electrophoresis glycine-based polymeric surfactants with...

Shahab A. ShamsiRashid IqbalCevdet Akbay

Department of Chemistry,Center of Biotechnologyand Drug DesignGeorgia State University,Atlanta, GA, USA

Glycine-based polymeric surfactants with variedpolar head group: II. Chemical selectivity in micellarelectrokinetic chromatography using linearsolvation energy relationships

A series of four acyl and four alkenoxy glycinates (i.e., mono-, di-, tri-, and tetra-derivatives of polysodium N-undecenoyl glycinate (poly-SUGs) as well as poly-sodium N-undecenoxy carbonyl glycinates (poly-SUCGs)) were compared forsimultaneous separation of nonhydrogen bonding (NHB), hydrogen-bond acceptor(HBA), and hydrogen-bond donor (HBD) solutes. An increase in the number of gly-cine units in the polar head group of polymeric surfactant decreases both theretention and the migration window of all solutes with some changes in separationselectivity. The poly(sodium N-undecenoxy carbonyl-glycinate) (poly-SUCG1) withone glycine unit was the least polar surfactant and has the lowest phase ratio, butthis monoglycinate surfactant provided the best simultaneous separation of 10-NHBs and 8-HBAs. On the other hand, 9-HBDs were well separated using any ofthe six mono-, di-, and triglycinate surfactants compared to the two tetraglycinates.Linear solvation energy relationships (LSERs) and separation of the geometricalisomers studies were also performed to further envisage the selectivity differences.From LSER studies, the phase ratio and hydrogen-bond-donating strength of thepoly-SUG series of surfactant were found to increase with an increase in the size ofthe head group, but no clear trends were observed for poly-SUCG surfactants. Thecohesiveness for all poly-SUG and poly-SUCG was positive, but the values weregenerally lower (with exception of the poly(sodium N-undecenoyl glycyl-glycyl-gly-cinate)) at a higher number of glycine units. Finally, the poly(sodium N-undecenoylglycinate) and poly-SUCG1 were found to be the two best polymeric surfactants asthey provided relatively higher shape selectivity for separation of two of the threesets of geometrical isomers.

Keywords: Hydrogen-bond acceptor benzene derivatives / Hydrogen-bond donor benzenederivatives / Isomer selectivity / Nonhydrogen bonding benzene derivatives / Polysodium N-undecenoxy carbonyl glycinates DOI 10.1002/elps.200500363

1 Introduction

In recent years, the linear solvation energy relationship(LSER) model has been applied for the characterization ofretention and selectivity differences between differentpseudostationary phases in MEKC [1–7]. This model,which was initially developed by Kamlet et al. [8, 9],describes solvation effects on physicochemical pro-cesses. More recently, Platts and Abraham [10] showedimproved accuracy of some of the solute descriptors withnew symbols and modified the LSER model which can bewritten as

log K’ = C 1 vV 1 eE 1 sS 1 aA 1 bB (1)

Correspondence: Professor Shahab A. Shamsi, Department ofChemistry, Center of Biotechnology and Drug Design GeorgiaState University, P.O. Box 4098, Atlanta, GA 30302-4098, USAE-mail: [email protected]: 11-404-651-1416

Abbreviations: HBA, hydrogen-bond acceptor; HBD, hydrogen-bond donor; LSER, linear solvation energy relationship; NHB,nonhydrogen bonding; poly-SUCG1, poly(sodium N-undece-noxy carbonyl-glycinate); poly-SUCG2, poly(sodium N-undece-noxy carbonyl glycyl-glycinate); poly-SUCG3, poly(sodium N-undecenoxy carbonyl-glycyl-glycyl-glycinate); poly-SUCG4,poly(sodium N-undecenoxy carbonyl-glycyl-glycyl-glycyl-glyci-nate); poly-SUG1, poly(sodium N-undecenoyl glycinate); poly-SUG2, poly(sodium N-undecenoyl glycyl-glycinate); poly-SUG3,poly(sodium N-undecenoyl glycyl-glycyl-glycinate); poly-SUG4,poly(sodium N-undecenoyl glycyl-glycyl-glycyl-glycinate)

4138 Electrophoresis 2005, 26, 4138–4152

2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Electrophoresis 2005, 26, 4138–4152 Glycine-based polymeric surfactants with varied polar head group 4139

In Eq. (1), known solute descriptors, V, E, S, A, and B, arecorrelated to the logarithmic retention factor (log k0). V andE are measures of a solute’s characteristic volume and theexcess molar refraction, respectively. The solute polarityand polarizability are represented by the S term. The A andB terms represent the solute hydrogen-bond-donatingand the solute hydrogen-bond-accepting abilities,respectively. The system coefficients (C, v, e, s, a, and b)refer to differences in the two bulk phases, i.e. the aqueousand the pseudostationary phases, between which thesolute is transferring. The constant C represents the inter-cept and includes information about the phase ratio of thesystem. The v term is a measure of the relative ease ofcavity formationor hydrophobicityof the pseudostationaryphase for the solute. The coefficient e verifies the differ-ence in ability of the pseudostationary phase and theseparation buffer to interact with solute n- or p-electronswhile the s coefficient measures the polarizibility differencebetween the pseudostationary phase and the separationbuffer. The coefficients a and b are the hydrogen-bond-accepting and hydrogen-bond-donating strengths of thepseudostationary phase, respectively. Therefore, variouspolymeric surfactants can be compared in MEKC systemsemploying the same aqueous buffer.

Over the past ten years, several groups have utilizedLSER to characterize a variety of conventional micelleswith varied polar head groups [11–22] and chain lengths[23–26]. In contrast, only a small number of reports com-paring polymeric surfactants with different hydrocarbonchain lengths [1, 6, 27, 28] and polar head groups [29, 30]have been published. Palmer’s group [27] reported theuse of siloxane polymers as well as copolymers of2-acrylmido-2-methyl-1-propane sulfonic acid andmethacrylates with chain lengths of C8-C18 [6] to investi-gate selectivity differences. Although no substantial dif-ferences in chemical selectivity were observed usingeither class of polymers, alkyl-modified siloxane polymerswere found to be more cohesive and less polar and havehigh degrees of dipolarity/polarizibility as compared toSDS. In a related LSER study, the same research groupalso compared the allyl glycidyl ether N-methyl taurinesiloxane polymer to sulfite-modified siloxane [7]. Theynoted that the use of later polymer with a shorter linkerarm (between the siloxane backbone and the ionic headgroup) as well as the absence of tertiary amine group onthe polar head results in a significant change in chemicalselectivity. Recently, in a series of two publications, ourresearch group investigated four polymeric sodium alke-nyl sulfate surfactants with chain lengths of C8-C11 [1, 28].The LSER studies suggested that an increase in thehydrocarbon chain length of these polymeric surfactantsdecreases the polar nature and effectiveness of acidstrength, but increases the polarizibility.

