*5.6. Additives*

The presence of additives in the reaction mixture may affect the precipitation by changing the solubility of a forming substance, influencing the reactive crystallization rate, nucleation, and crystal growth; the selective stabilization of a less stable polymorph; and the morphology of the forming crystals. The selected compounds that were additives in the reaction mixture used for the precipitation of calcium carbonate and their effect on the formation of vaterite are presented in Table 3.

The selection of the solvent for the precipitation of calcium carbonate has been intensively investigated. The addition of an organic solvent may change supersaturation [80], because usually both the solubility of each polymorphs and the activities of ions decreases. Therefore the promotion of vaterite precipitation in aqueous solutions of organic solvents is frequently reported [10,31,73,74]. Moreover, organic solvent and water molecules may be inhomogeneously dispersed. Such phenomena are observed, e.g., in ethanol-water mixtures, and are enhanced by an increase of the ethanol concentration and the addition of an inorganic salt [81]. Another feature of an aqueous ethanol solution is the ability to perform selective solvation as confirmed by a molecular dynamic simulation [81]. In this case, carbonate ions were mostly solvated by water and hardly solvated by ethanol, while calcium ions were solvated by both water ethanol molecules. These can lead to changes in the morphology of the precipitated particles of vaterite. As mentioned before, the addition of an organic solvent to the reaction mixture increases the supersaturation of the solution, hence the formation of vaterite particles considerably smaller in size is observed [10,74]. This is a result of the higher nucleation rate that provides a large number of nucleation sites in these systems. Moreover, organic solvent molecules that have negatively charged hydroxyl groups can adsorb at the forming vaterite surface, change the surface energy of the vaterite, and, as a result, stabilize these phases, thus preventing its transformation into more stable forms as has been reported for vaterite precipitation using the solvothermal method [23].

The formation of the adsorption layer at the produced crystals at the early stage of precipitation by different organic molecules added into the solution is often raised in the discussion of the role of organic additives. The adsorbed layer can inhibit the dissolution step of vaterite that is attributed to the remaining more unstable phases. If the adsorption energy of organic molecules at the solid surface interface is not enough to overcome the hydration energy of the hydrophilic part of these molecules, they are removed from the crystal surface. Then, the precipitated vaterite is re-dissolved into water and a more stable calcite is formed by the recrystallization process [82].


**Table 3.** The influence of selective additives on vaterite precipitation.

Abbreviations: Solv.—a solvothermal method; G-S—a gas-slurry system, ta—an ambient temperature; EDTMPA—ethylenediamine-tetrakis-N,N,N,N,-(methylenephosphonic acid); SDSN—sodium dodecylsulfonate; SDBS—sodium dodecylbenzenesulfonate.

The stabilization of a vaterite polymorph was reported for alcohols, polyalcohols [10,23,31,81], and aminoacids [82,86]. Vaterite particles were produced in the presence of aminoacids. Both polar interactions from the hydrophilic groups of additives and the hydrophobic interactions due to the van der Waals forces from the hydrophobic alkyl groups play an important role in stabilizing vaterite particles [82]. The stabilizing effect of oleic acid molecules was also demonstrated and the adsorption of oleic acid at the vaterite surface was confirmed by FTIR analysis [83]. Also, tetrazole [61] and fulvic acid [34] were identified as compounds that were able to absorb at the vaterite surface and retard vaterite dissolution. The stabilization of vaterite particles was also observed when polypeptides [33], bovine serum albumin, or soluble starch [87] was added into the reaction mixture. Hydrophilic and hydrophobic parts are present in surfactant molecules, therefore, these molecules can easily adsorb on the hydrophilic surface of vaterite particles. In the same conditions, the vaterite polymorphic form was precipitated in the solution containing surfactant while calcite formation was favored in a medium without the addition of surfactant molecules [75,88]. Moreover, changes in the morphology of vaterite particles precipitated in the presence of surfactant were observed [49,66].

