Wikipedia:Reference desk/Archives/Science/2015 July 27

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July 27

What chemical properties make soap useful for cleaning?

Why is soap good at cleaning things? What chemical processes underlie the effectiveness of all soaps and detergents? I remember pondering this question during organic chemistry in college, and I vaguely recall coming up with something about "micelles" being especially good at isolating dirt and making it capable of being scrubbed away by physical force or washed away by water, but I feel like there is more to it than that. Why does soap create useful micelles when other compounds with hydrophobic epitopes don't? Is it perhaps related to the reason why soap makes long-lived bubbles with proper agitation?

Also, how is a soap's effectiveness increased or decreased by time and temperature? Does hot water really make certain soaps and detergents more effective than if they were used with cold water? Is there an ideal amount of time for which a given dirty item should be exposed to soap's chemistry, for maximum cleaning power? Thanks for your help! PJsg1011 (talk) 06:39, 27 July 2015 (UTC)

Regarding the chemical processes, this is due to the fact that one end of the molecule is hydrophobic, whereas the other is hydrophilic -- so the hydrophobic end sticks to the greasy dirt, whereas the hydrophilic end is attracted to water and pulls the grease away from the surface being cleaned. In effect, this makes the surface of the grease particles hydrophilic as well (by coating it with hydrophilic molecules), which makes them miscible with water when normally they're immiscible. One end hydrophobic, the other hydrophilic -- THAT is the key. And yes, higher temperatures usually increase soap's effectiveness -- but this is due to the normal increase in solubility with temperature. 2601:646:8E01:9089:F88D:DE34:7772:8E5B (talk) 08:58, 27 July 2015 (UTC)

Surfactants make the water grap nonsolvable stuff. Cheap kitchen cleaners use lactic acid to remove fat and oil. Surfactants are more expensive and some you do not taste or smell them. For that reason such cleaners have added parfume or substances we can detect by smalling them. Whe getting enging oils on your hands and use soap, the oils part is removed by water and soap but your hands feel like sill put into vinegar which is not beeing removed by the soap. Then the soap contains some glycerol which is a part of soap production, the ester can solve in water. For that reason shower gels or shampoo might be more effective aginst engine oil on the hands. Todays full synthetic engine oils are based on hydrcracked substances, some very stable compared to mineral oil are esters. Calcite is being removed by citric acid oder formic acid. This show you the major difference between bath and kitchen cleaners. --Hans Haase (有问题吗) 09:14, 27 July 2015 (UTC)

ESV

What rescue equipment and capabilities does a modern oilfield ESV (Emergency Support Vessel) like the Iolair have? 2601:646:8E01:9089:F88D:DE34:7772:8E5B (talk) 08:50, 27 July 2015 (UTC)

Anyone? 2601:646:8E01:9089:F88D:DE34:7772:8E5B (talk) 05:46, 29 July 2015 (UTC)

End of domino impulse

Is it possible to estimate how many dominoes it would take to completely stop the impulse imparted by finger or hand to the first domino? This states that 3,847,295 dominoes is still insufficient to completely exhaust the impulse, resembling a perpetual motion. By impulse I mean standard force to provide watchable collapse speed, not too strong and not too slow. Brandmeister^talk 14:45, 27 July 2015 (UTC)

Each domino contains gravitational potential energy. That means that it releases energy when it falls down. That energy goes into knocking down the next domino. You can even have each domino get slightly larger than the previous one, since the gravitational potential energy of the smaller one is more than the force required to tip over the larger one. So, there is no theoretically limit to the number of dominoes that can be knocked down. (There is a practical limit, though, as the more you have the greater the chance of them being knocked down prematurely or being misaligned so they don't all fall down.) StuRat (talk) 15:07, 27 July 2015 (UTC)

I saw a this video of a chain of dominoes of increasing size just the other day. AndrewWTaylor (talk) 16:34, 27 July 2015 (UTC)

And there are some even bigger ones here. AndrewWTaylor (talk) 18:13, 27 July 2015 (UTC)

This is an example of a chain reaction. To achieve perpetual motion, each domino would have to bounce back up into its upright state after it had toppled. You could then create a circle of dominos that repeatedly toppled and bounced back up - but this is, of course, impossible. Gandalf61 (talk) 15:14, 27 July 2015 (UTC)

