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Lamellar-to-isotropic phase transition is observed in the system of TX-100/n-C8H17OH/H2O induced by neutral water-soluble polymer poly(ethylene glycol...
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J. Phys. Chem. B 2009, 113, 1993–2000

1993

PEG-Induced Lamellar-to-Isotropic Phase Transition in the System of TX-100/n-C8H17OH/ H2O Lingling Ge, Rong Guo,* and Xiaohong Zhang School of Chemistry and Chemical Engineering, Yangzhou UniVersity, Jiangsu ProVince 225002, P. R. China ReceiVed: September 16, 2008; ReVised Manuscript ReceiVed: NoVember 12, 2008

Lamellar-to-isotropic phase transition is observed in the system of TX-100/n-C8H17OH/H2O induced by neutral water-soluble polymer poly(ethylene glycol) (PEG) with molecular weight ranging from 400 to 20 000. The location of PEG in the lamellar liquid crystal and the microstructure change of the lamellar phase during phase transition are investigated by means of 2H NMR, small-angle X-ray diffraction (SXRD), rheology, polarized optical microscopy (POM), and freeze-fracture transmission electron microscopy (FF-TEM). Calculations based on the “swelling model” show that 0.92-2.58 wt % PEG2000 penetrates into the amphiphile layer and the rest resolve in the water layer. Both of these two kinds of locations induce the lamellar-to isotropic phase transition. The longer the chain length of PEG, the higher the efficiency is. In addition, a critical molecular weight of PEG is observed before phase transition occurs, with which the disturbance of PEG on the microstructure of lamellar liquid crystal is most prominent. The critical molecular weight of PEG is independent of the thickness of water layer. The value is 2000 for the system of TX-100/n-C8H17OH/H2O. Introduction Lamellar liquid crystal (LR), which usually appears in the system of surfactants, is one kind of special molecular assemblies repeated by surfactant amphiphile bilayer and solution layer. It has been intensively studied for a long period of time1-6 and has already been widely used in pharmacy, cosmetics, biology, and material synthesis.7,8 Recently, several groups addressed the question how additional parameters can control the structure of LR phase.9,10 One aspect is the influence of macromolecules as guest components in the lamellar phase, because the addition of polymers into water/lipid system adds extra flexibility to the shape and possible deformation of interface. Although the entropy is expected to reduce due to the constraint on the chain conformation, it has been reported that both lamellar and hexagonal phases can incorporate a considerable amount of polymer.11-13 Most related studies focused on the system of polymer and ionic surfactant because of their stronger electrostatic interactions.1,13-16 The fundamental requirements in understanding the behavior of these mixed systems are the knowledge of the location of polymer in the special neat structure and the microstructure change of the liquid crystal induced by polymer. Depending on the molecular structure of the polymer and the properties of the lamellar phase, the polymer can occupy a variety of sites in the lamellar liquid crystal: location either in the water layers or in the surfactant bilayers, penetration into the membranes, and absorption onto the bilayer.2,17 For instance, the location of carbohydrates, with different compositions and chain lengths in the LR phase, has been studied.17 The result shows that small sugars penetrate into the aqueous layers of the LR phase; that the macromolecular carbohydrates are sterically excluded from the aqueous layers and form an isotropic phase; and that the hydrophobically modified carbohydrates is introduced into the aqueous layers. Zhang et al.12 have reported that nonionic * Corresponding author. E-mail: [email protected]. Fax: (+86)-5147311374.

