Indian Journal of Chemical Technology Vol. 8, July 2001, pp. 244-251

Investigation of the migration behaviour of aromatic amines on silica layer using nonionic surfactant containing mobile phases: Mixed surfactants

assisted separations

Ali Mohammad* & Iftkhar Alam Khan

Analytical Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202 002, India.

Received 27 September 2000; accepted 23 February 2001

Migration behaviour of important aromatic amines on silica gel layers has been investigated using mobile phases containing a nonionic surfactant (Triton X-100) over a wider concentration range (0.001-0.1M). Effect of factors such as pH of mobile phase and the presence of electrolyte (NaCI, NaBr), non-electrolyte (urea) and alcohol (MeOH, EtOH, PrOH, BuOH) additives on the mobility of amines was also examined. The results obtained with surfactant containing mobile phases were compared with those obtained in the absence of surfactant. The main chromatographic characteristics and separation conditions for mixtures of amines on silica plates with Triton X-100 containing mobile phases were determined. As a first attempt, aqueous solutions of mixed surfactants (Triton X-100 plus sodium dodecyl sulphate, SDS) have been utilized for superior thin layer chromatographic separations of aromatic amines.

The use of micellar mobile phases in thin layer chromatography (TLC) was first suggested by Armstrong and Terrill in 19791

• The use of aqueous micellar mobile phases was later on extended to TLC separation of phenols2

, polynuclear aromatic hydrocarbons and substituted benzoic acids3

, amino acids4

, alkaloids5, dyes6

•7

, drugs8, amines9 and heavy

metal cations 10.

Despite evident favourable features, micellar mobile phases have not been widely used in TLC compared to their use in HPLC. The TLC methods reported to date involve the use of normal or reversed­phase stationary phases with micellar solutions of cationic, anionic and nonionic surfactants as mobile phases. None of these studies refer to the use of aqueous solutions of mixed surfactants as mobile phase in TLC.

Because of toxicological, pharmaceutical and industrial importance, the separation and identification of .a wide range of aromatic amines by TLC have received considerable attention. Several sorbent phases including alumina, cellulose, polyacrylonitrile, silica gel, NH2 -modified silica gel and silica gel impregnated with inorganic metallic salts and organic nitro-compounds 11

-16 have been

used. Few studies have also been performed on TLC separation of amines via charge-transfer complexation

*For Correspondence (E-mail: mohammadali 4u@rediffmail.com)

with nitro-compounds 17'18

• The mobile phases used include aqueous organic acids, mixed aqueous­organic systems and aqueous mineral acids containing H20 2, ammonium salts or lower chain alcohols.

Recently, a systematic study on the use of surfactant -mediated mobile phases in TLC separation of organic and inorganic substances has been initiated. The present study was undertaken to determine the feasibility, effectiveness and advantages of using mixed aqueous surfactants as mobile phase in TLC separation of aromatic amines on silica gel layer. The mixed surfactants mobile phases were obtained by addition of aqueous nonionic surfactant, Triton X-100 solution into an anionic sodium dodecyl sulphate (SDS) surfactant solution. The retention behaviour of amines with individual surfactant solutions, with and without alcohol or electrolyte additives was also examined.

Experimental Procedure Apparatus

A TLC apparatus (Toshniwal, India) was used to prepare thin layers (0.25 mm) of various adsorbents on 20x3 em glass plates. Glass jars (29x6 em) were used for the development of TLC plates.

Chemicals and reagents Triton X-100 (lso-octylphenoxypolyethoxy ethanol)

termed as TX -100 in this paper was from Loba

MOHAMMAD & KHAN: MIGRATION BEHAVIOUR OF AROMATIC AMINES ON SILICA 245

Chemie, India. SDS (BDH, India); methanol and ethanol (Qualigens, India); urea (GSC, India); sodium chloride (s.d.fine chemicals, India); propanol, N­cetyl-N,N,N-trimethyl ammonium bromide (CTAB) and calcium chloride (CDH, India) were used.

