J. Phys. Chem. B 1998, 102, 831-838
831
Effect of Oil on the Surfactant Molecular Curvatures in Liquid Crystals Hironobu Kunieda,* Kazuyo Ozawa, and Kuo-Lun Huang Graduate School of Engineering, Yokohama National UniVersity, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240, Japan ReceiVed: August 19, 1997; In Final Form: NoVember 7, 1997
The effect of added decane, m-xylene, or squalane on the phase behavior of the polyoxyethylene dodecyl ether (C12EOn) -water system was investigated as a function of polyoxyethylene (EO) chain length at 25 °C. When the surfactant is relatively lipophilic (more lipophilic than the balanced state), the type of liquid crystal is changed to the more lipophilic one upon addition of decane. The lamellar (LR) to lipophilic reverse hexagonal (H2) transition takes place in the C12EO3 system. On the other hand, the hexagonal (H1) to hydrophilic discrete cubic (I1) liquid crystal transition occurs in the hydrophilic C12EO7 system. There are two kinds of effects of oil on the self-organizing structures. One is the “penetration effect”, in which oil molecules penetrate into the surfactant palisade layer and expand the effective cross-sectional area, aS. The other is “swelling effect”, in which oil molecules are solubilized in the core of aggregates and expand the volume of aggregates. In this case, the aS is almost constant. Due to the swelling effect, the H1-I1 phase transition takes place in the decane-C12EO7 system, whereas the LR-H2 transition occurs in the decanelipophilic C12EO3 system due to the penetration effect. This causes the opposite tendency of the change in the curvature of the surfactant layers in liquid crystals on each side of the balanced state. As a result, the phase behavior of surfactant is quickly changed from forming micelles to reverse micelles within a narrow range of the EO chain in the presence of oil. On the other hand, m-xylene tends to penetrate the surfactant palisade layer, and the H1-LR transition occurs even in the hydrophilic C12EO7 system.
Introduction Various self-organizing structures are formed in binary water-polyoxyethylene-type nonionic surfactant systems.1-7 In the binary systems, the phase diagrams are usually drawn as a function of temperature, since the hydrophile-lipophile property of the surfactant is highly influenced by temperature change due to the conformational change in the hydrophilic polyoxyethylene (EO) chain.8,9 Although only few types of surfactant aggregates appear in each phase diagram, almost all the types of surfactant supramolecular assemblies are formed in the water-surfactant systems if the EO chain length of the surfactant is successively changed at constant temperature.10 As Winsor revealed,11-13 with decreasing the hydrophilicity of surfactant, the self-organizing structures are changed from aqueous micelles to reverse micelles via hexagonal, lamellar, and reverse hexagonal liquid crystals.10 On the other hand, in the presence of oil, it is well-known that the phase behavior is dramatically changed at certain temperature called the HLB temperature or PIT (phase inversion temperature in emulsion) in polyoxyethylene-type nonionic surfactant systems.14-19 Below the temperature, the surfactant forms an aqueous micellar phase that coexists with an excess oil phase, whereas a reverse micellar solution phase and an excess water phase appear above the HLB temperature. At the HLB temperature, the so-called bicontinuous microemulsion phase (or surfactant phase) coexists with both excess water and oil phases, and the three-phase body is observed. Microemulsions have been widely used for practical applications.20 In ionic surfactant or mixed nonionic surfactant systems, the same phase behavior is observed by changing the mixing ratio of hydrophilic * To whom correspondence should be addressed.