Or just have an infinite number of dominoes. That could take a while to set up, though. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:13, 27 July 2015 (UTC)

Not if you have an infinite number of people to do it! --65.94.50.73 (talk) 19:33, 27 July 2015 (UTC)

That is a genuinely brilliant response to a (no offense to Bugs who was clearly just trying to make light) nonsensical statement. Snow ^{let's rap} 12:59, 28 July 2015 (UTC)

An infinite number of dominoes lined up would continue to fall, one by one, without ever ending. Any extremely large but finite number will likewise continue to fall until the last one, barring the application of some external force. But it doesn't qualify as a perpetual motion machine, given all the work expended just to get them set up. ←Baseball Bugs ^{What's up, Doc?} carrots→ 13:52, 28 July 2015 (UTC)

What tickled me about the response is that "infinity" is a conceptual construct, not a real number, so outside of certain mathematical principles, any practical application of the term to represent physical phenomena defies both sense and the basic principles of the nature of the universe. Which is why I assumed, especially with the inclusion of the second statement, that you were going for humour. Regardless, the IP responded in a way that I felt hilighted the dubious usage, but did so by replicating it in a statement which "played by the same rules" even as it underscored how such usages lead to obvious paradoxes. I'm not even sure that was entirely the intent, but it made me laugh in any event. :) Snow ^{let's rap} 09:31, 29 July 2015 (UTC)

It's a bit of both. And it's kind of a variant on the "turtles all the way down" story. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:13, 29 July 2015 (UTC)

Desert chimney

I wonder if this concept has ever been tried. In a desert, where the outside is hot but dry, if you use an inside evaporative cooler, that makes the inside air cool but potentially overly humid. If a chimney was added, wouldn't that let the humidity out of the house, but not the cool air, since cool air sinks ? You would need to arrange the chimney so sand wouldn't blow in and fall down it, and some convolutions would also reduce radiative heating. The flue on the chimney could then be opened or closed, to control the house humidity level. So, is this approach ever used ? (I realize there are also swamp coolers that evaporate outside the house, then circulate the coolant inside the house, but I'm not asking about those.) StuRat (talk) 16:27, 27 July 2015 (UTC)

See qanat, windcatcher ... there may be more useful terms for this tech, which has been used a very long time in the Middle East. Wnt (talk) 18:52, 27 July 2015 (UTC)

That's a bit different, in that they use wind to replace the air, while I'm talking about not replacing the air, but only allowing the humidity in it to diffuse away, even without winds. (In fact, in my design, you might want to close the flue when windy, to prevent replacing cool, inside air with hot, outside air.) StuRat (talk) 18:58, 27 July 2015 (UTC)

Desert termite mounds are humid and cool. They are basically very tall chimneys. The termites make them humid. They don't have swamp coolers in them, but I mention this because they remain humid and cool even though they are of the design you mention. 209.149.113.45 (talk) 19:21, 27 July 2015 (UTC)

Certainly the interior would remain somewhat more humid than outside, and, in a desert, that would be a good thing. The idea is just to keep it from becoming oppressively humid, to the point where water no longer evaporates and evaporative cooling fails to work. StuRat (talk) 19:32, 27 July 2015 (UTC)

Enthalpy_of_vaporization, latent heat, sensible heat, adiabatic cooling, relative humidity, Evaporative_cooler#Physical_principles. Evaporative cooling works until the relative humidity is 100%. You can't let out "humidity" without letting out the cooler air. The air has less sensible heat because it has higher latent heat. That's how evaporative cooling works. SemanticMantis (talk) 23:00, 27 July 2015 (UTC)

Precisely. You can't "unmix" the humidity from the cool air unless you add another processing step (e.g., an air conditioner). Short Brigade Harvester Boris (talk) 00:53, 28 July 2015 (UTC)

This runs contrary to my experience. When it's cold and humid outside, and hot and dry inside, just opening the windows seems to make it hot and humid inside. I believe the water vapor achieves equilibrium much faster than the temperature does, and in this Q's scenario, the "unwillingness" of cold air to rise up the chimney should exaggerate this effect even more. And it's not "unmixing" the air, but really mixing it, with regards to water vapor, more quickly than with regards to temperature. StuRat (talk) 12:01, 28 July 2015 (UTC)

