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We have used molecular dynamics to study a two-dimensional system of point particles with screened Coulomb interactions, in a ‘‘box’’ whose size is allowed to vary. In addition to determining the familiar energy–temperature relations at both constant area and constant pressure, we have obtained pressure–area isotherms, diffusion constants, and pair-correlation functions. The results of our simulation are expected to be relevant to such systems as polar monolayers on the surface of water (Langmuir films) and thin colloidal suspensions. By monitoring the energy–temperature relation and the diffusion constant, we have observed first order solid–liquid transitions with hysteresis under both constant area and constant pressure conditions. However, the discontinuities in the pressure–area isotherms at the transition are almost imperceptible. Other thermodynamic relations such as area vs temperature at constant pressure, and the derivative of pressure vs area, are presented to support the analysis. [ABSTRACT FROM AUTHOR]

We study the condition of the occurrence of the rim instability in the solvent-induced dewetting process. Our experimental results show that the film thickness not only greatly influences the occurrence of the rim instability, but also influences the wavelength λ as characterized by the undulation of the deformed contact line. The molecular weight of polymer does not almost influence the occurrence of the rim instability and the wavelength λ. The wavelength λ is proportional to the width of the rim in the rim instability region. The receding contact angle θ of polymer solutions on substrates in the dewetting process is an important factor to influence the rim instability in the solvent-induced dewetting. [ABSTRACT FROM AUTHOR]

Reports on the use of specular x-ray reflectivity to probe the structures of self-assembled monolayers and multilayers deposited using a three-step siloxane self-assembly technique that is repeated to form periodic multilayers. Increase in the film thickness as a function of the number of trilayers; Bragg peaks corresponding to the inter-trilayer spacing.

X-ray diffraction data from water-supported monolayers of fatty acids with chain lengths from 19 to 22 is presented. The structures of the tilted mesophases L2’, L2, and Ov are characterized in detail. The contributions to the unit cell distortion from the tilt and the ordering of the backbone planes of the molecules are separated. It is shown that at the swiveling transition L2’–L2, not only the tilt azimuth but also the packing of the backbone planes change discontinuously. We demonstrate that the tilting transition LS–L2 is accompanied by the ordering of the backbone planes and may be discontinuous. Evidence is presented for a herringbone ordering transition within the L2 region. The distortions are related to symmetry of the phases and described by the order parameters responsible for tilt and herringbone ordering of the backbone planes of the molecules. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have studied Langmuir monolayers composed of mixtures of heneicosanoic acid and heneicosanol using π-A isotherms and x-ray diffraction. We find that the two materials are miscible in monolayer form. We have determined how the phase diagram of the acid evolves to that of the alcohol as the composition varies. X-ray diffraction data along isotherms and isobars show that the head–head interactions, like the tail–tail interactions, are anisotropic. The ‘‘swiveling transition’’ between phases with different tilt directions is accompanied by a rotation of the molecules about their long axes. [ABSTRACT FROM AUTHOR]

We have studied Langmuir monolayers of CH3(CH2)20OH (heneicosanol) using x-ray diffraction. Over the temperature range 7–55 °C, the monolayer isotherms show only one discontinuity, which may be either a ‘‘flat section’’ or a ‘‘kink;’’ we find that these are transitions between tilted and untilted phases. There are several untilted phases, with hexagonal and distorted-hexagonal structures, as in monolayers of the corresponding (heneicosanoic) acid. However, in contrast to the acid, when the pressure is lowered the molecules tilt only towards a next-nearest neighbor. The dependence of the lattice spacings on pressure and temperature indicate that the monolayer does not form a ‘‘tilted hexagonal’’ phase, even at temperatures where the untilted phase is hexagonal (rotator-II). [ABSTRACT FROM AUTHOR]

We have studied the effect of pressure and pH on Langmuir monolayers of heneicosanoic acid in the presence of calcium ions in the subphase. At very low pH the monolayer is unaffected by calcium ions; as the pH is increased, the head groups are pulled closer together and the chain tilt at zero pressure decreases. As a result, the phase transitions seen along the isotherms occur at lower pressures. When the isotherms become featureless, the chains are untilted at all pressures. At very high pH, a new untilted structure is observed. [ABSTRACT FROM AUTHOR]

Studies the kinetics of a structural phase transition in Langmuir monolayers of amphiphilic molecules. Microscopic relaxation mechanism in monolayers; Relationship between temperature and monolayer structure; rate of nucleation of a hexagonal phase from a metastable shear-induced structure.

We have studied the structure of a monolayer of C21H43OH on water, in the region near close packing, by grazing incidence in-plane x-ray diffraction. For all temperatures studied the isotherms in the πa plane show a kink, signaling a phase transition. Along an isotherm, and for pressures above the kink, we observe that the transverse structure factor has one peak which has constant position, width, and intensity; below the kink the diffraction peak shifts to smaller scattering vector (larger separation) and the amplitude decays as the surface pressure decreases, but the width of the peak remains constant. We rationalize these observations in terms of the influence on the transverse structure factor of gauche configurations in the amphiphile tails, with the kink representing the point at which the last of the gauche configurations is squeezed out of the chain. Along an isobar which is at higher pressure than the kink pressures of all isotherms crossed, the transverse structure factor has a single peak above a transition temperature and two peaks below that temperature; for π=30 dyn/cm the transition temperature is in the range 16.3

Specular x-ray reflectivity has been used to probe the microstructures of siloxane-based self-assembled electro-optic superlattices composed of high-hyperpolarizable organic chromophore arrays intercalated with Ga and In oxide sheets. The film thickness increases linearly as a function of the number of layers, underscoring the high structural regularity and efficiency of the synthetic approach. Relatively dense metal oxide structures are detected in these systems. The x-ray reflectivity data also indicate that the dependence of the relative surface roughness on the number of layers is nearly identical for self-assembled organic and organic–inorganic hybrid film structures. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]