Amines Studied Aromatic amines used in the present study include:

aniline (AL), diphenylamine (DPA),a-nitroaniline (a­NAL), m-nitroaniline (m-NAL), p-nitroaniline (p­NAL), a-chloroaniline (a-CAL), m-chloroaniline (m­

CAL), p-chloroaniline (p-CAL), a-naphthylamine (a­NPA), p-bromoaniline (p-BAL), p-anisidine (p-AS), p-phenylenediamine (p-PND), dimethylaniline (DMAL), indole (ID) a-toluidine (a-TLD), m­toluidine (m-TLD), p-toluidine (p-TLD) and p­dimethylaminobenzaldehyde (p-DAB). All amines were procured from CDH, India and were of Analytical Reagent grade.

Test solution The test solutions, 1% of all amines were prepared

in methanol.

Detection All amines were detected by exposing TLC plates

to iodine vapours and were visualized as dark brown/yellow spots.

Preparation of TLC plates The TLC plates were prepared by mlXlng the

sorbent with demineralized water in 1:3 ratio by weight with constant shaking to obtain a homogenous slurry. The resultant slurry was applied to clean glass plates with the help of an applicator to give a 0.25 mm thick layer. The plates were dried at room temperature and then activated at 100±5°C by heating in an electrically controlled oven for 1h. The activated plates were stored in closed chamber at room temperature until used.

Method Chromatography

The activated plates were marked with two horizontal lines 2 and 12 em from the base. The test solution (1% amines), 10 J.!L were spotted separately on the base line of the activated thin layer plates with the help of a micropipette. The spots were allowed to air dry and the plates were developed in chosen mobile phase by one-dimensional ascending technique, in glass jars. The solvent ascent was fixed

to 10 em from the point of application in all cases. After development, the TLC plates were dried at

room temperature. These plates were then exposed to iodine vapours for 10 min and then the spots were visualized, the amines show yellowish brown spots. The RF values [RF= (RL +RT)/2] were calculated from RL (RF of leading front) and RT (RF of trailing front) values of detected spots on TLC plate. The reproducibility (or precision) of RF values was checked by determining the RF value of the same sample by the same analyst on different days under identical experimental conditions, in the same laboratory using the same apparatus. The variation in RF values differs by a factor of ±0.12 (i.e. ±12%) from the average value, indicating a good reproducibility.

Limits of detection of some amines were determined by loading different amounts of amines on the TLC plates, developing the plates and detecting the spots. The method was repeated with successive lowering of amount of amines until no spot was detected. The lowest detectable amount of amines on TLC plate was taken as the limit of detection. The limit of dilution was determined using the expression:

Dilution limit : [(Volume of test solution x 106)/Limit of detection (mg)].

Results and Discussion The results of this study have been presented in

Tables 1-5 and Fig. 1. The chromatography of amines was performed using a wider concentration range (0.001-0.1M) of aqueous TX-100 (M 1-M8) in order to understand the mobility pattern of amines as a function of structural changes of surfactant in water. It is assumed that (a) at very low concentration (0.001M) of TX-100 (M 1) which is below its critical micelle concentration (CMC value, 0.028M), TX-100 is present entirely in the form of monomers, (b) at concentration level 0.02-0.0SM (M6, M7) which falls in the vicinity of CMC, TX-100 monomers are in a dynamic equilibrium with TX-100 micelles or submicelles, and (c) at higher concentration O.lM (M8), TX-100 is predominantly in the form of micelles along with a smaller number of "free" monomers in solutions.

The mobility of amines was found to be influenced by the microenvironment of aqueous surfactant mobile phase. The mobility of arnines does not follow a regular pattern. The RF value versus TX-100 concentration plots (not given) show curvilinear nature (with few exceptions) and pass through maxima and minima.

