Drying of Salt Solutions from Porous Media: Effect of Surfactants

Abstract

The evaporation of salt (NaCl) solutions from porous media is studied in the presence of surfactants, because surfactants are often used as cleaning agents for salt-contaminated stones. We show that, contrary to what is commonly assumed, the presence of the surfactant and the changed wetting properties do not affect the drying kinetics: The impact of the surfactants is rather that of a crystallization modifier for the salt. Upon adding a cationic or nonionic surfactant to salt solution, the drying rate is unchanged initially, but can slow down dramatically at later times due to the formation of a salt crust at the surface. When this happens, the total drying time increases compared to pure NaCl solutions without surfactants, at least for very porous stones for which the pores become completely blocked. Surprisingly, for a low-porosity stone the small pores at the surface remain open. The longer drying time for the large porosity stone increases the risk of, e.g., frost or fungal damage to the stones. Consequently, the use of surfactants in conservation treatments should be done with caution.

Salt creeping as a self-amplifying crystallization process

“…To confirm our hypothesis and gain further insight into the creeping mechanism, we take two different types of additives (surfactants) known to affect salt crystallization and test their influence on the creeping of NaCl. We use CTAB (cetyltrimethylammonium bromide), which is cationic, and Tween 80, which is nonionic; these have been shown to act as either nucleation promoters or inhibitors for salt crystallization, rather than changing the wetting properties of these highconcentration salt solutions (18,19). Although the addition of surfactants to the salt solutions lowers their surface tension, it does not substantially affect the initial contact angle of the liquid with the substrate.…”

“…On the other hand, Tween 80 is a nonionic surfactant, and consequently, it does not interact with the ions in the solution. One notable consequence of this is that adding NaCl in a Tween 80 solution does not change the CMC (critical micelle concentration) of this surfactant, whereas for CTAB, adding salt reduces the CMC, because the electrolytes screen the electrostatic interactions between the polar heads of this surfactant (18,19). The adsorption of CTAB at both interfaces (liquid/air and solid/liquid) then promotes the nucleation there, while the adsorption of Tween 80 inhibits it.…”

Self-Lifting NaCl Crystals

“…The physical mechanism of the formation of legs is likely to be similar to the growth in very thin films at high evaporation rates, as seen in microgravity 18 and in porous media. 19 In our experiments, at the last stages of evaporation, when cubic hollow crystals point toward the surface, tiny capillary bridges form, and it is only in this thin liquid film around the points of the macrocrystals that there is still salt solution from which salt can precipitate; see the Supporting Information for more details.…”

A low‐cost environmental chamber to simulate warm climatic conditions

“…Environmental chambers (also termed climate or climatic chambers) enabling temperature and relative humidity regulation during laboratory experiments are required in a range of disciplines, including ecology, geology, and hydrology. They are, for example, used to study salt weathering of rocks (Goudie & Parker, 1998), nutrient leaching in soils (Grant, Macrae, Rezanezhad, & Lam, 2019), and water repellency of soils (Jiménez‐Pinilla et al., 2016), or in evaporation studies (Huang, Bruch, & Barbour, 2013; Merz et al., 2018; Qazi, Bonn, & Shahidzadeh, 2018; Shokri‐Kuehni, Norouzi Rad, Webb, & Shokri, 2017; Song, Cui, Tang, Ding, & Tran, 2014). Moreover, environmental chambers serve to test monitoring equipment (Papapostolou, Zhang, Feenstra, & Polidori, 2017; Prechsl, Gilgen, Kahmen, & Buchmann, 2014; Windhorst, Waltz, Timbe, Frede, & Breuer, 2013).…”