Iridescent Solutions Resulting from Periodic Structure of Bilayer Membranes

Article abstract of DOI:10.1021/j100311a015

Dilute aqueous solutions of alkenylsuccinic acid (ASA) have been found to exhibit iridescent phenomena in the concetration range of about 1 to 2 wt percent.The structures of the iridescent solutions and the more concetrated solutions have been studied mainly by the ultraviolet (UV) and X-ray diffraction techniques.The experimental results of X-ray diffraction for the concentrated C16ASA solutions (10-60 wt percent) indicate that the bilayer membranes of 4.1 nm thickness are stacked periodically to form a lamellar liquid crystal.The UV diffraction method was applied to iridescent dilute solutions (0.7-2.4 wt percent) of the same surfactant.Strong but broad Bragg reflections corresponding the spacing of 300-150 nm were obtained.Because of the lack of higher order reflections in UV diffraction patterns, the detailed structure of iridescent solutions could not be made clear.A simple model is adopted to estimate the structure of dilute solutions.If the lamellar liquid crystal is uniformly formed in the whole space of the solution, the Bragg spacing, d, is expressed as d=<((1-c)/c)ρ₁/ρ₂+1>d₁, where c is the weight fraction of surfactant, ρ₁ and ρ₂ are the densities of surfactant and water layer, respectively, and d₁ is the thickness of surfactant layer.The Bragg spacing obtained from both X-ray and UV diffraction increases with decreasing concetration,c, of ASA and is proportional to (1-c)/c, and two lines from different methods are shown to be identical.These results strongly suggest that the iridescent dilute solutions also consist of lamellar liquid crystals having extremely long spacing distance as above and the iridescent color appears by the interference of reflected light by single bilayer membranes of ASA.