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The microgels made of poly( N -vinylcaprolactam) (PVCL) and a few percent of sodium acrylate (SA) were used as a model system to study the cation induced aggregation of colloid particles. Using such microgels make the aggregation controllable and reversible because PVCL can gradually change from hydrophilic (soluble) to hydrophobic (insoluble) in water when the temperature is increased in the range 25–35°C. The aggregation induced by different kinds of cations and at different temperatures was investigated by using a combination of static and dynamic laser light scattering (LLS). At temperatures lower than ∼30°C, the microgel shrinks with increasing the cation concentration due to the increase in the ionic strength and the intra-microgel complexation. The extent of the shrinking induced by different cations follows the order of Hg 2+ ≫Cu 2+ >Ca 2+ >Na + . At temperatures higher than ∼32°C, Ca 2+ and Cu 2+ can induce inter-microgel aggregation. The Ca 2+ induced aggregation is essentially reversible in the heating-and-cooling cycle, but there exists a large hysteresis in the Cu 2+ induced one. As expected, monovalent Na + was not able to induce the inter-microgel aggregation. The complexation between Hg 2+ and carboxylic groups is so strong that the intra-microgel complexation becomes so dominant that Hg 2+ was also not able to induce the inter-microgel aggregation. |
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