The water-soluble vitamins niacinamide (a neutral compound),...

The water-soluble vitamins niacinamide (a neutral compound), riboflavin (a neutral compound), niacin (an anion), and thiamine (a cation) were separated by micellar electrokinetic chromatography in $15 \mathrm{mM}$ borate buffer (pH 8.0) with $50 \mathrm{mM}$ sodium dodecyl sulfate. The migration times were niacinamide ( $8.1 \mathrm{~min}$ ), riboflavin ( $13.0 \mathrm{~min}$ ), niacin (14.3 min), and thiamine (21.9 min). What would the order have been in the absence of sodium dodecyl sulfate? Which compound is most soluble in the micelles?

Key Concepts

Capillary Electrophoresis and Micellar Electrokinetic Chromatography

These techniques separate analytes based on their electrophoretic mobility within a narrow capillary. In MEKC, added surfactants like SDS form micelles that serve as a pseudo?stationary phase, enabling the separation of neutral compounds in addition to charged ones by partitioning the analytes between aqueous and micellar environments.

Electrophoretic Mobility

This concept describes the intrinsic movement of charged molecules in an electric field. In capillary electrophoresis, the direction and speed of each analyte’s migration are determined by its net charge and size, causing cationic and anionic species to move differently relative to the electroosmotic flow.

Electroosmotic Flow (EOF)

EOF is the bulk flow of the electrolyte solution inside the capillary, generally moving toward the cathode. It affects all analytes, regardless of charge, by adding a common velocity component that can either enhance or counteract the molecules’ own electrophoretic movements, especially noticeable when the surfactant is absent.

Partitioning within Micelles

This concept refers to how analytes distribute between the aqueous phase and the hydrophobic interior of surfactant micelles. Differences in hydrophobicity and charge determine the extent of this partitioning; compounds that partition more extensively into the micelles experience longer effective migration times due to the slower movement of the micelle versus the bulk solution.

Effect of pH on Ionization

The pH of the buffer influences the ionization states of analytes, altering their effective charge and, consequently, their electrophoretic mobility. Understanding the ionization behavior at a given pH is crucial for predicting and interpreting the migration order both in the presence and absence of surfactants.

Transcript

00:01 Okay, this problem is having us rank these compounds in order of decreasing solubility in water.

00:05 So the ability to be soluble in water is characterized by polarity and also by its ability to undergo hydrogen bonding with water.

00:13 So as we can see with these four molecules up top, they're all polar except for this one.

00:20 And that is characterized by the non -polar bonds, specifically the carbon hydrogen nature of this compound.

00:27 So we know that in terms of polarity, this one is going to be the most non -polar, meaning it's going to be the least soluble in water.

00:35 So i'll label that one as four.

00:38 Okay, what about the other ones? so these are all polar, and it's kind of hard to measure polarity as a quantitative value.

00:44 So let's measure it by its ability to undergo hydrogen bonding with water.

00:48 So i know that water looks like this, right? and it can hydrogen bond with a oxygen, potentially with this hydrogen, or a hydrogen.

00:57 Potentially with this oxygen, right? and then same thing with this hydrogen.

01:01 We can bond it with an oxygen.

01:02 So we know that each of these three compounds down below, this one, this one, and this one, they can all undergo hydrogen bonding with water.

01:11 It's just a matter of which one can do it to a greater extent.

01:15 So i know that this one has three oxygens and two hydrogens, okay, and those two hydrogens are each participating with an oxygen, meaning that they can participate in hydrogen bonding.

01:24 This one has the single oxygen and the single hydrogen and then this one has my two oxygens and my single hydrogen...

Please give Ace some feedback