Surfactants and Thermodynamics of Micelles
ฝัง
- เผยแพร่เมื่อ 23 ก.พ. 2020
- This video lecture follows along with part of chapter 3 in An Introduction to Interfaces and Colloids. The Bridge to Nanoscience
The lecture focuses on surfactants and the thermodynamics of micellization.
Learning is all about making mistakes, so if I made a mistake, let me know so that I can fix it!
At around 4:00, it's probably more accurate to describe the surface tension as being due to compression. The water molecules beneath the surface are pulling on the surface molecules, while nothing is pulling on the top surface. Thus, the surface molecules get pulled toward each other a little more.
Wonderful explanation sir!!
Great video, thanks!
Hi Mister Ford, Thank you for making this video! it clarifies a lot for me. However, I was curious about why exactly the water molecules are stuck around the hydrophobic parts of the surfactant? I thought the water molecules would be more sticky (and thus move less) around the hydrophilic part due to hydrogen bridges and the water molecules are distancing themselves from the hydrophobic part (due to polarity differences). Therefore i reasoned that because there is a space around the hydrophobic part of the molecule where the water molecules don't want to go, there is, in total, less total space for the water molecules to move and therefore less entropy.
And why can't the water molecules induce partial polarity in the hydrophobic part (Debye forces)?
I'm a biologist trying to find my way in the world of chemistry, but it seems a quite tough one :)
The "stuckness" is generally described as an "iceberg effect". This iceberg effect has been used to explain experimental data that measured the solubility of hydrocarbons as a function of temperature. It's called "iceberg" because the experimental data can look like like a solid-to-liquid phase transition for the water. So the water surrounding the hydrophobic tail is modeled as a solid (and thus has much lower entropy than the liquid water.)
For me, I thought of it like this: the water has to go somewhere around the hydrophobic tail. Otherwise you have empty space, which is much less favorable. Each water molecule tries to "fit in" around the hydrophobic tail, but it much rather "fit in" with its neighbors. So the water molecules will pack together in a specified way that maximizes the interactions with surrounding water molecules while minimizing interactions with the tail (and yes, those interactions would be induced dipole interactions, most likely.) That is less favorable entropically even relative to the molecules around the head, where the water molecules can vibrate and rotate happily even if their interaction with the head is strong.
I think your thinking makes sense: there is a lot of space where the water molecules don't want to go. But they have to go there - otherwise you have empty space. And when they get to that space, they organize in a very specific way.
I'll note that this "iceberg effect" is a nice model that has been used to describe a lot of phenomena related to hydrocarbon and surfactant solubility - but it might not perfect, especially with more complicated systems.
@@michaelford8335 Many thanks for this clear and structured answer! I'll try to look a bit more into this iceberg effect. The way i see it now is that the hydrophobic tail actually can be seen as an independent surface (not really, but the packing of watermolecules sounds very familiar as to the packing of water molecules at the surface reating surface tension). A chemist also explained to me that water also forms small crystal like structures in arcs around PEG monomers (with hydrogen bonds from a PEG O to a couple of H2Os and than to another O in the PEG). Can i view this the same way, only on a larger scale?