From specular reflectivity to time-resolved grazing incidence X-ray scattering out of the specular plane (GISAXS): equilibrium and nonequilibrium states of organic/inorganic monolayers at liquid surfaces.

The grazing incidence diffuse X-ray scattering out of the specular plane technique (GISAXS) is presented, and its capabilities are compared to the more classical X-ray specular reflectivity technique (XSR). Three experimental illustrations are given to prove the efficiency of GISAXS. First, the structure of a DPPC phospholipid monolayer is analyzed. Second, the time-resolved kinetics of lipid desorption from a monolayer interacting with a mineral gel subphase is studied. Third, the structural evolution of biomembranes at extremely low temperatures is illustrated. The GISAXS technique appears to compete efficiently with neutron reflection techniques by taking advantage of remarkably short acquisition times crucial in kinetics experiments.

Self-assembly of organic monolayers below the freezing threshold.

One measure that arctic fish and amphibians use to minimize damage to cellular membranes during cooling and freezing processes is the production of cryo-protective substances. We have mimicked this biological "trick" by using the surface of a cryo-protectant as a liquid subphase for the preparation of organic membranes. Following this innovative approach, quasi two-dimensional amphiphilic monolayers were cooled to -40 degrees C at a liquid/gas interface. To date, the low temperature region of the generic phase diagram for alkane chain molecules has been only "virtually" accessible by tuning the molecular chain length. By extending the temperature range well below the freezing point of water, we gained new insights into membrane stability, morphology, and reorganization at low temperatures. Upon cooling relaxed monolayers at a surface pressure of 4.5 mN/m, we find a transition from a mesophase with tilted chains at ambient temperature toward a crystalline phase with upright chains at low temperatures. Structure factor calculations reveal that the chain alignment in the crystalline phase differs from the classical herringbone configuration.

Grazing incidence X-ray scattering to probe the self-assembly of phthalocyanine nanorods on a liquid surface.

The structural properties of Langmuir monolayers on aqueous substrates of a metal free phthalocyanine, 2,9,16,23-tetrakis(phenylthio)-29H,31H-phthalocyanine and an Aluminum centered phthalocyanine, Aluminum 2,9,16,23-tetrakis(phenylthio)-29H,31H-phthalocyanine chloride are reported here. Their structure is investigated under progressive lateral compression by grazing incidence diffuse X-ray scattering out of the specular plane to determine specular reflectivity-like information where the phase change of the molecules from "flat-lying" on the surface to "edge-standing" perpendicular to the surface was directly observed. Furthermore grazing incident X-ray diffraction is used to investigate the in-plane ordering of the system where it has been found that at high density states the systems can be considered as monolayers consisting of arrays of side-by-side cofacially aggregated cylindrical rodlike entities.

Observation of zinc phthalocyanine aggregates on a water surface using grazing incidence X-ray scattering.

Reported here are the structural properties of a zinc 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine Langmuir monolayer on a water surface under progressive lateral compression investigated by grazing incidence X-ray scattering. Grazing incidence diffuse X-ray scattering out of the specular plane (GIXOS) is exploited to determine specular reflectivity-like information where the phase change of flat-lying molecules on the surface to edge-standing molecules perpendicular to the surface is directly observed. Furthermore, grazing incident X-ray diffraction (GIXD) is used to investigate the in-plane ordering of the system where it has been found that in the high-density state (approximately 0.35 nm2 per molecule) the system can be considered to be a monolayer consisting of arrays of side-by-side lying cofacially aggregated cylindrical rodlike entities.

Nanocrystal induced organization of a langmuir phospholipid monolayer.

The influence of crystal surface charge on the thermodynamic and structural behavior of phospholipid monolayers has been investigated. We present how charged nanocrystals in the vicinity of an inherently nonordered lipid membrane provoke strong effects on the molecular arrangement within the monolayer. Apart from the induction of phase shifts and nucleation processes, the molecules were forced to adopt an ordered phase. A very recently developed X-ray scattering method is used for the first time to replace time-consuming specular reflectivity measurements. We conclude on the potential effects of crystal surface charge on cellular membranes.