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Nanotechnology can be defined as the manipulation of atoms and molecules at nano scale to produce devices, structures or systems having at least one novel/superior property. Materials having at least one dimension in the nano scale are called nanomaterials. Nanotechnology was first conceptualized by Richard Feynman in 1959 in his revolutionary talk "There's plenty of room at the bottom". The significance of nanotechnology is that, when the size of bulk materials is reduced to nanometer size range, the nanomaterials exhibit properties different from bulk materials, enabling unique applications. For example, opaque materials become transparent (copper), insoluble substances become soluble (gold), stable materials become combustible (aluminium). In fact, at normal scales, gold is chemically inert but at nanoscales, gold nanoparticles can serve as potent chemical catalysts.
Properties of nanomaterials:
Nanomaterials exhibit unique size-dependent properties because of two main reasons:
Firstly, the surface area to volume ratio of the nanomaterials is relatively larger than that of bulk materials of the same mass. This increases the chemical reactivity, affects strength and electrical properties of the material.
Secondly, quantum confinement is observed at nanometer sizes that changes the optical, electronic and magnetic properties of the material. The bandgap increases as the size of the material is reduced to nanometer range. This effect is caused by the phenomenon resulting from electrons and electron holes being constricted into a dimension which approaches the critical quantum measurement, known as the exciton Bohr radius.
Nanochemistry involves the study of synthesis and characterization of nanomaterials. Professor Geoffrey Ozin is regarded as the father of nanochemistry.
Applications of nanochemistry:
Nanochemistry has a wide range of applications in a variety of fields from medicine to electronics. Nanochemistry is employed in the formation of various useful materials. Some of them are:
Nanoparticles of various sizes and shapes like, gold, silver nanoparticles
Nanooxides such as iron oxide, cadmium oxide nanoparticles
Carbon nanotubes and other fullerenes
Nanopolymers and nanomembranes
Nanoformulations for cosmetics, example, in sunscreens, anti-ageing creams and so on
These materials have in turn, applications in various domains:
In semiconductor devices, such as nanocapacitors
As therapeutic molecules, nanomedicine, for diseases like cancer, Pakinson's
In targeted drug delivery
As taste enhancers (nano-foods) and for packaging in food industry
In fuel cells
In chemical and biosensors
In fabric technology
In solar cells
For manufacture of self-cleaning surfaces
MATERIAL SELF ASSEMBLY
Nanomaterial synthesis involves two strategies: top-down approach and bottom-up approach. Top -down approach is a subtractive process using physical and lithographic techniques to pattern the required structure whereas bottom-up approach is an additive process by assembly of the precursor atoms, molecules using molecular synthesis and chemistry.
Self assembly process is generally considered a bottom-up technique but can combine top-down and bottom-up approaches when small structures synthesized bottom-up are arranged on a template synthesized by top-down method. The classic definition of self assembly is "the spontaneous and reversible organization of molecular units into ordered structures by non-covalent interactions". Self assembly occurs due to the local interactions between the units without any external intervention.
The significance of self assembly is due to the spontaneity and precision of the process which is also reversible. Self assembly is important because, in nature, self assembly is the process by which several complex structures are synthesized ranging from galaxies to assemblies at nanoscale. Self assembled structures are ubiquitous in biological systems and chemistry. There are several examples of self assembly, for example, formation of lipid bilayer membrane, folded proteins, DNA helix are self assembly at the nanoscale. The formation of liquid crystals, colloids, micelles and folding of nucleic acids are some examples of self assembly at molecular scale.
Self assembly is now being utilized for synthesizing structures with precision and accuracy. The possibilities of synthetic chemistry have been extended by applying self assembly. Self assembly can create a delicate functional nanostructure without the time consuming process of one-by-one manipulation of its building unit. Several useful materials have been synthesized by self assembly such as dendrimers, carbon nanotubes, zeolites, mesoporous silica. These in turn have a wide range of applications in various fields.
