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J. Phys. Chem. B 2007, 111, 4082-4088
Formation of Amphiphile Self-Assembly Phases in Protic Ionic Liquids Tamar L. Greaves,† Asoka Weerawardena,† Celesta Fong,† and Calum J. Drummond*,‡,§ CSIRO Molecular and Health Technologies (CMHT), Bag 10, Clayton, Victoria 3169, Australia, CSIRO Molecular and Health Technologies (CMHT), P.O. Box 184, North Ryde, New South Wales 2113, Australia, and CSIRO Industrial Physics (CIP), P.O. Box 218, Lindfield, New South Wales 2070, Australia ReceiVed: October 3, 2006; In Final Form: January 9, 2007
A range of protic ionic liquids (PILs) have been identified as being capable of supporting the self-assembly of the nonionic surfactants myverol 18-99K (predominantly monoolein) and phytantriol. PIL-surfactant penetration scans have provided a high throughput technique to determine which lyotropic liquid crystalline phases were formed in the 40 PIL-surfactant systems investigated. Lamellar, inverse hexagonal, and bicontinuous cubic phases that are stable in excess PIL have been observed in surfactant-PIL systems. The studied PILs possess a wide range of solvent properties, including surface tension and viscosity. The nature of the formed amphiphile self-assembly phases is discussed in terms of the PIL structure and solvent properties.
Introduction Generally recognized protic solvents which have been broadly accepted as amphiphile self-assembly media for a number of years include water, glycerol, short chain alkane diols (e.g., ethylene glycol, propylene glycol, and butylene glycol), low molecular weight amides (e.g., acetamide, formamide, and N-methyl formamide), hydrazine, and ethylammonium nitrate (EAN).1-3 Until recently, EAN has been unique as an amphiphile self-assembly solvent because it is an ionic liquid (IL).1,4-10 The lyotropic phase behavior of a phospholipid 10 and polyoxyethylene nonionic surfactants4 in EAN and the formation of micelles in EAN6 were all similar to what had been seen in water. The ability of EAN to promote aggregation has been attributed to its protic nature and high cohesive energy which acts as a driving force for aggregation of surfactant molecules in the solvent.1 The ability to hydrogen bond was considered to govern the driving force for the self-assembly process.1,4 The properties and applications of the ionic liquid (IL) solvent class have been widely discussed.11-17 Protic ionic liquids (PILs) are an interesting subset of ILs.18-22 PILs, such as EAN, are formed through the stoichiometric combination of a Brønsted acid and a Brønsted base. Among other potentially useful properties, many different PILs are capable of supporting the self-assembly of amphiphiles, as has been demonstrated very recently with the cationic surfactant hexadecyltrimethylammonium bromide (CTAB).21 To date, PILs represent the largest known group of solvents which can be tailor-made to have specific physicochemical properties while retaining the ability to act as self-assembly media.20,21 The PIL group represents a very large number of protic solvents which have huge potential as new self-assembly media, and also in helping in the understanding of the self-assembly process.21 In this work, we extend our recent work with PIL-CTAB systems and present the lyotropic liquid crystalline phase * To whom correspondence should be addressed. E-mail: calum. drummond@csiro. † CSIRO Molecular and Health Technologies (CMHT), Clayton, Victoria. ‡ CSIRO Molecular and Health Technologies (CMHT), North Ryde, New South Wales. § CSIRO Industrial Physics (CIP), Lindfield, New South Wales.
behavior observed in 40 new PIL-surfactant systems. On the basis of high throughput surfactant-PIL penetration scans, we report the lyotropic liquid crystalline phases observed in a range of PILs as a function of temperature for the nonionic surfactants myverol 18-99K and phytantriol. Myverol 18-99K and phytantriol were used in the screening study reported herein because they are nonionic surfactants that possess lyotropic liquid crystalline phases, namely, bicontinuous cubic and inverse hexagonal phases, that are stable in excess water.23-26 In the present investigation, we were particularly interested in determining whether or not thermodynamically stable one-dimensional (1D) lamellar, two-dimensional (2D) inverse hexagonal, and three-dimensional (3D) bicontinuous cubic phases can exist in excess PIL. Stable submicron particles, namely, liposomes, hexosomes, and cubosomes, could exist in this situation and could potentially be used in a number of applications.26,27 Myverol consists of a number of monooacylglycerol components, with the main component being monoolein (approximately 70%).23 The structures of monoolein and phytantriol are shown in parts a and b of Scheme 1, respectively. Experimental Section The preparation and characterization of the PILs used herein have been described previously.20 The surfactants used were myverol 18-99K (approximately 70% monoolein, gift from Bronson and Jacobs, supplied by Quest International, melting point 35 °C) and phytantriol (96% purity, Aldrich, melting point 5-10 °C, boiling point 130 °C). PIL Penetration Behavior-Surfactant Self-Assembly. Penetration scans28 were conducted differently for the PILs which were liquid at room temperature compared to those which were solid. For liquid PILs, myverol and phytantriol were compressed between microscope slides and coverslips (myverol was melted by heating up to 35 °C) and allowed to cool prior to addition of the PILs to the edge of the coverslip. For the solid PILs at room temperature, a small portion was melted onto a glass slide and covered with a coverslip, then heated until melted, and then cooled and then either a drop of phytantriol or a drop of melted myverol was added to the edge of the coverslip. The PIL or
