Hindrance of solute diffusion within membranes as measured with microporous membranes of known pore geometry

1. 1. Mica sheets were made into membranes by a process of bombardment with fission fragments from a U235 source and subsequent etching with hydrofluoric acid. Pores formed by this process were essentially straight through the membrane, extremely uniform in size and elliptical in cross-section. On eight of these membranes, with pore radii ranging from 45 to 300 A, air flow, water flow, and diffusion rates for a graded series of 7 solutes were measured. From measurements of the diffusion rate of mostly non-electrolytes, with radii between 2.5 and 22.5 A, the true hindrance effect on diffusion within pores was determined.2. 2. Restriction of diffusion for even relatively small solutes is a very significant effect and can be adequately described by the Renkin equation for membranes which have well-defined, straight through pores, where is the ratio of solute diffusivity in the membrane to that in free solution, and Rs/Rp is the ratio of solute radius to pore radius. An approximation to the Renkin equation, , which is much simpler to use, correlates as well with the data in the range 0 Rs/Rp 3. 3. Water flow under small pressure drops is well described by the assumption of Poiseuille flow for this range of pore diameters, and therefore no effects due to "anomalous" water were apparent.4. 4. In most membrane operations there is a considerable resistance to diffusion due to the presence of a liquid film boundary layer along the surface of the membrane. This boundary layer resistance was not inversely proportional to the solute diffusivity as has often been assumed in the "unstirred layer" theory, but instead was found under these experimental conditions to be proportional to the -0.6 power of the solute diffusivity.5. 5. Boundary layer diffusion resistances were obtained by two independent methods: an electrochemical polarographic method and a membrane substitution method using membranes of known permeability to calibrate the diffusion cell.6. 6. Heteroporous membranes do not differ from isoporous membranes very much in regard to the relative hindrance of solute molecules, as long as the ratio of solute radius to mean pore radius is less than 0.2. For larger solute molecules, heteroporous membranes become increasingly less effective in hindering diffusion rates through the membrane.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34175/1/0000464.pd

Experimental And Theoretical Study Of The Electron Spectra In 66.7350-keV/u C++He Collisions

Measurements of the differential and total cross sections for production of free electrons are reported for collisions of 66.7350-keV/u C+ ions with He. The experimental data are compared with theoretical calculations that include electron emission from both target and projectile using the classical-trajectory Monte Carlo method and the independent-electron approximation. Good agreement is observed between theory and experiment, and structures appearing in the electron spectra can be explained in terms of either, or both, target and projectile ionization. © 1990 The American Physical Society

Electron Emission From Both Target And Projectile In C⁺ + He Collisions

The first classical trajectory Monte Carlo calculation of the electronic spectra arising from both target and projectile ionization is presented and compared with experimental measurements of the differential cross section for electrons emitted in C++He collisions. The theoretical treatment is based on an independent-electron model in which the interactions between the electrons are approximated by quantum model potentials. Good agreement is obtained between theory and experiment and structures appearing in the measurements can be explained in terms of the electron emission from either, or both, target and projectile. Further, the relative importance of the electron capture to the continuum and the electron loss to the continuum peaks is analyzed for different impact energies and the conclusions obtained from this analysis are found to be in agreement with experimental works

The fundamental problem of command : plan and compliance in a partially centralised economy

When a principal gives an order to an agent and advances resources for its implementation, the temptations for the agent to shirk or steal from the principal rather than comply constitute the fundamental problem of command. Historically, partially centralised command economies enforced compliance in various ways, assisted by nesting the fundamental problem of exchange within that of command. The Soviet economy provides some relevant data. The Soviet command system combined several enforcement mechanisms in an equilibrium that shifted as agents learned and each mechanism's comparative costs and benefits changed. When the conditions for an equilibrium disappeared, the system collapsed.Comparative Economic Studies (2005) 47, 296–314. doi:10.1057/palgrave.ces.810011

Chemical characteristics of air from differing source regions during the Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A)

Ten‐day backward trajectories are used to determine the origins of air parcels arriving at airborne DC‐8 chemical measurement sites during NASA\u27s Pacific Exploratory Mission‐Tropics A (PEM‐T) that was conducted during August‐October 1996. Those sites at which the air had a common geographical origin and transport history are grouped together, and statistical measures of chemical characteristics are computed. Temporal changes in potential temperature are used to determine whether trajectories experience a significant convective influence during the 10‐day period. Those trajectories that do not experience a significant convective influence are divided into four geographical categories depending on their origins and paths. Air parcels originating over Africa and South America are characterized by enhanced mixing ratios of O3, CO, HNO3, and PAN. The backward trajectories travel at high altitudes (∼10–11 km), covering long distances due to strong upper‐tropospheric westerly winds. The observed enhancement of combustion‐related species is attributed to biomass burning from distant sources to the west, extending even to South America. The relatively large value of Be‐7 probably is due either to less efficient removal of aerosols from upper tropospheric air or to small stratospheric contributions. Aged marine parcels are found to have relatively small concentrations of burning‐related species. Although these trajectories arrive at a wide range of aircraft altitudes, they do not pass over a land mass during the preceding 10‐day period. Air passing over Australia but no other land mass exhibits a combustion signature; however, photochemical product species such as O3 and PAN are less enhanced than in the long‐range transport category. These trajectories travel shorter distances and are at lower altitudes (∼5–8 km) than those reaching Africa and/or South America. The combustion influence on these parcels is attributed to biomass burning emissions injected over Australia. That burning is less widespread than in Africa and South America. Finally, trajectories originating over Southeast Asia appear to receive a weak combustion influence. However, compared to Africa and South America, Southeast Asia has a relatively small incidence of biomass burning. There is little combustion input from Australia due to the high transport altitudes compared to the lower heights of the convection. The Southeast Asian parcels exhibit the greatest NOx to ∑NOi ratio of any category, perhaps due to lightning. Parcels experiencing a significant convective influence also are examined. Most of these parcels pass through widespread, persistent convection along either the South Pacific Convergence Zone or Intertropical Convergence Zone approximately 5 days prior to arriving at the aircraft locations. Thus the category mostly represents marine convection. Mixing ratios of peroxides and acids in the convective category are found to be smaller than in parcels not experiencing convection. Small mixing ratios of Be‐7 and Pb‐210 suggest particle removal by precipitation