246

Solvent System

Aqueous surfac­tant solution

Buffered surfac­tant solution

Mixed aqueous alcoholic surfactant solution

Aqueous solution of mixed surfactant

Electrolyte and non-electrolyte added surfactant solution Surfactant free solvent system

Symbol

M1 M2 MJ M 4

Ms M 6

M1 Ms M9

Mw M11 Ml2 Mn M14

M1s M1 6

Ml7 M1 s M1 9

M2o M21 Mn M 23

M 24

M2s M26

M21 M2s M 29

M3o M 31

M 32

M 33

M34

Stationary phase : Silica gel G

INDIAN J. CHEM. TECHNOL., JULY 2001

Table !-The solvent systems used as mobile phase

Composition

O.OOIM TX-100 0.002M TX-100 0.005M TX-100 0.007M TX-100 0.014M TX-100 0.02M TX-100 0.05M TX-100 0.10M TX-100 0.10M TX-100 (pH2.3) 0.10M TX-100 (pH 3.4) 0.10M TX-100 (pH 5.7) 0.10MTX-IOO(pH 11.9) 0.02M TX-100 (pH 2.3) 0.02M TX-100 (pH 2.4) 0.02M TX-100 (pH 5.7) 0.02M TX-100 (pH 11.9) Methanol+0.02M aqueous TX-100 (5 :95) Methanol+0.02M aqueous TX-100 (10:90) Methanol+0.02M aqueous TX-100 (15:85) Ethanol+0.02M aqueous TX-100 (5:95) Ethanol+0.02M aqueous TX-100 (10:90) Ethanol+0.02M aqueous TX-100 (15:85) Propanol+0.02M aqueous TX-100 (5:95) Propanol+0.02M aqueous TX-100 (10:90) Propanol+0.02M aqueous TX-100 (15:85) O.OIM TX-100+0.001M SDS (1:1,2:1,1:2) O.OIM TX-100+0.01M SDS (1:1,2:1,1:2) 0.02M TX-100+0.01M SDS (1:1,2:1,1:2) 0.02M TX-100+0.01M SDS (1:3,3:1,1:9) 0.10M TX-100+0.01M SDS (1:1,2:1,1:2) 0.10M TX-100+0.01M SDS (1 :3,3:1,1:9) 0.01M NaC1+0.02M aqueous TX-100 (1:1) 0.10M CaCI2+0.02M aqueous TX-100 (1:1) O.IOM Urea+0.02M aqueous TX-100 (1:1)

Distilled water Distilled water+ methanol (95:5) 0.10M NaCl O.IOM CaC12

Mixed mobile phases were prepared by mixing different volumes of individual solution/solvent

The effect of pH value of aqueous 0.02 and 0.1M TX-100 (pH range 2.3-11.9) on the mobility of amines was examined using mobile phases MwM,6•

An irregular pattern showing both decrease apd increase in mobility (or RF value) with the increase in pH value was noticed. The results of aniline and substituted anilines (o-, m-, p- chloro- and nitro anilines) over the entire pH range show their minimum and maximum mobilities at pH values of 3.4 and 5.7 respectively. A secondary aromatic amine (DPA) has mobility very close to the mobility of tertiary aromatic amine (p-DAB) over the entire pH range. However, the higher mobility of ID at pH 3.4

and 11.9 can be utilized for its selective separation from DPA and p-DAB. Similarly, the variation in pH from 2.3 to 11.9 of aqueous 0.1 M TX-100 mobile phase induces a minute change in the mobility of amines. The most effected amines include a-and p­CAL, a-NPA and DMAL which can be separated from other amines at pH value of 3.4 or 11.9. Amongst ID, p-DAB and DPA, the highest mobility of ID at all pH values facilitates its good separation specially at pH 3.4.

In order to provide a more clear picture regarding the effect of TX -100 at concentration levels below and above its CMC (0.028M) at fixed pH value on the

0.4

t 0.2

LL a:

<:1

!

0.4

i 0.2

-0.4

-0.6

MOHAMMAD & KHAN: MIGRATION BEHAVIOUR OF AROMATIC AMINES ON SILICA

(a)

a-NPA m-NAL p-BAL o-NAL p-NAL AL

Amines~

-<>----e- Plot of ~RF ( RFwith 0.02 M TX- 100 at pH 2.3-RF with buffer

of pH 2.3 ) VS Amines

--ts---A- Plot of i).RF ( RFwith 0.1 M TX- 100 at pH 2.3-RF with buffer

of pH 2.3) VS Amines

(b)

