4138
Shahab A. Shamsi
Rashid Iqbal
Cevdet Akbay
Department of Chemistry,
Center of Biotechnology
and Drug Design
Georgia State University,
Atlanta, GA, USA
Electrophoresis 2005, 26, 4138–4152
Glycine-based polymeric surfactants with varied
polar head group: II. Chemical selectivity in micellar
electrokinetic chromatography using linear
solvation energy relationships
A series of four acyl and four alkenoxy glycinates (i.e., mono-, di-, tri-, and tetraderivatives of polysodium N-undecenoyl glycinate (poly-SUGs) as well as polysodium N-undecenoxy carbonyl glycinates (poly-SUCGs)) were compared for
simultaneous separation of nonhydrogen bonding (NHB), hydrogen-bond acceptor
(HBA), and hydrogen-bond donor (HBD) solutes. An increase in the number of glycine units in the polar head group of polymeric surfactant decreases both the
retention and the migration window of all solutes with some changes in separation
selectivity. The poly(sodium N-undecenoxy carbonyl-glycinate) (poly-SUCG1) with
one glycine unit was the least polar surfactant and has the lowest phase ratio, but
this monoglycinate surfactant provided the best simultaneous separation of 10NHBs and 8-HBAs. On the other hand, 9-HBDs were well separated using any of
the six mono-, di-, and triglycinate surfactants compared to the two tetraglycinates.
Linear solvation energy relationships (LSERs) and separation of the geometrical
isomers studies were also performed to further envisage the selectivity differences.
From LSER studies, the phase ratio and hydrogen-bond-donating strength of the
poly-SUG series of surfactant were found to increase with an increase in the size of
the head group, but no clear trends were observed for poly-SUCG surfactants. The
cohesiveness for all poly-SUG and poly-SUCG was positive, but the values were
generally lower (with exception of the poly(sodium N-undecenoyl glycyl-glycyl-glycinate)) at a higher number of glycine units. Finally, the poly(sodium N-undecenoyl
glycinate) and poly-SUCG1 were found to be the two best polymeric surfactants as
they provided relatively higher shape selectivity for separation of two of the three
sets of geometrical isomers.
Keywords: Hydrogen-bond acceptor benzene derivatives / Hydrogen-bond donor benzene
derivatives / Isomer selectivity / Nonhydrogen bonding benzene derivatives / Polysodium Nundecenoxy carbonyl glycinates
DOI 10.1002/elps.200500363
Correspondence: Professor Shahab A. Shamsi, Department of
Chemistry, Center of Biotechnology and Drug Design Georgia
State University, P.O. Box 4098, Atlanta, GA 30302-4098, USA
E-mail: [email protected]
Fax: 11-404-651-1416
Abbreviations: HBA, hydrogen-bond acceptor; HBD, hydrogenbond donor; LSER, linear solvation energy relationship; NHB,
nonhydrogen bonding; poly-SUCG1, poly(sodium N-undecenoxy carbonyl-glycinate); poly-SUCG2, poly(sodium N-undecenoxy carbonyl glycyl-glycinate); poly-SUCG3, poly(sodium Nundecenoxy carbonyl-glycyl-glycyl-glycinate); poly-SUCG4,
poly(sodium N-undecenoxy carbonyl-glycyl-glycyl-glycyl-glycinate); poly-SUG1, poly(sodium N-undecenoyl glycinate); polySUG2, 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)
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
In recent years, the linear solvation energy relationship
(LSER) model has been applied for the characterization of
retention and selectivity differences between different
pseudostationary phases in MEKC [1–7]. This model,
which was initially developed by Kamlet et al. [8, 9],
describes solvation effects on physicochemical processes. More recently, Platts and Abraham [10] showed
improved accuracy of some of the solute descriptors with
new symbols and modified the LSER model which can be
written as
log K’ = C 1 vV 1 eE 1 sS 1 aA 1 bB
(1)
Glycine-based polymeric surfactants with varied polar head group
In Eq. (1), known solute descriptors, V, E, S, A, and B, are
correlated to the logarithmic retention factor (log k0 ). V and
E are measures of a solute’s characteristic volume and the
excess molar refraction, respectively. The solute polarity
and polarizability are represented by the S term. The A and
B terms represent the solute hydrogen-bond-donating
and 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 aqueous
and the pseudostationary phases, between which the
solute is transferring. The constant C represents the intercept and includes information about the phase ratio of the
system. The v term is a measure of the relative ease of
cavity formation or hydrophobicity of the pseudostationary
phase for the solute. The coefficient e verifies the difference in ability of the pseudostationary phase and the
separation buffer to interact with solute n- or p-electrons
while the s coefficient measures the polarizibility difference
between the pseudostationary phase and the separation
buffer. The coefficients a and b are the hydrogen-bondaccepting and hydrogen-bond-donating strengths of the
pseudostationary phase, respectively. Therefore, various
polymeric surfactants can be compared in MEKC systems
employing the same aqueous buffer.
