Elsevier

Journal of Colloid and Interface Science

Volume 526, 15 September 2018, Pages 356-365
Journal of Colloid and Interface Science

Manipulating the fractal fiber network of a molecular gel with surfactants

https://doi.org/10.1016/j.jcis.2018.05.007Get rights and content

Abstract

Supramolecular assembly governs the formation and properties of many soft materials. Hence, it is significant to develop convenient approaches to control the assembly process. In this work, it is demonstrated that by using surfactants consisting of a sorbitan group (either ethoxylated or not) and an aliphatic chain, as additives, the fractal fiber network structure of a π gelator (with two alkyl chains) can be engineered. The two surfactants, which have the same hydrophobic tails but different hydrophilic heads, demonstrate different effects on the fiber network of the gelator. The surfactant with a large hydrophilic head (ethoxylated sorbitan) promotes fiber tip branching and that with a smaller hydrophilic head (non-ethoxylated sorbitan) enhances fiber side branching. Fractal analysis based on the Avrami model also demonstrates enhancement of fiber branching by the surfactants. Furthermore, the fluorescence emission of the gelator is enhanced by more than 30%. The observations have significant implications in engineering a class of supramolecular materials.

Introduction

Molecular gel is a class of supramolecular materials that have three-dimensional hierarchical structures [1], [2], [3], [4]. The multilevel structures, which have fractal geometries, determine the macroscopic properties and performance of the gels [5], [6], [7], [8], [9]. Controlling the assembly and structures of molecular gels has been a significant research effort in recent years. However, most of attention has been focused on the molecular modifications of available gelators or design of new gelators [10], [11], [12]. Controlling the molecular assembly with a non-chemical approach, on the basis of the nucleation and growth mechanism of gelation, has been demonstrated to be a more convenient approach, although it has received less attention [13], [14]. On the basis of this mechanism, the formation of fibers in a gel is initiated by primary nucleation of the gelator followed by growth of fibers. This process can be controlled by tuning the thermodynamics and kinetics of nucleation [13]. The most important thermodynamic parameter is supersaturation/supercooling [15], [16]. The nucleation kinetics can be tuned by suitable additives that have strong interfacial adsorption properties [14], [17], [18]. Such an additive can adsorb on the surface of substrates (e.g. dust particles and air bubbles) in a solvent to affect the primary nucleation of a gelator. During fiber growth, the adsorption of additives can hinder the integration and nucleation of gelator molecules onto fiber surface to retard fiber growth. It can also enhance structural mismatch between the new crystal layer and the existing crystal surface so that fiber branching is enhanced.

Due to their strong interfacial adsorption properties, surfactants have been generally used to tune crystal growth and to control the formation of nanostructures [19], [20]. Their uses for control of molecular gelators are very limited [21], [22]. During the nucleation and growth of gelator fibers, an enormous interfacial area is created between the fibers and solvent. Surfactant molecules can adsorb on the interface to affect the nucleation and growth of fibers so that the hierarchical structure of a gel can be tuned. In the case of growth of inorganic crystals, molecules of a surfactant generally adsorb on a certain crystal facet, which interrupts the integration of new crystal layers on to the facet, to affect the growth kinetics and morphology of crystals and the surfactant molecules desorb from the facet afterwards. In contrast, in an organic system like a molecular gel, the gelator molecules can interact with surfactant molecules through various non-covalent forces, depending on their molecular structures, and surfactant molecules may be incorporated in gel fibers. Although the feasibility in using surfactants to tune the nucleation and growth of gelator fibers has been demonstrated [21], [22], a good understanding on the role of surfactant molecular structure has not been achieved. Such an understanding is significant to the design of a convenient approach for manipulating the structure of a molecular gel. A surfactant molecule is amphiphilic with a hydrophilic head and one or two hydrophobic tails. The tails of most surfactants are hydrocarbon chains, while the heads are quite different in terms of chemical composition, charge and sizes. Hence, understanding the effects of hydrophilic heads on the fiber network of molecular gels is interesting and significant. Due to their low toxicity, non-ionic surfactants especially those obtainable from natural resources have replaced synthetic surfactants in many applications. Therefore, such surfactants are targets of this work.

In this work, we studied the fiber formation of a molecular gelator 2,3-di-n-decyloxyanthracene (DDOA) in dimethyl sulfoxide (DMSO) in the presence of two non-ionic surfactants, Polyoxyethylene (20) sorbitan monooleate (Tween 80) and Sorbitan monooleate (Span 80). These two surfactants have the same hydrophobic tails, but different hydrophilic heads. The sorbitan groups in Tween 80 are ethoxylated so that the hydrophilic head of this surfactant is much bigger and more rigid than that of Span 80. The selection of these surfactants will help illustrate how the hydrophilic head of a surfactant affect the assembly of a molecular gelator. During gel formation, fibers grow faster in the longitudinal direction; hence, the fiber tip surface is more energetically unstable which favors the adsorption of more rigid molecules to reduce the interfacial tension between the tip surface and the solvent [13], [14]. On the basis of energetic and kinetics considerations, Tween 80 may prefer to adsorb on fiber tip surface. The surfactants are derived from food products and hence are biocompatible/edible. For example, Tween 80 is generally used as an emulsifier in foods (e.g. ice cream) and drug formulations [23]. DDOA, with an aromatic moiety and alkyl chains, represents a class of molecular gelator that assemble through π-π stacking and van der Waals force. The molecular structures of DDOA and the surfactants are shown in Chart 1. The fiber networks of DDOA were characterized using confocal microscopy and scanning electron microscopy, and 1H NMR spectroscopy was used to investigate the interactions between surfactant and DDOA molecules.

Section snippets

Materials

The surfactants Tween 80 (premium, product P6224), Span 80 (product 85548), Tween 20 (premium, product P2887) and Span 40 (product 388920), dimethyl sulfoxide (DMSO) (>99.0%), and deuterated DMSO (D, 99.96%), as well as all the chemicals for the synthesis of DDOA were obtained from Sigma-Aldrich. DMSO was dried with molecular sieve before use. All the other chemicals were used as received.

Methods

Synthesis of DDOA: DDOA was synthesized and purified following reported procedures [24].

Gel formation and

Influence of surfactants on fiber network structure of DDOA

In the absence of any surfactant, DDOA crystallizes into spherulitic fiber networks. In such a network, a primary nucleation center is evident. Fibers nucleate and grow radially from the nucleation center. Both the confocal and SEM images demonstrate limited branching of the DDOA fibers (Fig. 1a and b). The median diameter of the fibers are 127 nm (Fig. 1c). As shown in Fig. 1c, fiber bundling can be identified in the SEM image. Only single fibers (not bundled) were used to calculate the fiber

Conclusions

Molecular π gelators are a class of key materials for fabricating functional materials with many applications such as foods, cosmetics and optoelectronics [5], [6], [25]. The fiber network structure, especially fiber branching, determines the porous structure of a gel, which is important for its functions, for example controlled release of nutrients or active compounds. Controlling fiber branching using suitable additives is a convenient approach to manipulate the fiber network structure of

Acknowledgment

This work was supported by the Australian Research Council (ARC) through a Future Fellowship Grant (FT130100057). JL Li also acknowledges support of the ARC through an Industrial Transformation Research Hub in Future Fibres (IH140100018).

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