Adsorbing Wall Research Articles

Hydrodynamics of adsorbed polymer layers: A new approach.

We consider fluid flow through a layer of adsorbed homopolymer, which we decompose into an equivalent set of polymeric loops, each tethered to the adsorbing wall and obeying Zimm dynamics. Using a multiple scattering analysis, we obtain an integral equation for the velocity profile. New results are presented for flow profiles in steady and oscillatory weak shear past an adsorbed layer at the theta temperature. Near the wall we find a universal behavior under steady shear for the fluid velocity u(z)\ensuremath{\sim}${\mathit{z}}^{\mathit{y}}$ at height z, with an exponent y\ensuremath{\simeq}2.0.

A grand canonical Monte Carlo study of Lennard-Jones mixtures in slit shaped pores

The grand-canonical Monte Carlo (GCMC) technique has been used for simulating the adsorption of mixtures in slit pores with graphite properties. Spherical Lennard-Jones models were used to model methane and ethane at super critical temperatures. A GCMC algorithm for mixtures which included attempts to change identities of particles was found to be more effective than conventional GCMC. Results were compared with density functional theory (DFT) calculations of Tan and Gubbins (1992; J. phys. Chem., 96, 845) and with ideal adsorbed solution theory (IAST) isotherms derived from single component data. Our simulation results were found to agree qualitatively rather than quantitatively with the DFT mixture results, the IAST was found to work well for the system studied. Adsorption selectivity was found to depend on packing considerations as well as the relative potential well depths of the adsorbate wall interactions.

Dilute and semidilute polymer solutions near an adsorbing wall

The variation of the monomer density ρ with distance z from an impenetrable and adsorbing wall is studied for polymer solutions with excluded volume interaction. Both the limits of weak and strong overlap between the polymer chains are considered. For comparison, solutions of ideal noninteracting chains and chains with an excluded volume interaction of the mean-field type are also discussed. The main emphasis is on a temperature region near the adsorption temperature Ta below which a single long chain, fixed with one end close to the wall, condenses a finite fraction of its monomers to the wall. The relation to an n-component field theory in a half-space is used to show that in this temperature region ρ(z) displays crossover scaling behavior involving two macroscopic lengths. The power law exponents for the z dependence in the various regions determined by the two lengths are obtained as well as the qualitative form of the whole profile ρ(z) for various limits of interest.

Scaling theory of polymer adsorption : proximal exponent

We interpret some recent calculations of Eisenriegler, Kremer, and Binder [1], and related multi-critical results of Diehl and Dietrich [2]. They show that the monomer concentration profile Φ(x) for isolated polymer chains in a good solvent and in close proximity to an adsorbing wall exhibits a singular behaviour ; i.e., Φ(x) ∼ (a/x) m, where a is a monomer dimension and m ≅ 1/3. Taking into account the proximal exponent m, we correct earlier scaling results [3, 4] for the interfacial tension of semi-dilute polymer solutions in good solvents. We also find good agreement with a recent result by Ishinabe [5] on the single chain energy.

Continuous measurement and speciation of sulfur-containing aerosols by flame photometry

A sulfur-specific flame photometer has been used for the real time measurement of sulfur-containing aerosols. Specificity for the aerosols was achieved with a diffusion tube stripper which removed sulfur-containing gases from the air stream by diffusion to an adsorbing wall but transmitted particles to the flame photometer. The sensitivity of the flame photometer to (NH 4) 2SO 4 and NH 4HSO 4 aerosols was identical, but a reduced response was found for H 2SO 4 aerosol. This occurred because of the high temperature (145°C) of the flame photometer burner block which caused the evaporation and loss of H 2SO 4 to the wall before reaching the flame. The addition of NH 3 to the sample air just upstream of the burner converted H 2SO 4 to (NH 4) 2SO 4 or NH 4HSO 4 and produced the expected increase in the flame photometer response. Heating the aerosol upstream of the diffusion stripper converted the sulfates to sulfur-containing gases over a temperature range characteristic of the aerosol being sampled. These gases were removed in the stripper, thereby decreasing the flame photometer output. The normalized response to aqueous H 2SO 4 aerosol decreased from unity at 50°C to 0.04 at 110°C and for (NH 4) 2SO 4 and NH 4HSO 4 aerosols from unity at 115°C to zero at 190°C. When the aerosol contained both H 2SO 4 and (NH 4) 2SO 4, the resultant thermogram was a function of both the (NH 4) 2SO 4/H 2SO 4 ratio and the manner in which the two components were mixed in the particles comprising the aerosol (i.e. homogeneously with constant (NH 4) 2SO 4/H 2SO 4 ratios or heterogeneously with varying ratios).