The time dependent adsorption of a series of phospholipids at the cyclohexane-water interface has been studied by measuring interfacial tensions between cyclohexane solutions of the phospholipids and pure water, using the modified Wilhelmy plate technique. An electronic microforce balance was used for the measurements of interfacial tensions as a function of time. The four phospholipids investigated were Egg phosphatidylcholine, dipalmitoyl phosphatidylcholine, dilauroyl phosphatidylcholine, and dimyristoyl phosphatidylcholine.
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The interfacial tension data were examined in accordance with diffusion theory and in terms of first order kinetics for a reversible process in order to elucidate the mechanisms responsible for the time dependent adsorptions. It was found that the adsorption process as a whole can be described in two stages. The initial stages of adsorption, up to between one and two hours, are dependent upon the diffusion of solute from the bulk solution to the interfacial region (sub-surface). From then on, the adsorption is described by a first order equation for a reversible process, the rate controlling step being the transfer of solute molecules from the sub-surface to the interface.
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From the first order kinetics equation, desorption coefficients have been evaluated for the phospholipids. It is concluded from the results that increasing the size of the non-polar group of the phospholipids leads to a larger desorption coefficient, indicating a faster rate of desorption from the interface into the cyclohexane phase.