Over the past ten years, several groups have utilized
LSER to characterize a variety of conventional micelles
with varied polar head groups [11–22] and chain lengths
[23–26]. In contrast, only a small number of reports comparing polymeric surfactants with different hydrocarbon
chain lengths [1, 6, 27, 28] and polar head groups [29, 30]
have been published. Palmer’s group [27] reported the
use of siloxane polymers as well as copolymers of
2-acrylmido-2-methyl-1-propane sulfonic acid and
methacrylates with chain lengths of C8-C18 [6] to investigate selectivity differences. Although no substantial differences in chemical selectivity were observed using
either class of polymers, alkyl-modified siloxane polymers
were found to be more cohesive and less polar and have
high degrees of dipolarity/polarizibility as compared to
SDS. In a related LSER study, the same research group
also compared the allyl glycidyl ether N-methyl taurine
siloxane polymer to sulfite-modified siloxane [7]. They
noted that the use of later polymer with a shorter linker
arm (between the siloxane backbone and the ionic head
group) as well as the absence of tertiary amine group on
the polar head results in a significant change in chemical
selectivity. Recently, in a series of two publications, our
research group investigated four polymeric sodium alkenyl sulfate surfactants with chain lengths of C8-C11 [1, 28].
The LSER studies suggested that an increase in the
hydrocarbon chain length of these polymeric surfactants
decreases the polar nature and effectiveness of acid
strength, but increases the polarizibility.
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
4139
To the best of our knowledge, only few reports comparing
polymeric surfactants with different anionic polar head
group have been published [29, 30]. Fujimato [29] observed
higher separation efficiency with significantly different elution pattern of several benzene derivatives when polysodium 11-acrylamidoundecanoate poly(Na 11-AAU) was
compared to SDS micelles or poly(sodium 10-undecylenate) in MEKC. The comparison on solvation properties of
the three aforementioned micellar systems suggested that
strong dipole-dipole and dipole-induced-dipole properties
of poly(Na 11-AAU) result in unique selectivity for several
polar solutes. Leonard and Khaledi [30] compared the solvation properties of the triblock copolymer (poly(methyl
methacrylate-ethyl acrylate-methacrylic acid), Elvacite
2669), SDS, and a mixed surfactant system of SDS and
Elvacite 2669. They observed significant selectivity differences for nonhydrogen bonding (NHB) and hydrogen-bond
acceptor (HBA) solutes when Elvacite 2669 and SDS are
used as individual or mixed systems. In addition, the mixed
SDS-Elvacite 2669 pseudophase was found to be less
cohesive and weaker hydrogen-bond donor (HBD) than the
Elvacite 2669 or SDS alone.
The present study is a continuation of Part I in which the
synthesis, characterization, and application of monomeric
and polymeric forms of sodium N-undecenoyl glycinates
(SUGs) and sodium N-undecenoxy carbonyl glycinates
(SUCGs) surfactants in MEKC were evaluated. In this second
part, the effect of polar head for the simultaneous separation
of 10-NHBs, 8-HBAs, and 9-HBDs solutes using all eight
polymeric surfactants was first compared. Next, LSER was
applied to further evaluate the effect of hydrophilic head
group on retention behavior and selectivity of 27-benzene
derivatives. Since the selectivity differences between benzene derivatives were not very large in magnitude using the
tetraglycinate derivatives of poly(sodium N-undecenoyl glycyl-glycyl-glycyl-glycinate) (poly-SUG4) and poly(sodium
N-undecenoxy
carbonyl-glycyl-glycyl-glycyl-glycinate)
(poly-SUCG4), no LSER analysis was performed using these
two polymeric surfactants. Finally, the six mono-, di-, and triglycinates of poly-SUG and poly-SUCG series were compared for the separation of geometrical isomers of phenylimidazoles, 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-phenylimidazole, 2-phenylimidazole, 4-phenylimidazole), nitrotoluenes (2-nitrotoluene, 4-nitrotoluene, 3-nitrotoluene), and
CE and CEC
Electrophoresis 2005, 26, 4138–4152
4140
S. A. Shamsi et al.
methoxyphenethylamines (4-methoxyphenethylamines, 3methoxyphenethylamines, 2-methoxy-phenethylamines)
were obtained from Aldrich (Milwaukee, WI, USA). Other
chemicals are essentially the same as in Part I [31].
2.3 Preparation of micellar buffer solutions and
solute 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 this
series [31].
2.5 Calculations
The retention factors, k0 , of the test solutes were calculated as previously reported [1, 28]. The system coefficients (v, e, s, a, and b) described in Eq. (1) were determined by multiple linear regression using SAS software
(SAS Institute, Cary, NC, USA).
Electrophoresis 2005, 26, 4138–4152
3 Results and discussion
3.1 Effects of the head group on MEKC separation of NHB, HBA, and HBD solutes
Figures 1 and 2 demonstrate the simultaneous separation
of 10-NHB solutes (solutes 1–10, Table 1) in MEKC using
poly-SUG and poly-SUCG surfactants as pseudostationary phases, respectively. A steady decrease in retention times of NHB solutes was observed with an increase
in the number of glycine molecules in polar head group of
both poly-SUG and poly-SUCG surfactants. Thus, it
appears that decrease in elution window and migration
time is due to two important reasons: (i) increase in chromatographic polarity of the polymeric surfactant and
(ii) increase in zeta potential at the capillary surface.
