Molecule

Chemical formula

Properties

Amines

C_nH_2n+1NH₂

Alternate with acids

Acids

C_nH_2n+1COOH

Easy to vary ‘n’

Amides

C_nH_2n+1CONH₂

Form alternate multilayers

Alcohols

C_nH_2n+1OH

Difficult to create the Langmuir Blodgett layers

Figure 2: commonly used amphiphiles in Langmuir film construction (Edwards, Bergren, & Porter, 2007)

Procedures

Once the desirable multi or single component composition is prepared, it is subjected to lateral pressure using a movable barrier placed inside a Langmuir trough. As compression force is applied to the film, it undergoes a phase transitional series. Finally, this pressure increases the concentration of the two-dimensional film (Taylor & Laurén, 2017).

Once the compressed film is produced, it is transferred to the surface of an electrode. the manner in which the initial layer is transferred and the resulting Langmuir-Blodgett film quality can be significantly affected by the substrate characteristics (Taylor & Laurén, 2017). Thus, the Langmuir film is transferred by lifting the hydrophilic substrate vertically. The electrode surface absorbs the film based on the attractive forces between the head groups and the surface. These attractive forces are so strong that solvent molecules between the film and solid surface interface are excluded from the transfer. The carefully transferred electrode supported layer retains the precursor Langmuir film orientation and packing density (Taylor & Laurén, 2017). In case multiple layers of hydrophilic head groups are required, they are directly deposited on the surface of the substrate through repetition of the raising or/and lowering of the substrate through the Langmuir film (Wilczek, Zhu, Chi, Thiele, & Gurevich, 2016).

Technique

Different deposition cycles develop complex architectural layers. The commonly formed layers are the ABA types that result from XYZ deposition cycles (Edwards, Bergren, & Porter, 2007). Repetition of extended deposition that enhances the orientation of head-to-tail molecules on the adjacent layers creates the x film types. The recession of deposition cycles successively forms the z layer types. The y-film type is formed by alternate receding and deposition cycles that enhance the orientation of tail-to-tail and head-to-head configuration of monomolecular layers. The use of a multiple tank trough and more than one amphiphile facilitates the formation of mixed (A-B-A layers) multilayers that possess varying structural features and characteristics (Edwards, Bergren, & Porter, 2007).

Machine Parts

The Langmuir-Blodgett machine, also referred to as the Langmuir-Blodgett trough is laboratory equipment made of four vital components as shown in figure (3) below.

Figure 3: parts of a Langmuir-Blodgett machine resistance (Taylor & Laurén, 2017)

The barriers are movable parts that slide parallel to the trough walls. These barriers made of hydrophilic material are in contact with the fluid top forming a meniscus to enable retaining of molecules within regardless of the packing density or chemical resistance (Taylor & Laurén, 2017). The barrier is made of flexible Teflon tape which is wound around the three roller pairs of which one is fixed while the remaining two moves on trolleys to maintain a constant tape length despite the working zone area variations (Taylor & Laurén, 2017).

The balance on the other hand is used to measure the surface pressure. Surface pressure which is expressed as the pure subphase surface tension less the amphiphiles subphase surface tension is a critical property of the L-B system (Taylor & Laurén, 2017). one of the monolayer properties indicators is the molecular area-surface pressure isotherm. The implication is that to maintain LB film uniformity, a constant surface pressure should be maintained. The Langmuir balance thus measures the surface pressure in terms of the directly exerted force on the movable barrier (Taylor & Laurén, 2017).

The Teflon tough is the main part of the L-B system that holds the subphase on which the monolayer is spread (Taylor & Laurén, 2017). The electronic dipper on the other hand is attached to the frame of the trough. The dipper facilitates monolayer transfer from the liquid-gas interface to the substrate (Taylor & Laurén, 2017).

L-B J-curve and formula

Figure 4: L-B J-curve (De Oliveira, De Barros, & Ferreira, 2017)

The L-B formula is expressed as:

Where  represents the surface pressure,   is the subphase tension in the air and  is the subphase surface tension on lipid monolayer (Osborn & Yager, 1995). To ensure the film remains at a relatively condensed state, the surface pressure shown in the j-curve above is maintained at an adequate value through film area adjustments.