Journal of Molecular Liquids 90 (2001) 157-166 www.elsevier.nl/locate/molliq
Phase Behavior of Mixed Polyoxyethylene-type Water
Nonionic Surfactants in
Hiinobu Kunieda’*, Hamidul Kabits, Kenji Aramaki’, and Kszuki Shige& ‘Division of Artificial Environment and Systems, Graduate School of Engineering,Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan b Division of Material Science and Chemical Eng&er&, Faculty of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240-8501, Japan * To whom correspondence should be addressed. Cloud temperatures, phase behavior, and the structures of liquid crystals were investigated in the aqueous systems of homogenous hexaethylene glycol dodecyl ether(ClzE06) and tied C1~E04-C12EOs,C1~EO&~EOs, and C~ZEOO-C~ZEO~. In the mixed surfactant systems, the average polyoxyethylene- (EO-) chain lengths ate kept constant, the same as CIzE06. The change in cloud temperatures is small in all the systems, whereas the phase behavior is successively changed with increasing the difference in EOchain length in the mixture. Lamellar liquid crystal is developed in the phase diagmms and it intrudes in the two-phase region above the cloud temperature. Hence, the phase pattern of the present mixed surfactant systems resembles that of ClzEOs system, but both cloud point and W+L, region appear at much hi& temperature. Hence, the Hydrophile-Lipophile Balance of the surfactant is not largely changed by mixing the surfactants but the SAXS results show that the surfactant molecules are more tightly packed in the hexagonal and lamellsr phases by mixing. It is considered that when surfactants of different EO-chain lengths are mixed, the considerable reduction in repulsion between the hydrophilic moieties takes place and the surf-t molecules are more tightly packed. 0 2001 Elsevier Science B.V. All rights reserved. l.INTRODUCTION Hydrophile-lipophile balance (HLB) of polyoxyethylene(POE)-type sur&tant is highly influenced by temperature [I]. Mitchell et al  constructed the phase diagrams in aqueous POE alkylether (C,&O,) systems with char&g surfactant concentration and temperature. Most of the ever found phases in surfactant-solvent mixture exist in the phase diagmms although one can not see all of them in each d&ram. Recently, Kunieda et al constructed the phase diagram in aqueous POE dodecyl ether  and oleyl ether  systems with changing the number of ethylene oxide unit (n) instead of temperature. The advantage of these phase diagrams is to see all the phases in one phase diagmm, and these allow one to 0167-7322/01/$ - see front matter 0 2001 Elsevier Science B.V. All rights reserved. PI1 SOl67-7322(01) 00118-O
understand the phase transition mechanism of liquid crystals. To vary n successively, two surfactantshavingffermtintegralnlrmbersofn,e.g.6and7,aremixed. Hence,onc argument comes up, i.e. How is the ejkct ofmixing surjbctant on the phase behavior and the structure of liquid crysrals? Generally, the physico-chemical properties of the mixtures are different fYomthat of
homogeneous surfactant solutions although the mixing of the homologues in aggregates is considered to be rather ideal. It is known that the solubilization of oil and water in bicontinuous microemulsion dramaticslly increases by mixing the su&ctants compamd with a single homogeneous surfactant . Especially, a large difference of HLBs between the surfactants used leads to the more increase of solubilization of water or oil in the microemulsions. It is also known that the aqueous mixed surfa&& solutions show the considerably lower surface tension at the cmc than that in the single surfactant solutions [6-81 and the emulsion stability is improved if the mixed surfactant is employed instead of homogeneous surfactant . Recently, we improved Israelachvili’s equation following to a new theory to consider the effect of mixing surfactant having different EO-chain lengths on the effective cross sectional area per surfactant molecule at the interface, a, [lo]. The value of a, in mixed surfactant systems is smaller than that in the homogeneous surfactant system due to the reduction of repulsive forces between hydrophilic moieties. This closer packing of surfactant molecules at the interface may be the reason for above phenomena in mixed surfactant systems and it should also affect the phase behavior and the self-organizing structure. In this context, we investigated the effect of mixii on the cloud point, phase behavior, and effective cross sectional area per surfactant molecule in liquid crystals in polyoxyethylene dodecyl ether systems by means of small-angle X-ray scattering (SAXS). We used homogenous hexaoxyethylene dodecyl ether as a standard surf&ant, and the average number of EO unit of the mixtures was always kept 6.
