Effect of Solvent Dielectric Constant on the Formation of Large Flat Bilayer Stacks in a Lecithin/Hexadecanol Hydrogel.
We investigated the effect of dielectric properties of the aqueous medium on the novel type of hydrogel composed of a crude lecithin mixture (PC70) and hexadecanol (HD), in which charged sheet-like bilayers are kept far apart due to interbilayer repulsive interaction. We used dipropylene glycol (DPG) as a modifier of the dielectric properties and examined its effect on the hydrogel by synchrotron X-ray diffraction, differential scanning calorimetry (DSC), polarized optical microscopy, and freeze-fracture electron microscopy. We found that at a DPG weight fraction in the aqueous medium WDPG ≈ 0.4, the bilayer organization is transformed into unusually large flat bilayer stacks with a regular lamellar spacing of 6.25 nm and consequently disintegration of the hydrogel takes place. Semiquantitative calculation of the interbilayer interaction energy based on the Deyaguin-Landau-Verwey-Overbeek (DLVO) theory suggested that the reduction of the aqueous medium dielectric constant ε by DPG may lower the energy barrier preventing flat bilayers from coming closer together. We inferred that the size of the bilayer sheet increases because the reduction of ε promotes protonation of acidic lipids that work as edge-capping molecules.
Beauty Care Laboratory, Kracie Home Products, Ltd., 134, Goudo-cho, Hodogaya-ku, Yokohama, Kanagawa 240-0005, Japan.
Yasuharu
Y
Nakagawa
Department of Physics, School of Science & Technology, Kwansei Gakuin University , 2-1, Gakuen, Sanda, Hyogo 669-1337, Japan.
Hiromitsu
H
Nakazawa
Department of Physics, School of Science & Technology, Kwansei Gakuin University , 2-1, Gakuen, Sanda, Hyogo 669-1337, Japan.
Satoru
S
Kato