To the best of our knowledge, only few reports comparingpolymeric surfactants with different anionic polar headgroup have been published [29, 30]. Fujimato [29] observedhigher separation efficiency with significantly different elu-tion pattern of several benzene derivatives when poly-sodium 11-acrylamidoundecanoate poly(Na 11-AAU) wascompared to SDS micelles or poly(sodium 10-undecyle-nate) in MEKC. The comparison on solvation properties ofthe three aforementioned micellar systems suggested thatstrong dipole-dipole and dipole-induced-dipole propertiesof poly(Na 11-AAU) result in unique selectivity for severalpolar solutes. Leonard and Khaledi [30] compared the sol-vation properties of the triblock copolymer (poly(methylmethacrylate-ethyl acrylate-methacrylic acid), Elvacite2669), SDS, and a mixed surfactant system of SDS andElvacite 2669. They observed significant selectivity differ-ences for nonhydrogen bonding (NHB) and hydrogen-bondacceptor (HBA) solutes when Elvacite 2669 and SDS areused as individual or mixed systems. In addition, the mixedSDS-Elvacite 2669 pseudophase was found to be lesscohesive and weaker hydrogen-bond donor (HBD) than theElvacite 2669 or SDS alone.

The present study is a continuation of Part I in which thesynthesis, characterization, and application of monomericand polymeric forms of sodium N-undecenoyl glycinates(SUGs) and sodium N-undecenoxy carbonyl glycinates(SUCGs) surfactants in MEKC were evaluated. In this secondpart, the effect of polar head for the simultaneous separationof 10-NHBs, 8-HBAs, and 9-HBDs solutes using all eightpolymeric surfactants was first compared. Next, LSER wasapplied to further evaluate the effect of hydrophilic headgroup on retention behavior and selectivity of 27-benzenederivatives. Since the selectivity differences between ben-zene derivatives were not very large in magnitude using thetetraglycinate derivatives of poly(sodium N-undecenoyl gly-cyl-glycyl-glycyl-glycinate) (poly-SUG4) and poly(sodiumN-undecenoxy carbonyl-glycyl-glycyl-glycyl-glycinate)(poly-SUCG4), no LSER analysis was performed using thesetwo polymeric surfactants. Finally, the six mono-, di-, and tri-glycinates of poly-SUG and poly-SUCG series were com-pared for the separation of geometrical isomers of phenyl-imidazoles, nitrotoluenes, and methoxy-phenethylamines.

2 Materials and methods

2.1 Instrumentation

Same as described in Part I [31].

2.2 Materials

The geometrical isomers of phenylimidazoles (1-phenyli-midazole, 2-phenylimidazole, 4-phenylimidazole), nitroto-luenes (2-nitrotoluene, 4-nitrotoluene, 3-nitrotoluene), and


CE

and

CE

C

4140 S. A. Shamsi et al. Electrophoresis 2005, 26, 4138–4152

methoxyphenethylamines (4-methoxyphenethylamines, 3-methoxyphenethylamines, 2-methoxy-phenethylamines)were obtained from Aldrich (Milwaukee, WI, USA). Otherchemicals are essentially the same as in Part I [31].

2.3 Preparation of micellar buffer solutions andsolute solution

These are the same as described in Part I [31].

2.4 CE procedure

The procedure is the same as described in Part I of thisseries [31].

2.5 Calculations

The retention factors, k0, of the test solutes were calcu-lated as previously reported [1, 28]. The system coeffi-cients (v, e, s, a, and b) described in Eq. (1) were deter-mined by multiple linear regression using SAS software(SAS Institute, Cary, NC, USA).

3 Results and discussion

3.1 Effects of the head group on MEKC separa-tion of NHB, HBA, and HBD solutes

Figures 1 and 2 demonstrate the simultaneous separationof 10-NHB solutes (solutes 1–10, Table 1) in MEKC usingpoly-SUG and poly-SUCG surfactants as pseudosta-tionary phases, respectively. A steady decrease in reten-tion times of NHB solutes was observed with an increasein the number of glycine molecules in polar head group ofboth poly-SUG and poly-SUCG surfactants. Thus, itappears that decrease in elution window and migrationtime is due to two important reasons: (i) increase in chro-matographic polarity of the polymeric surfactant and(ii) increase in zeta potential at the capillary surface.These observations are consistent with the data in Part Iof this series [31], which shows that the use of more polartriglycinates (e.g., poly(sodium N-undecenoyl glycyl-gly-cyl-glycinate) (poly-SUG3) and poly(sodium N-un-decenoxy carbonyl-glycyl-glycyl-glycinate) (poly-SUCG3)),as pseudostationary phase in MEKC, results in lower k0

for all 27 benzene derivatives than the less polar mono-glycinates (e.g., poly(sodium N-undecenoyl glycinate)

Figure 1. Comparison of (a)poly-SUG1, (b) poly-SUG2, (c)poly-SUG3, and (d) poly-SUG4

for the separation of NHB ben-zene derivatives. Conditions:17.4 mM at equivalent monomerconcentration (EMC) (mM) ofeach surfactant, 20 mM

NaH2PO4/Na2HPO4, pH 7.0.Pressure injection: 20 mbar, 4 s;130 kV applied voltage; UVdetection at 200 nm; 257C. Peakidentifications are the same asthose listed in Table 1.



Figure 2. Comparison of(a) poly-SUCG1, (b) poly-SUCG2,(c) poly-SUCG3, and (d) poly-SUCG4 for the separation ofNHB benzene derivatives. Otherconditions are the same asthose given in Fig. 1. Inset in (d)represents the expanded elec-trokinetic chromatogram of thefirst ten solutes in poly-SUCG4

surfactant. Peak identificationsare the same as those listed inTable 1.