It's physically impossible that "water vapor achieves equilibrium [i.e., complete mixing] much faster than the temperature" or for water vapor to mix more quickly than heat, except in perfectly laminar flow (where the molecular diffusivity of water vapor in air is slightly higher than the thermal diffusivity of air). And I can guarantee that your example is not laminar. It may feel worse because human comfort can be more sensitive to humidity than to temperature. Short Brigade Harvester Boris (talk) 14:00, 28 July 2015 (UTC)

Sounds like we need a chemist's input here. I recall from chemistry class that different molecules diffuse through the air at very different rates. Can we find a chart listing the diffusion rates of water vapor, diatomic oxygen and nitrogen in air ? StuRat (talk) 14:18, 28 July 2015 (UTC)

The molecular diffusivity of water vapor in air is slightly higher than the thermal diffusivity of air, but that's utterly irrelevant because the situation you're describing is nowhere close to laminar (i.e., the Reynolds number is >>1). Short Brigade Harvester Boris (talk) 14:59, 28 July 2015 (UTC)

So you think there will be turbulent flow up the chimney ? Why ? There should be little or no flow, only diffusion. We could even add baffles to ensure that, if necessary. BTW, what are those "diffusivity" numbers, and does the heat diffusivity take into account the tendency of heat to rise ? StuRat (talk) 15:06, 28 July 2015 (UTC)

For conditions distinguishing between laminar and turbulent flow, see Reynolds number, which you can calculate. (TL;DR version -- it's challenging to create laminar flow even in laboratory conditions.) Regarding the definition of the various diffusivities, I have been told that there is a large online encyclopedia that has articles on such topics. (smiley) Diffusivity is a molecular process and takes no account of buoyancy (which in any case promotes mixing). Short Brigade Harvester Boris (talk) 15:31, 28 July 2015 (UTC)

I've looked, but I haven't found a table, only equations. Surely somebody has done the math already ? It's not like I'm asking about the diffusivity of some rare gases in each other, after all. As for the tendency of hot air to rise promoting mixing, I have to disagree with that, as that mixing would result in an even distribution of heat in the air, but air is measurably hotter in upper floors of a house, or just by the ceiling of a single story, where it is blocked from rising further. StuRat (talk) 15:38, 28 July 2015 (UTC)

As the saying goes, Google is your friend.

OK, I'll tell you the secret: the thermal diffusivity of air is about 1.9*10-5 m2/s and the diffusivity of water vapor in air is about 2.5*10-5 m2/s. With these values we can use scaling arguments to show that a significant role for molecular diffusivity in transporting heat up the chimney is falsified by comparison to observations. The thermal diffusivity of air is about K = 1.9 * 10-5 m2/s. Then the distance that heat diffuses in time T can be scaled as d ~ sqrt(KT), or conversely the time T to diffuse by a distance d would be T ~ (d^2)/K. So the time for heat to diffuse up a 3 m (10 ft) chimney would be order T ~ (9 m2) / (1.9 * 10-5 m2/s) ~ 474,000 s or about 5.5 days.

Does it take nearly a week for heat to travel up your chimney? I doubt it. The dominant heat transfer process in the atmosphere is bulk turbulent motion, not molecular diffusion. Short Brigade Harvester Boris (talk) 15:52, 28 July 2015 (UTC)

Great info, thanks. Do you have a source for those diffusivity rates ? (Not that I doubt your numbers, I just want direct access for the future.) StuRat (talk) 15:59, 28 July 2015 (UTC)

Meant to respond to this earlier, but ran out of time; it's a bit redundant on SM's comments now, but I'll comment briefly anyway. The thing is, unless I have misunderstood you in some way, the system which you describe is basically the standard implementation for a swamp cooler. That is to say, the system is almost always more efficient and effective if you allow the space to vent. A flue is probably an atypical means of achieving this in the context of a small domicile, but a window serves just as well under most circumstances; there might be some minimal advantage to this end-point being at elevation, but usually the cooling unit itself (which is typically set at the other end of the system in a stream-lined setup) employs a powerful fan as part of the evacuation mechanic, so any propensity for hot air to rise is going to have very little total effect on either the total heat or humidity of the system as a whole. There are variations on evaporative cooling (and hybrid approaches which employ air conditioning) in which a closed system would be more optimal, but the system you describe is generally the most energy-efficient and common form, especially in the arid context you stipulate. Snow ^{let's rap} 00:20, 28 July 2015 (UTC)