polymers poly(ethylene oxide) (PEO) are entrapped in the aqueous domains of the lamellar phase, and the solubilization of polymers diminishes with increasing molecular weight. Investigations on the adjunction effect of PEG on the phase behavior of sodium dodecyl sulfate/dodecane/hexanol/water solutions show that PEG is dissolved in the aqueous solvent at high water/oil ratio, and it is incorporated into waterswollen bilayers at a low water/oil ratio.2 The importance of charge density has been proposed by Ruppet et al.15 based on the fact that the highest charged polymers absorb flat onto the SDS/decanol/bilayer, the lowest charged polymers present loop adsorption and the noncharged polymer does not lead to such absorption structure at all. For the microstructure and elastic properties of the lamellar liquid crystal, they may be affected markedly by specific polymer/surfactant interactions, and the phase behavior may thereby be significantly changed. Indeed, a phase separation into two lamellar phases is observed upon addition of PEG to the lamellar phase made of ionic surfactant SDS,18 and the softening of the repulsive steric interactions between the bilayers was the main reason for phase separation. Besides, investigation by Ficheux et al.19 showed that surfactant layer was made less stiff and easy to separate a polymer rich aqueous solution by addition of both absorbing and nonabsorbing polymers. In this report, polymer-induced phase separation is observed in the mixed system of nonionic polymer PEG and nonionic lamellar phase composed of TX-100, n-C8H17OH, and water. The effect of the chain length of PEG on the microstructure change of the lamellar phase is investigated. In addition, the newly reported “swelling model” in nonionic surfactant system is first employed to calculate the distribution of the polymer in the repeated lamellar structure. What is more, 2H NMR, rheology measurements combining FF-TEM are employed to follow the microstructure and elastic property change of the surfactant membrane during phase transition.

10.1021/jp808218j CCC: $40.75  2009 American Chemical Society Published on Web 01/22/2009

1994 J. Phys. Chem. B, Vol. 113, No. 7, 2009

Ge et al.

Experimental Section

Results

Materials and Sample Preparation. Triton X-100 (TX-100, >99%, Sigma) was used as received. The molecular structure of TX-100 was listed as follows:

To study the influence of added PEG on the microstructure of the lamellar liquid crystal, the weight ratio of TX-100:nC8H17OH:H2O is fixed at 48.6:11.4:40, and water is replaced by PEG2000 aqueous solution. The appearance of samples depend on the polymer concentration, divided into four polymer concentration regions: (I) WPEG2000 < 3.47 wt %. The samples appear transparent and show clear maltese crosses between crossed polarizers (Figure 1A), which are typical optical patterns of lamellar liquid crystals.22 The quadrupolar splitting in the 2 H NMR spectrum in Figure 2a b confirms the pure lamellar phase in the system.1,23 All of these suggest that a small amount of neutral polymer PEG can be resolved in the nonionic lamellar phase without changing the physical characteristics of the phase. (II) 3.47 wt % < WPEG2000 < 18.29 wt %. The samples become progressively more and more turbid with rising PEG content, and a white “milky” phase appears. The remaining maltese crosses under polarizers as shown in Figure 1C indicate the existence of lamellar phase, which can also be verified by the quadrupolar splitting in the 2H NMR spectra (Figure 2c,d). Meanwhile, a central peak appears between the quadrupolar splitting, which suggests that some of the lamellar phase has evolved into an isotropic phase in this region.1,23 Besides, the micrograph under normal light in Figure 1D shows that the isotropic drops dispersed in the lamellar phase with the diameter of about 5 µm. This might result in the turbidity that we observe in the samples. But no separated phases can be obtained by high-speed centfiguration (>20 000 rpm) or long-period equilibrium (>1 year), especially at lower concentration of PEG, because the viscosity of the samples is very high. So, all the measurements are performed with homogeneous samples. The relative proportion of the two microphases can be obtained by deconvoluting the central peak from the quadrupolar doublet in 2H NMR spectrum. The area ratio of central peak to total calculated by the least-squares fitting as described in ref 24 is associated with the ratio between the water in the isotropic phase to the total water in the sample. The estimated fraction of isotropic water is zero in PEG2000 concentration region I, and it increases from 0.03 to 0.91 when PEG2000 concentration increases from 3.47 to 18.29 wt %. (Table 1) This suggests that the lamellar phase evolves to isotropic phase induced by PEG2000. (III) 18.29 wt % < WPEG2000 < 52.60 wt %. The samples are no longer turbid, and two transparent phases are obtained. Both the upper and lower phases are dark under crossed polarizers indicating that the whole lamellar phase has evolved to isotropic phase in this region. Water and PEG dominate the 1H NMR spectrum of the lower phase, and only weak signals of protons from TX-100 and n-C8H17OH are observed (data not shown). These suggest that the lower phase is a polymer-rich O/W microemulsion. Most of the TX-100 and n-C8H17OH exist in the upper oil phase, and there also a certain amount of water exists in the upper phase, probably forming W/O microemulsion. The FF-TEM results discussed below will confirm this assumption. The small-angle X-ray diffraction measurements are performed to further investigate the effect of PEG on the microstructure of the lamellar liquid crystal. The first and second Bragg peaks are observed in regions Ι and II, and the positions of these two peaks correspond to the characteristic sequence of a lamellar liquid crystal (Q, 2Q, 3Q, ...).25 The shape and position of diffraction peaks still agree well with the characteristic of pure lamellar phase in region II though the isotropic phase appears, because the isotropic phase does not contribute to the X-ray diffraction.26 When the concentration of PEG enters region

