  The effect of different ionic cosolutes (NaCl, Na$_2$SO$_4$, Li$_2$SO$_4$,
NaSCN, Na$_2$[Fe(CN)5NO], and Na$_3$[Co(NO)$_6$]) on the interaction between
sodium dodecyl sulfate (SDS) and poly(ethylene oxide) (PEO) was examined by
small-angle X-ray scattering (SAXS) and isothermal titration calorimetric
techniques. The critical aggregation concentration values (cac), the saturation
concentration ($C_2$), the integral enthalpy change for aggregate formation
($\Delta H_{\mbox{agg}}$(int)) and the standard free energy change of micelle
adsorption on the macromolecule chain ( $\Delta\Delta G_{\mbox{agg}}$) were
derived from the calorimetric titration curves. In the presence of 1.00 mmol
L$^{-1}$ cosolute, no changes in the parameters were observed when compared
with those obtained for SDS-PEO interactions in pure water. For NaCl,
Na$_2$SO$_4$, Li$_2$SO$_4$, and NaSCN at 10.0 and 100 mmol L$^{-1}$, the
cosolute presence lowered cac, increased $C_2$, and the PEO-SDS aggregate
became more stable. In the presence of Na$_2$[Fe(CN)5NO], the calorimetric
titration curves changed drastically, showing a possible reduction in the
PEO-SDS degree of interaction, possibility disrupting the formed nanostructure;
however, the SAXS data confirmed, independent of the small energy observed, the
presence of aggregates adsorbed on the polymer chain.
