Porosimetry

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Porosimetry is an analytical technique used to determine various quantifiable aspects of a material's porous nature, such as pore diameter, total pore volume, surface area, and bulk and absolute densities.

The technique involves the intrusion of a non-wetting liquid (often mercury) at high pressure into a material through the use of a porosimeter. The pore size can be determined based on the external pressure needed to force the liquid into a pore against the opposing force of the liquid's surface tension.

A force balance equation known as Washburn's equation for the above material having cylindrical pores is given as:[1]

P_L - P_G = \frac{4 \sigma \cos \theta}{D_P}
P_{L} = pressure of liquid
P_{G} = pressure of gas
\sigma = surface tension of liquid
\theta = contact angle of intrusion liquid
D_{P} = pore diameter

Since the technique is usually done under vacuum, the gas pressure begins at zero. The contact angle of mercury with most solids is between 135° and 142°, so an average of 140° can be taken without much error. The surface tension of mercury at 20 °C under vacuum is 480 mN/m. With the various substitutions, the equation becomes:

D_P = \frac{1470 \ \text{kPa} \cdot \mu \text{m}}{P_L}

As pressure increases, so does the cumulative pore volume. From the cumulative pore volume, one can find the pressure and pore diameter where 50% of the total volume has been added to give the median pore diameter.

References[edit]

  1. ^ A.B. Abell, K.L. Willis and D.A. Lange, "Mercury Intrusion Porosimetry and Image Analysis of Cement-Based Materials", Journal of Colloid and Interface Science, 211, pp. 39-44 (1999). [1]