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I heard that there is a standard about cleaned glass, namely

Chemically clean glass supports a uniform film of water, with no hanging droplets visible.

I tried to clean my dishes at home using detergent. After several efforts, the ceramic-made dish looks like a uniform film of water just after finishing the cleaning. If I hold the dish for a while, the water starts to concentrate at regions, forming a droplet.

In addition, in this video http://www.youtube.com/watch?v=m7liHlmv5Kc at 2 min, there is water droplet in the flask.

My questions are:

(i) Does the clean standard have any time range? Does it apply to arbitary time scale?

(ii) If the answer to question (i) is yes, how should I do to make the dishes so clean that there is absolutely no water concentrating at certain regions? I do not have a dishwasher.

P.S. Video demonstration is highly appreciated.

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DO you want an answer for chemical glassware or for home dishes and glasses? –  G M Feb 2 at 12:46
    
for home dishes and glasses –  user26143 Feb 2 at 13:32

3 Answers 3

I am not that familiar with cleaning procedures, but the 'rule' to support a uniform film of water without droplets seems to be an easy proxy for checking whether there are still contaminated spots on the surface. In that case, a phenomenon called contact line pinning will occur, which results in droplets. The uniform film will only be possible if there are no contaminated spots.

The fact that there are droplets however doesn't necessarily indicate that the glass is not clean, because it can also be a result of just too much water being present. The maximum thickness of the uniform water film is governed by a balance between gravity and surface tension as $\sqrt{\frac{\gamma}{\rho g}}$ and for water is approximately 2.7 mm (see e.g. capillary length or the great book by de Gennes on capillarity and wetting phenomena).

If the water film becomes thicker then the 2.7 mm it will start to form droplets as well. So the 'cleaning rule' is only useful if you have removed sufficient water from the surface. In the video I'm quite sure that they have not removed sufficient water yet, so the rule cannot be applied.

In your case you are holding the glassware probably such that water starts to drain from 1 side to the other (due to gravity), at the moment that the layer at the bottom exceed the capillary length you will start to see droplets.

An additional point to consider: drying a water-wet piece of glassware in an oven only yields a clean surface when you have used deionized water (as they do in the video). If you use normal tap water then you will have some calcification due to the salts in the water that end up on your glass surface.

So to try and answer your questions based on these considerations:

i) Yes, there is a time range if you have gravitational drainage of the film. For anything that is not completely horizontal you will see this.

ii) Water concentrating is not the issue as long as it is deionized. If you really want spotlessly clean glasses: use deionized water after the whole cleaning procedure and just dry the glass in a warm place. If you don't have deionized water: you can make reasonably well deionized water by boiling tap water and catching the steam in a clean cup.

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Thanks for your answer. If too much water caused the droplet, why in that youtube video, the droplet did not form in the bottom of the flask? –  user26143 Feb 1 at 17:02
    
@user26143 You are right that there is a droplet at the side which suggests pinning thus a non clean surface. I do think that this droplet is on the outside of the flask, not on the inside, but that is hard to see. –  Michiel Feb 2 at 9:23

There are several laboratory techniques to rigorously clean glass. Here are a few of the more common ones, to add to the other answers. There are probably more procedures, and it's also common to perform more than one treatment to maximize efficiency.

Practically none of these have any use at home, because they are too dangerous, too expensive, and just outright unnecessary. A soapy scrub will do just fine cleaning anything in 99% of cases, even if water still beads.

1 - Ultrasonication in pure solvents or detergent solutions.

Often, the main impurities on the surface of glass are organic substances which are present at all times in air outside of strictly clean environments, and which strongly adhere to the glass when they come into contact. By dipping them into a solution with some affinity for the organic substances, it is possible to strip them off. Common options are regular dishwashing liquid (usually sulphonic acids with large alkyl chains), solutions of quaternary ammonium salts, aqueous ethanolanime, or pure deionized water, ethanol, isopropanol, acetone, among many others.

It is much more effective, however, to couple the aforementioned compounds with ultrasonication rather than simply dipping or scrubbing. In an ultrasonicator, the liquid is vibrated at frequencies higher than $10\ kHz$. Though macroscopically the liquid doesn't seem to change much, on the microscopic level there is chaos. The high frequency vibrations locally produce very fast solvent jets and cavitation bubbles, which effectively sandblast anything closeby. Glass manages to stay mostly unaffected due to its stiffness and strong bonding, but anything not very firmly attached to its surface will be flung away.

