Liquid, glass, gel: The phases of colloidal Laponite

Laponite is a synthetic disc-shaped crystalline colloid that is widely used to modify rheological properties of liquids in applications such as cosmetics, paints, and inks so that understanding its flow properties and aging behavior is of considerable practical as well as fundamental importance. However, some recent studies of the liquid-glass and sol-gel transitions in aqueous Laponite suspensions have produced results that do not fully agree with each other. Because Laponite is sensitive to sample preparation procedures, it is not straightforward to compare results reported by different groups. We have begun a study of the dynamics of Laponite suspensions during aging using photon correlation spectroscopy to explore the consequences of specific sample preparation procedures which may underlie these differences, including: (1) filtration of the sample through filters with different pore sizes before beginning the experiments, (2) adjusting and monitoring the pH of the solution, (3) varying the Laponite concentration, (4) carrying out the sample preparation in either ambient air or dry nitrogen atmospheres, (5) baking the 'dry' powder to remove adsorbed water, and (6) modifying the ion concentration by the addition of salts. We will compare the effects of different methods of preparation on the intermediate scattering function F(q, t) and its time evolution. In this report we will describe experiments that explore (1)-(3). The other three will be discussed in a future publication. (C) 2007 Elsevier B.V. All rights reserved.