The Dispersion And Gelation Of Laponite In Acid Condition

Laponite,a kind of synthetic clay,has been widely used in the preparation of nanocomposite hydrogel(NC)to improve the mechanical strength of the hydrogel.Laponite can be dispersed into a lamellar platelet with a diameter of about 25 nm and a thickness of 1 nm in water.There are a large number of negative charges on the surface of the platelets while a small amount of positive charges on the edges.When adding ionic monomers into Laponite dispersion or changing the ionic strength and pH of dispersion,the Laponite platelets will form a "house of cards" structure and lead to formation of a weak gel.So the system becomes heterogeneous,and then have a negative impact on optical and mechanical properties of hydrogels.In our previous study,we found that ionic monomer 2-acrylamido-2-methylpropanesulfonic acid(AMPS),which was directly added to Laponite dispersion can effectively prevent the aggregation and gelation of Laponite in a large amount.However,it still remains unclear how AMPS affects the dispersion and stability of Laponite.Therefore,in present study,we prepared a series of Laponite dispersion with different AMPS concentrations,with particular focus on the effect of AMPS on the dispersion behaviors and stability of Laponite.The results showed that low concentration of AMPS(<2.0 wt%)caused aggregation of Laponite due to the attraction of Van der Waals forces and reduction of electrostatic repulsion.With the increase of AMPS concentration(?2.0 wt%),the phenomenon of aggregation or gelation of the particles in the dispersion gradually disappeared due to chemical degradation under acid attack.After aged for 7 days,the dispersion showed different rheological behaviors depending on the AMPS concentration.6.0 wt% of AMPS concentration was the critical concentration in which Laponite/AMPS dispersions changed from pseudoplastic fluids to Newton fluids.It was electrolyte effect and orientation of AMPS that behaved as a dominant factor on the dispersion at low concentration of AMPS,while strong acidity of AMPS acted a key role in the dispersion at high concentration of AMPS.The chemical degradation would happen when Laponite was dispersed in acid solution.We further discussed the dispersion and chemical stability of Laponite in different acid solutions.The results showed that the acid chosen for dispersing Laponite is changed from strong acid to weak acid,the concentration of H+ would increase to reach stable dispersion.With the increase of the concentration of H+,the smaller the pKa value,the greater damages to Laponite structure.The time also has an obvious effect on the stability and chemical stability of Laponite in acid solution.When the concentration of H+ is less than 0.4 mol/L,the p H of dispersion would change after standing for a certain time,mainly due to the OH-released from the edge of Laponite layers under the affect of H+.Moreover,the state of the dispersion also changed due to the crosslinking of Mg2+ released from the process of decomposition.With the prolongation of time further,there was no obvious change on Laponite structure in strong acid,while the damage on Laponite structure became more serious in weak acid.And the damage of Laponite structure is close to each other gradually.A new NC composite hydrogel was prepared by introducing polyvinyl alcohol(PVA)and acrylamide(AM)in different acidic Laponite dispersions.The effects of Laponite in acid system as a crosslinking agent on the morphology and gelation of gel were investigated.The results showed that with the increase of acid content in the system,structure of Laponite was damaged more seriously,so the amorphous silicon products and the metal ions in the system increased,resulting in more dense three-dimensional network structure of the gel,and the smaller pores.Laponite was inclined to aggregate in weak acid condition and the gelation time was longer.With the increase of acidity,more silicon products and inorganic metal ions formed by the decomposition of Laponite in the system which promoted the formation of gel,and the gelation time decreases.However,when the acidity increased to a certain extent,the gelation time was prolonged,the gelation rate decline,and the high temperature resulted in the decrease of hydrogel strength.