(poly-SUG1) and poly(sodium N-undecenoxy carbonyl-glycinate) (poly-SUCG1)). In general, the elution order ofNHB solutes remained unchanged irrespective of the typeof head group except that the elution order of pro-pylbenzene (solute 8) and naphthalene (solute 9) wasreversed. Using both poly-SUCG1 and poly(sodiumN-undecenoxy carbonyl glycyl-glycinate) (poly-SUCG2),naphthalene was retained longer than propylbenzene.However, the elution order of these two solutes reverseswhen poly-SUCG3 and poly-SUCG4 were used as pseu-dostationary phases. These observed elution order rever-sals are most likely a result of poly-SUCG3 and poly-SUCG4 being more polar phases than poly-SUCG1 or poly-SUCG2. However, it was interesting to note that suchreversal was not observed in poly-SUG series of surfac-tants. Significant fronting was observed with respect to themost hydrophobic NHB analyte (e.g., biphenyl) using mostof the polymeric surfactants. This suggests that poor sol-vation capacity and polydispersity of the polymeric sur-factants are likely contributors to the peak asymmetry,especially for more hydrophobic compounds.

Several trends in differences in resolution and separationselectivity for certain NHB pairs were observed. First, theRs of all NHB solute pairs (with the exception of CTOL/

IBZ; for abbreviations see Table 1) are at least two to threetimes higher for mono-, di-, or triglycinates of poly-SUCGsurfactants (Fig. 2) as compared to poly-SUG surfactants(Fig. 1). This is consistent with the larger elution range(tmc/to) observed with the former class of polymeric sur-factant (Table 2, row 4 vs. row 9, Part I). For tetra-glycinates (poly-SUG4 and poly-SUCG4) very poor Rs

values were obtained (data not shown). This is probablybecause of smallest electrophoretic mobility (Table 2,column 9, Part I) and lowest phase ratio (due to smallestV values, Table 1, column 6) of tetraglycinates (comparedto mon-, di-, or triglycinates), which in turn decreases theelution window (Table 2, Part I). Second, the four (TOL/CBZ, CBZ/EBZ, EBZ/BrBZ, and IBZ/PBZ) out of ninepairs of NHB solutes were better resolved with poly-SUCG3 compared to poly-SUCG1 or poly-SUCG2, mainlydue to higher selectivity. For example, the selectivityvalues for these four pairs were 2.47, 1.60, 1.47, and 1.43using poly-SUCG3 versus 1.63/1.58, 1.34/1.19, 1.13/1.16,and 1.32/1.10 obtained with poly-SUG1/poly(sodium N-undecenoyl glycyl-glycinate) (poly-SUG2). Third, a com-parison between various pairs of NHB solutes shows thatthe most hydrophobic pair (e.g., NAP/BP, peak 9/10 inFigs. 1, 2) provided the highest Rs and a values. Interest-ingly, for this most hydrophobic pair of solutes, the poly-



Table 1. Test solutes and their solvation parameters

No. Solutes Solvation parameters

V E S A B

NHB(1) Benzene (BZ) 0.716 0.610 0.52 0.00 0.14(2) Toluene (TOL) 0.857 0.601 0.52 0.00 0.14(3) Chlorobenzene (CBZ) 0.839 0.718 0.65 0.00 0.07(4) Ethylbenzene (EBZ) 0.998 0.613 0.51 0.00 0.15(5) Bromobenzene (BrBZ) 0.891 0.882 0.73 0.00 0.09(6) 4-Chlorotoluene (CTOL) 0.980 0.705 0.67 0.00 0.07(7) Iodobenzene (IBZ) 0.975 1.188 0.83 0.00 0.12(8) Propylbenzene (PBZ) 1.139 0.604 0.50 0.00 0.15(9) Naphthalene (NAP) 1.085 1.360 0.92 0.00 0.20

(10) Biphenyl (BP) 1.324 1.360 0.99 0.00 0.22

HBAs(11) Phenethyl alcohol (PEA) 1.057 0.784 0.83 0.30 0.66(12) Acetophenone (AP) 1.014 0.818 1.01 0.00 0.48(13) Nitrobenzene (NBZ) 0.891 0.871 1.11 0.00 0.28(14) Methyl benzoate (MBZT) 1.073 0.733 0.85 0.00 0.46(15) 4-Nitrotoluene (NTOL) 1.032 0.870 1.11 0.00 0.28(16) 4-Chloroacetophenone (CAP) 1.136 0.955 1.09 0.00 0.44(17) Ethyl benzoate (EBZT) 1.214 0.689 0.85 0.00 0.46(18) 4-Chloroanisole (CAN) 1.038 0.838 0.86 0.00 0.24

HBA(19) Benzyl alcohol (BA) 0.916 0.803 0.87 0.33 0.56(20) Phenol (P) 0.775 0.805 0.89 0.60 0.30(21) 4-Fluorophenol (FP) 0.793 0.670 0.97 0.63 0.23(22) 3-Methylphenol (MP) 0.916 0.822 0.88 0.57 0.34(23) 4-Chloroaniline (CANL) 0.939 1.060 1.13 0.30 0.31(24) 3,5-Dimethylphenol (DMP) 1.057 0.820 0.84 0.57 0.36(25) 4-Ethylphenol (EP) 1.057 0.800 0.90 0.55 0.36(26) 3-Chlorophenol (CP) 0.898 0.909 1.06 0.69 0.15(27) 3-Bromophenol (BP) 0.950 1.080 1.17 0.67 0.20

Table 2. Head group effects of poly-SUG surfactants on the migration behavior of benzene derivatives in MEKC (n = 27)

Surfactantsystems

System constants Statistics

C E E S a b R2 a) SEb)

Poly-SUG1 22.566(60.091)c)

2.393(60.102)

0.608(60.078)

20.295(60.085)

0.298(60.047)

21.899(60.088)

0.986 0.055

Poly-SUG2 22.296(60.085)

2.067(60.095)

0.482(60.073)

20.268d)

(60.079)0.230(60.044)

21.845(60.082)

0.984 0.051

Poly-SUG3 22.000(60.124)

1.707(60.140)

0.648(60.108)

20.479d)

(60.116)0.293(60.064)

21.612(60.120)

