Investigation of the physical properties of highly curved membranes is important in biology, for example, in fusion intermediates, and in pharmaceutical or chromatographic applications, where nanoscale features may affect substrate binding. However, vesicle fusion below 40 nm precludes study of this size regime. In this investigation, the effect of high surface curvature on the adsorption and morphology of phosphotidylcholine lipids with alkyl chain lengths of 14 (DMPC), 16 (DPPC), and 18 (DSPC) onto silica (SiO2) nanobeads was investigated by thermogravimetric analysis (TGA), high sensitivity nanocalorimetry, and vibrational spectroscopy. The SiO2 beads ranged in size from 5 to 100 nm. Stable supported bilayers were formed on all bead sizes by vesicle fusion of the parent MLVs at temperatures above the main phase transition temperature (T(m)) of the lipids. A downward shift in T(m), and a broadening (deltaT1/2) of the transition with respect to the parent MLVs, was observed for the 100 nm beads. With decreasing bead size, T(m) first decreased, but then increased. On the smallest bead size, whose dimensions were comparable to those of the adsorbed lipids, T(m)'s were higher than those of the parent MLVs. The increase in T(m) indicated a stiffening of the supported bilayer, which was confirmed by Raman spectroscopic data. Narrowing of the phase transition or the appearance of peak doublets occurred at the smaller bead sizes. The results were consistent with a model in which the high free volume and increased outer headgroup spacing of lipids on highly curved surfaces induced interdigitation in the supported lipids.

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Supported lipid bilayers (SLBs) of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were formed on 20-100 nm silica (SiO(2)) nanobeads, and the formation was accompanied by an 8 nm increase in diameter of the SiO(2), consistent with single nanobeads surrounded by a DMPC bilayer. Complete SLBs were formed when the nominal surface areas of the DMPC matched that of the silica, SA(DMPC)/SA(SiO2) = 1, and required increasing ionic strength and time to form on smaller size nanobeads, as shown by a combination of nano-differential scanning calorimetry (nano-DSC), dynamic light scattering (DLS), and zeta potential (zeta) measurements. For 5 nm SiO(2), where the nanoparticle and DMPC dimensions were comparable, DMPC fused and formed SLBs on the nanobeads, but it did not form single bilayers around them. Instead, stable agglomerates of 150-1000 nm were formed over a wide surface ratio range (0.25 < or = SA(DMPC)/SA(SiO2) < 2) in 0.75 mM NaCl. At ionic strengths > 1 mM NaCl, charge shielding, as measured by zeta potential measurements (zeta --> 0), resulted in precipitation of the SLBs.

The interpretation of the infrared and Raman spectra of polymethylene chains is improved when Fermi resonances are considered in both the CH2 bending and stretching regions. This paper proposes a method for dealing with /e-dependent frequencies of chains either isolated or in the crystalline state. Previous intensity studies in infrared and Raman are used to account quantitatively for intensities and band shapes. The dynamically uncoupled chain is treated as a model for gauche structures, and the ratio, R, of the Raman intensities of the lines at 2850 and 2930 cm"1 is proposed as a measure of conformational disorder.

FTIR, Raman spectroscopy, and thermogravimetric analysis (TGA) were used to compare the structure and thermal properties of polyoctadecylsiloxane (POS) and octadecyl trichlorosilane (OTS) monolayers. The octadecyl chains in POS had higher conformational and intermolecular order than the same chains of OTS adsorbed as self-assembled monolayers (SAMs) on 106 nm hydrated silica beads. The latter were identical to the structures observed for OTS SAMs on superhydrated fumed silica that had primary particle sizes an order of magnitude smaller than the 106 nm beads, indicating that curvature did not affect the chain packing. The chains on POS were also more thermally stable to conformational and intermolecular disorder than the OTS chains, and this was attributed to increased free volume in the latter case. The differences in structure were partially determined by steric restrictions that arise because the Si−O−Si bond distance is less than the van der Waals radii of the alkyl chains. In POS, the most likely structure is therefore one in which the octadecyl chains are pointing in the same direction from every other Si atom, permitting good lateral chain packing. In the OTS SAMs, the octadecyl chains must all point in the same direction, away from the silica surface. Therefore, linear chains would be excluded but small clusters of dimers or trimers could be accommodated. These restraints increase the nearest neighbor distance between alkyl chains on SAMs and do not permit them to be as closely packed as for POS. This in turn contributes to the increased irreversible disordering of the chains with temperature.