The investigated poly(ethylene glycol)s (PEGs, MW ) 400, 1000, 2000, 4000, 6000, 10 000, 20 000) were Aldrich products, and the distribution of the MW agreed with Poisson distribution. n-Octanol was purchased from Fluka. Deuterated water (D2O, 99%) was Aldrich products, and doubly distilled water was used for all the experiments. The lamellar liquid crystal was prepared according to the phase diagram of TX-100/n-C8H17OH/H2O system.20 The weight ratio of TX-100:n-C8H17OH was kept constant at 81:19. The mixture of TX-100 and n-C8H17OH was homogenized using a vortex mixer at about 70 °C. Then water or PEG aqueous solution was added and mixed sequentially. The concentration of PEG in this report means the weight percentage of PEG in aqueous solution. The bubbles were removed by centrifugation at 3 × 103 rpm. Time-effect measurements showed that the samples reach equilibrium 1 week after sample preparation, so all samples were sealed and stored in thermostat for 1 week to ensure that slow kinetics did not influence the results. D2O was used to replace H2O for deuterium nuclear magnetic resonance (2H NMR) experiments. Small-Angle X-ray Diffraction (SXRD). Data were collected on Bruker AXS D8 Super Speed diffractometer, which allowed the angle 2θ to range between 0.8° and 5°. The X-ray radiation was Cu KR filtered by monochromator to yield a wavelength of 0.154 nm at 40 kV and 20 mA. A small amount of the sample was placed in a special holder, and the sample-to-detector distance was 250 cm. The scanning speed was 0.4°/min. All the SXRD measurements were repeated more than three times. Nuclear Magnetic Resonance (NMR). 2H NMR experiments in this study were carried out on a Bruker AV-600 NMR spectrometer with a deuterium frequency of 90.102 MHz. D2O was used to replace H2O for 2H NMR experiments. Sixty-four time accumulations were acquired. 1H NMR experiments were also conducted on this equipment with a 1H frequency of 600.13 MHz. Thirty-two times of accumulations was acquired, and D2O was used as external standard. Rheological Measurements. Rheological measurements were performed with a Haake RS600 rheometer. A plate-plate geometry with a diameter of 35 mm was used. The sample thickness in the middle of the sensor was 1 mm. Samples were kept in saturated water vapor during the whole period of measurements in case of evaporation. Because the rheological properties of such systems depend on the shear deformation history,21 the samples were gently put onto the top of the sensor plate, and then the plate was slowly elevated to its measuring position with constant velocity. Measurements were carried out after 10 min to allow stress relaxation. Polarized Optical Microscopy, POM. Samples were placed between a glass slide and a coverslip, and examined with a Leica DMLP microscope with a maximum magnification of 50× provided with a DFC320 CCD camera and cross polarizers. The Linkam TMS94 thermostation was provided. The texture of each sample was observed after 10 min equilibrium. The temperature for this experiment was kept at 25.0 ( 0.1 °C.

Phase Transition in TX-100/n-C8H17OH/H2O

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Figure 1. Polarization micrographs of samples at different concentrations of PEG2000 in the TX-100/n-C8H17OH/PEG2000(aq) system. The component is kept at TX-100:n-C8H17OH:PEG2000(aq) ) 48.6:11.4:40. The weight percentages of PEG2000(aq) are (A) 0, (B) 0.38, and (C) 3.92. Micrograph D is taken in normal light of sample C.

Figure 2. 2H NMR spectra of the TX-100/n-C8H17OH/PEG2000(aq) system with increasing concentration of PEG2000: (a) 0, (b) 2.97, (c) 3.47, and (d) 5.0. The component is kept at TX-100:n-C8H17OH:PEG(aq) ) 48.6:11.4:40.

