How it Works
Amphiphiles are chemical compounds possessing both hydrophilic and hydrophobic parts. They are generally, water insoluble. The hydrophobic group is typically a large hydrocarbon moiety, such as a long chain of the form CH3(CH2)n, with n>4. The hydrophilic group may consist of anionic carboxylates: (RCO2-), sulfates (RSO4-) sulfonates (RSO3-) etc. and cationic amines (RNH3+). Neutral/zwitterionic hydrophilic groups like diacylglycerol, DPPC phospholipids, etc. are also present. Molecules having several hydrophilic and hydrophobic domains are also exists. Typical examples are protein and enzymes. Following is the typical example of amphiphiles at air-water interface.
A Langmuir monolayer is a one-molecule thick layer of an insoluble organic material spread onto an aqueous subphase. Langmuir-monolayers are extensively studied for the fabrication of Langmuir-Blodgett film (LB films), which are formed by transferred monolayers on a solid substrate.
Gibbs Monolayer is a monolayer of partially soluble amphiphile. Only difference with Langmuir is the solubility of the amphiphile, otherwise structurally identical with Langmuir Monolayer. Substances, which are used to form Langmuir monolayers, are insoluble, so molecules are trapped at the air-water interface. In Gibbs monolayer molecule essentially hops and jumps in and out from the water surface. Since, it almost rare to find absolutely insoluble substance, so there is no rigid dividing line between these two types of monolayer. Division of these two layers can only be distinctly understood on the depth of the water under the monolayer and the time scale on which the experiment is performed. Generally surfactants and proteins are producing Gibbs monolayer.
A Langmuir-Blodgett film contains one or more monolayers of an amphiphile, deposited from the surface of a liquid onto a solid by immersing (or emersing) the solid substrate into (or from) the liquid. A monolayer is added with each immersion or emersion step, thus films with very accurate thickness can be formed. The monolayers are usually composed of polar molecules with a hydrophilic head and a hydrophobic tail (example: fatty acids).
It was discovered by Langmuir in 1918. 16 years later Katherine Blodgett discovered the process can be repeated, thus it leads to multi-layer.
Followings are the three types of film structure are possible by vertical lifting method.
There is another lifting method called Schaefer Method by horizontal lifting. Here the slide is coming down horizontally touches the monolayer and moving up horizontally to lift the film. Here slide should be hydrophobic in nature. Below is the schematic diagram of Schaefer method of lifting.
The surface pressure, π is defined as π = S0 - Sf where S0 and Sf are surface tension of clean air-water interface and subphase with material spread. It is actually the change in surface tension of water in negative sense due to the addition of other molecule at the air-water interface
Pressure (π)-Area (A) Isotherm:
Isotherm is curve of the surface pressure against area per molecule at a fixed temperature, and it often shows sharp bends or kinks, indicates the phase transitions in the two dimensional layer.
In the figure of π-A isotherm, distinct regions of differing compressibility can be seen. First, at the lowest pressure, molecules are in gas phase (G). Then with increasing pressure, liquid expanded (LE) phase region will appear after coexistence of G and LE phases. With higher pressure liquid condensed (LC) phase will appear after coexistence of LE and LC phases. With more pressure near-vertical condensed or solid (S) phase will appear. With further increase of pressure monolayer becomes unstable and a collapse showing sharp change or decrement of pressure will appear. π-A isotherm is a signature of a molecule. All these phases may not be distinct for a particular molecule and also depends strongly on temperature and compression speed.
The transfer ratio tr is defined as tr = Am/As, where Am is the decrease in monolayer area during a deposition stroke and As = Area of the substrate supposed to be covered. For ideal case transfer tr = 1.
Stability curve is the fractional change in area with time of a monolayer maintained at constant pressure. By measuring Area(A) Vs Time(T) at constant pressure one can obtain stability curve. Characteristic feature of the curve not only shows how stable is the monolayer but also can predict what type of process is going on in the monolayer. Adjacent figure shows the typical features of stability curve.
Time vs Pressure (T-V-P) curve:
This is the plot of change of pressure with time when a monolayer is maintained at constant area. This curve also represents how the monolayer is stable. But its main use is the measurement of adsorption kinetics of molecule present in the water subphase to the pre fabricated monolayer at the interface. Adjacent figure shows typical protein (ovalbumin) adsorption kinetics to different lipid monolayer (octadecylamine, stearic acid, DPPC) pre compressed at 6 mN/m air-water interface.
Generally two types of Wilhelmy plate are used for surface pressure measurement. One is filter paper type and other of rough surfaced platinum plate. We are using plate of filter paper which is completely soaked with water and effectively becomes an extension of the subphase. Guarantees zero contact angle. Platinum plate surface should be roughened by sandblasting. Roughened platinum plate can be wetted completely by water and hence makes zero contact angle. Smooth plate does not show zero contact angle. Plate should be very thin. Width of the plate is generally taken as 1 cm precise. Followings are the principles showing how Wilhelmy plate measures surface pressure.
Let a plate of length l, width w and thickness t be immersed into water by h. Then the resultant force F, acting on the plate is given by,
where, 𝜌 is the density of the plate, 𝜌0 is the density of water and g is the acceleration due to gravity. Now the surface pressure is defined as π = S0 - Sf Where, S0 and Sf are surface tension of clean subphase and subphase with material. Then the change in force due to material on subphase is 𝝳F = 2 (w+t). 𝝳S = 2 (w+t) π (Considering h = constant and θc ~ 0 i.e. cos θc =1) If the plate is thin enough i.e. t is negligible compared to w and if the plate width w = 1 cm then, 𝝳F = 2π or π = 𝝳F/2. So in this condition surface pressure is the half of weight measured by microbalance after zeroing microbalance in pure water.
Surface tension is a property of liquids arising from unbalanced molecular cohesive forces at or near the surface, as a result of which the surface tends to contract and has properties resembling those of a stretched elastic membrane. Polar liquid shows strong surface tension.
Surface Tension of Various Air-Liquids interface at 293 K (Weast, R. C. (Ed.). Handbook of Chemistry and Physics, 61st ed. Boca Raton, FL: CRC Press, p. F-45, 1981.)
|Common Liquid||Surface Tension (mN/m)|
Variation of surface tension of air-water interface with temperature (Weast, R. C. (Ed.). Handbook of Chemistry and Physics, 61st ed. Boca Raton, FL: CRC Press, p. F-45, 1981.).
|Temperature (⁰C)||Surface Tension (erg cm-2)|
The equilibrium angle of contact of a liquid on a rigid surface, measured within the liquid at the contact line where three phases (liquid, solid, gas) meets.
For example, water sheeting on glass has zero contact angle, but water beading on an oily surface or plastic may have a contact angle of 90° or greater.
Hydrophilic & hydrophobic surface:
A substrate surface is said to be hydrophobic when the contact angle with water is greater than 90°. Example Figure A above.
A substrate surface is said to be hydrophilic when the contact angle with water is less than 90°. Example Figure B above.