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Author Topic: Surfactant vs Wetting Agent  (Read 1169 times)

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nicamarvin

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Surfactant vs Wetting Agent
« on: September 25, 2018, 06:31:00 AM »

On metalizing non conductors like plastics it is always recommended to use a wetting agents, I use a mixture of coffee extracts, acetic acid and alcohol on my wetting agent, this wetting changes the surface of the plastic and makes it hydrophilic after it's applied and during consecutive rinsing, in fact if I let it dry and then pour some water on the surface the surface is still hydrophilic instead of the hydrophobic nature of plastics so my question is what is happening at the molecular level on the surface of the plastic? Is this really a wetting agent or surfactant? Perhaps something else I am not aware of? I have always understood that wetting agents will let a liquid cover the entire surface of an hydrophobic surface without beading but after the liquid is rinse the surface is hydrophobic again
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Enthalpy

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Re: Surfactant vs Wetting Agent
« Reply #1 on: September 25, 2018, 09:53:49 AM »

Yes, surfaces react chemically with liquids and often get permanently modified.

You typically obtain a layer of one or very few molecules that stick very strongly to the surface. They won't go away by rinsing unless you rinsing liquid too reacts chemically with the surface. This layer defines the hydrophobic or hydrophilic nature of the surface.

The chemistry of surfaces is a true and authentic mess. Volume chemistry applies partly there. Too few people are interested (essentially the semiconductor industry), so knowledge is very incomplete.
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wildfyr

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Re: Surfactant vs Wetting Agent
« Reply #2 on: September 25, 2018, 10:34:25 AM »

There is a strong adsorption reaction between a surface and whatever compound in a solution is has the strongest affinity (call it compound A). This affinity can be van der Waals forces, hydrogen bonding, ionic, or in some specific cases covalent bonding. This is driven by the fact that a solid surface does not want to be in direct contact with the environment. I'm guessing here, but having gas molecules adhered to a surface is probably a high energy state, whereas having a semi-organized monolayer of liquid or solid is lower.

Once this monolayer is established you need to disrupt it to replace it with something else (compound B), unless it is quite volatile. Sometimes you can overcome it with sheer concentration gradient if the difference in affinity for the surface between compound A and B isn't too large (say, replacing one alkane with another alkane).

Just because that surface molecule is soluble in a rinsing solvent doesn't mean the monolayer will be removed, often it has a much higher affinity for the surface than it does to be dissolved in a solute.

Here is how sensitive this can be, machines like XPS are incredibly sensitive to contamination, and if they are contaminated with outside gaseous hydrocarbons from local air those molecules will stick to the walls, and slowly outgas tiny amounts which foul up measurements. The whole machine has to be baked out at several hundred °C to force it all to stop adhering to all the metal surfaces, go into the gas phase, and get sucked out by a vacuum. Keep in mind, this is compounds like methane that got in there by being gas at room temperature!

Ligand exchange in metallic nanoparticles is also great example of this phenomenon.
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nicamarvin

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Re: Surfactant vs Wetting Agent
« Reply #3 on: September 26, 2018, 07:20:02 AM »

Thanks, I appreciate all of your reply

What do you think is happening when the components of the surfactant are combined?

I have tried each individuallly and none of them work as a surfactant on their own, for example treating a polystyrene article with concentrated instant coffee liquid the surface tension remains the same, then with diluted acitic acid(white vinegar) and it's the same. When I combine concentrated coffee with white vinegar the surface tension of the liquid mixture prevents any of the liquid to stick to the polystyrene(even if submerged on the liquid), is not until I add 100% isopropyl alcohol is when the surface tension of the liquid mixture is lowered and the polystyrene becomes treated.

I think the triacylglycerols and fatty acids on the coffee reacts with acetic acid to form some form of non-ionic surfactant but I am not very sure, what I can say is that besides changing the surface tension of plastics with a flame torch this is the most effective way to increase the surface energy of said surfaces allowing fluids to effectively wet the surface and permits a strong adhesive molecular and mechanical bond.

if anyone would like to try it out, it's about 20 grams of instant coffee, 500ml of vinegar(somehow the apple cider kind seems to work better than plain white vinegar) and about 100 ml of 100% isopropyl alcohol
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