TABLE 1: Ratio of Areas, (central peak area)/(total area), in the Deuterium Signal of 2H NMR Spectra from TX-100/ n-C8H17OH/PEG(aq), as a Function of PEG Concentration WPEG400/ wt % 0 9.63 18.72 19.73 21.74

ratio of area

WPEG2000/ wt %

ratio of area

WPEG20000/ wt %

ratio of area

0 0 0 0.74 0.99

2.97 3.47 4.96 10.17 18.29

0 0.03 0.05 0.40 0.91

0.54 1.03 1.46 5.01 10.00

0 0.03 0.07 0.37 0.71

III, no diffraction signal is observed, confirming that all the lamellar phase has been transferred to the isotropic phase. The repeated distance d of the lamellar liquid crystal can be obtained by the first Bragg peak and the Bragg equation nλ ) 2d sin θ. The plot of the lamellar repeated distance d as a function of PEG concentration within regions I and II in TX-100/nC8H17OH/PEG2000(aq) system is shown in Figure 3. The d value remains constant at lower concentrations of PEG2000, followed by a linear decrease with further increase of PEG2000 content. The concentration of PEG2000 corresponding to the transition point is about 3.50 wt %, which agrees with the boundary content of PEG2000 between regions I and II determined by 2 H NMR (3.47 wt %). These results suggest that the solubilization of PEG2000 exerts no apparent effect on the width of the lamellar phase,12 and the appearance of the isotropic polymer-rich aqueous phase makes the interlayer space decrease.

Figure 3. Dependence of layer thickness (d) on the weight percentage of PEG in the system of TX-100/n-C8H17OH/PEG(aq) with constant weight ratio TX-100:n-C8H17OH:PEG(aq) ) 48.6:11.4:40.

The distribution of the solvent and the added component in the liquid crystal is vital in the microstructure and subsequently the phase behavior. Ficheus et al.19 have proposed that the fraction of polymer in the bilayer can be obtained with the smectic period d and volume fractions of each component by assuming that surfactant is entirely contained in the bilayers

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Figure 4. (A) Dependence of the interlayer thickness d on the volume ratio of solvent in the system of TX-100/n-C8H17OH/PEG(aq) with constant weight ratio TX-100:n-C8H17OH ) 81:19. (B) Dependence of water penetration Rw on PEG content RPEG2000 and penetration of PEG RPEG2000 on solvent content Rw. The insert is the illustration of the microstructure of LR phase in the system of TX-100/n-C8H17OH/H2O.

and that the polar part of the amphiphile layer is constant. However, for the lamellar liquid crystal with ethoxylated nonionic surfactant, the increased penetration of either water molecules or polymer chains into the polar part of the amphiphile layer will cause a lateral expansion. And the variation in interlayer spacing with added water becomes more complex. The “swelling model” for ethoxylated nonionic surfactant/water lamellar liquid crystal provides a method to obtain the penetration of added component into the amphiphile layer by introducing an structure parameter k of the ethoxide chain of surfactant.27 In this work, the concentration of PEG is fixed within region I to ensure pure lamellar phase, and the “swelling model” is employed to determine the penetration of water and PEG into the amphiphile bilayer. The microstructure of the lamellar liquid crystal is illustrated in the insert of Figure 4A, where the interlayer thickness d is composed of an aqueous layer dw and an amphiphile layer d0. The relationship between the interlayer spacing and the penetration of added component is as follows:27

d ) d0(1 + R)/(1 + RRk)

(1)

The first derivative of eq 1 gives

∂d/∂R ) d0 /(1 - Rk) Rf0

(2)

Here R is the fraction of water or PEG penetrating into the amphiphile layer. R means the volume ratio of solvent to the other components. The volume is obtained by dividing the weight with density, and the density of the polymer is that of the melted one. k is the structure parameter of the ethoxide chain of surfactant. It is defined as k ) d0/d0P, where d0P means the thickness of surfactant polar group lager. k is calculated by the number of atoms in the chain.28 For TX-100, the value of k ) (7 + 9.5 × 3 + 2)/(9.5 × 3 + 3) ) 1.23, and we assume it constant within all the RPEG and Rw range investigated.

