Kinetics for the Collapse of Trilayer Liquid-Crystalline Disks from a Monolayer at an Air−Water Interface

Abstract

Unlike surfactants considered in previous studies, when phosphatidylcholine (PC) monolayers collapse at constant surface tension to form a 3D bulk phase, surface area decreases at rates that slow. The different kinetics could result from collapse by a distinct mechanism. Rather than the transfer of molecules all along the interface between the monolayer and bulk phase, PC films can collapse by the folding and subsequent sliding of a bilayer over the monolayer. By this mechanism, molecules can transfer to collapsed trilayers through a locus of constant size. In this article, we use the theory of nucleation and growth to show analytically that during collapse, the area can decrease at rates that decelerate when each individual structure grows through a region of fixed dimensions. We also show that binary films of 30% dihydrocholesterol (dchol) and dipalmitoyl phosphatidylcholine (DPPC), which have previously been shown to form a homogeneous monolayer from which trilayer disks grow through a point, collapse with rates of area decay that slow, in agreement with our analytical expressions.

Distribution of Coexisting Solid and Fluid Phases Alters the Kinetics of Collapse from Phospholipid Monolayers

“…27 Theoretical analysis shows that the kinetics of such a process should have an n of 1 if nucleation is instantaneous, confined to the very beginning of collapse, or if nucleation occurs at a constant rate but growth stops when the collapsed structures reach a certain size. 21 For mixtures of 30% dihydrocholesterol with DPPC, liquidcrystalline collapse, documented microscopically, 27 proceeds under isobaric conditions along a simple exponential. 21 Because our analysis makes no requirement that we know the mechanism of collapse, we have not proven here that our LE phase exhibits liquidcrystalline collapse.…”

“…21 For mixtures of 30% dihydrocholesterol with DPPC, liquidcrystalline collapse, documented microscopically, 27 proceeds under isobaric conditions along a simple exponential. 21 Because our analysis makes no requirement that we know the mechanism of collapse, we have not proven here that our LE phase exhibits liquidcrystalline collapse. That process, however, seems likely.…”

The biophysical function of pulmonary surfactant

“…Although viscoelastic relaxation below γ e could occur either by collapse or by effects within the monolayer, experimental results indicate that collapse is much more important. Following extensive, fast compressions from high γ to values just above γ e , where collapse does not occur, area changes <2% (Smith et al, 2003; Rugonyi et al, 2005), indicating that viscoelastic relaxation within the interface is minimal. The much larger changes in area following compression to γ just below γ e must reflect collapse.…”

Collapsed bipolar glycolipids at the air/water interface: Effect of the stereochemistry on the stretched/bent conformations

“…Indeed, the collapse mechanism can significantly disrupt the organization of lipid films. [46,47] The process of collapse and the transition of a monolayer to a three-dimensional phase are not yet well understood and depend on the studied molecules. [48] Formation of multilayers beyond the air/liquid interface was reported in previous studies with phospholipids such as DMPC (dimerystoylphosphatidylcholine) and DOPS (dioleoylphosphatidylserine) [49] or mixture of DPPC/cholesterol [46].…”