Davis Jonathan R, Panagiotopoulos Athanassios Z
Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08540, USA.
J Chem Phys. 2008 Nov 21;129(19):194706. doi: 10.1063/1.3009183.
Grand canonical Monte Carlo simulations on a cubic lattice are used to examine aggregation and phase separation of model amphiphiles with bulky head groups. The amphiphiles studied consist of a rigid, roughly spherical nanoparticle attached to one or more flexible chains. Overlapping distributions of energy and density are combined via histogram reweighting to obtain the free energy and osmotic pressure as a function of temperature and concentration. Finite size effects are used to distinguish between first order transitions to a disordered liquid or lamellar phase and continuous transitions to micelles. The transition type depends on the relative size of the solvophobic and neutral portions of the amphiphiles; none of the systems studied here exhibit both types of transition. The critical micellar concentration increases with temperature over the range of conditions examined. Solvophobic nanoparticles with neutral chains phase separate when the attached chain is short and form micelles for longer attached chains. For structures with neutral nanoparticles and solvophobic chains, amphiphile geometry plays a key role in determining whether the micelles that form are spheres or flat bilayers. Nanoparticles with many chains tend to form flat bilayers, while those with only one or two chains form nearly spherical aggregates. Particles with long chains undergo macroscopic phase separation instead of micellization, and the temperature range over which the first order transition occurs depends on the total volume occupied by the solvophobic segments.
在立方晶格上进行的巨正则蒙特卡罗模拟，用于研究具有庞大头部基团的模型两亲分子的聚集和相分离。所研究的两亲分子由附着在一条或多条柔性链上的刚性、近似球形的纳米颗粒组成。通过直方图重加权将能量和密度的重叠分布相结合，以获得作为温度和浓度函数的自由能和渗透压。利用有限尺寸效应来区分向无序液体或层状相的一级转变和向胶束的连续转变。转变类型取决于两亲分子的疏溶剂部分和亲水部分的相对大小；这里研究的所有体系都没有表现出两种类型的转变。在所研究的条件范围内，临界胶束浓度随温度升高而增加。当连接链较短时，带有中性链的疏溶剂纳米颗粒会发生相分离，而当连接链较长时则会形成胶束。对于具有中性纳米颗粒和疏溶剂链的结构，两亲分子的几何形状在决定形成的胶束是球形还是扁平双层方面起着关键作用。带有多条链的纳米颗粒倾向于形成扁平双层，而只有一条或两条链的纳米颗粒则形成近似球形的聚集体。带有长链的颗粒会发生宏观相分离而不是胶束化，并且一级转变发生的温度范围取决于疏溶剂段所占的总体积。