These observations are consistent with the data in Part I
of this series [31], which shows that the use of more polar
triglycinates (e.g., poly(sodium N-undecenoyl glycyl-glycyl-glycinate) (poly-SUG3) and poly(sodium N-undecenoxy carbonyl-glycyl-glycyl-glycinate) (poly-SUCG3)),
as pseudostationary phase in MEKC, results in lower k0
for all 27 benzene derivatives than the less polar monoglycinates (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 benzene derivatives. Conditions:
17.4 mM at equivalent monomer
concentration (EMC) (mM) of
each
surfactant,
20 mM
NaH2PO4/Na2HPO4,
pH 7.0.
Pressure injection: 20 mbar, 4 s;
130 kV applied voltage; UV
detection at 200 nm; 257C. Peak
identifications are the same as
those listed in Table 1.
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Electrophoresis 2005, 26, 4138–4152
Glycine-based polymeric surfactants with varied polar head group
4141
Figure 2.
Comparison of
(a) poly-SUCG1, (b) poly-SUCG2,
(c) poly-SUCG3, and (d) polySUCG4 for the separation of
NHB benzene derivatives. Other
conditions are the same as
those given in Fig. 1. Inset in (d)
represents the expanded electrokinetic chromatogram of the
first ten solutes in poly-SUCG4
surfactant. Peak identifications
are the same as those listed in
Table 1.
(poly-SUG1) and poly(sodium N-undecenoxy carbonylglycinate) (poly-SUCG1)). In general, the elution order of
NHB solutes remained unchanged irrespective of the type
of head group except that the elution order of propylbenzene (solute 8) and naphthalene (solute 9) was
reversed. Using both poly-SUCG1 and poly(sodium
N-undecenoxy carbonyl glycyl-glycinate) (poly-SUCG2),
naphthalene was retained longer than propylbenzene.
However, the elution order of these two solutes reverses
when poly-SUCG3 and poly-SUCG4 were used as pseudostationary phases. These observed elution order reversals are most likely a result of poly-SUCG3 and polySUCG4 being more polar phases than poly-SUCG1 or polySUCG2. However, it was interesting to note that such
reversal was not observed in poly-SUG series of surfactants. Significant fronting was observed with respect to the
most hydrophobic NHB analyte (e.g., biphenyl) using most
of the polymeric surfactants. This suggests that poor solvation capacity and polydispersity of the polymeric surfactants are likely contributors to the peak asymmetry,
especially for more hydrophobic compounds.
Several trends in differences in resolution and separation
selectivity for certain NHB pairs were observed. First, the
Rs of all NHB solute pairs (with the exception of CTOL/
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
IBZ; for abbreviations see Table 1) are at least two to three
times higher for mono-, di-, or triglycinates of poly-SUCG
surfactants (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 surfactant (Table 2, row 4 vs. row 9, Part I). For tetraglycinates (poly-SUG4 and poly-SUCG4) very poor Rs
values were obtained (data not shown). This is probably
because of smallest electrophoretic mobility (Table 2,
column 9, Part I) and lowest phase ratio (due to smallest
V values, Table 1, column 6) of tetraglycinates (compared
to mon-, di-, or triglycinates), which in turn decreases the
elution window (Table 2, Part I). Second, the four (TOL/
CBZ, CBZ/EBZ, EBZ/BrBZ, and IBZ/PBZ) out of nine
pairs of NHB solutes were better resolved with polySUCG3 compared to poly-SUCG1 or poly-SUCG2, mainly
due to higher selectivity. For example, the selectivity
values for these four pairs were 2.47, 1.60, 1.47, and 1.43
using 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 Nundecenoyl glycyl-glycinate) (poly-SUG2). Third, a comparison between various pairs of NHB solutes shows that
the most hydrophobic pair (e.g., NAP/BP, peak 9/10 in
Figs. 1, 2) provided the highest Rs and a values. Interestingly, for this most hydrophobic pair of solutes, the poly-
4142
S. A. Shamsi et al.
Electrophoresis 2005, 26, 4138–4152
Table 1. Test solutes and their solvation parameters
No.
