Shrinking of anionic polyacrylate coils induced by Ca2+, Sr2+ and Ba2+: A combined light scattering and ASAXS study

Abstract

Abstract. Anionic polyacrylate chains (NaPA) form precipitates if alkaline earth cations are added in stoichiometric amounts. Accordingly, precipitation thresholds were established for three different alkaline earth cations Ca 2+, Sr 2+ and Ba 2+. Close to the precipitation threshold, the NaPA chains significantly decrease in size. This shrinking process was followed by means of combined static and dynamic light scattering. Intermediates were generated by varying the ratio MCl 2 /[NaPA] with M denoting the respective alkaline earth cation. All experiments were performed at an inert salt level of 0.01 M NaCl. Similar coil -to -sphere transitions could be observed with all three alkaline earth cations Ca 2+ , Sr 2+ and Ba 2+. Based on these findings, supplementary conventional and anomalous small angle x-ray scattering experiments using selected intermediates close to the precipitation threshold of SrPA were performed. The distribution of Sr-counterions around the polyacrylate chains in aqueous solution provided the desired scattering contrast. Energy dependent scattering experiments enabled successful separation of the pure-resonant terms, which solely stem from the counterions. The Sr 2+ scattering roughly reflects the monomer distribution of the polyacrylate chains. Different ratios of the concentrations of [SrCl 2 ]/[NaPA] revealed dramatic changes in the scattering curves. The scattering curve at the lowest ratio indicated an almost coil-like behaviour, while at the higher ratios the scattering curves supported the model of highly contracted polymer chains. Most of X-ray scattering experiments on intermediate states revealed compact structural elements which were significantly smaller than the respective overall size of the NaPA particles.

Probing the extent of the Sr2+ ion condensation to anionic polyacrylate coils: A quantitative anomalous small-angle x-ray scattering study

“…The phase boundary for Sr 2+ -PA precipitation was approached by diluting the stock solution ͑III͒ with the solvent ͑IV͒. By means of this procedure, 25 four different ratios of ͓Sr 2+ ͔ / ͓NaPA͔ were obtained at constant concentrations of ͓Sr 2+ ͔ and ͓Na + ͔ ͓͑Sr 2+ ͔ / ͓NaPA͔ = 0.451, 0.4575, 0.458, and 0.464͒, denoted as samples D, C, B, and A, respectively. As the shape of the polymer chains in solution depends on the extent of complexation, different ratios correspond to different intermediates, bordering the precipitation threshold of SrPA.…”

“…͑5͒ provides a method which gives access to the pure-resonant scattering contribution by measuring the small-angle scattering at only three suitable energies. 15,24,25 An alternative route to separate all three integrals was achieved by Ballauff and co-workers, 26,27 which could afford to record scattering curves at 4 ͑Ref. 26͒ and 13 ͑Ref.…”

Finite-Size Networks from Cylindrical Polyelectrolyte Brushes and Porphyrins

“…[12][13][14][15][16][17] Studies on aggregates in solution predominantly cover the association of two oppositely charged polyelectrolytes 18,19 and polyelectrolytes with multivalent counterions. [20][21][22][23][24][25][26] Recently, we could show that a larger structural control can be realized if ionic components with a certain geometry are applied and secondary interactions between counterions in addition direct the structure formation. [27][28][29] For example, multivalent organic counterions are able to connect polyelectrolyte dendrimers into defined assemblies of different shape.…”

Anomalous small-angle X-ray scattering from charged soft matter

“…They investigated the collapse process of sodium polyacrylate chains in aqueous solutions with the addition of strontium counterions. The pure-resonant scattering term was used to analyze the extent of Sr 2+ condensation onto the polyacrylate coils [24,25]. A new methodology was proposed to determine the amount of cations localized around the collapsed chains with respect to the total amount of Sr ions in the solvent by analyzing the integral of the separated form factor of the counterions (so-called 'resonant invariant') [26].…”