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Chapter 26: Q28P (page 748)

Electroosmotic velocities of buffered solutions are shown for a bare silica capillary and one with aminopropyl groups (silica鈥擲iOCH2CH2CH2NH2) covalently attached to the wall. A positive sign means that flow is toward the cathode. Explain the signs and relative magnitudes of the velocities.

Short Answer

Expert verified

Negative wall - flow towards the cathode.

Positive wall - flow towards the anode.

鈫� the protonation of silanol and amines has an effect on the velocity.

Step by step solution

01

Define the electroosmotic velocity:

Electroosmotic flow is the bulk liquid motion that results when an externally applied electric field interacts with the net surplus of charged ions in the diffuse part of an electrical double layer.

02

Calculation for bare silica capillary:

Bare silica capillary

At pH=10

The wall of the bare silica capillary is negatively charged containing groups -Si-O groups- electroosmotic flow towards the cathode.

At pH=2.5:

The wall is almost neutral (+0.2) containing -Si-OH groups - electroosmotic flow -Si-O is barely present (few groups are present and give rise to a flow towards the cathode).

03

Calculation for aminopropyl silica capillary:

aminopropyl silica capillary

At pH =10:

The same as bare silica capillary, but with half the velocity.

At pH =2.5 :

The aminopropyl groups are protonated - electroosmotic flow towards the anode.

04

The signs and relative magnitudes of the velocities:

Negative wall - flow towards the cathode.

Positive wall - flow towards the anode.

鈫� the protonation of silanol and amines has an effect on the velocity.

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