Self-Organization, Phase Behavior, and Microstructure of Poly

22

J. Phys. Chem. B 2002, 106, 22-29

ARTICLES Self-Organization, Phase Behavior, and Microstructure of Poly(oxyethylene) Poly(dimethylsiloxane) Surfactants in Nonpolar Oil Carlos Rodrı´guez,† Md. Hemayet Uddin,† Kenichi Watanabe,† Haruhiko Furukawa,‡ Asao Harashima,‡ and Hironobu Kunieda*,† Graduate School of EnVironment and Information Sciences, Yokohama National UniVersity, Tokiwadai 79-7, Hodogaya-ku, Yokohama 240-8501, Japan, and Dow Corning Toray Silicone Co. Ltd., Chigusa-Kaigan 2-2, Ichihara 299-0108, Japan ReceiVed: June 5, 2001; In Final Form: October 22, 2001

Linear, long poly(oxyethylene) poly(dimethylsiloxane) surfactants, formula Me3SiO-(Me2SiO)m-2-Me2SiCH2CH2CH2-O-(CH2CH2O)nH (SimC3EOn), form reverse micelles in oil such as poly(dimethylsiloxane) and hydrocarbons. The critical micellar concentration (CMC) decreases dramatically with increasing the hydrophilicchain length of the surfactant, whereas the difference in hydrophobic chain length has less influence on the CMC. Hence, the segregation of the poly(oxyethylene) (EO) chain from nonpolar medium is a main factor to form aggregates in oil. Since the lipophilic surfactants used in this study have very long hydrophilic and hydrophobic chains compared to conventional nonionic surfactants, they also form liquid crystals in nonpolar medium such as discontinuous reverse micellar cubic and reverse hexagonal phases at a high surfactant concentration and even in the absence of solvent. Judging from SAXS data, oil penetrates in the palisade layer of surfactant, increasing the preferred negative curvature and relaxing the packing restriction of the hydrophobic chain. Although a normal micellar cubic phase is always changed to a micellar solution upon dilution with water, the present reverse micellar phase coexists with oil in a wide range of composition in the squalane system.

Introduction The self-aggregation of diluted surfactant solutions in nonaqueous solvents has been investigated for many years.1-4 It is known that surfactants can form reverse micelles, with a polar core and a lipophilic shell in contact with the solvent. The selforganization in nonaqueous solvents is strongly influenced by the interaction between headgroups. It has been reported that the micellar aggregation numbers of ionic systems are much larger than that of nonionic systems, which sometimes cannot even form micelles.2 A long hydrophilic chain is needed for copolymer-type nonionic surfactants, but the available range of hydrophobic chain length (C8-C18) restricts the hydrophilelipophile balance of the surfactant. In other words, it is not possible to use long hydrophobic chains in conventional surfactants to produce the reverse type of self-organized structures. For example, in the water-polyoxyethylene oleyl ether system the polyoxyethylene (EO) chain length must be shorter than 4 to form a reverse hexagonal phase.5 If the hydrophilic chain is longer, the surfactant layer curvature becomes zero or positive.5 On the other hand, the use of long, linear hydrophobic chains is limited because they increase the Krafft point. Furthermore, the incompatibility between EO and * Corresponding author. Phone & Fax: +81-45-339-4190. E-mail: [email protected]. † Yokohama National University. ‡ Dow Corning Toray Silicone Co. Ltd.