Homogenous polyoxyethylene dodecyl &hers (CrzEO, n = 1-8) were obtained from Nikko Chemicals Co. Extra-pure grade l-dodecanol (Cr2EOc) was obtained from Tokyo Kasei Kogyo Co. These chemicals were used without further purification. 2.2.Methods Determination ofPhase Boundary.
Phase boundaries were dcmrmined by direct visual observation with polarizers. The types of liquid crystal were identified by the SAXS (smallangle X-ray scattering) peak ratio.
Molar Volumes of Each Functional Group in the Surf&ant. The molar volume of surfactant is calculated by the following equation:
where A& V, and ps are the molecular weight, the molar volume and the density of surf&ant.
Arithmetic additivity approximately holds concerning to the molar volumes of each tunctional group in the surfacutnt [4, 111. Then, the molar volume of su&ctant is the sum of molar volumes of each group in the surfactant aud the following retation holds:
where V,, VE, and V& a~ the molar volumes of the lipophihc part, the oxyethylene (EO) group and the hydroxyl group, respectively, and n is the number of EO unit. VEOis 38.8 and VaH is 8.8 cm3 mol-’ according to the previous data on CtrEO, [3, 41. From these data and Eq. , the yL is calculated to 215 cm3moP. These vahres were used to analyze the detail structure of liquid crystals. The interlayer spacing of liquid crystal was measured by means of small-angle X-ray scattering (SAXS) perfotmed on a smah+u& scattering goniometer with an 18kW Rigaku De&i rotating anode goniometer (RINT-2500) at about 25°C. The samples of liquid crystal were lapped by plastic films for the measurement (Mylar seal method).
Analysis of Liquid Ctystalline Structures.
Hexagonal Liquid Crystal
Fig. 1. Schematic representations of the interlayer spacing, d, the radius of cylinder in hexagonal phase, dH, and the half thickness of hpophilic part in lamellar phase, dL, the effective cross sectional ama per surfactant molecule at hydrophile-lipophile interface, a,.
It is assumed that the lamellar (LJ phase consists of infinitely spread bilayers. Thus, the effective cross sectional area per surfactant at the interface, a, is calculated by the following equations using the interlayer spacing, d, obtained from the SAXS m-t.. For L, phase,
where & is the half thickness of the lipophilic part in the L, phase, & is the volume fraction of the hydrophobic part of the surfactant in the system, and VLis the volume of the lipophilic part of one surthctant molecule. For H1 phase,
where & is the radius of hydrophobic part of the cylindrical micelle. Note that a, changes very slightly with the surfactant concentration at constant n, if the type of liquid crystal is unchanged [4, 121. 3.RESULTS
3.l.The effect of mixing surfactants on the cloud temperature. In the present study, homogenous CIZEOd is considered as a standard stictant.
The The cloud average number of EO unit of C12EOs-C12E0, (rM-4) mixtures are always kept 6. point temperatures for the aqueous solutions of the homogenous C,zEOa and the mixtures are
E06 EON EOys EOIYB EO,/,, EOm
Fig. 2 Effect of mixing surf&tants on cloud w at 2wt% of surfactant in the system. EO, and EOda indicate the systems with pure ClzE0~ and mixture of CIZEO, and C12E08, respectively.
shown in Fig. 2, where the surfktant concentration is 2 wt?Ain the system. The smfactants used are indicated on the horizontal axis. The cloud temperatme slightly decreases and then, increases with increasing the difkence in the EO-chain lengths of the mixture. However, the decrease in the cloud temperature is only 3°C at most and the Hydrophile-Lipopbile BaIance (HLB) of surfactant is not largely changed by mixing the surfktants. 3.2.The effect of mixing snrfactants on the phase behavior.
