In this work, the classical theory of polymer/polyelectrolyte gel swelling is reviewed. This formalism is easy to understand and has been widely applied to gels and microgel particles. Nevertheless, its limitations and obscure aspects should be known before use. The case of temperature-sensitive gels is discussed in some detail because it deserves particular clarification. The application to experimental swelling data (of both gels and microgels) is also reviewed. In this way, strengths and weaknesses of this approach can be elucidated. Moreover, other formalisms are also outlined. Many of them are inspired by the classical one. Their improvements are briefly commented in this case. Others are based on different grounds.

The use of nanotechnology in biotechnological applications has attracted tremendous attention from researchers. Currently many nanomaterials, such as soft nanoparticles, are under investigation and development for their use in biomedicine. Among soft nanoparticles, polymeric gels in the nanometre range, known as nanogel particles, have received considerable attention. Nanogel particles, which are formed by polymeric chains loosely cross-linked to form a three-dimensional network, swell by a thermodynamically good solvent but do not dissolve in it. Nanogels are composed of hydrophilic polymers capable of undergoing reversible volume-phase transitions in response to environmental stimuli. Among them, temperature-sensitive nanogels showing a volume phase transition temperature (VPTT) near physiological temperature have been investigated in detail. Nanogels based on biocompatible and temperature-sensitive polymers having a lower critical solution temperature (LCST) around 32 °C in aqueous solutions swell at low temperatures and collapse at high ones. This unique behavior makes these nanogels attractive for pharmaceutical, therapeutical, and biomedical applications. In this review, different synthesis strategies to produce this type of nanogels in dispersed media are revised. Special attention is paid to poly(N-vinylcaprolactam) (PVCL)-based nanogels due to their proven biocompatibility. On the other hand, an extensive review on the characteristics, preparation, and physicochemical properties of another type of soft nanoparticles, which are the bicelles, is presented. The different morphologies obtained depending on experimental conditions such as temperature, lipid concentration, and long- and short-chain phospholipids molar ratio are revised, emphasizing on an important property of bicelles: their alignment in the presence of a magnetic field, and presenting the most important applications of bicelles as membrane models in diverse conformational studies of proteins and membrane peptides, together with the possibilities of administration of such vesicles by systemic routes. A key challenge for the characterization of both soft nanoparticles (nanogels and bicelles) involves the elucidation of their colloidal properties. In this work, some colloidal features of these nanoparticles such as their size, electric double layer or the internal structure and motions of their chains are analyzed. In addition, an overview on the previous and current understanding of the methods and techniques employed in this colloidal characterization is presented, mainly from an experimental point of view. Finally, the most recent results on polyelectrolyte gels and bicelles obtained from computer simulations are also briefly commented. Concerning polyelectrolyte gels, this review is mainly focused on the most important feature of these systems, their large capacity of swelling, which has been explored by simulation in the last decade.

Some theories claim that the Poisson–Boltzmann approach could fail to describe the electric double layer of colloids under certain conditions as a result of neglecting ion size correlations. For instance, if the surface charge density and/or the electrolyte concentration are high enough, the counterion local density in the vicinity of charged surface could become so large that the particle charge would be overcompensated. This phenomenon is theoretically known as overcharging and, sometimes, should involve a ζ-potential reversal. Accordingly, this work looks into overcharging through electrophoresis experiments. The electrophoretic mobility has been measured for latex particles with moderate and large surface charge density in solutions of asymmetric electrolytes $\text{z:1}$ (symmetric electrolytes have been studied in a previous work). In order to find out the relevance of ion size correlations, results are analyzed within the so-called hypernetted-chain/mean-spherical approximation (HNC/MSA) as well as a Poisson–Boltzmann approach. In the case of divalent counterions $\text{(z=2),}$ the HNC/MSA seems to justify why a mobility reversal is hardly observed. For $\text{z=3,}$ our results suggest that ion correlations could play an important role and be mostly (or partially) responsible for mobility reversal in certain cases.

The knowledge of the potential between interacting colloidal particles is essential to describe and control the aggregation processes in which they are involved. However, interaction forces also bring out spatial ordering phenomena, which has been used to obtain helpful information on them in three-dimensional dispersions. In this paper we look into this possibility for colloidal monolayers spread at the water–air interface. The radial distribution function has been determined for an assembly of negatively charged polystyrene particles at low concentration and without an additional electrolyte. A simple inversion scheme based on the HNC closure has been proved to give reliable information on u(r), providing that particles are not strongly correlated. Apart from the expected repulsive forces, the application of the inversion procedure suggests the possibility of a long-ranged attractive interaction.

In this paper, the structure of the electric double layer in the presence of (mostly) multivalent counterions is investigated through Monte Carlo simulations. Unlike previous similar studies addressing this matter, the difference of this study lies in the use of realistic hydrated ion sizes. Additionally, two different methods for calculating energies in the Metropolis algorithm are applied. The obtained results show that the conclusions of preceding papers must be revised. In particular, our simulations suggest the existence of certain ion layering effects at high surface charge densities, which are not accounted for by integral equation theories in the case of divalent counterions. These layering effects could justify why the overcharging phenomena due to ion size correlations are hardly observable in real colloids with divalent counterions. The existence of charge inversion due to ion size correlations (and without requiring specific counterion adsorption) is probed for trivalent counterions. Moreover, the hypernetted-chain/mean-spherical-approximation is tested under conditions not studied yet.