Classification of self assembly:
One way of classifying the self assembly process is by means of the equilibrium state of the system. Based on this, self assembly can be static or dynamic. Static self assembly systems are stable, as the system approaches equilibrium on forming the ordered structure and therefore do not dissipate energy. The formation of molecular crystals, folded and globular proteins are some examples of static self assembly. In dynamic self assembly, the interactions between the components can occur only when the system dissipates energy. Reaction-diffusion patterns in oscillating chemical reactions like Belousov-Zabatinski reaction, bacterial colonies are some examples of dynamic self assembly.
Self assembly can also be classified based on the size or nature of the building units: atomic, molecular and colloidal (Figure 1); self assembly therefore has a broad length scale, ranging from Angström to centimetre. Different forces act depending on the length scale: at molecular scale, non-covalent forces like hypdrophobic, hydrophilic, Van der Waals, electrostatic interaction act whereas at micro-/mesoscale, capillary, magnetic and gravitational forces are involved.
Figure: Classification of self assembly based on size or nature of the building units.
Another way of classifying self assembly is based on the system where it occurs: biological or interfacial. Several intriguing self assembly processes occur in a biological system. Some examples are the formation of lipid bi-layer membrane by self assembly of lipids, the formation of quaternary structures of proteins, double-stranded DNA helix by hydrogen bonding. Langmuir monolayer which is formed by the self assembly of amphiphiles at the gas-liquid interface is an example of interfacial self assembly.
Forces involved in self assembly:
In terms of forces acting in the process, self assembly occurs when a balance is established between three different classes of forces namely, attractive (driving) forces, repulsive (opposition) forces and directional forces. Formation of micelles or micellization from surfactant molecules is a classic example of self assembly. Micelles are surfactant aggregates formed when surfactant molecules are dispersed in aqueous solution at a particular concentration known as the critical micelle concentration (CMC). In this case, the driving force is the hydrophobic attraction between the hydrophobic tails of the surfactant and the opposing force is the electrostatic repulsion resulting from charge-bearing and hydrated head groups. The hydrophobic attraction is a long-range force and acts as the primary force that brings the individual surfactant molecules together. However, as the process proceeds, repulsive forces begin to impose. At a certain point, a balance between the attractive and repulsive forces is attained and at that point micelles are formed.
The self assembly processes in biological systems have, in addition to the attractive and repulsive forces, a directional force that distinguishes the biological systems from micelles and colloidal systems. But directional forces are not only seen in biological systems; many bio-mimetic systems such as dendrimers, synthetic amino acids show distinctive directionality.
When only attractive and repulsive forces are involved in the process, self assembly is random, usually takes place in a single step and the self assembled aggregates have a non-hierarchical structure. But when directional force acts along with the two forces, self assembly is directional, mostly involves multiple steps and the aggregates have a hierarchical structure.
Characteristics of self assembly:
There are certain features that characterize the distinctiveness of self assembly from other chemical reactions which form larger structures from atoms and molecules:
A self assembling system comprises a group of atoms, molecules or aggregates that have been formed through self assembly or conventional chemistry. These components may be similar or different in chemical composition. The shape, topology and surface properties of the building units determine the characteristics of the structure formed.
The interaction of the components always results in a transition from less ordered state to a highly ordered state. This transition from to a higher order may not be true in a chemical reaction which, depending on its thermodynamic properties, may proceed to a less ordered state.
In self assembly, interactions between the components are based on weak interactions like Van der Waals interactions, hydrogen bonding, electrostatic interactions, capillary forces, Ï€-Ï€ interactions and solvophobic forces in contrast to more conventional covalent, metallic and ionic bonds. Among these, hydrogen bonding is most frequently used in self assembly. The complementary shapes of the building units are also essential.
The self assembled systems are thermodynamically stable with a lower Gibbs free energy than the unassembled units. As a result, the self assembled structures are generally defect-free compared to structures formed by other methods.
