Invited Feature Article pubs.acs.org/Langmuir
Molecular Gels as Intermediates in the Synthesis of Porous Materials and Fluorescent Films: Concepts and Applications Rong Miao,† Junxia Peng,‡ and Yu Fang*,‡ †
Key Laboratory of Applied Surface and Colloid Chemistry of the Ministry of Education, School of Materials Science and Engineering and ‡School of Chemistry and Chemical Engineering, Shaanxi Normal University Xi’an 710062, People’s Republic of China ABSTRACT: Low-molecular-mass organic gelator (LMOG)based molecular gels are known as one of the most attractive soft materials and have received great attention since the early 1990s. In the last few decades, many LMOGs have been synthesized, and a series of theories have been proposed to better understand molecular gels. However, only limited applications of LMOGs have been realized for a variety of reasons, such as their lack of stability compared to chemical gels. Therefore, efforts to explore the applications of these materials are especially meaningful. As an example, this feature article mainly introduces studies on the application of LMOGs as intermediates in porous materials and fluorescent sensing films. Particular attention will be paid to gelator design, LMOG emulsion preparation, solid surface modification, and the practical application of the obtained materials. Concepts that are related to these studies, such as organic gel−water interface equilibria and molecular gel strategies, will be comprehensively illustrated. Finally, we will conclude with a study of LMOG-based intermediates. Some challenges and future perspectives related to these research areas will also be presented. It is anticipated that this feature article will not only contribute to the further understanding of LMOG-based intermediates but also will help to promote the practical application of molecular gels and facilitate development in related research areas. gels obtained have been better studied.12−16 Meanwhile, several reliable theories about such gels have been established.14,17−19 Thus, there is now more reliable guidance for gelator design, and the preparation of molecular gels has become much more predictable. Though great efforts have been made to promote further applications of these gels in recent years, only limited gains have been achieved.20,21 There are two obvious obstacles that hinder the application of LMOG-based gels: (1) evaporation of the organic liquids will probably result in the collapse of the gels and (2) many molecular gels are sensitive to the environment such that the gels may be destroyed by subtle stimuli from the environment. To avoid these drawbacks, organic liquids with high boiling points and/or encapsulation techniques can be involved under some circumstances.20 However, these methods often limit the number of the gelators, make the preparation process more complex, or restrict the application of the gels. An alternative path to practical applications of molecular gels is their effective use as intermediates in some special material preparations. Here, the gels are used as intermediates, and the volatile organic liquids are allowed to evaporate or the gels are polymerized afterward. In addition, features of LMOG-based gel
1. INTRODUCTION Gels usually appear as semisolid materials and demonstrate certain advantages over liquids and solids in some aspects.1−3 Low-molecular-mass organic gelator (LMOGs)-based gels are commonly recognized as supramolecular or molecular gels where organic molecules (often with a molecular weight of less than 3000) self-assemble into three-dimensional (3D) networks by noncovalent interactions, such as hydrogen bonding, π−π stacking, electrostatic interactions, van der Waals forces, and hydrophobic interactions.4−6 Except for common features of gels, including their 3D network structure, nonflowing nature, and abundance of solvent, LMOG-based molecular gels have several prominent characteristics:7−11 (1) Thermal reversibility. They are thermally reversible with the sol phases because of their physical gel nature. (2) Stimulus response. There is a delicate balance between the dissolution and crystallization of the LMOGs in the gels, and any disturbance (chemical or physical) may break down the gels. (3) Diverse choices for gelators. A large variety of gels can be obtained through the self-assembly of tailormade organic molecules that can be synthesized through wellestablished routes. (4) Broad adjustability. Many LMOGs can form gels in different organic liquids, and they show distinct selfassembly behaviors when the liquid or concentration varies. For nearly three decades, we have witnessed the increase and flourishing of studies of molecular gels. With the rapid development in characterization techniques and organic synthesis, increasing numbers of LMOGs have been synthesized, and the © 2017 American Chemical Society
Received: December 28, 2016 Revised: February 22, 2017 Published: February 27, 2017 10419
DOI: 10.1021/acs.langmuir.6b04655 Langmuir 2017, 33, 10419−10428
Langmuir
Invited Feature Article
Figure 1. Preparation of LMOG-based gel emulsions and the emulsion-templated porous materials. The photographs on right are adapted with permission from ref 31 (copyright 2012, American Chemical Society).
they are employed to prepare gel emulsions; (2) it is difficult to avoid phase inversion in solely particle-stabilized emulsions (Pickering emulsions) when the internal phase volume fraction is approximately 0.7, so surface modification is usually necessary to improve the emulsifying capability of the colloidal particles. These disadvantages result in poor stability of the gel emulsions and bring great challenges to porous material preparation. It is known that the gelator is only a very small part (
