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Last week I was discussing with a friend how we thought the stripeless cleaning of windows is achieved when using a cleaner like Windex Glass Cleaner as opposed to just using hot water, in which case you do get stripes left behind.

At first we thought that the stripeless nature was caused by surfactants in the cleaner that allow the (polar) dirt on the windows to be collected in the water through micellation. However, when we use normal cleaner not specifically designed to clean glass/windows, we also get the stripes even though we are sure that there are also surfactants in the normal cleaner.

The question is: what is the difference between the glass cleaner and the regular cleaner that makes sure that windows become stripeless clean?

One thought we considered but couldn't really work out was that the stripe formation would be something caused by a combination of the marangoni effect and evaporation, similar to the coffee-stain effect which can be overcome by specific tuning of the surfactants as shown here.

Could someone explain how the stripeless cleaning works? Can fluid dynamics explain what happens or is it really something chemical going on?

When searching I also ran into a solution that (at least the manufacturers claim) will also give stripeless clean windows. The interesting thing is that they use pure $\ce{H2O}$ so without any surfactants but also without any dissolved salts like regular tap water has.

Cross-post: https://physics.stackexchange.com/q/56800/7433

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After quick digging I found, that glass cleaner solutions usually contain alcohols and surfactants. This additions reduce surface tension and allows the solution to interact with glass more easily, distributing the solution over the entire surface.

The white stripes visible after water dried are usually solid salts. If the water does not contain any salts, but only compounds, that can quickly evaporate, it will not leave stripes. But it is not addition, but absence of additions that do the trick. For example, most bleacher/toilet cleaners contain hypoclorites, salts of unstalble $\ce{HOCl}$ acid and leave $\ce{NaCl}$ after evaporation

When tap water is allowed to dry, it usually leaves white stripes, but they are not visible on most surfaces

The only ideas I can get why the salts don't deposit if using a glass cleaner is that salts are removed (and if you have a cleaner not meant to be diluted, the water in it may be deionized) or to trap salt ions into organic ligands so resulted salts were not crystalline (possible, but I do not like the idea).

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  • $\begingroup$ But this doesn't explain why regular cleaner does leave stripes. In regular cleaner we also have alcohol and surfactants. $\endgroup$
    – Michiel
    Mar 25, 2013 at 11:25
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    $\begingroup$ The white stripes visible after water dried are usually solid salts. If the water does not contain any salts, but only compounds, that can quickly evaporate, it will not leave stripes. But it is not addition, but absence of additions that do the trick. For example, most bleacher/toilet cleaners contain hypoclorites, salts of unstalble $HOCl$ acid and leave $NaCl$ after evaporation. $\endgroup$
    – permeakra
    Mar 25, 2013 at 14:58
  • $\begingroup$ Ok, that makes sense, thanks! But that brings to mind another question: normal tap water does contain salts (quite a bit in some locations) so how come this does not deposit? $\endgroup$
    – Michiel
    Mar 25, 2013 at 16:02
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    $\begingroup$ @michielm The only ideas I can get is either to remove salts (and if you have a cleaner not meant to be diluted, the water in it may be deionized) or to trap salt ions into organic ligands so resulted salts were not crystalline (possible, but I do not like the idea). $\endgroup$
    – permeakra
    Mar 25, 2013 at 16:22

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