Hydrogel networks, spanning the bulk of the material and entrapping water, are
especially advantageous for creating solid-like materials. Can we create noncovalent
materials that will represent a viable alternative to conventional covalent systems? It
has been suggested that materials consisting mostly of water, having a robust and
uniform 3D network (such as hydrogels), based on strong noncovalent interactions,
may lead to adaptive, versatile, and environmentally friendly “water-based plastics”
that are stimuli-responsive and recyclable, unlike conventional polymer-based plastic
materials [5]. The challenge lies in the rational design of supramolecular hydrogels,
since it involves at least two levels of hierarchy: the formation of fibers and their
further interaction to form a 3D network that entraps water [9].
Robustness and rational design of water-based assemblies are crucial for developing
functional noncovalent polymeric materials. An additional important property to consider is the intrinsic complexity of aqueous self-assembly. Whereas the structure of
aqueous equilibrating systems can be rationalized using Israelachvili packing parameters, kinetically controlled assemblies do not comply with them [10–13]. Thus, when
strong noncovalent interactions are involved, kinetic control allows the formation of
multiple products and may dominate the outcome of self-assembly processes
[14–17]. Furthermore, specific molecular interactions can result in very complex
assemblies, where both kinetic control and multiple interaction modes play a significant
role. For example, complex and diverse structures (dendrimerosomes) were shown to
self-assemble from relatively simple dendritic amphiphiles [18], allowing access to a
new family of complex nonconventional nanostructures in water. Aqueous selfassembly of simple polyalcohol-based amphiphiles such as glycerol monoolein or
phytantriol leads to cubosomes, which are bicontinuous nanostructures having cubic
morphology created by bilayers with complex periodic structures [11]. Advantageously,
aqueous self-assembly promotes unique structural complexity, yet the factors leading to
such complexity are presently not well understood. Lack of a mechanistic understanding
and the complex nature of the systems present a significant challenge for rational
noncovalent synthesis in water.
In this review we focus on two key questions: (1) Can we encode noncovalent
structural motifs rationally employing strong hydrophobicity? (2) How can we
fabricate noncovalent water-based materials that rival covalent ones? Clearly,
both goals are directly linked because if we can control the structure we can control
the function. We begin this review with an overview of the properties of aromatic
amphiphiles and their assembly aptitudes, followed by examples from our work that
address structure encoding and the development of noncovalent water-based
materials.
2 Aromatic Amphiphiles
Hydrophobic interactions between extended hydrophobic surfaces can be very strong
[19, 20]. For rigid, flat hydrophobic surfaces of 5 nm
2 in size, the “unfavorability” of
their exposure to water is more than 30 kcal/mol, as estimated using oil–water
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
365
especially advantageous for creating solid-like materials. Can we create noncovalent
materials that will represent a viable alternative to conventional covalent systems? It
has been suggested that materials consisting mostly of water, having a robust and
uniform 3D network (such as hydrogels), based on strong noncovalent interactions,
may lead to adaptive, versatile, and environmentally friendly “water-based plastics”
that are stimuli-responsive and recyclable, unlike conventional polymer-based plastic
materials [5]. The challenge lies in the rational design of supramolecular hydrogels,
since it involves at least two levels of hierarchy: the formation of fibers and their
further interaction to form a 3D network that entraps water [9].
Robustness and rational design of water-based assemblies are crucial for developing
functional noncovalent polymeric materials. An additional important property to consider is the intrinsic complexity of aqueous self-assembly. Whereas the structure of
aqueous equilibrating systems can be rationalized using Israelachvili packing parameters, kinetically controlled assemblies do not comply with them [10–13]. Thus, when
strong noncovalent interactions are involved, kinetic control allows the formation of
multiple products and may dominate the outcome of self-assembly processes
[14–17]. Furthermore, specific molecular interactions can result in very complex
assemblies, where both kinetic control and multiple interaction modes play a significant
role. For example, complex and diverse structures (dendrimerosomes) were shown to
self-assemble from relatively simple dendritic amphiphiles [18], allowing access to a
new family of complex nonconventional nanostructures in water. Aqueous selfassembly of simple polyalcohol-based amphiphiles such as glycerol monoolein or
phytantriol leads to cubosomes, which are bicontinuous nanostructures having cubic
morphology created by bilayers with complex periodic structures [11]. Advantageously,
aqueous self-assembly promotes unique structural complexity, yet the factors leading to
such complexity are presently not well understood. Lack of a mechanistic understanding
and the complex nature of the systems present a significant challenge for rational
noncovalent synthesis in water.
In this review we focus on two key questions: (1) Can we encode noncovalent
structural motifs rationally employing strong hydrophobicity? (2) How can we
fabricate noncovalent water-based materials that rival covalent ones? Clearly,
both goals are directly linked because if we can control the structure we can control
the function. We begin this review with an overview of the properties of aromatic
amphiphiles and their assembly aptitudes, followed by examples from our work that
address structure encoding and the development of noncovalent water-based
materials.
2 Aromatic Amphiphiles
Hydrophobic interactions between extended hydrophobic surfaces can be very strong
[19, 20]. For rigid, flat hydrophobic surfaces of 5 nm
2 in size, the “unfavorability” of
their exposure to water is more than 30 kcal/mol, as estimated using oil–water
Aqueous Supramolecular Polymers Based on Aromatic Amphiphiles: Rational. . .
365