Indian Journal of Chemistry Vol. 41A, July 2002, pp. 1391 - 1396

Host-guest chromatographic behaviour of ketones in micellar pape( chromatographic separation

Zhengsheng Fu*, Guoqin Li", Aimei Yang, Wei dong Liangb & Zhiguo Wang

Department of Chemistry , Northwest Normal University, Lanzhou, 730070, P R China

"Department o f Basi s Course, Gansu Agricultural University , Lanzhou, 730070, P R China

bCollege of Petrochemistry and Engineering, Gansu University of Technology, Lanzhou, 730050, P R China

Received 29 January 2001 .. revised 27 December 2001

Separation of 14 ketone compounds by paper chromatography using pure water mice llar solution of hexadecy ltrimethyl ammonium bromide (CTAB ) and/or sodium dodecylsulphate (SDS) as the mobile phase has been reported. Based on the calculated molecular structural factors (e.g. length, width and height, etc.) and the cavities of mice lles, the ketones can be divided into three molecular groups. Each group has its own charac teri stics and all the groups show the suitability between the solute molecules (guest) and micelles (host). The suitability is the main driving force of chromatographic migration and reveals the relationship between solutes' molecular structures and the cavities o f surfac tants. Furthermore, the suitability can be directly used to expl ore molecular structure such as sizes and geometry.

Host-guest chemistry is one of the most spectacular fields for chemists I. Spectrofluorimetry , spectral methods and ion-molecule probe techniques have mainly been used to study this field2

.3

. While liquid chromatography does not give information about molecular structures, host-guest chromatography (HGC), which is based on recognition, can be used directly to explore molecular structures, such as size, geometry, etc. Several studies have been reported to explain the relationship between the guest and host in explain terms of the suitability of the molecular structures of solutes (guest) for the micellar shapes and sizes (host)4.5.

We can study the relationship between molecular structures of solutes and micellar shapes and sizes with normal micellar paper chromatography besides separations. Several types of organic compounds for example, alcohols, phenols, vitamin B and amino acids4.5 have been studied with this method in our laboratory . Thi s note reports the separation of 14 ketone compounds by micellar paper chromatography (MPC). Their HGC behaviours have also been described.

Materials and Methods p-Phenylacetophenone, p-phenoxyacetophenone, p­

methoxylphenyl ethylketone, 2,S-cyclohexadienyl ketone and 2-pentyl-2-cyclopentenyl ketone were prepared and purified in Ollr laboratory. The other reagents were purchased from Lanzhou Universi ty and used as received. SDS was washed five times

with absolute ethyl alcohol before use . Distilled water was used to make the stock surfactant solutions.

Traditional methods of paper chromatography were applied. Spot visualization was performed with 2,4-dinitrophenylhydrazine. All compounds showed yellow to red colour spots after spraying with the reagent. Retardation factors (RJ ) were calculated from four data points for every compound in two experiments with two pieces of paper used each time .

Results and Discussion According to the basic tendency the ketones be

divided into three groups. There are many factors influencing the solute's migration6

.7 novel concept has

been introduced to compare better the relationship between molecular structures and micellar shapes and

sizes: I"lRFRf2-RJl where Rfl is RJ value obtained when the concentration of surfactant is equal to its CMC, and Rf2 is the va lue at concentration higher than CMC. So, all the influential factors can be considered

relatively, and t1RJ value expresses only the "net contribution" for "net micellar increasing" . According to Armstrong 's equation6

:

1 K",C", ---+ -"'--=--K s[A]8 K \.[A]8

... (1)

where Ks and K", are binding constants of the stationary phase and pseudophase respectively, [A J is the concentration of the stationary phase binding sites ,

1392 INDIAN J CHEM, SEC A, JULY 2002

8 is the phase ratio, Cm is the micelle concentration , and Rf is the retardation factor. Equations 2 and 3, derived from Eq. 1 are used to discuss the suitability between the solute molecules and micelles5.

M = K ,.[A]8 X KII/lC",

f K.,[A]8 + 1 K , [A]8 + 1 + KmtlCm ... (2)

or

M = K,[A]8 x 1 f K .[A]8+1 K ,.[A]8+1

.I . + 1 .. . (3)

KmtlCm

where IlCIIl = C, .. 2 -C",j.

