At that time, Franklin focused only on the wave-damping phenomenon and did not realize that this effect was related to the formation of a monomolecular layer at the surface. Dividing the oil volume by the area it occupies gives the oil layer thickness, which is on the nanometer scale [6]. This simple calculation was not performed until years later, when Lord Rayleigh (1842–1919) determined the thickness of oil layers. However, he did not know that the calculated value corresponded exactly to the length of the molecule used in the experiments, indicating that the layer was one molecule thick [6].

During the same period, Agnes Pockels (1862–1935) made an important advance in the science of monolayer formation by creating a prototype of the Langmuir trough (Fig. 4.1). This prototype was a primitive device in which barriers were used both to compress oil molecules scattered on a water surface and to remove impurities from the surface. Pockles also developed a method for measuring the surface tension of water in a container [5]. In recognition of these important contributions, Rayleigh helped Pockles publish her results in the prestigious journal Nature. A few years later, Irving Langmuir (1881–1957) introduced the concept of molecular conformations.

For Langmuir, molecules were asymmetrical and would therefore have identical orientations on the water surface, depending on their hydrophilic or hydrophobic character. Langmuir also estimated the size of the molecule used in his experiments, which had a large impact on the scientific community at the time. These studies motivated Langmuir to investigate monolayer formation on water surfaces, and he was awarded the Nobel Prize in Chemistry for this work in 1932. Langmuir suggested that the monolayers formed on a water surface could be transferred to a solid surface. His assistant Katherine Blodgett (1898–1979) performed the corresponding experiments, and the initial results were published in 1934 and 1935. Blodgett was awarded her own Nobel Prize for this work. During this period, the LB monolayer deposition method was developed. Several years later, Langmuir and Vincent Schaefer (1906–93) studied protein deposition on solid substrates and discovered a new approach called the Langmuir–Schaefer (LS) method for depositing Langmuir monolayers. In the LS method, a Langmuir monolayer is deposited on a surface horizontally instead of vertically, as in LB films [5,7].

The formation of Langmuir monolayers and LB and LS films was not actively studied for a long time because they had no practical application. Around 1980, research interest in this area was renewed by the use of LB films in organic electronics. Since then, physicists, chemists, biologists, and engineers have actively collaborated to develop this technique using various types of materials for a wide range of applications.

Langmuir monolayers and LB films are fabricated by pouring a known volume of a given solution at the air–water interface. Ideally, the material in the solution is soluble in an organic solvent or solvent mixture. During film fabrication, the solvent is evaporated after a given amount of time, and the molecules are then compressed until maximum order is achieved.

Initially, the materials employed in the Langmuir technique were necessarily amphiphilic compounds, that is, molecules with a polar head group (hydrophilic) and a nonpolar tail group (hydrophobic) essentially insoluble in polar solvents. These compounds self-organize with their polar part in the water and nonpolar part in the air. The hydrophobic portion of these molecules generally consists of aliphatic chains, which reduce its solubility in the aqueous subphase. The hydrophilic part is responsible for spreading the molecules because it interacts strongly with the water. The molecular orientation of these molecules at the air–water interface minimizes their free energy [5,7–9].

A typical isotherm for amphiphilic molecules is illustrated in Fig. 4.2. Before compression by the barriers, the molecules are initially in the gas phase (stage A); they are fully dispersed and do not interact. As the molecules are compressed, their proximity leads to the formation of an expanded liquid phase (stage B). When the molecule surface density increases, the regular arrays in the film form, resulting in a compact structure called the condensed-liquid phase (stage C). However, further compression of the monolayer causes molecular disorder and a phenomenon commonly called collapse (stage D) [5,7–9].

The use of LB films composed of common amphiphilic molecules is limited by their properties. To search for new technological applications of LB films, researchers have investigated the use of various materials, including polymers [10–13], phospholipids [14–16], enzymes [17–20], and peptides [21,22]. However, the formation of monolayers of other types of molecules is not trivial because they might not be completely soluble in volatile organic solvents or might be unstable, making them difficult to spread at the surface and consequently deposit homogeneously on a substrate [8].

To minimize these difficulties, several strategies have been employed depending on the type of film material. For example, miscible solvents can be used, or the material can be spread in the subphase [10,23]. The organization, homogeneity, deposition quality, and formation of Langmuir monolayers and LB films can be evaluated experimentally by characterization methods such as Brewster angle microscopy [24,25], polarization modulation infrared reflection-absorption spectroscopy [24,26,27], and atomic force microscopy (AFM) [28,29].

where γ₀ and γ_A are the surface tensions of pure water and due to the monolayer, respectively [5,7].

