Spontaneous repeptization of colloidal dispersions

Repeptization by Dissolution in a Colloidal System of Iron(III) Pyrophosphate

“…Colloidal iron pyrophosphate shows the characteristic properties for electrocratic systems like the dependence of zeta potential on background ionic strength and excess precursor salt (see Supporting Information). However, the repeptization reported here shows some important differences with that of well-studied redispersible electrocratic systems such as the silver iodide sols. , First of all, the iron pyrophosphate system can be repeptized even after having aged for more than a month, while for most systems, the repeptization drastically decreases over time. , Second, colloidal iron pyrophosphate can be repeptized when coagulated with multivalent ions, something that is impossible for other electrocratic systems. ,, This unexpected repeptization of iron pyrophosphate is likely caused by the amorphous nature and open structure of the precipitate, which is observed independent of the preparation method . During coprecipitation of FePP i , the 5–20 nm particles immediately form ∼200 nm clusters that slowly aggregate further until they are macroscopically large.…”

“…While repeptization is important from a practical viewpoint as it governs the ability for colloidal dispersions to be dried, stored, and redispersed, it is almost exclusively studied when going from the dried to the dispersed state, , comprising only a minor part of the phenomenon. However, current literature agrees that while repeptization from the primary minimum can be experimentally realized when coagulation is induced by monovalent background ions and short aging times, the repeptization of systems aggregated by polyvalent ions is not possible by simply lowering the ionic strength of the medium: it needs additional means such as addition of surfactants or complexing agents . In this work, we further investigate the long-term stability of colloidal systems of iron pyrophosphate and unexpectedly find that they can be repeptized independent of aging time and valence of background ions and without any further additives.…”

“…2,3 Second, colloidal iron pyrophosphate can be repeptized when coagulated with multivalent ions, something that is impossible for other electrocratic systems. 8,9,16 This unexpected repeptization of iron pyrophosphate is likely caused by the amorphous nature and open structure of the precipitate, which is observed independent of the preparation method. 17 During coprecipitation of FePP i , the 5−20 nm particles immediately form ∼200 nm clusters that slowly aggregate further until they are macroscopically large.…”

“…However, current literature agrees that while repeptization from the primary minimum can be experimentally realized when coagulation is induced by monovalent background ions and short aging times, the repeptization of systems aggregated by polyvalent ions is not possible by simply lowering the ionic strength of the medium: it needs additional means such as addition of surfactants 8 or Figure 1. Schematic representations of interparticle interaction potentials in an electrocratic system: the double layer repulsion (long dash) and the van der Waals attraction (short dash) together yield the total interaction potential either without (solid line) or with (dash-dotted line) a secondary minimum depending on the repulsion strength.…”

Self-association and gel formation during sedimentation of like-charged colloids

“…Conditions for repeptization have been discussed by Frens and Overbeek 30 and Poovarodom and Berg. 31 Ion bridging has been discussed by Liu et al 32 Discussion on the background of counterion attraction is given in SI 9 (ESI †). Several, commonly provided, reasons leading to interparticle attractive force and gel formation were considered and ruled out, as discussed in the following: (1) residual van der Waals attraction: in spite of the careful refractive index matching, residual van der Waals attraction might be still present due to a slight refractive index mismatch and density fluctuations in the particles.…”