Influence of the pH on the Formation of the Catanionic n-Tetradecylammonium Cholate Surfactant Salt

Abstract

The catanionic n-tetradecylammonium cholates, with the 1:1 and 1:2 cationic:anionic surfactant molar ratio, have been synthesized by reaction of tetradecylammonium chloride and sodium cholate. The stoichiometry of the compounds formed is strongly dependent on pH of the aqueous reaction solution. In the neutral and slightly acidic medium the isolated surfactant was detected as 1:1, at acidic medium (pH ≈ 3) as 1:2, while the cholic acid alone precipitated from the strong acidic solution (pH ≈ 2). The catanionic surfactants have been characterized by elemental and spectroscopic (IR, ¹H, and ¹³C NMR) analyses, and microscopic observations. The mole fractions of individual components in precipitate present in the heterogeneous aqueous system have been determined as a function of pH. The data interpretation in the pH regions where two different kinds of precipitates coexist pointed to the complex equilibrium in the examined systems. The evaluation of the relevant equilibrium constants was solved by the nonlinear regression analysis. In the acidic region where cholic acid and 1:2 precipitate coexisted the equilibrium constants of dissolution were K ₁ = 8.4 × 10⁻¹⁰ and K ₄ = 3.7 × 10⁻²⁷, respectively. In the region where 1:2 and 1:1 precipitate coexisted the equilibrium constant of dissolution of 1:1 tetradecylammonium cholate was K ₂ = 4.2 × 10⁻²⁰.

Keywords:

• Alkylammonium cholate
• catanionic surfactant
• equilibrium constant
• pH

Acknowledgments

The authors thank Mr. Ž. Marinić and Dr. K. Wolsperger for recording the NMR spectra. The financial support from the Croatian Ministry of Science, Education and Sport (Projects No. 098-0982915-2949, 119-1191342-2961, and 098-0982915-2950) is gratefully acknowledged.

Notes

^a KBr pellets.

Abbreviations: vs, very strong; s, strong; m, medium; w, weak; vw, very weak; br, broad; vbr, very broad; sh, shoulder; as, asymmetric; s, symmetric.

^a Spectra recorded in CD₃OD. Values in parentheses refer to signal multiplicity and coupling constant (J in Hz); singlet, s; doublet, d; triplet, t.

^{b 1}H NMR data confirm the tetradecylammonium:cholate ratio 1:2 in this compound.