CONFORMATIONAL STATE OF NONIONIC SURFACTANTS - A RAMAN-SPECTROSCOPIC STUDY OF ALPHA[4-(1,1,3,3-TETRAMETHYLBUTYL)PHENYL]-OMEGA-HYDROXYOLIGO(OXYETHYLENE)S

Raman spectra have been measured for polydisperse nonionic surfactants alpha-[4-(1,1,3,3-tetramethylbutyl)phenyl]-omega-hydroxyoligo(oxyethylene)s (Triton X-100, X-45, and X-405) and their related compounds which include 4-(1,1,3,3-tetramethylbutyl)anisole, 4-tert-pentylanisole, 4-tert-butylanisole, 4-methylanisole, 1,1,3,3-tetramethylbutyl chloride, 1,1,3,3-tetramethylbutyl bromide, 1,1,3,3-tetramethylbutyl mercaptan, poly(ethylene glycol), etc. The Raman bands of the surfactants have been assigned to the respective constituent atomic groups, i.e., 1,1,3,3-tetramethylbutyl (tC8), oxyethylene (OE), and phenyl groups, by careful spectral comparisons with the pertinent model compounds. The bands associated with the OE chain have been assigned to particular local conformations on the basis of the conformation-spectrum correlations previously derived. It has been shown that the present band assignments are not consistent with those proposed by previous authors, and the conformational state of the surfactants has been reexamined accordingly. The temperature dependence of the Raman intensities of Triton X-100 shows that the gauche conformation of the hydrophobic tC8 group is more stable than the trans. The conformational state of the tC8 group varies persistently with water addition throughout in the isotropic micellar solution phase. The conformation of the hydrophilic OE chain of neat liquid Triton X surfactants consists of diverse conformational fragments, and an overall conformational state of the chain is substantially disordered. In aqueous solutions, the gauche conformation of the OCH2CH2O group is more favored than the trans with more water dilution. The conformational state of the OE chain of the surfactants is suggested to be appreciably ordered in very dilute solutions by assuming a reduced number of uniform conformations, which are probably similar to the helical conformation as found for solid poly(oxyethylene). The extended zigzag conformation of the OE chain is almost unlikely to exist. The predominance of the uniform helical conformations of the OE chain, which should be greatly stabilized in aqueous media but not in the neat liquid, is suggestive of a rather distinct boundary between the hydrophobic core and the head-group shell of the micelles. The persistent conformational change of the hydrophobic tC8 group with water addition is interpretable by the change of the size and shape of the aggregates.