Measurements of limestone in 100% seawater (1.15M ionic strength), 50% seawater, and 25% seawater were conducted on the Brookhaven Instruments NanoBrook ZetaPALS. Below is the procedure for preparing these samples as well as the measurement parameters used in their analyses.
Zeta Potential is relatively easy to measure in cases of high charge and low ionic strength. A challenge presents itself when the sample is near the isoelectric point or when the mobility is low. Low mobility occurs when the sample is suspended in organic solvents, viscous media, or in this case, high ionic strength solutions.
In this technical note, we present the measurement technique for analyzing zeta potential in >1 Molar salt conditions.
Each sample was prepared by weighing out 0.04 grams of limestone and suspending it in 10 mL aliquots of 100%, 50%, and 25% seawater. The solutions of 50% and 25% seawater were diluted from 100% seawater using deionized water with a resistivity of 18.2 MΩ*cm. Each of these suspensions were then allowed to sit for a minimum of 48 hours prior to analysis being conducted.
The electrode assembly to be used was conditioned in 1M NaCl using 350 cycles in the NanoBrook ZetaPALS, with frequency and voltage set to auto. This conditioning procedure produces a uniform black coating on the electrodes, which is vital for zeta potential analysis of suspensions in high ionic strength.
Following the 48 hour time period, each sample was shaken by hand to ensure it was homogeneous. One drop of the limestone suspension was added to 2 mL of sea water inside a BI-SCP plastic cuvette. A top was placed on the BI-SCP cuvette and rocked back and forth to further ensure homogeneity. The electrode assembly was then carefully placed into the suspension while holding the BI-SCP at a 45° angle to avoid trapping air bubbles between the electrodes. Visual inspection for bubbles on the surface of the BI-SCP cuvette or between the electrodes is needed at this point to ensure proper measurements. Air bubbles can often be dislodged by gently tapping the BI-SCP on a hard surface. The cuvette was then placed in the instrument and allowed to equilibrate to the measurement temperature of 25°C for a period of 5 minutes.
For each measurement, instrument parameters of 1 run of 100 cycles were used. The frequency and voltage of the instrument were set to auto, which came out to 2.5 Volts and 2 Hz. This high amount of cycles for one run was used in order to build up a phase in the measurement. No more than 1 run was used as to not degrade the sample.
The phase graph above shows the raw data points for the measurement at 100% seawater, an ionic strength of 1.15 Molar. Only a true phase analysis light scattering instrument such as the NanoBrook ZetaPALS can measure samples at this high concentration of salt. Although the measurement may appear noisy, the phase is measured correctly for the mobility of limestone as shown by the zeta potential measured for the 3 concentrations of seawater.
|Table 1: Zeta Potential of limestone measured at 3 seawater concentrations.|
|100% Seawater – 1 run/100 cycles:|
|50% Seawater – 1 run/100 cycles:|
|25% Seawater – 1 run/100 cycles:|
1- Alotaibi, M.B., Nasr-El-Din, H.A., and Fletcher, J.J. 2011. Electrokinetics of Limestone and Dolomite Rock Particles. SPE Res Eval & Eng 14 (5): 594-603