Department of Radiation, Radionuclides & Reactors, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands.
J Phys Chem A. 2010 Feb 25;114(7):2412-26. doi: 10.1021/jp9067735.
The sol-gel transition of fat-free milk by acidification was studied with neutron scattering experiments and Monte Carlo simulations. Spin-echo small angle neutron scattering (SESANS) and ultrasmall angle neutron scattering (USANS) experiments were performed to measure the static structure of milk and yogurt, as well as the aggregation kinetics. Colloidal gelation was simulated from a reaction limited domain (RLCA) to the diffusion limited regime (DLCA) as cluster-cluster aggregation of adhesive, hard spheres on a three-dimensional lattice. Comparisons were drawn between experimental and numerical correlation functions. Milk was modeled as a suspension of casein micelles in water, and its structure was described with a dilute system of solid spheres with a log-normal distribution of sizes. The structure and formation of yogurt were described with a self-affine model, used for systems containing heterogeneities with a wide range of sizes. Simulation speed was increased by 1 order-of-magnitude using a new algorithm to eliminate dead time. Observations by SESANS and USANS of milk particle sizes and yogurt length scales were consistent and agreed well with literature. Kinetic USANS data yielded reliable information about the growth of typical length scale during aggregation. The simulation model predicted the measurement data qualitatively best staying close to the RLCA regime until large structures had formed. Correlation lengths were in good quantitative agreement, but longest simulated length scales were a of factor 2(1)/(2) below experimental findings. We conclude that small, mobile aggregates are formed during the first 3 h, mostly influencing the dimensionality of the system and that large, inert structures are formed from 2 up to 8 h, which determine the typical length scale.
通过酸化研究了无脂牛奶的溶胶-凝胶转变，采用了中子散射实验和蒙特卡罗模拟。进行了自旋回波小角中子散射（SESANS）和超小角中子散射（USANS）实验，以测量牛奶和酸奶的静态结构以及聚集动力学。从反应受限区域（RLCA）到扩散受限区域（DLCA）模拟胶体凝胶化，作为在三维晶格上的粘性硬球的簇-簇聚集。比较了实验和数值相关函数。将牛奶模拟为在水中的酪蛋白胶束悬浮液，其结构用尺寸呈对数正态分布的稀固态球体的稀释系统来描述。酸奶的结构和形成用自仿射模型来描述，该模型用于具有广泛尺寸异质性的系统。通过使用一种新算法消除死时间，将模拟速度提高了 1 个数量级。SESANS 和 USANS 对牛奶颗粒尺寸和酸奶长度尺度的观察结果是一致的，并且与文献相符。动力学 USANS 数据提供了有关聚集过程中典型长度尺度增长的可靠信息。模拟模型定性地最佳预测了测量数据，直到形成大结构为止，它都接近 RLCA 区域。相关长度的定量吻合较好，但最长的模拟长度尺度比实验结果低 2^(1)/(2)。我们的结论是，在最初的 3 小时内形成了小的、可移动的聚集体，主要影响系统的维度，并且在 2 到 8 小时之间形成了大的、惰性的结构，决定了典型的长度尺度。
