Aggregate Structures of Asymmetric Di-Alkyl Phosphate Anions and the Role of Conformations about the Polar Region: SANS, Cryo-TEM, Raman Scattering, ¹³C NMR, and Selective NOE Studies

Abstract

The aggregate structures of two di-n-alkyl-phosphate anions (di-n-alkyl: n-butyl(n-dodecyl)(BDoP) and n-hexyl(n-dodecyl)(HDoP)) in water were studied by the use of SANS, cryo-TEM, Raman scattering, (13)C NMR, and selective NOE ((1)H{(1)H}) techniques. The results of SANS indicated that the different -CH(2)- lengths of the short chain led to a marked difference in the aggregational behavior of BDoP and HDoP. Cryo-TEM added direct images to support the average aggregation size and shape predicted by the SANS analyses. Raman scattering, (13)C NMR, and selective NOE results provided further evidence that variation of molecular conformations strongly contributed to variation in the shape of the aggregates. In particular, selective NOE was a powerful technique for investigating the dynamic structures of the hydrocarbon chains during growth of the micelles.

Complex, Dynamic Behavior of Extremely Asymmetric Di-n-Alkylphosphate-Anion Aggregates, the Long-Chain Effect and the Role of a Limiting Size: Cryo-TEM, SANS, and X-Ray Diffraction Studies

“…The plot for HDoP, which was reported in a previous paper, also provided two straight lines crossing at a concentration (3.90 × 10 –2 mol/l) very close to the second cmc, indicating that the growth of TLMs increases markedly above this concentration, estimated from the concentration-dependence of the cryo-TEM images. Thus, for HDoP the regions below and above the second cmc are termed regions I and II, respectively.…”

“…The slope in region III was greater than that in region II, reflecting the rapid increase in growth of highly ordered RLAs, as imaged by cryo-TEM. For HDoP, the increase of n from 109 to 220 in region II, also reflects the growth of TLMs.…”

“…HDeP is more likely to adopt a highly ordered RLA in water, compared with HDoP and HCeP. This structure may come from the difference in the main hydrocarbon chain length, since the shorter n -decyl chain conforms easily to an ordered structure upon aggregation …”

“…For HDoP, the first and second cmcs (1.2 × 10 –3 and 3.7 × 10 –2 mol/l, respectively) were cited from our previous paper …”

“…The axis ratios of the minimal micelles were 1.7 for HDeP and 1.6 for HCeP, indicating a spheroidal shape for these minimal micelles. For HDoP, the n 0 value and axis ratio were 27 and 1.0, respectively.…”