Benjamin Franklin’s report to the British Royal Society in 1774 is a good starting point for the history of Langmuir or Langmuir-Blodgett films. In his report Franklin stated:

“At length at Clapham where there is, on the common, a large pond, which I observed to be one day very rough with the wind, I fetched out a cruet of oil, and dropped a little of it on the water. I saw it spread itself with surprising swiftness upon the surface the oil, though not more than a teaspoonful, produced an instant calm over a space several yards square, which spread amazingly and extended itself gradually until it reached the leeside, making all that quarter of the pond, perhaps half an acre, as smooth as a looking glass.”'

Had Franklin known that the maximum extension of an oil film on water represents a layer one molecule thick (2 nm), he would have found out that a teaspoonful (2 mL) would cover an area of 2000 m2, which is half an acre as he suspected. Benjamin Franklin had created a ‘thin film’ – more precisely, a monolayer at the air-water interface.In the late 19th century, Agnes Pockles, a German pioneer in chemistry, discovered the influence of impurities on the surface tension of water by using a rudimentary surface balance in her kitchen sink. This system was a precursor to the Langmuir trough.In the early 20th century Irving Langmuir, an American chemist and physicist, was the first to perform systematic studies on floating monolayers on water. He was the first to give a modern understanding of monolayer structure at the molecular level, in particular that the molecules show a preferential orientation. The systems to study floating monolayers on water are now named after him: Langmuir films. In 1932 he was awarded the Nobel Prize for his work on surface chemistry. Langmuir was also the first to show that monolayers can be transferred from the air-water interface to solid substrates for further study. Together with his assistant, Katherine Blodgett, he showed that it was possible to go further and to deposit many monolayers onto the same substrate, thus building up a multilayer film of any required thickness. Deposited monolayers of any thickness are now known as Langmuir-Blodgett films or LB films. Since the late 20th century KSV NIMA has made a number of innovations that dramatically improved the Langmuir and Langmuir-Blodgett troughs. KSV NIMA continues to develop innovative products to give the next generation of researchers the tools they need to push the boundaries of thin film research.

Adhesion is the attraction between two dissimilar phases. Current theory divides adhesion into the three categories: fundamental adhesion, practical adhesion and thermodynamic adhesion.

• Fundamental adhesion is the sum of all interfacial intermolecular interactions between the two phases. It corresponds to the energy required to break chemical bonds at the weakest plane of the two-phase system. The fundamental adhesion cannot be directly measured since there are always other factors affecting in a measurement system.
• Practical adhesion is a function of fundamental adhesion and other factors, including internal stresses and the error caused by the measurement method. Practical adhesion can be measured by using variety of different techniques, such as Scotch tape test or stylus method.
• Thermodynamic adhesion is defined as the reversible work done in creating a unit area of the interface between two phases (see figure below). Typically, the unit is mN/m.

WAB = γA + γB – γABwhere WAB is the thermodynamic work of adhesion, γA the surface tension of substance A, γB the surface tension of substance B and γAB the interfacial free energy.

For two solid phases, the work of adhesion is not useful since there are various unknown parameters in the equation. If, however, one of the phases is liquid and the other one solid, the work of adhesion can be defined with Young-Dupré equation:WAB = γB (1 + cos θ)where γB is the surface tension of the liquid and θ is the contact angle between the liquid and solid. Now, the work of adhesion can be calculated by measuring surface tension of the liquid and contact angle with Attension tensiometers.

A microscopy trough is a special kind of Langmuir trough for characterization of Langmuir films before, during and after fabrication.

A microscopy trough is a special kind of Langmuir trough that includes a sapphire window in the top base of the PTFE trough that allows high optical transmission down to a wavelength of 200 nm (suitable for visible light or UV microscopy). It enables characterization of Langmuir films before, during and after fabrication. Troughs suitable for both upright and inverted microscopes are available.

UV-visible absorption spectroscopy of monolayers on the Langmuir Trough can be used to determine how light-absorbing molecules interact with one another when a monolayer of the molecules, or mixtures of molecules, are compressed to various surface pressures. The molecular neighbor-neighbor separation and packing density is made possible by controlling the surface pressure of the monolayer.

When photoluminescent materials are studied, the Inverted Microscopy Trough allows a microscope objective lens to reach within 2.2 mm of the monolayer, from beneath, to enable stimulation of the monolayer with very low wavelength light from above. The transmission properties of the sapphire microscopy window and the glass inverted microscopy window are shown below.