the hydrocarbon chain in conventional polyoxyethylene-type surfactants is not very high in the absence of water; therefore, they do not show much amphiphilicity in nonaqueous medium and, consequently, they do not form micelles in oil.6,7 Hence, not much research has been done on the effect of the hydrophilic chain on the critical micelle concentration of nonionic surfactants in nonpolar systems. Nonionic A-B type silicone surfactants are fluid even at high molecular weights due to the flexibility of the lipophilic moiety;8 therefore both hydrophilic and lipophilic chain lengths can be increased beyond the range of conventional surfactants, making this compound a kind of “suprasurfactant”. The amphiphilicity of high molecular weight silicone surfactants is suitable for the formation of reverse aggregates, although research has been focused on their aqueous phase behavior.9-11 Several studies have been published on the lyotropic phase behavior in nonaqueous solvents at high surfactant concentrations, but mainly in polar systems as polyols.12 There are a very few reports on liquid crystal formation in nonpolar solvents,13-16 and their microstructure is little known. Binary oil-amphiphile systems usually exist as isotropic solutions or in a solid state.17 In our previous study,18 however, we found a liquid crystal phase (inverse discontinuous cubic, I2) in pure, linear-chain silicone surfactant systems, which remains stable after the addition of oil. To our knowledge, this is the first observation of a reverse micellar cubic phase in a linear, polyoxyethylene-type surfactant.

10.1021/jp0121264 CCC: $22.00 © 2002 American Chemical Society Published on Web 12/08/2001

POE-PDMS Surfactants in Nonpolar Oil

J. Phys. Chem. B, Vol. 106, No. 1, 2002 23

TABLE 1: Data for SimC3EOn Surfactants as Received m

n

purity (wt %)

14 14 14 25 25 25 25

7.8 12 15.8 3.2 7.8 12.2 15.8

99.9 96.4 97.7 97.9 96.2 93.7 93.1

a

b

MS

1440 1625 1792 2053 2256 2449 2608

c

nVEO/VS PI m 0.215 0.294 0.354 0.063 0.136 0.195 0.239

d

1.19 1.19 1.19 1.20 1.20 1.20 1.20

PI n

d

1.26 1.16 1.20 1.21 1.26 1.16 1.20

water (wt %) 0.345 0.425 1.004 0.105 0.223 0.537 1.010

a Excluding water content. The main impurity is water soluble unreacted polyether, CH2dCHCH2sOs(CH2CH2O)nH. b MS is the average molecular weight of the surfactant. c nVEO/VS indicates the volume ratio of the oxyethylene blocks to the surfactant. d PI m and PI n stand for the polydispersity index of poly(dimethylsiloxane) and poly(oxyethylene) chains, respectively. PI ) Mw/Mn, where Mw is the weight average molecular weight and Mn the number average molecular weight.

As a matter of fact, the inverse cubic phase is very rare in surfactant systems. Moreover, the inverse micellar phases discovered so far, seemed to form only in the presence of solvent,17,19-21 which is not the case of the I2 phase in silicone surfactant systems. We think that this observation opens new possibilities for the study of inverse aggregation phenomena and may serve as an experimental support for models of the energetics of inverse mesophases. In this context, we investigated the aggregation, phase behavior, and microstructure of oil-poly(oxyethylene) poly(dimethylsiloxane) systems. The effect of the type of oil on the stability and structure of mesophases will be discussed. Experimental Section

rI ) C

Materials. The series of surfactant with the general formula Me3SiOs(Me2SiO)m-2sMe2SiCH2CH2CH2sOs(CH2CH2O)nH, abbreviated as SimC3EOn, were obtained from Dow Corning Toray Silicone Co. Ltd., Japan. Me is a methyl group attached to Si, m is the total number of silicone, and n is the average number of ethylene oxide (EO) units. Synthesis of silicone polyether surfactants involves three step process: (1) preparation of a siloxane hydride (tSiH) intermediate, (2) preparation of an allyloxy (CH2dCHCH2sOs(CH2CH2O)nH) intermediate, and (3) hydrosilylation of the allyloxy polyether with the siloxane hydride to form the final copolymer.8 These polymeric siloxane surfactants are generally hydrolytically stable at least few months. Table 1 shows data corresponding to the surfactants as received. Water content was measured by the Karl Fischer method with a Mitsubishi CA-06 moisturemeter. The surfactants were treated by washing with methanol and, after evaporation of the solvent, drying over P2O5. The remaining humidity was considered to be hydration water. Octamethylcyclotetrasiloxane (C8H24O4Si4 or D4) was obtained from Dow Corning Toray Silicone Co Ltd. n-Decane (99%), n-hexadecane (98%), and squalane (2,6,10,15,19,23-hexadecylmethyltetracosane, 98%) were used as received. The fluorescence probe 8-anilino-1-naphthalenesulfonic acid (ANS) was obtained from Sigma Chemical Co. and used without further purification. Calculation of the Volume Fraction of the Lipophilic Part of Surfactants. The volume fraction of the lipophilic part of surfactant was calculated by using the following equation:

φL )

where VS and VL are the molar volumes of surfactant and its lipophilic moiety. Fo is the density of oil, MS is the molecular weight of surfactant, and WS′ is the weight fraction of pure surfactant (excluding impurity) in the solvent-surfactant system. The values of VL are 1137 cm3/mol, or 1.89 nm3/ molecule for Si14C3-, and 1985 cm3/mol, or 3.29 nm3/molecule for Si25C3-.22 The molar volumes of lipophilic parts are more than 5 times larger than that of the conventional dodecyl group (215 cm3/mol or 0.36 nm3/molecule).23 Determination of Phase Diagrams. Various amounts of constituents were weighed and sealed in ampules. Samples were mixed using a vortex mixer and homogeneity was attained by repeated centrifugation through a narrow constriction in the sample tubes. The phase equilibria were determined by visual observation using crossed polarizers. The structural characterization of the liquid crystal was determined by means of smallangle X-ray scattering (SAXS) measurements. Structural Characterization Using Small-Angle X-ray Scattering (SAXS). The interlayer spacing of liquid crystals was measured by SAXS, performed on a small-angle scattering goniometer with an 18 kW Rigaku Denki rotating anode generator (Rint-2500) at about 25 °C. The samples were covered with plastic films (Mylar seal method) for the measurement. It is assumed that spherical reverse micelles are packed in a cubic array in the I2 phase. According to the geometry of the I2 phase24 (Fd3m space group) the following equations can be derived for the calculation of the radius of the hydrophilic core of micelle, rI, and the effective cross sectional area per surfactant molecule, as.

VL MS(1 - W′S) VS + FoW′S

(1)

( ) 3 4πnm

as )

1/3

(1 - φL - φo)1/3d

3VL 1 - φL - φo rI φL

(2)

(3)

where d is the interlayer spacing measured by SAXS, nm is the number of micelles per unit cell (24 for the I2 phase), C is a constant (C ) (h2 + k2 + l2)1/2, where h, k, and l are the Miller indices corresponding to the diffraction peak). VL is the volume of the lipophilic part of the surfactant molecule and φO is the volume fraction of oil in the system. Fluorescence Spectroscopy. Samples were prepared by dissolving ANS in methanol, introducing the adequate amount of this solution in sample tubes, and then evaporating the solvent under vacuum; after that, the surfactant solution was added. The samples were sonicated by a water bath-type ultrasonicator in order to enable the incorporation of the probe into the aggregates. Fluorescence spectra of ANS were obtained using a Shimadzu RF-5300 PC spectrofluorimeter (λexcitation ) 322 nm). Spectra were averaged over two scans. All measurements were made at constant temperature of 25 ( 0.2 °C. Figure 1 shows a representative fluorescence spectrum for the SimC3EOn systems. The wavelength of maximum fluorescence of ANS (λmax) is known to change with the polarity of the microenvironment in which it is located; therefore the change of λmax with surfactant concentration can be used to detect the formation of surfactant aggregates.25,26 Light Scattering. Static light scattering measurements were performed at a scattering angle of 90° with an Otsuka Electronics SLS-6000 equipped with a computer controlled stepping-motor driven goniometer, and a He-Ne laser (λ ) 632.8 nm). All measurements were made at a constant temperature of 25 (

24 J. Phys. Chem. B, Vol. 106, No. 1, 2002

Figure 1. Representative fluorescence spectrum for SimC3EOn-D4 systems.

Rodrı´guez et al.

Figure 3. Relative light scattering intensity at 90° as a function of surfactant concentration in Si25C3EOn-D4 systems for different values of n: 0 3.2; O 7.8; 4 12.2; ] 15.8. Lines are only a guide to the eyes.