DPA m-CAL p-BAL p-AS D-MAL p-TLD o-CAL ~NAL AL ~PND

Amines~

--()---(r- Plot of ~RF ( RFwith 0.02M TX- 100 at pH 11.9 RF with buffer

of pH 11.9 ) VS Amines

~ Plot of ~RF ( RFwith 0.1 M TX- 100 at pH 11 .9-RF with buffer

of pH 11 .9 ) VS Amines

p- DAB

247

Fig. 1-Plot of MF (RF with 0.1 or 0.02M TX-100 at pH 2.3 or 11.9- RF with buffer solution of pH 2.3 or 11.9) versus amines

248 INDIAN J. CHEM. TECHNOL., JULY 2001

Table 2-Mobility of amines on silica layer developed with various mobile phases in the presence and absence of surfactant

Amine M3s M6 M36 M37

DPA 0.44 0.32 0.65 0.42 o-CAL 0.82 0.40 0.71 0.56 m-CAL 0.72 0.69 0.79 0.72 p-CAL 0.85 0.65 0.90 0.77 a -NPA 0.67 0.67 0.77 0.75 o-NAL 0.65 0.87 0.77 0.72 m-NAL 0.70 0.87 0.77 0.77 p-NAL 0.82 0.92 0.82 0.77 p-BAL 0.95 0.75 0.78 0.80 AL 0.76 0.77 0.77 0.75 p-AS 0.75 0.80 0.83 0.75 p-PND 0.70 0.72 0.69 0.80 DMAL 0.00 0.45 0.0 0.38 10 0.67 0.70 0.83 0.90 o-TLD 0.80 0.65 0.70 0.80 m-TLD 0.72 0.70 0.74 0.65 p-TLD 0.82 0.73 0.85 0.82 p-DAB 0.37 0.38 0.37 0.60

mobility of amines, the plots of M?F [RF in 0.1 or 0.02 M TX -100 (pH 2.3 or 11.9)- RF in buffer solution (pH 2.3 or 11.9)] versus amines were constructed (Figs 1a and 1b). The positive and negative M?F values of amines clearly indicate the significant influence of TX-100 at both (below and above CMC value) concentration levels irrespective of the fact that either TX-100 is present in the form of monomers (0.02M)or micelles (0.1M) in the mobile phase. Secondly, the difference in magnitude of M?F values and the shapes of curves (Figs I a and 1 b) reveal that the surfactant-mediated mobile phase systems exert a determinant effect on the mobility of amines. From these figures following conclusions may be drawn:

(i) Compared to pH in the alkaline range (pH = 11.9), most of the amines show higher mobility at lower pH e.g. acidic range (pH = 3.4) of the mobile phase in the presence of surfactant as evident by positive values of t-.RF.

(ii) a-and m-TLD are more strongly retained by the stationary phase in the presence of surfactant (negative t-.RF) compared to their retention in pure buffer solution of pH 11.9. At pH 2.3, similar behaviour (negative M?F) with 0.1M TX-100 and a reverse behaviour (positive M?F) with 0.02M TX-100 was observed. Conversely, p-DAB is weakly retained (positive !J.RF) by stationary phase in 0.1 M TX-100 whereas it is strongly retained (negative M?F) by stationary phase in 0.02M TX-100.

(iii) Certain amines such as m-CAL, m-NAL and p-TLD at pH 2.3 and p-AS, p-NAL, ID and p-TLD at

M3s M32 M33 M34

0.43 0.65 0.50 0.70 0.70 0.65 0.68 0.65 0.72 0.69 0.80 0.70 0.77 0.77 0.85 0.82 0.72 0.70 0.62 0.82 0.77 0.60 0.67 0.76 0.90 0.77 0.80 0.77 0.92 0.80 0.83 0.80 0.87 0.65 0.72 0.70 0.85 0.75 0.72 0.90 0.87 0.62 0.75 0.85 0.77 0.67 0.85 0.82 0.40 0.35 0.38 0.31 0.87 0.80 0.82 0.77 0.70 0.77 0.75 0.80 0.67 0.75 0.70 0.79 0.76 0.78 0.82 0.87 0.58 0.62 0.65 0.55

pH 11.9 show almost identical retention behaviour (M?F:::; 0.0) in the presence or absence of surfactant.

