Study of surfactant alcohols with various chemical moieties at the hydrophilic-hydrophobic interface

Article abstract of DOI:10.1039/c3ra40704g

The melting behavior, the solubility, and the influence of hydrogen bonds were analyzed for a series of single- and double-tailed surfactant alcohols. Various effects such as the presence of free amides or the intermolecular spacing were found to be important factors for increasing or decreasing the melting temperature of a surfactant. Furthermore, we present a model for the packing of diamido-lipids and study the temperature-dependence of the IR signals. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra40704g

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Study of surfactant alcohols with various chemical
moieties at the hydrophilic–hydrophobic interface3
Cite this: RSC Advances, 2013, 3, 7237
a
b
b
´
Pierre-Leonard Zaffalon, Vincenza D’Anna, Hans Hagemann*
Received 8th February 2013,
Accepted 20th March 2013
and Andreas Zumbuehl*ac
DOI: 10.1039/c3ra40704g
www.rsc.org/advances
The melting behavior, the solubility, and the influence of
hydrogen bonds were analyzed for series of single- and
Here, we provide a concise list of single-tail and double-tail
surfactants containing various chemical moieties at the hydro-
philic–hydrophobic interface and study their melting behavior and
their solubility in various solvents. In order to ease the comparison
between molecules, we use the nomenclature that we proposed for
artificial phospholipids.1 For single-tail surfactants the C16 or
palmitic aliphatic chain is abbreviated by the capital letter P. This
is followed by the functional group present, i.e., et for ether, es for
ester, and ad for amide. For double-tail surfactants the positions of
the glycerol backbone are numbered 1 through 3 following the
Hirschmann notation and the aforementioned abbreviations for
the tails and the chemical moieties are used.3 Therefore, N,N9-(2-
hydroxypropane-1,3-diyl)dipalmitamide is now named Pad–OH–
Pad (10), making the comparison between molecules straightfor-
ward.
a
double-tailed surfactant alcohols. Various effects such as the
presence of free amides or the intermolecular spacing were found
to be important factors for increasing or decreasing the melting
temperature of a surfactant. Furthermore, we present a model for
the packing of diamido-lipids and study the temperature-
dependence of the IR signals.
Introduction
Recently, we have reported on the synthesis of Pad–PC–Pad, a 1,3-
diamidophospholipid with chemical modifications of the natural
glycerol backbone: the ester linkages were replaced with amido
groups and the natural 1,2-diacyl substitution pattern was
changed to a 1,3-arrangement.1 The vesicles formulated from this
lipid showed promising mechanosensitive properties: a dye-loaded
vesicle did not release its cargo spontaneously but did so when it
was exposed to shear-stress or simple shaking. Such vesicles might
find interesting applications as drug delivery devices.2 During the
synthesis of the phospholipid, we were confronted by the
surprising near-insolubility of the free alcohol N,N9-(2-hydroxypro-
pane-1,3-diyl)dipalmitamide (10). Intermolecular forces such as
hydrogen bonding and van der Waals interactions between the
hydrophobic chains are present in a balanced fashion and will be
influenced by chemical modification of the hydrogen donor–
acceptor moieties involved. The low solubility of this molecule
might be induced by intermolecular hydrogen bonds and this
hypothesis prompted us to have a closer look at various one- and
two-tailed hydrocarbon surfactants with different end-group
moieties.
