Revisiting the Fundamentals in the Design and Control of Nanoparticulate Colloids in the Frame of Soft Chemistry

Abstract

This review presents thoughts on some of the fundamental features of conceptual models applied in the design of fine particles in the frames of colloid and soft chemistry. A special emphasis is placed on the limitations of these models, an acknowledgment of which is vital in improving their intricacy and effectiveness in predicting the outcomes of the corresponding experimental settings. Thermodynamics of self-assembly phenomena illustrated on the examples of protein assembly and micellization is analyzed in relation to the previously elaborated thesis that each self-assembly in reality presents a co-assembly, since it implies a mutual reorganization of the assembling system and its immediate environment. Parameters used in the design of fine particles by precipitation are discussed while referring to solubility product, various measures of supersaturation levels, induction time, nucleation and crystal growth rates, interfacial energies, and the Ostwald-Lussac law of phases. Again, the main drawbacks and inadequacies of using the aforementioned parameters in tailoring the materials properties in a soft and colloidal chemical setting were particularly emphasized. The basic and practical limitations of zeta-potential analyses, routinely used to stabilize colloidal dispersions and initiate specific interactions between soft chemical entities, were also outlined. The final section of the paper reiterates the unavoidable presence of practical qualitative models in the design and control of nanoparticulate colloids, which is supported by the overwhelming complexity of quantitative relationships that govern the processes of their formation and assembly.

Keywords: co-assembly; micellization; nanoparticulate colloids; self-assembly; soft chemistry.

Fig. 1

Reverse micelles drawn (a) and as outcomes of simulations (b) look different. Unlike in the idealized drawing, the surfactant head groups do not completely shield aqueous interior of the modelled reverse micelle. Blue spheres represent the surfactant head-groups, whereby smaller yellow spheres denote counterions. Reprinted with permission from [49, 50].

Fig. 2

A pyramid depicting the intertwined nature of crystallization, protein assembly and proteolysis during the process of amelogenesis.

Fig. 3

Solubility of silver chromate heavily depending on the presence of other salts in the solution (left) and a table displaying formation constants for some complex ions (right). Reprinted with permission from [80].

Fig. 4

Free energy change during the nucleation of a new phase, with ΔG_n denoting the net free energy and n_c being the critical nucleus size.

Fig. 5

Different pH vs. time curves (A, B, C) for three identically prepared metastable solutions (4 mM CaCl₂, 2.5 mM KH₂PO₄, 150 mM KCl, 1 mM Bis-Tris/HCl at room temperature), which precipitate calcium phosphate phases via different phase transition pathways.

Fig. 6

Concentration of the nuclei formed by homogeneous (HON) and heterogeneous (HEN) nucleation as a function of the supersaturation, Δμ/kT. Reprinted with permission from [83].

Fig. 7

Schematic illustration of the model for the protein-controlled crystal growth of tooth enamel apatite based on the adsorbed amelogenin assemblies (blue spheres), channeling calcium and phosphate ions from the solution onto the growing crystal surface (left), and the mechanism of growth of silicon nanowires in the vapor-liquid-solid (VLS) process. Reprinted with permission from [99].

Fig. 8

Nucleation rate as a function of supersaturation (a); different nucleation rates depending on whether nucleation proceeds homogeneously (C), heterogeneously on a foreign phase with variable nucleation efficiency (B), or heterogeneously on a foreign phase with uniform nucleation efficiency (A) (b); nucleation rate as a function of the interfacial tension between the crystal and the fluid (c); and the size of a critical nucleus as a function of supersaturation estimated for a hypothetic system by means of vibrational frequency calculations (d).

Fig. 9

Distribution of counter-ions in the double layer surrounding a negatively charged colloidal particle (left) and an electron micrograph showing negatively charged nanosized gold particles adsorbed on electronegative plate-shaped kaolin crystals (right). Although kaolin platelets are negatively charged as a whole, their edges are electropositive and as such attract the gold particles onto them. Reprinted with permission from [148, 149].

Fig. 10

Together with the first two images, this yellow line gives rise to a smiling face. An imperfection, a loop and a drive: a key to it all. Hans-Georg Gadamer noticed that the meaning of every letter and symbol, including this simple line, can be seen as an intersection of horizon of potential meanings ascribed to it by the creator and a similar horizon created by the interpreter. A simple line such as this one can thus be seen as arising from the touch between mind and Nature, I and Thou, implicitly telling stories about both.