It is evident from Table 2 that the addition of alcohol (methanol or ethanol) into distilled water brings about a slight increase in the mobility of amines compared to 0.1M NaCl (or O.lM NaCl plus 0.02M TX-100). Mobility of amines in 0.01M CaC12

(or 0.1M CaCh plus 0.02M TX-100) either remains unchanged or increases marginally. Despite their comparable ionic sizes (Na+, 102 pm and Ca2+, 100 pm), dipositive calcium cation has much stronger electrostatic attraction to the oxygen of water molecules because of its much larger charge-to-size ratio. However, this factor has little effect on migration behaviour of amines as indicative from the data listed in Table 2. Little increase in mobility of some amines was also observed when 0.1M urea solution instead of 0.1M NaCl or CaC}z solution was added to 0.02M TX-100.

In addition to examining the effect of pH and inorganic electrolytes on the mobility of amines, the effect of added alcohols in surfactant-mediated aqueous mobile phase has also been investigated. The data presented in Table 3 indicate that the mobility of amines is influenced by both the type and concentration of added alcohol in the mobile phase. In general, enhanced mobility of amines was noticed when a more polar alcohol (i.e. methanol) was replaced by a moderately polar alcohol (i.e. propanol) in the mobile phase. The higher mobility of para substituted amines (p-CAL and p-TLD) compared to

MOHAMMAD & KHAN: MIGRATION BEHAVIOUR OF AROMATIC AMINES ON SILICA 249

Table 3-Effect of type and concentration of added alcohol to 0.02M aqueous TX-100 on the mobility of amines

Amine Mi7 M,s M,g M2o M21 M22 M23 M24 M2s

DPA 0.50 0.60 0.65 0.60 0.67 0.65 0.72 0.80 0.77 o-CAL 0.70 0.68 0.74 0.82 0.65 0.70 0.87 0.87 0.87 m-CAL 0.80 0.80 0.87 0.90 0.85 0.87 0.80 0.90 0.90 p-CAL 0.85 0.83 0.88 0.87 0.89 0.90 0.82 0.95 0.90 a-NPA 0.75 0.80 0.85 0.95 0.85 0.90 0.85 0.94 0.95 o-NAL 0.82 0.85 0.85 0.82 0.87 0.87 0.80 0.88 0.87 m-NAL 0.85 0.80 0.70 0.90 0.77 0.85 0.85 0.92 0.95 p-NAL 0.85 0.85 0.80 0.90 0.80 0.72 0.90 0.93 0.95 p-BAL 0.90 0.77 0.77 0.77 0.85 0.85 0.90 0.90 0.95 AL 0.88 0.85 0.80 0.70 0.72 0.75 0.90 0.90 0.90 p-AS 0.87 0.87 0.82 0.85 0.85 0.87 0.85 0.92 0.95 p-PND 0.77 0.92 0.77 0.80 0.85 0.90 0.95 0.84 0.92 DMAL 0.47 0.62 0.32T 0.40T 0.35T 0.42T 0.30T 0.43T 0.48 ID 0.75 0.90 0.90 0.82 0.90 0.87 0.86 0.90 0.92 o-TLD 0.55 0.57 0.85 0.60 0.70 0.75 0.85 0.88 0.78 m-TLD 0.62 0.60 0.80 0.70 0.70 0.78 0.89 0.90 0.85 p-TLD 0.70 0.73 0.85 0.79 0.77 0.83 0.93 0.90 0.95 p-DAB 0.55 0.50 0.55 0.57 0.60 0.58 0.60 0.77 0.75

*The concentration of alcohol (5- I 5%) increases from M 17 toM 19, M20 to M22 and M23 to M25

T refers to tailed spot (RL- RT > 0.3)

ortho substituted amines (a-CAL and o-TLD) may be attributed to the steric hindrance factor at ortho position of amine. From the following trend of decrease in RF value i.e. NAL>TLD>p-DAB (Table 3), it may be concluded that the presence of electron­donating group such as -CH3 in the molecule of amine plays an important role in lowering the mobility of amine. For example, p-DAB with two-CH3 groups in its molecule has lower RF than TLD which has only one-CH3 group. Conversely, the presence of electron donating group such as - N02 in the molecule of amine promotes the mobility.