The literature on single- or double-chain aliphatic ethers,
esters, or amides at the hydrophobic–hydrophilic interface spans
several decades and is therefore contradictory in itself. The
situation is easiest for single-chain aliphatic amides where the
amides are both hydrogen bond donors and acceptors: two
molecules pack head-to-head and form a centrosymmetric amide
pair, a hydrogen bonded dimer.4,5 The force attributed to these
…
two palmitic amide hydrogen bonds (CLO H–NH) is estimated at
33 kJ mol21.6 The additional free amide protons are expected to
form hydrogen bonds to the neighboring pair of molecules.7 Each
of these additional hydrogen bonds is contributing a stabilizing
energy of approximately 19 kJ mol21.7 It is interesting to note that
the long-chain N-methyl amides ‘‘are peppery in taste, provoke an
intense burning taste and numb the tongue’’, making them
candidates for local anaestetics.8
1,3-Diglycerides have been suggested as drug delivery vectors.9
These lipids are symmetrical molecules with 2 aliphatic chains in
the 1 and 3 positions and a free alcohol in position 2 of the
glycerol backbone. Initially, the X-ray data were interpreted to
show a head-to-head arrangement of the alcohols.10 In analogy to
the crystal structure of the 1,3-diglyceride of 3-thiadodecanoic
acid,11 the molecules are now thought to pack in an extended
form.12 The 1,3-diamido lipid Pad–OH–Pad (10) has already been
reported previously and its low solubility has been mentioned
aDepartment of Organic Chemistry, University of Geneva, Quai Ernest-Ansermet 30,
1211 Geneva, Switzerland
bDepartment of Physical Chemistry, University of Geneva, Quai Ernest-Ansermet 30,
1211 Geneva, Switzerland. E-mail: hans-rudolf.hagemann@unige.ch
c
´
Department of Chemistry, University of Fribourg, Chemin du Musee 9, 1700
Fribourg, Switzerland. E-mail: andreas.zumbuehl@unifr.ch
Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra40704g
3
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several times.9,13–15 However, a concise study of this phenomenon
has been missing until now.
15.6 mmol) in H2O (13 mL). At 20 uC, palmitoyl chloride (2.62 mL,
8.57 mmol) was added in one step. After stirring for 3 h, the
precipitate was washed with 100 mL of diethyl ether (100 mL),
H2O (100 mL) and methanol (100 mL). 2.00 g, 3.53 mmol, 84%) of
a white powder were obtained. 1H NMR (500 MHz, CDCl3): d 6.13
(s, 2H), 3.82 (s, 1H), 3.76 (q, J = 5.0 Hz, 1H), 3.44–3.36 (m, 2H), 3.27
(dd, J = 20.1, 5.2 Hz, 2H), 2.43–1.86 (m, 4H), 1.63 (q, J = 7.1 Hz, 4H),
1.27 (s, 48H), 0.91–0.87 (m, 6H). IR (cm21): 3294, 3100, 2916, 2849,
1640, 1565, 1466, 1439, 1376, 1268, 1250, 1230, 1116, 1086, 843,
721, 634. HRMS (ESI) m/z calcd for C35H71N2O3 [M + H]+ 567.5459,
found 567.5456. Mp: 132–134 uC.
Experimental
General remarks
Starting compounds and solvents were purchased from Sigma-
Aldrich/Fluka or Acros and were used without further purification.
Column chromatographic separations were carried out using 230–
1
400 mesh silica gel. TLC plates were developed with KMnO4. H
and 13C NMR spectra were recorded (as indicated) on either a
Bruker 300 MHz, 400 MHz or 500 MHz spectrometer and are
reported as chemical shifts (d) in ppm relative to TMS (d = 0). Spin
multiplicities are reported as a singlet (s), doublet (d), or triplet (t)
with coupling constants (J) given in Hz, or multiplet (m). Broad
peaks are marked as br. HRESI-MS were performed on a QSTAR
Pulsar (AB/MDS Sciex) spectrometer and are reported as mass-per-
charge ratio m/z. IR spectra were recorded on a Perkin-Elmer
Spectrum One FT-IR spectrometer (ATR, Golden Gate). Additional
IR spectra were performed using a Biorad Excalibur Instrument
equipped with high and low temperature Specac Golden Gate ATR
setups. DSC traces were obtained with a Mettler Toledo DSC1 Star
Systems differential scanning calorimeter from 3 to 5 mg samples
(5 uC min21, under N2).
