Size experiments with DLS

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4. change of solution chemistry to reach the new condition (B);

5. electrophoretic mobility and size measurements under the new condition (B);

5+1. Also, electrophoretic mobility and size measurements were conducted directly under condition B and upon addition of the “IEP-dose” related to condition A. In this case, condition B was not reached upon a change in chemical condition.

All the results obtained during these experiments and presented in this chapter were graphed as bars with three different colours: green, wine-red and blue. Green column indicates the size value determined at initial conditions A (step 3 of the protocol), wine-red column represents particles size after the change solution chemistry from condition A to B (step 5 of the protocol), and the blue column reports the size of the particles measured directly under condition B (step +1 of the protocol). The background value, instead, is the size of bare silica particles under condition A, i.e., the size measurement conducted at initial condition A before addition of PDADMAC, and it must be considered as a benchmark to appraise whether aggregation has occurred upon addition of PDADMAC.

Particle size is related to aggregation, which is a direct consequence of the surface potential and thus of the amount of PDADMAC adsorbed on the particle. As such, there is a strong correlation between PDADMAC adsorption and particle size. The purpose of these experiments is to assess whether some of the mechanisms discussed above lead to an interesting aggregation/disaggregation behavior. Specifically, our experiments were designed to observe any eventual disaggregation upon changes of solution chemistry in systems comprising particles and PDADMAC at doses lower than the saturation dose: as a matter of fact, step 2 of our protocol promotes aggregation as it is conducted to reach the system IEP.

Under such condition A, and based on the assumption of irreversibility of polymer adsorption, all PDADMAC chains are adsorbed onto the particle surface and none is left in solution. This

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condition is the most favorable one to induce coagulation, as electrostatic repulsion is cancelled. This condition also allows investigation of how subsequent pH or IS change to reach condition B can influence the zeta potential and the possible disaggregation.

All the size values obtained from these experiments are referred to the size average value (Z-avg), which was graphed as the base-10 logarithm in the figures that follow. Figure 3.3.1 shows the results of the first experiment, in which condition A is 3 mM NaCl pH 8 and condition B is 3 mM pH 4. The background particle size value in the absence of PDADMAC at pH 8 is 47.8 nm, which correspond to 1.68 on a base-10 logarithm scale; from previous experiments, it was determined that 2.8 mg/L (28 mg/g of PDADMAC dose) is the concentration with which the potential is about 0 μmcm/Vs at pH 8. The size measurement, conducted with this concentration, gave value of about 2157 nm. Therefore, as expected, there was an increase in size due charge neutralization. Changing the pH solution from 8 to 4, the mobility becomes positive increasing to 2.68 μmcm/Vs and this result was predictable given the results presented in a previous chapter showing that the isoeletric point (IEP) was reached with a higher dose of polymer at pH 8 compared to pH 4. This implies “overcharging” and the main consequence of this phenomenon is some disaggregation of the silica particles coated with an excess of PDADMAC and leading to a reduction of the size value from about 2157 nm to roughly 166 nm. If a fresh solution is prepared directly at pH 4 with addition of 28 mg/g of PDADMAC dose in solution with final conditions, aggregation does not occur significantly because the particles are positively charged and electrostatic repulsion are important and the size of these aggregates is small: 112 nm although the mobility is 2.11 μmcm/Vs, lower than that related to the middle bar in Figure 3.3.1. The small discrepancy between the size measurements implies that the initial condition influences the adsorbed amount of PDADMAC on silica particles. This result suggests that, similar to what discussed above regarding the desorption of previously adsorbed chains on a surface, here there is a

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barrier to disaggregation that needs to be overcome, such that already aggregated particles will have difficulties disaggregating even for aqueous conditions under which aggregation would not be promoted in the first place.

Figure 3.3.1 Size and mobility measurements in 3 mM NaCl solution at pH 8, from pH 8 to 4 and at pH 4 with 100 mg/l of LUDOX TM-50.

