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Actually this is going to be a list of interconnected questions. Please bear with me.

Cyclodextrin structure

Wikipedia article on cyclodextrin (CD) says that it is hydrophobic from inside.

(1) How is it hydrophobic when there are so many hydroxyl groups on the interior surface ?

(2) This image labels the interir cavity as apolar rather than hydrophobic. I guess the apolarity is because of mutual cancellation of the dipole moments. Am I right on this?

In any case I don't understand how the cavity excludes water.

There are reports on trapping organic substances such as naphthalene, toluene and crude oil in the CD cavity. Another report says that nitrosamime is trapped in CD by interacting with the hydroxyls.

(3) How does this trapping exactly occur ?

(4) Do the hydroxyls play a role in restricting the diffusion of the trapped organic molecules out of the cavity ?

All of this, I started investigating because of a recent news about an alcoholic beverage in powder form- palcohol. Initially I didn't believe it but later I found that alcohol is absorbed in CD to make a dry powder.

(5) What prevents alcohol from adsorbing on the exterior surface and not make the powder sticky ?

(6) Other studies say that there is a 1:1 ratio of trapping but palcohol claims that it is 58% alcohol by weight. Even this old patent says that CD absorbs a lot of alcohol. How does that happen ? In all it seems that CD behaves differently with different molecules.

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  • $\begingroup$ 1: www1.lsbu.ac.uk/water/images/cyclo.gif looking at this rendition you can get a different picture of the inner loop. While there are O-H in there, be sure to check out the carbon edges also extruding in. 2: I thought apolar == non-polar, which generally leads to hydrophobicity. Linking this to 3 and 4, it might be that water isn't excluded altogether, but these other groups are just preferred over water. 6: I'm not sure if we can assume palcohol is JUST Cyclodextrin + alcohol. Also 1:1 ratio of trapping is by moles of the molecule, right? Typically these aren't shown by weight... $\endgroup$
    – rch
    Commented Apr 25, 2014 at 10:04
  • $\begingroup$ Guessing for 5, preference? At first glance it seems way more favorable for the alcohol to simultaneously interact with many sites than to just hang off of one. $\endgroup$
    – rch
    Commented Apr 25, 2014 at 10:05

1 Answer 1

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Cyclodextrin hydroxyls are the external surface (hydrophilic and hydrogen-bonding). They circle the larger open end. The carbon and hindered ether skeleton is internal (hydrophobic). A cyclodextrin looks like a bonded hollow detergent micelle in water.

beta-cyclodextrin

Before you believe advertising hype, remember that underivatized cyclodextrins are nephrotoxic, crystallizing upon concentration, probably as the cholesterol endohedral complex. Look it up.

Ethanols insert their hydrocarbon tails into the center, then hydrogen bond their polar ends with each other and the rim.

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