Synthesis of cyclo-Pad–OH–Pad (11)
Hexahydropyrimidin-5-ol (18) (402 mg, 3.94 mmol) was dissolved
in a mixture of CH2Cl2 (18 mL), toluene (7 mL), NaOH (693 mg,
17.3 mmol) in H2O (13 mL). At 20 uC, palmitoyl chloride (2.4 mL,
7.9 mmol) was added in one step. After stirring for 3 h, the
solution was extracted with saturated NaHCO3 (50 mL) and
washed twice with 50 mL of CH2Cl2 (50 mL). The organic phases
were dried over MgSO4 and the solvents were removed under
reduced pressure. The crude material was purified by silica gel
column chromatography using a gradient of CH2Cl2 then CH2Cl2–
ethyl acetate 1 : 1. Cyclo-Pad–OH–Pad (11) was isolated as a white
powder (935 mg, 1.62 mmol, 41%). Rf = 0.5 (CH2Cl2–ethyl acetate
1 : 1). 1H NMR (500 MHz, CDCl3): d 5.69 (d, J = 13.1 Hz, 1H), 4.54
(d, J = 13.1 Hz, 1H), 4.16 (dd, J = 13.6, 3.4 Hz, 1H), 3.95–3.85 (m,
1H), 3.73 (dd, J = 13.8, 4.1 Hz, 1H), 3.54 (d, J = 13.6 Hz, 1H), 3.46 (d,
J = 13.5 Hz, 1H), 2.67–2.50 (m, 2H), 2.33–2.39 (m, 2H), 1.66 (d, J =
46.1 Hz, 9H), 1.26 (s, 49H), 0.89 (t, J = 6.8 Hz, 6H). 13C NMR (126
MHz, CDCl3): d 174.35, 172.91, 64.20, 55.91, 51.01, 47.83, 33.16,
32.91, 31.94, 29.71, 29.38, 25.29, 24.95, 22.71, 14.21. HRMS (ESI)
m/z calcd for C36H71N2O3 [M + H]+ 579.5459, found 579.5466. IR
(cm21): 3414, 2918, 2850, 1658, 1619, 1468, 1255, 1145, 885, 722.
Mp: 84–86 uC.
Synthesis of Pet–CH3 (4)
The synthesis was improved from an existing procedure.16
1-Bromohexadecane (1.00 g, 3.28 mmol) was dissolved in 18 mL
of dry MeOH containing cesium carbonate (1.04 g, 3.28 mmol).
The solution was put under microwave radiation at 150 uC for 1 h.
50 mL of CH2Cl2 was added and the mixture was filtered in order
to remove any insoluble salts. Then the organic solvents were
evaporated under reduced pressure and the compound was
purified by column chromatography (pentane then pentane–
CH2Cl2 3 : 1). The product was obtained as a colorless oil (683 mg,
1
2.66 mmol, 81%). Rf = 0.5 (pentane–CH2Cl2 3 : 1). H NMR (300
Synthesis of Pad–OTIPS–Pad (12)
MHz, CDCl3): d 3.39–3.19 (m, 5H), 1.59–1.43 (m, 2H), 1.22 (s, 26H),
0.84 (t, J = 6.5 Hz, 3H). 13C NMR (101 MHz, CDCl3): d 72.85, 58.29,
31.96, 29.75, 29.66, 29.55, 29.42, 26.18, 22.68, 13.99. Mp: 19–21 uC.
