| Competitive adsorption to air-water interfaces occurs anytime multiple surface-active species co-exist in solution. It is especially common in biological processes and is believed to be a causative factor in the development of Acute Respiratory Distress Syndrome (ARDS), a potentially fatal disease that afflicts ∼140,000 annually. Lung surfactant (LS) is a unique mixture of lipids and proteins that adsorbs to the alveolar interface and acts to lower the surface tension in the lungs, which makes normal breathing effortless. The surface tension control imposed by LS is compromised during ARDS; serum proteins in the alveolar fluid compete with lung surfactant for the interface. Understanding the physics and chemistry of this competitive adsorption process is necessary to develop new approaches which safely and effectively treat ARDS.;Langmuir troughs are used as an in vitro model for the lung to study LS/serum protein competitive adsorption. When serum proteins at ARDS concentrations are present, they quickly adsorb and saturate the air-water interface, preventing LS adsorption. Using classic theories from colloid science, protein adsorption can be modeled as an energy barrier to LS; the theory provides quantitative predictions of how best to reverse the inhibition of surfactant. Adding non-ionic hydrophilic polymers, electrolytes and polyelectrolytes can provide attractive interactions between the lung surfactant and the interface, thereby lowering the barrier to LS adsorption; each additive overcomes the repulsive barrier to LS adsorption by different colloidal scale forces. For example, non-ionic hydrophilic polymers generate a depletion attraction between the LS aggregates and the interface by the elimination of the "excluded volumes" of the LS aggregates and the interface. LS adsorption increases exponentially with polymer concentration and is molecular weight dependent as predicted by a simple depletion attraction model. Electrolyte additives lower the electrostatic barrier to LS adsorption; comparison between enhanced LS adsorption of mono and divalent electrolytes agrees with the sixth power of ion valence scaling predicted by the classic Schultz-Hardy rule. Polyelectrolyte additives act as a bridge to lower the electrostatic energy barrier imposed on the LS by the interfacial albumin; the concentration range which most effectively enhances LS adsorption corresponds to the charge neutralization condition. |
