Langmuir
Article
Syntheses of Racemic Hydroxy Fatty Acids. Esters of the hydroxyl-
substituted fatty acids were hydrolyzed to obtain the corresponding
fatty acids.21 In a typical procedure, ethyl 8-hydroxyoctadecanoate (1.4
g, 4.3 mmol) was refluxed with 95% ethanol (10 mL) and 9 mL of 2.5
N aqueous KOH for 4.5 h. After the reaction, ethanol was evaporated,
and the reaction mixture was diluted with 50 mL of water. The
resulting solution was acidified to pH 7 with 1:1 concd hydrochloric
acid:water. The precipitated solid was filtered and dried at 60 °C for
24 h. It was recrystallized from acetone to obtain 1.1 g (84%) of 8-
hydroxyoctadecanoic acid.
refluxing conditions, but a small amount of starting material remained
unreacted. As noted below, this material melted over a range of 5.1 °C.
It was purified by column chromatography using silica gel after
converting the acid to an ester (see below). 14-Hydroxyoctadecanoic
acid: mp 70.5−75.6 °C. 1H NMR (CDCl3, 400 MHz) δ 0.89−0.92 (m,
3H), 1.27−1.65 (m, 33H), 2.33−2.37 (m, 3H), 0.359 (s, 1H); IR
(neat) 2915, 2848, 1698, 1463, 1430, 1382, 1305, 1283, 1262, 1233,
1211, 1187, 1128, 1054, 938, 909, 766, 728, 720, 684, 624, 603 cm−1.
MS-ESI: m/z calculated 300 + 23; observed 323 (M + Na, C18H36O3).
Synthesis of Methyl 14-Hydroxyoctadecanoate. Impure 14-
hydroxyoctadecanoic acid from above was esterified using methanol/
H2SO4 mixtures. In a typical reaction, 14-hydroxyoctadecanoic acid
(485 mg, 1.6 mmol) dissolved in 20 mL of methanol and 1 mL of
concentrated H2SO4 was refluxed for 12 h. The reaction mixture was
then cooled, and excess methanol was evaporated under reduced
pressure using a rotary evaporator. The residual solid was then
extracted using dichloromethane (4 × 20 mL), and the combined
extracts were washed with water and dried over anhydrous Na2SO4.
The dichloromethane was evaporated on a rotary evaporator, and the
product was further purified by silica gel column chromatography
using 1:19 ethyl acetate:hexane as the eluent to get 386 mg (76% yield;
∼99% pure by GC) of methyl 14-hydroxyoctadecanoate, mp 52.6−
54.6 °C. 1H NMR (CDCl3, 400 MHz) δ 0.89−0.93 (t, 3H, J = 7 Hz),
1.26−1.43 (m, 28H), 2.28−2.32 (t, 2H, J = 7.8 Hz), 3.59 (s, 1H), 3.67
(s, 3H); IR (neat) 3477, 2916, 2849, 1739, 1464, 1435, 1383, 1359,
1314, 1295, 1275, 1254, 1238, 1222, 1198, 1174, 1134, 1116, 1062,
1046, 1018, 993, 973, 952, 885, 864, 793, 759, 728, 702, 603 cm−1.
Methyl 14-hydroxyoctadecanoate (300 mg, 0.95 mmol) was
refluxed with 95% ethanol (10 mL) and 9 mL of 2.5 N aqueous
KOH for 4.5 h. Ethanol was evaporated, and the reaction mixture was
diluted with 50 mL of water. The resulting solution was acidified with
a solution of concd hydrochloric acid and water in a 1:1 ratio. The
precipitated solid was filtered and dried at 60 °C for 24 h. The white
solid was recrystallized from acetone to obtain 276 mg (90%) of 14-
8-Hydroxyoctadecanoic acid (8HSA): mp 80.6−82.4 °C (lit. mp
78.5−79 °C31); 1H NMR (CDCl3, 400 MHz) δ 0.86−0.90 (t, 3H, J =
6.8 Hz), 1.26−1.67 (m, 28H), 2.33−2.37 (t, 2H, J = 7.4 Hz), 3.58 (m,
1H); IR (neat) 3374, 2916, 2849, 1734, 1693, 1464, 1433, 1314, 1297,
1280, 1263, 1240, 1228, 1204, 1194, 1131, 1115, 1103, 1047, 1022,
996, 898, 857, 839, 720 cm −1. MS-ESI: m/z calculated 300.5;
observed 323 (M + Na, C18H36O3). Elemental analysis for C18H36O3;
C 71.95, H 12.08 (calculated); C 71.92, H 12.53 (observed).
10-Hydroxyoctadecanoic acid (10HSA): yield (72%), mp 78.1−79.3
1
°C; H NMR (CDCl3, 400 MHz) δ 0.86−0.90 (t, 3H, J = 6.8 Hz),
1.28−1.65 (m, 28H), 2.32−2.36 (t, 2H, J = 7.4 Hz), 3.58−3.6 (m,
1H); IR (neat) 3389, 2919, 2849, 1714, 1690, 1465, 1433, 1337, 1315,
1295, 1281, 1260, 1223, 1192, 1132, 1116, 1101, 1033, 997, 891, 842,
795 cm −1; MS-ESI: m/z calculated 300.5; observed 323 (M + Na,
C18H36O3); Elemental analysis for C18H36O3;C 71.95, H 12.08
(calculated); C 72.11, H 12.52 (observed).