0.961 0.075

a) Correlation coefficient of the linear regressionb) Standard error of the calculated log k0 valuesc) SD for each coefficientd) Values are not statistically significant at the 95% confidence level.



meric surfactant (i.e., poly-SUCG1) with the lowestaggregation number and highest partial specific volume(within its own series) provided the highest resolution(Rs = 45.2) and highest selectivity (a = 19.96). In addition,separation between 4-chlorotoluene (solute 6) and iodo-benzene (solute 7) was dramatically dropped whenswitching from poly-SUCG1 (Fig. 2a) to poly-SUCG3

(Fig. 2c), and then slightly increased when poly-SUCG4

was used. Nevertheless, a comparison of poly-SUG sur-factants (Fig. 1) versus poly-SUCG (Fig. 2) clearly showsthat the use of former class of polymeric surfactant alwaysresulted not only in shorter migration time, but also innarrow elution range than the latter. Thus, it appeared thatthe presence of extra carbon atom in the aliphatic tail ofpoly-SUCG series renders this class of surfactant morehydrophobic resulting in longer migration times. Overall,the mono-, di-, and triglycinate of poly-SUCG and poly-SUCG class of surfactants showed satisfactory separa-tion of NHB solutes. In contrast, using either poly-SUG4

(Fig. 1d) or poly-SUCG4 (Fig. 2d), the same NHB soluteseluted very close with lower Rs and a as well as narrowelution windows of only 4 and 7 min, respectively.

Electropherograms in Figs. 3 and 4 show the separationof 8-HBA solutes using four poly-SUG and four poly-SUCG surfactant systems, respectively. The HBA solutesshowed similar elution pattern in all eight surfactant sys-

tems. Similar to NHB analytes, an increase in size of thehead group of either poly-SUG or poly-SUCG providedfaster separation of HBAs. The Rs of various HBA pairsunder investigation were affected differently with changesin the polar head group. Similar to NHB solutes, HBAsconsistently provided significantly higher Rs using alke-noxy compared to acyl polymeric surfactants. For exam-ple, Rs values were at least two- to threefold higher withpoly-SUCG than with poly-SUG surfactants (data notshown). Furthermore, it can be seen from Fig. 3a thatpoly-SUG1 could not resolve acetophenone (solute 12)and nitrobenzene (solute 13), whereas the same pair wassuccessfully resolved with poly-SUCG1 (Fig. 4a) as wellas with other polymeric surfactants. Similarly, 4-chlor-oacetophenone (solute 16) and ethyl benzoate (solute 17)pair was very well resolved using poly-SUCG1 (Fig. 4a),almost baseline resolved using poly-SUG1 (Fig. 3a) andpoly-SUCG3 (Fig. 4c), partially resolved with poly-SUCG4,(Fig. 4d), but was unresolved using any of the remainingpolymeric surfactants. Overall, poly-SUCG1 appears tobe the best polymeric surfactant since it provided thehighest Rs (mainly due to higher selectivity and large elu-tion range) toward most of the of HBA solutes. Althoughthis particular surfactant has the same number of carbonatoms in the hydrophobic tail (e.g., SUCG2 and SUCG3),the smaller and less sterically hindered polar head groupallows easy access to H-bonding interactions.

Figure 3. Comparison of (a)poly-SUG1, (b) poly-SUG2, (c)poly-SUG3, and (d) poly-SUG4

for the separation of HBA ben-zene derivatives. Other condi-tions are the same as thosegiven in Fig. 1. Peak identifica-tions are the same as those list-ed in Table 1.



Figure 4. Comparison of (a)poly-SUCG1, (b) poly-SUCG2,(c) poly-SUCG3, and (d) poly-SUCG4 for the separation ofHBA benzene derivatives. Otherconditions are the same asthose given in Fig. 1. Peak iden-tifications are the same as thoselisted in Table 1.

The simultaneous separation of 9-HBDs is illustrated inFig. 5 (poly-SUG surfactants) and Fig. 6 (poly-SUCG sur-factants). It is clear that almost all the mono-, di-, and tri-glycinate polymeric surfactants (Figs. 5a–c, 6a–c) aregood HBAs, showing simultaneous baseline or nearbaseline separation of HBD solutes. Since all of these sixpolymeric surfactants possess multiple HBA functionality,favorable separation selectivities of HBD solutes are nottoo surprising. Similar to the trends observed for NHB andHBA solutes, the use of poly-SUCG surfactants as pseu-dostationary phases always provided longer retentiontimes for HBD solutes than the corresponding poly-SUGsurfactants (at equivalent number of glycine units). Again,the separation selectivities were not as remarkable for 9-HBDs using poly-SUG4 (Fig. 5d) or poly-SUCG4 (Fig. 6d).Therefore, these two polymeric surfactants were notincluded in LSER analysis, which is discussed below.

3.2 LSER analysis

A total of 27 benzene derivatives used in this study andtheir descriptor values are listed in Table 1. Based on theirhydrogen-bond abilities, the solutes in Table 1 can becharacterized as NHB (solutes 1–10), HBAs (solutes 11–18), and HBDs (solutes 19–27).

3.2.1 Normalized residuals as a function ofsolute number

The retention behavior of all 27 test solutes in polymericSUG and SUCG surfactants was examined, and the sys-tem constants were calculated by multiple linear regres-sion. The statistical validity of LSER was evaluatedthrough the correlation coefficient (R2) and the standarderror of the estimate (SE). Relatively lower SE values wereobserved with poly-SUG surfactant systems (Table 2)relative to poly-SUCG surfactants (Table 3) due to a fewoutlying solutes. Figures 7a–f show the outliers in all sixpseudostationary phases. The following approach wasfollowed to determine outlier solutes. First, residualvalues of log k0 (experimental log k0 minus calculatedlog k0) were calculated. Then, normalized residuals (i.e.residual divided by the SD of the residual) were com-puted. Finally, the normalized residual values of log k0

were plotted against the solute number. When the nor-malized residuals were zero or nearly zero, the best fitbetween experimental log k0 and calculated log k0 valueswas obtained. However, normalized residuals in a rangeof 12 to 22 are reasonable for statistically sound corre-lations. Note that biphenyl (solute 10) is an outlier in allthree poly-SUCG surfactants (Figs. 7d–f), whereas benzylalcohol (solute 19) and benzene (solute 1) are the main



Figure 5. Comparisonof (a)poly-SUG1, (b) poly-SUG2, (c) poly-SUG3, and (d) poly-SUG4 for theseparation of HBD benzene deri-vatives. Other conditions are thesame as those given in Fig. 1.Peak identifications are the sameas those listed in Table 1.

