Laboratory Products
Surface area of metallic stearate excipients (zinc and calcium stearate) – a case study
Dr Katie Struckhoff, Head of Product Competence for Surface Area and Pore Size Analyzers, and Nathalia Prieto, Product Specialist Anton Paar GmbH,
www.anton-par.com
Zinc and calcium stearate (metal soaps) are among the most commonly used stearates across diverse industries. Their surface area, as characterised with the Anton Paar Nova 600 and Kaomi for Nova 21 CFR Part 11 software, for example, confi rms manufacturing quality and specifi cation values in controlled, regulated processes.
Metal soaps, also known as metallic stearates, are typically produced from the reaction between stearic acid and metals or metal oxides. As a result, highly hydrophobic compounds with high melting temperatures are obtained, making them ideal as stabilisers, lubricants, and anti-caking agents, among other applications. [1] Metal soaps are used for a wide variety of purposes – water resistance, stabilisation, thickening, lubrication (and others) – across a range of industries, such as:
• Cosmetics • Food processing • Polymer and rubber manufacturing • Powder metallurgy • Pharmaceutical production
The versatility of metal soaps has created a steadily growing market, the value of which is expected to reach $4.9 billion by 2028 [2]. This growth compels industries – especially highly-regulated ones that must comply with governmental regulations such as those imposed by the United States Food and Drug Administration (US FDA) – to evaluate (characterise) and manufacture raw materials as quickly, effi ciently, and transparently as possible.
Gas adsorption is a trusted technique for determining the specifi c surface area of stearates. Anton Paar’s Nova Series delivers this and other information – such as homogeneity of the evaluated materials – rapidly, reliably, and reproducibly.
Table 1: Analysis parameters for both stearates Parameter
Adsorbate
Bath temperature Sample cell
p/p0 target points
Number of data points Void volume mode p0 mode
Thermal delay
Equilibrium band % Equilibrium interval Equilibrium timeout
Equilibrium minimum time
0 minutes for others Re-dose tolerance Re-dose maximum
Materials and methods
Two commercially-available lab grade samples (zinc stearate and calcium stearate) were used to perform BET-specifi c surface area analysis. To assess the reproducibility of the data, two aliquots of each of the samples were prepared. The characterisation was done using a Nova 600 (a vacuum volumetric analyser that has two analysis stations for simultaneous runs).
Figure 1: Overlay of calcium stearate isotherms Value Nitrogen 77 K (liquid N2)
9 mm large bulb long cell with fi ller rod 0.05, 0.075, 0.1, 0.125 and 0.15 5
Helium measure From ambient 150 seconds 3 1
0 minutes for all points 3 minutes for p/p0 = 0.05;
50 % 7.00 Torr
To collect more data for reproducibility purposes, the two aliquots (per material) were analysed in three separate experiments with degassing between each.
No details of specifi c surface area appear in the calcium or zinc stearate monographs. Therefore, the guidance in USP monograph 846 on magnesium stearate for specifi c surface area was adopted. Specifi cally, the following:
• Sample preparation (degassing conditions) • Correlation coeffi cient > 0.9975 • Relative pressure measurement range
Two aliquots of each of the stearates were prepared in long cells with large bulbs with enough material to reduce noise and improve reproducibility. According to USP, the sample cells were degassed at 40°C for two hours under N2
fl ow on the Nova
600 degassing stations. After the samples reached ambient temperature and were reweighed, they were analysed simultaneously on the same instrument (previously qualifi ed to guarantee correct pressure transducer readings) using the parameters given in Table 1.
According to the monograph, the relative pressure range used to calculate BET surface area was 0.05-0.15 p/p0.
Kaomi for Nova 21 CFR Part 11 software and fi rmware were used to show the capabilities applicable for multiple industries that require Good Manufacturing Practices (GMPs) such as electronic signatures and records keeping (audit trail) as well as data integrity [3].
Results and discussion
The six total measurements for calcium stearate are overlaid in Figure 1. The BET specifi c surface area (SSA) calculated for each of the two aliquots analysed in triplicate for calcium stearate are given in Table 2. The results for surface area ranged from 6.262 m2 to 6.289 m2
/g all with correlation coeffi cients > 0.998. In addition, according to the BET
estimation, the C constant and intercept have positive values, confi rming that the data are valid and in line with BET requirements.
The six total measurements for zinc stearate are overlaid in Figure 2. In the case of zinc stearate, the surface area values for all data sets ranged from 5.047 m2 m2
/g to 5.098 /g and all data were within specifi cation with positive values for the C constant and
intercept. Because BET is a linear equation, Y = mX + b, a high correlation coeffi cient (linearity) is desirable. For zinc stearate, the correlation coeffi cient values were all > 0.999.
/g
INTERNATIONAL LABMATE - NOVEMBER 2025
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