Drying of Salt Solutions from Porous Media: Effect of Surfactants
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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.
KeywordsDrying kinetics Surfactants Efflorescence Sandstones Crystallization modifiers
Drying of saturated porous media is a widely studied phenomenon owing to its relevance to soil mechanics, food production, carbon dioxide sequestration, etc. (Rad et al. 2015; Eloukabi et al. 2013; Shahidzadeh-Bonn et al. 2007; Yiotis et al. 2003; Coussot 2000). Recently, it also received significant attention from the fields of building physics and cultural heritage conservation (Desarnaud et al. 2015; Derluyn et al. 2014). Buildings made of porous materials often become saturated by water along with salts from various sources like rain, groundwater uptake, marine sprays, salts sprayed for deicing of roads and cleaning processes. This invasion of salt solutions is eventually followed by drying and concomitant crystallization of the dissolved salts. The latter significantly influence the drying kinetics of porous structures, and previous studies have reported counterintuitive trends in drying kinetics when salts are present (Veran-Tissoires et al. 2012; Desarnaud et al. 2015; Shokri-Kuehni et al. 2017; Bergstad and Shokri 2016; Bergstad et al. 2017). Depending on the environmental conditions, the salt precipitation can alter the drying process by forming a salt crust that blocks the evaporation from the core of the stones (Desarnaud et al. 2015; Gupta et al. 2014; Grementieri et al. 2017). This often happens at low relative humidity, where the initial drying is rapid. On the other hand, for drying at higher relative humidity, different salt precipitation dynamics at the surface enables continuous hydraulic conductivity and can even enhance the evaporation process (Desarnaud et al. 2015; Gupta et al. 2014; Shokri-Kuehni et al. 2017; Veran-Tissoires et al. 2012). This then leads to the counterintuitive result that the total drying time can be longer at a low relative humidity. Other than environmental conditions, salt crystallization is also affected by the nature of the salt and the porous material, like its porosity, pore size distribution, and wetting properties (Rad et al. 2015; Eloukabi et al. 2013; Bergstad and Shokri 2016; Bergstad et al. 2017).
In addition, the environmental fluctuations, which lead to repeated cycles of drying and wetting, have been identified as a major cause of deterioration of building structures. The crystallization, which takes place during the drying process, can occur either on the surface of the stones in the form of ‘efflorescence’ or inside the porous structure known as ‘subflorescence’ (Nachshon et al. 2011). Both these modes of crystallization are detrimental to the mechanical strength and aesthetics of the buildings. The aspect of damage by subflorescence and efflorescence has been investigated in significant detail (Desarnaud et al. 2013, 2016; Flatt et al. 2014; Steiger 2005; Veran-Tissoires et al. 2012; Sghaier and Prat 2009). However, little attention has been invested in accounting for the damage resulting from the prolonged entrapment of moisture in porous structures. In this context, the drying kinetics of building structures is an important subject, as the time for which the stones remain saturated with water contributes to the susceptibility of these structures to frost damage (Winkler 1968) and biological attack from various algae (Vojtková 2017). Altering various parameters like the wettability of the porous media, pore size distributions and different environmental conditions have thus revealed very useful insights into the drying kinetics of porous stones yielding prospects to control it favorably.
Surfactants are popular ingredients in cleaning products. As a result, they inevitably end up in the buildings along with salts. Surfactants are amphiphilic compounds with the versatile property of adsorbing at the interfaces of different phases (hydrophilic and hydrophobic). In this way, they are known to lower the liquid/vapor and solid/liquid surface tensions and improve the spreading properties of the liquid.
