- Email: [email protected]
Determination of “solvent” concentration in “chromatographic solutions” on the cellulose sorbents
MICROCHEMICAL
30, 418-424
JOUKNAL
Determination “Chromatographic
(1984)
of “Solvent” Concentration Solutions” on the Cellulose TERESA
Imfitutr
of C:hemistty,
KOWAIAKA
Silesinrt
Received
in Sorbents
University,
February
Katowice,
Poland
5, 1983
INTRODUCTION
In the previous papers (2, 3) we discussed behavior of higher fatty alcohols and acids on the Whatman No. 2 chromatographic paper, and an attempt was made to give a physicochemical interpretation to the observed phenomena. This intcrprctation based on simplification allowed to regard the three-component chromatographic system (investigated substance + sorbent + mobile phase) as a “saturated binary solution,” containing two components, i.e., “solute” and “solvent.” Our “saturated binary solution” was then approached by the classical thermodynamic treatment, which depended upon calculation of the change of chemical potential, accompanying transfer of substance from the pure state (the molar fraction of a substance xi = 1) to the “binary solution” on chromatographic sorbent. Besides, due to the incremental nature of the change of chemical potential (Ap+), the partial changes of chemical potential were defined and calculated upon the experimental results, characteristic of such functional groups, as >CH,, -OH, and -COOH (i.e., APCH~,
ACLOH~
and
APCOOH).
In this paper we will discuss the following problem. When approximating a chromatographic system to the “binary solution,” we face the necessity to determine concentrations of both “solute” and “solvent.” As “solute” is the chromatographically investigated substance, it is relatively easy to define and determine its concentration in “chromatographic solution” (2). The situation changes when we start discussing the “solvent” concentration, and this is the topic of our actual considerations. DISCUSSION
In paper (2) we presented the following empirical relationship, confirmed both with higher fatty alcohols and acids: log c’; = A * rzCH2 + B 418
0026-265X/84 CopyrIght All
n&s
Q of
$1.50 1984
reproductmn
by
Academic m any
Press, form
Inc reserved.
(1)
SOLVENT
CONCENTRATION
IN CHROMATOGRAPHIC
419
SOLUTIONS
where Ci- concentration Of “SOlUte” in the “binary SOlUtiOU,” &Hznumber of >CH, groups in the molecule of an “i”-th compound, belonging to the given homologous series (for sake of simplicity the terminal-CH, group is included), A and B-the empirical equation constants, characteristic of the given homologous series (and the experimental conditions). Taking advantage of the empirical A and B constants, we further defined change of chemical potential, accompanying transfer of one> CH, group from the monocomponent system to the “binary solution” C&c& 1 as AFCH2z 2.303 - A * R - T and the analogical.change of chemical of one -OH or -COOH group, as Akou(cmn)
potential,
s 2.303 RT [B -
lOg(Ci
(2)
accompanying +
transfer (3)
c,.,,)]
where c,h-concentration of “solvent” (all experiments were performed on the Whatman No. 2 chromatographic paper). In Ref. (2) we estimated the At.~ou(coou) number values by assuming c,~ as equal to 0.002 and 0.03 M. This assumption based on some additional experimental results, from which it was clear that the number values of ci and c,,, were comparable in magnitude. Now let us discuss the exact c,,? value more carefully. Although the whole range of the physically sensible values can be given by dependence 0 < cc,, < + x, the previously quoted ones (2), i.e., 0.002 and 0.03 M, drastically limit this on the other hand very broad scope. The additional testing of the above-mentioned two concentrations (4) allows to judge the magnitude order of 1 x lo-* M as still too high, which means that 1 x lO-3 M is more probable. We will but shortly explain the applied testing method. Namely we will present the At.~ou values (Table 1), determined
CONCENTRATIONS ALCOHOL
TABLE 1 OF LAURYL ALCOHOL IN Ccl, (c,,,J AND CONCENTRATIONS IN THE “SATURATED BINARY SOLUTION" ON CHROMATOGRAM
No.