Figure 6. Comparison of (a)poly-SUCG1, (b) poly-SUCG2,(c) poly-SUCG3, and (d) poly-SUCG4 for the separation ofHBD benzene derivatives. Otherconditions are the same asthose given in Fig. 1. Peak iden-tifications are the same as thoselisted in Table 1.



Figure 7. Normalized residualsfrom (a) poly-SUG1, (b) poly-SUG2, (c) poly-SUG3, (d) poly-SUCG1, (e) poly-SUCG2, and(f) poly-SUCG3 surfactant sys-tem. Other conditions are thesame as those given in Fig. 1.

outliers in poly-SUG3 (Fig. 7c) and poly-SUCG3 (Fig. 7f),respectively. These data indicate that the number andtype of outliers may vary depending on the polymericsurfactant used.

3.2.2 Effect of the polar head group on phaseratio

The LSER constants and the statistics for all the pseudo-stationary phases using solvation parameter model (Eq. 1)are listed in Tables 2 and 3. As mentioned previously,the constant C represents the intercept and reflects differ-ences in phase ratio. The regression constant C is largeand negative for all the surfactant systems studied. Thephase ratio increases with an increase in the number ofhydrogen bonding sites in the head group of poly-SUGsurfactants (e.g., Cpoly-SUG3 . Cpoly-SUG2 . Cpoly-SUG1). Thistrend differs with the partial specific volume results repor-ted in Part I [31]. For example, for poly-SUG series of sur-factants the chromatographic phase ratio (b) calculatedfrom the proposed equation [1] follows the order:(bpolySUG1 = 0.0038, bpolySUG2 = 0.0037, bpolySUG3 = 0.0037,

bpolySUG4 = 0.0031), which seems essentially constant andis slightly lower for poly-SUG4. In contrast, the polymericSUCG surfactants do not show the same trend ofb seen in polymeric SUG surfactants. For the poly-SUCGseries, the b values follow the order: (bpolySUCG1 = 0.0033,bpolySUCG2=0.0031,bpolySUCG3=0.0020,bpolySUCG4=0.0014).Comparison of b and C values indicates that poly-SUCG1

provided essentially the same b value as poly-SUCG2, butthe C values for these two polymers are significantly dif-ferent (Table 3, column 2). However, the same two poly-meric surfactants have higher b values compared to poly-SUCG3 and poly-SUCG4. Thus, the trend in b values foronly poly-SUCG2 and poly-SUCG3 was found to be con-sistent with the C values.

3.2.3 Effect of the polar head group oncohesiveness and dispersion interactions

The vV term indicates the free energy change due tohydrophobic interactions. Larger coefficient v valuesindicate smaller cohesive energy of the pseudostationaryphase. In other words, less energy is required to form a



Table 3. Head group effects of poly-SUCG surfactants on the migration behavior of benzene derivatives in MEKC (n = 27)

Surfactantsystems

System constants Statistics

C v e s a b R2 a) SEb)

Poly-SUCG1 23.154(60.209)c)

3.277(60.235)

0.884(60.182)

20.442(60.196)

0.222(60.108)

22.349(60.203)

0.959 0.126

Poly-SUCG2 22.220(60.112)

2.085(60.126)

0.219(60.098)

20.085d)

(60.105)0.233(60.058)

21.757(60.109)

0.967 0.068

Poly-SUCG3 22.904(60.235)

3.024(60.264)

0.492(60.204)

20.099d)

(60.220)0.140(60.122)

22.564(60.228)

0.941 0.142

a) Correlation coefficient of the linear regressionb) Standard error of the calculated log k0 valuesc) SD for each coefficientd) Values are not statistically significant at the 95% confidence level.

cavity on the pseudostationary phase to accommodatethe solutes. Positive sign of the coefficient v indicatesthat solutes prefer to transfer from more cohesive (i.e.more polar) aqueous phase to less cohesive (i.e. morenonpolar) pseudostationary phases. For interpretationpurposes, it is also important to understand that cohe-siveness is a measure of the free energy (that includesboth enthalpy and entropy contribution) of cavity for-mation with respect to water. The value of coefficient vdecreases with an increase in the number of glycine inthe head group of the polymeric SUG surfactants(Table 2). For example, poly-SUG3 micelles with threeglycines on its head group are more compact and lessorganic-like (smaller coefficient v) as compared to poly-SUG1 and poly-SUG2 micelles. On the other hand, poly-SUG1 shows more organic character and requires lessenergy to create a cavity for a solute. It is noticeablefrom Tables 2, 3 that poly-SUCG1 has the highest vvalue (3.277) among all six surfactant systems; thus, it isthe least polar surfactant system. Poly-SUCG3 (v = 3.024)is slightly more polar than poly-SUCG1 but less polarthan poly-SUCG2. Thus, the six surfactant systemscan be ordered according to their v coefficient as:vpoly-SUCG1 . vpoly-SUCG3 . vpoly-SUG1 . vpoly-SUG2 <vpoly-SUCG2 . vpoly-SUG3. Although there are exceptions(e.g. poly-SUCG3), the values are generally lower at ahigher number of glycine units in the polar head group.This would indicate that as the number of hydrogenbonding group is increased cohesiveness increases.Poly-SUCG1 and poly-SUG3 show greatest and smallesthydrophobic interactions with the solutes, respectively.Thus, hydrophobic NHB solutes have strongest affinityfor poly-SUCG1 (Fig. 1a) than the remaining polymericsurfactants. Furthermore, cavity formation in poly-SUCG1 micelles requires relatively smaller energy.

Hence, partitioning of solutes will be more favorable inthe poly-SUCG1 phase than with the aqueous bufferphase.

