Reason for C18

[chandusabini]

What is reason behind that why C18 only why not C19 or C16 is there any scinetific story?

[Consumer Products Guy]

first one popularly used, tradition.

[tom jupille]

Consumer Products Guy is right. A more verbose version goes like this:

Many years ago, when bonded-phase technology was being developed, octadecyltrichlorosilane was commercially available in "food grade" purity at reasonable cost. As CPG said, they tried it and it worked. As it happens, the effect of chain length on chromatography is essentially exponential, so a small change has very little effect (the most common columns are C18, C8, C4). There wasn't much point is using a specially made (and thus very expensive) reagent to make a C19 column that wasn't a whole lot different from the easily available C18.

[Victor]

Tom,

I am intrigued when you use the phrase "food grade".

What uses were made of this reagent in the food industy? I remember being told a long time ago that the reagent was used to silylate the glass on cola bottles, to render the surface hydrophobic and make the drops stand out on the surface in an attractive fashion. I thought this explanation rather incredible at the time, but now you have got me thinking with the reference to food grade....

[tom jupille]

I remember being told a long time ago that the reagent was used to silylate the glass on cola bottles, to render the surface hydrophobic and make the drops stand out on the surface in an attractive fashion.

That's the story I'm referring to (minor difference: the silanization of the outside of the bottle was allegedly to minimize spalling of the glass in contact with the steel "pincers" in the vending machines of the day).

In detail, it's probably apocryphal, but I'm sure there's a grain of truth in there somewhere! The pertinent point is that a 1 or 2-carbon change in chain length makes little difference.

[Parsival]

I could imagine that the origin for the C18 chain is the corresponding fatty acid, because that would make the raw material very cheap.

Fatty acids are essentially even-numbered, hence 18, and not 19.

Of course, there are also C16, C12 etc. fatty acids. But maybe C18 would be the most prevalent?