NEUTRON REFLECTION FROM COUNTERIONS AT THE SURFACE FORMED BY A CHARGED INSOLUBLE MONOLAYER

Isotopic substitution has been used in conjunction with neutron reflec tion to make a selective study of the counterion layer beneath a charg ed insoluble monolayer spread on water. The insoluble monolayer consis ted of a mixture of sodium docosyl (C-22) sulfonate and docosanol, and its charge was adjusted by varying the relative amounts of anionic an d neutral species in the layer. The counterion was the tetramethylammo nium ion, TMA(+). The counterion distribution was determined at three surface charge densities, corresponding to areas per negative charge o f about 30, 60, and 85 Angstrom(2), and at three bulk electrolyte (TMA Cl) concentrations, 0.1, 0.01, and 0.001 M. There was found to be sign ificant penetration of the counterions into the headgroup region of th e amphiphile to form a well defined ''Stern'' layer, and there was als o a thicker layer corresponding to part of the diffuse layer. The chan ges in the behavior of the counterions with changes in surface charge density and bulk electrolyte concentration were qualitatively consiste nt with double-layer theory. However, a complication in applying theor y to the diffuse part of the double layer was that not all the counter ions could be observed by the neutrons. This may have been because of preferential adsorption of Na+ present in the original spreading solut ion or because of roughness of the counterion distribution. The oberva tion by neutron reflection of penetration of the amphiphilic layer by the counterions was consistent with the unusual behavior of the pi-A i sotherms on addition of TMACl.