45 research outputs found
Engines with ideal efficiency and nonzero power for sublinear transport laws
It is known that an engine with ideal efficiency ( for a chemical
engine and for a thermal one) has zero power because a
reversible cycle takes an infinite time. However, at least from a theoretical
point of view, it is possible to conceive (irreversible) engines with nonzero
power that can reach ideal efficiency. Here this is achieved by replacing the
usual linear transport law by a sublinear one and taking the step-function
limit for the particle current (chemical engine) or heat current (thermal
engine) versus the applied force. It is shown that in taking this limit exact
thermodynamic inequalities relating the currents to the entropy production are
not violated
Effect of ion hydration on the first-order transition in the sequential wetting of hexane on brine
In recent experiments, a sequence of changes in the wetting state (`wetting
transitions') has been observed upon increasing the temperature in systems
consisting of pentane on pure water and of hexane on brine. This sequence of
two transitions is brought about by an interplay of short-range and long-range
interactions between substrate and adsorbate. In this work, we argue that the
short-range interaction (contact energy) between hexane and pure water remains
unchanged due to the formation of a depletion layer (a thin `layer' of pure
water which is completely devoid of ions) at the surface of the electrolyte and
that the presence of the salt manifests itself only in a modification of the
long-range interaction between substrate and adsorbate. In a five-layer
calculation considering brine, water, the first layer of adsorbed hexane
molecules, liquid hexane, and vapor, we determine the new long-range
interaction of brine with the adsorbate {\em across} the water `layer'.
According to the recent theory of the excess surface tension of an electrolyte
by Levin and Flores-Mena, this water `layer' is of constant, i.e.\
salt-concentration independent, thickness , with being the
hydrodynamic radius of the ions in water. Our results are in good agreement
with the experimental ones.Comment: 27 pages, 2 tables, 4 figure