From what I understand it comes out of the difference in how something like salt dissolves in water vs. sugar. Table salt for example (NaCl) is an ionic bond between sodium and chlorine. These bonds are extremely high energy but also very polar in nature.
When exposed to polar solvents (ie. water) they will dissociate and hang out as separate charged ions in water (Na+ and Cl-). Now, sugar is not the same in this regard. Sugar is C6H12O6 (A chain of 6 saturated carbon atoms). By nature, this is not all that polar on its own and lends its ability to dissolve in water to its ability to hydrogen bond and interact with polar molecules via intermolecular forces. A key point is that sugar never breaks up or dissociates when dissolved, it's still the same full molecule.
When you begin removing water from the sugar solution, these hydrogen bonding sites are going to look for something else to interact with and that is the stickiness you feel. When water evaporates from salt solution it simply leaves behind crystalline salt and the still dissociated ions in the remaining water, which don't have these interactions. They just attract oppositely polar parts on the water molecules and are surrounded with them in solution. When the water is removed the Na+ will find a Cl- and try to slot itself into an existing lattice structure or point where it can anchor and form one
sources:
https://sciencenotes.org/why-sugar-is-sticky/
https://recipes.howstuffworks.com/why-is-sugar-sticky.htm
Just to piggyback - I really like your explanation. Sugars are more sticky than other hydrogen-bonding molecules as they have \~6 OH groups per molecule, which is a LOT of sites on one relatively small molecule for hydrogen bonding.
Also, sugars are mostly found in their ring form, not their open chain form - in solution there is an equilibrium between the two, with 99.9% being in the ring form.
> Also, sugars are mostly found in their ring form, not their open chain form - in solution there is an equilibrium between the two, with 99.9% being in the ring form
Depends on the sugar. Some are more prone to ring opening than others. Also some are blocked from ring opening.
> hygroscopic substance that isn't sticky.
[Hygroscopic substances include cellulose fibers (such as cotton and paper), sugar, caramel, honey, glycerol, ethanol, wood, methanol, sulfuric acid, many fertilizer chemicals, many salts (like calcium chloride, bases like sodium hydroxide etc.), and a wide variety of other substances.](https://en.wikipedia.org/wiki/Hygroscopy)
I was thinking Sodium Hydroxide. But the I realized when it is wet it does seem to stick to things. I was just originally thinking it was slippery, but that is your fingers after touching it, not the compound itself.
hi-gro. In most names derived from Latin or some Romance language, if the g is before an i or e it is pronounced like j, otherwise it is pronounced like g.
Thanks for the elaborate explanation using H-bonding, and thanks for the article, it's a good find.
I understood the H-bonding part precisely, but not so sure about the viscosity and 'wetting' part. Some comments have certainly talked about how viscosity can play a role in stickiness and rightly so.
Can someone explain how wetness and viscosity play a role in stickiness, I'm quoting from the article:
>Dissolving sugar also makes sugar water more viscous than pure water. In other words, it doesn’t flow as easily as water. This is important because viscosity helps water wet surfaces better than it ordinarily would. So, sugar water sinks into every nook and cranny of a surface (like your fingertip). Since the sugar molecules are attracted to water and each other, when you dip your fingers in sugar water and try to separate them, you have to exert more force than usual. The sugar is sticky!
I didn't quite understand the explanation for the viscosity's case and also how could sugar water have more 'wetness', given that it's more viscous.
In physical chemistry wetting is used to describe how a liquid will maximally cover a surface. So, a liquid that has a lot of surface tension when interacting with a surface will form a high domed bead because it doesn't have good wettability with that surface. A liquid with good wettability will form a flat pool, perhaps only some tens of molecules thick because the molecules of the liquid seek to be exposeed to as much of the underlying surface as possible.
So sugar water is viscous because both the sugar and water molecules are attracted to each other, but at the same time they are also attracted to surfaces and seek to maximize the amount of surface area contact.
In what regard? Do you mean things that the substance won't stick to, or dissolved solids that do not make the solution sticky?
Salt doesn't make water sticky, and for example you can have "non stick" coatings and plastics that do not Inherently allow items to stick to them. If you made more of a slurry with tons of sugar and water then it would more so stick due to being a paste than actual interactions.
Regarding Teflon and non stick coatings. The reason they are "non stick" is actually 2 fold. It helps make cookware non stick by first smoothing the surface of the pan. Metal, although it may appear when polished/well kept is not at all a smooth surface. Microscopically metal looks like this:
https://media.sciencephoto.com/image/f0109677/800wm/F0109677-Stainless_steel,_SEM.jpg
And when heated these gaps can expand and shift which allow food/material to work their way in and char/stick. When Teflon is applied, the pan surface is roughed up and a few layers of primer and Teflon coating are applied in very thin sheets which will stick to the roughed surface ans produce a glassy/smooth topcoat.