The self assembled systems are susceptible to perturbations exercised by the external environment due to the weaker interactions and its thermodynamic stability. These perturbations may cause variations in the thermodynamic variables and the interactions between the components can be modified. The course of the process may be strongly influenced by the interaction of the components with the environment.
The variations in the thermodynamic variables may, in some cases, bring the thermodynamic variables back to the initial condition which in turn causes the structure to revert to its initial configuration. In order to generate ordered structures, it is essential that the interactions between the components are reversible or such that the components can adjust their arrangement with respect to one another after the formation of the structure. Thus the strength of the association between the components must be similar to that of the opposition forces inclined to disrupt the structure.
Mobility of the components is necessary for self assembly process for bringing the components into contact. Although during self assembly, the complementary surface contacts on a growing structure can be maximized by reorganization so that a final structure with a thermodynamic minimum can be obtained. For molecular self assembly in solution, thermal mobility plays a key role in bringing the components into contact. Vibrational and mixing forces may be required at larger scales as Brownian motion becomes irrelevant with increase in size of the components.
Applications of self assembly:
Self assembly is the preferred method for the synthesis of complex structures over other chemical routes because the structures produced by self assembly are more accurate, precise and reproducible. Nanofabrication and micro-fabrication techniques employ self assembly to produce structures with great potential applications. Some of the structures obtained by self assembly and their applications are listed below:
Self assembled monolayers (SAM):
A self assemble monolayer (SAM) is formed by the spontaneous assembly of molecules into an ordered lattice on a substrate surface. Figure 2 represents the structure of a SAM. Initially, a specific group of the organic molecule binds to the substrate via one end (head) while the other end (tail) of the molecule forms the exposed surface of the SAM. This is followed by a rearrangement of the molecules adsorbed so that maximum surface coverage is achieved. The intramolecular interactions between the molecules adsorbed on the surface drives this process. These monolayers are highly oriented, ordered and packed. SAMs have gained attention because of the unique structural and chemical properties are exhibited by these monolayers and the opportunities it provides for tailoring solid surfaces.
Figure: Structure of a SAM
The canonical example of SAMs is the formation of alkanethiol monolayers on the surface of coinage metals like gold, silver, copper, platinum and palladium. The thiol group forms the head group which binds to the surface of the metal by losing the hydrogen atom. Thiol groups have high affinity towards coinage metals which acts as the primary driving force in the self assembly. The alkyl chain forms the tail group the terminal of which may be modified with a functional group. The resulting SAM can thus be tailored to be hydrophobic or hydrophilic or have functionality according to the terminal group. SAMs also form on the surfaces of semiconductors like GaAs and InP.
The SAMs have applications in various fields including electronics, biology, NEMS, MEMS and electrochemistry. The precise positioning of SAMs on substrates makes them excellent candidates as molecular electronic devices. The surface properties of electrodes can be modified using SAMs for electrochemistry, MEMS and NEMS. Patterned SAMs can also be used to functionalize biosensors.
Self assembly of carbon nanotubes:
Carbon nanotubes (CNTs) are carbon allotropes of fullerene structural family. They have a long, hollow cylindrical structure with walls formed by graphene (one-atom-thick carbon sheets). The CNTs have a high aspect ratio with diameter in nanoscale and length up to a few microns. CNTs are categorized as single-walled (SWCNT) or multi-walled (MWCNT) nanotubes. Significance of CNTs is owing to its extraordinary electrical, thermal conductivity and mechanical properties. As a result, CNTs find wide range of applications such as AFM (atomic force microscopy) tips, as carbon nanotube field effect transistors (CNTFET), in solar cells, in cancer therapy, ultracapacitors, biosensors, as structural composite materials.
CNTs can be synthesized by a variety of methods. CNTs synthesized by self assembly, though not very stable, offer easy functionalization and modification. In addition, by self assembly method, various morphologies other than nanotubes can be obtained like helical, twisted ribbon-like, rope-like fibrils.