A similar experiment was conducted reversing the pH and the results were graphed in Figure 3.3.2. The size of bare particles was 40 nm, consistent with the test discussed just above.

However, the silica electrophoretic mobility at pH 4 is lower (see Figure 3.2.1.1) the PDADMAC dose used to reach IEP was roughly 5 mg/g which led to an increase of the particles size value up to about 1528 nm. Increasing the pH to 8, the mobility decreased because silica charges ionize and the current amount of adsorbed PDADMAC is no longer sufficient to reach the IEP. The negative charge is expected to promote disaggregation. As a matter of fact, the particle size was reduced to 471 nm. However, also in this case

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disaggregation was not sufficient to return to the smaller particle size observed if the same PDADMAC dose were added directly to the suspension under condition B (pH 4).

Summing up, the main finding of the results represented in the two figures just above is that the disaggregation of already aggregated particles does occur upon changes of solution chemistry but needs to overcome an energy barrier. This is summarized as the particle sizes displayed in the wine-red middle columns are larger than those related to the last blue column, although in the second situation the lower absolute value of electrophoretic mobility should in principle promote more aggregation.

Figure 3.3.2 Size and mobility measurements in 3 mM NaCl solution at pH 4, from pH 4 to 8 and at pH 8 with 100 mg/l of LUDOX TM-50.

Subsequently, similar experiments were performed in a solution of 250 mM NaCl and the results are presented in Figure 3.3.3 and 3.3.4. Contrary to what discussed above, in these cases the increase in the magnitude of electrophoretic mobility did not result in any

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observable particle disaggregation (i.e., the middle column is at least as high as the left green column). Certainly, this behavior is due to the high number of ions dispersed in solution whose effect on particle stability is dominant compared to other phenomena.

Figure 3.3.3 Size and mobility measurements in 250 mM NaCl solution at pH 8, from pH 8 to 4 and at pH 4 with 100 mg/l of LUDOX TM-50.

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Figure 3.3.4 Size and mobility measurements in 250 mM NaCl solution at pH 4, from pH 4 to 8 and at pH 8 with 100 mg/l of LUDOX TM-50.

Just to confirm the observations just discussed, about the dominance of high IS on particle stability behavior, the last sets of experiments were conducted to investigate the influence of the IS increase at constant pH. Firstly, pH 4 was considered and the results are plotted in Figure 3.3.5. At this pH value, the change in IS did not significantly change the mobility, according to the statement above that at pH 4 the IEP does not depend strongly on the concentration of the PDADMAC (see Figure 3.2.2.3). Accordingly, no significant change in particle size was observed upon addition of IS.

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Figure 3.3.5 Size and mobility measurements at pH 4 in 3 mM NaCl solution, from 3 to 250 mM NaCl solution and in 250 mM NaCl solution with 100 mg/l of LUDOX TM-50.

On the contrary, in the last test at pH 8, the expected increase in mobility upon IS increase did not result in disaggregation. See results in Figure 3.2.2.4. The particle size was the same for the tests relative to all the three bars in Figure 3.3.6, confirming that a high IS causes aggregation and thwarts disaggregation in the presence of PDAMAC regardless of the way in which the solution is prepared or the final condition is achieved. It is important to underline that no significant aggregation was observed for bare particles, even in solutions of high ionic strength, even at relatively low values of mobility. On the other hand, aggregation was observed in the presence of PDAMAC also for mobility absolute values of 1 μmcm/Vs or higher. This observation suggests that phenomena are occurring during PDADMAC addition to the system at high IS that may promote aggregation during the adsorption process, or that forces other than DLVO forces might be at play in these systems. Regardless, the conclusions

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discussed above about disaggregation upon changes of solution chemistry stand under all circumstances.

Figure 3.3.6 Size and mobility measurements at pH 8 in 3 mM NaCl solution, from 3 to 250 mM NaCl solution and in 250 mM NaCl solution with 100 mg/l of LUDOX TM-50.

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