Freezing behavior of argon layers confined in mesopores

Abstract

Adsorbed layers of argon in mesoporous Vycor glass exhibit a peculiar phase behavior. We have studied their liquid-solid transition in pores with a mean pore diameter of 8 nm via ultrasonic measurements. An effective medium analysis reveals that during isothermal filling at low temperatures ($T<{T}_{\mathrm{fr},\mathrm{bulk}}$) frozen layers form on top of liquidlike surface layers. These frozen layers are bulklike (i.e., ${G}_{\mathrm{Ar},\mathrm{SL}}\ensuremath{\approx}{G}_{\mathrm{Ar},\mathrm{bulk}}$ holds for their intrinsic shear modulus). We detect that both the first and the second layer undergo a continuous phase transition in a broad temperature range of at least 40 K. A complete freezing is only achieved at very low temperatures of approximately 20 K (i.e., far below the bulk freezing temperature). Whereas those two adsorbed layers remain stable during cooling, the third one becomes unstable around 66 K, rearranges, and solid capillary condensate is formed (delayering transition).