Monitor adsorption of acetone vapor to a room temperature ionic liquid 1-octyl-3-methylimidazolium bromide by a langasite crystal resonator

In this work, the responses of a Y-cut langasite crystal resonator (LCR) in liquid phases were investigated by an impedance analysis method. The resonant frequency (fS) of the LCR decreases with increasing mass loading on the active surface of the resonator. The LCR can be operated at the resonant frequency that is down to about 60% of the fundamental frequency (f0) under foreign mass loading. The frequency–mass coefficient of the Y-cut LCR is theoretically derived to be −1.282 × 10−6f02, which is supported by the experimental results. The resonant frequency of the LCR decreases linearly while its motional resistance (Rm) increases linearly with increasing (ρη)1/2, where η and ρ are the viscosity and density of the liquid phase, respectively. The slopes of the plots of fS versus (ρη)1/2 and Rm versus (ρη)1/2 are related to the region of (ρη)1/2 because of the influence of surface roughness of the LCR. The changes in viscodensity of a room temperature ionic liquid (RTIL), 1-octyl-3-methylimidazolium bromide ([C8MIM][Br]), were investigated in acetone vapor adsorption and ascending temperature processes by the LCR. The adsorption of acetone into [C8MIM][Br] causes a significantly drop in viscosity of the [C8MIM][Br] film, which induces an increase in fS and a decrease in Rm for the RTIL modified LCR. When the thickness of [C8MIM][Br] film is less than the decay distance of the thickness-shear wave, a mass effect model is observed in the early adsorption process. Based on the responses of the LCR, the viscodensity of the [C8MIM][Br] film as well as the adsorbed amounts of acetone into film were monitored in real time during the adsorption or desorption processes.