SURFACE PROPERTIES OF SOME ANTIMICROBIAL CLAYS

Several natural and modified clay minerals are known to have antimicrobial activity. In order to inactivate or kill microbes, the mineral particles must come close to or touch the microbes. Such interactions are describable in terms of forces of attraction or repulsion and can be measured directly or indirectly. This study has applied two complementary but unrelated techniques in an attempt to discover what attributes confer antimicrobial activity on some clay materials but not on others. The surface thermodynamic properties of clays can be determined from contact angle measurements of small drops of liquids whose surface tension component and parameter values are known using the Young equation for polar materials. This study examined two green French clays, Argiletz and Argricur, which have been studied by others for their antimicrobial activity. These are mixtures of illite and smectite with a high iron content. Contact angles were measured on thin films of these materials deposited on clean glass slides. The γ^LW for both clays is about 42 mJ/m² which is in line with measurements of other layer silicate materials. The Lewis base parameters, γ^-, were very large (on the order of 50 mJ/m²) with smaller values for the Lewis acid parameter, γ⁺, (between 0 and 1.4 mJ/m²) indicating a very high energy material. Such a high energy surface is near the limit that can be measured reliably by the conventional contact angle approach. At this level the two French clays are not distinguishable. However, data collected using atomic force microscopy indicate the surface properties of the two clays are significantly different. Forces of adhesion were measured between a silicon nitride cantilever and each clay mineral. Adhesion forces between the cantilever and Agricur (25 nN) were nearly an order of magnitude larger than those measured between the cantilever and Argiletz (4nN).