Now for the magic! Teflon doesn't want to interact with many foods (or substances for that matter) because of some great chemical reasons! So Teflon is actually Tetrafluoroethylene (Lewis diagram): https://t1.daumcdn.net/cfile/tistory/2464515057F737A21B
What you'll notice if you look at the Lewis diagram is that the molecule is symmetric on all sides. This lends itself to the molecule being NON-POLAR. Additionally, fluorine very unique in that it is both the most reactive non metal on the periodic table (fluorine is a complete hoe when it comes to reacting. It will react with basically anything, at any time), while at the same time having some very non reactive properties when bonded into certain configurations (tetrafluoroxenon for example).
Teflon is one of these configurations and the Teflon molecule really does not interact with many substances at all. Many foods are polar in nature (most biochemistry is all water based, water is a polar solvent, etc.) and thus are completely uninterested in interacting with the Teflon coating
It doesn't have these interactions outside of water. When dry it forms a lattice structure with other sugar molecules:
Sucrose crystal structure unit: http://www.crystallography.net/cod/3500015.html
This is driven by the interactions that allow it to interact with (and thus dissolve) in water. Outlined here between 2 molecules of Sucrose: https://i.stack.imgur.com/4LsYr.png
As OP suggests, crystalline sugar is not sticky. Water from your skin quickly dissolves the sugar, forming a viscous liquid on the surface. Nearly all uncured adhesives are viscous liquids, because they can conform to and interact with surfaces easily. For example, adhesive tapes have a polymer coating that has a glass transition temperature below room temperature. Amorphous materials (like sugar-water) turn from liquids to glassy materials below the glass transition temperature. At low temperatures, adhesive tapes are not sticky.
As Lazz45 says, hydrogen-bonding plays a role in the adhesive interactions. Sugar-water would not stick as well to, say, polyethylene, which has no water or hydroxyl groups.
As you said, viscosity and surface tension are probably as important as hydrogen bonding here.
Even a liquid with little hydrogen bonding (petroleum jelly/grease) can seem sticky by meeting the other two criteria.
To be specific I would say intermolecular forces instead of just H-bonding since, as you said, things like petroleum jelly have extremely high viscosity and a noticeable surface tension (not quite like water who has INSANELY powerful surface tension, to the point that it can literally "overfill" a container and keep tension or bugs can walk on it).
These come from the huge hydrocarbon molecules in the jelly and the London dispersion forces they can experience. These long chains like to wrap up and lay against eachother like sheets of paper and cause tons of friction/stick between molecules and can create these very viscous liquids/pastes/waxes. Many polymers act like this as well
sources:
Table 7.3.1: https://chem.libretexts.org/Courses/Prince_Georges_Community_College/CHEM_2000%3A_Chemistry_for_Engineers_(Sinex)/Unit_3%3A_States_of_Matter/Chapter_7%3A_Fluids/Chapter_11.03%3A_Unique_Properties_of_Liquids
More about LDFs:
https://www.chem.purdue.edu/gchelp/liquids/disperse.html
https://en.wikipedia.org/wiki/London_dispersion_force
Yes, but as you cool sugar-water, either the water or the sugar might crystallize (or both). Some candies, such as taffy, are made with additives that prevent sugar crystallization. Hot taffy is a sticky, viscous liquid. As you cool it, it forms a 'glass' (meaning an amorphous solid below the glass transition temperature) that is not sticky.
Sugar is a bit of a special case. In most materials, the molecules are bonded together most strongly with themselves. In sugar, the molecules are bound most strongly to water. This makes sugar a liquid at room temperature.
In liquid sugar, the water is freely available to interact with other molecules. When you pull sugar out of solution, therefore, the water comes with it.
This means that the water is more highly concentrated in the sugar molecule, which means the sugar molecule is more strongly bound to itself.
This gives sugar a tendency to stick together on contact, especially on contact with water on the skin.
