Water null reflecting

Under some experimental circumstances, equations of the form of Eq. 17 can be simplified to gain information more easily (see later). For example, if null reflecting water is used as the subphase, then in Eq. 17 we have = 0, and hence for the DPPC monolayers we have... 

If deuterium-substituted surfactant is dissolved in this null reflecting water, the neutron reflection occurs only when the surfactant forms an adsorption layer at the air-solution interface. For this case the general expression for reflectance is considerably simplified, namely... 

Figure 3.12. The reflectivity of 1 mg m polymethyl methacrylate (PMMA) spread on a 0.1%wt/vol polyethylene oxide (PEO) solution in water (a) for a null reflecting subphase (o, dPMMA/dPEO A, dPMMA/hPEO and +, hPMMA/dPEO) and (b) lower set for a subphase of D2O (o, dPMMA/hPEO A, hPMMA/dPEO and +, hPMMA/hPEO). (S. K. Peace, PhD thesis. University of Durham, 1996.)...

From a fnndamental point of view, direct experimental information on the interface profile would be desirable. To get direct structural information with respect to the interface profile, neutron reflection wonld be the ideal technique to use. Neutron reflection has been used extensively to shed light on the strnctnre of air-liquid interfaces [44,45]. A liquid-liquid interface brings along the complication that the nentron beam has to pass an air-liquid interface before it reaches the water-water interface. The air-liquid interface is in general a much stronger transition in reflective index than the water-water interface and will obscure the reflection of the water-water interface, which is beyond it. Therefore, a water-water phase-separated system has to be used with a H2O/D2O ratio with zero neutron-scattering cross-section. The polymer in the top phase should also have an H/D composition rendering the polymer in the top phase invisible for neutrons. In principle, the polymer profile at the water-water interface can in this way be obtained, because the polymer composition at the interface differs from that with the null H/D composition in the top phase. The solvent profile can only be indirectly determined from the polymer excess. 

---