X-ray reflectivity from insoluble monolayers spread on aqueous subphases

The technique of X-ray reflection is used to study the structure of an insoluble monolayer spread on an aqueous subphase. The feasibility of the technique as a non-invasive structural probe for liquid surfaces is demonstrated. A comparison of the X-ray reflectivity from docosanoic acid spread on 2.5 × 10⁻⁴molar solutions of caesium chloride, cadmium chloride and lanthanum chloride shows that practically all of the cadmium and lanthanium ions are within 15 Å of the acid head groups whereas there is no evidence for such binding with caesium. The pH dependence of the amount of bound cadmium is used to show that a pK value of 5.35 ± 0.05 describes the conversion from acid to cadmium soap.