Salt effects on the thermoprecipitation of poly-(N-isopropylacrylamide) oligomers from aqueous solution

Simple salts are known to influence the cloud point temperature of the aqueous solution of thermoresponsive compounds, such as poly-(N-isopropylacrylamide), PNIPAM. The effect of one series of anions (six potassium salts) and two series of cations (five alkali-metal hydroxides and chlorides, respectively) on the cloud point temperature of PNIPAM oligomers is investigated. All salts with the exception of low (<0.5 M) concentrations of KI are found to lower the cloud point temperature; the relationship between the temperature and the concentration of the added salt tends to be linear. The intensity of the effect of a given salt corresponds to its position in the Hoff-meister series. However, while the anions differ significantly in force, the cations-with the exception of Li+-show little difference in this regard. The observed effects are interpreted based on the structure making/structure breaking potential of the involved ions as evidenced by their viscosity B coefficients. While a relationship could be established for the anions this is less obvious for the cations. Moreover, while the structure breaking ability of the I--anion to some extent explains its salting in ability at low concentrations, it does not account for the linear salting out effect observed at higher concentrations. The second attempt to interpret the results is based on the solvophobic theory, i.e., takes the contribution of both electrostatic and hydrophobic interactions into account. Thermodynamic data on the phase separation obtained through differential scanning calorimetry are used to calculate the unitary free Gibbs energy as a function of the molal salt concentration.