To Pad–OH–Pad (440 mg, 0.78 mmol) and NEt3 (430 mL, 3.10
mmol) in CH2Cl2 (12 mL) was added TIPS-triflate (470 mL, 1.75
mmol) and the mixture was stirred for 20 h at 20 uC. Then the
mixture was extracted with saturated NaHCO3 (50 mL) and washed
2 times with CH2Cl2 (2 6 50 mL). The organic phases were dried
over MgSO4 and removed under reduced pressure. The crude
material was purified by silica gel column chromatography using a
solvent gradient (pentane–EtOAc 9 : 1 then 4 : 1) to give the
product as a white powder (480 mg, 0.66 mmol, 94%). Rf = 0.5
Synthesis of Pes–OH–Pes (9)
2-(Benzyloxy)propane-1,3-diyl dipalmitate (275 mg, 0.417 mmol)
was dissolved in 8 mL of THF and Pd/C 10% was added (180 mg,
0.152 mmol). Hydrogen was injected into the system with Parr
apparatus and the solution was stirred at 50 psi overnight. The
catalyst was filtered with celite and the product was purified by
silica gel column chromatography (pentane–ethyl acetate 9 : 1) to
give the product as a white solid (175 mg, 0.308 mmol, 74%). Rf =
1
(pentane–ethyl acetate 4 : 1). H NMR (400 MHz, CDCl3): d 6.23
(dd, J = 8.6, 3.8 Hz, 2H), 3.94 (tt, J = 6.6, 3.3 Hz, 1H), 3.88–3.77 (m,
1H), 2.71 (ddd, J = 13.7, 6.6, 4.2 Hz, 2H), 2.31–2.11 (m, 3H), 1.73–
1.53 (m, 4H), 1.26 (d, J = 15.9 Hz, 43H), 1.07 (d, J = 4.7 Hz, 19H),
0.87 (t, J = 6.8 Hz, 6H). 13C NMR (101 MHz, CDCl3): d 174.18, 69.03,
41.44, 36.88, 31.98, 29.72, 29.55, 29.40, 25.80, 22.74, 18.09, 14.17,
12.28. IR (cm21): 3303, 2923, 2854, 1650, 1549, 1465, 1380, 1225,
1118, 1012, 883, 734, 679. HRMS (ESI) m/z calcd for C35H71N2O3
[M + H]+ 723.6793, found 723.6791. Mp: 51–53 uC.
1
0.18 (pentane–ethyl acetate 9 : 1). H NMR (400 MHz, CDCl3): d
4.20–4.11 (m, 5 H), 2.41 (s, 1 H), 2.34 (t, J = 7.3 Hz, 2 H), 1.63–1.61
(m, 4 H), 1.25 (s, 48 H), 0.88 (t, J = 8.0 Hz, 6 H). HRMS (ESI) m/z
calcd for C37H71O7 [M + MeCOO2]2 627.5205, found 627.5205.
Synthesis of Pad–OH–Pad (10)
The synthesis was performed similar to a literature procedure.13
1,3-Diamino-2-propanol (385 mg, 4.19 mmol) was dissolved in a
mixture of CH2Cl2 (18 mL), toluene (7 mL) and NaOH (625 mg,
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Synthesis of hexahydropyrimidin-5-ol (17)
The synthesis was performed according to a literature procedure.17
A solution of 1,3-diaminopropan-2-ol (1.00 g, 11.1 mmol) and
paraformaldehyde (0.31 g, 10.4 mmol) in methanol (17 mL) was
heated under reflux for 48 h. Then the solvent was removed under
reduced pressure. The resulting solid residue was washed with
THF to afford a yellowish crystalline solid (1.05 g, 10.3 mmol,
1
93%). H NMR (300 MHz, D2O): d 3.68 (d, J = 13.0 Hz, 1H), 3.61
(dd, J = 7.2, 3.5 Hz, 1H), 3.50 (d, J = 12.9 Hz, 1H), 3.11 (dd, J = 13.4,
3.2 Hz, 2H), 2.64 (dd, J = 13.3, 7.4 Hz, 2H). 13C NMR (101 MHz,
CDCl3): d 63.78, 58.94, 49.26.
Synthesis of 2-(benzyloxy)propane-1,3-diyl diparmitate (19)
Scheme 1 Synthesis of cyclo-Pad–OH–Pad (11).
Palmitoyl chloride (3.74 mL, 12.0 mmol) was added dropwise to 2-
(benzyloxy)propane-1,3-diol (18, 960 mg, 5.22 mmol) and pyridine
(1.10 mL, 13.6 mmol) in 20 mL of CH2Cl2 at 0 uC. Then the
solution was stirred for 20 h at rt under a N2 atmosphere. After
addition of H2O (20 mL), the solution was stirred for 5 min.
