60th Annual Report on Research 2015

The formation of sparingly soluble and insoluble inorganic salts (i.e., scale deposits) is a major problem in the petroleum industry operations. To avoid formation damage and well productivity reduction, it is crucial to remove scale deposits and control scale growth. The use of chemical scale inhibitors (i.e., antiscalants) is a common method for controlling scale deposits.  Commercial scale inhibitors for divalent cation carbonate and sulfate scales, such as calcium carbonate [(CaCO₃) calcite and aragonite] and barium sulfate [(BaSO₄) barite], include the polyelectrolytes that can dissociate phosphonates, carboxylates, and sulfonates anionic groups. However, it is imperative to identify highly efficient polymeric inhibitors and environmentally friendly ones to replace phosphonate inhibitors due to their environmental risks.

The efficiency of a polyelectrolyte inhibitor mainly depends on the properties of the charges, such as the strength, the number, the availability, and the conformation of the charges. Certain polypeptides with negatively and/or positively charged groups have been extracted from organisms and found to efficiently control the nucleation and crystallization of minerals (e.g., calcium carbonate). Their structures are attractive models for better understanding the inhibitor-mineral interactions and designing next generation antiscalants. Antifreeze polypeptides (AFPs) from cold-adapted organisms (e.g., fish, insects, and plants) can inhibit the nucleation and crystallization of ice and some non-ice like compounds. Analogs of the alpha-helical type I AFP from winder flounder have been found to control calcite crystal growth.

In this project, we aim to correlate the charge and molecular properties of the polyelectrolytes with their efficiencies in inhibiting the scale crystal formation. In year 1, we prepared an AFP from a fire-colored beetle (DAFP) and examined the surface charges of the polypeptide. DAFP is a small repeat polypeptide with short beta-sheets forming a helical structure. There are 5 aspartic acids, 1 glutamic acid, 2 lysines, and 4 arginines in one molecule of DAFP. The side chains of these charged residues are on the surface of the polypeptide.

We investigated the characteristics of calcium carbonate formation by mixing of equal volumes of equimolar sodium carbonate (Na₂CO₃) and calcium chloride (CaCl₂) in the absence and the presence of the prepared polyelectrolyte at room temperature. The formation of CaCO₃ particles and the suspensions of the formed CaCO₃ particles were monitored by absorbance. The pH of each mixture solution was also measured. The hydrolysis reaction of carbonate ion (CO₃^2-) in aqueous solution release hydroxide ions, resulting in a basic solution. Upon the formation of CaCO₃, the pH of the solution decreases.

The presence of DAFP results in a visible right shift of the maximal absorbance (Figure 1), suggesting that the precipitation rate of CaCO₃ decreases by the polyelectrolyte. The presence of DAFP also significantly increases of the sedimentation time (Figure 1), indicating that the precipitated CaCO₃ crystals in the presence of DAFP are with smaller sizes. The results suggest that interactions exist between the polyelectrolyte and CaCO₃ crystals.

To better understand the surface charge of CaCO₃ particles and the potential interactions between the polyelectrolyte and CaCO₃ particles, electrophoretic mobility measurements were performed in the absence and the presence of the prepared polyelectrolyte. Smoluchowski’s equation was applied for zeta potential calculations. The small positive zeta potential (ZP) of freshly precipitated CaCO₃ (~ +10 mV) suggests that the surface of the fresh precipitation has small positive net charge (e.g., the surface –Ca²⁺ sites) (Figure 2). Negative ZP can be observed when excess dianionic carbonate ions adsorb on the hydrolyzed –Ca²⁺ sites of CaCO₃.

The presence of DAFP delays the decrease of the ZP for about 10 minutes since the instant precipitation. The effect of DAFP on the ZP of CaCO₃ crystals may result from the specific adsorption of the polyelectrolyte on the hydrolyzed layer of the already precipitated CaCO₃ crystals and the negatively charged aspartic acids in DAFP may favorably bind to the –Ca²⁺ sites on the CaCO₃ crystals. The basic groups in DAFP may expose to the solutions and interact with carbonate anions. We have identified a promising scale inhibition system. To shed lights into the mechanism, further investigations are carrying out in more details (e.g., different concentrations, analogs of the polyelectrolyte).

Figure 1. A typical graph of the light absorbance changes of the systems in the absence (green dots) and the presence (purple filled squares) of the prepared polyelectrolyte versus time.

Figure 2. A typical graph of the zeta potential changes of the systems in the absence (green dots) and the presence (purple filled squares) of the prepared polyelectrolyte versus time.

This support has enabled new discoveries in the field of scale inhibition by the PI and her group. The grant has also enabled more students who are interested in chemical science to experience chemical research and the PI to mentor these students. It is noteworthy that most of these students are underrepresented minorities and two of them are going to present their results at a research retreat conference in the mid-September and more of them will present at a state research symposium. We are also preparing a manuscript on this project. These activities have great positive impacts on the students’ lives as well as on the career of the PI as a researcher, teacher, and mentor.