(‘XII” WI
1 2 3 4 5
0.010 0.025 0.040 0.050 0.100
OF LAURYL (c;)
&OH [kJ/moll 0.0133 0.0185 0.0200 0.0227 0.0351
Note. The correspondmg A).L~~ number values, calculated values from (2) (sorbent: the Whatman No. 2 chromatographic sample aliquots: 20 ~1, height of developing: 16 cm, working
+4.281 +.5.164 +4.312 +4.538 +5.300 for c,,, = 0.03 M and the B paper, mobile phase: decalin, temperature: 21°C).
420
TERESA
KOWALSKA
for lauryl alcohol in accordance with Eq. (3), depending on the amounts of the applied sample, and consequently on the obtained ci values (the experimental conditions were given in (2, 3)). As it comes out from the data, given in Table 1, for c,equal to 0.0133 M the corresponding Al.~ou value is lower, than for ci equal to 0.0351 M. Such a result lacks physical meaning, which can be explained in the following way. Practically all -OH groups, which are at the starting point of chromatogram, i.e., before the beginning of the regular chromatographic procedure, are connected with the neighboring -OH groups by two hydrogen bonds (i.e., alcohol appears as a line multimer, with a significant number of the “monomer” bricks in the chain):
I . . . O-H..
I
. O-H..
I
. O-H..
.
(4)
Transfer of a single hydroxyl group to the “binary chromatographic solution” takes place with the simultaneous dissociation of the associative multimer, i.e., with disruption of one hydrogen bond. In the extreme case, when the associative multimer entirely splits into a number of monomers, each dissociated -OH group turns out to be bonded by but one hydrogen bond with the sorbent: I I I I dissociation . . . O-H.. . O-H.. . O-H.. . -3 O-H.. . (5) Thus the change of chemical potential At.~ou is a measure of energy needed for transportation of 1 mol of the -OH groups to the “chromatographic solution” connected with the disruption of a certain number of hydrogen bonds. Obtainment of the “chromatographic solution,” its concentration 0.0133 M, is joined with the disruption of a greater number of hydrogen bonds per 1 mol of -OH groups, than, e.g., obtainment of the 0.0351 M “solution.” This is the reason why the At.~ou number value should in case no. 1 be higher than in each remaining case (Table 1). Thus, after having stated that cCh< 1 x 10e2 M, one tends to decide to calculate concentration of cellulose in the Whatman No. 2 chromatographic paper, expecting that it might eventually constitute the cChnumber value. From literature (1) one found out that 1 dm3 of Whatman No. 2 weighs 541.7 g. As it is made of cotton cellulose, one can first compute its molecular weight knowing that 1 mol of cotton cellulose contains ca. 3000 glucose units (8). The molecular weight is in this case ca. 540,000, and consequently the cellulose concentration in the Whatman No. 2 paper equals 0.001 M. We can see that this value well tits in the range, in which we look for cCh, and one can suppose that this regularity is not coincidental. Thus we might conclude that with chromatographic systems containing chromatographic paper as a sorbent the molar concentration of
SOLVENT
CONCENTRATION
IN
CHROMATOGRAPHIC
SOLUTIONS
421
“solvent” (cc& equals the number of moles of the cotton cellulose per 1 dm3 of paper. In the consecutive Table 2 the ApoH(cooHj values are gathered, computed for the selected higher fatty alcohols and acids from paper (2), with the c,~ value assumed as 0.001 M. All the remaining data, necessary for our computations were taken from (2). The results given in Table 2, although burdened with the error of paper chromatography, properly reflect the following relationship. The longer is the aliphatic chain of alcohols and acids, the greater is the change of chemical potential, accompanying transfer of 1 mol of the -OH or -COOH groups to the “chromatographic solution.” It is so due to the fact that the longer is the aliphatic chain of a given substance, the easier it dissociates on a chromatographic sorbent (6, 9), in accordance with the scheme of Eq. (.5), and proportionally the greater number of hydrogen bonds undergoes splitting. The questions discussed above allow the following conclusion. If we examined a still higher alcohol or acid, which is able to totally dissociate [see scheme (5)], then the absolute number value of the respective Al.~ou or Al.~~oou potential would equal enthalpy of 1 mol of hydrogen bonds (AH), in accordance with the dependence 40~~~00~)~
(6)
-AH.
