Abstract
Purpose: :
The purpose of this study was to evaluate the effects of amphiphilic lipids on the ability of nonpolar lipids to form thin lipid films (LF) on the surface of aqueous subphases using a Langmuir trough (LT) and a Brewster angle microscope (BAM). The tested lipids, their ratios, and the conditions of the experiments were chosen to model the interactions of the different lipid classes found in human meibum and in the tear film lipid layer.
Methods: :
Individual nonpolar lipids (behenyl stearate and behenyl oleate; BO and BS, respectively), and their mixtures with more polar compounds (such as palmitic acid, 12-hydroxystearic and 16-hydroxypalmitic acid; PA, 12HSA, and 16HPA), were dissolved in CHCl3 in varying ratios, spread onto the surface of Tris-buffered saline (TBS) in the LT to form there a LF on the TBS subphase. Multiple pressure-area (π-A) isotherms were recorded in cycles, and pictures of the LF were taken using a BAM. The data were analyzed to determine the rheological properties and topography of the LF using approaches described in our earlier publications.
Results: :
BS formed a very rigid layer on the TBS subphase. The lipid was poorly dispersible by itself during the expansion/compression cycles. However, in the presence of amphiphilic lipids, BS dispersed much more readily. Among the three tested amphiphilic lipids, the strongest effect was observed for 12HSA - a lipid with two hydrophilic groups and a hydrophobic tail. Its presence resulted in the formation of net-like structures that resembled films formed by human meibum. PA (a lipid with just one hydrophilic group and one hydrophobic tail) did not help in the dispersion of BS on TBS, but facilitated spreading of BO, to form discrete structures on TBS subphase. The 16HPA lipid with two hydrophilic tails, was unable to disperse BS. A pronounced hysteresis was observed in the PA:BS=1:9 (mol/mol) film. The hysteresis was found to be smaller for the 12HSA:BS=1:9 film, while an even smaller hysteresis was observed for the 16HPA:BS=1:9 film.
Conclusions: :
Our data demonstrate the role of amphiphilic lipid geometry in dispersing nonpolar lipids at the air/water interface. Tighter binding/mixing of 12HSA and PA (two lipids with well-defined hydrophobic tails) with the wax esters may contribute to the increased hysteresis, and the more efficient spreading of the hydrophobic lipids. In contrast, the looser binding (or poorer mixing) with 16HPA may contribute to the lower hysteresis, higher rigidity of the lipid film, and poor dispersibilty of the mixture.
Keywords: lipids • cornea: basic science • cornea: tears/tear film/dry eye