1 research outputs found
Controlling Interfacial Film Formation in Mixed Polymer–Surfactant Systems by Changing the Vapor Phase
Here we show that transport-generated
phase separation at the air–liquid
interface in systems containing self-assembling amphiphilic molecules
and polymers can be controlled by the relative humidity (RH) of the
air. We also show that our observations can be described quantitatively
with a theoretical model describing interfacial phase separation in
a water gradient that we published previously. These phenomena arises
from the fact that the water chemical potential corresponding to the
ambient RH will, in general, not match the water chemical potential
in the open aqueous solution. This implies nonequilibrium conditions
at the air–water interface, which in turn can have consequences
on the molecular organization in this layer. The experimental setup
is such that we can control the boundary conditions in RH and thereby
verify the predictions from the theoretical model. The polymer–surfactant
systems studied here are composed of polyethylenimine (PEI) and hexadecyltrimethylammonium
bromide (CTAB) or didecyldimethylammonium bromide (DDAB). Grazing-incidence
small-angle X-ray scattering results show that interfacial phases
with hexagonal or lamellar structure form at the interface of dilute
polymer–surfactant micellar solutions. From spectroscopic ellipsometry
data we conclude that variations in RH can be used to control the
growth of micrometer-thick interfacial films and that reducing RH
leads to thicker films. For the CTAB–PEI system, we compare
the phase behavior of the interfacial phase to the equilibrium bulk
phase behavior. The interfacial film resembles the bulk phases formed
at high surfactant to polymer ratio and reduced water contents, and
this can be used to predict the composition of interfacial phase.
We also show that convection in the vapor phase strongly reduces film
formation, likely due to reduction of the unstirred layer, where diffusive
transport is dominating