As it comes out from the results given in Refs. (2, 9), independently from sample aliquots, spotted on chromatographic paper, concentrations of higher fatty alcohols (as well as acids), obtained in the “binary chromatographic solutions” (c;) always preserve the same relative ratio, as shown in Table 3. From the results given in Table 3 it comes out that concentrations of TABLE
2
THE AP~~(~~~~,NuMBER VALUES, DETERMINED FORTHE SELECTED HIGHER FATTY ALCOHOLSANDACIDSONTHE WHATMAN No.2 CHROMATOGRAPHICPAPERANDDEPENDING ON CONCENTRATIONSOF THE APPLIED CCI, SOLUTIONS (c,,,&
~~~~~~~~~~lkJ/moll depending on c,,,~”Ml Substance
0.010
0.025
0.040
0.050
0.100
Myristyl alcohol Cetyl alcohol
+ 6.990 + 7.840 +9.113
+7.391 + 8.345 + 9.663
+ 6.433 i7.550 +8.451
+ 6.492 + 7.562 + 8.530
+ 6.741 + 7.709 +9.164
Laurie acid Myristic acid Palmitic acid
+ 4.991 + 5.633 + 6.669
+ 5.866 + 6.590 + 7.839
+ 5.493 +5.870 +7.307
+5.801 + 6.339 +7.761
+ 5.963 + 6.689 +8.151
Lauryl alcohol
Note.
Mobile
phase:
decalin,
the applied
sample
aliquots:
20 ~1.
422
TERESA
KOWALSKA TABLE
CONCENTRATIONS
OF THE SELECTED
HIGHER
3
FATTY
CHROMATOGRAPHIC
ALCOHOLS
ON THE WHATMAN
No.
2
PAPER”
Substance Lauryl alcohol Myristyl alcohol Cetyl alcohol Stearyl alcohol
12 14 16 18
100 65.1 38.2 28.4
’ Given as the relative percents (c,) (2)
higher fatty alcohols on chromatographic paper gradually diminish with prolongation of the aliphatic chain, aiming toward a certain boundary, which we decided to determine. With this purpose we considered and tested several algebraic functions and the best correlation with the data from Table 3 was found for ci%
= a [exp( - Y - +)I + P,
(7)
where (Y, p, and y are equation constants. Taking advantage of the data from Table 3, we established that the demanded, boundary value of the alcohol concentration on chromatographic paper was
while the interpolation 3 was ci%
lim c.. ,+‘z = 6.5 k 0.2, nc form of Eq. (7), fulfilled
= (1.93 5 0.02) x 103{exp[ -(0.252
by the data from Table
I O.OOl)] * nc} + 6.5 k 0.2. (9)
Thus the extremal concentration value of higher fatty alcohols on chromatographic paper (under the employed chromatographic conditions) is ca. 6.5% of the lauryl alcohol concentration. Thus we decided to examine the A).~ou value for a hypothetical alcohol, its concentration in the “chromatographic solution” being 6.5% of this of lauryl alcohol. The obtained results are given in Table 4. The absolute value of mean potential Aport, given in Table 4 is very well comparable with those of the hydrogen-bond enthalpy, quoted for higher fatty alcohols and acids (7). It is the proof that both the way of determining the “chromatographic solvent” concentration c,h and of interpreting the physical meaning of the A~oH(cooHI parameter [see scheme (5) and Eq. (6)] are correct.
