AFM study of paraffin wax surfaces

Paraffin wax surfaces cast against NaCl crystals, silicon wafers and a silicone elastomer used for microcontact printing, Sylgard®, were the subject of AFM morphology and surface roughness investigations. A distinctive stepped texture has been found on the paraffin wax surface, suggestive of layered crystals. Step heights measured across paraffin flat surface were in the range between 3 and 30 nm, with the most frequent occurrence being around 4–8 nm. A paraffin wax cross-section measured by AFM shows lamellae of individual sheets 7.6 ± 0.15 nm. The surface roughness of the paraffin samples shows a clear correlation with the roughness of the substrate, but the paraffin roughness is an order of magnitude greater than that of the mould roughness. Evidently the paraffin does not produce a faithful replica of the mould in the way that Sylgard does. The slight flexibility of Sylgard after curing allows it to be peeled off a rigid mould, and again to be peeled away from paraffin after the wax has set. It is demonstrated, however, that separation does not occur exactly at the interface between the two materials; rather it occurs through cohesive failure of the paraffin, close to the interface, and images reveal a laminar contour-like stepped morphology texture characteristic of paraffin crystals on both the separated surfaces.