surfactant/lipophilic surfactant at constant temperature.21, 22 This mixing ratio or composition is called the HLB composition, at which a microemulsion phase also coexists with excess water and oil phases. In both cases (HLB temperature or HLB composition), the three-phase body is, in general, very narrow, and the phase behavior is dramatically changed from aqueous micellar to reverse micellar phases with a slight change in temperature or surfactant mixing ratio. On the other hand, as mentioned above, the spontaneous curvature of the surfactant molecular layer is gradually changed with the above variables in the binary watersurfactant system.10 Hence, it is considered that the interaction between the surfactant layer and oil molecules causes the drastic change in the phase behavior. It is known that oil penetrates into the palisade layer of surfactant molecules at a water-oil interface.23,24 This “penetration” effect makes the spontaneous curvature negative or less positive. When the surfactant layer is convex toward water, the curvature is regarded to be positive. In fact, the HLB temperature in the polyoxyethylene-type nonionic surfactant system is shifted to lower temperatures when aromatic hydrocarbons or low-molecular-weight saturated hydrocarbons are used instead of high-molecular-weight saturated hydrocarbons.15 However, the drastic change in the phase behavior in the presence of oil cannot be explained only by this effect. Recently, we constructed the phase diagrams of polyoxyethylene oleyl ethers-water and polyoxyethylene dodecyl etherswater systems as a function of polyoxyethylene chain length at constant temperature.10,25 Almost all the types of self-organizing structures including reverse types appear in the systems. Hence,
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832 J. Phys. Chem. B, Vol. 102, No. 5, 1998
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the effect of oil on the self-organizing structures can be figured out by the addition of oil to the polyoxyethylene-type nonionic surfactant systems in which the EO chain is continuously changed. In this context, we investigated the effect of added oil to the self-organizing structures in a polyoxyethylene dodecyl etherwater system to know how the curvatures of surfactant molecular layers in liquid crystals are changed. The result will explain how the HLB temperature or HLB composition appears in a ternary water/surfactant/oil system. Experimental Section Materials. Homogeneous polyoxyethylene dodecyl ethers were obtained from Nikko Chemicals Co. They are abbreviated as C12EOn where n ) 1-9 is the number of EO units. Extrapure grade m-xylene, n-decane, squalane, and 1-dodecanol were obtained from Tokyo Kasei Kogyo Co. Cyclic silicone oil, octamethylcyclotetrasiloxane (SH244), was obtained from Toray Dow Corning Co. These materials were used without further purification. Two surfactants were mixed to obtain C12EOn, where the EO number, n, is not an integer. Small-Angle X-ray Scattering. Interlayer spacing of a lamellar liquid crystal was measured using small-angle X-ray scattering (SAXS), performed on a small-angle scattering goniometer with an 18 kW Rigaku Denki rotating anode generator (RINT-2500) at ∼25 °C. The samples were covered by plastic films for the SAXS experiment (Mylar seal method). Lamellar and hexagonal liquid crystalline phases were also distinguished by the SAXS peaks.26,27 The ratio of interlayer spacing from first and second peaks is 1:1/2 for the lamellar type and 1:1/x3 for the (reverse) hexagonal type, respectively. Since only one peak was observed in the I1 phase in the presence of oil, the detail structure is not identified. However, judging from the previous results and its positions in phase diagrams, the I1 phase is considered to be body- or face-centered discrete cubic phase.10,25,28 The type of liquid crystal was also identified by a polarizing microscope. Calculation of the Volume Fraction of the Dodecyl Group. The densities of surfactants were measured by the digital density meter (Anton Paar 40). We assume that the density of surfactant in a liquid state is unchanged even in solutions or liquid crystals at constant temperature. The molar volume of surfactant is calculated by the following equation
VS ) MS/FS
(1)
where MS and VS are the molecular weight and the molar volume of surfactant, respectively. It is also assumed that arithmetic additivity is held concerning the molar volumes of each functional groups in the surfactant.10,29 Then, the molar volume of C12EOn is the sum of molar volumes of each group in the surfactant, and the following relation is held
VS ) VL + nVEO + VOH
(2)
where VL, VEO, and VOH are the molar volumes of the lipophilic chain (dodecyl group), the oxyethylene unit, and the hydroxyl group, respectively. The n is the number of oxyethylene units. VS, VL, VEO, and VOH were determined from the density data for homogeneous polyoxyethylene dodecyl ethers (Nikko Chemicals Corp., C12EO1-6) and pure dodecanol because they are in a liquid state at 25 °C. The obtained values of VL, VEO, and VOH are 215, 38.8, and 8.8 cm3 mol-1, respectively. From these data, we can calculate the molar volume of C12EOn by eq 2. The above values were also used to calculate the volume fraction
Figure 1. Effect of added decane on the phase behavior of the waterpolyoxyethylene dodecyl ether system as a function of the EO chain of the surfactant at 25 °C. The volume fraction of the dodecyl chain of surfactant in water + surfactant () φL/(1 - φO)) is constant, 0.2.