The surface pressure isotherms are obtained using a Wilhelmy sensor, which is suspended by a wire from an electrobalance and partially immersed in the water. The electrobalance measures the force exerted to keep the sensor stationary as the surface tension varies. The vertical force exerted by the surface tension is detected by the balance and converted into a voltage [5,7]. A typical isotherm of an amphiphilic material (stearic acid) is illustrated in Fig. 4.2.

One disadvantage of this technique is related to the contact angle (θ) formed between the liquid subphase and the balance sensor, as shown in Fig. 4.3 [5,7]. Another disadvantage is that the position of the Wilhelmy sensor relative to the barrier positions can affect the pressure isotherm during monolayer formation. These problems can be minimized by placing the sensor in the center of the trough [8].

The surface pressure isotherms might exhibit hysteresis. This phenomenon arises due to differences in the molecular behavior during compression and decompression. Specifically, as the barriers compress the molecules, the area that they occupy decreases, resulting in an increase in their interactions and consequently an increase in the surface pressure. When the monolayer is decompressed, the surface pressure gradually decreases, but not in the same way that it increases during the initial compression. The hysteresis in the area occupied by the molecules indicates that some of the material is lost to the aqueous subphase or to aggregation. A large hysteresis implies that the stability of the monolayer is low. Another method for characterizing monolayer stability is based on compressing the monolayer to the maximum pressure at which molecule order is observed for long periods of time. If large variations in the area are observed during this period, the monolayers are unstable and therefore unsuitable for film deposition [7–9].

Another widely used technique for characterizing Langmuir films is surface potential (∆V) measurements. Surface potential is defined as the difference in the potentials of the aqueous subphase covered with the monolayer and the pure aqueous subphase. Surface potential can be measured using a Kelvin probe or a vibrating plate capacitor, which is more common [5,7–9,30]. In the vibrating capacitor method, surface potential is measured by a plate positioned above the water that detects the vibrations of the spread molecules. The potential of the pure water is used as a reference and is measured by a metal plate immersed in the subphase. The ∆V value is the change in the permanent electric dipoles at the air–water interface due the presence of a partially or fully ionized film. To gain insight into the effect, theoretical models were developed to relate the measured potential to the dipole moments of the film material. The most commonly used model was developed by Demchak and Fort (DF) [30]. In this model, the Langmuir monolayer is considered to be a three-layer capacitor in which each layer has a different relative permittivity, as illustrated in Fig. 4.4 [8,9,30].

According to the DF model, the surface potential is given by:

where A is the average area per molecule; ɛ₀ is the vacuum permittivity; μ₁, μ₂, and μ₃ are the dipole moments attributed to the polarization and reorientation of the molecules; and ɛ₁, ɛ₂, and ɛ₃ are the permittivities due to changes in the dipole moments of the hydrophilic and hydrophobic groups. More specifically, the component μ₁/ɛ₁ corresponds to the reorientation of water molecules that is induced by the presence of the monolayer, μ₂/ɛ₂ is the contribution of the hydrophilic portion of the molecule, μ₃/ɛ₃ is the contribution of the hydrophobic portion, and Ψ₀ is attributed to the electrical double layer that forms when the film is partially or fully ionized [5,7–9]. In polymer films, for example, it is impossible to estimate values for μ₁/ɛ₁ + μ₂/ɛ₂ + μ₃/ɛ₃ and Ψ₀, mainly because the polymer is often processed under different conditions, which induce structural changes in the polymer chain. These changes make it difficult to explain the experimental results using a theoretical surface potential model.

Molecular behavior during the surface potential measurement can be described as follows. Initially, the molecules spread on the subphase cover an area so large that their interactions are too weak to induce a detectable change in the surface potential of the aqueous subphase. During compression, the area covered by the molecules reaches a critical value at which the potential is no longer zero, and it increases sharply as the area per molecule is further decreased. Due to this critical area, surface potential measurements are more sensitive to the film organization than surface pressure measurements [5,7–9]. Fig. 4.5 shows a typical surface potential curve for stearic acid based on the DF model, which considers the changes in the dielectric constants of the three layers as functions of the area per molecule.

where A_L is the decrease in the area occupied by the molecules at the air/water interface (at constant pressure) and A_S is the area of the substrate covered with the monolayer [7].

