Phase Transitions in Dipalmitoylphosphatidylcholine Monolayers

Abstract

A self-assembled phospholipid monolayer at an air–water interface is a well-defined model system for studying surface thermodynamics, membrane biophysics, thin-film materials, and colloidal soft matter. Here we report a study of two-dimensional phase transitions in the dipalmitoylphosphatidylcholine (DPPC) monolayer at the air–water interface using a newly developed methodology called constrained drop surfactometry (CDS). CDS is superior to the classical Langmuir balance in its capacity for rigorous temperature control and leak-proof environments, thus making it an ideal alternative to the Langmuir balance for studying lipid polymorphism. In addition, we have developed a novel Langmuir–Blodgett (LB) transfer technique that allows the direct transfer of lipid monolayers from the droplet surface under well-controlled conditions. This LB transfer technique permits the direct visualization of phase coexistence in the DPPC monolayer. With these technological advances, we found that the two-dimensional phase behavior of the DPPC monolayer is analogous to the three-dimensional phase transition of a pure substance. This study has implications in the fundamental understanding of surface thermodynamics as well as applications such as self-assembled monolayers and pulmonary surfactant biophysics.

Gas–liquid phase equilibrium of a model Langmuir monolayer captured by a multiscale approach

“…Different regions in the P-A isotherms are attributed to various phases and their coexistence regions of the corresponding Langmuir monolayers: a horizontal line in the P-A isotherm is interpreted as the first-order G-LE phase coexistence regime, 9,11 or a G-LC phase transition for temperatures below the G-LE-LC triple point, 12,13 whereas a non-horizontal line is found to exist for coexistence of condensed phases. 7 The formation of 2D surface micelles, 2,3,7,14,15 which has been observed experimentally by using e.g. fluorescence microscopy, is found to be the main reason for the non-horizontal line in P-A isotherms.…”

Surface phase transitions in foams and emulsions

“…It is worth mentioning that several recent studies [38][39][40] warned about possible problems with some of these experimental methods. Thus, it was shown that Langmuir trough-based techniques may give unreliable results for the reversibility of the surface phase transitions, when reaching low surface tensions, due to the leakage of material [38]. An approach to overcome this problem was proposed in [39] where reversible phase transition in phospholipid layers was studied by a new method, called "constrained drop surfactometry", which is claimed to provide a leakage-proof environment.…”

Surface patterning of single-walled carbon nanotubes enhances their perturbation on a pulmonary surfactant monolayer: frustrated translocation and bilayer vesiculation

“…To model different respiration conditions, we xed the surface tension to 30 mN m À1 and 10 mN m À1 , respectively. 40,41 MD simulations suggested that the pristine SWCNTs can insert into, translocate across or be wrapped by the PSM, depending on the tube size and PSM tension. Specially, ultrashort SWCNTs with lengths comparable to the monolayer thickness (L ¼ 1.25 nm) were found to vertically insert into the PSM, regardless of the PSM tension ( Fig.…”