Interfacial response of a novel gemini metallo-surfactant to ionic guest species

Article abstract of DOI:10.1016/j.inoche.2014.02.006

The surface properties of a novel metallosurfactant with twin dodecyl aza-crown aliphatic tails (L3) were studied by surface pressure isotherms on aqueous subphases of selected Group I and Group II metals. Mean molecular areas of the headgroup at the mono

Full text of DOI:10.1016/j.inoche.2014.02.006

Inorganic Chemistry Communications 43 (2014) 15–18
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Interfacial response of a novel gemini metallo-surfactant to ionic
guest species
⁎
Gina E. Jaggernauth, Richard A. Fairman
Department of Chemistry, The University of the West Indies, St. Augustine, Trinidad and Tobago
a r t i c l e i n f o
a b s t r a c t
Article history:
The surface properties of a novel metallosurfactant with twin dodecyl aza-crown aliphatic tails (L3) were studied
by surface pressure isotherms on aqueous subphases of selected Group I and Group II metals. Mean molecular
areas of the headgroup at the monolayer interface (gas phase) are observed to decrease in response to aqueous
phase complexation of L3 by metal ions in the order Ca2+, Li+, Mg2+, Na+, Ba2+ and K+. The surface-tension de-
rived critical micelle concentration of L3 at 25 °C was 0.072 mM, while atomic force microscopy of aggregates ob-
served after preparing Langmuir Blodgett multilayers on mica confirmed the formation of vesicles of up to 100
nm diameter as opposed to wormlike micelles formed by the monomeric analogs.
Received 9 November 2012
Accepted 1 February 2014
Available online 14 February 2014
Keywords:
Diamsar
Ion selective response
Langmuir isotherms
© 2014 Elsevier B.V. All Rights Reserved
A common approach to the molecular engineering of surfactant mol-
ecules is that of a dimeric or gemini construct, where variations of the
spacer [1], aliphatic tail [2] and headgroup [3] together afford access to
unique physicochemical properties [4–7], interfacial phenomena [8], or
even cytotoxicity [9–11]. Typically, the spacer unit has dual and compet-
ing functions: to connect monomeric amphiphiles at a critical minimum
distance for both headgroups to behave as one in condensed mesophase,
while keeping the centers of like charge optimally distant to exert wider
influence on the negatively charged surfaces of, for example, cellular
membranes [12]. In this report, we describe the synthesis, surface and
aggregation behavior of a novel gemini-type metallosurfactant, L3, in
which a bulky and highly-charged cobalt(III)sarcophagine acts as both
spacer group and center of extensive headgroup charge density. Addi-
tionally, L3 bears a metal coordinating aza-oxa crown ether functionality
and is therefore likely to respond to different metal ions with configura-
tional changes at interfacial regions and through varying modes of ag-
gregation in bulk solution [13]. We also anticipate that the added
crown functionality will confer enhanced antimicrobial activity of the
amphiphile in analogous manner to natural [14] and synthetic macrocy-
clic ionophores [15]. Our investigations of the solution properties with
surface tension, Langmuir surface pressure measurements and atomic
force microscopy are herein described.
hydrochloride salt. The generally low, un-optimized yield (7%), can be
attributed to the hydrolysis of the amide during workup from the aque-
ous acid and also by the large numbers of coordinated solvent molecules
in the precursor amine which reduce available acid chloride. However,
all complexes were satisfactorily characterized by NMR, IR, and mass
spectral analysis as described in the supplementary data. We include
studies of the precursor organic amine without the cobalt cage
appended (L1) and the single tail analog (L2) for comparison of mono-
meric vs gemini surfactant properties (Fig. 1).
Despite its size, the crown and cage functionalities in L3 result in
high amphiphilicity, evident from its solubility in aqueous and organic
solvents such as water, methanol and chloroform. The critical micelle
concentration (cmc) was measured by surface tension measurements
(Fig. 2) at 25 °C with ultrapure water (18.2 MΩ). The break-point in
the graph of surface tension versus log surfactant concentration gives
a cmc of 0.072 mM and the lowest surface tension of 37 mN m−1 for
L3. Corresponding cmc values for L1 and L2 (2.45 and 1.07 mM) accen-
tuate the trend of significantly lower cmc values for geminis (34 times
lower for L3) relative to their monomeric analogs. Differences in bulk
aggregation behavior due to the twin tail-group of L3 were evaluated
by coating a mica substrate four times using the computer controlled
dipping accessory and performing tapping-mode atomic force micros-
copy after drying the substrate at room temperature. In contrast to the
2–3 nm wide wormlike micelles observed for the monomeric ligand
L2 [17], gemini L3 forms spherical vesicles as large as 100 nm in diam-
eter (Fig. 3). Such a marked difference in morphology is consistent
with an increased surfactant packing parameter [18,19] (volume/area
× length) as the effective hydrophobic volume is significantly increased
by twin-tails being attached to the single headgroup in L3.
Compound L3 was prepared by overnight condensation of the ap-
propriate cobalt(III)diamsar acid chloride [16] and unsymmetrically
functionalized diaza-18-crown-6 precursor amine [17], each prepared
as cited, at room temperature in the presence of K2HPO4. The condensa-
tion products were separated by ion exchange chromatography, L3
being eluted from the column by dilute hydrochloric acid as an orange
Estimates of the mean molecular areas (mma) of L1–L3 were ex-
tracted from the Langmuir isotherms of monolayers on either a pure
⁎
Corresponding author.
E-mail address: Richard.Fairman@sta.uwi.edu (R.A. Fairman).
http://dx.doi.org/10.1016/j.inoche.2014.02.006
1387-7003/© 2014 Elsevier B.V. All Rights Reserved