TABLE 2: Values of CL (Light Scattering) and CO (Fluorescence) as a Function of Ethylene Oxide Number (EON) in Si25C3EOn Systems

Figure 2. Change of ANS spectral maximum λmax with surfactant concentration in Si25C3EOn-D4 systems for different values of ethylene oxide chain length, n: 0 3.2; O 7.8; 4 12.2; ] 15.8.

0.2 °C. The solutions were filtered through 0.1 µm Micropore filters before the measurement. Results and Discussion Formation of Micelles in Oil. Figure 2 shows the change of λmax with surfactant concentration in the Si25C3EOn-D4 systems. λmax initially increases and then becomes almost constant at a given surfactant concentration CO, indicating that the probe changes from a nonpolar environment to a polar environment as surfactant concentration increases, namely, there is formation of inverse surfactant aggregates. The value of λmax in the plateau corresponds to that of ANS dissolved in polyethylene glycol (aproximately 468 nm), which suggests that the probe is located near the hydrophilic core of inverse aggregates. The break point in the λmax concentration curves was found in all surfactant systems except Si25C3EO3.2. In the case of this surfactant, λmax monotonically increases but remains at values far below 468 nm, which suggests that there are no aggregates present and the probe just interacts with the surfactant monomers. This fact can be attributed to the low amphiphilicity of the Si25C3EO3.2 molecule, namely,

EON

CL (mmol/L)

CO (mmol/L)

7.8 12.2 15.8

1.8 0.6 0.1

1.6 0.5 0.1

the ethylene oxide chain length is too short to form reverse micelles in a dilute region when compared with the hydrophobic moiety. Figure 3 shows the light scattering results for Si25C3EOnD4 systems. The formation of aggregates is considered to take place at the breaking point CL of each curve. As it is shown in Table 2, the values for CL are only slightly higher than the values of CO obtained by the fluorescence measurements. Therefore, we consider that CO is close to the critical micellar concentration (CMC). In the case of Si25C3EO3.2 almost no solute scattering was found at all, confirming the fluorescence results regarding the absence of aggregates in this system. It should be pointed out that the existence of aggregates in oil was also confirmed by SAXS. A strong dependence of the scattering intensity with the scattering vector as well as the presence of a correlation peak at high concentrations was observed. Change in CMC as a Function of m and n. Figure 4 shows the change of the CMC with ethylene oxide chain length, n. CMC decreases with n as a general trend. Although the number of experimental points is not enough to determine precisely the dependence of the CMC in oil with n, it seems that the CMC is a decreasing exponential function of n, which is analogous to the relationship between the CMC in water and the length of the lipophilic moiety. Regarding the influence of type of oil on the CMC (Figure 4a) it should be pointed out that in this case the formation of micelles is driven by a lipophobic effect. The bigger the affinity between the ethylene oxide chain and oil, the higher the CMC. The segregation tendency of the ethylene oxide chain from silicon oil is larger when compared with hydrocarbon oil, as observed in Figure 4a. Figure 4b shows that the values of the CMC decrease when the lipophobic chain length is increased from m ) 14 to m ) 25, although this change is not as pronounced as in the case of varying the ethylene oxide chain. Two factors should be



ln XCMC ) -

Figure 4. Change of the CMC with ethylene oxide chain length, n: (a) Si25C3EOn in D4 (O) and decane (0); (b) Si14C3EOn (0) and Si25C3EOn (O) in D4.

considered to explain this result. First, the solubility of a hydrophobic polymer tends to decrease with increasing molecular weight,27 and consequently, the lipophobic effect will increase. Second, with increasing poly(dimethylsiloxane) chain length the surfactant becomes increasingly conical in shape. Cone-shaped surfactants more easily form closed aggregates.28 In this case, the geometrical constraints in the aggregation process seem to be important in determining the CMC. The total free energy change ∆G following inverse micelle formation can be expressed by

