Gauche conformations, defect bands

Considerable experimental evidence (1-6,11) suggests that the methylene chains inside of a spherical micelle are almost as disordered as in the bulk liquid state (i.e. they contain a significant proportion of gauche conformers). The FTTR spectra of micellar SDS support this assertion, exhibiting CH2 stretching and scissoring band frequencies which are comparable to those found in the spectra of liquid hydrocarbons (1-6,11). A recent quantitative analysis of the CH2 defect modes of SDS has shown that the disorder of me methylene tails is similar to that found in liquid tridecane (11). 

Time - resolved spectra of a solid hydrocarbon layer on the surface of an internal reflection element, interacting with an aqueous solution of a nonionic surfactant, can be used to monitor the detergency process. Changes in the intensity and frequency of the CH2 stretching bands, and the appearance of defect bands due to gauche conformers indicate penetration of surfactant into the hydrocaibon layer. Perturbation of the hydrocarbon crystal structure, followed by displacement of solid hydrocaibon from the IRE surface, are important aspects of solid soil removal. Surfactant bath temperature influences detergency through its effects on both the phase behavior of the surfactant solution and its penetration rate into the hydrocaibon layer. 

Vibrational spectroscopy can be used to observe directly the gauche conformations involved in defects of polymer crystals. The methods used involve associating specific absorption bands active in the IR or Raman spectra with gauche conformations or even gauche-trans sequences. Selective deuteration is often used to identify the position of the defect... 

Antisymmetric CH2 Band Positions. The position of the antisymmetric methylene band in normal alkanethiol SAMs is sensitive to the conformational order of the hydrocarbon chains [10,23-25]. Shifts of these bands to higher wavenumbers are associated with decreases in interchain van der Waals interactions and increases in intrachain gauche defects (i.e., more liquid-like conformational states) [10,23-25]. Figure 6 shows the antisymmetric methylene band position as a function of the total number of carbon atoms in SAMs derived from a series of normal alkanethiols. The results are consistent with a model in which the conformational order (or crystallinity) of the films increases with increasing chain length [10,23]. 

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