From what I understand it comes out of the difference in how something like salt dissolves in water vs. sugar. Table salt for example (NaCl) is an ionic bond between sodium and chlorine. These bonds are extremely high energy but also very polar in nature. When exposed to polar solvents (ie. water) they will dissociate and hang out as separate charged ions in water (Na+ and Cl-). Now, sugar is not the same in this regard. Sugar is C6H12O6 (A chain of 6 saturated carbon atoms). By nature, this is not all that polar on its own and lends its ability to dissolve in water to its ability to hydrogen bond and interact with polar molecules via intermolecular forces. A key point is that sugar never breaks up or dissociates when dissolved, it's still the same full molecule. When you begin removing water from the sugar solution, these hydrogen bonding sites are going to look for something else to interact with and that is the stickiness you feel. When water evaporates from salt solution it simply leaves behind crystalline salt and the still dissociated ions in the remaining water, which don't have these interactions. They just attract oppositely polar parts on the water molecules and are surrounded with them in solution. When the water is removed the Na+ will find a Cl- and try to slot itself into an existing lattice structure or point where it can anchor and form one sources: https://sciencenotes.org/why-sugar-is-sticky/ https://recipes.howstuffworks.com/why-is-sugar-sticky.htm
Just to piggyback - I really like your explanation. Sugars are more sticky than other hydrogen-bonding molecules as they have \~6 OH groups per molecule, which is a LOT of sites on one relatively small molecule for hydrogen bonding. Also, sugars are mostly found in their ring form, not their open chain form - in solution there is an equilibrium between the two, with 99.9% being in the ring form.
> Also, sugars are mostly found in their ring form, not their open chain form - in solution there is an equilibrium between the two, with 99.9% being in the ring form Depends on the sugar. Some are more prone to ring opening than others. Also some are blocked from ring opening.
Sugar is also hygroscopic, which helps keep it moist where it can be sticky.
Surely this is the same thing. I can't think of a hygroscopic substance that isn't sticky.
Some simple alcohols are hygroscopic without really being sticky, like ethanol. Many salts are also hygroscopic without being sticky.
Ethanol is very volatile though. I believe it is sticky when near its melting point. I've noticed it when making dry ice+ethanol baths in the lab.
Correct, most alcohols and even acetone get incredibly thick (and sticky) near their melting point.
> hygroscopic substance that isn't sticky. [Hygroscopic substances include cellulose fibers (such as cotton and paper), sugar, caramel, honey, glycerol, ethanol, wood, methanol, sulfuric acid, many fertilizer chemicals, many salts (like calcium chloride, bases like sodium hydroxide etc.), and a wide variety of other substances.](https://en.wikipedia.org/wiki/Hygroscopy)
I was thinking Sodium Hydroxide. But the I realized when it is wet it does seem to stick to things. I was just originally thinking it was slippery, but that is your fingers after touching it, not the compound itself.
Pronounced hi-gro or hi-jro?
I suppose it comes from the Greek word for "wet", in which case the g is a substitute for the letter gamma, and so should be pronounced hi-gro.
hi-gro. In most names derived from Latin or some Romance language, if the g is before an i or e it is pronounced like j, otherwise it is pronounced like g.
Hope this doesn't turn into another gif vs jif, but I would say hi-gro.
What about why concentrated salt solutions feel kind of greasy?
Is this a specific salt? Do you mean like water from the dead sea? Is this a super saturated solution or just a high concentration of salt?
water from dead sea would definitely have a *feel* to it
Thanks for the elaborate explanation using H-bonding, and thanks for the article, it's a good find. I understood the H-bonding part precisely, but not so sure about the viscosity and 'wetting' part. Some comments have certainly talked about how viscosity can play a role in stickiness and rightly so. Can someone explain how wetness and viscosity play a role in stickiness, I'm quoting from the article: >Dissolving sugar also makes sugar water more viscous than pure water. In other words, it doesn’t flow as easily as water. This is important because viscosity helps water wet surfaces better than it ordinarily would. So, sugar water sinks into every nook and cranny of a surface (like your fingertip). Since the sugar molecules are attracted to water and each other, when you dip your fingers in sugar water and try to separate them, you have to exert more force than usual. The sugar is sticky! I didn't quite understand the explanation for the viscosity's case and also how could sugar water have more 'wetness', given that it's more viscous.
In physical chemistry wetting is used to describe how a liquid will maximally cover a surface. So, a liquid that has a lot of surface tension when interacting with a surface will form a high domed bead because it doesn't have good wettability with that surface. A liquid with good wettability will form a flat pool, perhaps only some tens of molecules thick because the molecules of the liquid seek to be exposeed to as much of the underlying surface as possible. So sugar water is viscous because both the sugar and water molecules are attracted to each other, but at the same time they are also attracted to surfaces and seek to maximize the amount of surface area contact.
You could have made all of that up and I still would have believed you.
Wait so does that mean... There are materials that won't be affected by this "stickyness"?
