Micelle-water distribution coefficients of nitroxides. Correlation with dodecane-water distributions and interfacial activity

Abstract

Micelle-water distribution coefficients have been determined for 2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPO) and 4-oxo-2,2,6,6-tetramethylpiperidinyl-l-oxy (OTEMPO) in sodium dodecyl sulfate (SDS) and cetyltrimethylammonium chloride (CTAC) and in several other micellar systems of different charge types, including the phospholipid palmitoyl lysolecithin, for TEMPO. In order to interpret the distribution coefficients and the overall solubilizing powers of the micellar systems, we used a recently proposed two-state model which invokes an equilibrium distribution between a "dissolved state", associated with the hydrocarbon core, and an "adsorbed state", associated with the interface region. According to this model, solublization is due in part to solubility in the hydrocarbon core and in part to adsorption to the interface arising out of the interfacial activity of the solubilizate. Dodecane-water bulk partition coefficients have been determined for TEMPO and OTEMPO. On a comparative basis, the overall micelle-water distribution coefficients of TEMPO and OTEMPO in SDS are 6 and 100 times higher than the corresponding dodecane-water partition coefficients. When corrections for solubility in the hydrocarbon core are made for the high Laplace pressure in SDS micelles calculated from the estimated interfacial tension and the radius, the micelle-water distribution coefficients for SDS are found to be higher by factors of 71 and 1600 for TEMPO and OTEMPO. This calculation indicates that less than 2% of the total solubility in the micelles should be ascribed to the hydrocarbon core and more than 98% of the solubilized molecules are in the adsorbed state. Evidence confirming the interfacial activity of the nitroxides has been obtained from the lowering of the tension of the dodecane-water interface by both TEMPO and OTEMPO. These data also indicate that about 99% of solubilized TEMP and OTEMPO molecules are in the adsorbed state, the large fraction arising from the very high surface-to-volume ratios of micelles. The interfacial tension data indicate furthermore a higher adsorbability of OTEMPO as compared to TEMPO, in agreement with deductions from micelle-water distribution equilibria. From the dodecane-water partition coefficients and the use of model hydrocarbon partition data, it is shown that the contribution of the nitroxide group to the free energy of transfer from dodecane to water is approximately -7.5 kcal/mol whereas that of the C=0 group of OTEMPO is approximately -2 kcal/mol. The hydrophilicity of the nitroxide group, arising mainly from hydrogen-bonding interactions, provides a molecular basis for the effects observed. The hydrophilicities of the nitroxide group and the 0=0 group, and the predominance of the adsorbed state in solubilization inferred here, are of importance in interpreting the microenvironmental effects experienced by TEMPO, OTEMPO, and other nitroxides in micellar systems, as reported in the previous paper. These were investigated by using solvatochromic visible absorption bands of the nitrox...

Different Micellar Packing and Hydrophobicity of the Membrane Probes TEMPO and TEMPOL Influence Their Partition Between Aqueous and Micellar Phases Rather than Location in the Micelle Interior

“…The K b values for TEMPO in (17, 19). According to this model two regions are distinall cases are higher than those for TEMPOL, in agreement guished in the micelle structure: (a) the liquid paraffin-like with the higher hydrophobicity of TEMPO and its higher micelle core composed of hydrophobic tails and characteraffinity for the micellar phase (5,6,25,26). ized by a low dielectric constant (e Å 2-3) and (b) the…”

“…With exception The dielectric constant in region (b) is higher than that in of SDS, the same order is observed for TEMPOL as well. region (a): its value was estimated as varying in the range Lower binding constants of TEMPO to zwitterionic micelles from 5-7 to 15-17 depending on internal and external facas compared to charged ones (CTAC and SDS) were also tors such as spatial structure, size and charge of polar surfacobserved in (26). We believe this effect should be associated tant head, the counterion structure and concentration, and to the fact that denser packing occurs for the zwitterionic the water content (20-23).…”

“…These data should be taken into account when nitroxide It may seem surprising that the distribution of TEMPO and TEMPOL is similar in all micelles studied in this work, radicals are used as paramagnetic probes or excited-state quenchers in microheterogeneous systems. while the affinity to polar media (hydrophilicity) is much higher for TEMPOL (5,6,25,26). It is, however, necessary to take into account that both 1 H NMR and optical absorption…”

“…It is possible to conclude that both radicals are localized in the same micelle region, in agreement with the NMR data. The radical behavior in the micelle has been previously described by a ''two-state'' model (26). According to this model the radical should be localized either in the micelle hydrocarbon core (''dissolved state'') or at the micelle-water interface (''adsorbed state'').…”

Particle Size Effects in TEMPO‐Mediated Radical Polymerization of Styrene in Aqueous Miniemulsion

“…[38][39][40][41] Further support is provided in the form of interfacial activity of TEMPO, as shown by the presence of TEMPO resulting in a reduction in the interfacial tension of dodecane/water. [42] Some fraction of the nitroxide may thus be unable to participate in the NMP mechanism (i.e., the deactivation reaction), and the significance of this effect would increase with decreasing particle size (increasing interfacial area). The fact that the present particle size distributions are somewhat polydisperse (d w /d n ¼ 1.3) would result in different polymerization kinetics in different particles, and the experimentally observed quantities thus correspond to some weighted averages.…”