SOLVENT
CONCENTRATION
IN
CHROMATOGRAPHIC
TABLE THE
EXTREME
(BOUNDARY)
CONCENTRATIONS
4 IN THE “BINARY
CHROMATOGRAPHIC
SOLUTION” (c,) FOR THE HIGHER FATTY ALCOHOLS HOMOLOGOUS CHROMATOGRAPHED ON THE WHATMAN No. 2 PAPER, AND CORRESPONDING ApoH VALUES (MOBILE PHASE: DECALIN) CS”l” IW
No. 1
0.010
2 3 4 5
0.025 0.040 0.050 0.100
0.000864 0.001202 0.001300 0.001476 0.002282
Finally let us review the full empirical ci 1%
c;
+
4% cc,,
%H~
= 2.303 - R . T =
423
SOLUTIONS
SERIES, THE
A)LOH [kJ/mol]
ACLOH [kJ/mol]
+ 11.971 + 12.724 + 11.840 + 12.015 + 12.604
+ 12.231
equation, ’ AJ-LC~Z
+
derived in Ref. (2): ACL~~(~~~~)
2.303 - R * T
’
(10)
where ~~~ is the total change of chemical potential accompanying transfer of the “i”-th compound to the “chromatographic solution.” On the other hand it is well known that free enthalpy G of the binary solution at constant temperature and under constant pressure can be given by the equation G = vi . /A; + y; - pi
(1 I)
where v,and vj-volume fractions of “i” and “j,” while l+and kj-their chemical potentials. Besides free enthalpy G depends (in accordance with definition) on both the enthalpic and the entropic factor, as given def. G=H-TS
(12)
Comparing Eqs. (11) and (12) with (10) one can conclude that the partial change of chemical potential Apou(coou) is most probably the enthalpic factor of the A~i magnitude, while the partial change Aucu2 incorporates the entropic factor. These results seem to be coherent, correct, and in agreement with the results published earlier (5, 6, 9). CONCLUSIONS
The results presented in this paper add an entirely quantitative characteristic to the thermodynamic model of the chromatographic system, presented in (2). It only became possible after having found the way to establish concentration of the “chromatographic solvent,” c& The other achievement of this paper depends on establishing the
424
TERESA
KOWALSKA
method, enabling direct determination of the hyarogen-bond enthalpy from the PC experimental results. This achievement can be regarded as a very satisfactory one in the light of the references, although until now the GLC results were considered precise enough to furnish the quantitative data (enthalpies included). SUMMARY An attempt was undertaken to furnish an entirely quantitative characteristics to the thermodynamic model of the chromatographic system presented in (2). The attempt proved to be successful and one managed to establish simultaneously a new method enabling direct determination of the hydrogen-bond enthalpy from the PC experimental results.
REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9.
“Handbuch der Papierchromatographie” (I. M. Hais and K. Macek, eds.),Vol. I. VEB Gustav Fischer Verlag. Jena, 1963. Kowalska, T., Thermodynamic characteristics of the activity of selected chromatographic sorbents. Chromatographia 17, 315-317 (1983). Kowalska, T., Elucidation of the chromatographic behaviour of higher fatty alcohols and acids on cellulose sorbents. Chromatographia 15, 650-652 (1982). Kowalska, T., unpublished results. Kowalska, T., Flakus, H. T., and Sliwiok, J., The role of steric effects in the associative interactions of higher fatty alcohols. Chem. Ser. 13, 59-62 (1978-79). Kowalska, T., Sliwiok, J., and Flakus H. T., Investigation of the association of higher fatty alcohols on chromatographic paper. Chromatogruphia 13, 157-160 (1980). Pimentel, Cl. C., and McClellan, A. L., “The Hydrogen Bond.” Freeman, San Francisco, 1960. Roberts, J. D., and Caserio, M. C., “Basic Principles of Organic Chemistry.” Chap. 18-12. Benjamin, Incorporated, New York, 1965. Sliwiok, J., Walczak, B., and Kowalska, T., Investigation of the self-association of higher fatty alcohols and acids on chromatographic sorbents. Chromatographia 14, 197-202 (1981).
