Determination of pore size in porous materials by evaporative mass loss
A method for determination of pore-size distribution in a porous material called evapo porometry (EP) is capable of determining pore sizes from approximately the nanometer scale up to the micron scale. EP determines the pore size based on the evaporative mass loss at constant temperature from porous materials that have been pre-saturated with either a wetting or non-wetting volatile liquid. The saturated porous material is placed in an appropriate test cell on a conventional microbalance to measure liquid mass loss at a constant temperature as a function of time. The mass-loss rate is then related to the pore-size distribution. The microbalance permits measuring the mass as a function of time. The slope of the mass versus time curve is the evaporation rate. The evaporation rate is related to the vapor pressure at the interface between the liquid in the porous material and the ambient gas phase. The vapor pressure in turn is related to the pore diameter.
1. An apparatus for determining pore size in a porous membrane material comprising
a microbalance;
a test cell for receiving the porous membrane material placed upon the microbalance, wherein
the test cell is formed as a diffusion chamber with a top open to atmosphere and a base;
the test cell is configured to hold the porous membrane material as a sole covering to an opening in the base and
the test cell has a seal structure positioned around a perimeter of the opening in the base and outside a perimeter edge of the porous membrane material and configured to prevent lateral liquid and vapor loss between the porous membrane material and the base of the test cell;
a temperature sensor configured to measure a temperature within the test cell; and
a computing device connected to receive data output from both the microbalance of mass measurements over a period of time and the temperature sensor over the period of time and configured to calculate a pore size of pores in the porous membrane material based upon the data output.
2. The apparatus of claim 1 , wherein the seal structure is configured to contact the porous membrane material.
3. The apparatus of claim 1 wherein
the test cell further comprises a base plate and an upper portion, the upper portion comprising an upper opening defining the top and a lower opening, the lower opening positioned proximal to the base plate; and
the seal structure is positioned between the base plate and the upper portion and is configured to seal against the porous membrane material.
4. The apparatus of claim 3 , wherein the upper portion of the test cell is formed as a circular cylinder with an open upper end positioned distal to the base plate.
5. The apparatus of claim 3 , wherein the base plate comprises a recess for accepting the porous membrane material.
6. The apparatus of claim 3 , wherein a cross-sectional area of the lower opening is greater than or equal to a cross-sectional area of the upper opening.
7. The apparatus of claim 3 , wherein the upper portion of the test cell defines a length between the upper and lower openings that is equal to or greater than 10 times the diameter of the lower opening.
8. The apparatus of claim 3 , wherein the base plate is configured to receive the porous membrane material directly upon an upper surface thereof such that an entire surface area of the porous membrane material is fully supported by the base plate.
9. The apparatus of claim 1 further comprising an environmental chamber in which the test cell is positioned in order to maintain an isothermal temperature within the test cell.
10. The apparatus of claim 9 further comprising a temperature sensor positioned within the environmental chamber to monitor temperature and provide feedback to the environmental chamber for maintaining the isothermal temperature.
11. The apparatus of claim 1 further comprising a material, device, or mechanism positioned within the environmental chamber to mitigate static charge.
12. The apparatus of claim 11 , wherein the static mitigating material, device, or mechanism is a polonium source.
13. The apparatus of claim 1 further comprising an anti-vibration platform upon which the microbalance is supported.
14. A method for determining pore size and pore-size distribution in a porous membrane material comprising:
placing a porous membrane material sample at a base of a test cell that forms a diffusion chamber open to atmosphere above the porous membrane material sample;
introducing a volume of a volatile liquid within the test cell on an upper surface of the porous membrane material sample;
placing the test cell on a microbalance;
maintaining the test cell and microbalance at a constant temperature;
measuring a mass of the test cell over a period of time to determine an evaporation rate;
relating the evaporation rate to a vapor pressure at an interface between the volatile liquid in the porous membrane material sample and an ambient gas phase within the test cell; and
relating the vapor pressure to a pore diameter.
15. The method of claim 14 further comprising creating a vapor-tight seal between the base of the test cell, the upper portion of the test cell, and the porous membrane material sample.
16. The method of claim 14 further comprising saturating the porous membrane material sample with the volatile liquid before placing the porous membrane material sample at the bottom of the test cell.
17. The method of claim 14 , wherein the volatile liquid is a wetting liquid.
18. The method of claim 14 , wherein the volatile liquid is a non-wetting liquid.
19. The method of claim 14 , wherein the volatile liquid is selected from isopropanol or n-propanol.
20. The method of claim 14 further comprising housing the test cell in an environmental chamber.
21. The method of claim 20 further comprising positioning a material, device, or mechanism within the environmental chamber to mitigate static charge.
22. The method of claim 14 further comprising supporting the test cell and the microbalance on an anti-vibration table.
23. The method of claim 14 further comprising determining a mass loss of the volatile liquid for each of a transient period, a surface-liquid evaporation period, and a pore-liquid evaporation period to determine the evaporation rate.
24. The method of claim 14 , wherein the porous membrane material is one or more hollow fibers.
25. A method for determining pore-size distribution in a porous membrane material comprising
determining a pore diameter by
placing a porous membrane material sample at a base of a test cell to form a diffusion chamber open to atmosphere;
introducing a volume of a volatile liquid within the test cell on an upper surface of the porous membrane material sample;
placing the test cell on a microbalance;
maintaining the test cell at constant temperature;
measuring a mass of the test cell over a period of time to determine an evaporation rate;
relating the evaporation rate to a vapor pressure at an interface between the volatile liquid in the porous membrane material sample and an ambient gas phase within the test cell; and
relating the vapor pressure to one or more pore diameters;
determining a cumulative mass loss of the volatile liquid for each of the one or more pore diameters; and
calculating a number of pores of a particular pore size based upon a corresponding cumulative mass loss and by approximating a form of pores as circular.
26. The method of claim 25 further comprising determining the mass loss of the volatile liquid for each of a transient period, a surface-liquid evaporation period, and a pore-liquid evaporation period.
27. A method of determining pore size and pore-size distribution of a porous membrane material, the method comprising
placing a porous membrane material in a test cell that forms a diffusion chamber above the porous membrane material open to atmosphere;
saturating the porous membrane material with a volatile liquid;
directly covering the porous membrane material with a volume of the volatile liquid sufficient to cover the porous membrane material;
maintaining the test cell at constant temperature;
measuring the mass of the test cell as a function of time
recording time-dependent mass measurements and temperature values of the test cell and in a memory device; and
calculating the pore size and pore-size distribution of pores in the porous membrane material based upon the time-dependent mass measurements and temperature values recorded in the memory device using a computing device connected to the memory device.
28. A method for use in characterizing pore size and pore-size distribution of a porous membrane material, the method comprising:
placing a porous membrane material in a test cell that forms a diffusion chamber above the porous membrane material open to atmosphere, wherein the porous membrane material is saturated and overlaid with a volume of volatile liquid sufficient to cover the porous membrane material;
collecting mass readings from a microbalance at a series of first time intervals;
recording the mass and time;
averaging mass readings data over a second series of time intervals;
forward-differencing between consecutive average mass values to determine an evaporation rate; and
converting the evaporation rate to pore size.