Role of Hydrogen Bonding in the pH-Dependent Aggregation of Colloidal Gold Particles Bearing Solution-Facing Carboxylic Acid Groups

The kinetic stability of aqueous gold colloids containing particles of 11 nm diameter and bearing solution-facing carboxylic acid groups has been analyzed by experiment and theory. Three different types of particles were made whose surfaces were modified by three different thiolate-tethered acids of varying chain-length and pK(a): 6-mercaptohexanoic and 12-mercaptododecanoic acids (pK(a) 4.80 in H2O/EtOH, 80:20 by Volume), and N-(2-(S)-methyl acetic acid)-6-mercaptohexamide (pK(a) 3.85). As measured by a combination of UV-visible absorption spectroscopy and dynamic light scattering, the particles were reversibly aggregated and dispersed by cycling the pH of solution between low (pK(a)), or the ionic strength between high and low values, respectively. Conditions of aggregation were satisfactorily and quantitatively explained using a model incorporating a superposition of a repulsive electric double layer and van der Waals attraction. That is, colloidal kinetic stability was predicted accurately using the classical theory of Deraguin, Landau, Verwey, and Overbeek (DLVO), without resorting to the addition of ad hoe forces such as hydrogen bonding.