Solvatochromism in Cationic Micellar Solutions:  Effects of the Molecular Structures of the Solvatochromic Probe and the Surfactant Headgroup

Abstract

The solvatochromic behavior of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (RB), 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (WB), 1-methyl-8-oxyquinolinium betaine (QB), and sodium 1-methyl-8-oxyquinolinium betaine 5-sulfonate (QBS) has been studied as a function of increasing the length of R in the series C12H25N+R3Br- (R = methyl, ethyl, n-propyl, and n-butyl). The microscopic polarity of water at the solubilization site of the micelle-bound probe, E T in kcal/mol, has been calculated from the position of its intramolecular charge-transfer band in the UV−vis region. Calculated polarities depend on the length of R and the probe structure and charge. This is attributed to gradual “dehydration” of the interfacial region as a function of the increasing length of R, and different (average) solubilization sites of the probes. Thus, hydrophobic RB and WB are located in a less polar environment than hydrophilic QB and QBS. These conclusions have been confirmed by measuring 1H NMR chemical shifts of the discrete protons of both surfactant and probes. The “effective” water concentration at the probe solubilization site, [water]interfacial, has been calculated from solvatochromic data in bulk aqueous 1-propanol and aqueous 1,4-dioxane. Both reference binary mixtures gave consistent [water]interfacial; our data also agree with those based on the use of a micelle-bound arenediazonium ion.

The dependence of micellar rate effects upon reaction mechanism

“…For example, extensive evidence with spectroscopic and other probes indicates that properties of ionic interfacial regions are similar to those of aqueous-organic solvents of moderately high water content [34,35]. Provided that allowance is made for reactant partitioning between water and micelles, reaction rates, relative to those in water, can be predicted qualitatively for many ionic reactions in terms of the simple mechanistic concepts applied to reactions in solution.…”

Surfactants with an amide group “spacer”: Synthesis of 3-(acylaminopropyl)trimethylammonium chlorides and their aggregation in aqueous solutions

“…For aqueous micelles, this leads to a decrease of the degree of counter-ion dissociation, α mic , the critical micelle concentration, cmc, the area per molecule at the solution/air interface, A interface , and a independent acyl-transfer reactions) [9,10], or to desolvation of the attacking nucleophile (e.g., S N 2 reactions) are enhanced by increasing of the volume of the head-group. The reason is that hydrophobic head-groups "dehydrate" the interfacial region, a conclusion corroborated by independent techniques [9][10][11][12][13].…”

Solvation simplified

“…The reason is that their solubilization sites in the micellar pseudo-phase is different, QBS exchanges with the surfactant counter-ion (Br -or Cl -), therefore it samples the outer layer of the interfacial region. 19 As an example of the determination of the relative importance of solute-solvent interaction mechanisms, consider the solvation of solvatochromic probes. The procedure in order to extract this information is as follows: 10,11 we determine the effects of a group of solvents on the Uv-vis spectra of one or more probes; we apply Equation 2 in order to calculate E T (probe); we use Equation 3 in order to calculate the regression coefficients (a), (b), (s), and (p).…”