(620e) A Novel Method for Pore Size and Porosity Analysis of Meso-and Macroporous Solids Using Electroacoustics and High Frequency Conductivity Measurements

We present a new method of determining porosity, pore size of meso-and macroporous materials by transmitting ultrasound pulses of a single or multiple frequencies through the porous material. Propagation of ultrasound through porous materials saturated/wetted with a liquid generates an electric response. This electroacoustic/electrokinetic effect is called seismoelectric current [1- 2]. The liquid and the porous body matrix are compressible at high frequencies of ultrasound at MHz range. Their compressibilities are different, which leads to the relative phase motion. This in turn causes displacement of the diffuse layer and the surface charge within the double layers the (wetting) liquid forms on the pores surfaces. This charge displacement shows up as oscillating electric current. It is possible to measure the seismoelectric current with existing electroacoustic devices initially designed for characterizing liquid dispersions, but we demonstrate that it is possible to utilize such a technique also for a porosity/pore size characterization of porous materials [2, 3]. Based on older theoretical predictions [1,2] it is expected that the magnitude of the seismoelectric signal is independent of the pore size if the liquid inside the pores only forms thin and isolated double layers on the pore walls. However, in case one uses a liquid with sufficiently low ionic strength these double layer are very thick so that they overlap and fill-out the complete pore space. For such a scenario we find that the seismoelectric current depends on the pore size [2,3]. In addition we utilize the correlation between seismoeclectric current magnitude and pore diameter due to hydrodynamic relaxation effects for the pore size analysis of wider macropores (pore width up to 8 μm. [1] We have performed experiments for various systems covering a wide range of pores sizes covering a wide pore size range from ca. nanometers to micrometers (e.g. porous glasses, porous titania, materials, geological cores etc., organic hydrogels)Further, we will demonstrate in our paper that information about the porosity of this materials can be obtained in a simple way by measuring the experimentally determined  ratio of the electrical conductivity of the porous material to the equilibrium conductivity of the media (supernate); using this data the porosity can be calculated by applying the well-known  Maxwell-Wagner theory [3].Summarizing, in our contribution we will discuss a novel, fast experimental technique (experiments can often completed in less than 10 minutes) for pore size/porosity characterization.

 References

 [1] A.S. Dukhin, P.J.Goetz, M. Thommes, Patent Pending, A1 20110283800 ,M. Thommes, Method for determining porosity, pore size and zeta potential, US Patent, Patents pending:

[2] A.S. Dukhin, P.J.Goetz, M. Thommes, Journal of Colloid and Interface Science 345, (2010) 547”Method for determining porosity, pore size and zeta potential of porous bodies”

[3] A.S. Dukhin, Phil Goetz. A1 20110012627 “Method for determining porosity with high frequency conductivity measurement”