TX-100 is a polydisperse preparation of p-(1,1,3,3-tetramethylbutyl) phenoxypolyoxyethylene glycols, containing an average of 9.5 oxyethylene units per molecule. The polydispersity of the oxyethylene chains could result in a nonuniform distribution of oxyethylene groups around the micelles, allowing the overall shape to be closer to spherical. Addition of either anionic or cationic surfactant to solutions of nonionic surfactant has been found to increase the cloud point by introducing electrical double layer repulsion between micelles 19• This charge stabilization contribution by ionic surfactant addition increases effective colloidal stability. Thus, the mixed surfactant systems with altered micro-environment polarities can have a direct influence on the retention behaviour of solutes which can be distinctly different from that of aqueous nonionic surfactant eluants. In

his first report on TLC of organic substances with aqueous micellar mobile phases 1, Armstrong remarked "For normal phase TLC (using aqueous micellar solutions) it was found that SDS was the best surfactant as solutions of nonionic surfactant produced smear spots. CTAB, while satisfactory, appeared to bind somewhat to the stationary phase, silica gel was virtually useless as stationary phase while alumina was somewhat less satisfactory than polyamide".

Bearing in mind the above facts and the literature reports till date wherein no publication on the use of nonionic/ionic surfactant mixtures in TLC has appeared, it was decided to develop mixed surfactant mobile phases for TLC analysis of aromatic amines using silica gel, the most preferred layer material, as stationary phase. Consequently, TLC of amines was performed with mobile phases obtained by mixing different volumes of O.OlM aqueous solution of SDS, an anionic surfactant (CMC value 0.0081M) into O.lOM or 0.02M aqueous TX-100 maintaining the final volume to 100mL. From the non-linear relationship between volume fraction of SDS and RF value, it can be assumed that different amines experience various microenvironment polarities in their immediate vicinities in a given stationary/mobile phases composition. Consequently, it is not surprising that much improved separations of amines were obtained using aqueous TX-100 with added SDS. A

250 INDIAN J. CHEM. TECHNOL., JULY 2001

Table 4--Experimentally achieved separations of amines on silica gel layer with various mobile phases

Mobile Phase Separation (RF value)

MIO p-DAB (0.48)- 10 (0.85) M35 DPA (0.40)- p-NAL (0.80)

DMAL (0.0)- DPA (0.49)- p-NAL (0.77) M30 (2: I) p-DAB (0.42)- m-CAL (0.82) DPA (0.60)- m-CAL (0.86)

DMAL (0.20)- p-NAL (0.90) DMAL (0.20)- DPA (0.60)- m-CAL (0.83) p-DAB (0.45)- o-NAL (0.75) DMAL (0.17)- DPA (0.55) -p-NAL (0.88) (1:2)DMAL (0.25)- p-BAL (0.90)

DPA (0.52) - p-AS (0.85)

(1:2)

M2s (2: 1)

(1 :2)

DMAL (0.25) - p-NAL (0.86) p-DAB (0.48)- o-NAL (0.75) DMAL (0.26)- DPA (0.57)- p -NAL (0.87) DMAL (0.26) - p-DAB (0.52)- m-CAL (0.85) M27(2:1) DMAL (0.35)- m-CAL (0.85) p-DAB (0.55) - m-CAL (0.85) DMAL (0.40) - m-NAL (0.87) p-DAB (0.53) - m-NAL (0.82)

DMAL (0.32) - m-CAL (0.83) p-DAB (0.52) - m-NAL (0.86) DMAL (0.35 - m-NAL (0.87) p-DAB (0.40) - m-CAL (0.85) DPA (0.45)- m-CAL (0.82) DMAL (0.43)- p-NAL (0.88) DMAL (0.36)- o-TLD (0.70) p-DAB (0.45) - p-NAL (0.85)

DMAL (0.40)- a-NPA (0.75) p-DAB (0.40)- o-TLD (0.75) p-DAB (0.40)- p-BAL (0.70) DMAL (0.37)- m-NAL (0.77)

few such separations achieved experimentally are listed in Table 4. These separations are not possible if only TX-100 is used as mobile phase. On the basis of results obtained with these mixed surfactant mobile phases (H20-TX-100-SDS), following trends are noticeable.