16
G.E. Jaggernauth, R.A. Fairman / Inorganic Chemistry Communications 43 (2014) 15–18
s
s
O
O
T
O
O
O
O
O
O
O
O
(iii-vi)
(i,ii)
(vii)
H25
C12
H25
C12
N
N
N
NH2
l
C
H
O
O
O
T
L1
HN
HN
HN
HN
O
O
(viii)
+
o3
+
Co3
H
O
H
O
NH
a
NH
C
H2N
H2N
HN
HN
HN
HN
HN
HN
HN
HN
(ix)
H
H
O
O
HN
HN
N
N
O
O
O
O
O
O
O
+
Co3
NH
NH
H25
C12
H25
N
C12
N
N
N
HN
HN
HN
HN
O
a
L2 L
3
,
Fig. 1. Synthesis of double-tailed amphiphile L3 (a = 1), precursors L2 (a = 0), and L1. (i) C12H25NH2, DMF, K2CO3, 4d (ii) p-TsCl, NaOH/THF, 0 °C, (iii) BzNH2, MeCN, Na2CO3, 3d (iv) H2,
Pd/C, 50 °C, 8 h, (v) CH2CHCN, 12 h, (vi) = (iv), (vii) CH2ClCO2H, Sephadex CM C25, (viii) SOCl2, 3 h, (ix) K2HPO4, RT, 12 h.
water subphase or above solutions of different alkali/alkaline earth
metal chlorides. Whereas typical cationic-surfactant response to
added electrolytes is small [20] and ascribed to interactions between
counterions and the Stern layer at the interface [21], the π–A isotherms
of L3 show large responses to cations even at 1 mmol added electrolyte.
For example, among the group I metals, the maximal surface pressure is
reduced by as much as 50% with Na+ (Fig. 4), while the sharper K+ cur-
vature suggests even higher sensitivity of the ligand host to this ion. A
similar pattern obtains for Ca2+ and Mg2+, with Ba2+ evidently better
at reducing both the maximum surface pressure and mma (Fig. 5).
Such high susceptibility implies dominant configurational-change
contributions to the surface tension differences, which we ascribe to
the influence of guest cation complexation since the counterion of all
the added electrolytes is invariant. We develop schematically in Fig. 6,
a model of these changes based on the affinity and coordination
modes that obtain for crown macrocycles with cations of varying
size and charge. In general, the metal radius that best matches the cavity
size of the crown module is strongly coordinated [22], but mismatched
dimensions can also yield strong binding through a variety of sandwich-
type coordination modes [23]. First, we invoke the former to account for
strong binding among the group I metals and the latter to account for
the effectiveness of the group II ions at reducing the mma of the inter-
facial segments. Next, we consider that since free crowns align parallel
to the interface in compressed monolayers [23], metal encapsulation
and attendant increases in solubility of this component will reduce mo-
lecular separation at the interface as the crown is further immersed in
the subphase. Lastly, we posit an adverse effect on molecular separation
where the crown–ion complex is close to the bulky cobalt-cage
appendage, due to strong electrostatic repulsion.
Treating L1–L3 within this framework, we note a dramatic response
of the monomeric precursor L1 to Mg2+ ions and an enhancement of
this behavior with the added cage headgroup in L2, but the gemini
metallosurfactant L3 is striking in opposition: all interactions result in
a reduced mma, and the effect is the largest for K+ and Ba2+ in their re-
spective groups (Fig. 7). Since L3 bears two macrocyclic rings, the possi-
bility of either inter- or intra-molecular coordination with the larger
cations is germane. However, applying our model to the mma changes
across the series L1–L3 (Table 1) affords distinction between the
two modes based on the observed effects on the surface pressure:
60
L3
L2
50
L1
40
30
-5.5
-4.5
-3.5
-2.5
-1.5
Log Concentration
Fig. 2. Critical micelle concentration of L1, L2 and L3 by surface tension measurements.
Fig. 3. Atomic force microscopy of L3 vesicles deposited on mica substrate.

