Grazing incidence X-ray diffraction studies on a heneicosanoic (C-21) acid monolayer with cadmium ions in the subphase, at near-zero surface pressure and similar to 9 degrees C, show that there exists a pH "window" within which the monolayer takes on a chiral structure and an ordered superlattice is also observed. Below this pH window, the monolayer has a tilted structure like that of a pure fatty acid monolayer at low pressures, indicating negligible headgroup-ion interaction. Above the window, the monolayer goes to a structure very similar to the high-pressure untilted S phase seen in pure fatty acid monolayers. The chiral structure is never seen when cadmium is replaced by zinc, another divalent metal with only slightly lower electronegativity, or (as reported earlier) with calcium. Raising temperature within the cadmium pH window destroys the superlattice and takes the monolayer to the backbone-disordered Rotator-I structure.