An electrostatic approach to negatively charged substrate reactions with hydroxide ion in cationic CTAB micelles

Abstract

Received August 1, 1 98g2 C. DOLCET and E. RODENAS. Can. J. Chem. 68, 932 (1990).An electrostatic treatment is presented to explain the experimental kinetic data we obtained for the basic hydrolysis of the negatively charged substrates acetylsalicylic acid and 3-acetoxy-2-naphthoic acid. 'This treatment, based on the non-linearized Poisson-Boltzmann equation, considers specific interactions between the counterions and the micellar surface and explains the displacement of these substrates from the micellar to the aqueous phase through bromide counterion addition.Key words: electrostatic approach, cationic CTAB micelles. Introduction In earlier papers we have reported our results in the basic hydrolysis of some aromatic esters in cationic micelles (CTAB, CTACl, CTAOH), hexadecyltrimethilarnmonium bromide, chloride, and hydroxide respectively (1, 2). These results were accounted for with the pseudophase ion-exchange (3,4) and the mass-action models (5). To explain the results for negatively charged substrates like acetylsalicylic acid (2-acetoxybenzoic acid) and 3-acetoxy-2-naphthoic acid the micellar counterion effect had to be taken into account. For reactions in CTAB micelles the addition of an additional Br-counterion to the CTAB micelles displaces these substrates from the micellar to the aqueous phase (6). The opposite effect has been found with positively charged substrates for which Br-counterion addition produces a sharp increase in substrate to micelle binding (7-9). It is also known that the pseudophase ion-exchange model fails at high reactive ion concentrations according to some experimental evidence in literature (10) so that it becomes necessary to consider an increase in the fraction of neutralized micellar head groups (11,12) or to consider an additional reaction pathway in which the reactive ion in the aqueous phase reacts with the micelle bound substrate (13).From all of this evidence, it becomes necessary to develop a new theoretical treatment. For these negatively chargeh substrates, in an early paper, we assumed that the substrate distribution between aqueous and micellar phases was the same as that for ions in solution as did other authors in literature (14). > , This treatment neglects the micellar surface potential and is not able to explain our kinetic results (15).In trying to fit all our kinetic results we developed an electrostatic treatment that treats the ion distribution around micelles by the Poisson-Boltzrnann non-linearized equation (NLPB). The Poisson-Boltzmann ion distribution around micelles has been successfully used to explain the thermodynamic behavior of ionic colloids with monovalent ions (16,17) although it fails to describe the distribution of multi-

Comparative study of the cationic surfactants and their influence on the alkaline hydrolysis of acetylsalicylic acid

“…The alkaline hydrolysis of aspirin has been widely studied in micellar media, whose effects depend on the substrate and surfactant structure. The significant work of Broxton [25][26][27], Rodenas [28][29][30][31][32], Segovia [33], and Ferrit [34][35][36][37] and their co-workers deserves special mention in this context. Broxton et al [27] investigated hydrolysis of aspirin in the presence of cetyltrimethylammonium bromide (CTAB) at pH 6-8 and 7-9 and proposed a suitable mechanism.…”

Noncovalent Interactions of Tiopronin-Protected Gold Nanoparticles with DNA: Two Methods to Quantify Free Energy of Binding

“…Therefore, the free energy of binding can be written as the sum of two contributions: (i) an electrostatic-potential-independent contribution, Δ G nel (nonelectrostatic or intrinsic), and (ii) an electrostatic-potential-dependent contribution, Δ G el (electrostatic). This separation has been discussed extensively in [43–47]: $\begin{array}{c}\mathrm{\Delta }G=\mathrm{\Delta }{G}_{\text{nel}}+\mathrm{\Delta }{G}_{\text{el}}.\end{array}$ Thus, the free energy of binding Δ G can be written as a sum of two contributions: a nonelectrostatic contribution, Δ G nel , and an electrostatic one, Δ G el , which implies $\begin{array}{c}{K}^{\text{DNA}/\text{AuNPs}}=K_{\text{nel}}{K}_{\text{el}}.\end{array}$ In order to separate these contributions, we used Lippard's equation. According to Howe-Grant and Lippard [47], log  K el is proportional to −log[Na + ]; that is, $\begin{array}{c}\log K^{\text{DNA}/\text{AuNPs}}=\log K_{\text{nel}}-\beta \log \left[{\text{Na}}^{+}\right].\end{array}$ The values of log⁡ K AuNPs/DNA appearing in Table 1 are plotted in Figure 5.…”

Restricted Geometry Conditions Promoted by AlOOH Nanoparticles: Variable Strength and Character of AlOOH-Cluster/Charged Ligand Interactions As a Consequence of Changes in the Solvent

“…This free energy, Δ G , can be written as the sum of two contributions: (i) an electrostatic potential independent contribution, Δ G nel (nonelectrostatic or intrinsic), and (ii) an electrostatic potential dependent contribution, Δ G el (electrostatic). This separation has been discussed extensively in refs – Δ G = Δ G nel + Δ G el In this way: K = K nel K el We used Lippard′s equation in order to separate these contributions. According to Howe-Grant and Lippard, log K el is proportional to −log[Cl − ]; that is, log K = log K nel − β 1og [ C 1 − ] …”