However, apart from wettability enhancers, surfactants can also act as crystallization modifiers (Rodriguez-Navarro et al. 2000; Sonin et al. 1998; Myatt et al. 1994; Qazi et al. 2017; Chen et al. 2015). Surfactants, by virtue of their ability to adsorb at the interfaces, can preferentially adsorb on selective crystal faces, thereby altering their overall shape, size and other properties. Surfactants have also been shown to act as nucleation inhibitors; because they can passivate impurities by absorbing on them, they can induce high supersaturations (Qazi et al. 2017) prior to salt precipitation. The latter can lead to serious consequences as the crystals forming at higher supersaturation could eventually exert an even higher crystallization pressure (Flatt et al. 2007; Espinosa-Marzal and Scherer 2010). This potential of additives to influence the crystallization phenomenon has drawn the interest of researchers from the crystallization field to study surfactants more extensively (Lubelli and van Hees 2007). Notably, different surfactants have a different influence on the wettability (Afsar-Siddiqui et al. 2004; Beacham et al. 2009) and the crystallization behavior of salts; hence, the choice of surfactants to be used in cleaning product is of crucial importance. Despite their very common use and ubiquitous presence, the influence of surfactants on the drying of porous media has not been studied so far. In this study, we test the influence of different surfactant types on the drying of sodium-chloride-contaminated sandstones with a high (Mesne sandstone) and a low porosity (Fontainebleau sandstone).
Characteristics of sandstones used in this study obtained by mercury intrusion porosimetry
Porosity ϕ (%)
Pore diameter Dp (μm)
Vp/Se of samples
The influence of surfactants on the drying of saline solutions in sandstones was investigated by measuring the evaporation rate of surfactant–salt solution in the sandstones while keeping the pure NaCl solution as control. NaCl of 99% purity as supplied by Sigma-Aldrich was used to make an aqueous solution close to solubility [5.5 m, the molality (m) is defined as the number of moles of solute per kilogram of solvent]. Only those surfactants were used that were stable at 5.5 m NaCl solution, as reported in an earlier study (Qazi et al. 2017). The two surfactants used were: cationic surfactant cetyltrimethylammonium bromide (CTAB) (critical micelle concentration CMC = 0.9E−3 M, 99% purity, Sigma-Aldrich) and a nonionic surfactant Tween 80 (CMC = 10 mg/l, proteomics grade purity, Amresco Ltd). We have been unable to find an anionic surfactant that remains stable in solution for these high concentrations of salt. Firstly, pure surfactant solutions at the critical micelle concentration were prepared with demineralized water from millipore. Subsequently, the ternary surfactant–salt solutions were made by adding NaCl corresponding to 5.5 m to the prepared surfactant solutions. After preparation, the solutions were stirred and left to stabilize for at least one night. The liquid/air surface tension (γlv) of these solutions was measured using Kruss EasyDrop tensiometer. The addition of surfactants reduces the surface tension of the NaCl solution from 81 to 32 mN/m in the presence of CTAB and to 40 mN/m in case of Tween 80. In principle, the change in surface tension translates into change in the wettability. However, the change in wettability strongly depends upon the surface properties of the substrate. In this case, we could not measure contact angles on the pure material of stone itself because of the pores. However, on a glass substrate (similar chemistry to sandstone), the contact angle of 5.5 m NaCl was around 25 degrees. When CTAB was present with NaCl, the contact angle was measured to be around 33 degrees and with Tween 80, it was around 23 degrees. Hence, it can be said that surfactants, when present in high salt concentrations, do not change the contact angles significantly.
The drying experiments were performed after saturating the sandstone samples with the solutions by capillary imbibition using a vacuum pump. The samples were then left to dry (except from the bottom side) on a mass balance with enclosures, installed in a climatic chamber while recording the mass at intervals of 30 s. To ensure a minimum directional flow of the air around the sample, the air supply to the climatic chamber was installed at the lower part of the chamber and from opposite sides; the experiments were done at a constant temperature (21 °C) and at two different relative humidities: 60% and 20%, representative of summer and winter weather conditions, respectively. In order to determine the influence of surfactants alone on the drying rate, control experiments using pure water–surfactant solutions were also carried out. All the experiments were performed at least 5 times in order to check the reproducibility of the drying mechanisms.
After completion of the drying process, the morphology of the efflorescence was imaged using a Hitachi tabletop scanning electron microscope (SEM). In order to visualize the occurrence of subflorescence, the samples were carefully cracked open and the inside of the samples was imaged by SEM.
