Hindawi Publishing Corporation Advances in Condensed Matter Physics Volume 2015, Article ID 151683, 22 pages http://dx.doi.org/10.1155/2015/151683 Review Article Amphiphiles Self-Assembly: Basic Concepts and Future Perspectives of Supramolecular Approaches Domenico Lombardo,1 Mikhail A. Kiselev,2 Salvatore Magazù,3 and Pietro Calandra4 1 Consiglio Nazionale delle Ricerche (CNR), Istituto per i Processi Chimico-Fisici (IPCF), Viale F.S. D’Alcontres, No. 37, 98158 Messina, Italy 2Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Ulica Joliot-Curie 6, Dubna, Moscow 141980, Russia 3Dipartimento di Fisica e Scienze della Terra, Universita` di Messina, Viale F.S. D’Alcontres, No. 31, 98158 Messina, Italy 4Consiglio Nazionale delle Ricerche (CNR), Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Via Salaria km 29.300, Monterotondo Stazione, 00015 Roma, Italy Correspondence should be addressed to Domenico Lombardo; [email protected] Received 5 May 2015; Accepted 27 September 2015 Academic Editor: Ram N. P. Choudhary Copyright © 2015 Domenico Lombardo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Amphiphiles are synthetic or natural molecules with the ability to self-assemble into a wide variety of structures including micelles, vesicles, nanotubes, nanofibers, and lamellae. Self-assembly processes of amphiphiles have been widely used to mimic biological systems, such as assembly of lipids and proteins, while their integrated actions allow the performance of highly specific cellular functions which has paved a way for bottom-up bionanotechnology. While amphiphiles self-assembly has attracted considerable attention for decades due to their extensive applications in material science, drug and gene delivery, recent developments in nanoscience stimulated the combination of the simple approaches of amphiphile assembly with the advanced concept of supramolecular self-assembly for the development of more complex, hierarchical nanostructures. Introduction of stimulus responsive supramolecular amphiphile assembly-disassembly processes provides particularly novel approaches for impacting bionanotechnology applications. Leading examples of these novel self-assembly processes can be found, in fact, in biosystems where assemblies of different amphiphilic macrocomponents and their integrated actions allow the performance of highly specific biological functions. In this perspective, we summarize in this tutorial review the basic concept and recent research on self-assembly of traditional amphiphilic molecules (such as surfactants, amphiphile-like polymers, or lipids) and more recent concepts of supramolecular amphiphiles assembly which have become increasingly important in emerging nanotechnology. 1. Introduction we present a tutorial introduction to the basic aspect of traditional, head/tail(s) type, amphiphiles whose aggregation Self-assembly processes involving amphiphilic macromole- is driven by soft interactions such as hydrogen bonds and cules provide unique and new opportunities for designing steric effects and hydrophobic and electrostatic interaction. novel materials for advanced application in nanotechnology Moreover, we highlight important examples where complex [1–3]. The thermodynamic incompatibility between the dif- processes such as the structure modulation and control of ferent blocks causes a spatial organization into ordered mor- morphology by other structure directing interactions can phologies on the nanoscale with the production of novel stimulate advanced application in materials science as well structural features, as demonstrated by recent studies [3, 4]. as in biological and medicinal chemistry. Finally, we provide Leading examples can be found in biosystems where assem- insight into the novel structural features obtained by the blies of different amphiphilic macromolecular components precise tailoring of chemical structures and the efficient use and their integrated actions allow the performance of highly of noncovalent forces for the introduction of chirality, signal specific cellular functions [3–5]. In the first part of this review processing, and recognition processes. The reversibility of 2 Advances in Condensed Matter Physics O O + S Na O O− Sodium dodecyl sulfate (SDS) C10E4 Anionic O O O O O O O Na+ S Nonionic O − C8E6 O O O O O O O O O Polyoxyethylene alkyl ethers (CnEm) Sodium bis(2-ethylhexyl) sulfosuccinate (AOT) (a) (b) CH 3 Cationic Br− N+ Cetyltrimethylammonium bromide CH CH (CTAB) 3 3 (c) O O P O O O + − N O H O Phosphatidylcholine (PC) O (phospholipid) Zwitterionic (d) Figure 1: Example of most common nonionic (a), anionic (b), cationic (c), and zwitterionic (d) amphiphilic