Optimal cleanliness plays an important part in many application segments, e.g. preparation and processing of silicon wafers for semiconductor industry, and packaging materials for food and pharmaceutical products. Cleanliness control is also essential in food factories, hospitals and in process plants. The efficiency of cleaning and detergent formulations can be evaluated by contact angle measurement.

The surface free energies of clean and treated surfaces are directly correlated to the cleanliness and surface composition. Contact angle is one of the most sensitive of all surface analytical techniques since even the top nanometer of the surface influences wetting behavior. As a simple and fast measuring technique, contact angle is commonly utilized to follow the cleaning process and the effectiveness of cleaning solutions. Therefore, contact angle measurements are a highly suitable quality control method in areas where cleanliness control is crucial.

Attension provides a range of user-friendly instruments, the Theta and Sigma series, for surface energy determination and contact angle measurements. Automated Theta gives an user-independent and quick method for the quality control of surfaces and their cleanliness.

Langmuir-Schaefer film (or LS film) can be defined as one or more monolayers of material deposited from a liquid surface onto a solid substrate. The deposition can be made by dipping the substrate horizontally through a floating monolayer at a constant molecular density or by placing the substrate in contact with the monolayer. LS deposition can be used for example to create nanoparticle coatings with controlled packing density.

Langmuir-Schaefer films formed by one or several Langmuir films deposited onto a solid surface by horizontally dipping (rather than vertically) the solid substrate from the gas phase to the liquid phase or from the liquid phase through the gas phase. The films obtained can be highly organized ranging from ultrathin monolayer to multilayer structures built up of hundreds of monolayers.

The Langmuir-Schaefer technique is derived from the Langmuir-Blodgett technique, Langmuir-Schaefer films being very similar to Langmuir-Blodgett films.

Both methods can be used in creating highly organized nanoparticle coatings with controlled packing density and film thickness.

A monolayer is a single-molecule thick film or layer of closely packed atoms or molecules at an interface.
Monolayers are often formed at the air-water interface as a Langmuir film. Monolayers are often located on a solid surface or floating on a gas-liquid interface. However, some monolayers can also be formed at a liquid-liquid interface.

Many materials can be used to form monolayers including lipids, nanoparticles, polymers and proteins to name a few. Monolayers are expected to be highly useful components in many practical and commercial applications such as sensors, detectors, displays and electronic circuit components. The possibility to synthesize organic molecules and develop new inorganic materials has enabled the production of electrical, optical and biological nano-components.

Monolayer properties depend on many criteria including the material used, the film structure and the packing density of the monolayer. Langmuir film and Langmuir-Blodgett film deposition are techniques most used for the controlled study and fabrication of well organized monolayers at gas-liquid, liquid-liquid and gas-solid interfaces. Langmuir troughs are used to fabricate monolayers in a controlled way. Other methods such as Dip coating or spin-coating can be used but they do not enable as much control of the film structure and packing density.

In the context of a Langmuir film, surface pressure is the difference in surface tension measured between a clean sub-phase and a monolayer-covered sub-phase. Surface pressure greatly influences monolayer properties.

Monolayers and surface pressure

Substances that can form monolayers are often amphiphilic: the hydrophobic tail guarantees the molecule is insoluble in water while the hydrophilic head facilitate spreading and ensure that the molecules stay at the surface. When a solution of an amphiphile in a water insoluble solvent is deposited on a water surface with a microsyringe, the solution spreads rapidly to cover the available area. As the solvent evaporates, a monolayer is formed. When created at the air-water interface, the monolayer is called a Langmuir film.

When the available area for the monolayer is large the distance between adjacent molecules is large and their interactions are weak. The monolayer can then be regarded as a two-dimensional gas (A in the illustration below). Under these conditions the monolayer has little effect on the surface tension of water. If the available surface area of the monolayer is reduced by a barrier system the molecules start to exert a repulsive effect on each other (B and C in the illustration below). This two-dimensional analogue of a pressure is called surface pressure, Π, and is given by the following relationship:

Π=ϒ-ϒ0

where ϒ0 is the surface tension of the sub-phase in absence of a monolayer and ϒ is the surface tension with the monolayer present at the interface.

What is surface tension?

Surface tension is a measurement of the cohesive energy present at an interface. The molecules of a liquid attract each other. The interactions of a molecule in the bulk of a liquid are balanced by an equal attractive force in all directions. The molecules at the surface of a liquid experience an imbalance of forces as shown below.

The air/water interface (and other gas-liquid interfaces) possesses excess free energy originating from the difference in environment between the surface molecules and those in the bulk. This interfacial free energy is responsible for the surface tension.

How is surface pressure measured?

