Experiments on the kinetics o
f heteroaggregation between oppositely charged particles, using both dynamic light scattering and turbidity methods, are reported. The negatively charged particles were cross-linked poly(
N-isopropylacrylamide) [PNIPAM] microgel particles, prepared using a carboxylic-acid-based initiator; these particles are swollen at room temperature. The positive particles were poly(4-vinylpyridine) [P4VP] particles, prepared using an amidinium-based initiator; such particles do not respond to temperature changes but do swell below pH
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4, where the pyridine moieties become protonated. As expected, the rate o
f heteroaggregation was shown to be largely independent o
f added salt concentration (up to
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20 mM),
for a variety o
f alkali metal chlorides (MCl, where M = Li, Na, K, or Rb). However, an unexpected, signi
ficant decrease in the aggregation rate was observed at certain speci
fic
sodium chloride concentrations (typically at
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1 and also
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4 mM). Similar e
ffects were not seen with the other alkali metal chloride salts. This strange e
ffect was eventually attributed to the
fact that the net charge on the positively charged P4VP particles had been reduced by the adsorption o
f (anionic) silicate species leached
from the glassware container. Sodium silicates are known to be signi
ficantly more soluble than those o
f the other alkali metal ions, particularly at high pH. Moreover, P4VP particles dispersed in water, ostensibly at neutral pH, do bu
ffer the aqueous medium to pH values around 9 or higher. This mechanism was con
firmed by determining the electrophoretic mobility o
f the P4VP particles as a
function o
f pH in the presence o
f the various alkali metal chloride salts. The mobility remained positive in 1 mM salt solutions over the pH range 3 to 11
for all the salts,
except for sodium chloride; in that case the mobility reversed sign at alkaline pH values. A similar e
ffect was observed
for a cationic polystyrene latex sample, prepared with the same amidinium-based initiator. These experiments demonstrate the importance o
f soluble silicates, leached
from glass storage vessels, particularly in the presence o
f sodium ions. Needless to say, the “anomalous” e
ffects disappeared when plastic storage vessels were used in place o
f the glass ones.