of a lipophilic chain of surfactant, φL, in the system
φL )
VL ML + nMEO + MOH 1 - WS VS + FW WS
(
)(
)
(3)
where FW is the density of water and WS is the weight fraction of C12EOn. ML, MEO, and MOH denote the molecular weights of dodecyl, the EO unit, and end hydroxyl groups, respectively. Notation. We use the following notations to distinguish each phases. The subscript 1 denotes “hydrophilic” or “normal”type self-organizing structure or phase, in which the average curvature of surfactant layer is positive and convex toward water. The subscript 2 indicates “lipophilic” or “reverse”-type assemblies. H1 ) hexagonal liquid crystal; H2 ) reverse hexagonal liquid crystal; I1 ) discontinuous-type cubic phase (water-continuous); LR ) lamellar liquid crystal; D1 ) isotropic bicontinuous surfactant phase (normal type) which appears in an oil-rich region;6,19 D2 ) isotropic bicontinuous surfactant phase (reverse type), which is denoted by D′ or L3 phase in the previous papers;6,19 Wm ) aqueous phase containing surfactant aggregates; Om ) oily phase like reverse micellar solution phase or surfactant liquid; W ) excess water phase; O ) excess oil phase; LCP ) liquid-crystal-present region. Results Effect of Added Decane on the Phase Behavior in a Water-Polyoxyethylene Dodecyl Ether Systems. The effect of added decane on the phase behavior of a polyoxyethylene dodecyl ether-water system was determined as a function of the EO chain length of the surfactant, and the results are shown in Figure 1. In the absence of oil (on the bottom axis of the phase diagram), the volume fraction of dodecyl group of each surfactant in the system, φL, is 0.2. The volume fraction of decane, φO, in the system is plotted vertically. The single H1 and LR regions are indicated by the broken curves because the boundaries are not very accurate. The types of self-organizing structures on the bottom axis are H1, Wm, LR, D2, and Om depending on the EO chain length of the surfactant. These phases were confirmed by SAXS and other measurements.10 The Wm phase is an isotropic liquid
Effect of Oil on the Surfactant Molecular Curvatures phase connected to an aqueous micellar solution phase at low surfactant concentration.25 The D2 is an isotropic fluid bicontinuous phase often designated as the L3 phase. Since the curvature of surfactant layer in the D2 phase is slightly negative, we use the subscript “2”.6 When an EO chain length of surfactant decreases, the self-organizing structure is gradually changed from a hydrophilic to lipophilic one in the absence of decane. The LR phase exists in the absence of oil when the EO unit of C12EOn is approximately between 2.4 and 4.3. The spontaneous curvature of surfactant bilayers in the LR phase is considered to be zero, and the HLB of surfactant is optimum in the given system. When decane is added, the phase transition occurs as shown in Figure 1. On the right-hand side of the balanced region where liquid crystal swells, a large amount of oil, aqueous micelles (I1), coexists with an excess oil phase, whereas a reverse micellar solution phase coexists with an excess water phase on the lefthand side. Although the LR phase is present in the LCP region, the type and the number of liquid crystals were not identified. It is very clear from Figure 1 that the tendency of the phase transition is opposite upon addition oil. When the EO chain length is around 3, the spontaneous curvature of surfactant aggregates tends to be negative upon addition of decane. With increasing decane content, the LR-D2-H2-Om transition takes place. Since oil molecules penetrate into the palisade layer of surfactant molecules, it is natural that the spontaneous curvature becomes more negative. However, when the surfactant is hydrophilic (EO ) 7 or so), the opposite phenomenon occurs and the H1-I1 transition takes place. The I1 phase is a discretetype cubic phase containing aqueous micelles.28 Hence, the curvature becomes positive upon addition of decane in hydrophilic surfactant systems. This opposite tendency of the