Molecular Attachment Research Articles

The pattern of adsorption of cationic antiseptics to polymethylmethacrylate.

Few studies have reported the maximum potential of surfaces to adsorb the cationic antiseptics and few comparisons between antiseptics have been carried out. The purpose of this investigation was to measure the maximum uptake of alexidine, cetyl pyridinium chloride, chlorhexidine gluconate and chlorhexidine acetate to polymethylmethacrylate and by the construction of adsorption isotherms determine the pattern of molecular attachment to the recipient surface. The mean maximum adsorption findings for these antiseptics demonstrated significantly greater uptake of cetyl pyridinium chloride than the other antiseptics and significantly less uptake of alexidine compared with the other antiseptics. All four antiseptics showed a Langmuir type adsorption isotherm consistent with the formation of a monolayer of molecules on the recipient surface. For chlorhexidine preliminary studies demonstrated that the pattern of adsorption was not altered by saliva pre-treatment of surfaces. No evidence of molecular multilayering was observed even at higher concentrations of the antiseptics. However, the maximum uptake on to the surface was significantly increased when no post-treatment washings were carried out. It was concluded that this increased uptake would be unlikely to play an important role in antiplaque activity because of its relative instability. The pH dependency for the adsorption of these antiseptics was demonstrated such that as previously observed for chlorhexidine gluconate at low pH adsorption is almost completely, or completely, prevented.

Crystal Growth Mechanisms and Kinetics

AbstractThe scientific contributions by David Turnbull and his co-workers toward understanding crystal growth span over 35 years from the late 1940's to the present. Turnbull's early attempt (1950) to correlate interfacial energies derived from droplet supercooling measurements with other thermochemical data still provides a significant data base for estimating such energies in a variety of materials. His work with Hillig in 1956 on quantification of the screw dislocation mechanism for interfacial molecular attachment remains as a predictive theory for defect-assisted growth of faceted, and therefore kinetically hindered, interfaces. The Hillig-Turnbull screw dislocation mechanism is now ranked among such notable kinetic models for crystal growth as the Wilson-Frenkel random attachment theory and the nucleationlimited layer spreading models of Burton, Cabrera, and Frank. Turnbull's contributions and interest in elucidating crystal growth kinetics have continued throughout his long and productive career, as evidenced by his recent work with Coriell (1982) on estimates of collision limited rapid crystal growth in highly supercooled transition metals. Progress in unraveling the kinetic contributions of interfacial attachment from those of heat and solute transport will also be reviewed to provide a current context of Professor Turnbuli's contributions to the field of rapid crystallization.

Luminescence of the excited mercury diethylamine complex. Attachment by ligand exchange

The possibility of molecular attachment to excited atoms in gases by ligand exchange has been examined. It was first confirmed that mixtures of mercury vapor and diethylamine Et2NH give negligible luminescence when subject to irradiation with the 2537 Å mercury resonance line. The emission was then examined from mercury vapor with added amine up to 100 Torr, in the presence of an excess of NH3. With NH3 alone, a strong emission was observed due to HgNH*3, a complex of Hg(3P0) and NH3, with λmax∼3500 Å. The addition of Et2NH gave rise to a second band to the long wavelength, extending throughout the visible region, with λmax∼5140 Å; the ratio of the intensity of the long wavelength member to the residual emission in the 3500 Å region was found to be proportional to the amine pressure. For various [Et2NH] and fixed [NH3], the set of normalized spectral profiles are isosbestic, corresponding to the superposition of just two basic functions. From the cw experiments, it was adduced that the addition of quite small pressures of Et2NH,∼1 Torr, to NH3 at 1 atm pressure effectively extinguishes the HgNH*3 luminescence by a fast ligand exchange reaction HgNH*3+Et2NH→Hg(Et2NH)*+NH3. The two basic functions which comprise the spectra are due to Hg(Et2NH)* and the dimer Hg(Et2NH)*2, which is the carrier of the long wavelength component. This scheme was confirmed using the resonance flash technique to time resolve the exchange of ligands; the rate coefficient is ∼2×10−10 cm3 molecule−1 s−1 for the above reaction. The radiative lifetime of the Hg(Et2NH)* monomer is 1.15 μs. The large red shift and broadening associated with attachment of the second ligand arises because the emission is from an upper state which belongs to the Hg(3P1) asymptote, and which is more strongly bound than the state which correlates with Hg(3P0).

