Chemistry Reference
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repeating structures normal to the interface can generate Bragg diffraction
peaks. Also, lateral inhomogeneity across the surface can appear as intensity
signals in the off-specular scattering. Thus NR can help to reveal quantitative
structural and compositional information on adsorption of LCNP.
Recently, NR measurements were carried out to investigate the structure
of the interfacial layer upon adsorption of LCNP (Vandoolaeghe et al., 2008,
2009b,c ). The refl ectivity profi les show sharp peaks that are analogous to Bragg
peaks in the diffraction from a crystal but in this case arise from one-
dimensional order of repeating patterns. The intensity of the peak increases
with time (Fig. 10.7a) revealing the increased presence of an organized repeat-
ing structure perpendicular to the plane of the surface, which arises from the
ordered arrangement of the lipid molecules in the LCNP (Vandoolaeghe
et al., 2009b). The NR curve fi ts nicely to the cubic phase model with the scat-
tering length density profi le varying as a sinusoidal function (Fig. 10.7b). The
NR experiment demonstrates that LCNPs adsorb intact on the hydrophilic Si
surfaces. The particles adsorb randomly at the interface with the internal cubic
phase undisturbed and retain their internal crystalline structure at the inter-
face for over 44 h (Vandoolaeghe et al., 2009b).
10.6.2
Phase Transitions at the Interface
The intact LCNPs can undergo phase transition when in contact with other
lipids at an interface. As mentioned previously, the exchange of lipids can take
place when GMO-based CPNP interacts with the supported lipid bilayer. The
amount of lipid exchange can be quantifi ed with NR by determining the com-
position of hydrogenous components of CPNP (GMO and F-127) and the
deuterated bilayer (d-DOPC). The results show that the fi nal composition of
the lipid layer on a surface at the end of an exchange depends very much on
the initial bilayer coverage (Vandoolaeghe et al., 2009c). At high bilayer cover-
age, the addition of CPNP leads to an extensive exchange of lipids. The fi nal
bilayer is composed of 72% of the CPNP components and the surface contains
very little residual intact particles. At lower bilayer coverage, spreading of
CPNP components fi lls in the defects of the bilayer. At the same time, a sub-
stantial adsorption of the intact particles can be seen by the presence of the
Bragg diffraction peak (Vandoolaeghe et al., 2009c).
The lipid exchange can also change the composition of the adsorbed CPNP
and induce a phase change. An NR study shows the incorporation of d-DOPC
molecules from the bilayer into the GMO-based CPNP (Vandoolaeghe et al.,
2009c). The incorporation of DOPC shifts the composition from the cubic
phase toward the cubic/lamellar phase and possibly the lamellar phase based
on the GMO/DOPC/D
2
O phase diagram. Such a transition can lead to a con-
traction of the unit cell dimension of the cubic structure and shifts the Bragg
diffraction peak toward higher momentum transfer,
Q
, values. The shift and
broadening of the Bragg diffraction peak are observed over time, which are
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