Experimental evidence of the time scales of lipid transport to air/water interfaces is reviewed. Several one-dimensional models of diffusion and adsorption of surfactant at the air/water interface are examined. To obtain a model which yields a qualitatively realistic description of adsorption rates, one should use a finite diffusion length and a time-dependent boundary condition (or an adsorption isotherm) which can account for adsorption barriers and the finite adsorption capacity of the surface. The models can be used for either transport of individual molecules or particulates (complexes, liposomes, vesicles, etc.). In the latter case, an effective lipid concentration which is capable for adsorption should be used. Sample calculations and experimental observations for DPPC (dipalmitoylphosphatidylcholine), which is a key component of lung surfactant, suggest that particulates play a key role on the adsorption rate, either by being transported themselves or by affecting the concentration of dissolved or adsorbable DPPC. The models can be used for correlating results of laboratory experiments to in vivo behavior of lipids.
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