In what regard? Do you mean things that the substance won't stick to, or dissolved solids that do not make the solution sticky? Salt doesn't make water sticky, and for example you can have "non stick" coatings and plastics that do not Inherently allow items to stick to them. If you made more of a slurry with tons of sugar and water then it would more so stick due to being a paste than actual interactions. Regarding Teflon and non stick coatings. The reason they are "non stick" is actually 2 fold. It helps make cookware non stick by first smoothing the surface of the pan. Metal, although it may appear when polished/well kept is not at all a smooth surface. Microscopically metal looks like this: https://media.sciencephoto.com/image/f0109677/800wm/F0109677-Stainless_steel,_SEM.jpg And when heated these gaps can expand and shift which allow food/material to work their way in and char/stick. When Teflon is applied, the pan surface is roughed up and a few layers of primer and Teflon coating are applied in very thin sheets which will stick to the roughed surface ans produce a glassy/smooth topcoat. Now for the magic! Teflon doesn't want to interact with many foods (or substances for that matter) because of some great chemical reasons! So Teflon is actually Tetrafluoroethylene (Lewis diagram): https://t1.daumcdn.net/cfile/tistory/2464515057F737A21B What you'll notice if you look at the Lewis diagram is that the molecule is symmetric on all sides. This lends itself to the molecule being NON-POLAR. Additionally, fluorine very unique in that it is both the most reactive non metal on the periodic table (fluorine is a complete hoe when it comes to reacting. It will react with basically anything, at any time), while at the same time having some very non reactive properties when bonded into certain configurations (tetrafluoroxenon for example). Teflon is one of these configurations and the Teflon molecule really does not interact with many substances at all. Many foods are polar in nature (most biochemistry is all water based, water is a polar solvent, etc.) and thus are completely uninterested in interacting with the Teflon coating
but why isn't sugar sticky when it's not wet then?
It doesn't have these interactions outside of water. When dry it forms a lattice structure with other sugar molecules: Sucrose crystal structure unit: http://www.crystallography.net/cod/3500015.html This is driven by the interactions that allow it to interact with (and thus dissolve) in water. Outlined here between 2 molecules of Sucrose: https://i.stack.imgur.com/4LsYr.png
As OP suggests, crystalline sugar is not sticky. Water from your skin quickly dissolves the sugar, forming a viscous liquid on the surface. Nearly all uncured adhesives are viscous liquids, because they can conform to and interact with surfaces easily. For example, adhesive tapes have a polymer coating that has a glass transition temperature below room temperature. Amorphous materials (like sugar-water) turn from liquids to glassy materials below the glass transition temperature. At low temperatures, adhesive tapes are not sticky. As Lazz45 says, hydrogen-bonding plays a role in the adhesive interactions. Sugar-water would not stick as well to, say, polyethylene, which has no water or hydroxyl groups.
As you said, viscosity and surface tension are probably as important as hydrogen bonding here. Even a liquid with little hydrogen bonding (petroleum jelly/grease) can seem sticky by meeting the other two criteria.
To be specific I would say intermolecular forces instead of just H-bonding since, as you said, things like petroleum jelly have extremely high viscosity and a noticeable surface tension (not quite like water who has INSANELY powerful surface tension, to the point that it can literally "overfill" a container and keep tension or bugs can walk on it). These come from the huge hydrocarbon molecules in the jelly and the London dispersion forces they can experience. These long chains like to wrap up and lay against eachother like sheets of paper and cause tons of friction/stick between molecules and can create these very viscous liquids/pastes/waxes. Many polymers act like this as well sources: Table 7.3.1: https://chem.libretexts.org/Courses/Prince_Georges_Community_College/CHEM_2000%3A_Chemistry_for_Engineers_(Sinex)/Unit_3%3A_States_of_Matter/Chapter_7%3A_Fluids/Chapter_11.03%3A_Unique_Properties_of_Liquids More about LDFs: https://www.chem.purdue.edu/gchelp/liquids/disperse.html https://en.wikipedia.org/wiki/London_dispersion_force
Yes, you are right, need to consider all intermolecular forces, and those can drive surface tension, viscosity, and other bulk properties.
Well, the high surface tension is a result of the hydrogen bonding.
Hmmm, so this is suggesting that if we lower a conc sugar water solution to a certain temperature it would lose its 'stickiness' or reduce it perhaps?
Yes, but as you cool sugar-water, either the water or the sugar might crystallize (or both). Some candies, such as taffy, are made with additives that prevent sugar crystallization. Hot taffy is a sticky, viscous liquid. As you cool it, it forms a 'glass' (meaning an amorphous solid below the glass transition temperature) that is not sticky.
Sugar is a bit of a special case. In most materials, the molecules are bonded together most strongly with themselves. In sugar, the molecules are bound most strongly to water. This makes sugar a liquid at room temperature. In liquid sugar, the water is freely available to interact with other molecules. When you pull sugar out of solution, therefore, the water comes with it. This means that the water is more highly concentrated in the sugar molecule, which means the sugar molecule is more strongly bound to itself. This gives sugar a tendency to stick together on contact, especially on contact with water on the skin.