According to Eq. 2, when I1Rf >0, K ill >0 and the solute is binding type, micelles give "net positive

contribution " to solute; when I1Rf =0, Km= ° and solute is non-binding, the micelles are non-contributing. According to Eq. 3, when I1Rf < 0, K",< 0, the solutes belongs to an anti binding type and micelles have a "net negative contribution". The solutes have been classified by the molecular length, width, height and micellar size as small, medium, big and super molecules. Plots of IlC", vs Rf showed different curve tendencies for different kinds of solutes indicating different molecular groupS5.

The HGC behaviours of phenols, amino acids and diphenylmethyl alcohols (DPMA) showed the regularity as above5. 8. In order to have a clear view about suitability, the authors distinguished TLC and HPLC methods used in their laboratorl from other works6

. 'o, Data showed that the relationship was applicable to any type of liquid chromatography such as PC, TLC and HPLC9

. It was observed that adding organic additives improved the separation by micellar chromatography4.", however, the micelles changed their shapes and sizes in this case. Hence, for all further studies we used the pure water system.

Since there are no authoritative data on molecular structural factors (length, width, height, etc.) available at present, we have calculated the molecular structural factors and the longest possible length of surfactant' s longest chain (LPLS) for knowing the biggest possible size of micellar 'cavity,' 2. All the three dimensional organic molecules were grouped with atoms. The structural factors of the three dimensional ketone molecules can be calculated with different parameters, such as bond di stance, atomic radius and

hybridization of atomic orbitals l2 . The length, width and height of the ketones, LPLS, molecular structures and the water solubilities related to discussion are listed in Table 1.

Table 2 gives the Rf of ketones with paper chromatography using different concentrations of CT AB and/or SDS aqueous solutions as mobile phase. Data show that all the ketones can be separated by CT AB and lor SDS micellar solutions in pure water as mobile phase at suitable concentrations, or at different concentrations being combined with different surfactants. All the plots of Cm versus Rf l(l- Rf ) were saw toothed curves, conforming to the early results4

.5.8. This is because of migration of

ketones due to the static electricity, hydrophobic and stereo effects between solutes and micelle, as well as the dynamic equilibrium of micellar cavities, etc. This conformed to the results of Fu et al. 4.5.8 , and illustrated that the suitability between the solutes' molecular structures and micellar cavities is the main driving force in the separation of ketones.

On the basis of comparison of the calculated molecular structural factors (Table 1) with the 'cavities' of micelles, the ketones can be divided into three groups, A, Band C. All these suitabilities showed excellent regularity . The order of stereo effect is Al>A2>B>C. We noticed that the length of 2-octanone is much longer than that of m­nitroacetophenone, but, the stereo effect of /11-

nitroacetophenone is much stronger than that of the others. This may be because of m-nitroacetophenone being a planar molecule, is more rigid than linear 2-octanone.

HGC behaviour ofmoleeular Group A According to the molecular structural factors of

ketones in Table 1, the molecules in Group A are big molecules (longer length) in CT AS and how very strong stereo effect, namely, I1Rf »O, K",»O. However, the LPLS of SDS is shorter than that of CT AB. Group A shows mainly the super molecular characteristics and some big molecular characteristics in SDS, hence, the plots of I1C", versus I1Rf are almost parallel, though a little above the line of !1Rf =0. An example is shown in Fig. 1; the others are similar. The molecular factors of sub-group AI are a little different from sub-group A2 .. the curves show the difference between A 1 and A2. Lengths of A 1 are longer than that of A2 , the I1Rf of A I grows more rapidly than that of A2 and the curves of A 1 are genera II y higher than those of A2 (Fig. I).

FU el al.: HOST-GUEST BEHA VIOUR OF KETONES

Table I-Molecular structural factors of ketones

Group Compo L (nm) W (nm) H (nm)" Structure

o

I p-Tolyl phenyl ketone 1.106 0.431 0. 192 C HJ-()-LO

2 p- Isopropylphenyl ethyl ketone 1.1 0 I 0.431 0.371

A I 3 p- Phenylacetophenone 1.079 0.431 0.262

A 4 P- Phenoxyacetophenone 1.014 0.431 0.261

5p-Pentyl-2-cyc1opentenyl ketone 1.013 0.443 0.161

6 p-Methoxylphenyl ethyl ketone O. 965 0.431 0.360 A2

B

C

7 Phenyl ketone

8 P- N itroacetoprenone <

9 P- Methylacetoprenone d

10 p-Aminoacetophenonec

11 m-Ni troacetophenoned

12 Cyc1ohexanone

13 2,5-Cyc1ohexadienyl ketone

142-0ctanone

LPLSb

' L, Wand H express length, width and height respectively. bLPLS express the longest possible length of surfactant 's chain .

