Islam Md Nazrul, Kato Teiji
Venture Business Laboratory, Utsunomiya University, Yoto 7-1-2, Utsunomiya 321-8585, Japan.
J Chem Phys. 2004 Nov 22;121(20):10217-22. doi: 10.1063/1.1806818.
The surface phase behavior of condensed-phase domains formed during a first-order phase transition in Langmuir monolayers of diethylene glycol mono-n-hexadecyl ether at the air-water interface has been investigated by Brewster angle microscopy and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). A variety of two-dimensional (2D) structures are observed just after the appearance of the phase transition at different temperatures. At 10 and 15 degrees C, the domains are found to be small nuclei of irregular structures. Spiral structures are observed at 20 and 22 degrees C, while striplike structures at 24 degrees C. The spiral domains attain increasingly compact shape with increasing temperature, and finally become circular at >or=26 degrees C. Increases in temperature result in dehydration in the ethylene oxide chain, which increases the hydrophobicity, and impart to the molecules a longer-chain-like character. As a result line tension increases with increasing temperature, which probably outweighs the dipole-dipole repulsions showing circular domains at higher temperatures. The PM-IRRAS measurement reveals that the nu(as)(CH(2)) mode moves to lower wave numbers indicating that the LE-LC (liquid expanded-liquid condensed) phase transition during the compression of the monolayer involves changes in the conformational order of the molecules with a preferential increase in the planner trans zigzag conformation of the hydrocarbon chains. The nu(as)(CH(2)) mode in the LC region of the isotherm shows a constant value around 2917.8 cm(-1) indicating a stable state of the monolayer with an almost all-trans conformation of the hydrocarbon chains. The downward band at 1124 cm(-1) assigned to the nu(as)(C-O-C) mode indicates that the corresponding transition dipole moment is oriented perpendicular to the water surface.
通过布鲁斯特角显微镜和偏振调制红外反射吸收光谱（PM - IRRAS）研究了二甘醇单正十六烷基醚在空气 - 水界面的朗缪尔单层膜中一级相变过程中形成的凝聚相畴的表面相行为。在不同温度下，相变刚出现后就观察到了各种二维（2D）结构。在10和15摄氏度时，发现畴是不规则结构的小核。在20和22摄氏度时观察到螺旋结构，而在24摄氏度时观察到条纹状结构。随着温度升高，螺旋畴的形状越来越紧凑，最终在≥26摄氏度时变为圆形。温度升高导致环氧乙烷链脱水，增加了疏水性，并赋予分子更长链状的特征。结果，线张力随温度升高而增加，这可能超过了偶极 - 偶极排斥力，从而在较高温度下呈现圆形畴。PM - IRRAS测量表明，ν（as）（CH₂）模式向更低波数移动，这表明在单层膜压缩过程中的LE - LC（液体膨胀 - 液体凝聚）相变涉及分子构象顺序的变化，其中烃链的平面反式锯齿构象优先增加。等温线LC区域中的ν（as）（CH₂）模式在2917.8 cm⁻¹左右显示出恒定值，表明单层膜处于稳定状态，烃链几乎全为反式构象。在1124 cm⁻¹处归属于ν（as）（C - O - C）模式的向下谱带表明相应的跃迁偶极矩垂直于水面取向。