30, 418-424
JOUKNAL
Determination “Chromatographic
(1984)
of “Solvent” Concentration Solutions” on the Cellulose TERESA
Imfitutr
of C:hemistty,
KOWAIAKA
Silesinrt
Received
in Sorbents
University,
February
Katowice,
Poland
5, 1983
INTRODUCTION
In the previous papers (2, 3) we discussed behavior of higher fatty alcohols and acids on the Whatman No. 2 chromatographic paper, and an attempt was made to give a physicochemical interpretation to the observed phenomena. This intcrprctation based on simplification allowed to regard the three-component chromatographic system (investigated substance + sorbent + mobile phase) as a “saturated binary solution,” containing two components, i.e., “solute” and “solvent.” Our “saturated binary solution” was then approached by the classical thermodynamic treatment, which depended upon calculation of the change of chemical potential, accompanying transfer of substance from the pure state (the molar fraction of a substance xi = 1) to the “binary solution” on chromatographic sorbent. Besides, due to the incremental nature of the change of chemical potential (Ap+), the partial changes of chemical potential were defined and calculated upon the experimental results, characteristic of such functional groups, as >CH,, -OH, and -COOH (i.e., APCH~,
ACLOH~
and
APCOOH).
In this paper we will discuss the following problem. When approximating a chromatographic system to the “binary solution,” we face the necessity to determine concentrations of both “solute” and “solvent.” As “solute” is the chromatographically investigated substance, it is relatively easy to define and determine its concentration in “chromatographic solution” (2). The situation changes when we start discussing the “solvent” concentration, and this is the topic of our actual considerations. DISCUSSION
In paper (2) we presented the following empirical relationship, confirmed both with higher fatty alcohols and acids: log c’; = A * rzCH2 + B 418
0026-265X/84 CopyrIght All
n&s
Q of
$1.50 1984
reproductmn
by
Academic m any
Press, form
Inc reserved.
(1)
SOLVENT
CONCENTRATION
IN CHROMATOGRAPHIC
419
SOLUTIONS
where Ci- concentration Of “SOlUte” in the “binary SOlUtiOU,” &Hznumber of >CH, groups in the molecule of an “i”-th compound, belonging to the given homologous series (for sake of simplicity the terminal-CH, group is included), A and B-the empirical equation constants, characteristic of the given homologous series (and the experimental conditions). Taking advantage of the empirical A and B constants, we further defined change of chemical potential, accompanying transfer of one> CH, group from the monocomponent system to the “binary solution” C&c& 1 as AFCH2z 2.303 - A * R - T and the analogical.change of chemical of one -OH or -COOH group, as Akou(cmn)
potential,
s 2.303 RT [B -
lOg(Ci
(2)
accompanying +
transfer (3)
c,.,,)]
where c,h-concentration of “solvent” (all experiments were performed on the Whatman No. 2 chromatographic paper). In Ref. (2) we estimated the At.~ou(coou) number values by assuming c,~ as equal to 0.002 and 0.03 M. This assumption based on some additional experimental results, from which it was clear that the number values of ci and c,,, were comparable in magnitude. Now let us discuss the exact c,,? value more carefully. Although the whole range of the physically sensible values can be given by dependence 0 < cc,, < + x, the previously quoted ones (2), i.e., 0.002 and 0.03 M, drastically limit this on the other hand very broad scope. The additional testing of the above-mentioned two concentrations (4) allows to judge the magnitude order of 1 x lo-* M as still too high, which means that 1 x lO-3 M is more probable. We will but shortly explain the applied testing method. Namely we will present the At.~ou values (Table 1), determined
CONCENTRATIONS ALCOHOL
TABLE 1 OF LAURYL ALCOHOL IN Ccl, (c,,,J AND CONCENTRATIONS IN THE “SATURATED BINARY SOLUTION" ON CHROMATOGRAM
No.
