Monolayers and the Langmuir-Blodgett technique a brief review

Monolayers and the Langmuir-Blodgett technique a brief review


The molecular density at the air-water interface is manipulated with moveable barriers in a Langmuir-Blodgett (LB) film balance. A modern film balance is sealed with Polytetrafluorethylene (PTFE or Teflon®). The water subphase is temperature controlled. By moving PTFE barriers the film density decreases (compression) or increases (expansion), which changes the surface tension γlv of the air-water interface. The quasi two-dimensional lateral or surface pressure Π of the water surface can be calculated according to

Π = γ0(H2O) γlv

As a reference the surface tension of the pure water with γ0 (H2O) ≈ 73 mN/m at 20C is chosen. Plots of the surface pressure Π versus the molecular area of the amphiphiles at constant temperature T (isotherms) are characteristic for the chosen amphiphile. The plot shows the surface isotherm for DMPE at 20°C.

Isotherm of DMPE at 20°C
The lipid DMPE (1,2-Dimyristoyl-sn-glycero-phosphoethanol- amine) exhibits different phases as a monomolecular film (monolayer) at the air-water interface. The plateau hints at a phase transition from a non-ordered liquid-expanded (LE) phase to an ordered liquid-condensed (LC) phase. The red line indicates the pressure regime, where the DMPE monolayer is condensed during very slow transfer onto a hydrophilic silicon substrate. More details, see main text.

At high molecular areas the monolayer exhibit a quasi two-dimensional gas state. The pressure increases at 78 Å2/molecule with further film compression into the liquid-expanded (LE) phase, where the amphiphiles are in contact without molecular ordering. The plateau at Πc = 5 mN/m is a phase transition from the LE-phase to the liquid-condensed (LC) phase. In the LC-phase DMPE exhibit a short-range translational order of the headgroup positions. Simultaneously, the alkyl chains of the hydrophobic molecular part show a long-range order in their orientation, as indicated in the insets.

A monolayer can be transferred to a flat solid support, as e.g. a hydrophilic silicon substrate, by an upstroke of the immersed substrate through the monolayer. Since the substrate is hydrophilic, the headgroup of the monolayer will orient towards to the solid surface, whereas the hydrophobic alkyl chains are exposed to the air.

Hydrophilic Langmuir-Blodgett transfer
The sketch shows the orientation of lipids like DMPE at the solid-air interface for an hydrophilic LB transfer, i.e. an upstroke of a hydrophilic substrate (black line) like silicon through the monolayer at the air-water interface. During the transfer, the surface pressure of the monolayer is kept constant by automatically compressing the film by the barriers to account for the material loss.



At high transfer speeds the quality of the transferred monolayer will largely be dominated by hydrodynamics: the underlying water layer has to be removed from the monolayer in contact with the solid surface. At low transfer speeds, the monolayer quality after transfer will be increasingly dominated by molecular interaction between the monolayer and the solid surface. This has been shown by fluorescence microscopy and precise contact angle measurements, both for very low transfer speeds close to the equilibrium height of the water meniscus adjacent to the three-phase contact line [1,2].


Fluorescence image (top view) of a hydrophilic Langmuir-Blodgett transfer of a DMPE monolayer onto a silicon substrate
The image has been recorded during observation of the LB transfer from the side in the sketch above at a surface pressure of about 2 mN/m. The dashed line represents the three-phase contact line, where the monolayer is transferred from the bright appearing water surface (right) in direction (white arrow) of the dark appearing silicon substrate (left). The contrast comes from the presence of an amphiphilic fluorescence dye, added to the monolayer of DMPE, to visualize different monolayer phases. The dye demix from condensed monolayer phases. Therefore it can be concluded, that the monolayer on the water is still in the non-condensed LE-phase in accordance with the isotherm, and thus appears bright. In contrast, the monolayer already transferred onto the silicon surface appears dark and thus must be alrady in the LC-phase. This condensation effect of silicon can be observed in the range of 2 to 5 mN/m, i.e. below the phase transition of the monolayer at the air-water interface (see red line in the isotherm). Thus, the presence of the negatively charged silicon substrate shifts down the phase transition on the water surface.



The monolayer condenses in a pressure range of 2 to 5 mN/m, when it is transferred from the LE-phase onto a negatively charged flat silicon substrate, owing to the substrate-layer interaction (red line in the isotherm above). This can be achieved by an electrostatic interaction between the negatively charged silicon surface and the dipolar headgroup of DMPE. To get more insight into the substrate-layer interaction, the Monolayer Particle Interaction Apparatus (MPIA) has been constructed.



References:

  1. K. Graf, H. Riegler; Molecular adhesion interactions between Langmuir monolayers and solid substrates, Colloids and surfaces A: Physicochemical and Engineering Aspects, 131 (1998), 215-224. [Full text]
  2. K. Graf, H. Riegler; Is there a General Equation of State Approach for Interfacial Tensions?, Langmuir, 16(11) (2000), 5187-5191. [Full text]