High-pressure ATR spectroscopic study on conformation and hydration of oxyethylene

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Molecular conformation of nonionic surfactants alkyl / oligo(oxyethylene) diblock compounds H(CH_2)_n(OCH_2CH_2)_mOH (C_nE_n) and their hydration behavior in the C_nE_m-water binary system have been studied by high-pressure infrared spectroscopy. Molecular conformation, molecular packing, and hydration of the surfactants in the gel phase have been studies using a diamond attenuated total reflection cell (DATRC) and a diamond anvil cell (DAC).We have first several sampling method for high-pressure DATRC measurement, because no standard method is available in this procedure. Consequently, we have obtained a pressure up to ca. 30 MPa with opposed anvils arrangement between a DATRC element and a pressure shoe. Accordingly, infrared measurements in pressure range from ambient pressure to ca. 5 GPa became possible with DATRC and DAC.We have studied pressure dependent molecular form and packing for the C_nE_7 compounds, because C_nE_7s assume the * form, which is the molecular form different from the * form for the other C_nE_ms with m=3-6 and m【greater than or equal】 8 in the solid state. As the peculiar form for C_nE_7s results from molecular packing in the crystal, external pressure should perturb the molecular form and packing.For C_<10>E_7-H_2O system, Hydrogen bonding was strengthened with increasing pressure and a conformational transformation from the * form to the * form has been observed at about 1 GPa. For C_<16>E_7-H_2O system, on the other hand, conformational transformation has not been occurred but small wavenumber sift for the infrared bands attributed to the oligo (oxyethylene) part been observed.