Fehér Bence, Wacha András, Jezsó Bálint, Bóta Attila, Pedersen Jan Skov, Varga Imre
Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark; Institute of Chemistry, Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary.
Research Centre for Natural Sciences, Institute of Materials and Environmental Chemistry, Magyar tudósok körútja 2, 1117 Budapest, Hungary.
J Colloid Interface Sci. 2023 Dec;651:992-1007. doi: 10.1016/j.jcis.2023.08.002. Epub 2023 Aug 2.
In the last 20 years, it has been demonstrated that oppositely charged polyelectrolyte-surfactant (PE-S) mixtures are prone to forming kinetically arrested non-equilibrium aggregates, which are present in the prepared mixtures from rather low surfactant-to-polymer-repeat-unit ratios. Practically, this means that the PE-S mixtures used for the structural investigations of the formed PE-S complexes are typically a mixture of the primary PE-S complexes and large non-equilibrium aggregates of close to charge-neutral complexes.
In this work, we present a unique approach that allows the preparation of PE-S mixtures in the equilibrium one-phase region (surfactant binding β, is typically below 80%) without forming non-equilibrium aggregates. We used this method to prepare equilibrium, non-aggregated complexes of sodium poly(styrene sulfonate) (NaPSS, M = 17 kDa) and dodecyltrimethylammonium bromide (DTAB) (β = 10 - 70%) both in water and in an inert electrolyte (100 mM NaCl). The evolution of the complex structure was monitored by small-angle X-ray scattering (SAXS) as a function of increasing surfactant binding (β), and the measured scattering data were fitted by suitable structural models on an absolute scale where concentrations, compositions, and scattering contrasts calculated from molecular properties are used as restraints.
We could show that at low binding (β < 30%), the system is a mixture of bare polyelectrolyte coils and NaPSS-DTAB complexes containing a closed surfactant associates of low aggregation number wrapped by the polyelectrolyte chain. Once all polymer chains are occupied by a micelle-like surfactant aggregate, the aggregation number increases linearly with increasing surfactant chemical potential. Using the structural insight provided by the SAXS measurements, we could fit the experimental binding isotherm data with a physically coherent, simple thermodynamic model. Finally, we also compared the stoichiometric NaPSS-DTAB precipitate's structure with the equilibrium complexes' structure.
在过去20年中，已证明带相反电荷的聚电解质 - 表面活性剂（PE - S）混合物易于形成动力学上受阻的非平衡聚集体，这些聚集体在制备的混合物中，从相当低的表面活性剂与聚合物重复单元比例时就已存在。实际上，这意味着用于所形成的PE - S络合物结构研究的PE - S混合物通常是初级PE - S络合物与接近电荷中性络合物的大型非平衡聚集体的混合物。
在这项工作中，我们提出了一种独特的方法，该方法能够在平衡单相区域（表面活性剂结合β，通常低于80％）制备PE - S混合物，而不会形成非平衡聚集体。我们使用这种方法在水和惰性电解质（100 mM NaCl）中制备了聚（苯乙烯磺酸钠）（NaPSS，M = 17 kDa）和十二烷基三甲基溴化铵（DTAB）（β = 10 - 70％）的平衡、非聚集络合物。通过小角X射线散射（SAXS）监测络合物结构随表面活性剂结合增加（β）的演变，并将测量的散射数据用合适的结构模型在绝对尺度上进行拟合，其中根据分子性质计算的浓度、组成和散射对比度用作约束条件。
我们可以表明，在低结合（β < 30％）时，系统是裸聚电解质线圈和NaPSS - DTAB络合物的混合物，其中包含由聚电解质链包裹的低聚集数的封闭表面活性剂缔合体。一旦所有聚合物链被类似胶束的表面活性剂聚集体占据，聚集数随表面活性剂化学势的增加而线性增加。利用SAXS测量提供的结构见解，我们可以用一个物理上连贯的简单热力学模型拟合实验结合等温线数据。最后，我们还比较了化学计量的NaPSS - DTAB沉淀物的结构与平衡络合物的结构。
