Haller J, Behrends R, Kaatze U
Drittes Physikalisches Institut, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany.
J Chem Phys. 2006 Mar 28;124(12):124910. doi: 10.1063/1.2179424.
Using the equal volume criterion and also the pseudospinodal conception the critical demixing point of the triethylene glycol monoheptyl ether/water system (C7E3H2O) has been determined as Ycrit=0.1 and Tcrit=296.46 K (Y, mass fraction of surfactant). From density measurements the critical micelle concentration (cmc) followed as Ycmc=0.007 at 288.15 K and Ycmc=0.0066 at 298.15 K. The (static) shear viscosity etas and the mutual diffusion coefficient D of the C7E3H2O mixture of critical composition have been evaluated to yield their singular and background parts. From a combined treatment of both quantities the relaxation rate Gamma of order parameter fluctuations has been derived. Gamma follows power law with universal critical exponent and amplitude Gamma0=3.1 x 10(9) s(-1). Broadband ultrasonic spectra of C7E3H2O mixtures exhibit a noncritical relaxation, reflecting the monomer exchange between micelles and the suspending phase, and a critical term due to concentration fluctuations. The former is subject to a relaxation time distribution that broadens when approaching the critical temperature. The latter can be well represented with the aid of the dynamic scaling model by Bhattacharjee and Ferrell (BF) [Phys. Rev. A. 31, 1788 (1985)]. The half-attenuation frequency in the scaling function of the latter model is noticeably smaller (Omega12 (BF) approximately 1) than the theoretically predicted value Omega12 (BF)=2.1. This result has been taken as an indication of a coupling between the fluctuations in the local concentration and the kinetics of micelle formation, in correspondence with the idea of a fluctuation controlled monomer exchange [T. Telgmann and U. Kaatze, Langmuir 18, 3068 (2002)].
利用等体积准则以及拟旋节线概念，已确定三甘醇单庚醚/水体系（C7E3H2O）的临界分层点为Ycrit = 0.1且Tcrit = 296.46 K（Y为表面活性剂的质量分数）。通过密度测量，得出在288.15 K时临界胶束浓度（cmc）为Ycmc = 0.007，在298.15 K时Ycmc = 0.0066。对临界组成的C7E3H2O混合物的（静态）剪切粘度ηs和互扩散系数D进行了评估，以得出它们的奇异部分和背景部分。通过对这两个量的联合处理，推导出了序参量涨落的弛豫率Γ。Γ遵循具有通用临界指数的幂律，且幅度Γ0 = 3.1×10⁹ s⁻¹。C7E3H2O混合物的宽带超声光谱表现出一种非临界弛豫，反映了胶束与悬浮相之间的单体交换，以及由于浓度涨落引起的临界项。前者服从弛豫时间分布，在接近临界温度时会变宽。后者可以借助Bhattacharjee和Ferrell（BF）的动态标度模型[《物理评论A》31, 1788 (1985)]得到很好的描述。后一模型标度函数中的半衰减频率明显小于理论预测值Ω12 (BF) = 2.1（Ω12 (BF)约为1）。这一结果被视为局部浓度涨落与胶束形成动力学之间存在耦合的一个迹象，这与涨落控制单体交换的观点一致[T. Telgmann和U. Kaatze，《朗缪尔》18, 3068 (2002)]。
