Hello!
I am trying to make a deposition of a L-alfa-lecithin phospholipid in a gold substrate for electrochemical applications.
Nowadays, I am using a KSV minimicro through with a Wilhelmy paper plate. As subphase, using MilliQ 18.2MOHm resistance (pH 5.5), and dispersing 10 uL of a 1 mg/mL chloroform solution of lecithin on top of the subphase.
The problem is that:
- If I lock the barrier position, for example at 70 mm, the initial pressure is 30 mN/m, but right after the locking, the pressure keeps going down
- If I try to lock the pressure at 30 mN/m, the pressure keeps falling, so the barrier continues to compress, until it touches the paper plate.
Can some have any hypothesis of why the pressure keeps falling instead of stabilizing?
Dear Rodrigo Mancini ,
The (possible) answer to your question pretty much depend on the type of L-α-Lecithin you use.
If your lecithin is dipalmitoylphosphatidylcholine (L-alpha-lecithin) then you should read this relevant paper about meta-stable lecithin monolayers which might be answering your mystery:
https://core.ac.uk/download/pdf/82208187.pdf
The basic message is that the lipids should be able to spread in the ‘fluid’ state. If not rather unstable surfaces are formed.
If your lecithin is for example L-α-Lecithin, Soybean then it contains a large amount of unsaturated phosphatidylcholine and less of the above effect is to be expected. Then a possible answer to your observations is a bit hard to tell:
-Leave the system sufficiently long enough (at least 10 minutes) in order to let the organic solvent evaporate. Only then start measuring the surface tension. It sounds to me that you possibly have applied not enough lipid on the surface and while the system equilibrate not enough lipid positioned itself on the surface.
-It might have something to do with the use of a paper plate (as you know a platinum plate is the preferred way).
-Also the fact that you seem to use an unbuffered system might have something to do with it.
FYI. An old but still relevant study of the effects of ions (and pH) on different Lecithin’s might be interesting to read:
http://www.jlr.org/content/8/3/227.full.pdf
See for a well performed study on the effects of salts in a highly related lipid (DPPC):
https://bib-pubdb1.desy.de/record/140202/files/monolayerHOF3.pdf
It is recently well studied that even small amounts of Ca2+ (can) have substantial effects on certain types of lipids:
https://www.nature.com/articles/srep38035
Good luck!
PS. Have a look at an earlier discussion here on Researchgate where the topic of the water is discussed.
https://www.researchgate.net/post/What_is_the_difference_between_distilled_water_and_deionized_water_DI_water2
Dear Rodrigo, It could be that your phospholipid is soluble in water and the monolayer does not forms properly. In what solvent are you suspend your molecules for the deposition? Is better to use a non-polar solvent.
Try to run an isotherm, starting to 0 mN/m and see if the Surface pressure increase as the area becomes shorter, this experiment will give you some previous information about your molecules. If Surface pressure does not increase, it means that most of the molecules are going to the subphase.
Be shure that your balance is clean and that there is not evaporation. In my short experience, evaporation cases an apparent decrease in the Surface tensión as the medition is calibrated with the wilhelmy plate at a certain level.
Most natural phospholipids should be considered as surfactants sparingly soluble in the aqueous subphase. Using a conc. NaCl solution as a subphase seems a convenient practice for such monolayes compression.
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