Kurtz Rachel E, Lange Arno, Fuller Gerald G
Department of Chemical Engineering, Stanford University, Stanford, California, USA.
Langmuir. 2006 Jun 6;22(12):5321-7. doi: 10.1021/la060290i.
Langmuir monolayers of mixtures of straight-chain and branched molecules of hexadecanol and eicosanol were studied using surface pressure-area isotherms, Brewster angle microscopy, and interfacial rheology measurements. For hexadecanol mixtures below 30% branched molecules, the isotherms show a lateral shift to a decreasing area proportional to the fraction of straight chains. Above a 30% branched fraction, the isotherms are no longer identical in shape. The surface viscosities of both straight and mixed monolayers exhibit a maximum in the condensed untilted LS phase at pi = 20 mN/m. Adding branched chains results in a nonmonotonic increase in surface viscosity, with the maximum near 12% branched hexadecanol. A visualization of these immiscible monolayers using Brewster angle microscopy in the liquid condensed phase shows the formation of discrete domains that initially increase in number density and then decrease with increasing surface pressure. Eicosanol mixtures exhibit different rheological and structural behavior from hexadecanol mixtures. The addition of branched chains results in a lateral shift to increasing area, proportional to the fraction and projected area of both straight and branched chains. A phase transition is seen for all mixtures, including pure straight chains, at pi = 15 mN/m up to 50% branched chains. A second transition is seen at pi = 25 mN/m when the isotherms cross over. Above this transition, the isotherms shift in the reverse direction with increasing branched fraction. The surface viscosities of both straight and mixed monolayers show a maximum in the L2' phase near pi = 5 mN/m. The surface viscosity is constant for low branched fractions and decays beyond 15% branched chains.
使用表面压力-面积等温线、布鲁斯特角显微镜和界面流变学测量方法，对十六醇和二十醇的直链和支链分子混合物的朗缪尔单分子层进行了研究。对于支链分子含量低于30%的十六醇混合物，等温线显示出横向移动至面积减小，且减小的面积与直链部分的比例成正比。当支链部分超过30%时，等温线的形状不再相同。直链和混合单分子层的表面粘度在凝聚的未倾斜LS相（π = 20 mN/m）时均呈现最大值。添加支链会导致表面粘度非单调增加，最大值出现在约12%支链十六醇附近。在液体凝聚相中使用布鲁斯特角显微镜对这些不混溶的单分子层进行可视化观察，结果显示形成了离散的区域，这些区域的数量密度最初增加，然后随着表面压力的增加而减小。二十醇混合物表现出与十六醇混合物不同的流变学和结构行为。添加支链会导致横向移动至面积增加，增加的面积与直链和支链的比例及投影面积成正比。在π = 15 mN/m时，所有混合物（包括纯直链）直至50%支链都出现了相变。当等温线交叉时，在π = 25 mN/m处出现了第二次转变。在这个转变之上，等温线随着支链比例的增加向相反方向移动。直链和混合单分子层的表面粘度在L2'相（π ≈ 5 mN/m）时均呈现最大值。对于低支链比例，表面粘度是恒定的，而在超过15%支链时会下降。
