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Rubitherm RT 27 is a paraffin-based phase-change material (PCM) in which a change between a solid and liquid phase is used to store/release thermal energy. Its enthalpy and heat capacity, as measured in a quasistatic regime by adiabatic scanning calorimetry, has a single distinct jump and peak, respectively, at about 27.3 C. We present a microscopic development from which the jump and peak can be accurately fitted and that could be analogously applied even to other PCMs. It enables us to determine the baseline and excess part of the heat capacity and thus the latent heat associated with the phase change. It is shown to be about 84 % of the total enthalpy change that occurs within 5 C from the peak maximum position. The development is based on the observation that PCMs often have polycrystalline structure, being composed of many single-crystalline grains. The enthalpy and heat capacity measured in experiments are therefore interpreted as superpositions of many contributions that come from the individual grains., Comment: 10 figures

The competing influence of two types of interactions on surface diffusion is investigated: one of them, epsilon, acts between adsorbed particles, while the other one, eta, between activated and adsorbed particles. To this end, a specific lattice-gas model on a triangular, square, and hexagonal lattice is considered with an attractive epsilon and an attractive or repulsive eta, both restricted to nearest neighbors. For all three lattices the influence is qualitatively the same. Namely, when eta is neglected, then epsilon is shown to decelerate diffusion with a rate exponential in epsilon and inverse temperature. Moreover, when eta is present and epsilon is fixed, then a sufficiently attractive eta (relative to epsilon) accelerates diffusion exponentially fast in eta and inverse temperature. However, quite surprisingly, a repulsive or slightly attractive eta has practically no effect on diffusion. Finally, when eta is set proportional to epsilon via a parameter $a$, surface diffusion is exponentially accelerated (decelerated) for $a$ above (below) a threshold value equal to 4, 5/4, and 7/8 for the three lattices. Thus, an eta of strength comparable to epsilon is enough to boost diffusion on the square and hexagonal lattices, while an eta of a rather large strength is needed for the boost on the triangular lattice., 19 pages, 9 figures

We analyze the collective surface diffusion coefficient, $D_c$, near a first-order phase transition at which two phases coexist and the surface coverage, $\te$, drops from one single-phase value, $\te_+$, to the other one, $\te_-$. Contrary to other studies, we consider the temperatures that are sufficiently sub-critical. Using the local equilibrium approximation, we obtain, both numerically and analytically, the dependence of $D_c$ on the coverage and system size, $N$, near such a transition. In the two-phase regime, when $\te$ ranges between $\te_-$ and $\te_+$, the diffusion coefficient behaves as a sum of two hyperbolas, $D_c \approx A/N|\te - \te_-| + B/N|\te - \te_+|$. The steep hyperbolic increase in $D_c$ near $\te_\pm$ rapidly slows down when the system gets from the two-phase regime to either of the single-phase regimes (when $\te$ gets below $\te_-$ or above $\te_+$), where it approaches a finite value. The crossover behavior of $D_c$ between the two-phase and single-phase regimes is described by a rather complex formula involving the Lambert function. We consider a lattice-gas model on a triangular lattice to illustrate these general results, applying them to four specific examples of transitions exhibited by the model., Comment: 17 pages, 3 figures