Effect of IS on aggregation of CeO 2 NPs

3.3.1.1 Zeta potential measurements

Figure 3.1 shows the zeta potential of CeO2 NPs as a function concentration of

NaCl or CaCl2. The zeta potential of CeO2 NPs decreased from positive to negative values

in the presence of NaCl (Fig. 3.1a). When the concentration of NaCl was elevated from 1 to 30 mM, the zeta potential rapidly reduced from 35.52 ± 11.34 mV to -12.17 ± 35.46 mV. It then increased slightly, and even reached close to 0 mV with the increase of NaCl concentration. The zeta potential was 1.8 ± 40.29 mV when the concentration of NaClwas 50 mM. The zeta potential was negative at higher concentrations of NaCl (> 50 mM of NaCl). When the concentration of CaCl2 increased, the zeta potential of CeO2 NPs tended

to become less positive (except at 30 mM) and close to 0 mV (Fig. 3.1b). It ranged from 25.19 ± 9.11 mV (at 1 mM CaCl2) to -2.77 ± 17.45 mV (at 50 mM CaCl2).

3.3.1.2 Hydrodynamic diameter measurements

Figure 3.2 shows the hydrodynamic diameter of CeO2 NPs as a function of time at

different concentrations of NaCl and CaCl2. The hydrodynamic diameter of CeO2 NPs

increased as the time increased at each NaCl concentration (Fig. 3.2a). When the concentration of NaCl was 1 mM, the average hydrodynamic diameter of CeO2 NPs

increased by 19.54% (92.79 ± 2.96 nm) at ~10 minutes when compared to the diameter during the first 100 seconds (77.62 ± 2.86 nm) (Table 3.2). However, at larger NaCl concentrations (> 30 mM), the average hydrodynamic diameter greatly increased by 169%

(> 860 nm) at ~10 minutes when compared to the diameters during the first 100 seconds (> 320 nm).

The hydrodynamic diameter of CeO2 NPs increased as the time duration increased

at each CaCl2 concentration and generally increased as the CaCl2 concentration increased

(Fig. 3.2b). The average hydrodynamic diameters of CeO2 NPs, after about a 10-minute

incubation period, slightly increased at low CaCl2 concentrations (< 15 mM) but

significantly increased at high CaCl2 concentrations (≥ 15 mM). For example, the average

hydrodynamic diameter at ~10 minutes increased by 16.68% (98.56 ± 6.00 nm) and 156.65% (316.23 ± 67.59 nm), when compared to the diameters during the first 100 seconds for 1 mM CaCl2 (84.47 ± 6.17 nm) and for 20 mM CaCl2 (811.61 ± 66.02 nm), respectively. In

addition, during the first 100 seconds and at ~ 10 min, the average hydrodynamic diameters of CeO2 NPs at larger CaCl2 concentrations (≥ 15 mM) were more than two times larger

than at lower CaCl2 concentrations (< 15 mM). For example, during the first 100 seconds,

the average hydrodynamic diameter of CeO2 NPs increased by 210.74% at 50 mM CaCl2

(347.07 ± 56.93 nm) compared to at 10 mM CaCl2 (111.69 ± 17.02 nm). The size growth

of CeO2 NPs was similar in the presence of CaCl2 to in the presence NaCl. However, the

influence of the divalent cation Ca2+ _{is more efficient than the monovalent cation Na}+ _in

enhancing the aggregation of CeO2 NPs. For example, at ~10 minutes and for 10 mM, the

average hydrodynamic diameter was 174.49% greater in the presence of Ca2+ _{(297.33 ±}

Figure B-4 shows CeO2 NPs particle size distributed at different hydrodynamic

diameters as a function of NaCl/CaCl2 concentration over an approximately 10-minute

incubation period during the first 100 seconds and at ~ 10 min, respectively. During the first 100 seconds, when the concentration of NaCl increased from 1 to 10 mM, the size distribution remained in the range of 51 ~ 100 nm (Fig. B-4a). When the concentration of NaCl increased to 30 mM, size distribution mainly increased to 201 ~ 400 nm. When concentration of NaCl increased from 30 to 100 mM, size was chiefly distributed in the range of 201 ~ 400 nm. At ~ 10 min, when the concentration of NaCl increased from 1 to 10 mM, the size distribution mainly increased slightly from 51 ~ 100 nm to 101 ~ 150 nm (Fig. B-4b). When the concentration of NaCl increased to 30 mM, the size distribution mainly increased sharply to 801 ~ 1000 nm. When the concentration of NaCl increased from 30 to 100 mM, the size distribution remained largely in the range of 801 ~ 1000 nm. In the case of CaCl2, its concentration increased from 1 to 5 mM while its size distribution

remained in the range of 51 ~ 100 nm during the first 100 seconds (Fig. B-4c). When the concentration of CaCl2 increased from 5 to 30 mM, size distribution mainly increased from

51 ~ 100 nm to 401 ~ 600 nm. When concentration of CaCl2 increased to 50 mM, size

distribution mainly decreased to 201 ~ 400 nm. At ~ 10 min, when the concentration of CaCl2 increased from 1 to 50 mM, the size distribution mainly increased from 51 ~ 100 nm

to 801 ~ 1000 nm (Fig. B-4d).

The attachment efficiency, which was obtained from the aggregation rate normalized by the fast aggregation rate, as a function of NaCl or CaCl2, is shown in Figure

3.3. The attachment efficiency of CeO2 NPs in the presence of monovalent electrolyte Na+

first increased and then remained constant (Fig. 3.3a). It slowly increased from 0.03 to 0.05 when [Na+_{] increased from 1 mM to 10 mM. Then it rapidly reached 1 when the}

concentration of Na+ _{was 30 mM. At 30 mM of Na}+_{, the attachment efficiency changed}

from the reaction limited aggregation regime to the diffusion limited aggregation regime. After 30 mM, the attachment efficiency remained around 1.00. It indicates that the critical coagulation concentration (CCC) of monovalent Na+ _([Na+_]

ccc)was 30 mM.

The attachment efficiency of CeO2 NPs in the presence of monovalent electrolyte

Ca2+ _{first increased and then remained constant (Fig. 3.3b). It increased from 0.03 to 0.04}

as the concentration of Ca2+ _{increased from 1 mM to 5 mM. Then it increased from 0.04 to}

1.00 when the concentration of Ca2+ _{was elevated from 5 mM to 15 mM. The attachment}

efficiency remained around 1.00 when [Ca2+_{] was larger than 15 mM. The change in the}

attachment efficiency shows that the curve was divided into reaction limited aggregation regime and diffusion limited aggregation regime at 15 mM of Ca2+_{. It indicates that the}

CCC of divalent Ca2+ _([Ca2+_]

ccc) was 15 mM. In the CeO2 NP suspensions, [Na+]ccc was

twice as large as [Ca2+_]ccc.

The net energy between CeO2 NPs at each NaCl or CaCl2 concentration is shown

in Figure 3.4. When concentration of electrolyte increased, the energy barrier existed only at 1 mM NaCl (16.90 kT) and at 1 mM CaCl2 (0.04 kT).