∆GT ) ∆GL - ∆GH + ∆GI ) RT ln XCMC

(4)

where ∆GL and ∆GH are lipophilic and hydrophilic terms and ∆GI is a surface term related to the oil-hydrophilic chain interactions. It has been proposed2 that ∆GL is proportional to N2m1/2 for long lipophilic chains, where N is the aggregation number and m is the number of units in the lipophilic chain, i.e., the number of silicone units. ∆GT is less dependent on m, and the CMC is expected not to change very much with the lipophilic chain length. ∆GH should be proportional to the number of ethylene oxide units n. If the reduction in free energy of the EO unit due to association is expressed by ωEO, eq 4 can be written as

ωEO n+c RT

(5)

where c is a constant. This expression is the same as that corresponding to the change in CMC of surfactant aqueous solution as a function of m if ωEO is replaced by ω, the reduction of free energy per -CH2- unit. In the latter case, ω ) 1.08RT.29 In the present nonaqueous system the experimental points are not sufficient and we could not eliminate water completely from the systems. It is known that added water largely influences the formation of inverse micelles in oil.30 Therefore, an accurate value for ωEO cannot be obtained. However, from the slope of the line in Figure 4b, ωEO is roughly estimated as 0.3RT. Hence, the reduction in energy of the hydrophilic EO chain is considerably low when compared with hydrocarbon chains in water, because the incompatibility between the EO chain and D4 is not very large compared to that between water and hydrocarbon chains. Phase Behavior of Concentrated SimC3EOn-Oil Systems. Figure 5 shows the phase behavior of SimC3EOn-D4 systems. In all systems, liquid crystal phases are found even in the absence of solvent, which is not common among conventional nonionic surfactants.31 Note also the low melting point of the surfactants despite the long hydrophobic chains. The stability of these phases increases with the increasing length of both hydrophobic and hydrophilic parts of the surfactant molecule. As a matter of fact, no mesophases were found for n < 7.8, and the liquid crystal region is very narrow in Si14C3EO12 systems (Figure 5a). This is in good agreement with the CMC data shown in the previous section: when the EO chain is very short, the aggregation tendency is very weak. The ability of the poly(dimethylsiloxane) chain to swell with the solvent has a strong influence on the phase behavior, as it occurs with the hydrophobic block in copolymers.32 The packing parameter33 is defined as VL/asl, where VL is the volume of the hydrophobic part, as is the effective crosssectional area per surfactant molecule, and l is the effective length of the hydrophobic moiety. For reverse aggregates, the packing parameter is larger than 1 and increases in the order lamellar phase < reverse hexagonal phase < reverse cubic phase. When m is kept constant, the stability of the I2 phase increases dramatically with n (Figure 5b,c). This fact can be correlated with the results obtained for the critical micellar concentration: as the segregation tendency increases from n ) 12.2 to n )15.8, the stability range of the mesophases tends to increase. SAXS measurements were carried out on Si25C3EO12.2,15.8 systems at 25 °C. Due to limitations in the temperature control of the SAXS equipment, measurements could not be performed on Si14C3EO12.2 system. A typical SAXS diffraction pattern for the I2 phase of the 2.5 wt % D4 sample in the Si25C3EO12.2/ D4 system is shown in Figure 6. A total of five Bragg peaks were identified (indicated by arrows), which can be indexed as the 220, 311, 222, 400, and 331 reflections of the face-centered space group Fd3m (Q227). This is the typical space group for the reverse micellar cubic phase. Since the SAXS pattern we obtained is similar to that previously observed for the Fd3m cubic phase in a bulk (as received) Si25C3EO15.8 sample,18 it is possible that the I2 phase of both the bulk and solvent present Si25C3EO12.2,15.8 systems has the same structure as proposed by Luzzati et al.,24 namely a unit cell containing 24 quasi-spherical reverse micelles, 8 larger ones and 16 smaller ones. The interlayer spacing, d corresponding to the 311 Bragg reflection (the most intense one in the Figure 6) of the I2 phase


Rodrı´guez et al.

Figure 6. SAXS diffraction pattern obtained from an I2 sample of 2.5/97.5 wt % D4/Si25C3EO12.2 composition at 25 °C. The arrows indicate reflections corresponding to the Fd3m space group. q is the scattering vector.

