Three-Phase Intersection Points in Monolayers

Some phospholipid/dihydrocholesterol Langmuir monolayers form coexisting liquid phases. Gas domains form at the interface between the phospholipid-rich and dihydrocholesterol-rich liquid phases when these monolayers undergo expansion to low surface pressure. Analysis of the domain shapes thus formed yields the relative line tensions of the gas/phospholipid, gas/dihydrocholesterol, and phospholipid/dihydrocholesterol phase interfaces

Comment on "Local accumulation times for source, diffusion, and degradation models in two and three dimensions" [J. Chem. Phys. 138, 104121 (2013)]

In a recent paper, Gordon, Muratov, and Shvartsman studied a partial differential equation (PDE) model describing radially symmetric diffusion and degradation in two and three dimensions. They paid particular attention to the local accumulation time (LAT), also known in the literature as the mean action time, which is a spatially dependent timescale that can be used to provide an estimate of the time required for the transient solution to effectively reach steady state. They presented exact results for three-dimensional applications and gave approximate results for the two-dimensional analogue. Here we make two generalizations of Gordon, Muratov, and Shvartsman’s work: (i) we present an exact expression for the LAT in any dimension and (ii) we present an exact expression for the variance of the distribution. The variance provides useful information regarding the spread about the mean that is not captured by the LAT. We conclude by describing further extensions of the model that were not considered by Gordon,Muratov, and Shvartsman. We have found that exact expressions for the LAT can also be derived for these important extensions..

Topography and instability of monolayers near domain boundaries

We theoretically study the topography of a biphasic surfactant monolayer in the vicinity of domain boundaries. The differing elastic properties of the two phases generally lead to a nonflat topography of ``mesas'', where domains of one phase are elevated with respect to the other phase. The mesas are steep but low, having heights of up to 10 nm. As the monolayer is laterally compressed, the mesas develop overhangs and eventually become unstable at a surface tension of about K(dc)^2 (dc being the difference in spontaneous curvature and K a bending modulus). In addition, the boundary is found to undergo a topography-induced rippling instability upon compression, if its line tension is smaller than about K(dc). The effect of diffuse boundaries on these features and the topographic behavior near a critical point are also examined. We discuss the relevance of our findings to several experimental observations related to surfactant monolayers: (i) small topographic features recently found near domain boundaries; (ii) folding behavior observed in mixed phospholipid monolayers and model lung surfactants; (iii) roughening of domain boundaries seen under lateral compression; (iv) the absence of biphasic structures in tensionless surfactant films.Comment: 17 pages, 9 figures, using RevTeX and epsf, submitted to Phys Rev

Liquid-Liquid Immiscibility in Lipid Monolayers

Some binary lipid mixtures form coexisting liquid phases when spread at the air/water interface. This work describes the pressure–composition phase diagrams of binary mixtures of four unsaturated phosphatidylcholines with dihydrocholesterol. These four binary mixtures have critical compositions of approximately fifty mole percent, and average critical exponents of 0.25±0.07. The data can also be approximated by a regular solution thermodynamic model, yielding parameters for the non-ideality of these mixtures

Critical Pressures in Multicomponent Lipid Monolayers

Epifluorescence microscopy has been used previously to study coexisting liquid phases in lipid monolayers of dihydrocholesterol and dimyristoylphosphatidylcholine at the air/water interface. This binary mixture has a critical point at room temperature (22°C), a monolayer pressure of approx. 10 mN/m, and a composition in the vicinity of 20-30 mol% dihydrocholesterol. It is reported here that this critical pressure can be lowered, raised, or maintained constant by systematically replacing molecules of this phosphatidylcholine with molecules of a phosphatidylethanolamine, or an unsaturated phosphatidylcholine, or mixtures of the two, while maintaining the dihydrocholesterol concentration at 20 mol%. Thus, even complex mixtures of lipids may be characterized by a single, well-defined second-order phase transition. In principle, such transitions might be found in biological membranes

A PSEUDOVECTOR NUCLEAR HYPERFINE INTERACTION

In many studies of nuclear hyperfine interactions in polyatomic molecules and solids it has been assumed that the electron spin S and nuclear spin I are coupled through a symmetric dyadic