Solutes
Solvation parameters
V
E
S
A
B
NHB
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
Benzene (BZ)
Toluene (TOL)
Chlorobenzene (CBZ)
Ethylbenzene (EBZ)
Bromobenzene (BrBZ)
4-Chlorotoluene (CTOL)
Iodobenzene (IBZ)
Propylbenzene (PBZ)
Naphthalene (NAP)
Biphenyl (BP)
0.716
0.857
0.839
0.998
0.891
0.980
0.975
1.139
1.085
1.324
0.610
0.601
0.718
0.613
0.882
0.705
1.188
0.604
1.360
1.360
0.52
0.52
0.65
0.51
0.73
0.67
0.83
0.50
0.92
0.99
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.14
0.14
0.07
0.15
0.09
0.07
0.12
0.15
0.20
0.22
HBAs
(11)
(12)
(13)
(14)
(15)
(16)
(17)
(18)
Phenethyl alcohol (PEA)
Acetophenone (AP)
Nitrobenzene (NBZ)
Methyl benzoate (MBZT)
4-Nitrotoluene (NTOL)
4-Chloroacetophenone (CAP)
Ethyl benzoate (EBZT)
4-Chloroanisole (CAN)
1.057
1.014
0.891
1.073
1.032
1.136
1.214
1.038
0.784
0.818
0.871
0.733
0.870
0.955
0.689
0.838
0.83
1.01
1.11
0.85
1.11
1.09
0.85
0.86
0.30
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.66
0.48
0.28
0.46
0.28
0.44
0.46
0.24
HBA
(19)
(20)
(21)
(22)
(23)
(24)
(25)
(26)
(27)
Benzyl alcohol (BA)
Phenol (P)
4-Fluorophenol (FP)
3-Methylphenol (MP)
4-Chloroaniline (CANL)
3,5-Dimethylphenol (DMP)
4-Ethylphenol (EP)
3-Chlorophenol (CP)
3-Bromophenol (BP)
0.916
0.775
0.793
0.916
0.939
1.057
1.057
0.898
0.950
0.803
0.805
0.670
0.822
1.060
0.820
0.800
0.909
1.080
0.87
0.89
0.97
0.88
1.13
0.84
0.90
1.06
1.17
0.33
0.60
0.63
0.57
0.30
0.57
0.55
0.69
0.67
0.56
0.30
0.23
0.34
0.31
0.36
0.36
0.15
0.20
Table 2. Head group effects of poly-SUG surfactants on the migration behavior of benzene derivatives in MEKC (n = 27)
Surfactant
systems
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)
b)
c)
d)
Correlation coefficient of the linear regression
Standard error of the calculated log k0 values
SD for each coefficient
Values are not statistically significant at the 95% confidence level.
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Electrophoresis 2005, 26, 4138–4152
Glycine-based polymeric surfactants with varied polar head group
meric surfactant (i.e., poly-SUCG1) with the lowest
aggregation 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 iodobenzene (solute 7) was dramatically dropped when
switching 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 surfactants (Fig. 1) versus poly-SUCG (Fig. 2) clearly shows
that the use of former class of polymeric surfactant always
resulted not only in shorter migration time, but also in
narrow elution range than the latter. Thus, it appeared that
the presence of extra carbon atom in the aliphatic tail of
poly-SUCG series renders this class of surfactant more
hydrophobic resulting in longer migration times. Overall,
the mono-, di-, and triglycinate of poly-SUCG and polySUCG class of surfactants showed satisfactory separation of NHB solutes. In contrast, using either poly-SUG4
(Fig. 1d) or poly-SUCG4 (Fig. 2d), the same NHB solutes
eluted very close with lower Rs and a as well as narrow
elution windows of only 4 and 7 min, respectively.
Electropherograms in Figs. 3 and 4 show the separation
of 8-HBA solutes using four poly-SUG and four polySUCG surfactant systems, respectively. The HBA solutes
showed similar elution pattern in all eight surfactant sys-
4143
tems. Similar to NHB analytes, an increase in size of the
head group of either poly-SUG or poly-SUCG provided
faster separation of HBAs. The Rs of various HBA pairs
under investigation were affected differently with changes
in the polar head group. Similar to NHB solutes, HBAs
consistently provided significantly higher Rs using alkenoxy compared to acyl polymeric surfactants. For example, Rs values were at least two- to threefold higher with
poly-SUCG than with poly-SUG surfactants (data not
shown). Furthermore, it can be seen from Fig. 3a that
poly-SUG1 could not resolve acetophenone (solute 12)
and nitrobenzene (solute 13), whereas the same pair was
successfully resolved with poly-SUCG1 (Fig. 4a) as well
as with other polymeric surfactants. Similarly, 4-chloroacetophenone (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) and
poly-SUCG3 (Fig. 4c), partially resolved with poly-SUCG4,
(Fig. 4d), but was unresolved using any of the remaining
polymeric surfactants. Overall, poly-SUCG1 appears to
be the best polymeric surfactant since it provided the
highest Rs (mainly due to higher selectivity and large elution range) toward most of the of HBA solutes. Although
this particular surfactant has the same number of carbon
atoms in the hydrophobic tail (e.g., SUCG2 and SUCG3),
the smaller and less sterically hindered polar head group
allows 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 benzene derivatives. Other conditions are the same as those
given in Fig. 1. Peak identifications are the same as those listed in Table 1.
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. A. Shamsi et al.
Electrophoresis 2005, 26, 4138–4152
Figure 4. Comparison of (a)
poly-SUCG1, (b) poly-SUCG2,
(c) poly-SUCG3, and (d) polySUCG4 for the separation of
HBA benzene derivatives. Other
conditions are the same as
those given in Fig. 1. Peak identifications are the same as those
listed in Table 1.
The simultaneous separation of 9-HBDs is illustrated in
Fig. 5 (poly-SUG surfactants) and Fig. 6 (poly-SUCG surfactants). It is clear that almost all the mono-, di-, and triglycinate polymeric surfactants (Figs. 5a–c, 6a–c) are
good HBAs, showing simultaneous baseline or near
baseline separation of HBD solutes. Since all of these six
polymeric surfactants possess multiple HBA functionality,
favorable separation selectivities of HBD solutes are not
too surprising. Similar to the trends observed for NHB and
HBA solutes, the use of poly-SUCG surfactants as pseudostationary phases always provided longer retention
times for HBD solutes than the corresponding poly-SUG
surfactants (at equivalent number of glycine units). Again,
the separation selectivities were not as remarkable for 9HBDs using poly-SUG4 (Fig. 5d) or poly-SUCG4 (Fig. 6d).
