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

Stoichiometry of cholesterol–sphingomyelin condensed complexes in monolayers

AbstractSome binary mixtures of cholesterol and phospholipids in monolayers have thermodynamic phase diagrams with two upper miscibility critical points. This feature has been interpreted in terms of ‘condensed complexes’ between the phospholipid and cholesterol. The present work gives evidence for the formation of complexes with a common simple integral stoichiometry in binary mixtures of cholesterol and a series of five sphingomyelins where the amide-linked acyl chain length is varied. This indicates that these complexes have a distinct geometry even though they form a liquid phase

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

I. Investigation of possible interactions between thallium (I) and thallium (III)in solution and in the crystalline thallium sesqui-halides. II. Spectrophotometric investigations of the copper (II) chloro-complexes in aqueous solutions of unit ionic strength. III. Optical interaction between the chloro-complexes of copper (I) and copper (II) in solutions of unit ionic strength. Interpretations of the spectral absorption of a copper (I) - copper (II) dichloro-complex. IV. Spectrophotometric investigation of the interaction between iron (II) and iron (III) in hydrochloric acid solutions

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. A radiochemical and spectrophotometric investigation of the interactions between thallium (I) and thallium (III) is used to show that the majority of the thallium (I) and thallium (III) atoms in the crystalline thallium sesquihalides do not occupy equivalent positions in the crystalline lattice and that there is no strong optical interaction between thallium (I) and thallium (III) in aqueous solutions containing perchlorate or chloride ion. The absorption spectra of mixed solutions of copper (II) perchlorate, perchloric acid and hydrochloric acid at a constant ionic strength of 1.00 in the wavelength range 250-300 [...] are interpreted in terms of the equilibria, [...]. The equilibrium constants for these reactions are determined and the enthalpy of formation of [...] is estimated. The non-additive light absorption in the 400-600 mp wavelength range of solutions maintained at unit ionic strength with perchloric acid and containing copper (I) and copper (II) and low concentrations of chloride ion, has been interpreted in terms of an "interaction complex", [...]. At higher chloride ion concentrations in solutions of the same ionic strength, interaction complexes of higher chloride coordination (but still containing only one copper (I) and one copper (II) in each complex) are important. Two interpretations of the 400-600 [...] spectral absorption of [...] are advanced and discussed. At present no conclusions can be drawn as to which interpretation is to be preferred. The light absorption in the 450-900 [...] wavelength range by mixed solutions of iron (II) and iron (III) in hydrochloric acid is interpreted as evidence for the formation of unstable but strongly absorbing interaction complexes, each interaction complex containing one atom of iron (II) and one atom of iron (III) and a number of coordinating chloride ligands. The light absorption by interaction complexes decreases with decreasing hydrochloric acid concentration and there is no interaction absorption by solutions containing [...] and no chloride ion. Absorption spectra of iron (II) in solutions of varying hydrochloric acid concentration observed in the 700-900 [...] wavelength range are used to show the presence of iron (II) chloro-complexes