Dipartimento di Fisica, Università di Roma La Sapienza, Rome, Italy.
J Phys Chem B. 2011 Jun 9;115(22):7248-55. doi: 10.1021/jp109326e. Epub 2010 Dec 20.
The lateral correlated adsorption of polyions onto oppositely charged vesicles, leading to the formation of stable equilibrium clusters of mesoscopic size, is associated to the release of a fraction of counterions, initially condensed on the polyion chains. This ulterior release of counterions provokes an increase of the number of free ions, besides the ones due to the partial ionization of both charged particles and polyions, that can be appropriately monitored by means of electrical conductivity measurements of the whole system. We have investigated this behavior in a suspension of cationic vesicles made up by dioleoyl trimethyl ammonium propane (DOTAP) liposomial vesicles interacting with an anionic polyelectrolyte composed by polyacrylate sodium salt. This system has been in the past extensively studied by us by means of different experimental techniques, and its behavior has been sufficiently characterized, as far as hydrodynamic and electrical properties are concerned. In this note, we report on the dc electrical conductivity behavior during the whole aggregation process, from the single polyion-coated liposomal particles, to polyion-induced liposome clusters, to finally polyion-fully covered liposomes, in polyion excess conditions. We have evaluated the excess of released counterions on the basis of the standard theory of the electrical properties of aqueous charged solutions and compared this quantity with the one predicted by the lateral correlation adsorption model. The agreement is quite good, offering strong experimental evidence of the role played by the release of counterions in the aggregation process. Finally, we have considered a similar liposomial system, where the lateral correlation adsorption was inhibited by structural reasons, having replaced the polyion by a simple electrolyte, whose dissociated ions will adsorb randomly at the particle surface, rather than in a correlated manner. In this case, no counterion release upon complexation occurs, and the electrical conductivity of the suspension approaches the one theoretically expected.
聚离子在带相反电荷的囊泡上的横向相关吸附，导致介观大小的稳定平衡簇的形成，与最初凝聚在聚离子链上的部分抗衡离子的释放有关。这种抗衡离子的进一步释放会导致游离离子数量的增加，除了由于带电荷粒子和聚离子的部分电离而产生的游离离子之外，还可以通过整个系统的电导率测量来适当监测这些游离离子。我们研究了由二油酰基三甲基铵丙烷（DOTAP）脂质体囊泡组成的阳离子囊泡悬浮液与由聚丙烯酸钠盐组成的阴离子聚电解质之间的这种相互作用。过去，我们通过不同的实验技术对该系统进行了广泛的研究，并对其行为进行了充分的表征，就水动力和电特性而言。在本说明中，我们报告了在整个聚集过程中的直流电导率行为，从单个聚离子包覆的脂质体颗粒，到聚离子诱导的脂质体簇，最后是聚离子完全覆盖的脂质体，在聚离子过量的条件下。我们根据水溶液带电溶液电特性的标准理论评估了过量释放的抗衡离子，并将其与侧向相关吸附模型预测的数量进行了比较。这种情况相当吻合，为抗衡离子在聚集过程中所起的作用提供了强有力的实验证据。最后，我们考虑了一个类似的脂质体系统，其中由于结构原因，侧向相关吸附被抑制，聚离子被简单的电解质取代，其离解离子将随机吸附在颗粒表面，而不是以相关的方式吸附。在这种情况下，复合物形成时不会发生抗衡离子释放，悬浮液的电导率接近理论上预期的值。