The interlayer spacing d shows linear increase with Rw at different PEG2000 contents (Figure 4A). The amphiphile layer thickness d0 obtained by extrapolating the content of water or PEG(aq) component to zero17 increases with PEG concentration, and the water (or PEG2000(aq)) layer thickness dw obtained by dw ) d - d0 decreases with PEG content at a certain Rw value. Besides, the penetration of water RH2O in the amphiphile layer can be obtained by the straight line in Figure 4A and eq 2. Similarly, RPEG can be gained by plotting interlayer spacing d as a function of RPEG at constant Rw (data not shown). The PEG2000 content dependent on RH2O and the Rw content dependent on RPEG2000 are shown in Figure 4B. Clearly, increasing content of PEG2000 results in the enhanced penetration of water RH2O. What is more, the penetration of PEG2000 decreases with Rw, and the value is within the range of 0.92-2.58 wt %, indicating that most of the PEG2000 chains resolve in the water layer and, meanwhile, part of the polymer chains penetrates into the amphiphile bilayer. In order to investigate the effect of polymer chain length on the interaction between PEG and the lamellar liquid crystal, the same experiments with those mentioned above were performed on the systems of TX-100/n-C8H17OH/PEG(aq) with lower molecular weight (MW) of 400 and higher MW of 20 000. For the system of TX-100/n-C8H17OH/PEG20000(aq) with increasing PEG content, all the appearance of change of samples, the evolution of the 2H NMR spectra, and the change of SXRD profiles are the same as those in the system of TX-100/nC8H17OH/PEG2000(aq). However, for the system of TX-100/nC8H17OH/PEG400(aq), no white appears within the whole PEG400 concentration range investigated, from 0 to 100 wt %. The quadrupolar splitting in the 2H NMR spectrum disappears, and the portion of water in the isotropic phase increases dramatically (Table 1) within a very narrow PEG400 concentration region, indicating that the lamellar structure ruptures swiftly once the PEG400 content is over a certain value. The boundary PEG contents of each region are shown in Figure 5, from which we can find that all the critical transiting concentrations of PEG

Phase Transition in TX-100/n-C8H17OH/H2O

Figure 5. Dependence of critical transition concentrations (ctc) of PEG on molecular weight in the system of TX-100/n-C8H17OH/PEG(aq). Note: ctc1 and ctc2 correspond to the starting and ending of phase transition from LR to isotropic phase, and ctc3 corresponds to the over saturation of PEG in the isotropic phase.

(ctc) decrease with increasing MW of PEG, indicating that the efficiency of PEG inducing phase transition is higher for PEG with greater chain length. To follow the microstructure change of the lamellar liquid crystal during phase transition, rheology behaviors of TX-100/ n-C8H17OH/PEG(aq) systems are investigated. First, controlled stress measurement is performed in order to determine the extent of the linear viscoelastic region. Each of the samples has a linear viscoelasticity in 0-10.0 Pa range of applied stress amplitude (data are not shown), and 1.0 Pa is therefore chosen as applied stress for frequency-dependent oscillatory measurements to ensure that the lamellar structure is not destroyed. Ne´meth et al.29 and Hala´s et al.30 have pointed out that the response of frequency-dependent oscillatory measurement is extremely sensitive to the microstructure of the sample and that the spectra can be regarded as the “rheological fingerprint” of the system. The oscillatory rheological behavior of the sample in the TX100/n-C8H17OH/H2O system without PEG reveals a typical response of the lamellar phase22 (Figure S1 in the Supporting Information). Replacing the water component with PEG400 and PEG20000 aqueous solution, all the characteristic responses of the lamellar liquid crystal remain. This indicates the existence of the lamellar phase within a lower PEG concentration range. However, the signal of storage modulus disappears once the concentration of PEG goes beyond a certain value, indicating the disappearance of the lamellar phase and the sample left is pure viscous liquid. Interestingly, the change of the oscillatory response with PEG2000 content is rather different in the system of TX-100/n-C8H17OH/PEG2000(aq). The characteristic of the oscillatory response of the lamellar phase remains within the concentration range of 0-3.50 wt %. But the storage modulus G′ shows frequency dependence; i.e., G′ value rises with increasing frequency, when the concentration of PEG2000(aq) is higher than 3.50 wt %. This change feature remains till the concentration of PEG2000(aq) reaches 18.29 wt %. From there on the storage modulus disappears. The dependence of storage modulus G′ on the PEG content at ω ) 100 rad/s is shown in Figure 6. In the system of TX-100/n-C8H17OH/PEG(aq) with shorter and longer chain lengths (PEG400 and PEG20000), a slight linear decrease of G′ is followed by a relative rapid decrease and the G′ value disappears at higher PEG content.

