Surface-active ionic liquids in micellar catalysis: impact of anion selection on reaction rates in nucleophilic substitutions

A series of surface-active ionic liquids based on the 1-dodecyl-3-methylimidazolium cation and different anions was synthesized and applied for micellar catalysis of nucleophilic substitutions.


CMC determination and aggregation parameters calculation
For all the techniques implied the CMC was determined at the break point, i.e. the intersection of the two linear fittings of data points.

Surface tension
Π CMC is the effectiveness of the surface tension reduction, it is defined according to the following equation 1 (1): where γ 0 is the surface tension of water, and γ CMC is the surface tension of the solution at the CMC.
The Gibbs adsorption isotherm equation can be used to estimate the average area per molecule residing at the surface (A min ), assuming that the structure pattern corresponds to a monolayer, it is defined according to the following equations 2 (2-3): where N= Avogadro's number, and Γ=surface excess concentration [mol/1000 m 2 ] (3) where is the slope of the curve surface tension vs logC before the CMC, and the 1 concentration is expressed in mol/L, and R=8.31 J /mol K. The packing parameter 3 (P) which includes the volume (v) of the hydrocarbon chain imbedded in the hydrocarbon core of the aggregate and the maximum effective length (l c ) that the chain can assume, calculated according to Tanford equation (6-7) 37 : v= (27.4+26.9 * n)*10 -3 nm 3 (6) l c = (0.154 + 0.1265 * n) nm (7) n equals the actual number of atom in the hydrophobic core of the micelle, so generally one is left out. The packing parameter is then defined as P=v/a 0 l c , where a 0 is the cross-sectional area occupied by the hydrophilic group at the micelle-solution interface, it is in our case approximated with A min . A value of P < 0.33 corresponds to spherical micelles, and intermediate value 0.33<P<0.5 corresponds to non-spherical, ellipsoidal micelles, while higher values correspond to rod-like micelles and various interconnected structures. 36

Conductivity
The degree of counterion binding (β) can be estimated from the conductivity data, based on the ratio of the slopes above (S 2 ) and below (S 1 ) the cmc 4 (4): The Gibbs energy of micellization (ΔG 0 mic ) was calculated form the conductivity data according to the following equation 5 (5): Where the argument of the logarithms is the molar fraction of the surfactant, and water concentration is taken as W=55.4 mol/L.

Kinetic constants fitting
The investigated reaction between 4-nitrophenyl diphenyl phosphate (PNPDPP) and acetaldoxime follows a follows a S N 2(P) mechanism via in the attack of the nucleophile acetaldoxime at the phosphorous centre. Two leaving groups are possible, either the favoured p-nitrophenolate (NP), whose formation can be followed at 402.5 nm or phenolate (P) as a by-product.
For the determination of the rate constants for both, the desired reaction (k obs ) and the by-product formation (k'), a competitive reaction scheme (see figure above) is assumed which leads to a system of coupled differential equations: where the vector (t) represents the concentration of PNPDPP, NP and P at time t ⃗ (2) and the reaction matrix R looks like The general solution of the coupled differential equations in Eq. (1) depends on the Eigenvalues  1 =0,  2 =0 and  3 =-(k obs +k') and the corresponding Eigenvectors of the reaction matrix R: At time t=0, (0) = ([PNPDPP] 0 , 0, 0) since only the reactant is present. Using this initial condition, ⃗ the -coefficients can be determined: As a result, the time-dependent concentration of NP yields (5) In a least-square fit of the spectroscopic data to Eq. (5) using gnuplot, the sum of the two rate constants k obs + k' can be determined. The individual values of k obs and k' are evaluated from the ratio of the initial concentration of the reactant [PNPDPP] 0 and the final concentration of the product NP which also equals k obs / (k obs +k').

Additional SAXS data
The scaled medium contrast is obtained by the ratio of the electron densities (ref. 35 of the paper, P. Bartlett and R.H. Ottewill, J. Chem. Phys., 1992, 96, 3306) = -where the subscript denotes (surrounding) matrix, shell and core, respectively.  = 0 means that density of core and medium are identical, , and only the core scatters. For  = 1, and = = only the shell contributes to scattering.
The following Figures S1a to S1c show the shift towards a more uniform distribution of the radius of the core for higher concentrations. It may be not the case for Cl (as these data are difficult to evaluate due to the weakly pronounced maximum in the scattering intensities), but clearly visible for Br and OTS.