Spreading concentration effect on the morphology of phospholipid monolayers
Introduction
Phospholipids are frequently investigated for their important role in many systems. For example in living systems where they form a lipid matrix [1], or in food emulsions and foams, in which they participate in the formation and stabilisation of interfacial boundaries [2]. For this reason much attention has been paid to the study of phospholipids as soluble and insoluble monolayer at various interfaces, air/water [3]or oil/water interfaces 4, 5, 6, 7.
The adsorption isotherm of l-α-dipalmitoylphosphatidylcholine (DPPC) has been determined from adsorption kinetics experiments of the lipid at the chloroform/water interface [8]. It is of the Langmuir type, and a critical aggregation concentration (CAC) of 0.03 mM/l has been observed. Above this critical point, DPPC adsorption activity does not change with further increasing bulk concentration. However, in monolayer experiments one often uses a solution of 1 mM of lipid in chloroform or in mixed solvents as spreading concentration. According to the experimental adsorption results this means that the lipids were spread mainly as aggregates rather than monomeric molecules. Provided that the aggregation of molecules remains at the surface after spreading, from the thermodynamic point of view there must be some initial aggregation, which can hardly be observed by general optical microscopy, as nuclei to produce a large number of small size domains. If this is true, there must be an influence of monomers and aggregated molecules spread onto the fluid surface on their thermodynamics and rearrangement.
To clarify this problem, morphological changes in the coexistence region of monolayers with different spreading concentration have been investigated by Brewster angle microscopy (BAM) in the present work.
Section snippets
Materials
Dipalmitoylphosphatidylcholine (DPPC, +99%) obtained from Sigma was used without further purification. GC chloroform from Aldrich was used as spreading solvent. All experiments were performed with Millipore filtered water at 23±0.5°C.
Method
Adsorption of DPPC at the chloroform/water interface was measured by a pendent drop technique. The experimental set-up and conditions were exactly the same as described previously [8]. The determination of the DPPC adsorption isotherm was given by its equilibrium
Results and discussion
Fig. 1 shows the adsorption isotherm of DPPC at the chloroform/water interface. The solid line is the fitting result by a Langmuir adsorption isotherm given by Γ=Γ∞co/(aL+co), where Γ∞ is the saturation adsorption, and aL is the Langmuir adsorption constant. The fitting to the experimental data gives values for the two characteristic parameters Γ∞=2.71×10−10 mol/cm2; aL=2.79×10−10 mol/cm3. The critical aggregation concentration appears at CAC=0.03 mM. After this critical point, the interfacial
Acknowledgements
The work was financially supported by the research contract between the Max-Planck-Gesellschaft, Germany and the Chinese Academy of Sciences. J.B. Li would like to thank the support by the President Fund of the Chinese Academy of Sciences. We also thank G. Weidemann for his help in partial BAM experiments.
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