10.1021/jp066511a CCC: $37.00 Published 2007 by the American Chemical Society Published on Web 03/31/2007
Amphiphile Self-Assembly in Protic Ionic Liquids
J. Phys. Chem. B, Vol. 111, No. 16, 2007 4083
SCHEME 1: Structure of (a) Monoolein and (b) Phytantriol
TABLE 1: Protic Ionic Liquids (PILs) along with Their Water Content, Melting Point, Tm, Surface Tension, γLV, Viscosity, η, and Gordon Parameter, G (The Melting Point, Tm, Was Not Observable for All Samples in the DSC Traces; Tm, γLV, and η Are Reproduced from ref 20 and G from ref 21) PIL
abbreviation
water (% wt/wt)
methylammonium formate ethylammonium formate propylammonium formatea butylammonium formate pentylammonium formate 2-methyl-propylammonium formate 2-methyl-butylammonium formate 3-methyl-butylammonium formatea ethanolammonium formate 2-propanolammonium formate ethylammonium acetatea ethanolammonium acetate ethylammonium propionate ethylammonium butyrate ethylammonium glycolate ethylammonium lactate ethanolammonium lactate ethylammonium nitrate ethanolammonium nitrateb ethylammonium hydrogen sulfateb
MAF EAF PAF BAF PeAF 2MPAF 2MBAF 3MBAF EOAF 2POAF EAA EOAA EAP EAB EAG EAL EOAL EAN EOAN EAHS
0.46 0.38 0.61 0.32 0.26 0.71 0.64 0.41 0.55 0.45 0.12 0.47 0.42 0.26 0.50 0.85 0.51 0.22 0.72 0.31
Tm (°C)
γLV (mN/m) 27 °C
13 -15 50 2 12 26 -1 47 c c 87 c c c c c c 9 51 40
η (mPa‚s) 25 °C
G (J/m3)
43.1 38.5
17 32
1.041 0.867
33.3 31.9 31.2 30.8
70 78 225 229
0.669 0.614 0.629 0.596
65.0 46.2
220 854
1.448 0.977
51.5 31.5 29.6 49.3 39.3 57.2 47.3 50.6 56.3
701 75 208 1200 803 1324 32 113 128
1.099 0.644 0.576 1.056 0.793 1.149 1.060 1.097 1.215
a Denotes samples which were solid at room temperature. b Metastable liquid at room temperature when first prepared. c No crystalline to isotropic liquid phase transition, as this PIL forms a glass.
surfactant was drawn between the two glass surfaces by capillary action, establishing a concentration gradient with neat PIL and neat surfactant as the two extremes and liquid crystals, if any, in the middle. In order to obtain larger liquid crystalline regions to aid in the identification of the phases, the edges of the coverslip on the amphiphile-PIL slides were sealed using melted black wax. Prior to use, the myverol-PIL systems were stored in a desiccator overnight, and the phytantriol-PIL systems were stored for between 2 and 15 h. Some of the phytantriol systems could not be left for long times due to merging of the phytantriol with the PIL. The different liquid crystalline phases for each PIL-surfactant combination as a function of temperature were identified on the basis of their characteristic birefringent texture using polarized light microscopy (PLM). Olympus IMT-2 and Olympus IX 70 microscopes equipped with cross-polarizing lenses were used for the analysis. The samples were heated at 2-10 °C/min in a Mettler FP82HT hot stage controlled by a FP90 central processor. In general, the samples were heated up to 90 °C. A small stream of chilled nitrogen was passed through the stage to cool below room temperature. It was important to prevent exposure to air, since these PILs are extremely hygroscopic, with for example MAF imbibing up to 14% water if left exposed to air overnight. However, although the utmost care was taken in the handling and storage of the solvents, residual moisture was retained in the PILs (∼0.5
% wt/wt), so it is possible that this participates in the self-assembly process. Results and Discussion Physicochemical Properties of PILs. The relevant physicochemical properties for the PILs used in this investigation are given in Table 1. A detailed characterization of these PILs, including their thermal properties and structure-property trends, has been reported elsewhere.20 Phase Behavior of Myverol and Phytantriol in PILs. Most of the PILs described in Table 1 were capable of promoting the formation of liquid crystal phases (LCPs) with the nonionic surfactants myverol 18-99K and phytantriol. The phases observed were isotropic cubic (Q), birefringent lamellar (LR), birefringent inverse hexagonal (H2), and isotropic inverse micellar (L2).23-26 The isotropic phases were distinguished as cubic or inverse micellar based on rheology and the assumption that the overall phase progression was similar to that seen in water.23-26 The temperatures over which the LCPs were observed for myverol and phytantriol in each of the PILs are given in Tables 2 and 3, respectively, alongside their corresponding behavior in water (the PILs with no observable LCPs are omitted). In addition to the phases described in Table 2, a LCP with very poorly defined texture was observed for myverol in 2MPAF and EAB at 30 °C, which disappeared quickly and was insufficient to characterize the phase. Of the PILs listed in Table 1, only EAA did not form LCPs with either myverol or phytantriol, though it has previously been
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TABLE 2: Regions of Stability for the Liquid Crystal Phases of Myverol 18-99K in the PILs and Water [Q, Lr, and HII Refer to Isotropic (Assumed Bicontinuous Cubic), Lamellar, and Inverse Hexagonal Phases, Respectively, and Temperatures Are in °C] solvent
Q
LR
water23 a water29 b MAF EAF PAF BAF PeAF 3MBAFc EOAFd 2POAF EOAA EAP EAG EAL EOAL EAN EOAN EAHSe
20->50 20-86 23-44 23-40 25-36
27->50 20-48 23-44 23-56 24-58
HII 84-98
23-41 23-42 24-41 22-50 32-59
23-88 18-92 24-70 25-41
18-39 34-54 36-54
18-39
