H and ¹⁹F NMR Investigation on Mixed Hydrocarbon−Fluorocarbon Micelles

Abstract

H and 19 F NMR measurements on aqueous solutions of sodium perfluorooctanoate (SPFO) and sodium dodecanoate (SD) mixtures are reported. The surfactant concentration ranged from ∼0.3 to 10 times the critical micelle concentration (cmc = 0.03 mol L -1 ). The cmc of the SD/SPFO/water mixed system obtained from NMR data was in good agreement with that previously obtained by conductivity measurements. Below the cmc, the experimental chemical shift (δ) was independent of the total concentration for both surfactants. Above the cmc, however, the δ values for 19 F varied linearly with concentration, whereas the values for the hydrogenated surfactant deviated from linearity. These observations indicate that below the cmc each monomer is not affected by the presence of the others. Above the cmc, on increasing the total concentration, the chemical shift trends indicate that the fluorinated chains begin to aggregate, forming islands among hydrocarbon chain domains. Since the extended chain of the fluorinated surfactant is shorter than the inner micelle radius, some methyl groups of the longer SD must be segregated within the micelle. This patchwork distribution, involving an intramicellar phase separation, prevents the computation of the micelle composition; however, NMR data give information complementary to that obtained by a previous SANS study indicating the existence of mixed micelles having the same composition. Information on the structure of micelles and on the mean distribution of the two components in the system are obtained by SANS, while the NMR technique suggests details on the chemical environment of a single monomer and on the structural organization of the molecules within a micelle. Thus, the patchwork model here proposed is able to explain apparently conflicting data obtained from different techniques.

Aggregation behavior of N-alkyl perfluorooctanesulfonamides in dimethyl sulfoxide solution

“…Since the polarizability follows the trend of hydrocarbon > water > fluorocarbon [64], it was reported that when a hydrocarbon surfactant molecule is transferred from an aqueous environment to a HC-rich environment upon the aggregation of monomers into micelles, the shielding of the CH 3 group decreases and its chemical shift increases [65,66]. Likewise, the decrease of x-CH 3 shift above the cmc in the present system indicates that the CH 3 group is transferred from a bulk solution of DMSO to a fluorocarbon-rich environment.…”

In-plane distribution in mixtures of cationic and anionic surfactants

“…This property has been used several times in other characterizations of catanionic systems, e.g. bulk vs. aggregate composition [26][27][28] or size and morphology characterization. 18,27,[29][30][31][32][33][34][35] However, these studies exploit the mean contrast of the mixed micelles or bilayers.…”

Model-Free Analysis of Critical Micellar Concentrations for Detecting Demixing in Surfactant Mixtures

“…To validate the conclusions drawn on the basis of thermodynamic arguments, we performed NMR measurements on F 6 OM/DM and F 6 OM/DPC mixtures at the mixing ratios in question. For FC/HC mixtures, combined 19 F- and 1 H NMR spectroscopy provides information on the chemical environment and, thereby, the micellization of each surfactant individually. ,, The chemical shifts of atoms in the surfactant chains change when micelles form with increasing surfactant concentration, as exemplified for F 6 OM/DM at X F 6 OM = 0.25 (Figure a,b). Plotting the chemical shift change against the reciprocal surfactant concentration ,, yielded breakpoints indicating the onset of micellization.…”

“…Numerous studies on FC/HC surfactant mixtures have revealed that their mixing behavior is not only hard to predict but even difficult to ascertain experimentally, as vastly diverging (de)­mixing scenarios have been observed and proposed for various surfactant mixtures . These scenarios include mixing within one type of micelle, − segregation, that is, demixing leading to the coexistence of two micelle populations, − or both, depending on the mixing ratio of the two. − Moreover, data interpretation is often complex and ambiguous, and approaches that furnish robust results tend to be labor-intensive and require sophisticated instrumentation such as nuclear magnetic resonance (NMR), ,,,, small-angle neutron scattering (SANS), and cryo-transmission electron microscopy (cryo-TEM) . These difficulties arise chiefly because micellar (de)­mixing is often so complex that it evades reasonably simple thermodynamic models. , …”