What causes the maximum viscosity in ethanol concentrations of 50%?

[SqrachMasda]

I did an experiment to calculate viscosities of various ethanol concentrations, 0% to 100%
the graph is parabolic, and therefor the highest viscosity is 50% concentration

i don't understand why, since seperately water and ethanol are closer to 1 centipoise well ethanol more and water less, but still, why would the combination of the 2 slow it's rate down.

i found things of surface tension but it didnt help much

also the densities increase from .789 to 1
no variation really

I just can't find the logic of why mixing these 2 would increase the viscosity so much relative to what they are seperately.

basically as the concentrations went up
the viscosity rose until it hit 50% then decreases again
why rise at all?

 

[GCT]

what happens to the partial molar volume of water when it's dissolved in ethanol (pre-saturation)? That is if you were to add, for instance 5mL of water to 50mL of ethanol, would you observe a volume of 55mL?

 

[SqrachMasda]

well that makes me 10 times more confused
it's already done, but i never came to a concrete answer

i said it was so to intermolecular attractions, i figured if they were larger species they would move slower and 50% is when more of this would occur,...i assumed all this so if it's completely wrong, it's no surprise

 

[GCT]

also the densities increase from .789 to 1
no variation really

If you add water to ethanol for instance, you get a different volume than that expected simply by the addition of volumes. You may, for instance, add 5mL of water to 50mL of ethanol and expect 55mL of solution, but volume is actually smaller. Why is that? If water and ethanol did not interact, if it the solution were ideal, you would expect no changes in expected volume. But interaction does occur, is water repulsed by ethanol? Does it show a positive or negative deviation from ideality?

 

[SqrachMasda]

this is pretty old, but i asked my professor a while later and he said there was no real answer and in fact if i could get a very clear response i'd get an A for the semester.
turns out we were never really expected to get it right (i hate that stuff)
... so I guess an answer to this should not come very quickly because he's a pretty smart guy

 

[GCT]

"no real answer" to what?

 

[wangasu]

this maximum phenomena are often observed in binary solutions when making a plot of log_viscosity vs. mole fraction. the scale of deviation from the linear correlation, which connects the two liquids (x=0, and 1), might reflect the degree of interaction of liquid molecules. stronger interaction will slow down the molecular motions, and lead to the bigger deviation at around 50mol%.