Investigation of micelle structure by fluorine magnetic resonance. I. Sodium 10,10,10-trifluorocaprate and related compounds

Abstract

Soaps of the type CFa(CH2),COONa have been prepared with n = 8, 10, and 11. In aqueous solution each soap produces a single, concentration-dependent fluorine magnetic resonance signal. The data readily yield values of the critical micelle concentrations and of the chemical shifts of both the monomeric soap ions and the micellar material. As judged by the cmc values these soaps are essentially similar in their behavior to the ordinary alkyl carboxylates. Comparison of the micelle shifts with shifts observed for the soaps and for CF3(CH2)sCF3 in various solvents shows that the medium surrounding the CFa group when it is in the micelle has characteristics about midway between those of water and of hydrocarbon. This suggests that there is considerable penetration of water into the interior of the micelles, a conclusion also supported by determinations of the fluorine chemical shift of solubilized benzotrifluoride. The micelle shift is virtually unaffected by changes in the size of the micelles brought about either by changing the chain length of the soap or by adding simple electrolytes.( I ) Preliminary results of this work were presented at the 151st (19641, have reported small concentration-dependent changes in the position of the water nmr signal in some detergent solutions.

The demicellization temperature of potassium n-octanoate in deuterium oxide as estimated from 1H and 13C nuclear magnetic resonance spectra

“…The c.m.c. value from the Av versus log C T~ plot is similar to that from the proton nmr peak positions and the carbon- (26,31). It is also worth noting that the shape of the Av versus log C,' plot differs from that of 6 versus CT l, and this may be attributed to the different rates at which 6(a-CH2) and S(CH3) vary with the total surfactant concentration.…”

“…Extrapolation of the Sobs versus CT curve to CT = 0, i.e., infinite dilution, yields the 6, value for the unassociated monomer (6,). Also, extrapolation of the Sobs versus CT' curve to infinite concentration (C;' + 0 ) provides the value for the micellar state (SM) (26). The difference, A6 = SM -S,, is therefore a measure of the change in environment of a proton or set of magnetically equivalent protons upon entering the micellar state.…”

“…1) at the high temperatures. If a simple monomer micelle equilibrium reaction exists, then the nmr peak position, or chemical shifts, as a function of c;' will generate two straight lines (6 = 6M + (C,/CT)(6, -6,)), which intersect at the critical micelle concentration (26). Deviations from ideal behaviour result in curved 6 versus CT plots with larger radii of curvature for cases where the aggregates are small in size and the dominant mode of self-association is via multiple equilibria.…”

“…Corrections were not applied for bulk magnetic susceptibility in the coaxial samples as this was determined to be a small effect (25)(26)(27).…”

Low solubility of unconjugated bilirubin in dimethylsulfoxide – water systems: implications for pK a determinations

“…The claim that such turbidity did not affect 13 C-NMR measurements [3,5,6] contrasts with evidence that even small multimers can change NMR chemical shifts [25,26]. It should be noted also that, at high concentrations of B = , extensive, reversible self-association of B = can lead to apparently stable supersaturation with no separation of an insoluble phase [2].…”