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Figure 6. Dependence of the storage modulus G′ on the concentration of PEG(aq) within pure lamellar phase region (Ι) and lamellar/isotropic phase region (Π) in the system of TX-100/n-C8H17OH/PEG(aq) with PEG molecular weight of 400, 2000, and 20 000.

The concentrations of PEGs corresponding to the two transition points in each curve agree with the ctc values determined by 2 H NMR shown in Figure 5. Hence, we can safely assume that the starting and ending of the lamellar to isotropic transition is the reason for the rapid decrease and disappearance of the storage modulus, respectively. However, for the system of TX100/n-C8H17OH/PEG2000(aq), the decreasing rate of G′ in region I is much higher than in region II (Figure 6). Besides, comparing the three curves in Figure 6 we can find that the G′ value in the TX-100/n-C8H17OH/PEG(aq)system with intermediate polymer chain length is much lower than that with small and great chain lengths, indicating that the structure of the lamellar liquid crystal is disturbed more prominently by addition of PEG with intermediate chain length. The concentration of PEG in PEG(aq) is kept constant within region I (0.5 wt %), and the effect of the MW of PEG on the structure disturbance to lamellar liquid crystal is further investigated. From Figure 7 we can find that the storage modulus decreases to a minimum value with rising MW of PEG, and then increases with further rising MW of PEG. The MW value of PEG corresponding to the lowest point is determined as critical molecular weight, and the value is 2000 for the TX100/n-C8H17OH/H2O system. This value is dependent of the content of PEG(aq) in the lamellar phase, indicating that the value of critical molecular weight is independent of the content of solution in the system. In addition, comparing the three curves in Figure 7, we can find that the storage modulus decreases with rising content of solution at a certain MW value of PEG. Discussion PEG-Induced Phase Transition. The experimental results show clearly that the lamellar phase in the system of TX-100/ n-C8H17OH/H2O transfers gradually into isotropic phase upon addition of PEGs. An important question is where the PEGs are distributed and how they induce phase transition. PEG is a linear polymer composed of repeated EO unites. It may mainly resolve in the aqueous domain of the lamellar phase due to its inherent higher hydrophility. Investigations on the AOT/PEO/ water system also show that PEOs are entrapped in the aqueous domains of the lamellar phase,12 and the solubilization of the polymers exerts no apparent influence on the width of aqueous domain in the lamellar phase. Meanwhile, interactions of PEG

1998 J. Phys. Chem. B, Vol. 113, No. 7, 2009

Figure 7. Dependence of loss modulus G′ on the molecular weight of PEG in the Triton X-100/n-C8H17OH/0.5 wt %PEG(aq) system with constant weight ratio TX-100:n-C8H17OH ) 81:19.

with the hydrophilic part of TX-100 molecules, which is also composed of repeated EO groups, may drive some of the polymer to penetrating into the amphiphile layer. The fact that the penetrations of PEG into the surfactant bilayer calculated by “swelling model” are positive supports this assumption (Figure 4B). Besides, the penetration of PEG into the hydrophilic part of the amphiphile layer can be linked to our former investigations on the interaction between PEG and TX-100 micellar solutions, where PEG chains tend to insert into the hydrohiphilic layer of TX-100 micelle forming the TX-100/ PEG complex. 31 The penetration of PEG into the amphiphile bilayer results in the increased amount of EO chains there, and the initial neat assemblies of surfactant molecules are probably disturbed. Subsequently, more water molecules may penetrate into the amphiphile layer resulting from increasing water penetration (Figure 4B) and decreasing thickness of water layer. The increased penetration of water and EO chains of PEG results in the increased bilayer thickness d0 (Figure 4A). Besides, because the water layer thickness increases with rising water content (Rw) in the system (Figure 4A), the accommodation of PEG chains in the aqueous domains increases accordingly, which causes the decreasing penetration of PEG in the amphiphile bilayer (Figure 4B). The “less ordered” assemblies of the surfactant molecules in the amphiphile layer upon addition of small amount of PEG can be verified by the slight initial decrease of the storage modulus which has been related directly to the rigidity of the amphiphile layer.32,39 However, the repeated lamellar structure is initially preserved as seen in samples containing polymer within region I, which is verified by quadruple splitting in 2H NMR spectrum (Figure 2). The FFTEM pictures also show that the parallel undulating layers remain in the whole visual field. Based on these, the microstructure illustration of TX-100/n-C8H17OH/PEG(aq) system within region I is presented in Figure 8B. The disturbance of PEGs on the repeated lamellar structure increases with rising PEG content, resulting in the bend of the parallel undulating as shown in FF-TEM picture (Figure 9B). The mixture separates into a lamellar phase and a polymer-rich aqueous phase once the concentration of PEG enters region II, as manifested by the single peak between the quadruple splitting

