Theoretical and Experimental Investigation on the Moisture Sorption Kinetics of a PVA/LiCl Composite Membrane in a Dynamic Humidity Environment

Abstract

The PVA/LiCl composite membrane is an important material with a selective permeation to moisture. In this study, the sorption kinetics characteristics of the PVA/LiCl composite membrane are investigated in a dynamic humidity environment. The sorption kinetics curves of the PVA/LiCl composite membranes with different LiCl contents were measured in the range from 10 to 90% relative humidity, and the kinetics characteristics of the sorption components of the total moisture uptake were analyzed with a triple-mode sorption model. The results show that the process of pooling adsorption occurs only when the amount of Henry absorption exceeds a threshold. With the increase in LiCl content, the threshold gradually decreases, indicating that introducing LiCl in the membrane mainly affects the process of pooling adsorption. In the desorption part of the kinetics curve, the amounts of Henry absorption and pooling adsorption gradually decrease, while the moisture adsorbed by the hydrophilic hydroxyls, i.e., the amount of Langmuir adsorption, is still largely retained in the membrane, resulting in the water-retaining performance in a low-humidity environment. It is noteworthy that in a high-humidity environment, the amounts of Henry absorption and Langmuir adsorption are almost evenly distributed in the film, while the amount of pooling adsorption near the surface is significantly higher than that at the center. The reason is that with the increase in humidity, the rate of pooling adsorption near the surface increases significantly, resulting in the majority of the moisture diffusing into the dense membrane clusters near the surface. The established theoretical method and model parameters can be adopted to predict the moisture sorption and desorption processes of the PVA/LiCl composite membrane in a dynamic humidity environment.

Wearable Supercapacitive Temperature Sensors with High Accuracy Based on Ionically Conductive Organogel and Macro‐Kirigami Electrode

“…As previously reported in the literature, [82,83] PVA hydrogels experienced dehydration while being stored, without hermetic sealing, in ambient environmental conditions. [84,85] To improve the reproducibility of our supercapacitive sensors, Kapton tape was used to encapsulate the sensors to prevent absorption and desorption of water by the NaClorganogel from the ambient atmosphere. Both unencapsulated NaCl-organogels and the encapsulated NaCl-organogel sensors were placed in the lab environment (23 °C, Relative humidity 55%) and recorded their weight changes for 24 h. Figure 3c shows the weight loss of unpackaged NaCl-organogel with various water contents.…”

Critical Role of Boundary Conditions in Sorption Kinetics Measurements

“…There is a similar issue for the characterization of the sorption kinetics of molecules onto the solid surface of various porous materials such as silica particles or composites sorbents, , TiO2 films, aluminophosphate layers, metal–organic framework, , resins, polymer composites, , zeolites, carbon, or of water penetration in clay or cereals …”

“…The knowledge of the kinetics of the sorption process, i.e., the time variation of the absorbed mass under specific conditions, is also essential as it provides the characteristic time for a material to become wet (with bound water) under a high RH, to dry under a low RH, or the characteristic time needed for a textile or construction material to absorb the humidity of a room. There is a similar issue for the characterization of the sorption kinetics of molecules onto the solid surface of various porous materials such as silica particles or composites sorbents, 12,13 TiO2 films, 14 aluminophosphate layers, 15 metal−organic framework, 13,16 resins, 17 polymer composites, 18,19 zeolites, 20 carbon, 21 or of water penetration in clay 22 or cereals. 23 Typically, in order to measure the sorption kinetics, a sample of material is placed in a climatic chamber supposed to be able to maintain the air around the sample at a given value of RH, i.e., n ∞ , while the material has been prepared under a different RH, i.e., n 0 .…”

“…17,58 This leads to interpreting sorption kinetics data, taking into account the diffusion process inside the material along with the exchange with the solid surface. 13,[16][17][18]21,58 However, in those cases, simplified boundary conditions (such as constant adsorbate concentration along the free surface of the material) are generally assumed despite the complex shape of the sample (typically in the form of pellets or powder) and the poor control of the fluid flux along the sample.…”

“…Actually, this point has already been well identified within the framework of sorption of pollutants in porous materials, leading to the conclusion that a sorption process relies on several steps: transport of adsorbate in the bulk solution around the particles, diffusion of adsorbate inside the pores, and surface adsorption or desorption. , This leads to interpreting sorption kinetics data, taking into account the diffusion process inside the material along with the exchange with the solid surface. ,− ,, However, in those cases, simplified boundary conditions (such as constant adsorbate concentration along the free surface of the material) are generally assumed despite the complex shape of the sample (typically in the form of pellets or powder) and the poor control of the fluid flux along the sample.…”