Figure 5. Phase behavior of SimC3EOn-D4 systems as a function of D4 concentration: (a) Si14C3EO12-D4; (b) Si25C3EO12.2-D4; (c) Si25C3EO15.8-D4. (a) is also shown in an expanded scale. H2 ) inverse hexagonal phase; I2 ) inverse micellar (or discontinuous) cubic phase; Om ) inverse micellar solution; S ) solid-present region.

was measured by SAXS as a function of D4 concentration. Studies on other Fd3m cubic structures24 have shown that micelles display a high degree of spherical symmetry and that the difference in the size of the two types of micelles is not large. We therefore assumed monodisperse spherical micelles

ordered in a Fd3m structure to calculate the radius of the hydrophilic core of micelle, rI, and the effective cross-sectional area per surfactant molecule, as. Variation of d, rI, and as of the I2 phase in the Si25C3EO12.2,15.8/D4 systems at 25 °C is shown in Figure 7. It can be observed that as increases as oil is added. The penetration of oil between the poly(dimethylsiloxane) chains causes an increase in the effective hydrophobic volume in the aggregates, therefore the surface area per molecule increases by steric hindrance. However, In the case of Si25C3EO15.8 systems (Figure 7b), as first decreases and then increases with oil content. It is possible that D4 is incorporated first between the aggregates forming the I2 phase and then penetrates into the surfactant layer. The increase in the effective hydrophobic volume also induces geometrical or steric restrictions to the formation of aggregates, hence, the aggregation number and consequently rI, decrease in the I2 phase. Considering that the length of the extended oxyethylene unit in the solid state is approximately 0.36 nm,34 giving a total length of 4.4 nm for n ) 12.2 and 5.7 nm for n ) 15.8, the oxyethylene chains seem to be in a compressed state in Si25C3EOn systems. The values for as are larger than the effective cross sectional area of the extended silicone chain, approximately 0.50 nm2,22 which indicates that the hydrophobic chains are coiled. Effect of Oil on the Phase Behavior of Si25C3EO15.8 Systems. Figure 8 shows the phase behavior of Si25C3EO15.8 systems in decane, hexadecane, and squalane. The I2 cubic phase is the only liquid crystalline phase present. The concentration range in which the single I2 phase exists tends to decrease in the order D4 > decane > hexadecane > squalane. It is expected35 that the curvature free energy, Gc, plays an important role in the stability of inverse mesophases. In the inverse cubic phase, the large hydrophobic volume favors a negative spontaneous curvature. However, there is also a cost of nonzero packing energy, since the poly(dimethylsiloxane) chains must stretch to cover the regions between the micelles. Since curvature and packing considerations oppose, there is a situation of frustration35 of Gc. It is known that this frustration can be decreased by the addition of hydrophobic molecules.36,37 Namely, the stability of the cubic phase should



Figure 7. SAXS data at 25 °C for Si25C3EOn-D4 systems as a function of D4 concentration: d (0), interlayer spacing; rI (4), length of hydrophilic part in aggregates; as (O), effective cross-sectional area per surfactant molecule; (a) Si25C3EO12.2; (b) Si25C3EO15.8.

be influenced by the amount of oil that can be incorporated in the intramicellar space. On the other hand, the penetration of oil into the surfactant palisade layer favors the negative curvature of aggregates and hence the inverse cubic phase is stabilized. For D4 (Figure 5c), decane (Figure 8a), and hexadecane (Figure 8b), the I2 phase melts to an inverse micellar solution. However, in the case of squalane (Figure 8c), both the I2 phase and the inverse micellar solution coexist with excess oil. This fact can be explained in terms of the solubility of the lipophilic silicone chain in oil. Poly(dimethylsiloxane) is completely soluble in D4, decane, and hexadecane, but the mutual solubility of poly(dimethylsiloxane) and squalane is small, so that there is a phase separation between surfactant and oil phases. Figure 9 shows the SAXS data for Si25C3EO15.8 in different oils at 25 °C. In the I2 regions of all the systems, both the interlayer spacing, d(311), and the radius of the hydrophilic core, rI, tend to decrease because probably the aggregation number of reverse micelles is decreased. However, in the squalane

