3 research outputs found
Mesoscopic Diffusion of Poly(ethylene oxide) in Pure and Mixed Solvents
We present results from an experimental
dynamic light-scattering study of polyÂ(ethylene oxide) (PEO) in both
a pure solvent (water) and a mixed solvent (tert-butanol + water). The concentration dependence
of the diffusive relaxation of the PEO molecules is found to be typical
of polymers in a good solvent. However, the mesoscopic diffusive behavior
of PEO in the mixed solvent is very different, indicating an initial
collapse and subsequent reswelling of PEO caused by co-nonsolvency.
Furthermore, in the solutions of PEO with very large molecular weights,
we found additional hydrodynamic modes indicating the presence of
PEO clusters and aggregates similar to those found by some other investigators
Dual Action of Hydrotropes at the Water/Oil Interface
Hydrotropes are substances
containing small amphiphilic molecules, which increase solubility
of nonpolar (hydrophobic) substances in water. Hydrotropes may form
dynamic clusters (less or about 1 ns lifetime) with water molecules;
such clusters can be viewed as “pre-micelles” or as
“micellar-like” structural fluctuations. We present
the results of experimental and molecular dynamics (MD) simulation
studies of interfacial phenomena and liquid–liquid equilibrium
in the mixtures of water and cyclohexane with the addition of a typical
nonionic hydrotrope, tertiary butanol. The interfacial tension between
the aqueous and oil phases was measured by Wilhelmy plate and spinning
drop methods with overlapping conditions in excellent agreement between
techniques. The correlation length of the concentration fluctuations,
which is proportional to the thickness of the interface near the liquid–liquid
critical point, was measured by dynamic light scattering. In addition,
we studied the interfacial tension and water–oil interfacial
profiles by MD simulations of a model representing this ternary system.
Both experimental and simulation studies consistently demonstrate
a spectacular crossover between two limits in the behavior of the
water–oil interfacial properties upon addition of the hydrotrope:
at low concentrations the hydrotrope acts as a surfactant, decreasing
the interfacial tension by adsorption of hydrotrope molecules on the
interface, while at higher concentrations it acts as a cosolvent with
the interfacial tension vanishing in accordance with a scaling power-law
upon approach to the liquid–liquid critical point. It is found
that the relation between the thickness of the interface and the interfacial
tension follows a scaling law in the entire range of interfacial tensions,
from a “sharp” interface in the absence of the hydrotrope
to a “smooth” interface near the critical point. We
also demonstrate the generic nature of the dual behavior of hydrotropes
by comparing the studied ternary system with systems containing different
hydrocarbons and hydrotropes
Dual Action of Hydrotropes at the Water/Oil Interface
Hydrotropes are substances
containing small amphiphilic molecules, which increase solubility
of nonpolar (hydrophobic) substances in water. Hydrotropes may form
dynamic clusters (less or about 1 ns lifetime) with water molecules;
such clusters can be viewed as “pre-micelles” or as
“micellar-like” structural fluctuations. We present
the results of experimental and molecular dynamics (MD) simulation
studies of interfacial phenomena and liquid–liquid equilibrium
in the mixtures of water and cyclohexane with the addition of a typical
nonionic hydrotrope, tertiary butanol. The interfacial tension between
the aqueous and oil phases was measured by Wilhelmy plate and spinning
drop methods with overlapping conditions in excellent agreement between
techniques. The correlation length of the concentration fluctuations,
which is proportional to the thickness of the interface near the liquid–liquid
critical point, was measured by dynamic light scattering. In addition,
we studied the interfacial tension and water–oil interfacial
profiles by MD simulations of a model representing this ternary system.
Both experimental and simulation studies consistently demonstrate
a spectacular crossover between two limits in the behavior of the
water–oil interfacial properties upon addition of the hydrotrope:
at low concentrations the hydrotrope acts as a surfactant, decreasing
the interfacial tension by adsorption of hydrotrope molecules on the
interface, while at higher concentrations it acts as a cosolvent with
the interfacial tension vanishing in accordance with a scaling power-law
upon approach to the liquid–liquid critical point. It is found
that the relation between the thickness of the interface and the interfacial
tension follows a scaling law in the entire range of interfacial tensions,
from a “sharp” interface in the absence of the hydrotrope
to a “smooth” interface near the critical point. We
also demonstrate the generic nature of the dual behavior of hydrotropes
by comparing the studied ternary system with systems containing different
hydrocarbons and hydrotropes