Different LB film architectures can be obtained by varying the deposition parameters, substrate characteristics, and film material. The possible architectures are called X-, Y-, and Z-type films and are illustrated in Fig. 4.7. Y-type LB films are generally obtained when substrates with hydrophilic surfaces are used. For the X- and Z-type films, deposition occurs preferentially during substrate immersion and removal from the subphase, which favors the formation of films in which the polar and nonpolar groups in adjacent monolayers interact. It is important to note that Z-type films form on hydrophilic substrates, whereas X-type films form onto hydrophobic substrates [5,7,32].

LB and LS films are widely used in sensors, biosensors, and cell membrane model studies because their molecular organization can be highly controlled, which results in satisfactory experimental results. In one study, enzymes were immobilized on lipid monolayers of LS films, and the catalytic activity of the biomolecule-containing films in sucrose hydrolysis was evaluated. The experimental results indicated that 78% of the invertase enzymatic activity was maintained in this system [17]. In another study, the interactions between mucin and chitosan were studied using dimyristoyl phosphatidic acid (DMPA) Langmuir monolayers as cell membrane models. The experimental results for the DMPA–chitosan–mucin Langmuir and LB films indicated that a mucin–chitosan complex forms via electrostatic interactions that are crucial for the mucoadhesion mechanism [35].

De Barros et al. developed electrochemical sensors composed of polyaniline and montmorillonite clay using LB technique for the detection of copper, lead, and cadmium metal ions. The Langmuir isotherms and ultraviolet-visible (UV-vis) absorption spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy results indicated the existence of synergistic molecular-level interactions between the film materials, which were favorable for the sensor performance, that is, the sensors had high sensitivity and a low detection limit on the order of μg L⁻¹ [10]. Thin films of polythiophene derivatives were fabricated using LS and spin-coating techniques to compare the effect of the deposition method on the electrical properties of the films. The experimental results revealed an electrochromic effect during analysis, and the electrical and electrochemical measurements revealed the effects of the film organization on its electrical properties [11].

For further reading on the theoretical concepts and applicability of this technique, the reader is referred to the work of M. Petty titled “Langmuir–Blodgett Films: An Introduction” and other texts in this field.

The LbL technique was further developed in several subsequent studies that described, for example, the successive deposition of inorganic ionic compounds [48] and polyanion adsorption on cellulosic fibers containing preadsorbed polycations [49], among other systems [50]. Decher contributed not only to the rediscovery of this technique but also to its use as an alternative to the LB method for fabricating nanostructured films. He also demonstrated that it could be used to deposit films of various types of materials, including bipolar amphiphilic molecules [42], polyelectrolytes [42,44], and proteins and DNA [43,44]. According to Decher and Schlenoff [50], the field of nanostructure fabrication via the LbL method is still popular and growing more than 20 years after the technique was reintroduced.

Unlike the sophisticated LB method, the main advantages of the LbL technique are its simplicity and low cost. Fig. 4.8 illustrates the LbL deposition process and film formation.

Initially, a substrate is immersed in a solution containing charged species, for example, polyanions (step A). During this step, the anionic polyelectrolyte adsorbs on the substrate surface via simple electrostatic interactions, resulting in the formation of a negative charge network on the surface. Then, the substrate is immersed in a rinsing solution (step B) to remove weakly adsorbed material to prevent contamination of the next solution. The substrate is subsequently immersed in a polycation solution to generate a new network of positive charges on the surface (step C). Finally, the substrate is again immersed in a rinsing solution (step D). Thus, a bilayer of the ionic materials, in this case polyelectrolytes, is obtained. This procedure can be repeated as many times as desired to obtain multilayer films with controlled structures and thicknesses.

When the initial substrate charge density is small, multiplication of surface functionality occurs. In this process, the charge density of the first adsorbed layer is larger than that of the substrate, which favors the subsequent adsorption of oppositely charged species [46]. It should be noted that the surface charge and roughness of the substrate (glass, quartz, silicon, paper, cloth, etc.) might affect the deposition of the first few layers [53]. The influence of the substrate on the deposited layers becomes negligible after a few deposition cycles. Then, the film growth and structure are fundamentally governed by the film materials. This important feature allows a film to be fabricated on a different surface for a given application than on the substrate that was used to study the film properties [50].

During the second stage of the layer formation process, the conformations of the adsorbed polyelectrolyte chains change as they relax [54]. The relaxation process explains the interpenetration of adjacent polyelectrolyte chains observed in experimental studies. However, it is assumed that the adsorbed segments do not diffuse along the surface during LbL film formation because this process would involve the breaking of many electrostatic bonds simultaneously, which requires more energy than that gained by thermodynamic contributions to the system and is therefore highly improbable [54].