Ge et al. in 2H NMR spectrum (curves c and d in Figure 2). This phase separation behavior has already been observed in the system of polyacrylamide/cetylpyridinim chloride/hexanol,33 and the polymer coils lying between the bilayers of the lamellar phase have been regarded as the main reason for the curvature of the membrane. In the system under investigation, random coil of PEG in the aqueous domain and penetration of PEG in the amphiphile layer may both contribute to the phase separation. Water as well as PEG is expelled from the lamellar phase. This leads to the shrunk interlayer space as shown in Figure 3, because water content in the lamellar decreases. The drops of polymer-rich aqueous phase dispersing in the lamellar phase are of the same order of magnitude as the wavelength of light, which might result in the turbidity that we observe in the sample within region II. Besides, the isotropic phases generally do not contribute to the storage modulus,34 which causes the sharp decrease of G′ value once the concentration of PEG enters region II (Figure 6). With further increase of PEG content in the system, the lamellar phase is totally destroyed, as manifested by the disappearance of SXRD signal and G′ value. The W/O and O/W microemulsions in the upper and lower phases respectively are left, as shown in FF-TEM pictures (Figure 9C,D). The welldefined sphere morphology in the lower phase corresponds to O/W microemulsion, and the atactic sphere morphology in the upper phase is of W/O microemulsion. With further addition of PEG, the lower aqueous phase will be over saturated with polymer, and white solid of PEG deposits in the lower phase. Effect of Molecular Weight of PEG. Previous studies have reported that in the case of infinite surfactant films, such as lamellar phase, the polymer size is an important parameter that determines the resultant structure of film.35 In the present investigation, the interaction mechanism of PEG with the lamellar liquid crystal is found strongly dependent on the MW of PEG before phase separation occurs (region I). For PEG with shorter chain length, such as PEG 400, the steric hindrance and the conformational restriction of PEG chains penetrating into the hydrophilic parts of the amphiphile bilayer are relatively small.16 Thus, PEGs with shorter chain lengths tend to penetrate into the hydrophipilic part of the bilayer and act as the cosurfactant.36 This location enlarges the average distance of the surfactant chains there; i.e., the apparent area of surfactant is enlarged. It has been proposed by Gradzieliski et al.37 and Siddig et al.38 that the rigid structure of the liquid crystal will be disturbed with increasing molecular area of the surfactant, and the decreasing storage modulus value with increasing PEG400 content is a manifestation of the increased average molecular area of surfactant (Figure 6). It should be noted that the penetration of PEG molecules is stochastic and probably well proportioned; hence, the infinite film of the lamellar with PEG400 penetrated is uniform. So the rupture of the lamellar liquid crystal in TX-100/n-C8H17OH/PEG400 system occurs within a very narrow PEG concentration range (Figure 5). For PEG with longer chain length, such as PEG20000, most of the chains form random coils in solution.39 The effective coil size of the polymer in the aqueous domain is difficult to estimate, but it is probably of a similar magnitude to that in pure water. Dynamic light scatter experiments performed on PEG20000(aq) reveals that the hydrodynamic radius of the random coil is about 2.52 nm;31 i.e., the diameter is about 5.04 nm. The thickness of the aqueous layer is less than 3 nm (Figure 8B). So we can safely assume that the presence of the random coil in the aqueous layer may probably result in the curvature of the lamellar structure, and consequently the appearance of the isotropic phase.40 Besides, the number of end groups of

Phase Transition in TX-100/n-C8H17OH/H2O

J. Phys. Chem. B, Vol. 113, No. 7, 2009 1999

Figure 8. Illustration of microstructure change in the process of phase transition in the system TX-100/n-C8H17OH/PEG(aq).