Extraction was achieved with the addition of aqueous HCl 1 M (40
mL). The aqueous phase was washed with CH2Cl2 (50 mL). The
organic phases were washed with NaHCO3 10% (50 mL). The
aqueous phase was washed again with CH2Cl2 (50 mL) and all the
organic phases were dried with MgSO4. The crude material was
purified by silica gel column chromatography (CH2Cl2) to give 2-
(benzyloxy)propane-1,3-diyl dipalmitate as a white solid (3.42 g,
5.19 mmol, 99%). Rf = 0.5 (CH2Cl2). 1H NMR (400 MHz, CDCl3) d
5.66 (d, J = 11.5 Hz, 1H), 4.56 (d, J = 9.0 Hz, 1H), 4.27–4.03 (m, 1H),
3.90 (s, 1H), 3.73 (d, J = 8.5 Hz, 1H), 3.68–3.38 (m, 2H), 2.73–2.48
(m, 2H), 2.36 (d, J = 23.3 Hz, 3H), 1.62 (s, 5H), 1.26 (s, 48H), 1.02–
0.76 (m, 6H). 13C NMR (101 MHz, CDCl3) d 173.56, 137.84, 128.44,
127.82, 74.50, 72.12, 63.02, 34.20, 31.96, 29.73, 29.50, 24.93, 22.73,
14.15. FTIR (cm21): 2923, 2853, 1741, 1458, 1378, 1167, 1115, 1053,
735, 697. HRMS (ESI) m/z calcd for C42H78NO5 [M + NH4]+
676.5874, found 676.5892.
Single-tail surfactants
First, we focused on a series of single-tail surfactants containing 16
carbons and various chemical motifs at the hydrophilic end of the
molecule (see Table 1). A clear correlation between the melting
point of the pure surfactant and its propensity to form hydrogen
bonds was found: as expected, an amide functionality (capable of
both accepting and donating H-bonds) would increase the melting
temperature of a molecule compared to molecules containing
chemical moieties which would form weaker H-bonds such as
esters and ethers. Here, the amide Pad–H (3) melted 53 K higher
than the ether Pet–H (1) and 42 K higher than the acid Pes–H (2).
The introduction of a methyl end group on the hydrophilic side
of the molecule would be expected to reduce a tight packing of the
surfactants and thus a lower melting point was expected when
going from Pet–H (1) to Pet–CH3 (4). Indeed, a reduction by 30 K
was found. The same 30 K reduction was also found between Pes–
H (2) and Pes–CH3 (5), where the inhibition of the hydrogen bond
donation moiety apparently only had a minor effect on the
melting temperature. The mono-methylation of Pad–H (3) to Pad–
CH3 (6) only led to a reduction by 7 K meaning that neither the
packing nor the hydrogen bonding was significantly reduced (see
Results and discussion
Synthesis
Several C16-surfactants have already been reported in the
literature. However, in order to complete the picture and to be
able to draw meaningful conclusions, we have synthesized
additional new compounds.
Cyclo-Pad–OH–Pad (11) was available through a double amide
formation between pyrimidin-5-ol (17) and the corresponding
palmitoyl chloride (see Scheme 1). Pyrimidin-5-ol (17) itself was
made beginning from the commercially available 1,3-diaminopro-
panol (16) via a condensation reaction using paraformaldehyde.
Again, starting from 1,3-diaminopropanol (16), Pad–OH–Pad
(10) was made via a double amide formation using palmitoyl
chloride (see Scheme 2). A protection with triisopropyl triflate led
to Pad–OTIPS–Pad (12).
We have also included an improved synthesis of Pes–OH–Pes (9,
see Scheme 3):18 starting from 2-(benzyloxy)propane-1,3-diol, a
double esterification with two equivalents of palmitoyl chloride led
to the benzyl-protected Pes–OBn–Pes (19) that was readily
deprotected with H2 over Pd to give Pes–OH–Pes (9).
Scheme 2 Synthesis of Pad–OTIPS–Pad (12).
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Scheme 3 Synthesis of Pes–OH–Pes (9).
Fig. 1 Increasing intermolecular hydrogen bonding interactions leads to higher
melting temperatures (2 to 3). The effect is reversed by blocking the hydrogen bond
donating protons on the amide nitrogens (3 to 6 to 7).
Fig. 1). On the other hand, the fully methylated Pad–(CH3)2 (7)
melted at a temperature that was 58 K lower than Pad–H (3) and
both the disruption of hydrogen bonding and molecular packing
became evident.
would significantly reduce this effect. We have therefore
synthesized cyclo-Pad–OH–Pad (11), a molecule which has both
amides bridged by a methylene group. Indeed this molecule (11)
shows a large reduction of the melting point by 46 K compared to
Pad–OH–Pad (10). We hypothesize that this effect is mainly due to
the presence or absence of intermolecular hydrogen bonding
interactions.
