This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Invited Feature Article Cite This: Langmuir 2019, 35, 7603−7616
pubs.acs.org/Langmuir
Competition and Cooperation among Different Attractive Forces in Solutions of Inorganic−Organic Hybrids Containing Macroionic Clusters Jiancheng Luo and Tianbo Liu*
Downloaded via 5.62.152.23 on August 13, 2019 at 20:32:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States ABSTRACT: Hybrids composed of nanoscale inorganic clusters and organic ligands are ideal models for understanding the different attractive forces during the self-assembly processes of complex macromolecules in solution. The counterion-mediated attraction induced by electrostatic interaction from the large, hydrophilic macroionic clusters can compete or cooperate with other types of attractive forces such as hydrophobic interactions, hydrogen bonding, π−π stacking, and cation−π interactions from the organic ligands, consequently determining the solution behaviors of the hybrid molecules including their self-assembly process and the final supramolecular structures. The incorporation of organic ligands also leads to interesting responsive behaviors to external stimuli. Through the manipulation of the hybrid composition, architecture, topology, and solution conditions (e.g., solvent polarity, pH, and temperature), versatile self-assembled morphologies can be achieved, providing new scientific opportunities and potential applications.
■
INTRODUCTION Understanding the roles and interplay of multiple noncovalent interactions in solution is important because they regulate the solution behavior of numerous self-assembly and biophysical processes1−7 but also challenging because these forces have various interaction strengths and effective distances and different responses to the change in external conditions.8−11 The well-defined inorganic−organic hybrid macromolecules involving distinct moieties (e.g., hydrophilic and hydrophobic) and different types of interactions (e.g., electrostatic interaction, hydrogen bonding, hydrophobic interaction, and π−π stacking) are valuable models for understanding such forces.12 On the other hand, the coexistence of multiple forces and the tunable relative strengths among them impart to hybrid molecules more fascinating solution behaviors compared to those of classic inorganic ions/clusters,13 surfactants,14,15 block copolymers,16,17 and colloidal suspensions,18 with an important additional advantage of no or very small polydispersity. Several types of inorganic molecular clusters, including polyoxometalates (POMs), 19 polyhedral oligomeric silsesquioxane (POSS),20 fullerene (C60),21,22 and other small nanoparticles,23 have been broadly used as inorganic components to fabricate well-defined inorganic−organic hybrids, as summarized in a number of reviews.24−30 Consequently, these hybrids have been studied extensively in different states such as solution,12,31−33 liquid crystalline,34,35 ionic liquids,36 bulk crystalline,37 and interfaces.38−40 In this feature article, we will present solution studies with a series of inorganic−organic hybrids based on the charged inorganic molecular clusters (macroionic clusters) as shown in Figure 1. This feature article mainly covers the work from our group due to the requirement and the limited length, but it does © 2019 American Chemical Society
Figure 1. Representative molecular architectures of inorganic−organic hybrids in this feature article. The blue balls represent fully hydrophilic macroionic clusters, the yellow balls represent hydrophobic molecular clusters, and the red lines represent organic molecules. The incorporation of different organic molecules onto macroionic clusters provides a feasible way to study the coexistence of multiple attractive forces in solution.
not suggest that we ignore or undervalue the contributions from many other groups or work involving such hybrids in other areas beyond the solution state.28,41−48 The fundamental logic that we have applied to understand these complex solution systems is summarized in Table 1. When various intermolecular forces (especially attraction, as solvation, electrostatic repulsion, and particle Brownian movement keep Received: February 18, 2019 Revised: May 16, 2019 Published: May 22, 2019 7603
DOI: 10.1021/acs.langmuir.9b00480 Langmuir 2019, 35, 7603−7616
Langmuir
Invited Feature Article
Table 1. Features of Several Attractive Forces in Macroionic Solutionsa response to external conditions electrostatic interaction cation−π interaction hydrogen bonding π−π stacking van der Waals forces hydrophobic interaction
strength (kcal/mol)
effect distance
solvent polarity ↑
temperature ↑
extra electrolyte
1−20 5−40 2−10 0−10 0.1−1 0−10
long
