Detergent stocks and bubbles

SDS, Tween-20, Triton X-100, and CHAPS are all called detergents on the bottle. They do not share a charge, a cold-room habit, or a concentration at which micelles appear. This comparison is for choosing a class, storing a stock so it stays a solution, and keeping bubbles out of a pipette and a plate reader. It is not a lysis recipe. No percentages for a homemade extraction buffer belong here.

The water you dissolve them in still has a grade, discussed in laboratory water types and where they fail. If the detergent sits in a pH buffer, the meter work is preparing a buffer and checking pH. How a bubble wrecks a delivered volume is part of accurate micropipetting technique. The solids themselves are specified from the reagents and chemicals catalogue. Glass that last held a different detergent is a reason to look at beakers and flasks. Put the chemical name, not the word detergent, on the quotation request.

Charge first, nickname second

SDS, sodium dodecyl sulfate, is anionic. It denatures many proteins and binds them in a ratio that electrophoresis exploits. The charge is also why it crashes with potassium. Potassium dodecyl sulfate is poorly soluble, so a buffer that contains potassium chloride, potassium acetate, or potassium phosphate is a bad home for SDS even when the sodium form looked clear. The precipitate can appear when you mix two stocks that were each fine alone.

Tween-20, polysorbate 20, and Triton X-100, a tert-octylphenyl polyethylene glycol, are non-ionic. They are the milder pair people reach for when a wash buffer should reduce sticky binding without unfolding every protein. Milder is not inert. They still form micelles, they still foam, and they still show up in mass spectra and in some enzyme rates if the method assumed zero detergent.

CHAPS is zwitterionic, a cholate derivative with both a positive and a negative charge on the same molecule. It is used when an ionic detergent would denature the fold you still need and a non-ionic detergent is hard to dialyse away. The zwitterion is not a average of SDS and Tween. It has its own critical micelle concentration, its own solubility, and its own incompatibilities. Treat the class as a choice you write down.

The critical micelle concentration is the idea that matters

Below the critical micelle concentration, detergent is mostly present as monomers. Above it, extra detergent forms micelles and the monomer concentration stays near that threshold. Solubilising a membrane or carrying a hydrophobic solute in micelles is an above-threshold job. Coating a surface to reduce nonspecific binding can happen around and below it, which is why some wash buffers are deliberately dilute. A recipe that says "add detergent" without saying which side of the threshold it intends is incomplete.

Published values are real and condition-dependent, so this page will not pretend your buffer has the same number as pure water. As a map: SDS in pure water sits on the order of several millimolar, and added salt lowers that threshold, sometimes by a lot. Tween-20 and Triton X-100 sit well below 1 millimolar, so many dilute washes are still above their threshold. CHAPS sits on the order of several millimolar, higher than the non-ionic pair, which is one reason dialysis is more often realistic for CHAPS than for Triton X-100. Confirm the figure in a source that matches your salt and temperature before you design a new method. If you are following a sheet, follow the sheet's mass or percent and do not redesign it from a table of critical micelle concentrations on the day of the experiment.

Percent is a poor language for this comparison unless you also know the formula weight. One percent of a low-mass detergent and one percent of a high-mass detergent are different numbers of molecules. Convert, then compare. A stock labelled only as a percent, with no chemical identity, cannot be converted at all.

Cold, potassium, and light

SDS solutions that were clear on a warm bench often cloud on a cold shelf because the Krafft boundary for SDS in water lies below ordinary room temperature and above a typical cold room. The solid that appears is not contamination. It is the detergent leaving solution. Potassium makes a second, chemical precipitate that warming may not fully forgive if the insoluble salt has already formed. Keep SDS stocks at a temperature where they stay clear, keep them out of potassium buffers, and write both constraints on the bottle.

Triton X-100 absorbs strongly near 280 nanometres because of the aromatic ring. A protein concentration read at 280 in a Triton buffer is partly a detergent reading. Tween-20 is much quieter there. CHAPS lacks that aromatic absorbance. None of the three is invisible to a spectrophotometer once bubbles are present, because bubbles scatter at every wavelength. Blank with the buffer you actually have, bubbles included if you cannot get rid of them, and do not subtract a water blank from a foamy detergent well.