A schematic mechanism for striated growth of solid binary mixtures

It is shown, on the basis of a schematic model derived from that set up for the growth of plagioclase rocks by Haase et al., that the interplay between diffusional transport and two slow molecular attachment kinetics in non-linear competition is sufficient to account for the possibility of striated growth (i.e., growth of a solid binary mixture with planar periodic composition oscillations in the growth direction) La croissance a striations planes (oscillations periodiques de composition dans la direction de croissance) peut resulter de l'effet combine du transport diffusif et de deux cinetiques d'attachement moleculaire lentes en competition non lineaire

Isolation of a subunit of laminin and its role in molecular structure and tumor cell attachment.

Laminin, the glycoprotein of basement membranes, migrates as two components of 200 kilodaltons (kDa) (alpha subunit) and 400 kDa (beta subunit) after reduction on polyacrylamide gel electrophoresis. We have isolated the alpha subunit and studied its structure by electron microscopy and its function as an attachment factor for tumor cells. Using selective proteolysis of laminin by alpha-thrombin, the beta subunit was removed without any change in the quantity or size of the alpha subunit. Removal of the beta subunit caused a 35-40% decrease in the total mass of the laminin molecule. The alpha and beta subunits differed by 50-fold in the amount of reducing agent required for complete migration on polyacrylamide gels. By electron microscopy, the whole laminin molecule appeared as a "cross" with three identical short arms (37 nm) and one long arm (75 nm). The alpha subunit examined by electron microscopy was missing the long arm and had no change in the length of the three short arms. This subunit of laminin mediated the attachment of human squamous carcinoma cells to type IV collagen. Such attachment properties were lost after pepsin treatment which is known to remove the globular end regions of the short arms. We conclude that the beta subunit of laminin is embodied in the long arm of the molecule and that the alpha subunit consists of three similar chains of 200 kDa. The globular end regions of the laminin short arms are required for the attachment of tumor cells to type IV collagen.

Energetics analysis of polyethylene crystallization: single crystal growth surfaces

Semiempirical energy calculations have been used to investigate the structure and energetics of molecular attachment and mobility on the {110} and {200} growth faces of polyethylene single crystals. The results indicate that, while the crystallographic position of the first nucleating segment on the {110} face is a low energy position, other dispositions of the molecule have somewhat lower energy. Growth layers nucleated by molecules in the lowest energy position are substantially different from the crystallographic structure. After inclusion of a few molecules however, this growth layer must undergo a rearrangement requiring the cooperative motions of several molecules in order to assume the now lower energy crystalline structure. Comparisons of the energies and mobility of molecular segments on the {110} and {200} growth faces indicate that there is a greater attraction of the segments to the {200} face, but that chain mobility is substantially greater on the {110} face. Correlation of these factors with the observed dependence of degree of truncation of polyethylene single crystals on crystallization temperature and the concentration of polymer in solution suggests that the nucleation energetics take precedence over factors of chain mobility, but the total truncation behaviour observed is probably the result of an interplay between these two factors. Energetics calculations also provide an explanation for the observed lower melting temperature of the {200} sectors.

Excess Velocity Potential of the Platelet Crystal in a Supercooled Melt

Starting with the traditional time-invariant parabolic-cylinder model to represent the growth of a platelet-shaped crystal in a pure supercooled melt, the effect of thermal imbalance at the solid-liquid interface is calculated. In particular, the heat field equations are solved for the initial excess velocity distribution at the interface, on the assumption that the shape constraint is suddenly replaced by the constraint of heat conservation. The computed results resemble those found for the paraboloidal needle crystal. In particular, the hypothetical parabolic interface tends to bulge with a pronounced peak at one radius behind the leading edge of the platelet crystal. The maximum velocity principle, when applied to this model, yields a square-law relation between the dimensionless tip velocity and the supercooling for pure ice and water. The results parallel those calculated by Horvay and Cahn for the isothermal model. The curvature effect of surface tension and the effect of the molecular attachment kinetics are found to lower the dimensionless tip velocity by about 20% below that in the isothermal model, uniformly over the range of supercooling considered.