Figure 9. FF-TEM in the system of TX-100/n-C8H17OH/PEG2000(aq) with constant weight ratio TX-100:n-C8H17OH:PEG(aq)) 48.6:11.4:40. The weight percentages of PEG(aq) are (A) 0 and (B) 5. (C) and (D) correspond to the upper and lower phases at 30 wt % of PEG. Scar bars: (A) 50 nm, (B) 200 nm, (C) 200 nm, (D) 500 nm.

PEG20000 is approximately 1/50 of PEG400 at the same weight percentage of PEG, so the probability of PEG20000 penetrating into the hydrophilic layer is much smaller than that of PEG400. Therefore, we can infer that the main disturbance of PEG with longer chain length on the lamellar liquid crystal is the random coil in aqueous domain. Different from the film disturbed by the penetration of PEG chains, the disturbance of the random coil on the infinite film is partial and gradual. So the macroscopic phase transition occurs at relatively lower PEG contents, and the process covers a wider range of PEG contents (Figure 5). The interesting phenomenon is PEG with intermediate chain length. It also exists in the form of random coils. Meanwhile, for PEG with intermediate chain length, such as PEG2000, the amount of end group is about 10 times that of PEG20000 with the same weight content. The steric hindrance of the end groups to the penetration into the hydrophilic layer is relatively small.36 Hence, two kinds of interaction mechanism, penetration and random coil, coexist in the mixed system of lamellar liquid crystal and PEG2000. The random coil of PEG2000 is probably smaller than that of PEG20000 as Rg ) 1.47 × 10-2M0.58,41 where Rg is the radius of gyration of random coil and M is the molecular weight of the polymer, so the phase transition occurs at higher PEG2000 contents (Figure 5). However, the structure of the remaining lamellar liquid crystal may have been disturbed by the penetration of PEG chains. Thus, the storage modulus is maximal in the system of PEG2000 compared to that of PEG400 and PEG20000 (Figure 6). The microstructure illustration of TX-100/n-C8H17OH/PEG(aq) with shorter, intermediate and long chain length is shown in Figure S2 in the Supporting Information. Conclusions The neutral water-soluble polymer PEG with MW ranging from 400 to 20 000 can induce the phase transition of the

lamellar phase in the nonionic system of TX-100/n-C8H17OH/ H2O, from lamellar phase to isotropic phase. The efficiency of PEG inducing phase transition increases with rising MW. Both the solubilization of PEG chains in the aqueous domain of the lamellar liquid crystal and the penetration of the chains into the hydrophilic layer can contribute to the phase transition. The interaction mechanism between PEG and the lamellar liquid crystal strongly depends on the MW of PEG. A critical MW of PEG is found, with which the disturbance of PEG on the neat lamellar structure is most prominent. The critical MW value is 2000 for the lamellar liquid crystal in TX-100/n-C8H17OH/H2O system. Acknowledgment. This work was supported by the National Nature Science Foundation of China (No. 20633010 and 20773106). We gratefully acknowledge Professor Stig E. Friberg for his helpful instruction. Supporting Information Available: Results of frequencydependent measurements in the system of TX-100/n-C8H17OH/ PEG(aq) with PEG molecular weight: (A) 400, (B) 2000, (C) 20 000. Illustration of the microstructure of LR phase in the system of TX-100/n-C8H17OH/PEG(aq) with short chain (A), intermediate chain (B), and long chain (C) of PEG. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Pacios, I. E.; Renamayor, C. S.; Horta, A.; Lindman, B.; Thuresson, K. J. Phys. Chem. B 2002, 106, 5035. (2) Javierre, I.; Bellocq, A. M.; Nallet, F. Langmuir 2001, 17, 5417. (3) Yang, B. S.; Lal, J.; Richetti, P.; Marques, C. M.; Russed, W. B.; Prudhomme, R. K. Langmuir 2001, 17, 5834. (4) Firestone, M. A.; Wolf, A. C.; Seifert, S. Biomacromolecules 2003, 4, 1539.

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