Therefore, these two polymeric surfactants were not
included in LSER analysis, which is discussed below.
3.2 LSER analysis
A total of 27 benzene derivatives used in this study and
their descriptor values are listed in Table 1. Based on their
hydrogen-bond abilities, the solutes in Table 1 can be
characterized as NHB (solutes 1–10), HBAs (solutes 11–
18), and HBDs (solutes 19–27).
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3.2.1 Normalized residuals as a function of
solute number
The retention behavior of all 27 test solutes in polymeric
SUG and SUCG surfactants was examined, and the system constants were calculated by multiple linear regression. The statistical validity of LSER was evaluated
through the correlation coefficient (R2) and the standard
error of the estimate (SE). Relatively lower SE values were
observed with poly-SUG surfactant systems (Table 2)
relative to poly-SUCG surfactants (Table 3) due to a few
outlying solutes. Figures 7a–f show the outliers in all six
pseudostationary phases. The following approach was
followed to determine outlier solutes. First, residual
values of log k0 (experimental log k0 minus calculated
log k0 ) were calculated. Then, normalized residuals (i.e.
residual divided by the SD of the residual) were computed. Finally, the normalized residual values of log k0
were plotted against the solute number. When the normalized residuals were zero or nearly zero, the best fit
between experimental log k0 and calculated log k0 values
was obtained. However, normalized residuals in a range
of 12 to 22 are reasonable for statistically sound correlations. Note that biphenyl (solute 10) is an outlier in all
three poly-SUCG surfactants (Figs. 7d–f), whereas benzyl
alcohol (solute 19) and benzene (solute 1) are the main
Electrophoresis 2005, 26, 4138–4152
Glycine-based polymeric surfactants with varied polar head group
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Figure 5. Comparison of (a) polySUG1, (b) poly-SUG2, (c) polySUG3, and (d) poly-SUG4 for the
separation of HBD benzene derivatives. Other conditions are the
same as those given in Fig. 1.
Peak identifications are the same
as those listed in Table 1.
Figure 6. Comparison of (a)
poly-SUCG1, (b) poly-SUCG2,
(c) poly-SUCG3, and (d) polySUCG4 for the separation of
HBD benzene derivatives. Other
conditions are the same as
those given in Fig. 1. Peak identifications are the same as those
listed in Table 1.
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. A. Shamsi et al.
Electrophoresis 2005, 26, 4138–4152
Figure 7. Normalized residuals
from (a) poly-SUG1, (b) polySUG2, (c) poly-SUG3, (d) polySUCG1, (e) poly-SUCG2, and
(f) poly-SUCG3 surfactant system. Other conditions are the
same as those given in Fig. 1.
outliers in poly-SUG3 (Fig. 7c) and poly-SUCG3 (Fig. 7f),
respectively. These data indicate that the number and
type of outliers may vary depending on the polymeric
surfactant used.
3.2.2 Effect of the polar head group on phase
ratio
The LSER constants and the statistics for all the pseudostationary 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 differences in phase ratio. The regression constant C is large
and negative for all the surfactant systems studied. The
phase ratio increases with an increase in the number of
hydrogen bonding sites in the head group of poly-SUG
surfactants (e.g., Cpoly-SUG3 . Cpoly-SUG2 . Cpoly-SUG1). This
trend differs with the partial specific volume results reported in Part I [31]. For example, for poly-SUG series of surfactants the chromatographic phase ratio (b) calculated
from the proposed equation [1] follows the order:
(bpolySUG1 = 0.0038, bpolySUG2 = 0.0037, bpolySUG3 = 0.0037,
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
bpolySUG4 = 0.0031), which seems essentially constant and
is slightly lower for poly-SUG4. In contrast, the polymeric
SUCG surfactants do not show the same trend of
b seen in polymeric SUG surfactants. For the poly-SUCG
series, 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, but
the C values for these two polymers are significantly different (Table 3, column 2). However, the same two polymeric surfactants have higher b values compared to polySUCG3 and poly-SUCG4. Thus, the trend in b values for
only poly-SUCG2 and poly-SUCG3 was found to be consistent with the C values.
3.2.3 Effect of the polar head group on
cohesiveness and dispersion interactions
The vV term indicates the free energy change due to
hydrophobic interactions. Larger coefficient v values
indicate smaller cohesive energy of the pseudostationary
phase. In other words, less energy is required to form a
Electrophoresis 2005, 26, 4138–4152
Glycine-based polymeric surfactants with varied polar head group
4147
Table 3. Head group effects of poly-SUCG surfactants on the migration behavior of benzene derivatives in MEKC (n = 27)
Surfactant
systems
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)
b)
c)
d)
Correlation coefficient of the linear regression
Standard error of the calculated log k0 values
SD for each coefficient
Values are not statistically significant at the 95% confidence level.
cavity on the pseudostationary phase to accommodate
the solutes. Positive sign of the coefficient v indicates
that solutes prefer to transfer from more cohesive (i.e.
more polar) aqueous phase to less cohesive (i.e. more
nonpolar) pseudostationary phases. For interpretation
purposes, it is also important to understand that cohesiveness is a measure of the free energy (that includes
both enthalpy and entropy contribution) of cavity formation with respect to water. The value of coefficient v
decreases with an increase in the number of glycine in
the head group of the polymeric SUG surfactants
(Table 2). For example, poly-SUG3 micelles with three
glycines on its head group are more compact and less
organic-like (smaller coefficient v) as compared to polySUG1 and poly-SUG2 micelles. On the other hand, polySUG1 shows more organic character and requires less
energy to create a cavity for a solute. It is noticeable
from Tables 2, 3 that poly-SUCG1 has the highest v
value (3.277) among all six surfactant systems; thus, it is
the least polar surfactant system. Poly-SUCG3 (v = 3.024)
is slightly more polar than poly-SUCG1 but less polar
than poly-SUCG2. Thus, the six surfactant systems
can 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 a
higher number of glycine units in the polar head group.