oil effect on the curvature of surfactant layer is also observed in a single polyoxyethylene-type nonionic surfactant system at different temperatures.19 This phenomenon cannot be explained only by the oil penetration, and another factor has to be considered. At least two kinds of factors are acted to change the spontaneous curvature of self-organizing structures. In Figure 1, when the EO chain length of surfactant is around 5, the solubilization of oil in the Wm or LR phase increases. We fixed a water/oil ratio () 3/1) in Figure 1 and decreased the surfactant concentration; a three-phase body consisting of microemulsion, excess water, and oil phases was observed when the EO chain length is around 4.2. The composition of the threephase region is called the HLB composition. In other words, the HLB temperature is 25 °C at this EO unit. The three-phase body exists in a very narrow range of EO unit ()0.2) in which aqueous micelles are changed to reverse micelles. This is extremely narrow compared with the binary water-surfactant system. The Wm phase is gradually changed to the Om phase via LR, D2, reverse bicontinuous cubic phase region in a wide range of the EO chain ()2 EO unit) in a dilute region of the binary water-surfactant system.25 Consequently, on the both sides of the HLB temperature or composition, the effect of added oil on the curvature of the surfactant layer is opposite. This is the reason the phase behavior is quickly changed in the presence of oil. Effect of the Types of Oils. It is well-known that the HLB temperature of polyoxyethylene-type nonionic surfactant is highly influenced by the types of oils.15 In the case of saturated hydrocarbons, the HLB temperature increases with increasing the molecular weight of oil. The HLB temperature is low in the case of unsaturated aromatic hydrocarbons. At constant temperature, the surfactant tends to form aqueous micelles in
J. Phys. Chem. B, Vol. 102, No. 5, 1998 833
Figure 2. Effect of added m-xylene on the phase behavior of the water-polyoxyethylene dodecyl ether system as a function of the EO chain of the surfactant at 25 °C. The volume fraction of the dodecyl chain of surfactant in water + surfactant () φL/(1 - φO)) is constant, 0.2. “I” is an isotropic single-phase region in which the type of phase was not identified.
Figure 3. Effect of added squalane on the phase behavior of the waterpolyoxyethylene dodecyl ether system as a function of the EO chain of the surfactant at 25 °C. The volume fraction of dodecyl chain of surfactant in water + surfactant () φL/(1 - φO)) is constant, 0.2.
saturated large-molecular-weight hydrocarbon systems whereas reverse micelles are formed in aromatic or low-molecular-weight hydrocarbons. To investigate the difference in the types of oils, the effect of added m-xylene and squalane on the phase behavior was investigated, and the results are shown in Figures 2 and 3. In Figure 2, commercial polyoxyethylene dodecyl ethers (n ) 10 and 24) were used for surfactants whose chain lengths are more than 10. A isotropic single-phase region on the righthand side in Figure 2 originates from the surfactant liquid, Om, because the surfactant concentration is higher than that for the H1 phase. Since this phase coexists with O phase, we denote it the D1 phase, although the details of the structure are not known. In both phase diagrams, the H2 phase was not observed. It is clear from Figure 2 that m-xylene largely penetrates into the surfactant palisade layer and makes the curvature negative. At the EO chain ) 7, the H1-LR-D2-Om transition takes place. On the other hand, it is considered that the penetration of squalane molecules is very little even compared with the decane
834 J. Phys. Chem. B, Vol. 102, No. 5, 1998
Figure 4. Effect of added decane on the change in interlayer spacing of liquid crystals as a function of φL + φO or φO in the C12EO7 system: 9, interlayer spacing, d; 0, effective cross-sectional area per one surfactant molecule, aS, b, the radius of a lipophilic cylinder in the H1 phase, rH.
