Effects of Headgroup Structure on the Incorporation of Anions into Sulfobetaine Micelles. Kinetic and Physical Evidence

Abstract

Zwitterionic micelles of (tetradecyldialkylammonio) propane-, butane-and pentanesulfonate increase the rate of reaction of Brwith methyl naphthalene-2-sulfonate, MeONs, but this reaction is largely suppressed by addition of NaClO4. The NMR line width of 35 Cl in ClO4increases very sharply on incorporation into micelles of tetradecyldimethylammonio propanesulfonate, SB3-14. The 14 N line width of SB3-14 also increases significantly under these conditions, but there is no evidence for strong interactions with 23 Na + . Aggregation numbers, Nagg, of SB3-14 increase modestly with increasing [SB3-14], but not with the other surfactants. The increase is greater with added NaClO4, but NaOH, NaCl, and NaNO3 have little effect on Nagg. Interactions of the anions with the micelles are treated with equations of the form of the Langmuir isotherm, and binding parameters estimated kinetically and conductometrically agree. Second-order rate constants of the reaction of Brwith MeONs at micellar surfaces are similar in sulfobetaine and cationic micelles. The reaction of Iwith MeONs is also accelerated by micelles of SB3-14.

Structure of Micellar Head-Groups and the Hydrolysis of Phenyl Chloroformate − The Role of Perchlorate Ion

“…[7c] Cationic and sulfobetaine micelles have very similar effects upon values of kЈ M for S N 2 and other spontaneous hydrolyses, [7c,8a,18] indicating that their interfacial regions behave similarly as reaction media. This conclusion is consistent with other evidence from, for example, nonsolvolytic bimolecular [13,19] and E1cB [20] reactions involving nucleophilic or basic anions. However, reactions of nucleophilic anions are slower in sulfobetaine micelles than in cationic micelles, because of the higher affinity of the latter for anions.…”

“…Hydrolysis in sulfobetaine micelles is further inhibited by addition of NaClO 4 ( Figure 3 and Table 2, and Table S4 of the Supplementary Materials), which interacts strongly with micelles of SB3-14 and displaces water from the interfacial region. [13] These experiments were performed using 0.05  surfactant, where reaction is largely in the micellar pseudophase (Figure 2), and the solubilities of SB4-14 and SB5-14 are sharply increased by NaClO 4 . Pairing of ionic centers should lessen the solubilities of these sulfobetaines in water, but this should decrease on addition of ClO 4 Ϫ , which pairs with the cationic center.…”

“…However, provided that transfer equilibria are taken into account, second-order rate constants in the micellar pseudophase are similar for cationic and sulfobetaine micelles. [13][19b] Although sulfobetaine micelles have no net charge, the positive charge density at the cationic surface is higher than the negative charge density at the sulfonate surface, which accounts for their interaction with anions of high charge density, such as OH Ϫ and F Ϫ . [18,21] They also interact specifically with anions of low charge density, [13] and interactions with ClO 4 Ϫ are discussed later.…”

“…[13][19b] Although sulfobetaine micelles have no net charge, the positive charge density at the cationic surface is higher than the negative charge density at the sulfonate surface, which accounts for their interaction with anions of high charge density, such as OH Ϫ and F Ϫ . [18,21] They also interact specifically with anions of low charge density, [13] and interactions with ClO 4 Ϫ are discussed later. Many cationic micelles have low fractional charge α, i.e., the headgroups are largely neutralized by anions, [1cϪ1g] and sulfobetaine micelles behave, in many respects, like cationic micelles with α Ǟ 0.…”

Study of the Reaction 2-(p-Nitrophenyl)Ethyl Bromide + OH− in Sulfobetaine Aqueous Micellar Solutions in the Presence and Absence of Added Salts

“…These are, respectively, 1.3, 2.0, 4.3, and 7.1 mol −1 dm 3 . It is worth noting that the value corresponding to bromide ions agrees with that given by Di Profio et al (25). A larger value of K X means a stronger binding of the X − anion to the sulfobetaine micelles and a more effective displacement of reactive hydroxide ions from the micellar surface and, therefore, a larger decrease in k m .…”