Real-Time Crystal Growth Monitoring of Boric Acid from Sodium or Lithium Sulfate Containing Aqueous Solutions by Atomic Force Microscopy

Abstract

The crystal growth of boric acid from an aqueous solution in the absence and presence of sodium and lithium sulfate was studied by real-time monitoring. For this purpose, atomic force microscopy in situ has been used. The results show that the growth mechanism of boric acid from its pure and impure solutions is spiral growth driven by screw dislocation and that the velocity of advancement of steps on the crystal surface, and the relative growth rate (ratio of the growth rate in presence and absence of a salt) is reduced in the presence of salts. The reduction of the relative growth rate could be explained by the inhibition of advancement of steps of the (001) face mainly in the growth direction [100] caused by the adsorption of salts on the actives sites and the inhibition of the formation of sources of steps such as dislocations. The adsorption of the salts on the crystal surface is anisotropic and independent of the supersaturation and preferentially on the active sites of the (100) edge. Moreover, this information is of significance for the improvement of the quality of boric acid recovered from brines and minerals and the synthesis of nanostructures and microstructures of boron-based materials.

Figure 1

Schematic diagram of the experimental setup for single-crystal growth of boric acid. Modified from refs (42, 43). Reprinted (adapted) with permission from Gou, L.; Lorenz, H.; Seidel-Morgenstern, A. Investigation of a Chiral Additive Used in Preferential Crystallization. Cryst. Growth Des. 2012, 12 (11), 5197–5202. 10.1021/cg300042q. Copyright 2023 American Chemical Society.

Figure 2

Scheme of the estimation of the growth rate for the (001) face from a supersaturated solution at σ = 0.02 and 19 °C, using the optical images acquired in the single crystal growth cell.

Figure 3

Setup of the AFM open flow-through cell for crystal growth measurement.

Figure 4

(a) Boric acid crystal morphology predicted with WinXMorph^, from the crystalline structure reported by Zachariasen. (b) Single crystal of boric acid grown by evaporation from aqueous boric acid solutions saturated at 20 °C.

Figure 5

Single crystal growth of boric acid from a supersaturated solution at σ = 0.02 and 19 °C.

Figure 6

(a) Growth dynamics for the (001) face growth of boric acid from a supersaturated solution at σ = 0.02 and 19 °C. Time t: -●- (red), 0 h; -●- (green), 1 h; -●- (gray), 2; -●- (blue), 3 h; -●- (purple), 4 h; -●- (yellow), 5 h; -●- (sky blue), 6 h; -●- (golden), 7 h; and -●- (black), 8 h. (b) Resulting growth rate for the (001) face growth of boric acid in the growth directions: ● (red), [010]; ● (blue), ; ● (light green), ; ● (dark green), ; ● (purple), ; -●- (sky blue), [100].

Figure 7

(a) Topography of the (001) face of a single crystal of boric acid taken by AFM in air at room temperature. (b) Step edges and (c) profiles obtained from the AFM image given in (a). (d) Unit cell of boric acid elaborated with VESTA 3 using the crystalline structure reported by Zachariasen. For the boric acid molecule, the boron, oxygen, and hydrogen atoms are represented by green, red, and light red colors, respectively.

Figure 8

Evolution of steps on the (001) face of boric acid grown from its aqueous solutions at 23 °C and supersaturation σ = 0.07. Topography (gold color) and edges (gray color) at time t: (a) 8.01, (b) 9.05, (c) 12.17, and (d) 13.21 min.

Figure 9

Tracking of the steps’ edges on the (001) face of boric acid grown from its aqueous solutions at 23 °C and at σ = 0.07. Time t: (a) 8.01 and (b) 9.05 min.

Figure 10

(a) Advancement of steps’ edges on the (001) face of boric acid grown from its aqueous solutions at 23 °C and supersaturation σ = 0.07. Point changes are given for time increase from 8.01 to 12.17 min. (b) Velocity of advancement of steps v in the direction. (c) Velocity of advancement of steps v in the [100] direction.

Figure 11

Tracking of the width of the terraces λ and height of the steps h on the (001) face of boric acid during growth from its aqueous solutions at 23 °C and supersaturation σ = 0.07, at time t = 9.05 min.

Figure 12

Evolution of steps on the (001) face of boric acid grown from its aqueous solutions in the presence of sodium sulfate (5 wt %) at 23 °C and supersaturation σ = 0.07. Topography (gold color) and edges (gray color) for time intervals of 64 s (a–d).

Figure 13

Evolution of steps on the (001) face of boric acid grown from its aqueous solutions in the presence of lithium sulfate (5 wt %) at 23 °C and supersaturation σ = 0.07. Topography (gold color) and edges (gray color) for time intervals of 64 s (a–d).

Figure 14

Advancement of steps on the (001) face of boric acid at 23 °C and supersaturation σ = 0.07 at different salt concentrations c_imp of sodium sulfate: (a) 1, (b) 5, and (c) 16 wt %, and lithium sulfate at (d) 1, (e) 5 and (f) 8 wt %, and average velocity of advance of steps v of the (001) face in the directions (g) [100] and (h) at the same salt concentrations, supersaturation, and temperature stated above. ---, trend lines. —, experimental bar lines.

Figure 15

Relative growth rate G/G_o of boric acid in the presence of, c_imp, in wt % of Na₂SO₄, ●(red), and Li₂SO₄, ●(blue), at 23 °C and σ = 0.07.—, calculated from eq S7 for the (001) face in directions: (a) [100] and (c) . The coverage of active sites by an impurity, θ_l, for the boric acid crystal surface in the presence of, c_imp, Na₂SO₄, ●(red), and Li₂SO₄, ●(blue), —, calculated from eq S2 at 23 °C and σ = 0.07 for the (001) face in directions: (b) [100] and (d) . Shaded regions indicate 95% confidence intervals for the model forecast. —, experimental bar lines.