I think you are picturing the swamp cooler venting directly into the chimney, while I had in mind something like a misting system venting right into the living space, with a separate chimney elsewhere to let the excess humidity out. StuRat (talk) 12:09, 28 July 2015 (UTC)

No, if you review my post you will see that I understand that the living space is situated between the cooling unit and the chimney. Indeed, it would be a virtually useless setup in any other configuration. But again, the scenario you propose is the typical way in which a swamp cooler system is employed. And in this regard it doesn't mater whether your use a misting system or a conventional swamp cooler; the two vary in how quickly they will cool a given space, but that is a quality derived from the unit, not from the venting you propose, which I assure you is very much the typical manner in which venting is configured in a structure that employs a swamp cooler (again, other than the fact that for small domiciles an open window is usually employed rather than a chimney/flue). Snow ^{let's rap} 12:50, 28 July 2015 (UTC)

From my reading of our swamp cooler article, it sounds like they use a fan to blow air into the house, and then vent the house air back to the outside. Hence there is a high rate of air exchange. A disadvantage of this is that cool air is blowing back out of the house at the vents. My idea is to not force any air into or out of the house. Instead, the mister creates mist without pulling in any outside air, and only the excess humidity goes up the chimney. (A regular open window wouldn't work, as here hot outside air would indeed mix with the cool inside air.) StuRat (talk) 13:02, 28 July 2015 (UTC)

Sure, you can vent with a chimney, and it may make more sense to do that with a given architecture, but there is no way that you'll be letting humidity out without also letting cool air out. It is true the outdraft of a chimney may be less cool than the outdraft from a window. SemanticMantis (talk) 13:37, 28 July 2015 (UTC)

I realize that 100% of the humidity won't be vented nor that 0% of the cool air will be vented, but as long as more of the humidity is vented than the cool air, that seems like a successful design to me. StuRat (talk) 13:42, 28 July 2015 (UTC)

I don't think you get it. Cool air and humidity are not separate things. When the water evaporates, it cools the air by converting sensible heat to latent heat. I don't know how many other ways to say it. Your design is in fact very similar to things the Persians worked out a thousand years ago. I don't think anyone is saying your idea is bad, only that can't selectively exhaust humidity that way. You need a condenser for that. SemanticMantis (talk) 14:43, 28 July 2015 (UTC)

I don't understand why you think cool air is the same as humid air. You can have hot air which is either humid or dry, or cool air which is either humid or dry (although extremely cold air can't hold much humidity). This shows they can be separated, by natural processes. While water vapor itself carries heat, the water vapor is only a small portion of the total air, and hence that heat is only a small portion of the total. The water vapor is left cool from evaporation, but the heat from the inside air should quickly warm the water vapor back up, and in the process cool the rest of the air down. After that, it would be nice to be able to remove the now warmer water vapor. This is my goal. StuRat (talk) 14:50, 28 July 2015 (UTC)

Of course temperature and humidity can vary. But when you use an evaporative cooler, you are making the air both cooler and more humid. This is because of the latent heat of evaporation. I don't know what else to say, maybe someone else can put it in a way that you will understand. I think your explanation of how you think it works means you're not fully understanding latent heat, but I can't explain it any better than our article already does. It is true that a chimney will tend to let out the hotter portion of air in a room, if it is ducted properly. SemanticMantis (talk) 15:09, 28 July 2015 (UTC)

Let's try this. I will lay out all the logical steps, and you can point out which steps contain errors:

1) A mister sprays water droplets into the house air. No outside air is brought in.

2) Flash evaporation of water droplets into water vapor lowers the temperature of the water vapor molecules.

3) The water vapor mixes freely with the house air, and thus each reaches the same equilibrium temperature.

4) The chimney is arranged in such a way (perhaps with baffles) as to prevent turbulent motion of air. The lack of air blowing into or out of the house elsewhere prevents rapid movement of air up or down the chimney.

5) Since "The molecular diffusivity of water vapor in air is slightly higher than the thermal diffusivity of air" (2.5 vs 1.9*10-5 m2/s) per Short Brigade Harvester Boris, more water vapor will vent up the chimney than heat.