G.E. Jaggernauth, R.A. Fairman / Inorganic Chemistry Communications 43 (2014) 15–18
17
25
20
15
10
5
45
L1
L2
L3
30
H2O
15
LiCl
NaCl
0
KCl
-15
-30
0
100
H2O Li+ Na+ K+ Mg2+ Ca2+ Ba2+
140
180
220
260
Subphase
Mean Molecular Area ( Å2 )
Fig. 7. Percent change in mean molecular area with various cations for L1–L3.
Fig. 4. π–A isotherms for L3 monolayer above aqueous group I ions in subphase.
25
the mma by at least 50% as the lipophilic segments are brought to
their minimum separation distance in this alternative binding mode.
Our results therefore support an intermolecular coordination for the
largest cations.
20
H2O
The synthesis, amphiphilic properties and ion-selective interfacial
response of a new gemini metallosurfactant with a crown ether func-
tionality between the lipophilic tails are described. In concert, the
crown and cobalt cage affect metal and coordination-mode discrimina-
tion among group I and II ions, which is modeled according to confor-
mational changes of L3 at the interface, and supported by monolayer
compression studies. A low cmc value for L3 in aqueous solution exem-
plifies the observed trends of decreasing cmc in going from a monomer-
ic to a dimeric surfactant, and AFM images of its aggregates support the
Tanford relation [19] between lipophilic volume, critical packing param-
eter and observed aggregate morphology.
CaCl2
15
MgCl2
10
BaCl2
5
0
100
140
180
220
260
Mean Molecular Area ( Å2 )
Acknowledgments
Fig. 5. π–A isotherms for L3 monolayer above aqueous group II ions in subphase.
This work was supported by the Research and Publication Fund of
the University of the West Indies (UWI), St. Augustine. The authors
are grateful to Dr. Leonette Cox (UWI) for providing Atomic Force
Microscopy.
the mma values are only moderately reduced by inter-molecular com-
plexation, as confirmed by experiment, while intra-molecular binding
with two endogenous macrocyclic rings would be expected to reduce
Fig. 6. Proposed changes in surface area by metal ion binding at the air–water interface: row 1 for single-tailed ligands, row 2 for double-tailed ligand L3. Guest cation radii increase left to
right and solid bars depict effective radius of the host–guest complex.

18
G.E. Jaggernauth, R.A. Fairman / Inorganic Chemistry Communications 43 (2014) 15–18
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H2O
Li+
–
59.4
55.7
48.9
51.7
67.2
52.3
49.8
0
153.5
171.6
151.1
157.7
218.5
140.9
170.9
0
181.7
161.7
159.7
145.6
159.9
167.9
152.9
0
76
−6.2
−17.7
−13.0
13.1
−12.0
−16.2
11.8
−1.6
2.7
42.3
−8.2
11.3
−11.0
−12.1
−19.9
−12.0
−7.6
Na+
K+
102
138
72
100
135
Mg2+
Ca2+
Ba2+
−15.9
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