Pre-formed CNTs have been self assembled to form various structures. Self assembly of single-walled or multi-walled CNTs can form two-dimensionally and three-dimensionally ordered structures, even single crystals of CNTs. This is useful for fabrication of devices like CNT cathodes for field emission displays.
Self assembly can be used to prepare and modify structured colloids. Face centred cubic (fcc) single crystals of gold nanoparticles can be obtained through self assembly. Spheres of metal oxides such as ZnO, CuO have been assembled into ordered arrays of diameters ranging from 100 nm to a few µm. These are used to fabricate photonic bandgap crystals which have applications in lasers and waveguides. Structures of specific shapes with nanoscale features at desired locations can be created by hierarchical self assembly. Such structures have great potential applications.
Dendrimers are spherical molecules which are repetitively branched, monodisperse and highly symmetric. They are also referred to as 'star polymers'. The synthesis of dendrimers involves a series of repetitive steps; starting from a central initiator core. The process may then proceed either outwards, as in divergent method or inwards, as in convergent method. Each growth step represents a generation of a dendrimer. Thus, with each generation, the molecular weight and number of surface active sites increases. When compared to linear polymers, globular dendrimers have significantly improved chemical and physical properties, high reactivity due to the presence of increased surface groups and ability to host guest molecules in the internal cavities. Dendrimers have several applications such as carriers in drug delivery, vectors in gene delivery, in sensors, for synthesis of nanoparticles.
Dendrimers can also be synthesized by self assembly and also act as building blocks for self assembly. Dendrimers are assembled to form bolaamphiphiles which can form micelles. Bolaamphiphiles lower the CMC required for the synthesis of micelles. Organometallic dendrimers have multiple co-ordination centres and electron transfer redox centres which lead to applications as novel catalysts, photochemical devices and as electron conversion and transfer devices.
Directed self assembly:
Directed self assembly is quite distinct from spontaneous self assembly. Physical, chemical and geometrical cues can be added to a self assembly process so that a desired structure may be obtained. Directing a self assembly may involve lithographically patterning a substrate on which self assembly occurs, so that self assembly occurs only on specific regions. Honeycomb structured patterns, grid structures, trenches and so on, have been formed by self assembly directed by lithographic patterning. The addition of structure-directive additives along with building units also directs the self assembly process. Application of external force, such as mechanical force in the case of Langmuir monolayers, also guides self assembly. Epitaxial growth of crystals on other crystal surface is possible by directed self assembly. Structures obtained from directed self assembly are being extensively used in semiconductor manufacture, for construction of chips and other electronic devices. Furthermore, directed self assembly also helps to repair defects in guide structures. Electrostatic and electromagnetic fields can be used to guide the formation of nanowires and CNTs on desired substrates.
MOLECULAR SELF ASSEMBLY
Spontaneous assembly of molecules, without any external intervention is said to be molecular self assembly. Molecular self assembly is ubiquitously found in biology, chemistry and materials science. Formation of molecular crystals, phase separated polymers, colloids, lipid bilayers, protein folding, nucleic acid folding, self assembled monolayers (SAMs) all are examples of molecular self assembly. The principles and characteristics of molecular self assembly are the same as self assembly of materials. Molecular self assembly also proceeds from a less ordered state to a highly ordered structure. But there is a difference in the forces directing the process. The self assembly of materials is directed by a wide range of forces based on the nature of the building units, such as non-covalent weak interactions, elastic, colloidal, capillary, gravitational forces and so on. Whereas, molecular self assembly is restricted to the weak non-covalent interactions like hydrogen bonding, Van der Waals forces, metal coordination and Ï€-Ï€ interactions. The structure formed by molecular self assembly is determined by the molecular structures. A variety of structures of different sizes and shapes can be synthesized by molecular self assembly. It is feasible to construct challenging molecular topologies using molecular self assembly. For example, molecular analogue of Borromean rings have been constructed using DNA. Previously unachievable nanostructures can thus be obtained by accurate control of intermolecular forces on molecular scale.
