J. Phys. Chem. 1994,98, 3015-3023
3015
Intermediate Lyotropic Liquid Crystal Phases in the C&Os/Water System Sergio S. Funarit and Michael C. Holmes' Department of Physics and Astronomy, University of Central Lancashire, Preston PRl 2HE, U.K.
Gordon J. T. Tiddy Unilever Research, Port Sunlight Laboratory, Bebington, Wirral, Merseyside L63 3JW, U.K. Received: October 19, 1993; In Final Form: December 21, 19930
Mixtures of the nonionic surfactant hexaethylene glycol n-hexadecyl ether, C16EO6, and water were examined by optical microscopy, 2HNMR, and small-angle X-ray and neutron scattering. The phase diagram has been delineated between 28% and 70% by weight of Surfactant. Besides the conventional lamellar, La, and hexagonal, HI, phases, the system exhibits a wide range of "intermediate" behavior including a defected lamellar phase (LaH),a nematic phase of rod micelles, (Nc), and cubic or intermediate phases depending upon sample history, which both occur in the same region of the phase diagram. It is the first time that an N c phase has been identified in a nonionic surfactant system. The rich diversity of phases is explained in terms of the decreasing hydration of the polyoxyethylene polar head groups with increasing temperature and the increasing interaggregate interactions with increasing surfactant concentration.
1. Introduction
at least two str~ctures.~J These consist of spherical or short rod micelles which are positionally ordered in three dimensions onto a body-centered or primitive cubic lattice, respectively. However, another possible phase here is a lyotropic nematic phase composed of rod-shaped micelles, Nc, which do not possess any long-range positional order but possess orientational order.16 This phase, although normally restricted in extent on the phase diagram, gives a very characteristic opticaltexturel6-18 and allows the liquid crystal director to be oriented both by external fields and by surface forces. A similar nematic phase of disk-shaped micelles, Na13J6J8 provides the only "intermediate" phase between lamellar and isotropic phase when there is no intervening hexagonal phase. Hexagonal and lamellar phases remain the two most often observed phases in lyotropic liquid crystalline systems. The occurrence of other "intermediate" phases depends very much on the particular balance of forces between molecules. For example, it seems to be the case that noncubic intermediate phases are formed in preference to bicontinuous cubic phases when the alkyl chain portion of the surfactant molecule is more rigid, either because of fluorination9 or because of increased length.1lJ9 In this paper we present optical microscopy, 2H NMR, and neutron and X-ray scattering studies of the nonionic system hexaethylene glycol n-hexadecyl ether, C I ~ E O ~ / ~ H ZThis O. system has previously had its phase diagram determined qualitatively by the optical microscopy water penetration scan technique.lgJ0 It shows a wealth of details. In particular, there is a nematic phase bounding the isotropic to hexagonal transition and a region between the hexagonal and lamellar phase which appears to be cubic on heating but becomes birefringent on formation by cooling. Clearly, in this system the intra- and interaggregate forces are delicately balanced, providing an opportunity to study the occurrence of intermediate phases. It is shown to exhibit an Nc phase between LIand Hi phases and two intermediate phases between the HI and Laphases.
When surfactants are dispersed in water, a wide range of liquid crystal phases can form. The name "intermediate phase" has been usod for those noncubic phases occurring at compositions between hexagonal and lamellar phases. They form a fascinating group with a wide range of morphologies"intermediate" between those of the conventional hexagonal and lamellar phases. These latter phases have had their structures well established for some time.1-3 The hexagonal phase (HI) consists of surfactant molecules aggregated to form infinitely long rods which pack on a two-dimensional lattice perpendicular to the rod axes. In the lamellar phase (La) the aggregates are infinite continuous planes separated by water layers with one-dimensional positional order in the direction normal to the planes. These two phases can be regarded as a progression in which the aggregate curvature decreases as a result of changes in intermolecular interactions arising from changes in surfactant concentration, temperature, or additives to the system. Bicontinuous cubic phases (VI,Vz), of which there are a number of possible structures,495 have been regarded as forming the intermediate by which the hexagonal phase can undergo a morphological transition to the lamellar phase. There are also, however, a number of noncubicmesophases which have been found in this region.z*4.612 There appears to be a wide variety of possible structures ranging from lamellar phases in which the planes are broken by irregular defects, LH, which have no correlation between adjacent planes,11J3J4,tetragonal phases, Tu in which the lamellar planes are pierced by regular holes which are correlated from layer to Iay~r,2*~1&~O to rhombohedral networksof connected rod structures, Ra.2418J5 Clearly, the bicontinuous cubic phases may be regarded as part of a much larger groupof "intermediate" phases which provide these systems with the means to make a phase transition from hexagonal to lamellar phase. In a similar sense, it can be claimed that there is an "intermediate" region between the isotropic micellar phase and the hexagonal phase. Traditionally, this has been occupied by the micellar cubic phases (Il), of which there are believed to be
2. Experimental Section
Samplea. The hexaethyleneglycol n-hexadecyl ether, Cl6EO6 (purity >98%), was obtained from Nikko Chemicals Ltd. Co., Japan, and received no further treatment. TheZH20was obtained from Fluorochem, Glossop, U.K.(purity 99.8%), and was used without further purification. Samples were prepared by weighing
* To whom corrmpondence should be ad-. + h n t addrerr: Faobbereioh Phyik, Univelaitit Lcipzig, LinnQtraase 5, D 04103 Lcipzig, Germany. *Abtract published in Aduancr ACS Absrrocrs, February 15, 1994. OO22-3654/94/2Q98-3O 15$04.50/0
(Q
1994 American Chemical Society
3016 The Journal of Physical Chemistry, Vol. 98, No. 11,1994
Funari et al.
the desired amount of surfactant and 2H20 into constricted glass tubes and flame-sealing. They were mixed by heating to approximately 50 OC and centrifuging through the constriction. Once the samples were homogeneous, when viewed between crossed polars, Lindeman capillary tubes of 0.5" diameter were filled and sealed. NMR samples were contained in sealed 5-mm tubes. The alkyl chain volume fraction can be calculatedl by considering the polar head group, polyoxyethylene, to be in the aqueous region of the phase.