3.2.4 Effect of polar head group on dipolarityand polarizability

As described earlier, coefficient e represents the ability ofmicellar phase and aqueous phase to interact withsolutes n- or p-electrons. The data in Tables 2 and 3 showthat all the polymeric poly-SUG and poly-SUCG surfac-tants have positive e coefficient values. Hence, comparedwith aqueous buffer system, these surfactant systemscan easily become polarized through interacting withsolutes’ n- and p-electrons. Due to its relatively highestpositive e coefficient value (e = 0.884), poly-SUCG1

micellar phase shows the strongest interaction withneighboring solute n- and p-electrons and hencebecomes polarized most easily (Table 3). In contrast,poly-SUCG2 has the least polarizability among all surfac-tant systems (e = 0.219). Thus, the polymeric surfactantsystems used in this study can be ordered accordingto their polarizability strength: epoly-SUCG1 . epoly-SUG3 .

epoly-SUG1 . epoly-SUCG3 < epoly-SUG2 . epoly-SUCG2.

Except poly-SUCG2 and poly-SUCG3, all pseudosta-tionary phases have statistically significant negativecoefficient s values (Tables 2, 3). Thus, all pseudosta-tionary phases are less polar than the aqueous phase,except the two aforementioned polymeric surfactantswhose s coefficients are statistically insignificant at the95% confidence level, and thus their dipolarity is similar tothat of aqueous phase. In addition, less negative s coeffi-cients of the two pseudostationary phases (poly-SUG1

and poly-SUG2) show that they are relatively more polar



compared to other polymeric surfactants. Consequently,these two polymeric surfactants should interact morewith polar compounds. Based on coefficient s valuesreported in Tables 2 and 3, polarity of the surfactants canbe ordered as: spoly-SUG2 < spoly-SUG1 . spoly-SUCG1 .

spoly-SUG3. The polarity of the glycinate polymers arisesfrom the carbonyl functionality on the head group. More-over, the carbamates surfactants (e.g., poly SUCG1) areless polar than amide surfactants (e.g., poly-SUG1), andthis lower polarity resulted in longer retention with theformer polymeric surfactant.

3.2.5 Effect of the polar head group on hydrogenbonding

A positive coefficient a obtained for all six polymeric sur-factants (Tables 2, 3) indicates that the hydrogen-bond-accepting strength (i.e., basicity) of these pseudosta-tionary phases is greater than that of aqueous phase.However, the coefficient a value for one polymeric sur-factant (i.e., poly-SUCG3) is statistically insignificant. Inother words, the hydrogen-bond-accepting strength ofthis pseudostationary phase is not much different fromhydrogen-bond-accepting strength of the aqueousphase. In general, the poly-SUCG surfactants areexpected to have more basic character than the poly-SUG surfactant due to the presence of oxygen atomadjacent to the carbonyl group at the N-terminal end,which offers extra HBA site for the solutes. However, thedata in Tables 2 and 3 do not support this assumption.This is probably due to the fact that this extra oxygenatom in poly-SUCG surfactants is involved in intramo-lecular hydrogen bonding between surfactant monomersrather than hydrogen bonding with the solutes.

The coefficient b is related to the difference in hydrogen-bond-donating ability (i.e., acidity) of the pseudosta-tionary phase and that of aqueous phase. The negativesign of coefficient b indicates that all pseudostationaryphases are less acidic than the aqueous phase. This is notsurprisingsince the surfactant has fewer protons to donateas compared to water molecules in aqueous solution. Thepseudostationary phases with larger (or less negative)b values provide stronger HBD sites for solute interaction.All the polymeric SUG and SUCG surfactants havehydrogen-bond-donating sites in their head group. Basedon the coefficient b values listed in Tables 2 and 3, therelative hydrogen-bond-donating strength of the poly-meric SUG and SUCG surfactants can be ordered asbpoly-SUG3 . bpoly-SUCG2 . bpoly-SUG2 . bpoly-SUG1

. bpoly-SUCG1 . bpoly-SUCG3. Thus, the poly-SUCG1 andpoly-SUCG3 with the largest negative b coefficients pro-vide the weakest, while the poly-SUG3 and poly-SUCG2

provide the strongest proton-donating micellar environ-

ment for the solutes. The strength of the hydrogen-bond-donating ability of polymeric SUG surfactants decreaseswith a decrease in the number of glycine unit on the headgroup of the surfactants. Therefore, poly-SUG3 has thestrongest whereas poly-SUG1 has the weakest interac-tions with HBA solutes. Unlike polymeric SUG surfac-tants, no clear trend of hydrogen-bond-donating ability inpolymeric SUCG was observed.

3.3 Effect of the head group on isomerselectivity

Three sets of isomers (i.e., phenylimidazole, nitrotoluene,and methoxyphenethylamine derivatives) were used toexamine the shape selectivity (ashape) of six polymericsurfactant systems. All the pseudostationary phasesused in this study showed significant selectivity differ-ences toward the geometrical isomers. Figure 8 illustratesthe separation of three phenylimidazole isomers using thethree poly-SUG (a–c) and three poly-SUG (d–f) surfactantsystems. The elution order of phenylimidazole isomersemploying all six polymeric surfactants was essentiallythe same: 1-phenylimidazole , 2-phenylimidazole , 4-phenylimidazole. A closer look at the chemical structuresof phenylimidazole isomers (Fig. 8, insets) reveals thatone hydrogen-bonding site of 1-phenylimidazole is hin-dered by the bulky five-membered ring, thus eliminatinghydrogen bonding with the surfactant systems. Hence,1-phenylimidazole elutes in a much shorter time as com-pared to the remaining two isomers. All the remainingpolymeric surfactants (except poly-SUCG1) provided Rs

and ashape of the three geometrical isomers. As the num-ber of glycine units in poly-SUG surfactants increases theRs and ashape values between the solute pairs 1–2 and 2–3decrease (Figs. 8a–c, inset tables). However, as seen inFig. 8d, monoglycinate of SUCG (i.e., poly-SUCG1) sur-factant could not separate the last two phenylimidazoleisomers. In contrast, poly-SUG2 and poly-SUG3 surfac-tants successfully resolved all three phenylimidazole iso-mers in a relatively shorter time. Moreover, a quick com-parison of poly-SUCG2 versus poly-SUCG3 reveals thatfor the first pair (1-phenylimidazole/2-phenylimidazole)the Rs and ashape decrease, whereas for the second pair(2-phenyl imidazole/4-phenyl imidazole) the Rs and ashape

increase (Figs. 8e and f, inset tables). However, the effi-ciency is always better for all three isomers with the latterpolymeric surfactant.

