What makes a substance greasy or sticky on a molecular level?

Studying the molecular composition of everyday items has fascinated us ever since we could see them under a microscope. How and what a substance is composed of, understandably, goes on to decide how that substance behaves. We have brittle solids that appear rigid but snap when bended, elastic solids that can be bent out of shape without breaking, thick viscous liquids that slowly make their way along a surface, more runny liquids like water, and ultimately gases of all kinds.

When talking about sticky items – like the various forms of adhesive – on a molecular level, we need to be aware of the fact that molecules are always experiencing some sort of attraction or repulsion towards their peers. Adhesives and naturally sticky substances are different mind you, and we’ll see how that is.

When we talk about adhesives, like glue or tape, their molecules are bound together in long protein chains. Their magnetic properties, among many other more complicated reactions taking place, allow them to cling to each other with quite a lot of strength. When applied to a surface, adhesives have the property of actually pouring into any pores on that surface. When they do so, they stick to that surface as both substances’ molecules become intertwined with each other. This phenomenon does not effectively take place on surfaces without the presence of sufficient pores and irregularities, like smooth glass.

Naturally sticky substances on the other hand – for example honey – are like that simply because of how their molecules are set up. Their molecules can bond with each other for a variety of reasons, including hydrogen bonding. Hydrogen bonding is a very common element of the sugar family of chemical substances, and as such most substances containing any form of sugar tend to be sticky.

When comparing adhesives and sticky substances, the key difference is that adhesive substances can form bonded chains with other substances whereas natural sticky substances do not do so. Sticky substances’ molecules can be thought of as magnets that try to stay attracted to each other even when an external force is applied. For this reason, the substance appears to stick with itself. Only when an adequate external pressure is applied do the molecules break apart and the substance seems to divide itself in two.

Greasy substances are much like sticky substances in that their molecules stick to each other and thus pull them along when slid around. However, there is a minor difference between sticky substances and greasy substances in the manner of their molecular attraction. While sticky substances are capable of somewhat sticking to other surfaces – albeit not as adeptly as adhesives – greasy substances are not. You might observe that greasy substances “slide around” on other surfaces. That is not to say they cannot stick to other surfaces at all, but it requires more external pressure to ease them into it.

While we’re on the topic of molecular attraction, did you know this is the same principle soap works on? Water itself is not a very good candidate for substances to attach themselves to, thus washing your hands with just plain water is not as effective at cleaning your hands. Soap is engineered so that its molecules actually “grab on to” the grime present on your skin. And when washed away the soap molecules are swept away by the water, dragging and taking the grime along with them.