Sodium hexametaphosphate

Calcium chloride (CaCl 2 , 93%, Alfa Aesar), sodium carbonate monohydrate (Na 2 CO 3 • H 2 O, ≥ 99.5%, Sigma-Aldrich), sodium hexametaphosphate ((NaPO 3 ) 6 , Alfa Aesar), tetraethyl orthosilicate (TEOS, 98.0%, Aldrich), ethanol (EtOH, ≥ 99.8%, Sigma-Aldrich), ammonium hydroxide (NH 4 OH, 30.0%, Carlo Erba), hexadecyltrimethylammonium bromide (CTAB, >98.0%, Tokyo Chemical Industry Co., Ltd.), hydrochloric acid (HCl, 36% w/w, Alfa Aesar), sodium hydroxide (NaOH, 97%, Thermo Scientific), potassium hydrogen phosphate (K 2 HPO 4 , 98%, Alfa Aesar) and potassium dihydrogen phosphate (KH 2 PO 4 , 98%, Alfa Aesar) were used without additional purification.
An ultrasonic cup horn (19.5 kHz, Danacamerini prototype) was used to prepare nano-CaCO 3 and HMSNs, as shown in Fig. 1. The ultrasonic generator was used to regulate ultrasonic power, which can be adjusted up to 300 W (electric input power). The ultrasonic transducers consist of high-efficiency pre-stressed piezoelectric (PZT) rings (planar PZT Morgan Electronics, diameter 50 mm) compressed between two erga discs. The maximum capacity of the cup horn is 80 mL, which is cooled with cooling water [38] . The ultrasonic powers delivered were determined calorimetrically with water as heating media and all ultrasonic power mentioned below refers to the power measured calorimetrically [39] .

An additive-free, commercial MC powder containing ~87% protein and ~7% lactose was used for this study (Naked Nutrition, Coral Gables, FL) . Mineral content of the powder was ~36 mg per g of powder with ~20 mg being calcium. According to the information provided by the manufacturer, the MC was produced from skim milk following an acid-free, bleach-free cold process. Sodium phosphate monobasic (CAS 7558-80-7, phosphates = 1, MSP), sodium pyrophosphate tetrabasic (CAS 7722-88-5, phosphates = 2, TSPP), sodium polyphosphate (CAS 68915-31-1, molecular weight 1,733 g•mol -1 , phosphates = 17, linear, SPP), and HEPES buffer were obtained from Sigma-Aldrich (St. Louis, MO). Sodium tripolyphosphate (CAS 7758-29-4, phosphates = 3, linear, STPP), sodium trimetaphosphate (CAS 7785-84-4, phosphates = 3, ring, STMP), and sodium hexametaphosphate (CAS 10124-56-8, phosphates = 6, ring, SHMP) were purchased from Alfa Aesar (Ward Hill, MA). Water with a resistivity of ~18.2 MΩ•cm was used for the preparation of solutions. pH determinations were performed at room temperature (20 ± 2°C) using an Ion 700 bench-top pH meter (Oakton, Vernon Hills, IL) that was calibrated before measurements. The HCl and NaOH (concentrated and diluted solutions as required) were used for pH adjustments.