Quantitative Correlation of Counterion (X) Affinity to Ionic Micelles and X- and Temperature-Induced Micellar Growth (Spherical – Wormlike Micelles – Vesicles) for X = 5-Methyl- and 5-Methoxysalicylate Ions

The semiempirical kinetic method has been used to determine the ratio of cetyltrimethylammonium bromide, CTABr, micellar binding constants of counterions X (<i>K</i>_X) and Br (<i>K</i>_Br), i.e., <i>K</i>_X/<i>K</i>_Br (= <i>R</i>_X^Br) for X = dianionic 5-methyl- and 5-methoxysalicylate ions. The values of <i>K</i>_X and <i>K</i>_Br have been derived from the kinetic parameters obtained in the presence of spherical/nonspherical and spherical micelles, respectively. The values of <i>R</i>_X^Br remain essentially independent of [CTABr] within its range 0.005–0.015 M for both dianionic 5-methyl- and 5-methoxysalicylate ions. The increase in temperature from 35 to 55 °C decreases the values of <i>R</i>_X^Br from 796 to 53 for 5-methylsalicylate ions and from 89 to 7.0 for 5-methoxysalicylate ions. Rheological properties of 0.015 M CTABr solutions containing ≥0.01 M counterionic salt, M₂X, show indirectly the presence of unilamellar vesicles, ULV, and long linear, entangled, and branched wormlike micelles, WM, at, respectively, 35 and 55 °C for X = dianionic 5-methylsalicylate ion. However, such studies show WM and probable spherical micelles, SM, at, respectively, 35 and 55 °C for X = dianionic 5-methoxysalicylate ions. It has been shown that, at a constant [CTABr], the micellar structural transitions from SM-to-WM-to-vesicles may be correlated quantitatively with the values of <i>R</i>_X^Br regardless of whether such micellar structural transitions occur due to variation in the values of [M₂X] at a constant temperature or due to variation in temperature at a constant [M₂X]