The effect of poly(propylene glycol) on the formation of lyotropic liquid crystalline phases of amphiphiles containing glycerol head groups

Shin-Hong Ou, Sung-Chuan Hung, and Guo-Jhan Wong Institute of Biotechnology and Chemical Engineering, I-Shou University, Kaohsiung city, Taiwan Tzong-Ming Wu Department of Materials Science and Engineering, National Chung Hsing University, Taichung city, Taiwan

Abstract:

In order to prepare a nonlamellar lyotropic liquid crystalline structure, amphiphiles (PPG-Gly 340 and 1000) containing poly(propylene glycol)s of different molecular weights, 340 and 1000 respectively, as hydrophobic chains and glycerol as head groups were synthesized and investigated by optical microscopy and small angle X-ray scattering. The effect of poly(propylene glycol) on the developments of reverse bicontinuous cubic phase (V 2 ) and reverse hexagonal phase (H 2 ) was examined by the mixing of PPG-Gly with glyceryl monooleate (GMO). The addition of PPG-Gly to GMO expedites the lamellar-Ia3d (V 2 )-Pn3m (V 2 ) transformation of the hydrated GMO. Poly(propylene glycol)s of different chain lengths show rather different effects on both the phase behavior and the dimension of the structure. The Pn3m lattice parameter can be reduced by more than 20% on the addition of PPG-Gly1000. In contrast, the addition of PPG-Gly 340 increases the dimension by less than 5%. This result is explained by the hydrophobic chain splay. From polarizing optical microscopy, the addition of PPG-Gly 1000 to GMO enhances the formation of H 2 phase substantially. The appearance of the H 2 phase lasts for a temperature range of ca. 23℃. Keywords: poly(propylene glycol); glyceryl monooleate; lyotropic liquid crystal; small angle X-ray scattering.

Introduction:

In this study, we exploit the possibility of preparing lyotropic liquid crystalline amphiphiles with reverse phases by employing poly(propylene glycol) as hydrophobic chains. First of all, the methyl branching structure of PPG appears to be suitable for the formation of reverse phases. Secondly, lyotropic liquid crystalline phases are affected by hydrophobic chain length. Longer chain length may increase the bulkiness of hydrophobic region and thus favors the formation of reverse phases.

Poly(propylene glycol)s of different molecular weights are commercially available. By adjusting the molecular weight, one might be able to manipulate the liquid crystalline phase behavior and the dimension of the structure. Thirdly, the formation of H and facilitate the formation of H 2 phase.

2 phase is restricted by the packing constraint in the hydrophobic region. The employment of flexible PPG with suitable chain length may be able to effectively fill the energetically unfavorable space of reverse hexagonal structure with less energy cost

Results and discussion

Figure 4 Figure 5

85

Figure 1

HO HO O HO O HO O

PPG-Gly 18.7 Å

O O

GMO 17.5 Å

O O n O

90 85 80 60 55 50 75 70 65 1 2 3 PPG-Gly 1000 (mole %) 4 5 130 125 120 115 110 105 100 95 90 85 0 1 2 3 4 PPG-Gly (mole%) 5 6 7 80 75 70 65 60 55 20 30 40 50 Water (wt%) 60 70

Figure 2 Figure 6

Figure 1. Figure 2.

Molecular structure and dimensions of GMO and PPG-Glys. PPG Gly 340 was approximated by n = 4 and PPG-Gly 1000 by n = 15.

Figure 3. Figure 4.

SAXS profiles of the hydrated samples with various hydration values measured at 23℃. (A) GMO (B) PPG-Gly340/GMO= 3/97 (mole/mole) (C) PPG-Gly 1000/GMO = 3/97 ● indicate The Lα peaks. ▲ indicate the Ia3d peaks. ■ indicate the Pn3m peaks. At 20 wt% H2O, a single peak shows up for all of the threecompositions. The diffraction peak wasdetermined to be (001) plane from the Lα phases. New peaks appear for all samples at 30 wt% H2O.

Lattice parameters of the Pn3m phases of the hydrated samples with various compositions of PPG-Gly/GMO at 60 wt% H2O as determined by SAXS. Symbols: PPG-Gly 340/GMO (■), PPG-Gly 1000/GMO (▲), and GMO (●). at which the H 2 Temperature ranges of the H 2 phases of the hydrated samples with various compositions of PPG-Gly 1000/GMO at 50 wt% H finaltemperature at which the H 2 phases could be observed. 2 O as determined by polarizing optical microscopy. The triangles indicate the starting temperatures phases were observed. The squares indicate the

Figure 5.

Temperature ranges of the H 2 phases of the hydrated samples with the mole composition of PPG-Gly 1000/GMO = 3/97 at various weight percentages of water as determined by polarizing optical microscopy. The triangles indicate the starting temperatures at which the H 2 phases were observed. The squares indicate the final temperature at which the H 2 phases could be observed.

Figure 6.

Polarizing optical micrograph of PPG-Gly 1000/GMO = 3/97 (mole/mole) with 50 wt% H 2 O at 68℃