Figure 8. Phase behaviors of Si25C3EO15.8 in different oils: (a) decane; (b) hexadecane; (c) squalane.

system the d curve becomes flat beyond the solubilization limit of the I2 phase. The inflection point clearly indicates the phase boundary between I2 and I2 + O regions. In all the systems, as tends to increase. As explained previously, this fact indicates that there is oil penetration in all systems, and the penetration ability might determine the amount of oil that can be incorporated in the inverse cubic phase. For a given oil concentration, changing the oil has a little effect on the value of as, which suggests that the intermicellar interactions are not affected much. Therefore, the decrease in the melting point of the I2 phase might be attributed to changes in intramicellar interactions, which are expected to get weak with dilution. On the other hand, packing


Rodrı´guez et al. be found at surfactant concentrations much lower than the critical packing volume fraction. There is little possibility that such a solvent structure exists when oil is the dispersing media. Hence, the surfactant concentration at which inverse cubic phases are formed is always higher than the minimum critical volume fraction of packed spheres (0.52 for a primitive cubic packing). It is considered that the incorporation of oil in the inverse cubic phase occurs mainly by physical trapping in the lipophilic chains; therefore it decreases in the order decane > hexadecane > squalane, as the molar volume of the oil increases. The micellar aggregation number, Nagg, can be calculated by

(4/3)πrI3 Nagg ) VH

(6)

where VH is the volume of the hydrophilic part of the surfactant molecule, VH ) VS - VL. From the results of Figures 7b and 9a, Nagg for Si25C3EO15.8 is estimated to be 230 in D4 and 170 in decane at 30% oil (in the vicinity of the Om phase). These values are relatively large when compared to normal, globular micelles at high concentrations.38 The results for Nagg are in agreement with those concerning the CMC (Figure 4a); namely, the aggregation tendency of Si25C3EO15.8 in D4 is stronger than in decane. Conclusions The self-aggregation in long poly(oxyethylene) poly(dimethyl siloxane) surfactants was investigated. These surfactants are able to form not only inverse micelles but also liquid crystals in nonpolar oil. The self-aggregation behavior seems to be strongly influenced by the segregation tendency of the poly(oxyethylene) chain. The flexibility of the silicone chain relaxes the packing restrictions and allows obtaining the inverse structures not found in conventional hydrocarbon surfactant systems. These structures remain stable upon addition of nonpolar oil, which penetrates in the surfactant palisade layer. The incorporation of oil in the I2 phase decreases with the increasing molar volume of the surfactant, and finally, the I2 phase coexists with excess oil. Since these lyotropic phases are formed in a variety of solvents, much research can be done on the effect of the nature of dispersing medium on self-aggregation phenomena. References and Notes

Figure 9. SAXS data for Si25C3EO15.8 in different oils as a function of oil concentration: d (0), interlayer spacing; rI (4), length of hydrophilic part in aggregates; as (O), effective cross-sectional area per surfactant molecule; (a) decane; (b) hexadecane; (c) squalane.

considerations are also expected to play an important role. In nonionic surfactant/water systems, the presence of a hydrated structure surrounding the headgroups increases the effective volume fraction of aggregates and, hence, cubic phases can

