NEUTRON REFLECTION FROM COUNTERIONS AT THE SURFACE OF A SOLUBLE SURFACTANT SOLUTION

The distribution of counterions at a charged surface has been measured directly using neutron specular reflection with isotopic labeling bei ng used to highlight the scattering from the counterions. The system i nvestigated was the soluble surfactant tetramethylammonium dodecyl sul fate with and without added tetramethylammonium chloride. Depending on the conditions a fraction of ions were found to penetrate the surfact ant head-group region. The majority of the counterions observed formed a layer adjacent to the head-group layer which must be part of the di ffuse layer. These two layers did not, however, account for all the co unterions, and the remainder of the diffuse layer could not be observe d with certainty. The reason for this is not understood, although it i s probably associated with the roughness of the diffuse layer. Using t he measured charge density and surface coverage as parameters, the cou nterion distribution calculated from the Stern-Gouy-Chapman model of t he electrical double layer appears to account quite well for the shape of the distribution, although the effect of the ''missing'' ions made a detailed quantitative comparison impossible.