(‘XII” WI
1 2 3 4 5
0.010 0.025 0.040 0.050 0.100
OF LAURYL (c;)
&OH [kJ/moll 0.0133 0.0185 0.0200 0.0227 0.0351
Note. The correspondmg A).L~~ number values, calculated values from (2) (sorbent: the Whatman No. 2 chromatographic sample aliquots: 20 ~1, height of developing: 16 cm, working
+4.281 +.5.164 +4.312 +4.538 +5.300 for c,,, = 0.03 M and the B paper, mobile phase: decalin, temperature: 21°C).
420
TERESA
KOWALSKA
for lauryl alcohol in accordance with Eq. (3), depending on the amounts of the applied sample, and consequently on the obtained ci values (the experimental conditions were given in (2, 3)). As it comes out from the data, given in Table 1, for c,equal to 0.0133 M the corresponding Al.~ou value is lower, than for ci equal to 0.0351 M. Such a result lacks physical meaning, which can be explained in the following way. Practically all -OH groups, which are at the starting point of chromatogram, i.e., before the beginning of the regular chromatographic procedure, are connected with the neighboring -OH groups by two hydrogen bonds (i.e., alcohol appears as a line multimer, with a significant number of the “monomer” bricks in the chain):
I . . . O-H..
I
. O-H..
I
. O-H..
.
(4)
Transfer of a single hydroxyl group to the “binary chromatographic solution” takes place with the simultaneous dissociation of the associative multimer, i.e., with disruption of one hydrogen bond. In the extreme case, when the associative multimer entirely splits into a number of monomers, each dissociated -OH group turns out to be bonded by but one hydrogen bond with the sorbent: I I I I dissociation . . . O-H.. . O-H.. . O-H.. . -3 O-H.. . (5) Thus the change of chemical potential At.~ou is a measure of energy needed for transportation of 1 mol of the -OH groups to the “chromatographic solution” connected with the disruption of a certain number of hydrogen bonds. Obtainment of the “chromatographic solution,” its concentration 0.0133 M, is joined with the disruption of a greater number of hydrogen bonds per 1 mol of -OH groups, than, e.g., obtainment of the 0.0351 M “solution.” This is the reason why the At.~ou number value should in case no. 1 be higher than in each remaining case (Table 1). Thus, after having stated that cCh< 1 x 10e2 M, one tends to decide to calculate concentration of cellulose in the Whatman No. 2 chromatographic paper, expecting that it might eventually constitute the cChnumber value. From literature (1) one found out that 1 dm3 of Whatman No. 2 weighs 541.7 g. As it is made of cotton cellulose, one can first compute its molecular weight knowing that 1 mol of cotton cellulose contains ca. 3000 glucose units (8). The molecular weight is in this case ca. 540,000, and consequently the cellulose concentration in the Whatman No. 2 paper equals 0.001 M. We can see that this value well tits in the range, in which we look for cCh, and one can suppose that this regularity is not coincidental. Thus we might conclude that with chromatographic systems containing chromatographic paper as a sorbent the molar concentration of
SOLVENT
CONCENTRATION
IN
CHROMATOGRAPHIC
SOLUTIONS
421
“solvent” (cc& equals the number of moles of the cotton cellulose per 1 dm3 of paper. In the consecutive Table 2 the ApoH(cooHj values are gathered, computed for the selected higher fatty alcohols and acids from paper (2), with the c,~ value assumed as 0.001 M. All the remaining data, necessary for our computations were taken from (2). The results given in Table 2, although burdened with the error of paper chromatography, properly reflect the following relationship. The longer is the aliphatic chain of alcohols and acids, the greater is the change of chemical potential, accompanying transfer of 1 mol of the -OH or -COOH groups to the “chromatographic solution.” It is so due to the fact that the longer is the aliphatic chain of a given substance, the easier it dissociates on a chromatographic sorbent (6, 9), in accordance with the scheme of Eq. (.5), and proportionally the greater number of hydrogen bonds undergoes splitting. The questions discussed above allow the following conclusion. If we examined a still higher alcohol or acid, which is able to totally dissociate [see scheme (5)], then the absolute number value of the respective Al.~ou or Al.~~oou potential would equal enthalpy of 1 mol of hydrogen bonds (AH), in accordance with the dependence 40~~~00~)~
(6)
-AH.
