Methods and compositions for producing lipid-based cubic phase nanoparticles were first discovered in the 1990s. Since then a number of studies have been presented, but little is known about how to control key properties such as particle size, morphology, and stability of cubic phase dispersions. In the present work we give examples of how these properties can be tuned by composition and processing conditions. Importantly we show that stable particle dispersions with consistent size and structure can be produced by a simple processing scheme comprising a homogenization and heat treatment step.
Cloud point curves and conjugate coexistence curves for the
quasi-binary water systems of
ethyl(hydroxyethyl)cellulose (EHEC) and its hydrophobically modified analogue (HMEHEC) have
been determined. The cloud
point curves are remarkably independent of the polymer concentration up
to as high as 20 wt % polymer, and
they are clearly different from the coexistence curves. This is
interpreted as an effect of the multicomponent
nature of the polymer. Hydrophobic modification of EHEC lowers the
cloud point by approximately 15 °C.
On heating above the cloud point, EHEC redistributes to the
concentrated phase, but, however, a substantial
amount of the polymer remains in the dilute phase even far above the
cloud point. For HMEHEC, essentially
all polymer is found throughout in the concentrated phase.
Addition of the anionic surfactant, sodium
octylbenzenesulfonate (SOBS), affects the phase behavior. Small
amounts of SOBS cause a swelling of the
concentrated phase for both EHEC and HMEHEC, due to an electrostatic
repulsion between polymer−surfactant
aggregates. On further addition of SOBS, a dissolution of polymer
from the concentrated phase is observed
for EHEC. For HMEHEC, the surfactant binding swells the
concentrated phase until the one-phase region
is reached.
The effect of surfactants on the adsorption properties of ethyl(hydroxyethyl)cellulose (EHEC) and its
hydrophobically modified analogue (HM-EHEC) at the solid−liquid interface has been studied by
ellipsometry. The adsorption characteristics of EHEC and HM-EHEC without the presence of surfactants
are also presented. The polar silica surface and a hydrophobized silica surface were used as substrates.
On the polar silica surface, a small addition of the anionic surfactant sodium dodecyl sulfate (SDS) caused
a 3- to 5-fold expansion of the preadsorbed EHEC or HM-EHEC layers, while the adsorbed amount was
less influenced. On the hydrophobized silica surface, SDS could replace EHEC (>10 mM SDS), while some
adsorbed HM-EHEC still could be detected well above the critical micelle concentration of SDS in the bulk
(14 mM). The nonionic surfactant octa(ethylene oxide) dodecyl ether (C12E8) did not affect the adsorbed
layer structure on silica, and the cationic surfactant cetyltriammonium chloride (CTAC) on hydrophobized
silica showed similar effects as SDS but with a smaller magnitude. It is proposed that the adsorbed layer
structure mainly is governed by polymer−surfactant interfacial interactions.
Semidilute (above the overlap concentration, c*) aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC)
as a function of concentration were investigated with the surface force apparatus (SFA), and by static light
scattering (SLS) and osmotic pressure measurements. The anomalous excess in scattering intensity observed
in SLS experiments at small angles indicates concentration inhomogeneities in the samples, probably as
a result of association of the EHEC chains. The light scattering and osmotic pressure results show that
EHEC in aqueous soluttion does not behave as expected for a polymer in a good solvent. The results from
the SFA experiments show that EHEC adsorbs strongly on mica. These adsorbed layers give rise to a
short-range steric repulsion when the two EHEC-coated mica surfaces are brought into close contact. The
most remarkable result, however, is the very long-range attractive force observed at larger surface
separations. This attractive force features oscillations with a periodicity that decreases when the polymer
concentration increases. The periodicity corresponded to the mesh network size determined by SLS. To
our knowledge, this type of long-range attraction has not previously been reported. The origin of the force
could not be explained in terms of bridging or classical depletion effects. Rather, this long-range attraction
might be induced by segregation of different polymer fractions, with different surface activities, present
in the EHEC solutions.
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