1 LB
L1 +Lg
20 I
30
40
I
I
50
I
60
I
I
1
70
% by weight (&EO6
where M ais the molecular weight of the alkyl chain, M,,,is the molecular weight of the ethoxy chain, pa, p,,,, and pw are the densities of the alkyl chain, ethoxy chain, and the water, respectively, and c is the weight fraction of amphiphile. The densities of 2H20, polyoxyethylene, and alkyl chains are taken to be 1100,21 1150,22 and 832 kg m-3,21respectively. Optical Microscopy. A qualitative survey of the Cl6EOa mesophasewas obtained using the optical microscopy penetration technique.2423 For specified concentrations, the samples were contained in flame-sealed 0.2-mm-path length flat capillaries (Camlab, Cambridge, U.K.) and observed using a Vickers M72 polarizing microscope which was fitted with a Linkham TH600 temperature control stage. Generally, a heating/cooling rate of 2 OC/min was used, in order to allow time for temperature equilibriation of the sample. The transition temperatures were considered accurate to f l OC. *HN M R Spectroscopy. 2H MMR experimentswere made on a Brucker WM250spectrometer with samplescontainedin 5-mmdiameter tubes. Temperature control was accurate to f l OC. The 2H quadrupolar splittings, Av,of powder spectra were taken by measuring the inner peaks, corresponding to a director orientation of 90° to the magnetic field, Ho.Each spectrum was recorded after the sample had equilibrated at the preset temperature and in the magnetic field for at least 15 min. For the compositiondependence of Au in poly(ethy1ene oxide) surfactants, Rendall et al.aJ5 have proposed a model where water is bound collectively to the head groups. They concluded that the alkyl chain makes no direct contribution to the quadrupolar splitting. The 2H NMR splitting for the heavy water molecules reflects mostly the contribution of molecules bound to the first one or two EO groups attached to the alkyl chain and is expressed as
where pb is the number of bound water molecules, EQ,b,l is the splitting associated with a single oriented "20 molecule, and A1) is the water fraction with an order parameter &,I of the water molecules, associated with the first EOgroups (denoted by the 1). X-ray and Neutron Scattering. Nickel-filtered Cu Ka!X-rays were used with pinhole cameras with sample-to-film distances of 113 and 260 mm. Samples were contained in 0.5-mmLindeman capillary tubes. Repeated back and forth centrifugation of the sample in the capillaries gave some alignment of the L, phase parallel to the capillary wall. They were held in a copper block, the axis of the tube being perpendicular to the direction of the beam, and the temperature was controlled by a Haake F3 water bath and circulator, with an accuracy of f0.5 OC. Neutron scattering experiments at the small-angle scattering facility, LOQ at ISIS, the SERC's neutron spallation source, gave better contrast. The sampleswere held in l-mm-path length
Figure 1. Phase diagram of the Cl&O#H20 system. The phases observed areas follows: L1, isotropicmicellar phase; L,, classicallamellar phase: LaH,lamellar phase containingwater-filleddefects;HI,hexagonal phase; Nc, a nematic phase of rod micelles; Lp, gel phase. The phase marked'X"isa V1,cubicphaseonheating but a birefringentintermediate phase, Int on cooling. Twephase regions are marked 'A + B" where A and B are one of the above phases. Phase bondaries are marked by solid lines and have been determined from ?H NMR. Boundaries which are uncertain are marked by broken lines. The hatched region between HI and X is uncertain; no well-defined boundary has been observed.
quartz glass Helma cells with electrical temperature regulation (io. 1 "C). Samples were not aligned with respect to the neutron beam.
3. Experimental Results OpticalMicroscopy. The phase diagramobtained fromoptical microscopy and ZHNMR is shown in Figure 1. Two types of experiment with the optical microscopy were used for its delineation. First, the penetration scan technique was used to obtain a qualitativeidentificationof the sequence of phases present at a given temperature. Second, single-concentration samples were examined as a function of temperature. Figure 2 shows a series of penetration scans at several temperatures. The phase identificationis detailed in the figure, and the significant features are reported in Table 1. A significant feature of the penetration scan experiment is that a nematic phase is observed between LI and HI phases. It is more clearly seen in the penetration scan picture taken at 27.6 OC shown in Figure 3a and in a 32.8 wt % sample at 26.0 OC in which the characteristic schlieren texture is clearly visible (Figure 3b). The second significant feature is that the cubic VIphase observed on heating becomes a birefringent intermediate phase, "Int", on cooling. This region of metastability is labeled X in the phase diagram, Figure 1. Thegel phase (LB)was not studied in detail. It has a very high viscosity, is stable to low temperatures (
