Achieving Rapid Absorption and Extensive Liquid Uptake Capacity in Porous Structures by Decoupling Capillarity and Permeability: Nanoporous Modified Calcium Carbonate

Porous media with rapid absorption properties are greatly sought after in the fields of super absorbers and catalysts. Natural materials, such as diatomite, or synthetic zeolite feature strongly in industrial reaction processes. Most, or all, of such materials, however, are surface acidic. A novel rapidly absorbing alkaline porous structure, with a high absorption capacity, is presented here. As in the case of diatomite or zeolite, the pigment design incorporates strong capillarity within a highly permeable packed medium. A model is proposed for general use with highly absorbing media that can be proven microscopically to have separate domains of micro- or nano-capillarity embedded within a permeable matrix. The new pigment morphology, based on natural ground calcium carbonate (gcc), exhibits this property using special surface structure modifications. It is contrasted with standard gcc by using consolidated tablet blocks made from a suspension of the pigment and chosen mixtures thereof. The blocks are characterised after drying by mercury porosimetry, and the absorption dynamic of a selected liquid is studied. It is shown that using a self-assembly method of discrete pore structures provides a much faster absorption rate and a liquid capacity for up to 10 times more fluid than a conventional homogeneously distributed pore concept. In such unique discrete network systems, the mercury intrusion curve provides a separable analysis of permeability and capillarity in respect to the inflection point of the cumulative intrusion curve. The discrete decoupled properties each follow the absorption behaviour predicted by previous modelling (Ridgway and Gane, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 206(1–3), 2002). The absorption driving force is shown to be determined by the proportion of fine pores present up to a size equal to a Bosanquet inertially-defined optimum within the timescale of absorption. Combining the wetting force, from the capillarity-controlled fine pore structure, with the experimental flow resistance of the sample, consisting of the assembly of particles, it is possible to predict the trends in absorption dynamic using the pore and throat model Pore-Cor.* Use of this model allows existing materials as well as new synthetic designs to be modelled prior to manufacture. The novel alkaline material is compared with independent absorption data for diatomite and shown to be comparable. *Pore-Cor is a software product name of the Environmental and Fluid Modelling Group, University of Plymouth, Devon PL4 8AA, U.K.