The introduction of a bulky head-group will create a large steric
barrier and will force the surfactant molecules to increase their
intermolecular distance (see Fig. 2). This will reduce the
contribution of the hydrogen bonding and overrule this effect of
the chemical group linking together the backbone and the alkyl
chains. Indeed, Pad–OTIPS–Pad (12) shows a low melting point at
Double-tail surfactants
We next analyzed a series of double-tail surfactants (see Table 2).
Here, an ether-based backbone moiety would be expected to
lead to the lowest melting point of all the listed molecules due to
the absence of hydrogen bonding between molecules. A slightly
higher melting point would be reached for the more hydrogen
bonding prone esters and the highest melting point would be
expected for amides. Indeed, Pet–OH–Pet (8) does melt at 16.8 K
lower than its ester analog Pes–OH–Pes (9) and a full 76.8 K lower
than the amide (10).
If an amide such as Pad–OH–Pad (10) would form hydrogen
bonds, the substitution of the amide proton by an alkyl group
Table 1 Melting points of a library of C16 single-tail surfactants
Chemical Structure
Compound name
Melting point [uC]
Reference
Vendor
Pet–H (1)
49–51
Pes–H (2)
Pad–H (3)
61–62.5
Vendor
19
104–104.5
Pet–CH3 (4)
Pes–CH3 (5)
19–21
30–31
20
19
Pad–CH3 (6)
98.6–99.8
46–47.8
Pad–(CH3)2 (7)
19
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Table 2 Melting points of a library of C16 double-tail surfactants
Chemical structure
Compound name
Melting point [uC]
Reference
21
Pet–OH–Pet (8)
54–55.2
Pes–OH–Pes (9)
72–73
22
Pad–OH–Pad (10)
Cyclo-Pad–OH–Pad (11)
Pad–OTIPS–Pad (12)
132–134
84–86
51–53
Pet–racPet–OH (13)
Pes-Pes-OH (14)
58–60
Vendor
R: 62–64
S: 63–65
23
24
Pad–racPad–OH (15)
110–112.5
25
the same level as Pet–OH–Pet (8), a surfactant that cannot form
intermolecular hydrogen bonds.
membrane at the C2 position.26 Analogously, this will organize the
esters of Pes–OH–Pes (9) and the amides of Pad–OH–Pad (10) at
the same vertical level and allow a good preorganization for the
formation of hydrogen bonds. A 1,2-backbone moiety will place
the esters of Pes–Pes–OH (13) and the amides of Pad–Pad–OH (14)
on different vertical levels and the preorganization for hydrogen
bonding is less optimal. Although Pes–Pes–OH (13) and Pad–Pad–
OH (14) show a hydrogen bonding effect, it is less pronounced
than in Pes–OH–Pes (9) or Pad–OH–Pad (10) as can be seen by the
lowered melting temperatures (62 uC vs. 72 uC and 110 uC vs. 132
uC, respectively).
It is interesting to note that going from a single-tail to a double-
tail surfactant, the melting temperature should significantly
increase. Indeed, for all the reasonable comparisons (e.g. 4 and
8; 5 and 9; 6 and 10; 7 and 11), an increase of .30 K in the melting
temperatures can be found. The proximity of two H-bond forming
moieties in the same molecule (e.g. 10) does not lead to a larger
increase of the melting temperature. Apparently, the proximity
A change from a 1,3-binding moiety (as found e.g. in Pet–OH–
Pet (8)) to a more natural 1,2-binding moiety (e.g. in the 1,2-diether
lipid Pet–Pet–OH (13)) will lead to a rearrangement of the
hydrophilic portion of the crystal packing. However, this effect
apparently has only a minor influence on the melting point (54 uC
for Pet–OH–Pet (8) vs. 58 uC for Pet–Pet–OH (13)) and does not
significantly change the overall stability of the crystal packing.
