Surface and bulk interchange energy in binary mixtures of chain molecules.

The interchange (interaction) parameter, controlling the phase behaviour of a binary mixture, is determined for the bulk and the surface of binary mixtures of different types of chain molecules, using surface tensiometry and a mean-field theory. For all mixtures and concentrations studied an identical behaviour is observed at the surface, depending only on the square of the reduced chain length mismatch delta n/n, where delta n and dealta n are the difference in and average of the number of carbons of the two components.

Surface freezing in binary mixtures of chain molecules. I. Alkane mixtures.

X-ray surface scattering and surface tension measurements are used to study surface freezing in molten mixtures of alkanes. These binary mixtures consist of protonated and deuterated alkanes, as well as of alkanes of different lengths. As for pure alkanes, a crystalline monolayer is formed at the surface a few degrees above the bulk freezing temperature. The structure of the monolayer has been determined on an angstrom scale. A simple theoretical approach is used to account for the thermodynamical observations at the surface and in the bulk. The model is based on a competition between entropic mixing and a repulsive interaction due to chain-length mismatch. The surface and bulk liquid phases are treated as ideal mixtures, while the solid phases are treated as regular mixtures. The theory is found to account well for all the mixtures studied, both hydrogenated-hydrogenated and hydrogenated-deuterated. The repulsive interaction and its dependence on the chain lengths of the components are determined from fits to the measured data.

Surface freezing in binary mixtures of chain molecules. II. Dry and hydrated alcohol mixtures.

Surface freezing is studied in dry and hydrated alcohol mixtures by surface x-ray scattering and surface tension measurements. A crystalline bilayer is formed at the surface a few degrees above the bulk freezing temperature. The packing is hexagonal, with molecules aligned along the surface normal in all cases. The in-plane lattice constant reveals a qualitatively different behavior with composition for hydrated and dry mixtures. The simple theoretical approach used successfully for alkane and deuterated alkane mixtures accounts well also for the alcohol mixtures. The repulsive length-mismatch term opposing the mixing entropy term in the free energy of the mixtures is shown to have a universal behavior for all mixtures studied: protonated alkanes, deuterated alkanes, and dry and wet alcohols. This universality is somewhat counterintuitive in view of the different interactions (e.g., hydrogen bonding in alcohols) in the different mixtures.

Demixing transition in a quasi-two-dimensional surface-frozen layer.

A thin/thick transition was observed by x-ray reflectivity in a surface-frozen crystalline bilayer on the surface of a molten binary mixture of long alcohols. This rare example of a solid-solid phase transition in a quasi-2D system is shown to result from an abrupt temperature-driven change in the layer's composition, kinetically enabled by the layer's ability to exchange molecules with the underlying 3D liquid bulk. Mean-field thermodynamics yields a Gibbs-adsorption-like expression which accounts very well for the transition.

Compositionally modulated phase in crystalline n-alkane mixtures.

We report an x-ray scattering study of the compositionally modulated microphase separated state (muPSS) of binary n-alkane (C23H48:C28H58) mixtures. By employing a quenching technique, we obtained many orders of sharp lamellar and superlattice reflections. The muPSS is a regular superstructure consisting of pure C23 layers, and layers rich in C28 but containing up to approximately 14% C23 plus voids. A monolayer of the minority component regularly alternates with one or more layers of the majority. The mechanism for thermodynamic stability of the muPSS may be related to the entropy associated with longitudinal displacements of the molecules.

Surface freezing in n-alkane solutions: the relation to bulk phases.

Surface freezing (SF) was investigated in tricosane-dodecane alkane solutions as a function of temperature (T) and molar concentration of tricosane (phi), using surface tension and synchrotron x-ray surface diffraction techniques. A crystalline SF monolayer, having a rotator R(II) structure, was found to exist for 35 degrees C

Positional order and thermal expansion of surface crystalline N-alkane monolayers.

We report a high-resolution synchrotron grazing incidence x-ray diffraction measurement of a surface crystalline monolayer at the liquid-vapor interface of the n-alkane eicosane (C20H42) just above its melting temperature. The peak width of the surface monolayer rotator phase is shown to be resolution limited and implies positional correlations of at least approximately 1 microm. The high resolution allowed determination of the temperature dependence of the peak position over the narrow (3 degrees C) temperature range of the surface crystal phase. The two-dimensional thermal expansion was determined to be (dA/dT)/A=1.8(+/-0.1)x10(-3) degrees C-1, which is comparable to the expansion in similar chain length bulk n-alkane rotator phases. Our data are consistent with the power-law shaped scattering tails expected from quasi-long-range order in two dimensions.

Surface tension measurements of surface freezing in liquid normal alkanes.

Surface tension measurements reveal surface freezing in liquid n-alkanes. A solid monolayer of molecules is found to exist up to 30 degrees C above the bulk freezing point. This surface phase exists only for carbon numbers 14 n

Structure of membrane surfactant and liquid crystalline smectic lamellar phases under flow.

Synchrotron x-ray scattering studies were performed to probe the nonequilibrium structures of two layered systems at high shear rates: the smectic-A phase of the thermotropic liquid crystal 4-cyano-4'-octylbiphenyl (8CB) and the lamellar L(alpha) phases of surfactant membranes composed of sodium dodecyl sulfate and pentanol. Whereas the lamellar surfactant phases oriented primarily with their layers parallel to the shearing plates, as expected intuitively, in the corresponding high shear regime, the smectic-A liquid crystalline material oriented with the layers perpendicular to the shearing plates. A careful numerical study revealed that this surprising layer orientation results from nonlinear dynamics of the liquid crystal director and is caused by the flow distortion of thermal fluctuations.

X-ray Scattering Studies of Aligned, Stacked Surfactant Membranes.

X-ray scattering studies were performed to understand the structure and correlations in the lamellar phases of thick, freely suspeded films of (i) the hydrated phospholipid dimyristoylphosphotidylcholine (DMPC) and (ii) the ternar system consisting of the surfactant sodium dodecyl sulfate (SDS), cosurfactant (pentanol), and water. The films were drawn in a temperature- and humidity-controlled environment, where the layers were oriented to within 0.1 degrees . In the DMPC system, this made it possible to directly observe the orientation of the P(beta') modulation and to identify phase L(beta') as three distinct phases distinguished by the direction of chain tilt with respect to the lattice. In the L(alpha) phase of the ternary system, power law behavior of the (0,0,L) structure factor arising from the algebraic decay of layer correlatios was observed in single crystals.