0.935 0.431 0.392

0.740 0.431 0.247

0.732 0.431 0.255

0.726 0.431 0.255

0.666 0.564 0.253

0.505 0.319 0.273

0.443 0.445 0. 178

1.070 0.234 0.178

CT AB 1.982 (nm)

o

-0- 11 C H'-iH ~ # C- CH,cHJ

CHJ 0

0-0-- II ~ # ~ # C- C H,

o

0--0-11 ~ # 0 ~ # C- C H,

o

Q-C H'CH'CH,c H,c HJ

o

-0-11 NO, ~ J C--CHJ

o

C HJ-o-~-CHJ o

N H ,-o-~-CHJ o

t='>r-II y C- CH,

NO,

oD 0=0 0

CH .I -~-CH 2CH2C H2CH~ H2CH j

SDS 1.474 (nm)

<Soluble, and dlnsoluble (according to Lang's Chemistry Handbook (Thirteenth Edition, Science Publication House, 1991 , Beijing)

1393

The shapes and sizes of micelles are in a continuous equilibria system in aqueous solution. In an aqueous micellar solution, there are several kinds of micelles with different shapes and sizes. These micelles are spherical, rod-shaped, disk-shaped, etc. The cores (cavities) change with these shapes. The shapes and cavities of micelles are in dynamic equilibria\3. However, Eq. 2 or Eq. 3 arise from a special condition in which there is only one type of micelles, and the stoichiometry between the

pseudophase and solute is 1: 1. For 2: 1 complexes the correct pseudophase retention equation is6

:

... (4)

where K, K, and K2 are the binding constants to the stationary phase, first pseudophase and second

pseudophase respectively. !1Rr can be expressed as follows5

:

1394 INDIAN J CHEM, SEC A, JULY 2002

Table 2-Rr values of Ketones in CT AS /SDS Micellar Solution

Ketone" CT AS (mallL)

0.00 .00 1 .002 .003 .004 .005 .010 .020 .030 .040 .050 .060 .070 .080 .090 .100

1

2 3

4

5

6

7

8 9

10 11

12

13

14

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.88

0.00

0.60

0.72

0.85

0.77

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.06

0.87

0.01

0.59

0.71

0.90

0.88

0.87

0.07

0.07

0.03

0.04

0.02

0.02

0.06

0.91

0.06

0.61

0.75

0.91

0.88

0.93

0.13

0.17

0.07

0.11

0.07

0.06

0.12

0.93

0.13

0.62

0.74

0.92

0.86

•

0.1 7

0.23

0.12

0. 18

0.10

0.09

0.1 5

0.90

0.17

0.60

0.71

0.92

0.83

0.93

0.24

0.35

0.14

0.23

0.19

0.13

0.24

0.90

0.26

0.60

0.7 1

0.80

/

0.44

0.44

0.30

0.45

0.37

0.26

0.37

0.89

0.25

0.59

0.72

0.90

0.81

0.89

0.75

0.73

0.70

0.73

0.65

0.7 1

0.70

0.75

0.66

0.64

0.76

0.91

0.85

0.86

0.86

0.80

0.87

0.88

0.85

0.89

0.82

0.88

0.75

0.70

0.78

0.9 1

0.88

0.90

0.90

0.72

0.90

0.88

0.83

0.86

0.81

0.92

0.77

0.76

0.87

0.89

0.87

0.90

0.89

0.72

0.86

0.87

0.84

0.86

0.85

0.91

0.75

0.79

0.88

0.89

0.88

0.87

0.89

0.88

0.89

0.91

0.89

0.91

0.89

0.86

0.84

0.8 1

0.84

0.87

0.89

0.83

0.79

0.84

0.85

0.87

0.83

0.89

0.85

0.87

0.86

0.84

0.87

0.91

0.84

0.89

0.84

0.78

0.86

0.86

0.80

0.84

0.85

0.84

0.81

0.88

0.86

0.85

0.86

0.87

0.84

0.77

0.79

0.83

0.80

0.83

0.89

0.79

0.76

0.78

0.86

0.86

0.87

0.78

0.74

0.82

0.84

0.78

0.76

0.80

0.76

0.79

0.80

0.82

0.85

0.84

0.84

Ketone" SDS (mollL)