A second, more pronounced effect can be seen when hydrogen
bonds are involved. The case is analogous to phospholipids: 1,2-
phospholipids were found to have the glycerol backbone
organized almost parallel to the membrane normal.26 The acyl
chain at sn-1 extends the zig-zag conformation of the backbone
and immerses right into the membrane. The acyl chain at sn-2
first organizes parallel to the membrane surface and only turns
down into the membrane at C2. A 1,3-phospholipid is organized
more symmetrically and will have both acyl chains extending out
parallel to the membrane surface and bending down into the
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Table 4 Temperature dependence of the solubility of Pad–OH–Pad (10)
Solubility [mg mL21
]
THF
CHCl3
CH2Cl2
5 uC
23 uC
0.40
0.53
0.66
0.85
0.49
0.73
solvents. The solubility of Pad–OH–Pad (10) in organic solvents is
temperature dependent as can be seen in Table 4.
Differential scanning calorimetry measurements
Fig. 3 shows the DSC trace of Pet–OH–Pet (8) and Pad–OH–Pad
(10). The ether lipid shows a single peak at 60.5 uC corresponding
to the melting of the chains at a high transition enthalpy of DH =
126.9 kJ mol21 ¡ 2.13 kJ mol21. However Pad–OH–Pad shows two
peaks: one for the chain melting at 134.8 uC (DH = 14.05 kJ mol21
¡ 1.06 kJ mol21) and a broader peak at 119.2 uC (DH = 71.45 kJ
mol21 ¡ 1.83 kJ mol21). This finding is consistent with DSC
studies performed for the asymmetric 1,2-diamido-phospholipid
Mad–Mad–PC where a similar broad peak was attributed to the
breaking of the hydrogen bond region.27
If Pad–OH–Pad (10) were equipped with a phospholipid head
group such as phosphocholine, it would lead to the 1,3-diamido
phospholipid Pad–PC–Pad. This substitution has a profound effect
on the packing geometry and is expected to significantly reduce
Tm, which is found with the Tm of 35 uC at a transition enthalpy of
DH = 30.38 kJ mol21 (in a vesicle bilayer and a U-shaped
morphology).2
Fig. 2 Increasing hydrogen bonding from esters (9) to amides (10) leads to
increased melting temperatures. Weakening the intermolecular forces by spacing
apart the molecules (12) leads to a drastic drop in melting temperature.
effect between the two tails in a double-tail surfactant is mainly
influenced by van der Waals forces.
Infrared studies
The range of infrared signals between 3000 cm21 and 3500 cm21
can be used as a convenient means for detecting intramolecular
hydrogen bonds. In order to study the influence of increasing
temperature on hydrogen bonds, we analyzed a solid sample of
Pad–OH–Pad (10, see Fig. 4). The spectra show two hydrogen bond
related stretch vibrations at 3100 cm21 (N–H) and at 3300 cm21
(O–H). The position of the O–H band remains practically constant
between 24 uC and 100 uC, then the position increases by about 10
cm21 at 110–120 uC and increases further above 130 uC.
A significant line broadening is also observed above 130 uC.
These changes correlate with the transitions observed by DSC (see
Fig. 3). In the case of the IR data, the changes appear at a
somewhat lower temperature, but in this case, the temperature
was increased at a much slower rate (20 min for a 10 uC change)
Solubility studies
The presence of hydrogen bonds might be at the origin of the high
melting temperatures measured for the diamido phospholipids.
In parallel, such an intermolecular hydrogen bonding network
might also reduce the solubility of the molecule. Both Pad–OH–
Pad (10) and Pes–OH–Pes (9) are expected to form intermolecular
hydrogen bonds and both molecules show a reduced solubility in
organic solvents (see Table 3). In the case of cyclo-Pad–OH–Pad
(11) and Pad–OTIPS–Pad (12) the propensity to form intermole-
cular hydrogen bonds is reduced due to the methylation of the
amide nitrogen (11) or due to the increased intermolecular
spacing (12). Both molecules show a high solubility in organic
Table 3 Solubility study of selected double-tail surfactants (measured at 20 uC)
Pes–OH–Pes (9)
Pad–OH–Pad (10)
Cyclo-Pad–OH–Pad (11)
Pad–OTIPS–Pad (12)
Solubility [mg mL21
Solubility [mM]
]
THF
50
50
35
88
88
62
,0.8
,0.8
,0.8
,1.4
,1.4
,1.4
.100
.100
.100
.173
.173
.173
.100
.100
.100
.138
.138
.138
CHCl3
CH2Cl2
THF
CHCl3
CH2Cl2
7242 | RSC Adv., 2013, 3, 7237–7244
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Fig. 3 Differential scanning calorimetry scans of Pet–OH–Pet (8, solid line) and Pad–
OH–Pad (10, dashed line).
