X-ray Studies on the C12EO2\mathrm{C_{12}EO_2}/Water System

Binary mixtures of the poly(oxyethylene) surfactant C12EO2 and water have been investigated using optical microscopy and time-resolved X-ray diffraction during temperature scans. At concentrations in the range from 48 to 70 wt % of surfactant a thermal sequence from lamellar Lα to cubic Ia3d to cubic Pn3m to L2 was found upon heating. The geometrical parameters of the phases such as thickness of the hydrocarbon core of the lamellar phase and length and diameter of the rods forming the cubic structures were calculated for such conditions. In the lamellar Lα phase the surface area per molecule and the thickness of the hydrocarbon core showed little sensitivity to concentration. The thickness of the ethoxy groups was estimated to 1 nm. At high concentrations and temperatures near the cloud point, equilibrium conditions are difficult to reach, leading to the formation of cubic phases with apparently the same structure but different thermal behavior. One is insensitive to temperature changes while the other shrinks upon heating. The transition between the Ia3d and Pn3m cubic phases has the characteristics of a Bonnet transformation

Cubic topology in surfactant and lipid mixtures

Ternary systems of palmitoyl-oleoyl-phosphatidylcholine (POPC) and the non-ionic surfactant C12EO2 (di-ethylene-oxide-mono-dodecyl-ether) in water have been studied with optical microscopy, NMR, DSC and X-rays from ambient temperatures to 45 °C. Below 29 °C the system is in the lamellar liquid crystalline state. Between 30 and 32 °C it transforms into a cubic Ia3d structure which converts into the cubic Pn3m phase at 39 °C. The transitions are fully reversible. An epitaxial relationship between all three phases was found, which is an elegant and convenient way to rearrange molecules from lamellar bilayers to a network of curved surfaces. The la3d (Q230) to Pn3m (Q224) transition occurs without measurable enthalpy change. This, together with the metric relation of 1.60 between the cubic lattice constants is strong evidence for a Bonnet transformation, where the structural changes occur without change in curvature. The potential significance of the cubic phases as intermediate structures for biological processes, e. g. transport across a bilayer or fusion of membranes, are discussed

Comparative SAXS and DSC study on stratum corneum structural organization in an epidermal cell culture model (ROC):Impact of cultivation time

Cell cultured skin equivalents present an alternative for dermatological in vitro evaluations of drugs and excipients as they provide the advantage of availability, lower variability and higher assay robustness compared to native skin. For penetration/permeation studies, an adequate stratum corneum barrier similar to that of human stratum corneum is, however, a prerequisite. In this study, the stratum corneum lipid organization in an epidermal cell culture model based on rat epidermal keratinocytes (REK organotypic culture, ROC) was investigated by small-angle X-ray scattering (SAXS) in dependence on ROC cultivation time and in comparison to native human and rat stratum cornea. In addition, the thermal phase behavior was studied by differential scanning calorimetry (DSC) and barrier properties were checked by measurements of the permeability of tritiated water. The development of the barrier of ROC SC obtained at different cultivation times (7, 14 and 21 days at the air–liquid interface) was connected with an increase in structural order of the SC lipids in SAXS measurements: Already cultivation for 14 days at the air–liquid interface resulted overall in a competent SC permeability barrier and SC lipid organization. Cultivation for 21 days resulted in further minor changes in the structural organization of ROC SC. The SAXS patterns of ROC SC had overall large similarities with that of human SC and point to the presence of a long periodicity phase with a repeat distance of about 122 Å, e.g. slightly smaller than that determined for human SC in the present study (127 Å). Moreover, SAXS results also indicate the presence of covalently bound ceramides, which are crucial for a proper SC barrier, although the corresponding thermal transitions were not clearly detectable by DSC.Due to the competent SC barrier properties and high structural and organizational similarity to that of native human SC, ROC presents a promising alternative for in vitro studies, particularly as it can be obtained under overall rather straightforward cell culture conditions and thus low assay costs

Cold crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) by simultaneous measurements of X-ray scattering and dielectric spectroscopy

The isothermal cold crystallization of poly(ethylene naphthalene-2,6-dicarboxylate) was investigated by simultaneous small and wide angle X-ray scattering and dielectric spectroscopy (DS). By this experimental approach, simultaneously collected information was obtained about the specific changes occurring in both crystalline and amorphous phases during crystallization, namely about the chain ordering through wide angle X-ray scattering, about the lamellar crystals arrangement by means of small angle X-ray scattering, and about the amorphous phase evolution by means of DS. The results indicate that average mobility of the amorphous phase suffers a discontinuous decrease upon passing from the primary to the secondary crystallization regime. We interpret these results assuming that the restriction to the mobility of the amorphous phase occurs mainly in the amorphous regions between the lamellar stacks and not in the amorphous regions within the lamellar stacks. © 2002 Elsevier Science Ltd. All rights reserved.MCYT (grant FPA2001-2139) Spain. The experiments at HASYLAB (Hamburg, Germany) have been funded by the IHP-Contract HPRI-CT-1999-00040 of the European Commission (EC(ERBFMGEDT 950059) and II-00-015 EC).Peer Reviewe

Comparison of Small-Angle Scattering Methods for the Structural Analysis of Octyl-beta-maltopyranoside Micelles

Small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) experiments were performed to investigate the micelle structure of n-octyl-β-maltopyranoside (OM). Density measurements were carried out to obtain the volumetric characteristics of OM for micelle structure analysis. Both temperature and concentration were varied in scattering experiments to study their effects on micelle size. The scattering data were analyzed by the indirect Fourier transformation method (IFT) and model fitting. The IFT method gave the radius of gyration of the micelles and their pair distance distribution function. It was found that the radii of gyration from SANS data were much smaller than those from SAXS data at similar solution conditions. Moreover, pair distance distribution functions from SANS and SAXS data were also different. Model fitting indicated that a spherical shell model can be used to describe both SANS and SAXS data using similar structure parameters. Comparison of SAXS data in D2O and H2O shows that the OM micelle has a similar structure in both solvents. The size of the micelle does not increase with increasing concentration up to 188 mM. From 10 to 50 °C, the structure of the micelle is not sensitive to temperature changes. Comparison of the micelle structures of OM with those of its two closely related glycolipids, n-octyl-β-glucopyranoside (OG) and n-dodecyl-β-maltopyranoside (DM), suggests that the hydrophilic force plays an important role in the micelle structure of glycolipids