Fig. 3 The binary or pseudo binary phase diagmms of aqueous system of ClzEOa(a), CIzEO&2EOs(b), CIZE&-C,~EOS(C), and Cl2EO&2EOs(d). W, is the surf&ant concentration in system. W, W,,,,H,, VI, L,,, D2, S and II indicate water phase, aqueous micellar phase, hexagonalliquid crystalline phase, bicontinuous cubic phase, IameIIarliquid crystalline phase, sponge phase, solid phase and two isotropic equilibrium phases.
162 The binary or pseudo bii phase diagmms of C12EO,j,C&O&rEOs, Cr2Eo2’ Ci2EOs, and C1rE00-C~2EOsin water are shown m Fig. 3. In a dilute mgion, aqueous micellar solution (W,) forms at low tempemtums whereas the phase separation takes place just above 50°C. With incmasii the su&ctant concemratioon,the aqueous mice&r phase is changed to hexagonal liquid crystal (Hi) and lamellar liquid crystal (LJ as is shown in Fig. 3a. There is a bicontinuous-type cubic phase between Hi and L, phases. The phase diagram of Ci2EO4C12EOs-watersystem (Fig. 3b) is similar to that of C!r2EO,jsystem. When the difkence in the EO-chain lengths in the mixture is not very large, the phase behavior is not largely changed. However, the maximum temperature of L, phase is slightly raised and the cloud point curve close to the top of L,, phase is a little bit convex towards the two-phase region. In the phase diagmm of Cr2EO&2EOs-water system, the La-present region intrudes into the two-phase region although the cloud temperature is not very different as described before. The phase behavior resembles that of a binary water-C12EOssystem . However, in the water-C,sEOS system, the cloud temperature is about 32% at 1-2wt% of aqueous surfactant solution and the maximum temperatures for the La phase is around 60°C. These temperatures in the present mixed surfactant system are high+ Therefore, the HydrophileLipophile Balance of the mixed surfactant is still similar to that of homogenous CQEO~. The lamellar liquid crystal forms even in a dilute region, which may be due to increase in the rigidity of surfactant layer. The isolated isotropic phase, D2,which is also called Lr phase , appears inside the W+L, region. In general, this phase forms at temperatures above the W+L, region in homogenous polyoxyethylene alkyl ether systems. Since the present mixed surfactant system is a ternary system, some deviations from a homogeneous surfactant system may take place. The phase diagram in the Cr2EOo-C12EO~water system is shown in Fig. 3d. Since the melting temperature of C12EOsis approximately 24°C and it even increases by the hydration, the solid-present region is very wide in tbis system. We could not observe the VI phase between Hr and L, phases in this system. Apart from these phenomena, the phase pattern is simiiar to that of Ci2E02-C$EOs system. Note that the cloud tempemture does not differ so much than that of homogenous C12E0,jsystem. Consequently, the lamellar phase behavior is sensitively varied by the difference of EO-chain length between the mixed surfactants, but the cloud point and the hexagonalphase behavior are not sensitive.
3.3.Interlayer spacing of liquid crystal As described in the former section, the
HLB of mixed surfactant is not very different from that of homogenous surfactant if the average EO-chain length is the same. However, lamellar liquid crystal tends to develop in the phase diagramand intrudes into the two-phase region above the cloud temperature when the EO chains in the mixture are very different. It is considered that the surfactant layer in aggregates would be mom rigid by mixing the surfactants due to tightly packing of surf&ant molecules at the interface [14,15]. In order to co&km this, the interlayer spacing of hexagonal and lamellsr phases were measured by means
~P~$~:. &a: 0.0.
. . . . , . , . . , . , . . , . .
