Crystal Growth in Confinement studied in a Model Slit Pore

Precipitation of salts in confined space is the key mechanism for rock weathering, and damage to building materials. If a repulsive force acts between the growing crystal in a pore and the pore wall, a thin layer of aqueous solution remains between crystal and pore wall, which permits diffusion of the ions to the growing crystal surface [1]. Supersaturation of this confined solution is responsible for the crystallization pressure [2] that leads to tensile stresses in the material and eventually to material failure. Due to the high ionic strength of the solution, electrostatic double layer forces are screened, and the repulsion results mainly from structural forces between crystal and mineral surface, which depend on surface and confined thin film properties.

In this work we investigate the properties of nano-confined potassium nitrate (KNO3) solution in a model slit pore in a broad range of concentrations with an extended surface force apparatus (eSFA). The improved distance resolution of the eSFA allows the distinction of hydration structures and hydrated-ion correlations in confined films with thickness smaller than 3 nm that point at a short-range ordering of hydrated ions induced by confinement (Figure 1). We have also performed X-Ray reflectivity measurements combined with eSFA to resolve the structural features of this confined thin film (papers in preparation). Both our experimental results [3] and simulations [4] indicate that confinement-induced ion dehydration causes the concentration of the nano-confined film to be higher than that of the bulk solution during the experimental time. Our results also show that already at intermediate concentrations (100 mM) an attractive interaction leads to an energetic minimum at a film thickness of ≈ 2 nm, followed by a strong hard wall repulsion. Indeed, at salt concentrations below the saturation limit, the formation of an ionic condensate was observed whose nature is being investigated. The interfacial properties of mica change with concentration as indicated by the variation of the pull-off force (Figure 2).

The knowledge derived from this fundamental work will be relevant for future applications in building sustainability, conservation of cultural heritage, and damage protection of roads and tunnels by ice heave and salt. Indeed, the improvement of the durability of construction materials used in civil buildings, roads and tunnels is an important factor for a sustainable environment (alone the maintenance of roads in USA costs an estimated $50 billion per year).

Enlarged view: Figure 1 (from [3]): Film-thickness transitions measured at different salt concentrations. The gray background areas correspond to the expected diameter of a hydrated ion with the first hydration shell, and, the radius of a single hydration shell (cf. the size of water molecule). No transitions are observed in the DLVO regime where the van der Waals instability masks detection of any weaker structural correlation force. For comparison, the typical distance-measurement precision for conventional SFA data and the eSFA data presented here are illustrated on the left as error bars. The scatter of the transitions shown here is therefore real and contains physical information. Nonetheless, we note here that the final transition observed in many double-layer compression cycles is characteristically smaller (2.9 ± 0.3 A ) than all the others and interpreted as forced surface association of hydrated ions.
Figure 1 (from [3]): Film-thickness transitions measured at different salt concentrations. The gray background areas correspond to the expected diameter of a hydrated ion with the first hydration shell, and, the radius of a single hydration shell (cf. the size of water molecule). No transitions are observed in the DLVO regime where the van der Waals instability masks detection of any weaker structural correlation force. For comparison, the typical distance-measurement precision for conventional SFA data and the eSFA data presented here are illustrated on the left as error bars. The scatter of the transitions shown here is therefore real and contains physical information. Nonetheless, we note here that the final transition observed in many double-layer compression cycles is characteristically smaller (2.9 ± 0.3 A ) than all the others and interpreted as forced surface association of hydrated ions.
Enlarged view: Figure 2 (from [3]): (a) Pull-off force measured from maximally loaded mica–mica contact as a function of the concentration of KNO3 solution. Each result (dot) is the average of at least 3 measurements; most of the error bars are smaller than the symbol. The superimposed gray line is to guide the eye. A sudden decrease is observed between 200 mM and 1 mM concentration. The significant data scattering in the DLVO regime results from the influence of the particular properties of the mica surfaces in each experiment. (b) Schematic representation of hydrated-ion soft-sphere layering transitions of D = 4 ±1 A˚ in the ‘‘ordering’’ regime and equilibrium adsorbed ions. (c) Forced surface association giving rise to the final D=2.9 ± 0.3A film thickness transition.
Figure 2 (from [3]): (a) Pull-off force measured from maximally loaded mica–mica contact as a function of the concentration of KNO3 solution. Each result (dot) is the average of at least 3 measurements; most of the error bars are smaller than the symbol. The superimposed gray line is to guide the eye. A sudden decrease is observed between 200 mM and 1 mM concentration. The significant data scattering in the DLVO regime results from the influence of the particular properties of the mica surfaces in each experiment. (b) Schematic representation of hydrated-ion soft-sphere layering transitions of D = 4 ±1 A˚ in the ‘‘ordering’’ regime and equilibrium adsorbed ions. (c) Forced surface association giving rise to the final D=2.9 ± 0.3A film thickness transition.

References

  1. (a) Correns, C. W., Growth and Dissolution of Crystals under Linear Pressure. Discuss Faraday Soc 5 (1949), 267-271; (b) Flatt, R.; Steiger, M.; Scherer, G., A commented translation of the paper by C.W. Correns and W. Steinborn on crystallization pressure. Environ Geol 52 (2007), 187-203.
  2. (a) Steiger, M., Crystal growth in porous materials - I: The crystallization pressure of large crystals. Journal of Crystal Growth 282 (2005), 455-469; (b) Steiger, M., Crystal growth in porous materials - II: Influence of crystal size on the crystallization pressure. Journal of Crystal Growth 282 (2005), 470-481.
  3. Espinosa-Marzal, R. M. , Drobek, T., Balmer, T., Heuberger, M.. Hydrated ion ordering in electrical double layers, Phys Chem Chem Phys 14 (2012), 6085-6093.
  4. Malani, A.; Ayappa, K. G.; Murad, S., Effect of confinement on the hydration and solubility of NaCl in water. Chemical Physics Letters 431 (2006), 88-93.
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