Although electrostatic interactions are involved in the formation of polyelectrolyte films (Fig. 4.8), they are not the only interactions that can be exploited in LbL film formation. Depending on the characteristics of the selected materials, covalent bonds, van der Waals forces, and hydrogen bonds can also govern the film formation mechanism, further enhancing the versatility of the LbL technique [55].

Hydrogen bonds, for example, are sensitive to the medium and can be broken and reformed by changing the pH. This phenomenon is observed in the production of some porous films, such as polyacid acrylic (PAA)/polyvinyl pyridine (PVP) multilayers. In a basic solution, the PAA layers dissolve, allowing the PVP layers to subsequently rearrange to yield the porous structure [56]. The film characteristics, such as the pore size and pore size distribution, can also be tuned by varying the pH and temperature of the solution and the immersion time and nature of the substrate during treatment in a basic solution [55,57]. Hydrogen bonds also enable the production of multilayers of a single material; for example, films can be fabricated from a dendrimer with carboxylic acid groups that can act as both hydrogen bond donors and acceptors [56]. Other examples of different multilayer film formation mechanisms include charge transfer reactions (electron donor and acceptor materials are alternately deposited) [58] and covalent bonding [59].

In addition to these different interactions, other strategies, such as using avidin–biotin biospecificity [60], DNA hybridization [61], and other interactions [2,55,62,63], have been employed to obtain LbL films. In LbL films, different interactions not only drive the film formation mechanism but also participate to a minor degree in films formed via electrostatic interactions, influencing their properties, such as their stability, morphology, and thickness [53].

Of all the characteristics of a film, its thickness might be one of the most important [50]. The thickness depends on the deposition conditions of each layer, which in turn influence the subsequently deposited layer and, consequently, the final film properties, including the roughness, uniformity, chemical resistance, and so on. The intrinsic film material properties, such as the nature and density of the charged groups; the solution characteristics, such as the concentration, pH, and ionic strength; the fabrication operating parameters, such as the deposition and rinse times; and many other factors affect the film formation. For example, studies indicate that in general, increasing the ionic strength of a polyelectrolyte solution causes the polymer chains to contract, which leads to an increase in the surface density of the adsorbed segments and thus to an increase in film thickness [54]. However, exposing a polyelectrolyte film to a concentrated salt solution after fabrication leads to a decrease in film roughness due to increased chain mobility [50]. Moreover, changing the solution pH can alter the degree of dissociation and the conformation of some polyelectrolytes and also the enzymatic activity of biological films. Although many factors in the deposition process affect the film properties, understanding their effects are crucial to optimizing the film deposition process.

These techniques can be advantageous over the conventional method and therefore more appropriate, depending on the desired film characteristics and application. The spin-assisted LbL and spray-LbL methods are faster than the conventional immersion method, particularly in the production of thick (micrometer) films because they do not depend on the diffusion kinetics of the species in the liquid medium. They also consume smaller amounts of material. However, the spin-assisted LbL method can only be used with flat substrates and produces nonuniform films on large-area substrates. The spray technique is more appropriate for these types of substrates [64]. The characteristics of films produced via the spray and conventional methods are very similar, whereas films produced by the spin-assisted LbL method are generally smoother due to decreased chain interpenetration, for example, in the case of polyelectrolytes [64]. It should be noted that all three deposition methods can be automated, which increases the reproducibility of the fabricated films [65].

Several other techniques, such as infrared spectroscopy, Raman spectroscopy, AFM, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), are often used to characterize LbL films. Raman and infrared spectroscopies are used to determine the functional groups present in the film materials by recording their vibrational modes. Techniques such as AFM, SEM, and TEM are used to analyze film morphology, including its surface roughness, uniformity, and so on. The reader is referred to other chapters in this book for more information on various nanostructure characterization techniques.

LbL films are also used in electronic devices; they serve as charge injection layers in light-emitting diodes [76], are employed in solar cells [77], constitute the active layer in field-effect transistors [78], and are used in memory applications [79]. In addition, these films are used as ion-exchange membranes in fuel cells [80] and as antireflective surface coatings in optical applications [81], among other applications [2,3,9].

For a more comprehensive study of the concepts and applicability of the LbL technique, the reader is referred to the publication of G. Decher titled “Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials, 2nd Edition” and other texts in this field.