Adsorption Isotherms of Low-Pressure H₂O on a Low-Temperature Surface Measured by a Quartz Crystal Microbalance

Abstract

The low-pressure gas in the vacuum plume produced by the chemical thrusters contaminates the spacecraft when adsorbed on the low-temperature surface. To provide theoretical support for further research on gaseous plume pollutants, the adsorption isotherms of low-pressure H 2 O were measured by a quartz crystal microbalance (QCM) at temperatures ranging from 233 to 273 K. The measured isotherms are similar to the type-I and type-II isotherms and have been correlated by various models (e.g., the Langmuir, Dubinin–Radushkevich, Brunauer–Emmett–Teller (BET), and universal models). It shows that the universal model has a great advantage in predicting the adsorption at a specific temperature point in our study. To estimate the adsorption at the continuous temperature range, the critical parameters of the multi-Langmuir model were expressed in semiempirical formulas. Since the normalized isotherms of H 2 O at different temperatures converge well, a simplified multi-Langmuir (SML) model was proposed. The experimental results at the temperature and pressure ranges we explored are consistent with the results predicted by the SML model, suggesting that the SML model is more suitable and convenient to predict the low-pressure adsorption of H 2 O for a continuous low-temperature range. Moreover, the low-pressure adsorption behaviors of H 2 O and CO 2 on the low-temperature surface are compared and discussed.

Vibrational spectroscopic evaluation of hydrophilic or hydrophobic properties of oxide surfaces

“…[11][12][13] Further, in surface science, hydrophilic or hydrophobic properties of material surfaces have been widely studied using FT-IR, [14][15][16][17][18][19] Raman spectroscopies, [19][20][21][22][23][24] desorption temperatures, [25][26][27] and adsorption capacities of H 2 O. [28][29][30][31][32] However, when we discuss hydrophilic or hydrophobic properties by surface wettability, which is determined by the contact angles of a water droplet, a "clean" silica surface shows high wettability. [33][34][35][36] This is a reason for considering silica surfaces as hydrophilic.…”