Fig. 4 Interlayer spacing (a), radius of cylinder in hexagonal phase and half thickness of lipophilic part in lmellar phase (b), and efktive cross sectional mea per surfactant molecule at hydrophile-lipophile inter&e (c) as a fimction of surfactant concentration for the pure C12EO~system (O), the mixed CQEO~~~~E~~~~,~I~~~~-~I~~~s~~~,~I~E~o-CI~~~S~~~~Y~.
of SAXS in aqueous CtzE06, CtrEO&&EOs, CtzEO&~EOg, and CirEO&zEOs systems and the results am sho\ivn in Fig. 4a. The radius of the cylindrical micelles in the hexagona phase, the half-thickness of bilayer in the lamellar phase and the effective cross sectional area per surfactant molecule at the interface of hydrophobic core of aggregates were calculated from the interlayer spacing data by Eqs.  to  and the results are shown in Figs. 4b and c. The data for the homogenous CtrE06 system is in a good agreement with the previous data . In the L, phase, a, slightly increases with increasing the water content, but it becomes almost constant in the hexagonal phase. The hydrocarbon chain length of the dodecyl group is evaluated as 1.6 mn in its extended form by dividing 0.357 run3 (the molar volume of dodecyl chain) by 0.22 nm2 (the cross sectional area per chain of the close-packed hydrocarbon chains in a liquid state) [ 171. The extended length is slightly longer than the radius of cylindrical micelles except for the system with the mixture of CirEOs and CtzEOo, inferred the possibility of the solubilization of CtrE00 (ldodecanol) in the core of hexagonal cylinders. The half-thickness of the bilayer is considerably shorter than the hydrocarbon chain length. Therefore, in the hexagonal phase, the hydrophobic chain is almost in its extended form whereas it is shrunk in the lamellar phase. The a, decreases when the mixed surfactants are used as is shown in Fig. 4c. When the EO-chain lengths in the mixture is more different, the a, tends to be smaller. In other words, the surfactant molecules are packed more tightly in the mixed surfactant systems. ‘This is the reason why the larnellar-liquid-crystal region maintains at higher temperature and lower surfactant concentration in the mixed systems. %A.Reduction of effective cross sectional area per surfactant When the POE-type nonionic surfactants having di&rent EO-chain length am mixed, the reduction of a, takes place and the surfactant molecular layer becomes rigid as theories predict [ 14,151 although the HLB of the mixed surfactant is not largely changed. Fig. 5 shows the schematic representation of EO chains in homogenous and mixed surfactant in aggregates. It is considered that the hydrated EO chain shows the repulsion between them due to the steric hindrance and/or the hydration force whereas the hydrocarbon chain reveals the attraction due to the interfacial tension between the hydrophobic chain and water. In the present system, the hydrocarbon chain length is always the same in both single and mixed surfactant systems. Therefore, the change in us is attributed to the change in the repulsion between the hydrated EO chains. The repulsion is a rather short-distance force [ 181 and it does not act ifthe EO chains are considerably separated. Since the EO-chain length is homogenous in a single surfactant system, the repulsion takes place along the whole EO chain. On the other hand, when the surfactants are mixed, the distance between the longer part of the EO-chain of the hydrophilic surfactant is more separated as is schematically shown in Fig. 5. Consequently, the net repulsion would be reduced by mixing the surfactanta.
Fig. 5 Schematic representation of aggregates at the interface in homogenous and mixed surfactant systems.
4.CONCLUSION We investigated the cloud temperatures, phase behavior, and molecular parameters in the liquid crystals in the aqueous polyoxyethylene-type nonionic surfactant systems with a homogeneous surfactant (&EOJ and mixed surfactanta (C,+30&zEOs, CizEOz-CizEOs, Ci2EO&2EOs). Cloud temperatures are almost same in all the systems. The phase behavior in the homogeneous system and the mixed Ci2EO&t2EOs system is almost same whereas the other two systems resemble to the homogeneous CtzEOs system although the cloud temperatures and the temperature range of the lamellar phase in the latter system is much lower. The effective cross sectional area per surfactant at the interface, a,, was evaluated from the interlayer spacing measured by SAXS. a, in the mixed systems is smaller than that in the homogeneous system and furthermore, the value decreases with incmaaii the di%knce of EO-chain lengths in the mixed systems, which is due to the reduction of repulsive forces between hydrophilic moieties. Hence, mixing of surfactants does not affect the HLB of surfactant but it does the total phase behavior and, furthermore, the effect is emphasized when the difference of EO-chain length increases. s.AcKNoWLEDGMENT We acknowledge the organizers of 26th ICSC to give the opportunity to submit our paper to Journal of Molecular Liquids.
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