Influence of Changes in Water Properties on Reactivity in Strongly Acidic Microemulsions

Abstract

Replacing the counterion in sodium bis(2-ethylhexyl)sulfosuccinate (NaOT, usually known as AOT or Aerosol OT) with H+ (HOT) allows strongly acidic microemulsions to be obtained through the effect of a change in the solvation mechanism of the surfactant, where the Na+...OH2 interaction is displaced by a stronger H+...OH2 interaction. This raises the proportion of water bound to the counterion, which is reflected in the FT-IR spectrum for water trapped in the microemulsion and the 1H NMR spectrum for the hydrogen atoms in the water molecules. In NaOT microemulsions, the resonance signal for hydrogen atoms in the water molecules increases from delta approximately 3.9 ppm at W = 2 (with W = [H2O]/[NaOT]) to delta approximately 4.8 ppm at W = 50. In HOT microemulsions, the disparate strength of Na+...OH2 and H+...OH2 interactions results in a decrease in the resonance signal for the hydrogen atoms in the water molecules from delta approximately 8.6 ppm at W = 2 to delta approximately 4.9 ppm at W = 50. These changes in the physical properties of water alter chemical reactivity in a way that is clearly apparent in solvolytic processes in NaOT and HOT microemulsions. Thus, the rate constants of reactions involving an associative mechanism increase with decreasing W in NaOT microemulsions, but decrease with decreasing W in HOT microemulsions. The disparate behavior is a result of a decreased nucleophilicity of interfacial water in HOT microemulsions relative to NaOT microemulsions. For a dissociative process the rate constants are greater in HOT microemulsions than in NaOT ones, and increase with increasing W in both types of microemulsions, which can be ascribed to an increased electrophilicity of interfacial water in HOT microemulsions.

Characterization of Reverse Micelles Formulated with the Ionic-Liquid-like Surfactant Bmim-AOT and Comparison with the Traditional Na-AOT: Dynamic Light Scattering, 1H NMR Spectroscopy, and Hydrolysis Reaction of Carbonate as a Probe

“…Taking into account the above and according to the models developed in previous works, ,, the k obs values of the micellar system can be expressed by eq as follows where k i is the rate constant of hydrolysis at the micellar interface, K oi is the distribution constant of BNPhC between the continuous medium and the interface, and Z is defined as Z = [ n -heptane]/[surfactant]. Equation can be rewritten as follows As it can be seen, eq predicts that the relationship between the reciprocal of k obs and Z is linear, and K oi value can be obtained from the ratio between the intercept and slope. , The predicted linear relationship between 1/ k obs and Z at a constant value of W 0 was verified experimentally and K oi values were obtained from the data plotted in Figure : K oi (H 2 O/Na-AOT/ n -heptane) = 43 and K oi (H 2 O/bmim-AOT/ n -heptane) = 56.…”

“…This interfacial interaction increases the electron density on water protons, whereby the chemical shifts of hydrogen atoms from entrapped water in Na-AOT RMs appear at lower value than in bulk (4.85 ppm) . On the other hand, the interaction of the entrapped water with positively charged counterions makes the water more electrophilic, resulting in a downfield shift in comparison to bulk water. , Therefore, at low W 0 values, most of the water present in water/bmim-AOT/ n -heptane and water/Na-AOT/ n -heptane RMs interacts with anionic surfactant polar head group. By increasing the amount of water in both systems, the proportion of free water progressively increases, and water begins to recover its hydrogen-bond network structure.…”

“…Previous studies performed in Na-AOT RMs 76,78,79 report that as a consequence of the interaction of water with AOT polar head group, entrapped water molecules have better electron donor capability compared to bulk water. This interfacial interaction increases the electron density on water protons, whereby the chemical shifts of hydrogen atoms from entrapped water in Na-AOT RMs appear at lower value than in bulk (4.85 ppm).…”

“…As BNPhC is poorly soluble in water, therefore, it is assumed that it is partitioned between the oil phase (BNPhC o ) and the surfactant monolayer (BNPhC i ), whereby the reaction only takes place in the micellar interface where water and BNPhC molecules can meet to form the product (P) (Scheme 4). 64 Taking into account the above and according to the models developed in previous works, 63,64,78 the k obs values of the micellar system can be expressed by eq 1 as follows…”

Reactivity in organised assemblies

“…The properties of water in the strongly acidic microemulsions were significantly altered, e.g., the nucleophilicity was reduced in the acidic microemulsions. 34 This effect was clearly visible in solvolysis reactions proceeding via an associative mechanism e.g., hydrolysis of benzoyl halides or chloroformates showed decreased reactivity with decreased water content.…”