In a confined geometry, thermal activation gives rise to specific condensation or evaporation mechanisms. Accounting for this phenomenon is therefore important for characterizing porous materials from their sorption isotherms. However, most phenomenological models discussing thermal activation in strong confinement are only qualitative. Here, we use a semimacroscopic approach to compute the temperature and diameter dependences of the condensation and evaporation for a fluid confined in a cylindrical nanopore in the presence of thermal activation. In particular, our model predicts the effect of confinement on cavitation. Our results show how thermal activation controls the existence and shape of the hysteresis loop in porous materials with noninterconnected cylindrical pores, and shed a new light on the applicability of the classical Barrett−Joyner−Halenda method for determining pore diameter distributions in porous materials. This work paves the way to a phenomenological multiscale approach able to describe condensation and evaporation in materials with connected pores.

General studies of phase transitions PACS 64.70.F--Liquid-Vapour transitions PACS 67.25.bh -Quantum fluids and solids : 4 He -Films and restricted geometries.Abstract. -We report on thermodynamic and optical measurements of the condensation process of 4 He in three silica aerogels of different microstructures. For the two base-catalysed aerogels, the temperature dependence of the shape of adsorption isotherms and of the morphology of the condensation process show evidence of a disorder driven transition, in agreement with recent theoretical predictions. This transition is not observed for a neutral-catalysed aerogel, which we interpret as due to a larger disorder in this case.

Aggregation and other interactions are suppressed for a biological semiflexible polyelectrolyte, hyaluronan (HA), when it is embedded in a mixture with another negatively charged and flexible polyelectrolyte chain, sodium polystyrene sulfonate. We see directly HA only in the mixture using Small-Angle Neutron Scattering, isotopic labelling and contrast matching. At low ionic strength, for which aggregation is usually seen for pure HA solutions, an unambiguous set of experimental results shows that we neither observe HA aggregation nor a polyelectrolyte peak (observed for solutions of single species); instead we observe a wormlike chain behaviour characteristic of single chain with a variation of the persistence length with the square of the Debye screening length, L e~ -2 , as formerly predicted by Odijkand not yet observed on a polymer chain.

We combine high resolution isotherms measurements and light scattering technics to study over a broad temperature range collective effects during the condensation and evaporation of helium from Vycor, a prototypic disordered porous material. For evaporation, our results provide the first direct evidence for a crossover from a percolation collective mechanism at low temperature to a local cavitation mechanism at high temperature. No long range collective effects are detected during condensation. We compare these results to recent theoretical predictions emphasizing the specific role of disorder, and discuss their relevance for determining pores sizes distributions in disordered porous materials.