Lipid and Simple Liquid Surfaces

X-ray reflectivity of a liquid surface at glancing angles α well beyond the glancing angle αC for total reflection yields detailed information about the average electron density profile ρ(z) across the surface. It is easily shown1 that the ratio of the observed reflectivity R(α) and the Fresnel reflectivity R_F(α) corresponding to an infinitely sharp density profile measures the Fourier transform of the density gradient ρ'(z) : R(α)/R_F(α) = |∫ρ'(z)exp(iQz) dz|² with Q = 2ksinα being the wave vector transfer at specular reflection. The method has been used to investigate (i) thermal roughness of simple liquids² (ii) smectic layering of liquid crystals^3,4,5 (iii) monolayers on water of fatty acids⁶, surfactants⁷, phospholipids⁸ and of microemulsions⁹ Using thermal neutrons beams in stead of X-rays one can enhance the contrast between monolayer and subphase and of specific parts of molecules by isotope substitution¹⁰. Neutron reflectivity is also sensitive to surface magnetism. In-plane structures^6,7,8 can be deduced from grazing incidence diffraction for α near α_C.