Specificity of Chromatographic Adsorbents ARTHUR L. LEROSES, PATRICK H. RIOSAGHAN, CHARLES A. RIVET’, AUD EDGAR D. SRIITH? Loctisiana State University, Baton Rouge, La. The object of the present w-ork was to determine what factors w-ere responsible for the adsorption of a given substance on a given adsorbent. It was hoped that it would be possible to construct a mathematical function relating the interactions in the chromatographic system to the movement of the adsorptive zone. Preliminary work indicated that, for the compounds studied, the adsorption could be accounted for in terms of donor-acceptor and hydrogen-bonding interactions and that the carbon side chain acted to decrease adsorption. An arbitrary relationship was set up on this basis; donor values
were assigned to oxygen and nitrogen atonis and hydrogen-bonding values to OH and IVH hydrogens. The solvents were assigned arbitrary values as competitive agents, and the adsorbent strengths were experimentally evaluated. Rates of movement of chromatographic zones calculated on the basis of assigned values agreed reasonably well with experimental values. Using the methods proposed, i t should be possible to evaluate the strength of adsorbents quantitatively, and to determine their specificity toward compounds, so that optimum conditions for separating compounds may be selected.
T
D,, to the adsorbrd compound. Thwe subscripts are also applied similarly to terms listed below A = acceptor strength of a substance for an electron pair l j H = donor strength in terms of an electron pair donated to a hydrogen atom in hydrogen bond formation €1 = acceptor action of a hydrogen-bonding hydrogen for an electron pair.
HIS work was undertaken with the purpose of discovering
the forces responsible for reversible adsorption of chemical compounds as manifested in chromatographic behavior. It vias hoped that it would be possible to devise a mathematical function relating the properties of adsorbent, developing solvent, and adsorbed compound [called “adsorptive” by Weil-hlalherbe ( 6 ) ] to the R value of the substance (see Nomenclature). The authors feel that these efforts have been successful to the extent of a first approximation. As yet most of the theoretical treatments of the chromatographic process have been based on the adsorption isotherm of the adsorbed substance and have given relationships between the shape and rate of movement of zones and the isotherm constants. The authors have attempted to relate these constants to the structure of the adsorbed compound and to empirical donor, acceptor, and hydrogen-bonding strengths of the adsorbent and developer. Experiments have indicated that interactions involving the above variables may suitably account for the behavior of most common adsorbed compounds, and that it is possible in a large number of cases not only to calculate the order of magnitude of the R value, but even to achieve considelable accuracy in these calculations. The authors hope that this approach to chromatography Till provide a good method for espressing the strength and specificity of adsorbents, for translating results obtained with one lot of adsorbent to those obtained with another, and for discovering nevi adsorbents with special properties.
DISCUSS103 OF hiETHOD
The compounds used as adsorbed compound i n this work n-ere classified according to functional group as follows: electron donor, electron acceptor, hydrogen-bond hydrogen acceptor, hydrogen-bond hydrogen donor, odd-elect,ron, ionic, polar, and miscellaneous. Practically all of the compounds which are stable and sufficiently soluble in organic solvents for chroniatography belong to the electron donor, hydrogen donor or acceptor, polar, or miscellaneous groups. Experimental results indicate that of the adsorbents studied the last two classes are of minor importance. The proportionality factor in the adsorptiou isotherm is a nieasure of adsorption affinity; in dilute solut~ionsthe following relations may be sh0n.n behyeen this factor and ot,her terms: j’ = I:s = 1‘0 1’s = (1 - R ) R
+
hO3IENCLATURE
R
= the ratio of rate of movement of the adsorbed conipound
f
=
k
=
S
=
2’.
=
T.
=
;MaE =
n
=
1 2
+
I t may also be shown ( 2 , S ) that R = l/(f 1 ’ = T,/( T , 7’‘). The evaluation of T , and T , in terms of the variables affect.ing these terms will probably furnish the best approach to the accurate description of the chromatographic process since a considerable simplification of the problem is accomplished by the separat,ion of variables. Preliminary examination indicates that this process will be time consuming and, although more accurate, will probably not displace the approsimate treatment, given he-
in the column to the movement of the developing solvent in the column; RL ( 4 ) [or RF (2)]applies to the front edge of the zone, RT applies to the rear edge the proportionality factor in an adsorption isotherm such that the amount of substance adsorbed on the adsorbent in equilibrium with one unit volume of solvent is obtained by multiplying the concentration in solution by f. The value off may vary with concentration an equilibrium constant for the adsorption reaction surface area of the adsorbent in terms of moles per uiiit of adsorbent as defined for f statistical average time an adsorptive particle spends in solution between adsorptions statistical average time an adsorptive particle spends on the adsorbent during each adsorption applies to the sum of the molecular weights of all side chains in an adsorptive molecule donor strength of substance in respect to electron pair. D. refers to the adsorbent; Dd, to the developer;
lOW.
Both T , and T , as well as f are functions of the energy of sdsorption which may be expressed as a donor, acceptor, and hydrogen-bond int,eract,ion between adsorbent and the adsorbed compound, modified by the developing solvent. Esperiniental results have indicated that on the adsorbents used here increased size of the side chain decreases the adsorption affinity; consequently, the hypothesis Jvas made that the adsorption affinity could be expressed as:
f
=
(A)[(F)(2) +
+
Terms for other interactions may be added as necessity indicates. In using this approach to ackorption affinity it was necessary to
Present address, Esso Standard Oil Co., Baton Rouge, La. Present address, Buckeye Cotton Oil Co., Memphis, Tenn.
730
V O L U M E 2 3 , NO. 5, M A Y 1 9 5 1 Table I .
731
Interaction Tendencies of Substances A D DH
Developers Petroleum ether Benzene Adsorbents Special Filtrol V e i c k reagpnt Eilii-ic acid Florisil hIerck hea\-i. ~ n w i l e rcalciuta carbonate Calcium acid pljosphate dihydrate hlagnesium ox id^ Calcium hg-droxide
In 23
In
5 8 1,333 2,570 1,160
... ... ...
26 42 190 23
... ... ... ...
224
1 00"
...
...
0.17 0.20 0.04 0.002
Xitro group Aroiiiatic ring
1
4 3
1,300 120 260
3,350 11,500 I @U'
These values are arbitrarily
,.. .
H
.,
... ... .
... ...
1.00"
... ... . .
,
X.-IIIII~
