Water-solid interactions: Crystalline powder characterization and development of atomic force microscopy techniques

Moisture induced changes impact the stability of powder systems. Increasing water content influences the properties (chemical stability, caking, flowability, etc.) of solid materials through five major mechanisms: adsorption, absorption, hydrate formation, deliquescence, and capillary condensation. The deliquescence relative humidity (RHomix) in mixtures is lower than the individual ingredient deliquescence points. Capillary condensation occurs when water condenses to liquid within a capillary and is hypothesized to initiate deliquescence lowering. Water-solid interactions and moisture acquisition of non-nutritive sweeteners (NNS) with bulking agents were evaluated using a gravimetric sorption isotherm. Functional implications of formulation (pure, midpoint, eutonic composition), particle size (50, 150, ∼500μm), and storage time (0-28 days) on powder flow were examined with Revolution powder analysis. Atomic force microscopy investigated the surface forces on sucrose, citric acid, fructose, and sodium chloride with humidity. All NNS exhibit surface moisture adsorption. Aspartame, neotame, and Na-saccharin formed hydrates at 55%, 60%, 85%RH, respectively, and Na-saccharin deliquesces at 92% RH. Acesulfame-K and sucralose remained constant up to 95% RH. Blends of NNS with crystalline bulking agents reduced moisture while amorphous mixtures increased the moisture gained. The amount gained or lost varied with storage humidity, mixture composition, and ingredient ratio. Decreasing particle size increased the moisture acquired by capillary condensation. In general, relative humidity at 10% below the RHomix , increasing storage time, mixtures formulations, and decreasing particle size reduced powder flow. SEM imaging of citric acid and sucrose revealed increased coalescence with decreasing particle size when stored close to RH omix. Adhesions between crystals were higher at low and high humidities potentially by van der Waals forces and capillary forces, respectively. In mixtures, adhesion forces for contacting component at the RHomix were comparable to individually measurements at RHo. AFM determined deliquescence lowering is initiated by capillary condensation. Sucrose probe experiments revealed that capillary condensation between the surface of citric acid, fructose, and sodium chloride and the probe occurred within the predicted range at 300 to 400 nm. Food ingredients provide favorable protection or unfavorable increases in moisture and several factors cause physical degradation in powder formulations. This study also determined that deliquescence lowering is initiated by capillary condensatio