As it comes out from the results given in Refs. (2, 9), independently from sample aliquots, spotted on chromatographic paper, concentrations of higher fatty alcohols (as well as acids), obtained in the “binary chromatographic solutions” (c;) always preserve the same relative ratio, as shown in Table 3. From the results given in Table 3 it comes out that concentrations of TABLE
2
THE AP~~(~~~~,NuMBER VALUES, DETERMINED FORTHE SELECTED HIGHER FATTY ALCOHOLSANDACIDSONTHE WHATMAN No.2 CHROMATOGRAPHICPAPERANDDEPENDING ON CONCENTRATIONSOF THE APPLIED CCI, SOLUTIONS (c,,,&
~~~~~~~~~~lkJ/moll depending on c,,,~”Ml Substance
0.010
0.025
0.040
0.050
0.100
Myristyl alcohol Cetyl alcohol
+ 6.990 + 7.840 +9.113
+7.391 + 8.345 + 9.663
+ 6.433 i7.550 +8.451
+ 6.492 + 7.562 + 8.530
+ 6.741 + 7.709 +9.164
Laurie acid Myristic acid Palmitic acid
+ 4.991 + 5.633 + 6.669
+ 5.866 + 6.590 + 7.839
+ 5.493 +5.870 +7.307
+5.801 + 6.339 +7.761
+ 5.963 + 6.689 +8.151
Lauryl alcohol
Note.
Mobile
phase:
decalin,
the applied
sample
aliquots:
20 ~1.
422
TERESA
KOWALSKA TABLE
CONCENTRATIONS
OF THE SELECTED
HIGHER
3
FATTY
CHROMATOGRAPHIC
ALCOHOLS
ON THE WHATMAN
No.
2
PAPER”
Substance Lauryl alcohol Myristyl alcohol Cetyl alcohol Stearyl alcohol
12 14 16 18
100 65.1 38.2 28.4
’ Given as the relative percents (c,) (2)
higher fatty alcohols on chromatographic paper gradually diminish with prolongation of the aliphatic chain, aiming toward a certain boundary, which we decided to determine. With this purpose we considered and tested several algebraic functions and the best correlation with the data from Table 3 was found for ci%
= a [exp( - Y - +)I + P,
(7)
where (Y, p, and y are equation constants. Taking advantage of the data from Table 3, we established that the demanded, boundary value of the alcohol concentration on chromatographic paper was
while the interpolation 3 was ci%
lim c.. ,+‘z = 6.5 k 0.2, nc form of Eq. (7), fulfilled
= (1.93 5 0.02) x 103{exp[ -(0.252
by the data from Table
I O.OOl)] * nc} + 6.5 k 0.2. (9)
Thus the extremal concentration value of higher fatty alcohols on chromatographic paper (under the employed chromatographic conditions) is ca. 6.5% of the lauryl alcohol concentration. Thus we decided to examine the A).~ou value for a hypothetical alcohol, its concentration in the “chromatographic solution” being 6.5% of this of lauryl alcohol. The obtained results are given in Table 4. The absolute value of mean potential Aport, given in Table 4 is very well comparable with those of the hydrogen-bond enthalpy, quoted for higher fatty alcohols and acids (7). It is the proof that both the way of determining the “chromatographic solvent” concentration c,h and of interpreting the physical meaning of the A~oH(cooHI parameter [see scheme (5) and Eq. (6)] are correct.