The KSV NIMA L & LB Trough measures surface pressure by measuring surface tension using the Wilhelmy plate method. In this method, a measurement is made by determining the force due to surface tension on a plate suspended so that it is partially immersed in the sub-phase. This force is then converted into surface tension (mN/m or dynes/cm) with the help of the dimensions of the plate.

Wilhelmy plate at the air-water interface

The plate is very thin and made of platinum, but even plates of glass, quartz, mica and filter paper can be used. The forces acting on the plate consist of the gravity and surface tension downward, and buoyancy due to displaced water upward. For a rectangular plate of dimensions lp, wp and tp, of material density ρp, immersed to a depth hl in a liquid of density ρl, the net downward force is given by the following equation:

F=ρpGlpwptp + 2γ (tpwp)(cos θ) - ρlgtlwlhl

where γ is the liquid surface tension, θ is the contact angle of the liquid on the solid plate and G is the gravitational constant. The surface pressure is then determined by measuring the change in F for a stationary plate between a clean surface and the same surface with a monolayer present. If the plate is completely wetted by the liquid (i.e. cos θ = 1) the surface pressure is then obtained from the following equation:

Π=- Δγ=-[ΔF/2(tp+wp)]=-ΔF/2Wp

Using a very thin plate can thus increase the sensitivity. The force is in this way determined by measuring the changes in the mass of the plate, which is directly coupled to a sensitive surface pressure sensor.

Surface pressure—area isotherm or π-A isotherm can be defined as a measurement at constant temperature of surface pressure, as a function of the available area for each molecule in a floating monolayer (Langmuir film).

The most important indicator of the monolayer properties of an amphiphilic material is found by measuring the surface pressure as a function of the area of water surface available to each molecule. Usually an isotherm is recorded at constant temperature by compressing the film (with the barriers) at a constant rate while continuously monitoring the surface pressure. Depending on the material, repeated compression and expansion may be necessary to achieve a reproducible trace.

When the monolayer is compressed a number of distinct regions or phases can be seen on an isotherm. The phase behavior of the monolayer is mainly determined by the physical and chemical properties of the amphiphile, the sub-phase temperature and the sub-phase composition. For example, various monolayer states exist depending on the length of the hydrocarbon chain length and the magnitude of other cohesive and repulsive forces. Increased chain length increases the attraction between molecules and condenses the π-A-isotherm. On the other hand, if an ionizable amphiphile is used the ionization of the head groups induces repulsive forces that oppose phase transitions.A simple terminology used to classify different monolayer phases of fatty acids was proposed by W.D. Harkins in 1952. The monolayers mostly exist in the gaseous state (G) and on compression can undergo a phase transition to the liquid-expanded state (L1). Upon further compression, the L1 phase undergoes a transition to the liquid-condensed state (L2), and at even higher densities the monolayer finally reaches the solid state (S). If the monolayer is further compressed after reaching the S state the monolayer will collapse into three-dimensional structures. The collapse causes a rapid decrease in the surface pressure; this is seen as a horizontal break in the isotherm if the monolayer is in a liquid state.There are also many other critical points in a π-A-isotherm, such as the molecular area at which an initial, pronounced increase in the surface pressure is observed, Ai, and the surface pressures at which phase transitions occur between the L1 and L2 state and the L2 and S state.

Example

Typical isotherms of a fatty acid with a single hydrocarbon chain and a phospholipid with two hydrocarbon chains are illustrated in the graph below. Following the definitions above one can see that the fatty acid has three distinct regions gas (G), liquid (L1) and solid (S), while the phospholipid has an additional almost horizontal transition phase (L2-L1) between the two different liquid phases. This is very common for phospholipids and the position of this horizontal transition phase is very temperature dependent. As the temperature is increased the surface pressure value at which the horizontal transition phase occurs will increase and vice versa.

Typically, coatings and surface treatments should be homogeneously distributed over the whole surface that they applied to, and at times surface heterogeneity can be a desirable characteristic.
After applying any coating or surface treatment, heterogeneity control can be performed by contact angle measurement. Contact angle is one of the most sensitive of all surface analytical techniques since even the top nanometer of the surface influences wetting behavior. As a simple and fast measuring technique, contact angle is commonly utilized to study surface heterogeneity.Depending on the application, the contact angle measurement can be applied to study surface heterogeneity in different length scales:

Macroscale heterogeneity and cleanliness can be studied by contact angle mapping.

Contact angle hysteresis can be utilized to define nanoscale surface heterogeneity. Surface heterogeneity is particularly important for the coating of super-hydrophobic surfaces where nanoscale roughness is one of the major reasons for the so-called lotus effect.