As nanoparticles become more commonly used in everyday products it becomes increasingly important to understand " nanoparticle aggregation in the aqueous environment . . . for assessing the fate, transport and toxicity of nanomaterials". In an effort to increase the body of scientific knowledge in this area, Dongxu Zhou, Samuel W. Bennett, and Arturo A. Keller, all of the University of California Santa Barbara Bren School of Environmental Science and Management, in an article published on the PLOS One website "report for the first time . . . temperature variations can cause either agglomeration or disagglomeration . . . depending on the heating and cooling paths. This finding is very relevant . . . , since it indicates that ambient temperature change, constantly occurring in open waters, can alter nanoparticle mobility." Following studies cited in the article’s references, the authors define aggregates as "particle clusters bound by irreversible chemical bonds", while agglomerates are "clusters" held together by weak physical interactions. " Once released in the environment, nanoparticles will very likely exist as agglomerated aggregates, i.e. aggregate clusters that have weaker bonds between them. "
In experiments on clusters of three types of metallic oxides – titanium dioxide, zinc oxide, and cerium oxide – lead the authors to conclude
. . . that in open water these soft (weakly bonded) agglomerates can be disagglomerated by common environmental stimuli, such as exposure to sunlight or an increase in temperature from diurnal variations. Although not evaluated, it is likely that mechanical shocks may also result in temporary disagglomeration. The released aggregates can be much more mobile and bioavailable while the stimuli is present. Although in our experimental setting we observe reagglomeration once the stimuli are removed, in the environment it may be that the probability of interacting with another nanoparticle aggregate is much lower. . . . The effect of disagglomeration on toxicity has not been considered, or systematically evaluated. This phenomenon warrants attention since it is likely that these metal oxide nanoparticles will experience these natural stimuli during their transport in the environment.