Surface tension and surfactants

[rwooduk]

I am struggling to understand the relation between surface tension and surfactants. When surfactants are added to say water they may have charge and head groups which influence surface motion due to repulsion and their size. However often when people refer to surface tension they refer to a liquids surfaces ability or inability to be deformed.

Let's take alcohols for example, they have different surface tensions, but are they surfactants?

I am trying to understand how surfactants change viscosity / surface tension properties. And if alcohol is a surfactant especially here where it talks about the chain length:

It has been found that as the alkyl chain length of the alcohol increases the concentration at the surface decreases.

Also are gases surfactants if they change the surface tension of a liquid? but they have no head group?

And finally does the pH of a solution relate to its surface tension?

Apologies if the question doesn't make sense, I'm a little confused.

Thanks for any help

 

[Bystander]

... and to top it all off? It appears to have given the damned thing narcolepsy ... it goes to sleep in the middle of anything requiring more than a couple minutes typing.

I am struggling to understand the relation between surface tension and surfactants. When surfactants are added to say water they may have charge and head groups which influence surface motion due to repulsion and their size. However often when people refer to surface tension they refer to a liquids surfaces ability or inability to be deformed.

Let's take alcohols for example, they have different surface tensions, but are they surfactants?

Also are gases surfactants if they change the surface tension of a liquid? but they have no head group?

And finally does the pH of a solution relate to its surface tension?

Apologies if the question doesn't make sense, I'm a little confused.

Thanks for any help

Yes. Yes. No.

 

[rwooduk]

Yes. Yes. No.

Apologies for the delay, I had a brief holiday. Not sure what the first thing about narcolepsy is lol

Thanks for the reply!

You said no to the last question, but (as I understand it) this results from an equilibrium hydroxide vs hydronium balance i.e. i.e. (Γ(H+) = Γ(OH)) ---> if one is adsorbed the other compensates and the surface tension remains constant. But for more complex species they may change this equilibrium effect? that is a possible surface pH reduction causing a change in surface tension. Could you possibly confirm?

Some of what I'm trying to understand is given in this paper:

http://www.pnas.org/content/104/18/7342.short

Water autoionization reaction 2H2O → H3O− + OH− is a textbook process of basic importance, resulting in pH = 7 for pure water. However, pH of pure water surface is shown to be significantly lower, the reduction being caused by proton stabilization at the surface. The evidence presented here includes ab initio and classical molecular dynamics simulations of water slabs with solvated H3O+ and OH− ions, density functional studies of (H2O)48H+ clusters, and spectroscopic isotopic-exchange data for D2O substitutional impurities at the surface and in the interior of ice nanocrystals. Because H3O+ does, but OH− does not, display preference for surface sites, the H2O surface is predicted to be acidic with pH < 4.8. For similar reasons, the strength of some weak acids, such as carbonic acid, is expected to increase at the surface. Enhanced surface acidity can have a significant impact on aqueous surface chemistry, e.g., in the atmosphere.

Any addiditonal comments would be welcome, as I'm really not a chemist.

 

[Bystander]

Hmmm ---- pH along some nano-scale gradient vs. bulk properties? I'm not certain there are meaningful ways of discussing such things. If you'll allow me a bit to mull this over?

 

[rwooduk]

Hmmm ---- pH along some nano-scale gradient vs. bulk properties? I'm not certain there are meaningful ways of discussing such things. If you'll allow me a bit to mull this over?

Yes please, that would be helpful. Its meaningful to me because I'm dealing with the coalescence of bubbles where changes in surface tension have been seen to have an effect, especially in terms of a change in pH at the surface causing some sort of surface tension gradient. The above paper seems to be suggesting a relation between a change in pH and surface tension, while others suggest surface tension remains constant during a change in pH due to equilibrium conditions. As I understand it. So yes, any further comments / ideas would be more than welcome, thanks.

 

[Bystander]

Coalescence of bubbles implies a reduction of surface area, and of surface free energy; methinks this is more a matter of conventions than of actual physical properties.

 

[rwooduk]

Coalescence of bubbles implies a reduction of surface area, and of surface free energy; methinks this is more a matter of conventions than of actual physical properties.

yes the coalescence of bubbles is generally due to surface properties, in particular surface "drainage" and its ability to occur so rupture can then happen. but for low Bond numbers (high surface tension) it has been calculated that coalescence would occur less readily. therefore if there is a relation between pH and surface tension, this would imply a relation between pH and coalescence. But you are right in that it may not be so meaningful, as the studies on this are quite scarce :-/ Here's what I've written so far (with limited understanding) for anyone that is interested:

The suggested relations between pH and surface tension are somewhat conflicting. Beattie et al. show a relation between pH and the zeta potential, ζ (which describes the potential difference between surfaces in a liquid) and earlier work by Lorentz showed a distinct change from [H+] = 1mM to [H+] > 1mM (but a maximum with HCl concentrations) [205, 206]. The former authors report surface tension independence from pH 1 to pH 13 citing that absorption of hydroxide ions is always balanced by equilibrium conditions in the form of similar absorption of hydronium ions i.e. (Γ(H+) = Γ(OH)) [207]. However they do acknowledge a dynamic surface tension that causes surface active species to be adsorbed, which although equilibrates in milliseconds would certainly be apparent in a cavitational context. Tabor et al. controlled the pH value of solutions containing CO2 in view to observing the effect upon coalescence. It was suggested that the complex species environment that CO2 produces upon dissolution in water can displace hydroxide ions from the bubbles’ surface, which shifts the isoelectric point (a point where there is no net charge). Additionally, it was proposed that other species that are adsorbed can contribute to the steric repulsion between the bubbles [208]. The suggestion that these factors (including a change in pH) contribute to a reduction in coalescence was similarly reported by Brems et al. who relate a reduction in pH at the surface due to dissolved species (via influenced hydronium ion surface effects) to have an effect on coalescence [209]

If anyone has any comments on something that is incorrect, any feedback would be welcome. Also if anyone wants the references.

Thanks Bystander for your help! again any further comments would be welcome