There are two types of molecular self assembly: intermolecular and intramolecular self assembly.
Intermolecular self assembly:
Intermolecular self assembly occurs between molecules to form supramolecular assemblies. Some examples are formation of micelles, SAMs, Langmuir monolayer, liquid crystal phases and so on.
Intramolecular self assembly:
Intramolecular self assembly is more commonly known as folding. The molecules involved in intramolecular self assembly are complex polymers that have the ability to form a well-defined, stable structure from random coil conformation. Typical example is protein folding, where secondary, tertiary and quaternary structures are formed by folding of primary structures.
Examples of molecular self assembly:
There are various examples of molecular self assembly, but SAMs and Layer-by-Layer (LbL) assembly have received significant attention because of their potential applications.
Self assembled monolayers (SAMs):
SAMs are molecular assemblies spontaneously formed on a substrate surface. (The formation of SAMs, examples and its applications has been dealt in the previous section). Instead of simply adding the solution of building units on the substrate, techniques such as chemical vapour deposition and molecular beam epitaxy can be used for deposition of the components onto the substrate. To yield desired structures with required functionality, SAMs can be patterned. There are various methods by which SAMs can be patterned. Local deposition of the self assembling molecules by dip-pen lithography and micro contact printing is one method. By this method, nanostructures will be formed only at those specific regions where the molecules have been deposited. Another method involves covering the entire substrate surface with SAMs and locally removing from regions where nanostructures are not desired. This can be achieved by techniques like scanning tunnelling microscopy (STM), atomic force microscopy (AFM) and ultraviolet irradiation. SAMs can also be patterned by modification of terminal groups of the molecules so as to increase/decrease the affinity of the molecules to the substrate, thereby controlling the regions where nanostructures will be deposited. Such patterned SAMs have can be used as biosensors. The tail groups of the SAMs can be modified so that they have an affinity towards the analyte molecule (cells, proteins or molecules) to be detected. Patterned SAMs with two types of building units have been used for aligning SWCNTs.
Layer-by-Layer (LbL) self assembly:
Layer-by-Layer (LbL) self assembly, also known as electrostatic self assembly is a technique for the fabrication of thin films. The thin films are generally formed by ionic interaction and are multilayered with alternating cationic and anionic electrolyte layers. The process involves spontaneous adsorption of cationic and anionic electrolytes, alternatively on a suitable substrate. Typically, only one of the two layers is the active layer. The other layer enables the adsorption of the active layer and binds the multilayered film by electrostatic attraction. Originally, this method was restricted to fabrication by using polyelectrolytes. But now, the method has been extended to other molecules like conjugated polymers, dendrimers, proteins and so on; with adsorption being driven not only by ionic interactions but also secondary interaction such as hydrogen bonding. There are several advantages in this method over other thin film deposition techniques:
The thin films formed by this technique are highly stable.
LbL method offers high level of control over the thickness of the film because of the linear growth of the layers.
This method is simple, inexpensive and can be automated.
Formation of the thin film follows a number of repeated steps of charge neutralization and resaturation. For example, on a negatively charged surface, initially, cationic electrolytes adsorb, leading to charge neutralization. Over-adsorption of the cationic electrolytes leads to charge reversal and the surface now becomes resaturated with positive charge. Anionic electrolytes then adsorb leading to further charge neutralization and resaturation. These steps can be repeated a number of times to yield films with desired number of layers and sequence (Figure 1). The molecules can be deposited on the substrate using several techniques such as dip coating, spray coating, spin coating and flow based methods. Some examples of polycations are polyethyleneimine (PEI), polydiallyldimethylammonium chloride (PDDA) and polyallylamine hydrochloride (PAH); some examples of polyanions are polysodiumstyrene sulfonate (PSS) and polyacrylic acid (PAA).
Figure: Formation of thin film by LbL self assembly
The thin films fabricated by LbL assembly have applications in anti-reflective coatings, chemical sensors, superhydrophobic coatings, non-linear optics, corrosion control and bioelectronics.