Further studies of the dynamic scattering mode in nematic liquid crystals

Evaluation of fabrication procedures and operating life have indicated that moisture can play a major role in determining the performance of nematic liquid crystal displays based on the dynamic scattering mode (DSM) in anisylidene-p-aminophenyl-acetate. There is strong evidence that the presence of moisture promotes electron injection by lowering the potential barrier between the metal electrode and the liquid crystal. This could arise from a lowering of the effective work function of the metal by adsorption and/or increasing the electron affinity of the liquid crystal by molecular attachment. In any case the injected electron is captured by a neutral molecule and transported as a negative ion. The electron is removed by an oxidation process at the anode leaving the original molecular species free to repeat the process. It was also found that a short pulse (shorter than a space charge build-up time) could be used to electrically quench dynamic scattering and appreciably reduce the relaxation time. The use of this new scheme significantly improves the brightness and contrast of matrix addressed, real time displays based on dynamic scattering. A novel laminate structure for realizing such displays is also presented.

Measurement of atomic kinetics of solidification using peltier heating and cooling—I. theory

Periodic Pettier heating and cooling may be generated at a solid/liquid interface by the passage of an alternating electric current. Melting and freezing will then occur at rates which will be influenced by the kinetic law of molecular attachment relating solidification velocity to interface temperature. We describe here the theory of an experiment in which the equation describing the interface attachment kinetics is to be determined by measuring the response of a solid/liquid interface to square-wave Peltier heating and cooling. The mathematical methods required for extracting the kinetic equation by analysis of the waveform of interface motion are given and it is shown that with this technique, linear, parabolic and exponential kinetics should give distinguishably different waveforms. The advantages of the method are discussed, along with the precautions necessary to eliminate possible confusing effects.

On the dendritic growth of pure materials

The following three theoretical treatments of unconstrained dentritic growth are discussed: (i) The Ivantsov analysis, which assumes the existence of an isothermal liquid-solid interface and concludes that this dendrite body shape must be a paraboloid of revolution; (ii) and (iii) the non-isothermal treatments of both Bolling and Tiller, and Temkin. These latter two works take account of the variation in both curvature and interface attachment kinetics over the entire surface of the dendrite. They also enable one to calculate the velocity and curvature of the dendrite tip by assuming that the tip region can effectively be described as a paraboloid of revolution. It is proposed that Temkin's analysis is sensitive enough to determine the excess solid-liquid surface free energy, λ, and the linear kinetic coefficient for molecular attachment, μ o. Using the best available data for the ice-water system, the Temkin treatment yields 20 ± 2 erg/cm 2 and 16 ± 0.5 cm/sec °C for λ and μ o, respectively. These values are in remarkably good agreement with other experiments and theory.

Limitations of the Thermal-Wave Technique for Determining Molecular-Attachment Kinetics

Consideration of the particular mechanisms of molecular attachment is shown to impose serious limitations on the thermal-wave technique for quantitatively determining molecular-attachment kinetics at least as presently applied. For a layer-edge-attachment mechanism, the variation of layer-edge density as a function of position relative to layer sources on the surface leads to a nonuniform departure from equilibrium over the surface and possibly to capillarity effects. Thus, the plane-wave analysis of Sekerka and Tiller cannot be considered to be exact. The technique is further shown to be incapable of distinguishing between layer-edge motion and uniform interface advance. The discrepancies between available experimental results and theory are considered.

On the Flow and Exchange Diffusion of Electrons and Negative Ions Formed by Molecular Attachment

Formation of negative ions by attachment of electrons to molecules was first recognized by Thomson (1916). Huxley (1959) recently discussed the steady-state structure of a stream of electrons and ions drifting and diffusing in a gas, taking account of ionization by molecular attachment.

The Structure of a Stream of Electrons and Ions Drifting and Diffusing in a Gas When Ionization by Collision and Molecular Attachment are Present

The theory is developed of the structure of a stream of electrons and ions drifting under the action of a uniform field and diffusing in a gas when either or both ionization by collision and electron attachment are present. The cases considered include a point source and a line source, and in the latter case the influence of a magnetic field is discussed.

Molecular diffraction attachment for RCA microscope.

A new device for the distribution of vapors across an electron beam for molecular diffraction studies is described. The efficiency of the new method has made it possible to obtain molecular diffraction patterns with an electron microscope without need of alterations. The vapors are ejected from a circular crest of a diffusion box immediately surrounding the beam to give a heavy concentration of vapors in contact with the beam while releasing a small amount of vapor. The method has been standardized with well known patterns of CCl4 and CBr4.