3 Results and Discussion
3.1 Prague Sandstone (High Porosity)
3.1.1 Drying at Summer Weather Conditions (RH 60%)
In contrast to pure NaCl, when surfactants are added to the salt solution, the total drying of the sample slows down dramatically. With surfactants, the drying mechanism results in the emergence of three drying stages (capillary, exponential, diffusive) as it has been observed previously for evaporation of pure salt solutions but at low relative humidity (Desarnaud et al. 2015), as shown in Fig. 1. However, here because the exponential regime remains very small compared to the two other regimes, we will focus more on the capillary (first regime) and diffusive regimes (third regime) in the rest of analysis of the results. In the first capillary stage, 60% (Sr ~ 0.4) of the initial salt solution evaporated with a constant drying rate (CRP) equivalent to the pure NaCl solution dm/dt ~ 0.2 mg/min), irrespective of the surfactant. However, beyond this stage, surprisingly the drying slows down dramatically and the rate is no longer constant.
According to this model, the limiting factor for the drying kinetics is the presence of a salt crust, and the water vapor diffusion through this crust, so that the relative humidity in the environment becomes less relevant.
Upon fitting our experimental data with the diffusive transport model, we obtain again a very good agreement (Fig. 1, right).
The results confirm that although the presence of surfactants in the solution lowers the surface tension, it does not influence the evaporation rate of water and their main effect is on the salt precipitation dynamics.
3.1.2 Drying at Winter Weather Conditions (RH 20%)
The presence of surfactants combined with rapid evaporation (at low relative humidity) clearly accelerates the development of a crust. As it is shown in Fig. 5 (right), in the initial stage, drying appears to be unaffected by the surfactants and leads to the rapid evaporation of about 60% of the water that was initially present. At later times (the ‘crust stage’), the drying process slows down much earlier compared to pure NaCl. This can be understood from the observation that the presence of surfactants (as impurities) combined with rapid evaporation leads to the formation of more crystallites, each of them grows laterally and makes quickly an extensive crust of salt, thereby blocking the pores more rapidly (Fig. 6).
3.2 Fontainebleau Sandstone (Low Porosity)
3.2.1 Drying at Summer Weather Conditions (RH 60%)
The very limited influence of surfactants on drying kinetics is also related to the characteristics of the stone: Small pores and low porosity allow the salt solution to come to the surface of the stone by capillary forces where lower porosity leaves more surface area for the evaporation. The larger surface area also means that there is enough solid surface for the salt to crystallize without blocking the pores.
3.2.2 Drying at Winter Weather Conditions (RH 20%)
The mechanism of salt crust formation is shown to be due to the lateral steps growth of crystallites at the surface with defective junctions between them. Here, we show that such mechanism of growth occurs more readily when the drying process is accelerated by a low RH or when impurities such as surfactants are added to the solution. In many practical situations, detergents are used to clean salt-contaminated stones. Our results reveal that after adding a cationic or nonionic surfactant, the drying rate initially is unchanged but can slow down dramatically due to the formation of a salt crust even at high relative humidity. When this happens, the presence of the surfactant increases the total drying time when compared to pure NaCl solutions dried under the same conditions.
Compared to pure water or pure salt solution, the addition of surfactants reduces the equilibrium capillary height due to the lower surface tension. However, during drying, surfactants can rather stabilize the wetting film during the retraction of the liquid and therefore do not change the evaporation rate at the surface during the constant rate period. Hence, the impact of the surfactants is rather that of a crystallization modifier.
For stones with high porosity, with lower solid surface area, these additives can exacerbate the salt crust effect: By modifying the crystallization dynamics of the NaCl during drying, the water evaporation switches from being limited by capillary action through pores to being limited by diffusion through the salt crust covering the surface and subsurface. The water loss due to evaporation in the third stage of drying after the crust formation scales with the square root of time for both relative humidities, as is expected for diffusion-limited vapor transport through the crust.
For stones with low porosity, with much higher solid surface area (> 90%), the surfactants stabilize the wetting film covering the grains and induce a homogeneous crystals precipitation on the surface of grains. In addition, because of higher capillary flux due to smaller pores, the pores remain open at the surface.
Lowering the RH in all cases promotes the lateral growth of crystals, which will decelerate the evaporation due to the gradual formation of crust.
Surfactants are often used as cleaning agents. Slow drying can be detrimental for building materials as it makes them more susceptible to frost and biological damage. The properties of cleaning products should therefore be carefully considered in the context of conservation strategies in relation to the petrophysical properties of the stone and the environmental conditions.
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