From simple amphiphilic to surfactant behavior: analysis of (1)H NMR chemical shift variations.
Analysis of the (1)H NMR chemical shift variations for the methyl protons of sodium decanoate and decanoic acid in D(2)O solutions using reduced variables is consonant with a narrow distribution of sizes about the mean aggregation number for decanoate ion micelles, in contrast with decanoic acid polydisperse aggregates which increase their size with concentration, until phase separation is reached. At defined temperatures between 10 and 50 degrees C, the chemical shift coefficients for the methyl group protons exhibit a negative temperature slope (shielding) for decanoate ion micelles and a positive temperature slope (deshielding) for decanoic acid aggregates. These results suggest that an increase of temperature improves the mobility of the decanoate ion chains in the micelles, thus inducing the methyl groups of the decanoate ion micelles to spend more time near the micelle-water interfaces. In turn, the size of polydisperse decanoic acid aggregates increases with temperature.
Andrade-Dias
Cristiano
C
Department of Chemistry, University of Aveiro, CICECO, P3810-193 Aveiro, Portugal. teixeiradias@ua.pt
Lima
Sérgio
S
Teixeira-Dias
José J C
JJ