Use of water uptake and capillary suction time measures for evaluation of the anti-diarrhoeic properties of fibrous clays

This study explores the use of water sorption/retention measures as alternative methodologies to the official methylene blue adsorption test for the evaluation of the anti-diarrhoeic properties of some fibrous clay. Three fibrous phyllosilicates—palygorskite from Ciudad Real (PCR), and sepiolite from Vicalvaro (SV) and Yunclillos (SY)—have been studied. Two commercial anti-diarrhoeal products—Pharmasorb Colloidal© (PHC) and Pharmasorb Regular© (PHR)—each mainly consisting of palygorskite, were also included in the study. The 75–125-μm sieve fraction was selected and methylene blue adsorption (MBA) capacity and water uptake characteristics—total amount of water retained (Qmax) and water uptake velocities (Iw)—measured. Finally, the water retention capacity, with dispersions of 10% w/v, was assessed by means of capillary suction measures (CST). MBA capacity was similar for all the samples, with PHC showing the highest capacity to adsorb the dye. PHC was also the sample which retained the most water (Qmax>2.5 mg/mg), followed by PCR and PHR (Qmax≈1.5 mg/mg) and finally SV and SY (Qmax<1 mg/mg). Regarding the velocity for water entry, water uptake was almost total after 10 s for sepiolite samples, 100 s for PHR and 600 s for PCR and PHC. Finally, water retention time of the PHC dispersion (>160 s) was almost twice those of the rest of studied materials. A linear correlation was found between the amount of palygorskite in PCR, PHC and PHR and their water uptake capacities. This was not possible for the sepiolite samples. An inverse correlation was found between velocity of water suction (Iw) and capacity of methylene blue adsorption. Water uptake and water retention measures are proposed as complementary to the methylene blue adsorption assay in the evaluation as anti-diarrhoeics of the studied materials.