Aged ether-containing detergents can accumulate peroxides. An enzyme that died in an old Triton or Tween stock and lives in a freshly opened bottle of the same detergent is a reason to retire the old stock. This page does not set a peroxide limit. The laboratory's quality habit does, or the method sheet does.

Bubbles ruin the two measurements you trust

Absorbance and fluorescence in a plate or a cuvette assume a clean optical path. A bubble is a lens and a scatterer. Duplicates fan out, a standard curve jumps, and a kinetic trace looks like biology. The fix is physical. Pour or swirl instead of shaking, let the foam collapse, and if a well already has a bubble, pop it or rerun the well. Do not average a bubble into the result.

Pipetting fails more quietly. The air-displacement pipette draws air and liquid. A foam plug in the tip is air you paid for with lost volume. Aspirate below the foam, slowly, with the tip pre-wetted if the technique page's habits fit the viscosity. Dispense against the wall. A detergent stock that was vortexed ten seconds before the aliquot will defeat a careful hand. For viscous stocks, reverse pipetting may be what your laboratory already teaches; use that teaching rather than a new ritual invented for one afternoon.

Filtration of a detergent buffer foams and can change wetting, so an integrity impression based on "how hard it was to push" is unreliable. That is a side effect, not a reason to filter a detergent you could have dissolved in sterile water and added after the salts were sterilised.

When the control fails

If an absorbance standard curve scatters only in the detergent-containing wells, rerun after the foam has settled before you remake the chemistry. If it scatters after the foam is gone, then suspect the aromatic absorbance of Triton or a precipitate of SDS. If a cold SDS stock was cloudy when you pipetted it, you did not deliver the concentration on the label. Warm it back to a clear solution if the stability of that stock allows, mix, and aliquot again. If potassium and SDS met in the well, remake the buffer without that pair. Do not centrifuge the precipitate away and pretend the supernatant is still the recipe.

If an enzyme control fails in old Tween and passes in new Tween, retire the old bottle for that assay. Do not decide that the enzyme lot changed on the same day unless a no-detergent control also moved.

Conductivity will not rank these classes for you. Detergents that are non-ionic barely move it, and SDS moves it because it is a salt. A pH electrode in a foamy beaker is also a poor measurement. Degas nothing violently; let the foam fall, then read, as the buffer page already asks you to wait for a settled reading.

Heat, cold rooms, and a stock that sweated

A laboratory that is hot on the bench and cold in the store will cycle SDS across its solubility boundary every time someone refrigerates "all the buffers". Write "do not refrigerate" on an SDS stock that the method keeps at room temperature. Humidity does not create micelles, but it does dilute a hygroscopic solid if you weigh it slowly, and some detergent powders arrive that way. Weigh promptly. A power cut that warms a refrigerator and then cools it again is a reason to look for SDS crystals before the next gel, not a reason to shake the bottle into a foam and hope.

Safety

Detergent powders and concentrates irritate eyes and lungs. Weigh them in a way that does not aerosolise the dust, and wear eye protection. SDS in particular is unpleasant to inhale. This comparison does not approve a lysis of infectious material. If a detergent buffer will be used on a culture, the biosafety rules of that culture still apply, and a detergent is not a validated disinfectant just because it lyses cells in a method. Waste that contains detergent goes where the laboratory chemical procedure sends it.

What to ask for

Name the detergent, the grade, the package, and whether you need a powder or a solution of a stated percent or molarity. Ask for the formula weight if you must convert a percent to a molar concentration. Ask whether a non-ionic detergent's certificate says anything your assay needs about peroxides or carbonyls, and do not invent a limit the certificate does not have. On receipt, check the identity against the potassium and temperature constraints before you pour it into an existing buffer. A quotation request that says "a little detergent for washes" will come back as whatever was convenient. The class table is the specification.