The primary nucleation mechanism of the gypsum in a bulk aqueous medium was identified as a heterogeneous one for 0.05 and 0.03 mol·L⁻¹ CaSO₄·2H₂O solutions and 25 °C. By means of a particle counter and dynamic light scattering (DLS) technique, solid nano/microimpurities were found, and controlled in stock brines for gypsum supersaturated solutions preparation. It is demonstrated that the common procedure of reagent grade 0.10 mol·L^‒1 CaCl₂ and Na₂SO₄ aqueous solutions filtration via 200 nm membranes is capable to reduce the foreign solid microimpurities content (size > 100 nm) from 10⁶ to 10³ units in 1 mL, but fails to affect the more numerous nanofraction (size < 100 nm). Thus, the gypsum nucleation takes place in presence of a significant amount of “nano/microcodust” templates, and has a heterogenous character. The induction time, measured by conductivity for the similar supersaturation levels, reveals a well detectable dependence on nano/microdust concentent: an increasing background particle concentration substantially decreases the induction period at a constant saturation state and temperature, and thus increases the nucleation rate. Therefore, the gypsum nucleation reaction starts tentatively through the fast heterogeneous formation of well-defined, primary nuclei via [Ca²⁺], [SO₄^2‒], and [CaSO₄]^o species sorption on the surface of “nano/microdust” particles. Thus, the “nano/microdust”, naturally occurring in any high purity chemical, plays a key role in sparingly soluble salts nucleation in the bulk aqueous medium.

An attempt to reveal the mechanisms of scale inhibition with the use of two different fluorescent-tagged antiscalants at once is undertaken. To reach the goal, a novel 1,8-naphthalimide-tagged polyacrylate (PAA-F2) is synthesized and tested separately and jointly with 1,8-naphthalimide-tagged bisphosphonate (HEDP-F) as a gypsum scale inhibitor within the frames of NACE Standard TM0374-2007. Here, it is found that at a dosage of 10 mg·dm⁻³ it provides a much higher inhibition efficiency (96%) than HEDP-F (32%). A PAA-F2 and HEDP-F blend (1:1 mass) has an intermediate efficacy (66%) and exhibits no synergism relative to its individual components. The visualization of PAA-F2 revealed a paradoxical effect: an antiscalant causes modification of the CaSO₄·2H₂O crystals habit, but does not interact with them, forming particles of its own solid complex [Ca-PAA-F2]. This paradox is interpreted in terms of the “nano/microdust” concept, prioritizing the bulk heterogeneous nucleation step, while an ability of the scale inhibitor to block the nucleus growth at the next steps is proven to be of secondary importance. At the same time, HEDP-F does not change the gypsum crystals morphology, although this antiscalant is completely located on the surface of the scale phase. The PAA-F2 and HEDP-F blend revealed an accumulation of both antiscalants in their own [Ca-PAA-F2/Ca-HEDP-F] phase with some traces of HEDP-F and PAA-F2 on the CaSO₄·2H₂O crystals surface. Thus, the visualization of two different antiscalants separately and jointly applied to gypsum deposition demonstrates differences in phosphonic and polymeric inhibitors location, and a lack of causal relationship between antiscalant efficiency and scale particle habit modification. Finally, it is shown that the confocal microscopy of several fluorescent antiscalant blends is capable of providing unique information on their interrelationships during scale deposition.

Application of antiscalants is a worldwide practice for industrial scale formation mitigation. The range of reagents is constantly expanding, and new scale inhibitors are permanently elaborated, including biodegradable ones. An antiscalant-driven scale inhibition theory has formed in the mid-twentieth century, and is up to date with some minor refinements. However, in recent years, the classical views have been increasingly criticized on the grounds of such modern methods as dynamic light scattering, particle counter technique and fluorescent visualization of antiscalant location in industrial and model system’s deposits. These methods provide a better understanding of scale inhibition mechanisms. In a present review the major mechanisms of scale inhibition are critically examined, and a hypothesis on the dominating role of solid impurities interaction with antiscalant is formulated. According to this hypothesis, the scale crystals nucleation in the bulk aqueous medium is a heterogeneous process, catalyzed by foreign solid nano/microdust particles, serving as crystallization templates (seeds). Thus, an antiscalant competes for these templates with the scale forming ions, blocks the background seeds, and reduces therefore the number of potential crystallization centers. In this way, the scale inhibitor slows down the scale formation due to the foreign seeds isolation, but not via direct interaction with the nuclei of a sparingly soluble salt.