Fig. 5 Temperature dependent IR study of the OH stretch vibration region of cyclo-
Pad–OH–Pad (11).
compared to the DSC experiments (5 uC min21). This hints at the
possibility that an intermolecular O H–O hydrogen bond is
…
present between the secondary alcohol and an amide carbonyl
group of a neighboring molecule. This interaction seems to be a
determining feature both of the structure of Pad–OH–Pad and its
thermal behavior. The influence of the second hydrogen bond
between an amide N–H and the carbonyl group of a second amide
is clearly inferior to the first hydrogen bond as this stretching
vibration is virtually absent at 130 uC .
Similarly, cyclo-Pad–OH–Pad (11) was analyzed (see Fig. 5). As in
the case of Pad–OH–Pad (10), a strong hydrogen bond exists
between the free secondary alcohol and the carbonyl group of an
amide. This bond disappears above 80 uC, which is in perfect
agreement with the melting temperature of cyclo-Pad–OH–Pad
(84–86 uC). In contrast to Pad–OH–Pad, no amide N–H stretch
vibration was found nor expected. This finding corroborates our
analysis that hydrogen bonds are an important factor in
determining the melting characteristics of the double-tail surfac-
tants.
At low temperatures, the CH2 bending mode (at ca. 1460 cm21
)
as well as the CH2 scissoring mode (at 720–730 cm21) reveal a
splitting which is characteristic for the chain packing found in the
orthorhombic phase of the odd carbon containing n-alkanes such
as C21H44.28,29 At 120 uC (above the first phase transition), this
splitting disappears (see Fig. 6). This behavior is similar to the one
observed in alkanes at the orthorhombic to hexagonal phase
transition.28
Simultaneously, the amide I and, in a more pronounced way,
the amide II bands [e.g. ref. 30] are changed. It has been shown31
that the relative intensity of the two components of the amide I
band around 1640 cm21 is related to the backbone conformation
close to the amide groups. These observations show that at this
phase transition, the packing is somewhat loosened not only from
the inter-chain contacts, but also from some torsion of the peptide
backbone.
Above the melting point, both amide bands are shifted and
broadened significantly, and the CH2 bending mode has become
Fig. 4 Temperature dependent IR study of the OH and NH stretch vibration region
of Pad–OH–Pad (10).
Fig. 6 Comparison of the IR spectra of Pad–OH–Pad (10) at 30 uC, 120 uC, and 170
uC.
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RSC Advances
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Conclusions
In this article we have analyzed the melting and solubility
properties of several single- and double-tailed surfactants contain-
ing various chemical functionalities between the hydrophobic and
hydrophilic portions of the molecules. The presence of inter-
molecular hydrogen bonds between the free alcohol and the
carbonyl oxygen of a neighboring molecule is suggested to have a
major influence on the melting behaviour of the substances.
Furthermore, we show that diamido-lipids are capable of forming
an intermolecular hydrogen bonding network. The combination
of alcohol–amide carbonyl as well as amide–amide hydrogen
bonds is hypothesized to lead to the very low solubility of Pad–
OH–Pad (10).
Acknowledgements
The work was supported by the Swiss National Science
Foundation (200020_132035), COST Action D43, and the
NCCR in Chemical Biology. We thank the Mass Spectroscopy
and NMR facilities at the University of Geneva for analytical
services. We thank Jiri Mareda for fruitful discussions and
helpful comments.
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7244 | RSC Adv., 2013, 3, 7237–7244
This journal is ß The Royal Society of Chemistry 2013