Coaggregation of Two Anionic Azo Dyestuffs: A Combined Static Light Scattering and Small-Angle X-ray Scattering Study

Abstract

The formation of azo dyestuff aggregates in dilute aqueous solution induced by the addition of Mg, Ca, Sr, or Ba ions is followed by time-resolved static light scattering (SLS) and time-resolved small-angle X-ray scattering (SAXS). Time-dependent molar mass data of the growing aggregates is interpreted by means of a kinetic model introduced by Lomakin et al. ( Proc. Natl. Acad. Sci. U.S.A. 1996 , 93 , 1125 ) for the description of β-amyloid aggregation. This interpretation reveals significant trends within the homologous series of alkaline earth cations. The trends refer to the nucleation and the growth rate of the dyestuff fibers. Time-resolved SAXS experiments indicate that these first two stages are followed by a third one during which a network forms by partial lateral alignment of fibers. At high enough dyestuff concentrations, this network formation even leads to a gel-like phase. Anomalous SAXS (ASAXS) on such a gel phase formed upon the addition of Sr revealed the extent of neutralization of the dyestuff molecules within the gel by the specifically interacting alkaline earth cations.

Systematic Limitations in Concentration Analysis via Anomalous Small-Angle X-ray Scattering in the Small Structure Limit

“…In analogy Re(A M (q)A Pb (q)) denotes the (averaged) superposition of the scattering amplitudes of the monomer and the Pb 2+ -counterions. When solving the vector equation constituted by ASAXS measurements at a minimum of three energies by the Gaussian elimination procedure, the form factor |A Pb pqq| 2 can be determined in analytical form [15,[23][24][25][26][27]:…”

“…The distribution of the counterions is not accessible by conventional SAXS measurements, because the scattering contributions of the counterions and the macroions superimpose and cannot be distinguished. The first ASAXS experiments of counterion distributions were reported by Stuhrmann [16] and more recent results are published in [15,[17][18][19][20][21][22][23][24][25]. In several of those studies sodium polyacrylate (NaPA) chains were subjected to the addition of two different divalent cations [M 2+ ] with M 2+ equal to Sr 2+ or Pb 2+ as two examples for the impact of specifically interacting cations on the conformation and solubility of NaPA chains.…”