This would indicate that as the number of hydrogen
bonding group is increased cohesiveness increases.
Poly-SUCG1 and poly-SUG3 show greatest and smallest
hydrophobic interactions with the solutes, respectively.
Thus, hydrophobic NHB solutes have strongest affinity
for poly-SUCG1 (Fig. 1a) than the remaining polymeric
surfactants. Furthermore, cavity formation in polySUCG1 micelles requires relatively smaller energy.
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Hence, partitioning of solutes will be more favorable in
the poly-SUCG1 phase than with the aqueous buffer
phase.
3.2.4 Effect of polar head group on dipolarity
and polarizability
As described earlier, coefficient e represents the ability of
micellar phase and aqueous phase to interact with
solutes n- or p-electrons. The data in Tables 2 and 3 show
that all the polymeric poly-SUG and poly-SUCG surfactants have positive e coefficient values. Hence, compared
with aqueous buffer system, these surfactant systems
can easily become polarized through interacting with
solutes’ n- and p-electrons. Due to its relatively highest
positive e coefficient value (e = 0.884), poly-SUCG1
micellar phase shows the strongest interaction with
neighboring solute n- and p-electrons and hence
becomes polarized most easily (Table 3). In contrast,
poly-SUCG2 has the least polarizability among all surfactant systems (e = 0.219). Thus, the polymeric surfactant
systems used in this study can be ordered according
to their polarizability strength: epoly-SUCG1 . epoly-SUG3 .
epoly-SUG1 . epoly-SUCG3 < epoly-SUG2 . epoly-SUCG2.
Except poly-SUCG2 and poly-SUCG3, all pseudostationary phases have statistically significant negative
coefficient s values (Tables 2, 3). Thus, all pseudostationary phases are less polar than the aqueous phase,
except the two aforementioned polymeric surfactants
whose s coefficients are statistically insignificant at the
95% confidence level, and thus their dipolarity is similar to
that of aqueous phase. In addition, less negative s coefficients of the two pseudostationary phases (poly-SUG1
and poly-SUG2) show that they are relatively more polar
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S. A. Shamsi et al.
Electrophoresis 2005, 26, 4138–4152
compared to other polymeric surfactants. Consequently,
these two polymeric surfactants should interact more
with polar compounds. Based on coefficient s values
reported in Tables 2 and 3, polarity of the surfactants can
be ordered as: spoly-SUG2 < spoly-SUG1 . spoly-SUCG1 .
spoly-SUG3. The polarity of the glycinate polymers arises
from the carbonyl functionality on the head group. Moreover, the carbamates surfactants (e.g., poly SUCG1) are
less polar than amide surfactants (e.g., poly-SUG1), and
this lower polarity resulted in longer retention with the
former polymeric surfactant.
ment for the solutes. The strength of the hydrogen-bonddonating ability of polymeric SUG surfactants decreases
with a decrease in the number of glycine unit on the head
group of the surfactants. Therefore, poly-SUG3 has the
strongest whereas poly-SUG1 has the weakest interactions with HBA solutes. Unlike polymeric SUG surfactants, no clear trend of hydrogen-bond-donating ability in
polymeric SUCG was observed.
3.2.5 Effect of the polar head group on hydrogen
bonding
Three sets of isomers (i.e., phenylimidazole, nitrotoluene,
and methoxyphenethylamine derivatives) were used to
examine the shape selectivity (ashape) of six polymeric
surfactant systems. All the pseudostationary phases
used in this study showed significant selectivity differences toward the geometrical isomers. Figure 8 illustrates
the separation of three phenylimidazole isomers using the
three poly-SUG (a–c) and three poly-SUG (d–f) surfactant
systems. The elution order of phenylimidazole isomers
employing all six polymeric surfactants was essentially
the same: 1-phenylimidazole , 2-phenylimidazole , 4phenylimidazole. A closer look at the chemical structures
of phenylimidazole isomers (Fig. 8, insets) reveals that
one hydrogen-bonding site of 1-phenylimidazole is hindered by the bulky five-membered ring, thus eliminating
hydrogen bonding with the surfactant systems. Hence,
1-phenylimidazole elutes in a much shorter time as compared to the remaining two isomers. All the remaining
polymeric surfactants (except poly-SUCG1) provided Rs
and ashape of the three geometrical isomers. As the number of glycine units in poly-SUG surfactants increases the
Rs and ashape values between the solute pairs 1–2 and 2–3
decrease (Figs. 8a–c, inset tables). However, as seen in
Fig. 8d, monoglycinate of SUCG (i.e., poly-SUCG1) surfactant could not separate the last two phenylimidazole
isomers. In contrast, poly-SUG2 and poly-SUG3 surfactants successfully resolved all three phenylimidazole isomers in a relatively shorter time. Moreover, a quick comparison of poly-SUCG2 versus poly-SUCG3 reveals that
for 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 efficiency is always better for all three isomers with the latter
polymeric surfactant.