Polymeric substances were tested as additives in vaterite precipitation, as well. Polypeptides adsorbed at the interfaces of forming particles. They decreased in aggregation and changed the electrokinetic and morphological properties of the precipitate [33]. However the addition of the guar gum to the initial calcium chloride solution resulted in the production of hollow spherical vaterite particles [12]. However, core-shell vaterite microspheres composed of nanoparticles in the core and hexagonal nanoplates at the outer layer were precipitated using solutions of calcium chloride and ammonium carbonate with the addition of poly(styrene sulfonic acid) sodium salt [45]. When the synthesis was performed using a sodium carbonate solution as a carbonate source, vaterite microspheres covered by nanorods were precipitated [45].

The influence of additives on the rate of calcium carbonate precipitation was reported for some systems. In the solution method, it was found that the addition of ethylene glycol [80] reduced the precipitation rate. The retardation of crystal growth was observed when poly-aspartic acid and poly-glutamic acid were present in the solution [33]. In these systems, high supersaturation was preserved for a longer time and it resulted in a higher concentration of the vaterite phase in precipitated calcium carbonate. However, an increase in the vaterite growth was observed in the presence of ethanol, isopropanol, and diethylene glycol [31]. However, the stabilization of vaterite particles by organic solvent molecules prevented its transformation to more stable calcite. The opposite effect was found when citric acid was added [34]. Then, the precipitation was also slower, but the creation of calcite was privileged. It seems that the addition of citric acid increased the solubility of calcium carbonate and reduced the supersaturation in the system, which could result in the crystallization of calcite. However, when precipitation was carried out by carbonation of the calcium hydroxide suspension, the addition of amines, diamines, and amino acids resulted in a longer reaction time [82]. Vaterite was created when diaminooctane and amino acids were used. Although amino acids did not promote the CO2 absorption and the formation of high supersaturation, their stabilizing effect on the vaterite was prevalent. On the other hand, the addition of sucrose into the initial solution of calcium chloride and ammonia reduced the reaction time [85]. Sucrose facilitated the absorption of CO2 and caused the high supersaturation in the system, which promoted the creation of vaterite.

In summary, according to the Ostwald rule, the least stable vaterite precipitates first and subsequently transforms to the more stable one. In the absence of an additive, the kinetics is a dominant factor influencing the vaterite concentration in the produced calcium carbonate. As additives can affect each stage of crystallization, i.e., nucleation, growth, and transformation, they can therefore change the course of the precipitation process. Comprehensive information on the influence of tested additives on vaterite precipitation is not available. As shown in Table 3, only one study presented the effects of the additive in three of the highlighted areas [31]. Little information is available for carbonation-based precipitation, where additional substances can also affect the rate of CO2 absorption and the generation of supersaturation in the system.

#### **6. Summary**

Vaterite is a polymorphic form of calcium carbonate, which is the subject of many studies due to its unique properties and related potential applications. Especially, spherical polycrystalline particles of vaterite are indicated as the most promising ones for applications. Various utilizations require particles with defined characteristics to be obtained. In this review, methods used for spherical vaterite precipitation were presented. Classical routes and recently new proposed approaches for calcium carbonate precipitation were summarized. Favorable conditions for vaterite particles precipitation were also described.

A variety of proposed methods allows the selection of an approach due to the availability of substrates and equipment, which may have an impact on the cost of the produced calcium carbonate. The problem of separating the obtained particles, e.g., in the emulsion method, is not discussed, but it may decide on the choice of a specific method. Also, the recovery or recirculation of the liquid residue has not been investigated so far.

Spherical vaterite particles can be formed using the presented methods when appropriate process parameters are maintained. The most frequently indicated conditions conducive to the formation of vaterite are the relatively high supersaturation, temperatures up to 40◦C, and pH between 8 and 10. However, these parameters can be shifted because there are many relationships between them that are not fully understood. In particular, the presence of additives may affect the range of conditions favorable to the precipitation of vaterite. Therefore, the stabilizing role of additives seems to be a promising research area. Especially, the selection of non-toxic compounds is very important when vaterite particles are used in pharmaceutical and cosmetic preparations or for biomedical applications.

**Funding:** The research described in this paper was financially supported by Faculty of Chemistry, Gdansk University of Technology, grant number DS 033155.

**Conflicts of Interest:** The author declares no conflict of interest.