6) The fact that heat tends to rise and the air inside the house is cooler than that in the chimney will tend to retard the flow of heat even more. Specifically, hotter patches of air in the chimney will rise up and out the chimney, while cooler patches will stay in the house. Placing the chimney opening in the highest point of the house, distant from the mister, will also help to ensure that the air there is hotter than the rest of the house, so the air that is vented isn't cool air. StuRat (talk) 15:26, 28 July 2015 (UTC)

You seem to have invented a system which vents hot air out of the house without letting new air in. How long do you expect it to carry on doing that? AndyTheGrump (talk) 15:32, 28 July 2015 (UTC)

That would be a very slight flow, and all houses are "leaky" to air, so the air would come into the house through the normal leaks (around doors, etc.). This happens without a chimney, too, to ventilate the home. Presumably air is blown into such tiny leaks on the windward side and sucked out on the other side, although with this chimney design, perhaps it would no longer be sucked out by those leaks, but instead go up the chimney. StuRat (talk) 15:40, 28 July 2015 (UTC)

And you are expecting a 'very slight flow' to be sufficient to cool the inside of the house to a significant degree? AndyTheGrump (talk) 15:47, 28 July 2015 (UTC)

No. The airflow doesn't provide the cooling, the evaporation of the misted water does (step 2 above). StuRat (talk) 15:49, 28 July 2015 (UTC)

I don't have time to go in to this any more today, but I think your step 2) may be wrong. "Evaporative cooling therefore causes a drop in the temperature of air proportional to the sensible heat drop and an increase in humidity proportional to the latent heat gain." This is all about latent heat vs. sensible heat. I'm not sure, but I don't think the water itself loses sensible heat. For step 4) as pointed out above, truly laminar flow is very hard to achieve. Turbulence occurs at all scales. It may be useful to approximate this flow as laminar, it may not. As for step 6), this will be air that was once cooled by the mister, even if it is warmer than the air that is lower. You might want to look at Psychrometrics#Psychrometric_charts, and recall that evaporation is isenthalpic, i.e. it occurs at constant enthalpy. What you really need is thermodynamics here, not chemistry, but I've said all I can on the topic at present. SemanticMantis (talk) 17:35, 28 July 2015 (UTC)

The above seems too complicated to follow, so tell me if this gets the gist:

1) You have air that you pass over a wet pad (the "swampy" smell in the swamp cooler, which ironically enough doesn't work in a swamp). That can come from inside or outside, doesn't matter. 2) The evaporation of the water makes the pad, and the air passing over it, cooler. The humidity makes it heavier. Therefore, the wet air should tend to sink rather than rise. 3) Given an unlimited supply of water (indeed a fortunate thing in the desert!) one can therefore constantly cool the air passing over it to a degree determined by the starting humidity and temperature. 4) The need for air flow is dictated by the amount of heat the cooled air/water is called upon to absorb; the evaporative cooling of the water must equal that, which determines the air flow and thus the water consumption. Essentially, the moistened air can be seen as a stream of coolant at a certain temperature, and as you get arbitrarily close to that temperature the flow becomes arbitrarily large. 5) A chimney next to the house will indeed release some "coolth" through the wall for the inhabitants, but only to a limited degree. All the usual designs of car radiators and such would seem to apply - cooling fins, multiple ducts, countercurrent exchange and so forth. 6) However, that said, nothing is quite as efficient as simply releasing the cooled air into the house, together with its humidity, and if the desert is dry enough, that might bring few complaints. This system, together with a windcatcher to suck out hot air from the top of the building, would seem to be the usual Middle East installation AFAIK. Wnt (talk) 18:47, 28 July 2015 (UTC)

"The humidity makes it heavier." A common misconception. At a given temperature and pressure, humid air is less dense than dry air. Short Brigade Harvester Boris (talk) 01:34, 29 July 2015 (UTC)

D'oh! You're absolutely right, and if I'd been thinking instead of typing I would have realized it, since the ideal gas law still applies (more or less), and H2O is a lighter molecule than O2. [1] details this explanation. Wnt (talk) 13:38, 29 July 2015 (UTC)

Humid air feels "heavier", even though it isn't. ←Baseball Bugs ^{What's up, Doc?} carrots→ 17:11, 29 July 2015 (UTC)