A positive coefficient a obtained for all six polymeric surfactants (Tables 2, 3) indicates that the hydrogen-bondaccepting strength (i.e., basicity) of these pseudostationary phases is greater than that of aqueous phase.
However, the coefficient a value for one polymeric surfactant (i.e., poly-SUCG3) is statistically insignificant. In
other words, the hydrogen-bond-accepting strength of
this pseudostationary phase is not much different from
hydrogen-bond-accepting strength of the aqueous
phase. In general, the poly-SUCG surfactants are
expected to have more basic character than the polySUG surfactant due to the presence of oxygen atom
adjacent to the carbonyl group at the N-terminal end,
which offers extra HBA site for the solutes. However, the
data in Tables 2 and 3 do not support this assumption.
This is probably due to the fact that this extra oxygen
atom in poly-SUCG surfactants is involved in intramolecular hydrogen bonding between surfactant monomers
rather than hydrogen bonding with the solutes.
The coefficient b is related to the difference in hydrogenbond-donating ability (i.e., acidity) of the pseudostationary phase and that of aqueous phase. The negative
sign of coefficient b indicates that all pseudostationary
phases are less acidic than the aqueous phase. This is not
surprising since the surfactant has fewer protons to donate
as compared to water molecules in aqueous solution. The
pseudostationary phases with larger (or less negative)
b values provide stronger HBD sites for solute interaction.
All the polymeric SUG and SUCG surfactants have
hydrogen-bond-donating sites in their head group. Based
on the coefficient b values listed in Tables 2 and 3, the
relative hydrogen-bond-donating strength of the polymeric SUG and SUCG surfactants can be ordered as
bpoly-SUG3 . bpoly-SUCG2 . bpoly-SUG2 . bpoly-SUG1
. bpoly-SUCG1 . bpoly-SUCG3. Thus, the poly-SUCG1 and
poly-SUCG3 with the largest negative b coefficients provide the weakest, while the poly-SUG3 and poly-SUCG2
provide the strongest proton-donating micellar environ-
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3.3 Effect of the head group on isomer
selectivity
In Fig. 9 is shown the MEKC separation of three nitrotoluene 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 with
elution order as follows: o-(2-nitrotoluene) , p-(4nitrotoluene) , m-(3-nitrotoluene). However, only two
Electrophoresis 2005, 26, 4138–4152
Glycine-based polymeric surfactants with varied polar head group
4149
Figure 8. Head group effects on
the elution order, resolution (Rs),
and shape selectivity (ashape)
of (1) 1-phenylimidazole, (2) 2phenylimidazole, (3) 4-phenylimidazole isomers using (a) polySUG1, (b) poly-SUG2, (c) polySUG3, (d) poly-SUCG1, (e) polySUCG2, and (f) poly-SUCG3.
Other separation conditions are
the same as those given in
Fig. 9.
Figure 9. Head group effects on
the elution order, resolution (Rs),
and shape selectivity (ashape) of
(1) 2-nitrotoluene, (2) 4-nitrotoluene, (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 are
the same as those given in
Fig. 1.
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
4150
S. A. Shamsi et al.
isomers were resolved with the remaining four polymeric
surfactants. It is interesting to note that the polymeric
surfactants (e.g., poly-SUG1 and poly-SUCG1) with less
hydrogen bonding sites and more negative b values provided highest shape selectivity. In contrast, previous work
showed that increasing the number of hydrogen-bonding
sites on the polar head group of polymeric surfactant
improves chiral separations [32]. This is despite the fact
that no significant differences in migration times of geometrical isomers were observed when poly-SUG1 was
compared to poly-SUG2 and poly-SUG3. A further comparison of the three poly-SUG surfactants (Figs. 9a–c)
versus the three poly-SUCG (Figs. 9d–f) clearly shows
that the increase in migration times using the latter class
of polymeric surfactants did not improve the ashape of
nitrotoluene isomers. Although the differences in Rs were
obtained for nitrotoluene isomers using the six polymeric
surfactants, no change in the elution order was observed
and remained to be o , p , m.
Finally, the MEKC separation of three methoxyphenethylamine isomers (containing both HBD and
HBA sites) is shown in Figs. 10a–f. Unlike the separation
of nitrotoluene isomers it is interesting to note that
elution times of methoxyphenethylamine isomers were
Electrophoresis 2005, 26, 4138–4152
almost half when poly-SUG1 and poly-SUCG1 were
replaced by poly-SUG2 and poly-SUCG2, respectively.