(1) Debye, P.; Prins, W. J. Colloid Sci. 1958, 13, 86. (2) Konno, K.; Kitahara, A. J. Colloid Interface Sci. 1971, 35, 636. (3) Fendler, J. H.; Fendler, E. J.; Medary, R. T.; ElSeoud, O. A. J. Chem. Soc., Faraday Trans. 1 1973, 69, 280. (4) Attwood, D.; McDonald, C. Colloid Polym. Sci. 1974, 252, 138. (5) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (6) Kitahara, A. J. Phys. Chem. 1965, 69, 2988. (7) Kunieda, H.; Nakamura, K.; Olsson, U.; Lindman, B. J. Phys. Chem. 1993, 97, 9525. (8) Silicone Surfactants; Hill, R. M., Ed.; Surfactant Science Series 86; Marcel Dekker: New York, 1999. (9) Yang, J.; Wegner, G.; Koningsveld, R. Colloid Polym. Sci. 1992, 270, 1080. (10) Hill, R. M.; He, M.; Lin, Z.; Davis, H. T.; Scriven, L. E. Langmuir 1993, 9, 2789. (11) Li, X.; Washenberg, R. M.; Scriven, L. E.; Davis, H. T.; Hill, R. M. Langmuir 1999, 15, 2278. (12) Wa¨rnheim, T. Curr. Opin. Colloid Interface Sci. 1997, 2, 472. (13) Ekwall, P.; Mandell, L.; Fontell, K. J. Colloid Interface Sci. 1970, 33, 215. (14) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1979, 70, 577. (15) Herrington, T. M.; Sahi, S. S. J. Am. Oil Chem. Soc. 1988, 65, 1677.

POE-PDMS Surfactants in Nonpolar Oil (16) Angelico, R. Molecular diffusion and shear effects in giant wormlike micelles; Universita Degli Studi di Bari: Bari, Italy, 1998. (17) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1998, 14, 2627. (18) Uddin, Md. H.; Rodriguez, C.; Watanabe, K.; Lopez-Quintela, A.; Kato, T.; Furukawa, H.; Harashima, A.; Kunieda, H. Langmuir 2001, 17, 5169. (19) Seddon, J. M. Biochemistry 1990, 29, 7997. (20) Takahashi, H.; Hatta, I.; Quinn, P. J. Biophys. J. 1996, 70, 1407. (21) Seddon, J. M.; Zeb, N.; Templer, R. H.; McElhaney, R. N.; Mannock, D. A. Langmuir 1996, 12, 5250. (22) Kunieda, H.; Uddin, Md. H.; Horii, M.; Furukawa, H.; Harashima, A. J. Phys. Chem. B 2001, 105, 5419. (23) Kunieda, H.; Ozawa, K.; Huang, K. L. J. Phys. Chem. B 1998, 102, 831. (24) Luzzati, V.; Vargas, R.; Gulik, A.; Mariani, P.; Seddon, J. M.; Rivas, E. Biochemistry 1992, 31, 279. (25) Mast, R. C.; Haynes, L. V. J. Colloid Interface Sci. 1971, 53, 35. (26) Saito, Y.; Kondo, Y.; Abe, M.; Sato, T. Yakugaku Zasshi 1994, 114, 325 (in Japanese). (27) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley and Sons: Chichester, U.K., 1998; Chapter 8.

J. Phys. Chem. B, Vol. 106, No. 1, 2002 29 (28) Folmer, B. M.; Svensson, M.; Holmberg, K.; Brown, W. J. Colloid Interface Sci. 1999, 213, 112. (29) Nakagawa, T.; Shinoda, K. Colloidal Surfactants; Academic Press: New York, 1963. (30) Alexandridis, P.; Andersson K. J. Colloid Interface Sci. 1997, 194, 166. (31) Rodriguez, C.; Naito, N.; Kunieda, H. Colloids Surf. A 2001, 181, 237. (32) Alexandridis, P.; Olsson, U.; Lindman, B. Langmuir 1997, 13, 23. (33) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (34) Ro¨sch, M. In Nonionic Surfactants; Schick, M., Ed.; Marcel Dekker: New York, 1967; Chapter 22. (35) Safran, S. A. Statistical Thermodynamics of Surfaces, Interfaces and Membranes; Addison-Wesley: Reading, MA, 1994. (36) Tate, M. W.; Gruner, S. M. Biochemistry 1987, 26, 231. (37) Sjo¨lund, M.; Rilfors, L.; Lindblom, G. Biochemistry 1989, 28, 1320. (38) Danino, D.; Talmon, Y.; Zana, R. J. Colloid Interface Sci. 1997, 186, 170.

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