In Fig. 9 is shown the MEKC separation of three nitroto-luene isomers using both poly-SUG (a–c) and poly-SUCG(d–f) surfactant systems. The poly-SUG1 and poly-SUCG1

provided resolution of all three geometrical isomers withelution order as follows: o-(2-nitrotoluene) , p-(4-nitrotoluene) , m-(3-nitrotoluene). However, only two



Figure 8. Head group effects onthe elution order, resolution (Rs),and shape selectivity (ashape)of (1) 1-phenylimidazole, (2) 2-phenylimidazole, (3) 4-phenyli-midazole isomers using (a) poly-SUG1, (b) poly-SUG2, (c) poly-SUG3, (d) poly-SUCG1, (e) poly-SUCG2, and (f) poly-SUCG3.Other separation conditions arethe same as those given inFig. 9.

Figure 9. Head group effects onthe elution order, resolution (Rs),and shape selectivity (ashape) of(1) 2-nitrotoluene, (2) 4-nitroto-luene, (3) 3-nitrotoluene using(a) poly-SUG1, (b) poly-SUG2, (c)poly-SUG3, (d) poly-SUCG1, (e)poly-SUCG2, and (f) poly-SUCG3.Other separation conditions arethe same as those given inFig. 1.



isomers were resolved with the remaining four polymericsurfactants. It is interesting to note that the polymericsurfactants (e.g., poly-SUG1 and poly-SUCG1) with lesshydrogen bonding sites and more negative b values pro-vided highest shape selectivity. In contrast, previous workshowed that increasing the number of hydrogen-bondingsites on the polar head group of polymeric surfactantimproves chiral separations [32]. This is despite the factthat no significant differences in migration times of geo-metrical isomers were observed when poly-SUG1 wascompared to poly-SUG2 and poly-SUG3. A further com-parison of the three poly-SUG surfactants (Figs. 9a–c)versus the three poly-SUCG (Figs. 9d–f) clearly showsthat the increase in migration times using the latter classof polymeric surfactants did not improve the ashape ofnitrotoluene isomers. Although the differences in Rs wereobtained for nitrotoluene isomers using the six polymericsurfactants, no change in the elution order was observedand remained to be o , p , m.

Finally, the MEKC separation of three methoxy-phenethylamine isomers (containing both HBD andHBA sites) is shown in Figs. 10a–f. Unlike the separationof nitrotoluene isomers it is interesting to note thatelution times of methoxyphenethylamine isomers were

almost half when poly-SUG1 and poly-SUCG1 werereplaced by poly-SUG2 and poly-SUCG2, respectively.In contrast, only slight increase in elution time of thesame three isomers was observed when poly-SUCG2

was replaced by poly-SUCG3. Interestingly, poly-SUG3

provided longer elution times as compared to poly-SUG2 or poly-SUG1. Similar to the separation ofnitrotoluene isomers, both poly-SUG1 and poly-SUCG1

provided better separation selectivity of all threeisomers with the following elution order: 4-methoxy-phenethylamine , 3-methoxyphenethylamine , 2-me-thoxy-phenethylamine. The elution order of the isomerscan be attributed to both intramolecular H-bonding(between NH2 functional group and O atom) as well asintermolecular H-bonding between isomers and polarhead group of the surfactants. For example, in 2-meth-oxyphenethylamine intramolecular and intermolecularhydrogen bondings are stronger than in 4-methox-yphenethylamine. Hence, the former isomer is retainedlonger than the latter in all six polymeric surfactant sys-tems. However, due to the basic nature of these com-pounds severe peak tailings were observed. Again, theremaining four di- and triglycinate polymeric surfactantsprovided only partial or no Rs of the last two isomers ofmethoxyphenethylamine.

Figure 10. Head group effectson the elution order, resolution(Rs), and shape selectivity(ashape) of (1) 4-methoxyphen-ethylamine, (2) 3-methoxyphen-ethylamine, (3) 2-methoxyphen-ethylamine isomers using(a) poly-SUG1, (b) poly-SUG2,(c) poly-SUG3, (d) poly-SUCG1,(e) poly-SUCG2, and (f) poly-SUCG3. Other separation con-ditions are the same as thosegiven in Fig. 1.



4 Concluding remarks

Simultaneous separations of 10-NHB, 8-HBA, and 9-HBDsolutes were compared in MEKC using four poly-SUGand four poly-SUCGs. Each polymeric surfactant con-tains a C11 hydrophobic tail along with a head group ofmono-, di-, tri-, or tetraglycinate. It was seen (Figs. 1–6)that decrease in analysis time for 27-benzene derivatives(i.e., 10-NHB, 8-HBA, and 9-HBD solutes) was due to anincrease in the size as well as polarity of the head group ofpoly-SUG and poly-SUCG surfactants. Overall, simulta-neous separation of NHB and HBA solutes was bestachieved with poly-SUCG1, whereas HBD solutes pro-vided equally good Rs using any of the six mono-, di-, andtriglycinates of poly-SUG and poly-SUCG surfactants. Incontrast, poly-SUG4 and poly-SUCG4 showed very fastseparation, but poor selectivity for all three classes ofsolutes.

The retention behavior of the 27-benzene derivatives wasalso compared for three poly-SUG and three poly-SUCGsurfactants using the LSER model. The normalized resi-duals as a function of solute number for outlier studyshowed that the number and type of outliers might varydepending on the polymeric surfactant used. For poly-SUG surfactants, the phase ratio increases (C) with anincrease in the size of polar head group, whereas thepoly-SUCG surfactants do not show any such trend. Thecoefficient v value shows that cohesiveness increaseswith an increase in the size of head group of poly-SUGsurfactants. For poly-SUG series, poly-SUG1 and poly-SUG3 provided the least and the most polar environment,respectively. Although poly-SUCG1 provided the highestv value and was consequently the least polar surfactant,no significant and clear trend was seen for poly-SUCGseries of surfactants. The poly-SUCG1 and poly-SUG3 aremore easily polarized (e) upon interacting with the solutesn- and/or p-electrons. In addition, poly-SUG1 and poly-SUG2 have the highest polar character as compared tothe remaining surfactant systems. It should be noted thatthe amide surfactants (e.g., poly-SUG1) are more polarthan carbamates (e.g., poly-SUCG1). This is because thecarbonyl carbon is a part of the hydrophobic tail in theformer polymeric surfactant, while there is an extra car-bon atom in the hydrophobic tail of the latter surfactant.The more positive sign of coefficient a indicates that theaqueous buffer phase is less basic compared to thepolymeric SUG and SUCG phases. However, the smallerabsolute values for the a coefficient for all poly-SUCGapparently reflect that the hydrogen-bond-acceptingcharacteristics of these micellar phases are not muchdifferent from those in the bulk aqueous phase, andthereby has an insignificant influence on solute partition-ing. In general, poly-SUG1 and poly-SUCG2 are the

