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Kinetics of Dissolution and Recrystallization of Sodium Chloride at Controlled Relative Humidity†

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Kinetics of Dissolution and Recrystallization of Sodium Chloride at Controlled Relative Humidity† Kinetics of Dissolution and Recrystallization of Sodium Chloride at Controlled Relative Humidity† Marina Langlet1*, Frederic Nadaud2, Mohamed Benali1, Isabelle Pezron1, Khashayar Saleh1, Pierre Guigon1, Léa Metlas-Komunjer1 UTC/ESCOM, Équipe d’Accueil “Transformations Intégrées de la Matière Renouvelable” (EA 4297)1 UTC/ESCOM, Service d’Analyses Physico-Chimique2, Abstract Both producers and users of divided solids regularly face the problem of caking after periods of storage and/or transport. Particle agglomeration depends not only on powder water content, tem- perature and applied pressure, but also on the interactions between the solid substance and water molecules present in the atmosphere, i.e. on relative humidity (RH) at which the product is stored. Ambient humidity plays an important role in most events leading to caking: capillary condensation of water at contact points between particles, subsequent dissolution of a solid and formation of a saturated solution eventually followed by precipitation of the solid during the evaporation of water. Here, we focus on the kinetics of dissolution followed by evapo-recrystallization of a hygroscopic so- dium chloride powder under controlled temperature and RH, with the aim of anticipating caking by predicting rates of water uptake and loss under industrial conditions. Precise measurements of water uptake show that the rate of dissolution is proportional to the difference between the imposed RH and deliquescence RH, and follows a model based on the kinetic theory of gases. Evaporation seems to be governed by more complex phenomena related to the mechanism of crystal growth from a supersatu- rated salt solution. Keywords: Sodium chloride, hygroscopy, Knudsen law, vapor sorption, caking humidity, DRH, of 75% at 25℃. Examples of systems 1. INTRODUCTION at risk include also cosmetics, agricultural chemicals, Water being ubiquitous in the atmosphere, the explosives and pharmaceuticals. Caking of such pow- influence of moisture on the chemical and physical ders is generally strongly influenced by dissolution stability of many dispersed systems and its impact on followed by recrystallization of the hygroscopic solid product manufacturability, quality and shelf-life is of substance present. A crystalline substance is said to great concern. In particular, the presence of water in deliquesce if it forms an aqueous solution when the an atmosphere where formulated products contain- ambient relative humidity reaches a certain threshold ing deliquescent hygroscopic substances are manipu- value. Below this critical RH, crystal surrounded by lated can have a pronounced effect on their end-use water vapor is thermodynamically favorable1,2) while properties such as particle aggregation and ability above the critical RH, the aqueous solution is the of powder to flow. Many industrial formulations, es- thermodynamically favored phase. It is of fundamen- pecially food products, contain sodium chloride, a tal interest to understand events taking place when deliquescent substance with a deliquescence relative particles containing a hygroscopic substance are ex- posed to the atmosphere containing water vapor. For th example, it is well known that adsorbed water cannot † Accepted : September 10 , 2011 1 Rond-Point Guy Deniélou, 60200 Compiègne, France cause the dissolution of the solid substrate while 2 Rond-Point Guy Deniélou, 60200 Compiègne, France condensed liquid water can. Capillary condensation 3 1 allée du réseau Jean-Marie Buckmaster, 60200 Com- of water vapors leads to the formation of pendular piègne, France liquid bridges at the contact points between particles. * Corresponding author: Ｅ-mail: [email protected] This liquid water is likely to dissolve deliquescent TEL:（+ 33）344 234 744 crystalline substances present in the particles. Disso- ⓒ 2011 Hosokawa Powder Technology Foundation 168 KONA Powder and Particle Journal No.29 (2011) lution gives rise to the formation of a saturated solu- ture of small individual particles and agglomerates tion from which recrystallization of solute follows if of a hygroscopic substance is subjected to high RH, water is evaporated, for example due to the changes small particles might completely dissolve long be- of ambient conditions of relative humidity and/or of fore large agglomerates are dissolved (as presented temperature. Usually the re-crystallized solid bridges in step 5). This means that not all parts of the solid mechanically bind the particles in contact more ef- sample are in equilibrium with their environment. In ficiently than the pendular liquid bridges between order to better understand the kinetics of the deli- them. The mechanical strength of solid bridges quescence of hygroscopic substances under well-con- depends not only on temperature, humidity and pres- trolled ambient conditions, we propose the use of Dy- sure, but it depends also on the mass transfer, the namic Vapor Sorption (DVS) apparatus. We examine solubility of the powder in water and the number of dissolution of a solid followed by its recrystallization contact points, i.e. on the coordination number of via the precise measurements of water uptake and particles in the powder bed3). water loss at variable conditions of ambient relative A schematic representation of the adsorption of wa- humidity. In such a way we provide an original meth- ter molecules followed by capillary condensation and od for the precise determination of deliquescence dissolution of the solid phase is proposed in Fig. 1. relative humidity. Modeling the dissolution of a solid The time sequence of phenomena taking place when and the evaporation of water from a solution so ob- hygroscopic crystals are brought in contact with an tained is tempting on the basis of the kinetic theory atmosphere containing water vapor is the following: of gases with the aim of predicting the corresponding adsorption of water molecules on the solid surface at kinetics at different ambient conditions of RH similar low vapor pressure (steps 1 and 2), followed by the to those met in industrial applications. multilayer formation and capillary condensation at This work will focus on the partial dissolution and contact points/lines/surfaces at intermediate vapor recrystallization of sodium chloride, a classic model pressures (step 3). In the case of good wetting of the hygroscopic solid. Sodium chloride is present in at- solid by liquid water, capillary condensation can oc- mospheric aerosols where it represents the majority cur at quite low vapor pressures, i.e. at low RH, but of solid particles1). For that reason, a significant quan- the quantity of liquid water, determined by the so- tity of highly reliable data concerning NaCl-water called Kelvin radius, will remain small. At higher RH, binary systems, as well as various other mixtures both Kelvin radius and the corresponding quantity of relevant to atmosphere science, has been published liquid water will increase rapidly. Deliquescence of in the last few decades1,4,5). Moreover, salt dissolu- the solid takes place if RH becomes equal or higher tion in water is of particular interest also because of than DRH, i.e. when the ambient vapor pressure be- its occurrence in food and other industrial products. comes equal or higher than the vapor pressure of the Yet these formulations are often complex mixtures of saturated solution of NaCl (steps 4 to 6). When a mix- ingredients influencing each other’s behavior. Issues Fig. 1 Hygroscopic crystals in water vapors: adsorption of water vapor and condensation of liquid water followed by dis- solution of a solid (adapted from Peters4), Zasetsky5) and Mauer2)). KONA Powder and Particle Journal No.29 (2011) 169 concerning caking phenomena in multi-component of the ambient gas, P, linearly. The proportionality systems are currently under study in our laboratory. coefficient is dependent on temperature, molar mass In Fig. 2, one can find the well-known P-T diagram of the condensed phase and exchange surface. It is for pure water in the temperature range between 0℃ represented by a “Knudsen coefficient”, KKnudsen. If we and 30℃ (in dark continued line). It is completed by define the sticking-coefficient as the probability for a an analogous curve for a saturated solution of NaCl water molecule to remain at the surface after impact, in water (in light continued line). One can verify that and considering that the condensed phase has a non- the NaCl curve closely follows the shape of the pure negligible pressure, ps, the net flux of molecules, dm/ water curve, and that the vapor pressure of the satu- dt, entering the condensed phase is: rated solution of NaCl is approximately 75% to 76% of dm = K (P p ) Eq. (1) vapor pressure for pure water for this temperature dt Knudsen − s range in accordance with DRH values for sodium Moreover, as P corresponds to the relative humidity chloride cited in literature. The curves of vapor pres- and ps corresponds to the deliquescence relative hu- sure for pure water and the aqueous solution of NaCl midity, one can write: are completed by the vapor pressures corresponding dm K P = Knudsen 0 (RH DRH) Eq. (2) to 60, 70, 80 and 90% RH (dotted lines). These condi- dt 100 − tions of RH are chosen for their relevance in powder or, caking. One can also note that in this temperature dm = K P (a a∗ ) Eq. (3) range, moderate variations of vapor pressure per- dt Knudsen 0 w − w mit setting up conditions for the dissolution of NaCl where aw is the water activity of the substance in so- (when RH>DRH) and its recrystallization (when RH lution and aw the water activity of the saturated solu- <DRH). tion of the same substance at the same conditions of pressure and temperature. Note that water activity is RH Theoretical background linked to relative humidity: a = and to the im- w 100 We propose to describe the dissolution and crys- posed vapor pressure, P, via the saturated pressure tallization of NaCl on the basis of a simple model of pure water P0 taken from the kinetic theory of gases which was P aw = already successfully applied to another hygroscopic P0 salt, ammonium nitrate8,9).
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