Effect of Humidity Cycling on Salt Hydration and Dehydration: Quantitative Image Analysis and Kinetic Interpretation

Humidity strongly governs crystallization, deliquescence, and phase stability in hygroscopic salts, yet their dynamic response to cyclic humidity changes remains poorly quantified. Here, we investigate the hydration and dehydration behavior of potassium carbonate (K2CO3) and ammonium nitrate (NH4NO3) droplets under controlled humidity cycling using a noninvasive, imaging-based method. Quantitative analysis of the time-resolved intensity and morphology reveals distinct nucleation pathways, reversible phase transitions, and cycling-dependent memory effects. Crystallization and dissolution kinetics were analyzed using Avrami, Tobin, and first-order formalisms, showing that Tobin’s exponent effectively captures impingement-limited and cooperative growth regimes. The results demonstrate that humidity cycling provides mechanistic insight into salt–water systems by showing that (i) nucleation begins only after a distinct supersaturation buildup and at reproducible relative humidity (RH) thresholds, (ii) crystal growth proceeds through a rapid, front-propagating burst once the threshold is crossed, (iii) deliquescence occurs sharply and reversibly at a higher RH than crystallization, and (iv) hydration and dehydration follow different RH pathways, producing clear hysteresis in both intensity and area evolution. This work establishes humidity-controlled imaging as a versatile platform for investigating dynamic phase behavior relevant to environmental processes, material stability, and evaporite pattern formation.