Enthalpic pairwise self-association of l-carnitine in aqueous solutions of some alkali halides at T = 298.15 K

• Dilution enthalpies of l-carnitine in aqueous alkali halide solutions by ITC.
• The second virial coefficients of enthalpy (h2) have been calculated.
• The values of h2 increase with increasing molalities of aqueous salt solutions.
• The signs of h2 turn from negative in pure water to positive in salt solutions.
• The trends is ascribed to the salt effects on pairwise self-associations.

Knowledge of the influence of ions of various nature on intermolecular hydrophilic and hydrophobic interactions in solutions is required in many research fields. In this paper, dilution enthalpies of zwitterion l-carnitine in aqueous NaCl, KCl and NaBr solutions of various molalities (b = 0 to 3.0 mol · kg−1) have been determined respectively at T = (298.15 ± 0.01) K and p = (0.100 ± 0.005) MPa by isothermal titration calorimetry (ITC). In light of the MacMillan–Mayer theory, the 2nd virial enthalpic coefficients (h2) have been calculated. The h2 coefficients increase gradually with increasing molality (b) of the three aqueous alkali halides solutions, from small negative values in pure water to relatively larger positive values in solution. The trends of h2 coefficients are ascribed to the salt effects on the balance between hydrophilic and hydrophobic interactions in pairwise self-associations. It is considered that the size of cations and anions exert influences on h2 coefficients through their surface charge densities and hydration (or dehydration) abilities.

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