molecule. noncovalent interactions allows dynamic switching of nano- (Figure 1(a)). Anionic surfactants, which are widely used as structures morphology and functions in response to various detergents and soap for cleaning processes, consist generally external stimuli which further provides a flexible platform for of negatively charged headgroups and positively charged the design and fabrication of smart amphiphilic nanomateri- counterions (such as sodium, potassium, or ammonium als and functional supramolecular devices. ions). Carboxylate, sulfate, sulphonate, and phosphate are the commonly used polar groups (Figure 1(b)). Cationic surfac- tants consist of positively charged headgroups such as a qua- 2. Characteristic and Basic ternary ammonium and a halide ion as a counterion. Cetyl- Properties of Amphiphiles trimethylammonium bromide (CTAB) and sodium bis(2- ethylhexyl) sulfosuccinate (AOT) are the most employed Amphiphiles are compounds possessing both hydrophilic cationic amphiphiles (Figures 1(b) and 1(c)). Finally in zwit- (water-loving) and lipophilic (fat-loving) or water-hating terionic amphiphiles the headgroups possess both a positive components. In conventional head/tail(s) amphiphiles the and negative charge, as it happens, for example, in the vesicle- lipophilic part consists generally of a long (saturated or unsat- forming phospholipid phosphatidylcholine (Figure 1(d)). For urated) hydrocarbon chain, while the hydrophilic head can example, if zwitterions contain a carboxylate and a proto- be either nonionic or ionic. Nonionic surfactants have either nated ammonium ion, it may behave as an anion (at high pH) polyether or polyhydroxyl units as the hydrophilic group. or a cation (at low pH) assuming then an amphoteric charac- A large variety of conventional nonionic surfactants consist ter. Some traditional amphiphilic molecules are reported in generally of a hydrophilic poly(ethylene oxide) chain, often Figure 1. called ethoxylates, connected with a hydrophobic alkyl chain, Due to their amphiphilicity (or surface activity), the and are generally used in cleaning applications with anionic amphiphiles polar headgroup interacts with the water while surfactants. For example, polyoxyethylene alkyl ethers, CE, the nonpolar lipophilic chain will migrate above the interface are nonionic surfactants made of hydrophilic oxyeth- (either in the air or in a nonpolar liquid). In this case the yleneunitsandanalkylchainwith methylene groups disruption of the cohesive energy at the interface favors Advances in Condensed Matter Physics 3 O O H H H H Donor O Hydrogen bond H H Acceptor Water network O O H H H H Figure 2: Schematic representation of polarity of water molecules and hydrogen bond in water. a microphase separation between the selective solvent and Table 1: Strength of the main noncovalent interaction involved in the dispersed phase of the amphiphile with the formation amphiphiles self-assembly. of many smaller closed interfaces or micelles-like aggregates. Bonding and interaction type kJ/mol Due to their ability to reduce the interfacial tension amphi- philic molecules are often called surfactants (i.e., surface Covalent bond 100–400 active agents). For this reason amphiphiles play an important Ion-ion/ion-dipole/dipole-dipole 200–300/50–200/5–50 role as emulsifiers, detergents, dispersants, and wetting and Hydrogen bond 4–120 foaming agents in several applications [6, 7]. Cation-n() interaction 5–80 - interaction 0–50 van der Waals interaction (<5 kJ/mol) 3. Soft Interaction in Amphiphiles Hydrophobic effects Entropy Self-Assembly Metal-ligand 0–400 Despite the weakness of the forces involved in amphiphiles self-assembly, the relevant number of these soft interactions will produce an overall effect that is strong enough to hold Figure 2, the polarity of water molecules results in the attrac- different amphiphile molecules together as well as to ensure tion of the negative (oxygen) portion and positive (hydrogen) their stability in solution [8, 9]. Moreover, the weakness of partialchargesandisatthebasisforthewater hydrogen the involved interactions makes the structure more flexible bonding which have many important implications on the enabling the system to withstand minor perturbation while properties of water and its relevant functions in biological preserving the reversibility of the self-assembled structure. systems [10–13]. The main weak (noncovalent) forces acting in the amphi- Together with the hydrogen bond, the hydrophobic effect philes self-assembly are hydrogen bonding, hydrophobic isthesecondmaindrivingforceofamphiphileself-assembly effects, electrostatic interaction, and van der Waals forces. into various