strongest HBA phases, while poly-SUG2 and poly-SUCG3

are the weakest HBA phases. The hydrogen-bond-donating ability of micellar phase has a greater impact onMEKC retention and selectivity, as well as solute-micelleinteraction. The negative and large coefficient b for allpolymeric surfactants indicates that these pseudosta-tionary phases are less acidic than the aqueous buffersolution. The polymeric surfactant with the least negativeb coefficient is the strongest proton-donating phase.Based on the coefficient b values listed in Tables 2 and 3,poly-SUCG1 and poly-SUCG3 provided the weakest whilepoly-SUG3 and poly-SUCG2 provided the strongesthydrogen-bond-donating environment for the solutes.

The use of polymeric surfactants in MEKC providedunique separation capabilities for shape selectivity ofgeometrical isomers. Major selectivity differences wereobserved when poly-SUG and poly-SUCG surfactantswere utilized for MEKC separation of phenylimidazole,nitrotoluene, and methoxyphenethylamine isomers. Sur-prisingly, most of the polymeric surfactants (except poly-SUCG1) provided baseline separation of phenylimidazoleisomers. On the other hand, among the six polymericsurfactants the two monoglycinates (i.e., poly-SUG1 andpoly SUCG1) were clearly the most selective pseudosta-tionary phases being able to resolve all three geometricalisomers of nitrotoluenes and methoxyphenethylamines.

This work was supported by a grant from the NationalInstitutes of Health (Grant No. GM 62314-02) and thePetroleum Research Fund (Grant No. 35473-G7).

Received May 12, 2005Revised August 13, 2005Accepted August 14, 2005

5 References

[1] Akbay, C., Shamsi, S. A., Electrophoresis 2004, 25, 622–634.

[2] Poole, C. F., Poole, S. K., Abraham, M. H., J. Chromatogr. A1998, 798, 207–222.

[3] Chen, N., Zhang, Y., Terabe, S., Nakagawa, T., J. Chroma-togr. A 1994, 678, 327–332.

[4] Yang, S., Khaledi, M. G., Anal. Chem. 1995, 67, 499–510.[5] Rosés, M., Ràfols, C., Bosch, E., Martínez, A. M., Abraham,

M. H., J. Chromatogr. A 1999, 845, 217–226.[6] Shi, W., Peterson, D. S., Palmer, C. P., J. Chromatogr. A

2001, 924, 123–135.[7] Schulte, S., Palmer, C. P., Electrophoresis 2003, 24, 978–

983.[8] Kamlet, M. J., Taft, R. W., J. Am. Chem. Soc. 1976, 98,

2886–2894.[9] Kamlet, M. J., Doherty, R. M., Abraham, M. H., Marcus, Y.,

Taft, R. W., J. Phys. Chem. 1988, 92, 5244–5255.[10] Platts, A. J., Du, M. C., Abraham, M. H., J. Org. Chem. 2000,

65, 7114–7122.



[11] Yang, S., Bumgarner, J. G., Khaledi, M. G., J. Chromatogr. A1996, 738, 265–274.

[12] Muijselaar, P. G., Claessens, H. A., Cramers, C. A., Anal.Chem. 1997, 69, 1184–1191.

[13] Poole, S. K., Poole, C. F., Anal. Commun. 1997, 34, 57–62.

[14] Poole, S. K., Poole, C. F., Analyst 1997, 122, 267–274.

[15] Poole, S. K., Poole, C. F., J. High Resolut. Chromatogr. 1997,20, 174–178.

[16] Khaledi, M. G., Bumgarner, J. G., Hadjmohammadi, M., J.Chromatogr. A 1998, 802, 35–47.

[17] Liu, Z., Zou, H., Ye, M., Ni, J., Zhang, Y., J. Chromatogr. A1999, 863, 69–79.

[18] Trone, M. D., Khaledi, M. G., Anal. Chem. 1999, 70, 1270–1277.

[19] Trone, M. D., Khaledi, M. G., Electrophoresis 2000, 21,2390–2396.

[20] Trone, M. D., Khaledi, M. G., J. Chromatogr. A 2000, 886,245–257.

[21] Fuguet, E., Ràfols, C., Bosch, E., Rosés, M., Abraham, M.H., J. Chromatogr. A 2001, 907, 257–265.

[22] Jandera, P., Fischer, J., Jebavá, J., Effenberger, H., J. Chro-matogr. A 2001, 914, 233–244.

[23] Crosby, D., El Rassi, Z., J. Liq. Chromatogr. 1993, 16, 2116–2122.

[24] Takeda, S., Wakida, S., Yamane, M., Higashi, K., Terabe, S.,J. Chromatogr. A 1996, 744, 135–139.

[25] Vitha, M. F., Carr, P. W., Sep. Sci. Technol. 1998, 33, 2075–2100.

[26] Trone, M. D., Khaledi, M. G., J. Microcol. Sep. 2000, 12,433–441.

[27] Peterson, D. S., Palmer, C. P., Electrophoresis 2001, 22,3562–3566.

[28] Akbay, C., Shamsi, S. A., Electrophoresis 2004, 25, 622–634.

[29] Fujimoto, C., Electrophoresis 2001, 22, 978–983.[30] Leonard, M. S., Khaledi, M. G., J. Sep. Sci. 2002, 25, 1019–

1026.[31] Iqbal, R., Rizvi, S. A. A., Shamsi, S. A., Electrophoresis 2005,

26, 4127–4137.[32] Shamsi, S. A., Macossay, J. M., Warner, I. M., Anal. Chem.

1997, 69, 2980–2987.


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