Self assembly can also occur at interfaces. Interfaces direct self assembly along certain directions by providing physical, geometrical and chemical spaces. This type of self assembly is referred to as "Interfacial/Surface self assembly". There is an intrinsic restriction of at least one of the dimensions of self assembly in the length scale of the components. For this reason, it is also known as "two-dimensional self assembly". Two-dimensional self assembly is very unique compared to three-dimensional self assembly (or bulk self assembly) because of the inevitable interactions that take place between the interface, at which self assembly occurs and the building components. The strength of such interactions may vary, but is usually comparable to colloidal and intermolecular forces between the building units. This intrinsic factor furnishes three typical, unique characteristics to two-dimensional self assembly that differentiate it from three-dimensional self assembly:
For bulk self assembly, it is a prerequisite that the building units are amphiphilic, so that proper balance of forces is ensured. But in the case of interfacial self assembly, the building units may or may not be amphiphilic. Once the components are attracted onto the interfaces, the forces among themselves are sufficient to induce self assembly.
The intermolecular and colloidal forces among the building units are altered by their interaction with the interface. For example, on adsorption onto interfaces, even non-polar molecules can acquire dipole moments. This characteristic provides additional means of control over the self assembly processes at interfaces.
The self assembled aggregates formed in interfacial self assembly are found to have significantly different chemical and physical properties than those in bulk self assembly. This is because the self assembled aggregates in interfacial self assembly prevail in a state of confinement on the interfaces. For example, the tensile strength of Langmuir monolayers is different from that of bilayers formed in bulk from the same components.
Interfaces are formed spontaneously between any two phases. Interfaces have zero thickness geometrically and macroscopically and therefore are two-dimensional. However, microscopically and physicochemically, interfaces are three-dimensional; having a thickness in the range of approximately 1nm-1µm (Figure 1). Interfaces are the region where there is a gradual change of physicochemical properties from one phase to the adjacent phase.
Figure: Thickness of an interface
Forces involved in interfacial self assembly:
The interfaces, having thickness in the nanometer range, act as nanometer scale surface wells when the building units are adsorbed onto them. This is because the interplay of intermolecular forces of the building units is confined to the interfaces. As the physicochemical properties change gradually within the interface, the interaction between the building units become more favourable along the interface's direction and less favourable through either phase's direction (either phase 1 or 2).
The intermolecular and colloidal forces between the building units can act as either attractive or repulsive forces for the process of interfacial self assembly. But the direction of the interfacial self assembly is determined by the interaction between the building units and interfaces and is thus referred to as 'intrinsic directional force'.
General strategy of two-dimensional self assembly:
The general strategy of a two-dimensional self assembly can be explained as follows: the self assembly process is initiated when the building units are first located at the interface. This is a prerequisite condition, regardless of the type of the interface. This is usually achieved by adsorption, mainly physisorption. After this, when the force balance between the building units is fulfilled, the building units at the interface self assemble and form self assembled aggregates. The force balance determines the mode of packing of the building units, the specific structure of the self assembled aggregates and the prospect of subsequent higher order self assembly.
There are three different categories of building units for two-dimensional self assembly: amphiphilic, nonamphiphilic and building units with one or more functional groups. There is a distinctive packing pattern shown by each group of building units. The packing mode is determined by the type of the interface, the external force applied during self assembly and the surface density (degree of surface coverage). It is also influenced by experimental conditions such as pH, the deposition rate, molecular structure and properties and so on.
Amphiphilic building units can have two types of packing pattern, either upright or flat mode, based on the surface density. The amphiphilic building units show an upright mode of packing when the surface density is very high or if an external force is applied in the lateral direction (Langmuir monolayer). If the surface density is relatively low, then a flat mode if packing is expected. Interfacial self assembly with building units having functional groups produce self assembled aggregates with great structural richness and are highly selective and directional. This is because the functional groups also exert attractive forces, in addition to intermolecular forces that can overcome the repulsive forces. Building units with multifunctional units and single functional unit also show different packing pattern. For nonamphiphilic building units, there can be no upright packing, unless a strong external force is applied, which can overcome the adsorption, intermolecular and/or colloidal forces.
