[FHUGO]

Hi,

I am looking for a surfactant or mixture that might match these properties.

Wax-like solid at 25C and a flowable liquid when melted  with a melting point <100C. The particular desired ability is for the surfactant to temporarily mix with water and then seperate into two distinct phases so it can be easily recovered. Ideally very little residue would be left on surfaces the surfactant contacts and the phase separation could be controlled, for example, by aggregation of the surfactant above or below a certain temperature or with the addition of a flocculent that is easily separable from the surfactant.

[Enthalpy]

Welcome, FHugo!

I don't feel the melting point will bring much. If a melt mixes with water, so uses to do the solid, only a bit less. Just plain usual soap is a solid and it dissolves in water.

The very usefulness of a surfactant is to be hydrophilic at one end, that is, it dissolves in water. Perhaps you could try a surfactant less hydrophilic than soap, but I suppose that too much of it will remain in water after cooling.

Some chemical transformation, maybe. Let's see if other people find something.

If your goal is to clean the used water (is it?), why shouldn't you pass it through reverse osmosis?
https://en.wikipedia.org/wiki/Reverse_osmosis
It uses very little energy and separates the flux into very clean water and a small fraction of water containing concentrated impurities. You could find some existing device and extrapolate the size and speed to your needs to see how reasonable this is. A specialized company would tell how sensitive the device is to dirt.

[wildfyr]

What about using a pH shift to crash it out? I imagine some long chain carboxylic acids are decent surfactants at neutral or basic pH, but insoluble at lower pH when the acid is fully protonated.

Never seen this done in lit for surfactants, but removing carboxylic acids from water layer is common this way!

Temp is a tough way to do this because a surfactant is usually such a tiny amount of the total mass that if it's at all "soluble" it will be present in water layer to some degree at all liquid temps. A perfect one might exist, but hard to find.

Cetyl alcohol is a solid at room temp, liquid above 50C, has very poor room temp solubility in water, and has surfactant properties if you want to try that.

Edit: cetyl alcohol is used as a "liquid pool cover" to prevent evaporation. Interesting, maybe there is precedent.

[wildfyr]

Also it's nonionic so it won't stick to things too badly, and is easily removed with any organic solvent.

Jeez, this one might be good enough that I deserve to get paid a consulting fee!

[rolnor]

I gave you a "molsnack" wildfyr, good thinking!

[Enthalpy]

I just wonder about the use of the surfactant. If the resulting liquid shall only wet surfaces, OK. But if it shall remove dirt, this will stay somewhere, possibly at the "reused" surfactant.

[Enthalpy]

Purifying wastewater by reverse osmosis is long done. At chemical plants, it reduces the volume of the dirty effluents. In California during the last drought, it provided usable water from the sewage. What smaller units are conceivable?

Washing machines could save sweetwater. Between two uses, treat 0.1m³ in one or several days. Add tanks, accept to squander water when the machine serves more frequently. In 24h it's 1.2cm³/s. Recovering 80% of the water takes some 60 bar, at 70% efficiency 10W only.

Dishwashers are very similar.

Laundries, hotels, restaurants, collective houses would rather have a collective reverse osmosis unit. Reusing water only at washing machines or dishwashers is conceptually more acceptable.

0.2€/kWh electricity cost 0.5€/m³ saved water, less during nighttime. In rich and rainy Germany, water costs some 4€/m³ in house quantity.

To clean floors, professionals have trolleys with brooms, floorclothes, surfactants, water... Purifying the used water would waste less time renewing it. 5L in 15min need 50W, hence a motor and battery. Economics may tell "only where water is scarce".

Manned or robotic cars wash the floor at supermarkets, undergrounds... They already have electricity and collect the used water. 1L/min needs 150W if done in real time. This saves idle time and water cost. It may reduce the car's size.

The size, procurement cost and maintenance cost of reverse osmosis units must still be checked for these uses.

Marc Schaefer, aka Enthalpy

[FHUGO]

Thanks for all the replies!

Wildfyr is steering in the right direction. I have had some success destabilising non-ionics past their CP and giving it a little extra push by salting out with calcium and sodium salts. I will add that it is no issue having a moderate amount of surfactant in the aqueous phase so long as a good portion is separated out for reclamation.

Also the surfactant will not be doing any cleaning so ideally it is only water and surfactant or water and surf+wax.
The water and surfactant would in this case be reused.

I am trying to steer clear of organic solvents but could you have a combination of two substances that might only be water soluble in combination but when in water with the addition of something or a change in temp or ph they might separate into 3 phases?

[FHUGO]

Just a thought, could you combine two surfactants with a great difference in melting points to lower the melting point in combination then use this discrepancy later on to separate the phases from water?

[wildfyr]

I did mention that long chain carboxylic acids might do it in response to pH.