Scotti Andrea
Institute of Physical Chemistry, RWTH Aachen University, 52056, Aachen, Germany.
Soft Matter. 2021 Jun 9;17(22):5548-5559. doi: 10.1039/d1sm00277e.
The volume occupied by colloids in a suspension - namely the volume fraction - is the thermodynamic variable that determines the phase behavior of these systems. While for hard incompressible spheres this quantity is well defined, for soft compressible colloids such as microgels - polymeric crosslinked networks swollen in a good solvent - the determination of the real volume occupied by these particles in solution is particularly challenging. This fact depends on two aspects: first the surface and, therefore, the volume of the microgels is hard to define properly given their external fuzziness; second, microgels can osmotically deswell, deform or interpenetrate their neighbors, i.e. change their shape and size depending on the solution concentration. Here, the form factors of few hydrogenated microgels embedded in a matrix of deuterated but otherwise identical microgels are measured using small-angle neutron scattering with contrast variation. From the analysis of the scattering data, the variation of the volume of the microgels as a function of concentration is obtained and used to calculate the real microgel volume fraction in solution. Soft neutral microgels are shown to facet already at low concentrations while in contrast, harder microgels maintain their shape and change their volume.
悬浮液中胶体所占的体积（即体积分数）是决定这些系统相行为的热力学变量。对于硬的不可压缩球体，这个量定义明确，但对于软的可压缩胶体，如微凝胶（在良溶剂中溶胀的聚合物交联网络），确定这些颗粒在溶液中实际占据的体积极具挑战性。这一情况取决于两个方面：其一，由于微凝胶外部模糊，其表面进而体积难以恰当界定；其二，微凝胶会通过渗透作用收缩、变形或与相邻微凝胶相互渗透，即根据溶液浓度改变其形状和大小。在此，利用具有对比度变化的小角中子散射测量了嵌入氘代但其他方面相同的微凝胶基质中的少数氢化微凝胶的形状因子。通过对散射数据的分析，得出微凝胶体积随浓度的变化情况，并用于计算溶液中实际的微凝胶体积分数。结果表明，软中性微凝胶在低浓度时就会出现刻面，而相比之下，较硬的微凝胶则保持其形状并改变其体积。