Synthesis of Racemic 12-Hydroxystearic Acid (12HSA). D-12HSA
(3.0 g, 10 mmol) was added to a stirred solution of Na2Cr2O7 (2.1 g,
7.0 mmol) in 3−5 mL of DMSO. Concd H2SO4 (2.0 g, 2.5 equiv) was
added dropwise with stirring, maintaining the temperature below 80
°C. The mixture was heated and stirred at 70 °C for 2 h and stirred for
an additional 12 h at ambient temperature. The reaction mixture was
poured into ice-cold water, and the solid that precipitated was filtered.
The solid was chromatographed using a silica gel column and 1:9 ethyl
acetate:hexane as eluent. After recrystallization from acetone, 1.0 g
(33%) of 12-oxooctadecanoic acid was obtained as an off-white solid,
mp 78.7−81.4 °C. 1H NMR (CDCl3, 400 MHz) δ 0.86−0.90 (m, 3H),
1.27−1.65 (m, 24H), 2.33−2.40 (m, 6H); purity ∼99% by GC.
The 12-oxooctadecanoic acid was reduced to 12HSA by NaBH4 in
ethanol.21 In a typical reaction, 12-oxooctadecanoic acid (1 g, 3 mmol)
was added to 25 mL of absolute ethanol with stirring at room
temperature. The solvent was warmed slightly to dissolve the
compound. Sodium borohydride (0.1 g, 3 mmol) was then added,
and the mixture was stirred at room temperature for 1.5 h. Excess
sodium borohydride was destroyed by neutralizing with glacial acetic
acid. The ethanol and acetic acid were removed under vacuum, and the
yellowish-white residue was dried under vacuum for an additional 12 h.
It was washed with water (4 × 30 mL), dried, and recrystallized from
acetone to yield 0.90 g (96%) of DL-12HSA, mp 74.9−76.9 °C (lit.
mp 76.2 °C).25 1H NMR (CDCl3, 400 MHz) δ 0.87−0.90 (t, 3H, J =
6.6 Hz), 1.28−1.65 (m, 28H), 2.17−2.36 (t, 2H, J = 7.4 Hz), 3.58−
3.60 (m, 1H); IR (neat) 3327, 2915, 2849, 1706, 1464, 1436, 1410,
1328, 1314, 1295, 1277, 1263, 1241, 1222, 1189, 1133, 1116, 1081,
1026, 1000, 920, 898, 861, 837, 794, 728, 721, 684, 631, 619, 605
cm−1. Analytical data for C18H36O3: C 71.95, H 12.08 (calculated); C
71.54, H 12.62 (observed).
Synthesis of Racemic 14-Hydroxyoctadecanoic Acid (14HSA). 14-
Oxooctadecanoic acid was reduced to its hydroxyl derivative using
NaBH4/ethanol. In a typical reaction, 14-oxooctadecanoic acid (1.4 g,
4.3 mmol) was added to 25 mL of ethanol with stirring at room
temperature (solution was slightly warmed to dissolve the compound).
Sodium borohydride (0.12 g, 4.3 mmol) was then added to this
solution, and the reaction mixture was stirred at room temperature for
1.5 h. Excess sodium borohydride was destroyed by neutralizing with
glacial acetic acid. The ethanol and acetic acid were evaporated on a
rotary evaporator; the remaining residue was dried under vacuum for
an additional 12 h. It was then extracted with water (4 × 30 mL),
dried, and recrystallized from acetone. The product contained a small
amount of the starting material, and repeated crystallizations did not
remove it. The reaction was carried out at higher temperatures and at
1
hydroxyoctadecanoic acid, mp 75.8−77.7 °C. H NMR (CDCl3, 400
MHz) δ 0.89−0.93 (t, 3H, J = 7 Hz), 1.26−1.67 (m, 28H), 2.33−2.36
(t, 2H, J = 7.4 Hz); IR (neat) 3494, 3387, 2916, 2849, 1738, 1464,
1435, 1383, 1359, 1314, 1296, 1275, 1222, 1198, 1175, 1134, 1117,
1061, 1046, 1018, 993, 974, 884, 864, 793, 759, 728, 702, 624, 604
cm−1. MS-ESI: m/z calculated 300 + 23; observed 323 (M + Na,
C18H36O3).
RESULTS AND DISCUSSION
■
Viscous solutions instead of gels were formed when sols of up
to 2 wt % 2HSA or 3HSA in mineral oil were cooled from 110
°C to 30 °C isothermally (i.e., the sol was placed on the Peltier
plate at the preset temperature). Mineral oil samples at the 2.0
wt % 6HSA, 8HSA, 10HSA, 12HSA, and 14HSA could be
inverted in a glass vial and did not exhibit flow. Rheological
measurements confirmed that the mineral oil samples of 2 wt %
samples of 6HSA, 8HSA, 10HSA, 12HSA, and 14HSA are true
gels (Figure 2). Their G′ values are virtually independent of
frequency over a very broad range. Further, G′ is greater than
G″. However, 2HSA and 3HSA were not and exhibited a G′/G″
crossover. At this time, the strength of the gel samples is known
only qualitatively. Although none of the HSA were capable of
gelating mineral oil at concentrations <1 wt %, which would
make them ‘supergelators’, they were able to form gels at
relatively low concentrations. The CGCs were found to be
∼1.7 wt % for 12HSA and 14HSA and ∼1.9 wt % for 6HSA,
8HSA, and 10HSA.
Early work on 12HSA organogels indicated the presence of a
SAFiN whose nature is dependent on the ability of the
carboxylic acid head groups to dimerize and the secondary
hydroxyl groups on the fatty acid backbone to form hydrogen-
bonding arrays.5,8,29 All of the HSA/mineral oil dispersions,
whether viscous liquids or gels, were opaque. The opacity or
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dx.doi.org/10.1021/la204412t | Langmuir 2012, 28, 4955−4964