SOLVENT
CONCENTRATION
IN
CHROMATOGRAPHIC
TABLE THE
EXTREME
(BOUNDARY)
CONCENTRATIONS
4 IN THE “BINARY
CHROMATOGRAPHIC
SOLUTION” (c,) FOR THE HIGHER FATTY ALCOHOLS HOMOLOGOUS CHROMATOGRAPHED ON THE WHATMAN No. 2 PAPER, AND CORRESPONDING ApoH VALUES (MOBILE PHASE: DECALIN) CS”l” IW
No. 1
0.010
2 3 4 5
0.025 0.040 0.050 0.100
0.000864 0.001202 0.001300 0.001476 0.002282
Finally let us review the full empirical ci 1%
c;
+
4% cc,,
%H~
= 2.303 - R . T =
423
SOLUTIONS
SERIES, THE
A)LOH [kJ/mol]
ACLOH [kJ/mol]
+ 11.971 + 12.724 + 11.840 + 12.015 + 12.604
+ 12.231
equation, ’ AJ-LC~Z
+
derived in Ref. (2): ACL~~(~~~~)
2.303 - R * T
’
(10)
where ~~~ is the total change of chemical potential accompanying transfer of the “i”-th compound to the “chromatographic solution.” On the other hand it is well known that free enthalpy G of the binary solution at constant temperature and under constant pressure can be given by the equation G = vi . /A; + y; - pi
(1 I)
where v,and vj-volume fractions of “i” and “j,” while l+and kj-their chemical potentials. Besides free enthalpy G depends (in accordance with definition) on both the enthalpic and the entropic factor, as given def. G=H-TS
(12)
Comparing Eqs. (11) and (12) with (10) one can conclude that the partial change of chemical potential Apou(coou) is most probably the enthalpic factor of the A~i magnitude, while the partial change Aucu2 incorporates the entropic factor. These results seem to be coherent, correct, and in agreement with the results published earlier (5, 6, 9). CONCLUSIONS
The results presented in this paper add an entirely quantitative characteristic to the thermodynamic model of the chromatographic system, presented in (2). It only became possible after having found the way to establish concentration of the “chromatographic solvent,” c& The other achievement of this paper depends on establishing the
424
TERESA
KOWALSKA
method, enabling direct determination of the hyarogen-bond enthalpy from the PC experimental results. This achievement can be regarded as a very satisfactory one in the light of the references, although until now the GLC results were considered precise enough to furnish the quantitative data (enthalpies included). SUMMARY An attempt was undertaken to furnish an entirely quantitative characteristics to the thermodynamic model of the chromatographic system presented in (2). The attempt proved to be successful and one managed to establish simultaneously a new method enabling direct determination of the hydrogen-bond enthalpy from the PC experimental results.
REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9.
“Handbuch der Papierchromatographie” (I. M. Hais and K. Macek, eds.),Vol. I. VEB Gustav Fischer Verlag. Jena, 1963. Kowalska, T., Thermodynamic characteristics of the activity of selected chromatographic sorbents. Chromatographia 17, 315-317 (1983). Kowalska, T., Elucidation of the chromatographic behaviour of higher fatty alcohols and acids on cellulose sorbents. Chromatographia 15, 650-652 (1982). Kowalska, T., unpublished results. Kowalska, T., Flakus, H. T., and Sliwiok, J., The role of steric effects in the associative interactions of higher fatty alcohols. Chem. Ser. 13, 59-62 (1978-79). Kowalska, T., Sliwiok, J., and Flakus H. T., Investigation of the association of higher fatty alcohols on chromatographic paper. Chromatogruphia 13, 157-160 (1980). Pimentel, Cl. C., and McClellan, A. L., “The Hydrogen Bond.” Freeman, San Francisco, 1960. Roberts, J. D., and Caserio, M. C., “Basic Principles of Organic Chemistry.” Chap. 18-12. Benjamin, Incorporated, New York, 1965. Sliwiok, J., Walczak, B., and Kowalska, T., Investigation of the self-association of higher fatty alcohols and acids on chromatographic sorbents. Chromatographia 14, 197-202 (1981).