In contrast, only slight increase in elution time of the
same three isomers was observed when poly-SUCG2
was replaced by poly-SUCG3. Interestingly, poly-SUG3
provided longer elution times as compared to polySUG2 or poly-SUG1. Similar to the separation of
nitrotoluene isomers, both poly-SUG1 and poly-SUCG1
provided better separation selectivity of all three
isomers with the following elution order: 4-methoxyphenethylamine , 3-methoxyphenethylamine , 2-methoxy-phenethylamine. The elution order of the isomers
can be attributed to both intramolecular H-bonding
(between NH2 functional group and O atom) as well as
intermolecular H-bonding between isomers and polar
head group of the surfactants. For example, in 2-methoxyphenethylamine intramolecular and intermolecular
hydrogen bondings are stronger than in 4-methoxyphenethylamine. Hence, the former isomer is retained
longer than the latter in all six polymeric surfactant systems. However, due to the basic nature of these compounds severe peak tailings were observed. Again, the
remaining four di- and triglycinate polymeric surfactants
provided only partial or no Rs of the last two isomers of
methoxyphenethylamine.
Figure 10. Head group effects
on the elution order, resolution
(Rs), and shape selectivity
(ashape) of (1) 4-methoxyphenethylamine, (2) 3-methoxyphenethylamine, (3) 2-methoxyphenethylamine
isomers
using
(a) poly-SUG1, (b) poly-SUG2,
(c) poly-SUG3, (d) poly-SUCG1,
(e) poly-SUCG2, and (f) polySUCG3. Other separation conditions are the same as those
given in Fig. 1.
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Electrophoresis 2005, 26, 4138–4152
Glycine-based polymeric surfactants with varied polar head group
4 Concluding remarks
Simultaneous separations of 10-NHB, 8-HBA, and 9-HBD
solutes were compared in MEKC using four poly-SUG
and four poly-SUCGs. Each polymeric surfactant contains a C11 hydrophobic tail along with a head group of
mono-, 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 an
increase in the size as well as polarity of the head group of
poly-SUG and poly-SUCG surfactants. Overall, simultaneous separation of NHB and HBA solutes was best
achieved with poly-SUCG1, whereas HBD solutes provided equally good Rs using any of the six mono-, di-, and
triglycinates of poly-SUG and poly-SUCG surfactants. In
contrast, poly-SUG4 and poly-SUCG4 showed very fast
separation, but poor selectivity for all three classes of
solutes.
The retention behavior of the 27-benzene derivatives was
also compared for three poly-SUG and three poly-SUCG
surfactants using the LSER model. The normalized residuals as a function of solute number for outlier study
showed that the number and type of outliers might vary
depending on the polymeric surfactant used. For polySUG surfactants, the phase ratio increases (C) with an
increase in the size of polar head group, whereas the
poly-SUCG surfactants do not show any such trend. The
coefficient v value shows that cohesiveness increases
with an increase in the size of head group of poly-SUG
surfactants. For poly-SUG series, poly-SUG1 and polySUG3 provided the least and the most polar environment,
respectively. Although poly-SUCG1 provided the highest
v value and was consequently the least polar surfactant,
no significant and clear trend was seen for poly-SUCG
series of surfactants. The poly-SUCG1 and poly-SUG3 are
more easily polarized (e) upon interacting with the solutes
n- and/or p-electrons. In addition, poly-SUG1 and polySUG2 have the highest polar character as compared to
the remaining surfactant systems. It should be noted that
the amide surfactants (e.g., poly-SUG1) are more polar
than carbamates (e.g., poly-SUCG1). This is because the
carbonyl carbon is a part of the hydrophobic tail in the
former polymeric surfactant, while there is an extra carbon atom in the hydrophobic tail of the latter surfactant.
The more positive sign of coefficient a indicates that the
aqueous buffer phase is less basic compared to the
polymeric SUG and SUCG phases. However, the smaller
absolute values for the a coefficient for all poly-SUCG
apparently reflect that the hydrogen-bond-accepting
characteristics of these micellar phases are not much
different from those in the bulk aqueous phase, and
thereby has an insignificant influence on solute partitioning. In general, poly-SUG1 and poly-SUCG2 are the
 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
4151
strongest HBA phases, while poly-SUG2 and poly-SUCG3
are the weakest HBA phases. The hydrogen-bonddonating ability of micellar phase has a greater impact on
MEKC retention and selectivity, as well as solute-micelle
interaction. The negative and large coefficient b for all
polymeric surfactants indicates that these pseudostationary phases are less acidic than the aqueous buffer
solution. The polymeric surfactant with the least negative
b 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 while
poly-SUG3 and poly-SUCG2 provided the strongest
hydrogen-bond-donating environment for the solutes.
The use of polymeric surfactants in MEKC provided
unique separation capabilities for shape selectivity of
geometrical isomers. Major selectivity differences were
observed when poly-SUG and poly-SUCG surfactants
were utilized for MEKC separation of phenylimidazole,
nitrotoluene, and methoxyphenethylamine isomers. Surprisingly, most of the polymeric surfactants (except polySUCG1) provided baseline separation of phenylimidazole
isomers. On the other hand, among the six polymeric
surfactants the two monoglycinates (i.e., poly-SUG1 and
poly SUCG1) were clearly the most selective pseudostationary phases being able to resolve all three geometrical
isomers of nitrotoluenes and methoxyphenethylamines.
This work was supported by a grant from the National
Institutes of Health (Grant No. GM 62314-02) and the
Petroleum Research Fund (Grant No. 35473-G7).
Received May 12, 2005
Revised August 13, 2005
Accepted August 14, 2005
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