Fine Tuning of the Arrangement of Non-Close-Packed Structures: Specific Ion Effects of Lanthanide Cations

Specific ion effects are widespread and play a crucial role in many fields across chemical and biological systems. Here, the identity of the constituent ions is a determining factor, not just their charge or concentration. Therefore, these effects show a complex dependence on properties such as ion polarizability, size, and hydration shell. The formation of non-close-packed structures (NCPS) of colloidal particles on gold substrates in the presence of different ions enables the investigation of ion effects by examining particle arrangement. Due to their chemical similarity and common valence, lanthanide (Ln³⁺) ions were explored to determine the effect of polarizability exclusively. It is shown that the tendency of particles to aggregate correlates with the polarizability of the Ln³⁺ ions. Colloidal stability was increased in the presence of highly polarizable ions (La³⁺), whereas a less polarizable ions (Lu³⁺) led to the promotion of aggregation. Contrary, the nearest neighbor distances of the colloidal particles remain approximately constant through all lanthanide ions. Moreover, it could be shown that in the presence of Ln³⁺ ions, the maximum surface coverage of particles is consistently lower than for monovalent ions or in the random sequential adsorption (RSA) model because multivalent co-ions induce long-range interactions. Therefore, using trivalent co-ions allows for adjustment of interparticle distances over a larger range than monovalent ions. The maximum surface coverage of particles at ionic strengths from 0.1 to 1 mM decreases with increasing ion valency. A maximum surface coverage of 0.43 is achieved with GdCl₃ at an ionic strength of 300 mM.