0.00 0.0 I 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

2

3

4

5

6

7

8

9

10 11

12

13

14

0.00 0.84 0.85 0.91 0.91 0.89

0.00 0.72 0.85 0.89 0.91 0.82

0.00 0.73 0.88 0.92 0.92 0.89

0.00 0.75 0.88 0.93 0.92 0.92

0.00 0.86 0.85 0.88 0.92 0.90

0.00 0.91 0.92 0.93 0.94 0.88

0.00 0.84 0.84 0.90 0.89 0.88

0.88 0.89 0.82 0.78 0.76 0.75

0.00 0.77 0.87 0.92 0.9 1 0.88

0.60 0.65 0.72 0.81 0.72 0.91

0.72 0.57 0.89 0.91 0.92 0.87

0.85 0.87 0.92 0.92 0.93 0.88

0.77 0.91 0.93 0.91 0.92 0.90

0.00 0.90 0.88 0.92 0.93 0.90

·Compound names, same as in Table I.

0.91 0.91

0.92 0.89

0.92 0.88

0.90 0.92

0.89 0.90

0.92 0.93

0.91 0.87

0.70 0.74

0.94 0.94

0.82 0.90

0.85 0.93

0.93 0.91

0.93 0.92

0.92 0.89

0.90 0.89

0.86 0.9 1

0.90 0.93

0.92 0.87

0.91 0.89

0.93 0.92

0.89 0.90

0.71 0.7 1

0.91 0.92

0.87 0.91

0.90 0.92

0.89 0.93

0.92 0.90

0.92 0.92

0.88

0.88

0.89

0.90

0.91

0.92

0.89

0.70

0.90

0.86

0.90

0.88

0.89

M _ K[A]8K)[K 2 (C2 +C)+l]f1Cm

J - (K[A]8+1) 2 +(K[A]8+I)K)[K2 (Ci +C)2)

+(C2 +C)]+[K2 (C2 +C)

dynamic equilibria. Thi s means that KJ and K2 (even KJ , K4 , etc.) are dependent on different solutes which

. mayor may not be suitable for a certain cav ity (or cavities) of micelles.

... (5)

When K2 is zero and CJ is zero, this equation reduces to Eq. 2 or Eq . 3.

Either KJ or K2 or both can be negative, !'1RJ shows synthesis of KJ and K2• For a more complex system (or real system), the equation will include K3• K4 ... etc.4

.5.8.13 . As above, the shapes and sizes of micelles can cha'nge with increase in the concentration of surfactant solution, and the cavities of mice lles are in

In CT AB the curves of A become flat when Cm is higher than 0.04 moIlL, because the econd micellar phase (K2 area in Fig. 1) emergss and has become the main phase. The main driv ing force of chromatographic migration, in this case, is the suitability between ketone and the second micell ar phase. The second micellar cavity is bigger than the first one, so ketone becomes relatively smaller and shows the medium molecular characteristics . Fig. I also shows that molecular Group A has different characteristics in K) (first micellar phase) area and " K) + K2" (first, second mixed micellar phase) areas.

FU et at.: HOST-GUEST BEHA VIOUR OF KETONES 1395

1.0

0.8

0.6 ..:r <i

0.4

0.2

0.0

0 2 468

llr, llCmx100(moI/L)

10

Fig. I-Plots of t!.Cm versus M f of the ketones [p­phenyl acetophenone (. ) (Group A I), phenyl ketone (0 ) (Group A2) and p-methylacetophenone ( ~ ) (Group 8) in CT A8 (dashed line) or SDS (solid line) respectively. Cml of CTA8=0.002 moUL and of SDS =0.0 I moUL. K" K2 and "K, +K2" express first micellar phase area, second micellar phase area and first, second mixed micellar phase area respectively)

In KI area, !1RJ increases very fast, because the ketones are big molecules which are suited to the cavity of the first micellar phase completely.In " KI + K2" area, although the second micellar phase has emerged, the first micellar phase is still the main phase. An interesting phenomenon appears in the mixed phase; the curves show a crisscross plot indicating that different ketones shows different suitabilities in this rase.

