L-type Precipitation of Sodium Polyacrylate

Following this paper, two pioneering publications revealed the major aspects of this precipitation [23,25]. In both investigations, aqueous solutions of the sodium salt of poly-carboxylic acids were used instead of the polyacid. Michaeli [25] investigated in a highly systematic way the influence of an inert salt NaCl and the degree of ionization Q of the polyelectrolyte chains on 

[M2+]c and [C00-]c denote molar concentrations of M2+ and monomer respectively required for precipitation. The intersection of the line and its slope were best described according to 

Ikegami and Imai [23] extended our knowledge on NaPA, confirming Eq. (26) for Q=1 and [NaCl]=0 and M=Ag+, Mg2+, Ca2+, Ba2+ and La3+. If self-ionization was completely depressed by adding HC1, precipitation of PA did not follow Eq. (26) any more but required much larger amounts of bivalent metal cations (H-type), independent of [COO-]. At an intermediate neutralization of Q=0.25, Ba2+ resulted still in an L-type precipitation whereas Mg2+ could only induce a H-type precipitation [23]. 

In this context, reference is given once more to the results by Sabbagh and Delsanti [77] on solutions free of inert salt. They could show that deviations from Eq. (26) also occurred if the polymer concentration was low enough. A minimum in the diagram [M2+] versus [COO ]c appeared which was shifted to larger values of [COO ]c, the smaller the molar mass of the NaPA samples became. 

From the experiments just outlined [74-76], a few points are worth being emphasized A powerful procedure was developed to gradually approach phase boundaries of polyelectrolyte precipitation. The approaches can be performed in a highly systematic manner and lead to states which are located extremely close to the precipitation threshold. Approaches could successfully be accompanied by LS experiments. The experiments demonstrated, that the polyelectrolyte chains shrank dramatically in size immediately before the phase boundaries were reached. A sudden increase of the scattering intensity indicated the phase boundaries. These developments give rise to the hope that intermediates may be revealed which have not become accessible in preceding investigations [78-81]. 

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