Examples of two-dimensional self assembly:
Self assembly at the gas/liquid interface:
There are two typical examples of self assembly at gas-liquid interface: Langmuir monolayers and surface micelles.
Langmuir monolayers are formed at the gas-liquid interface by the mechanical force-induced self assembly of amphiphiles. Typically, the amphiphiles are insoluble in the liquid phase, often known as subphase. When the amphiphiles are adsorbed at the interface, they are loosely distributed; the area occupied by the individual components is large. When an external force is applied by the lateral motion of the barrier, it causes a decrease in the total area of the liquid surface occupied by the building units. The low solubility of the building units prevents its extraction into the liquid phase which leads to an upright packing of the amphiphiles into a monolayer (Figure 2). Langmuir monolayers are used for the fabrication of Langmuir-Blodgett (LB) films. Upon compression, the monolayers form a condensed film, which may be highly organized. The presence of the film is detected by the decrease in surface tension (or surface pressure) of the liquid subphase. LB film is formed by the transfer of the monolayers onto a solid substrate by immersion or emersion. Multiple immersions/emersions lead to a film of multiple layers.
Figure: Formation of Langmuir monolayer
LB films have several applications in various fields:
LB films can be used in Metal-Insulator-Semiconductor (MIS) as passive layers.
LB films have been used as models for biological membranes, especially for investigation of mode of drug action.
LB films have application in biosensors; films of poly(3-hexyl thiopene) are used as glucose biosensor.
LB films can be used as UV resists for lithographic techniques.
Surfactants and amphiphilic polymers having low solubility for the liquid subphase at the gas-liquid interface form surface micelles through flat packing. Here also, hydrophobic force acts as the attractive force that brings the building units together and electrostatic repulsion, hydration forces act as repulsive forces. The line tension around the surface micelle acts as an additional attractive force.
Self assembly at the liquid/solid interface:
Hemimicelles and semimicelles are standard types of self assembled aggregates formed at the liquid/solid interface. At low concentrations, amphiphilic molecules from the liquid phase adsorb onto the solid phase at the liquid/solid interface such as water/silica gel and water/alumina interface. As the concentration of the molecules increases to a specific, higher concentration of critical hemimicelle concentration (CHMC), there is a dramatic increase in the adsorption of the molecules, resulting in the formation of hemimicelles. The CHMC of a molecule may be greater than or equal to CMC of the molecule. Hemimicelles and semimicelles are considered to be two-dimensional projections of three-dimensional micelles formed in solution. Thus, the thermodynamic facts derived from micellization in solution hold true for hemimicelles also.
Self assembly at the liquid/liquid interface:
Langmuir monolayer can be induced at liquid/liquid interfaces by careful selection of building units. The interfaces usually are aqueous solution in contact with non-polar liquid. The building units should be afloat at the liquid/liquid interface and also fulfil the force balance process. Once the building units are dispersed at the interface, on application of an external force such as surface pressure, Langmuir monolayers can be formed. Surfactants and other amphiphilic molecules are thermodynamically favourable to form a surface micelle at an aqueous solution/oil interface. Nonamphiphilic molecules such as nanoparticles can also be self assembled at liquid/liquid interfaces into various morphologies.
Self assembly at the gas/solid interface:
At gas/solid interfaces, building units are deposited as gas phase or in high vacuum on solid surfaces for self assembly to occur. Self assembly can only occur when the building units can be adsorbed from vapour phase or under high vacuum. Variety of building units like porphyrin derivatives and alkyl chain based functional units can be introduced at gas/solid interfaces. Formation of molecular corral on semiconductor surface by the self assembly of haloalkane provides an excellent example. Such nano-corrals are used in molecular electronics for capturing surface electrons.