An estimation of the surface ionization constant of oleic acid in aqueous sodium chloride solution

Abstract

The zeta potential (ς) measurements and the site binding theory were utilized for calculations of the parameters of the electrical double layer (edl), ionization, and complexation constants for oleic acid-aqueous sodium chloride solution interface. Assuming thatς is equal to the diffuse layer potential (ψ _d ) of the edl, the charge of the diffuse part of the edl was calculated from the Gouy-Chapman equation. The intrinsic ionizaiton constant was then determined by an extrapolation method to be$pK_{a_2 }^{\operatorname{int} } = 4.4$. Subsequently, the surface potential (ψ ₀) was calculated, and it was found thatψ ₀ changes by 50 mV per pH unit (50 mV/pH) or 42.5 mV/pH for 10⁻³ and 10⁻² M NaCl, respectively. For further calculations, the integral capacity of the outer zone of the compact part of the edl was assumed to be$K_2 = 30\frac{{\mu F}}{{cm^2 }}$ for both ionic strengths. It was established that the intrinsic complexation constant for the binding of Na⁺ ions with the surface of oleic acid ispK ^int_Na = 2.9±0.5 if the integral capacity of the inner zone of the compact edl (K ₁) is 80$\frac{{\mu F}}{{cm^2 }}$ for 10⁻³ M NaCl, but 280$\frac{{\mu F}}{{cm^2 }}$ for 10⁻² M NaCl. The use of the sameK ₁ value for both ionic strengths gives a differentpK ^int_Na for different NaCl concentrations, and also provides unrealistic surface charge (σ _o ) values greaterfor 10⁻³ M NaCl than for 10⁻² M NaCl, at the same pH of the solution.