PROBING SOFT POLYMERIC COATINGS OF A CAPILLARY BY ATOMIC-FORCE MICROSCOPY

Atomic force microscopy (AFM) has been used to probe the surface of a capillary after coating with ''soft'' polymers, notably polyacrylamide s. The aim was the investigation of the efficiency of coverage of the silica surface, so as to reduce or eliminate the electroosmotic flow ( EOF), particularly noxious in the separation of macromolecules. The qu ality of such coating is strongly dependent on two variables: temperat ure and pH. In the first case, progressively higher temperatures produ ce open silica patches, where no polymer seems to be bound. The transi tion from coated to largely uncoated surfaces occurs at 50 degrees C. Also the pH of the polymerizing solution strongly affects the coating efficiency. Since in all coating procedures the monomer solution is no t buffered, addition of accelerator (TEMED, N,N,N'N'-tetramethylethyle ndiamine) induces polymer growth at pH 10-11. These pH values generate hydrolysis of the siloxane bridge anchoring the bifunctional agent (B ind Silane, onto which the polymer chain should grow) to the wall. Thu s, coating and de-coating occur simultaneously. Low temperatures durin g polymer growth (typically 10 degrees C) and buffered solutions (pH 7 , titrated after TEMED addition) ensure a most efficient and thorough coating, with virtual elimination of EOF: well coated capillaries exhi bit residual EOF values, at pH 10, of the order of 10(-7) cm(2) V-1 s( -1) vs. a standard value for uncoated capillaries of the order of 10(- 4) cm(2) V-1 s(-1). The AFM data have been fully confirmed by direct m easurement of EOF in coated and uncoated capillaries under an electric field.