(a) The RF value versus SDS volume fraction plots that pass through maxima and minima show a minima at 0.5 volume fraction of SDS for p-AS, 10, p-PND, a-and p-NAL, a-CAL and AL in mobile phases containing 1.0 or 0.02 M TX-100. ID is the most affected amine with largest dip at minima.

(b) Amines such as p-TLD, m-CAL and p-DAB have minima when SDS volume fraction reache:-: 0.75 in both 0.1 or 0.02 M TX-100.

(c) For m-NAL or a- NPA and for DPA, minima occurs at 0.5 and 0.75 volume fractions of SDS respectively in mobile phase containing 0.1M TX-100 only.

Rs

3.52 2.16

1.18 1.73 4.00

2.14

1.46 3.3 6.10 2.45

3.57 3.00 2.11 1.18

3.18 2.06 4.16 3.10 2.74 2.43 2.27 2.76

3.50 3.50 2.00 3.22

(d) In mobile phases containing 0.02M TX-100, the minima for m-TLD occurs at SDS volume fraction of 0.5 whereas in the case of p-BAL and a-TLD minima was noticed at 0.75 volume fraction of SDS.

(e) The minima for p-CAL occurs at lower volume fraction (i.e. 0.33) of SDS in 0.02 or 0.1M TX-100 containing mobile phase.From the observations summarized above, it can be safely concluded that 0.01M SDS (an anionic surfactant) is most effective when its volume fraction in the mobile phase containing TX-100 is kept near or above 50%(i.e.volume fraction 0.5) irrespective of the fact that either the concentration of nonionic surfactant is above or below its CMC value. The RF value (or mobility) of individual amine changes by the factor of 10±5% when it is chromatographed as mixture. The separations achieved experimentally with some mobile phases have been summarized in Table 4. It is clear from

MOHAMMAD & KHAN: MIGRATION BEHAVIOUR OF AROMA TIC AMINES ON SILICA 251

Table 5-Detection and dilution limits of some amines achieved on silica gel layer developed with M37 (2: I)

Amines Lower Limit of Dilution Limit Detection ()..lg)*

DPA 0.12 (0.25) I : 8.333xl04

p-DAB 0.33 (0.40) I: 6.06xl04

o-NAL 0.07 (0.10) I : 14.285xi04

p-NAL 0.08 (0.12) I : 12.5xl04

m-CAL 0.09 (0.11) I: ll.lllx!04

DMAL 0. 14 (0.25) I : 7.142xl04

*The values in brackets refer to the lower limit of detection an silica layer developed with distilled water (i.e. zero surfactant)

Table 4 that DMAL (Primary amine) can be selectively separated from most of amines including DPA (secondary amine) and p-DAB(tertiary amine). An important separation of ID from p-DAB can easily be achieved with mobile phase M 10• This separation is particularly important because of the fact that indole reacts with p-DAB in acidic medium to yield a red condensation product20

. Thus the presence of p-DAB as an impurity in indole will interfere in the detection of other aldehydes by indole. To demonstrate the separation efficiency, the value of resolution factor (Rs) for the separations of amines from their two­component mixtures was calculated using the formula.

R = D s 0.5(dl + d2)

where D is the distance between centres of the resolved spots from a mixture and d 1 and d2 are respective average diameters. From high Rs values (Rs> 1.0) listed in Table 4, it is evident that the separations are highly efficient.

The proposed method is highly sensitive to primary aromatic amines and the amounts lower than 0.1. )lg of nitro- as well as chloro-anilines can be easily detected on silica layer using mixed surfactant mobile phases (Table 5). It is interesting to note that an enhanced sensitivity can be achieved with surfactant mediated

mobile phases as the detection limits for all amines are higher with water (zero surfactant) as mobile phase (Table 5).

The sensitivity of the method was found to decrease in the order: Primary (CAL or NAL)> secondary (DPA)> tertiary_p-DAB) aromatic amines. It is interesting to note that the detection limit of DMAL is about 2.4 times lower than that of p-DAB in spite of the fact that both amines possess the same number of -CH3 groups in the molecule.

Acknowledgement The authors are grateful to Prof. K.G. Varshney,

Chairman, Department of Applied Chemistry, Aligarh Muslim University, Aligarh for providing research facilities.

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