2 - Prolonged exposure to high temperatures

Common glass is thermally stable and can be heated strongly before it softens, so it can take temperatures at which many other substance decompose, evaporate or burn (if exposed to oxygen). Different types of glass have different maximum temperatures depending on their composition. Also, the thermal expansion while heating and contraction while cooling can be problematic for delicate glassware.

3 - Soak in base bath

Strong bases react with many substances and help dissolve them off the surface of the glass. However, while very inert, glass itself will also slowly react with concentrated basic solutions at room temperature. This also helps clean because if very persistent stains manage to survive the basic conditions, the glass surrounding them will begin dissolving slightly, and after a while the stain ends up detaching from the substrate as the glass behind it will have dissolved away. The regular options are concentrated $\ce{NaOH}$ or $\ce{KOH}$ in water or ethanol.

4 - Soak in highly oxidizing solutions

Glass is composted mostly of silicon already in its most oxidized form ($\ce{SiO2}$). Hence exposure to very oxidizing conditions will not damage the surface of the glass, while anything else on it is likely to dissolve, be it organic or inorganic matter. Classic options for dipping the glass are acidic piranha solutions (a mixture of pure $\ce{H2SO4}$ and concentrated $\ce{H2O2}$), basic piranha solutions (concentrated aqueous $\ce{NH4OH}$ and concentrated $\ce{H2O2}$), sulphocromic solution ($\ce{H2SO4}$ + $\ce{H2Cr2O7}$, in disuse due to the toxicity of $\ce{Cr^{6+}}$) and sulphonitric solution ($\ce{H2SO4}$ + $\ce{HNO3}$). These are all pretty heavy-duty mixtures. For example, here's why piranha solutions earn their name.

5 - Treatment with ozone/oxygen plasma

This is a step up from even the piranha solutions. If all you want to do is oxidize organic matter on the surface of the glass, then why not cut out the middleman and directly bathe the glass in reactive oxygen? The glass sample is first put into a closed chamber containing oxygen (air works, but pure oxygen is better). If ozone is being used, then the chamber is lit from the inside by a mercury lamp, which produces lots of ultraviolet light. The light is capable of dissociating oxygen molecules and producing ozone, which then is directly exposed to whatever is on the surface of the glass. In the case of oxygen plasma, the chamber is pumped down to a low pressure of pure oxygen gas before a coil produces strongly varying electric and magnetic fields inside the chamber, generating all sorts of oxygen species such as singlet oxygen (an electronically excited form of standard, triplet oxygen), free oxygen atoms, and several ions and radicals. Here's a nice video on how this process works in practice.

After all the work you put into cleaning a glass surface, it will immediately start adsorbing impurities if outside a vacuum, and it may take as little as a few minutes before it is too dirty to use in very sensitive and specialized applications. I have found that water already beads significantly again less than 10 minutes after the final treatment.

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"Clean" depends on what you consider to be dirt. detergent leaves a spotty adsorbed layer of detergent. "Clean" glass by removing its surface. Begin by having a lab coat, chemical gloves, and a full face shield. If it gets on you it will dig holes. Instant blindness in an eye.

5-gallon plastic (not vinyl!) bucket with a cover, 4 gallons of biological denatured ethanol, pound jar of of potassium hydroxide pastilles. Shove crushed ice into the jar with mixing to form a viscous slurry (exotherm!). Pour with mixing into the alcohol (probably have to do this twice to get all the KOH dissolved). This is a KOH bath. (Standard denatured alcohol will react to go red and stinky, but still work.)

Soak Pyrex in a KOH bath overnight to remove everything except hydrocarbon grease and metal oxides. Soak soda lime glass for an hour or three, for it slowly dissolves. Rinse with DI water, soak in 0.1 M HCl for an hour [#SiO(-) K(+) to #SiOH], rinse well with DI water, store totally submerged in DI water. Water will perfectly sheet. If the surface is allowed to dry, start again. Never put your hand in there. Wear long heavy gloves and positive eye protection. Immediately, thoroughly wash off skin contact.

NEVER allow glass flats to stack flat under water. They will bond (a micron of water layer is the hydrogen bonding adhesive). Try sliding them apart without applying orthogonal pressure.

Chromate/conc. sulfuric acid) cleaning solution cover the glass with a layer of chromium(VI) species. Things like NoChromix work, but you still have the conc. sulfuric acid.

Note added in proof: NEVER put graduated glassware in a base bath, for they lose calibration. Frosted syringe barrels are iffy. Aluminum will fizz hydrogen, corrode, and disappear.

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And just another reason why I'm glad I'm a computational chemist. –  LordStryker Feb 4 at 16:32

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