Article pubs.acs.org/JPCB
Structural Characterization of Lecithin-Stabilized Tetracosane Lipid Nanoparticles. Part I: Emulsions M. Schmiele,† S. Busch,‡ H. Morhenn,§ T. Schindler,† T. Schmutzler,† R. Schweins,∥ P. Lindner,∥ P. Boesecke,⊥ M. Westermann,# F. Steiniger,# Sérgio S. Funari,▽ and T. Unruh*,† †
Professur für Nanomaterialcharakterisierung (Streumethoden), Friedrich−Alexander−Universität Erlangen−Nürnberg, Staudtstr. 3, 91058 Erlangen, Germany ‡ German Engineering Materials Science Centre (GEMS) at Heinz Maier-Leibnitz Zentrum (MLZ), Helmholtz-Zentrum Geesthacht GmbH, Lichtenbergstr. 1, 85747 Garching, Germany § Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstr. 1, 85747 Garching, Germany ∥ DS/LSS, Institut Laue-Langevin (ILL), 71 Avenue des Martyrs, CS20156, 38042 Grenoble CEDEX 9, France ⊥ European Synchrotron Radiation Facility (ESRF), 71 Avenue des Martyrs, CS40220, 38042 Grenoble CEDEX 9, France # Center for Electron Microscopy of the Jena University Hospital, Ziegelmühlenweg 1, 07743 Jena, Germany ▽ HASYLAB, Notkestr. 85, 22603 Hamburg, Germany S Supporting Information *
ABSTRACT: The structure of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-stabilized colloidal tetracosane emulsions was investigated by photon correlation spectroscopy and smallangle X-ray and neutron scattering, using emulsions with different neutron scattering contrasts. Special emphasis was placed on the structure of the DMPC stabilizer layer covering the emulsion droplets. A monolayer, structurally similar to a half DMPC bilayer, with a thickness of 16 Å is found. Thereby, the phosphocholine headgroups arrange flat at the oil−water interface. A deep penetration of the tetracosane oil into the stabilizer layer can be ruled out.
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INTRODUCTION Lipid nanoparticles can be in a liquid (nanoemulsion) and solid (solid lipid nanoparticles1,2) state or both, partially solid and liquid.3,4 The structure of the lipid−water interface in lipid nanodispersions is important in many respects in pharmaceutical and food science. The amount and type of emulsifier and further additives can have a significant impact on the crystallization behavior,5−17 polymorphic transitions and stability of the lipid nanoparticles as well as the release and protection of encapsulated active substances. In food science the structure of the stabilizer layer is important to inhibit the oxidation of encapsulated substances such as β-carotene and lipids rich in poly unsaturated fatty acids (e.g., ω-3 fatty acids).18−22 Despite their importance, little is known about the structure of the stabilizer layer, for both, nanoemulsions (Part I) and nanosuspensions (Part II, 10.1021/acs.jpcb.6b02520). In this study we use tetracosane (C24H50) nanoemulsions, stabilized by the lecithin DMPC, as a model system. Here in Part I, we focus on the structure of the interfacial lecithin stabilizer layer of these nanoemulsions. By a combined analysis of small-angle X-ray and neutron scattering (SAXS, © 2016 American Chemical Society
SANS) data we investigate the structure of the DMPC stabilizer layer at the oil−water interface and compare the results with the known structure of a half DMPC bilayer23−25 and molecular dynamic (MD) simulations of similar nanoemulsions.26−28 In the SANS experiments different neutron scattering contrasts were achieved by using protiated and deuterated compounds of tetracosane and DMPC. In Part II, the structure of the corresponding nanosuspensions is analyzed from the molecular level to the microscopic scale as a function of the temperature. Special attention is paid to the structure of the DMPC stabilizer layer covering the crystallized nanoparticles.
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MATERIALS AND METHODS Materials. Tetracosane (TCS, ≥ 99% purity, molecular structure in Figure S1a in the Supporting Information) and sodium glycocholate hydrate (NaGC, ≥ 97% purity, Figure S1c) were purchased from Sigma-Aldrich Chemie, Taufkirchen, Germany. 1,2-dimyristoyl-sn-glycero-3-phosphocholine Received: March 10, 2016 Revised: May 19, 2016 Published: May 27, 2016 5505
DOI: 10.1021/acs.jpcb.6b02519 J. Phys. Chem. B 2016, 120, 5505−5512
Article
The Journal of Physical Chemistry B Table 1. Chemical Composition of the TCS Dispersions (given in wt %)a sample
composition
H−H−D H−D−D D−H−D D-D−D D−HD−HD
2.71% 2.75% 3.11% 3.10% 3.20%
TCS TCS TCS-d50 TCS-d50 TCS-d50
H−H−D (15%)
13.78% TCS
0.92% 0.99% 0.92% 0.98% 0.71% 0.26% 4.66%
DMPC DMPC-d54 DMPC DMPC-d54 DMPC DMPC-d54 DMPC
0.092% 0.094% 0.095% 0.093% 0.095%
NaGC NaGC NaGC NaGC NaGC
96.28% 96.17% 95.87% 95.82% 29.79% 65.94% 81.10%
0.46% NaGC
D2O D2O D2O D2O H2O D2O D2O
a
In an equivalent fully protiated sample the amount of substance would correspond to 3% TCS, 1% DMPC, and 0.1% NaGC in the case of H−H− D, H−D−D, D−H−D, D−D−D, and D−HD−HD, and 15% TCS, 5% DMPC, and 0.5% NaGC for H−H−D (15%).
Table 2. Molecular Volumes, V, Neutron Scattering Lengths, b, NSLDs, ρn = b/V, Number of Electrons, Z, and ED, ρX = Z/V, for Liquid TCS(-d50), DMPC(-d54), and (Heavy) Water, Used in the Fitting of the SAXS and SANS Patterns of the DMPCStabilized Emulsions (T = 55 °C)a TCS (liq.) TCS-d50 (liq.) DMPC DMPC-h DMPC-ch DMPC-ch-d54 water heavy water
chem. formula
ρ (g/cm3)
V (Å3)
b (fm)
ρn (10−6 Å−2)
Z (−)
ρX (nm−3)
C24H50 C24D50 C36H72NO8P C10H18NO8P C26H54 C26D54 H2O D2O
0.776b 0.891c
723 723 1101d 319d 782d 782 30.3 30.4
−27.4 493.1 31.0 60.1 −29.1 533.0 −1.68 19.1
−0.38 6.82 0.28 1.88 −0.37 6.82 −0.56 6.29
194 194 374 164 210 210 10 10
268.3 268.3 339.7 514.1 268.5 268.5 329.6 328.7
0.986e 1.093f
For liquid TCS(-d50) and (heavy) water, V was calculated via V = M/(ρ·NA), where ρ denotes the weight density of the substance, M is the molar mass, and NA = 6.022 × 1023 mol−1 is the Avogadro constant. For DMPC(-d54) the values are given for the phosphocholine headgroup (h, including the two carbonyl groups) and the remaining molecular part of the two C13 acyl chains (ch). bTaken at 55 °C from ref 34. cObtained from ρ for TCS by respecting the different molar masses for TCS-d50 and TCS. dTaken for the Lα phase at 30 °C from ref 23, temperature effects are neglected on the basis that the DMPC volume increases by
