surfactant adsorption in the presence of a protein film are surprisingly similar to those that determine colloid stability. Albumin adsorbed to the interface induces an electrostatic energy barrier to surfactant diffusion of order 5-6 kBT, leading to a reduction in adsorption equivalent to reducing the surfactant concentration by a factor of more than 100! Adding hydrophilic, non-adsorbing polymers such as polyethylene glycol to the subphase provides a ?depletion attraction? between the surfactant aggregates and the interface that eliminates the energy barrier; the same polymers are used to flocculate colloidal particles. Surfactant adsorption increases exponentially with polymer concentration as predicted by the simple Asakura and Oosawa model of depletion attraction. Surfactant adsorption is also strongly affected by the ionic strength and valence (z) of added electrolytes: the ion concentration needed to restore surfactant adsorption scales as z-6, the same scaling as the classic Schulz-Hardy rule for colloid stability. Polyelectrolytes also show the same behavior in enhancing competitive adsorption as in colloid stability; oppositely charged polyelectrolytes first enhance surfactant adsorption, then as the polyelectrolyte concentration is increased, reduce adsorption, consistent with charge overcompensation.