Sorption isotherms of water in nanopores: the relation between hydrophobicity, adsorption pressure, and hysteresis

MATIAS FACTOROVICH; ESTEFANIA GONZALEZ SOLVEYRA; VALERIA MOLINERO; D. A. SCHERLIS

JOURNAL OF PHYSICAL CHEMISTRY C

AMER CHEMICAL SOC

Lugar: Washington; Año: 2014 vol. 118 p. 16290 - 16300

1932-7447

The motivation of this study is to elucidate how the condensation anddesorption pressures depend on the contact angle in water sorption isotherms. Thisquestion is investigated for cylindrical pores of 2.8 nm by means of molecular dynamicssimulations in the grand canonical ensemble, in combination with the mW coarse-grainedmodel for water. The contact angle is characterized for different sets of water−surfaceinteractions. In the first place, we show that desorption in open-ended pores is anonactivated process in which pressure is accurately described by the Kelvin equation insystems with moderate or low water affinity, with contact angles greater or equal than 24°.Then, we explore the influence of hydrophobicity on the capillary condensation and on thewidth of the hysteresis loop. We find that a small increase in the contact angle may have asignificant impact on the surface density and consequently on the nucleation free energybarrier. This produces a separation of the adsorption and desorption branches, exacerbatingthe emerging hysteresis. These results suggest that the contact angle is not as relevant as theadsorption energy in determining condensation pressure and hysteresis. Finally, we considernonequilibrium desorption in pores with no open ends and describe how homogeneous and heterogeneous cavitationmechanisms depend on hydrophilicity.