HGC behaviour oj molecular Group C Figure 2 shows the behaviour of molecular Group

C. It clearly ill ustrates that a ll the ketones of Group C are small molecules, both in CT AB and in SOS micellar solutions. All the curves are around the line

f1Rf = O. However, the curves in SOS are hi gher than those in CT AB because LPLS of SOS is small er and the ketones show larger stereo effects.

HGC behavior of molecular Group B Ketones of Group B are very close to medium sIze

molecules in CTAB ; their curves are below the curves of Group A (Figs 1 and 3).

All the ketones of Group B become close to super size molecul es in SOS micellar solutions for short LPLS ; their curves are hi gher than that in CT AB and

are almost parallel to the line of f1Rf = 0 (Fig. 3). All the curves show excellent regularity in SOS, the curves from top to bottom are in direct ratio to their length for the super size (Table 1, Fig. 3) . However,

0.4

0.2

0.4

0.2

'io.o -+-~;:;;;;~~B~~~~ ~=() o 2 8 10

Fig. 2-Plots of t!.Cm versus t!.RJ of the ketones [cyc!ohexanone (. ), 2,5-cyclohexadienyl ketone (0 ) and 2-octanone (M in CT AS (A) or SDS (8) respectively. Cm , of CTA8 is 0.00 1 moUL and of SDS is 0.01 moUL.)

p -ni troacetophenone shows typical super molecular characteristics.

Solubility and stereo factors of solutes and their

contribution to f1RJ values Under certain conditions, some factors such as

molecular inner hydrogen bond and/or solubility of solutes influence the HGC behaviour of ketones5

.

Since the micellar cavities are hydrophobic , the solubility and stereo factors of solute will compete with each other. The characteristics of super and big molecules are dependent on the stereo effect, and those of small or medium sized solutes on the polarity, hydrophobicity or hydrophilicity. The stereo effects are less significant for small molecules but increase in significance for medium sized molecules. It was observed that the hydrophilic - hydrophobic equilibria influence the suitibilities on certain cond itions.

The water solubili ties of Group B are different (Table I). In eTAB , the stereo effect of Grour B becomes smaller and hydrophilinoc ity becomes more important. The substituted methyl on benzene ring is a hydrophobic group, so, p-methylacetophene shows

hi gher molecular f1RJ values (Fig. 1 ) and its curve is below the curves of Group A. Group B shows medium sized mol ecules' characteristics (Fig. 3), and the curves show the competition between stereo effect and hydrophilicity . p-Nitroacetophenone shows typical super molecular characteristics for both stereo effect and high hydrophi licity in SOS (Fig. 3).

1396 INDIAN J CHEM, SEC A, JULY 2002

0.2

~O.O

-0.2

0.4

0.2

B

• * • 4 • • • • _9.- - - - -0 - - 0- - 9_ - 0 o----i.--- - - -0- --'"- - A- -A-_I!>. e .... - 0 A

~ 0.0 +---------------------~RrO

-0.2 • 0 2 4 6 8 10

~Cmx lOO(mol/L) , Fig. 3- Plots of /).Cm versus t:.Rf of the ketones [p­nitroacetophenone (. ), p-methylacetophenone (~), p­aminoacetophcnone (0 ) and m-nitroacetophenone ( "') in CT AB (A) or SOS (B) respectively. Cmt of CT AB is 0.00 i moUL, and of SOS is 0.01 moUL (for p-methylacetophenone in CTAB, please see Fig. I).]

The same regularity is also shown in Fig. 1. In K2 area of CT AB, the values of !1RJ ketones are in the order: p -phenylacetophenone > phenyl ketone> p­methylacetophenone in comparison with their lengths (Table 1) for their medium molecular characteristics. However, phenyl ketone shows more super molecular characteristics for its double benzene ring structure which is much more rigid and shows more stereoscopic effect in smaller cavities of SDS micellar solution (Fig. I).

Conclusion Liquid chromatography,

effecti ve, does not gi ve although simple and

information about the

molecular structures. However, HGC, a novel technique combining the host-guest chemistry and chromatographic methods, gives more structural information on the molecules . Results show . that the suitability between the molecular structures of solutes (guest) and the cavities of micelles (host) is the main driving force of chromatographic migration.

Acknowledgement Support of this work through Funds from Gansu

Province Education Bureau, PRC (No. 971-07) and funds from Gansu Province Nature Science, PRC (ZQ-97-018) is gratefully acknowledged.

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