[wtolf2]

Which will come out first through the column? Aliphatic or Aromatic amino acid?
They are uncharged!!

[Babcock_Hall]

Is this a homework problem?

[wtolf2]

Yes it is. Any idea?

[Arkcon]

What can you pull together for an answer given their structures.  You seem to know that there's no obvious charge to cause binding, but can you come up with other effects that would slow one or the other?

[wtolf2]

Yeah there is no other effect affects the order of amino acids coming out. It seems there is no answer...

[Babcock_Hall]

One, you need to show your attempt first before we can help with homework (Forum rules).  Two, you have not provided any information about pH or what matrix is employed, which would be helpful.  Three, there are other effects besides pure ion-exchange that sometimes come into play.  In my experience it depends on the matrix, however.

[Arkcon]

Well, for practical applications, you'll have to consider the matrix supporting the cation-exchange group, and the size of the pores vs the size of the molecules.  But for a homework question, looking for some sort of transient phenomena would be what's needed to answer the problem

[Babcock_Hall]

@OP, At low pH values, the amino acids will not be uncharged.  What charge will they have?