A Systematic Investigation and Insight into the Formation
Mechanism of Bilayers
of Fatty Acid/Soap Mixtures in Aqueous Solutions
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Abstract
Vesicles are the most common form
of bilayer structures in fatty
acid/soap mixtures in aqueous solutions; however, a peculiar bilayer
structure called a “planar sheet” was found for the
first time in the mixtures. In the past few decades, considerable
research has focused on the formation theory of bilayers in fatty
acid/soap mixtures. The hydrogen bond theory has been widely accepted
by scientists to explain the formation of bilayers. However, except
for the hydrogen bond, no other driving forces were proposed systematically.
In this work, three kinds of weak interactions were investigated in
detail, which could perfectly demonstrate the formation mechanism
of bilayer structures in the fatty acid/soap mixtures in aqueous solutions.
(i) The influence of hydrophobic interaction was detected by changing
the chain length of fatty acid (C<sub><i>n</i></sub>H<sub>2<i>n</i>+1</sub>COOH), in which <i>n</i> = 10
to 18, the phase behavior was investigated, and the phase region was
presented. With the help of cryogenic transmission electron microscopy
(cryo-TEM) observations, deuterium nuclear magnetic resonance (<sup>2</sup>H NMR), and X-ray diffraction (XRD) measurements, the vesicles
and planar sheets were determined. The chain length of C<sub><i>n</i></sub>H<sub>2<i>n</i>+1</sub>COOH has an important
effect on the physical state of the hydrophobic chain, resulting in
an obvious difference in the viscoelasticity of the solution samples.
(ii) The existence of hydrogen bonds between fatty acids and their
soaps in aqueous solutions was demonstrated by Fourier transform infrared
(FT-IR) spectroscopy and molecule dynamical simulation. From the pH
measurements, the pH ranges of the bilayer formation were at the p<i>K</i><sub>a</sub> values of fatty acids, respectively. (iii)
Counterions can be embedded in the stern layer of the bilayers and
screen the electrostatic repulsion between the COO<sup>–</sup> anionic headgroups. FT-IR characterization demonstrated a bidentate
bridging coordination mode between counterions and carboxylates. The
conductivity measurements provided the degree of counterion binding
(β = 0.854), indicating the importance of the counterions