Figure 5. Effect of added decane on the change in interlayer spacing of liquid crystals as a function of φL + φO or φO in the C12EO3 system. The volume ratio of the lipophilic moiety of surfactant to water + hydrophilic moiety of surfactant is 0.376/0.624. 9, interlayer spacing, d; 0, effective cross-sectional area per one surfactant molecule, aS; b, the radius of lipophilic cylinder in the H2 phase, rRH, and the half thickness of lipophilic part in the LR phase, rLR.
system,24 and the LR region is not shifted toward the hydrophilic surfactant region. Among three oils, m-xylene is considered to have the biggest power to change the spontaneous curvature negative. For this reason, the HLB composition appears in a hydrophilic surfactant system whereas it is almost unchanged in the squalane system. In addition to the decane system, the solubilization of squalane in the Wm phase increases as shown in Figure 3. On the other hand, the D2 phase swells a large amount of m-xylene as shown in Figure 2. Hence, in the latter system, the origin of microemulsion is the bicontinuous D2 phase. Structural Change in Liquid Crystals. To understand the change in liquid crystalline structure upon addition of decane, we measured the interlayer spacing in C12EO7 and C12EO3 systems. The results are shown in Figures 4 and 5. The volume ratio of the lipophilic moiety of surfactant to water plus the hydrophilic moiety of surfactant is kept constant, 0.2/0.8 for C12EO7 and 0.376/0.624 for C12EO3. At this ratio in the C12-
Kunieda et al.
Figure 6. Effect of added m-xylene on the change in interlayer spacing of liquid crystals as a function of φL + φO or φO in the C12EO7 system. There is no oil on the left-hand axis at which φL ) 0.2. 9, interlayer spacing, d; 0, effective cross-sectional area per one surfactant molecule, aS; b, the radius of lipophilic cylinder in the H1 phase, rH, and the half thickness of lipophilic part in the LR phase, rLR.
Figure 7. Effect of added m-xylene on the change in interlayer spacing of liquid crystals as a function of φL + φO or φO in the C12EO3 system. The volume ratio of the lipophilic moiety of surfactant to water + hydrophilic moiety of surfactant is 0.376/0.624. 9, interlayer spacing, d; 0, effective cross-sectional area per one surfactant molecule, aS; b, the half thickness of lipophilic part in the LR phase, rLR.
EO3 system, a single H2 phase appears upon addition of decane to the LR phase. The changes in interlayer spacing of the H1 phase of C12EO7 and the LR phase of C12EO3 systems upon addition of m-xylene are also shown in Figures 6 and 7. The cylinders are packed in a hexagonal array in the H1 phase as shown in Figure 8a. It is assumed that the H1 phase consists of infinitely long cylindrical rods. The radius of the lipophilic core, rH, and the effective cross-sectional area of one surfactant molecule, aS, in the H1 phase can be calculated by the following equations:
rH )
{
2 (φL + φO) x3π
aS )
}
( )
2VL φO 1+ rH φL
1/2
d
(4)
(5)
where φL is the volume fraction of lipophilic part of surfactant
Effect of Oil on the Surfactant Molecular Curvatures
J. Phys. Chem. B, Vol. 102, No. 5, 1998 835
Figure 8. Interlayer spacing, d, and rH, rLR, and rRH in the H1, LR, and H2 phases.
in the system, φO is the volume fraction of oil in the system, d is the measured interlayer spacing, and VL is the volume of the lipophilic part of one surfactant molecule. On the other hand, the half distance of the lipophilic part, rLR, and the effective cross-sectional area of one surfactant molecule, aS, in the LR phase (Figure 8b) is calculated by
rLR ) (d/2)(φL + φO) aS )
(6)
( )
VL φO 1+ rLR φL
(7)
{
2 (1 - φL - φO) x3π
aS )
is 0.5 nm2 for both types and approximately coincident with that in the H1 phase at the same point. It means that the radius of the spherical micelle, rI, is 1.5 times larger than the rH in the H1 phase. Hence, the mean curvature of the sphere, 1/rI ) 1/1.5rH , is larger than that in the H1 phase, 1/2rH, at the transition point. On the other hand, in the C12EO7 -m-xylene system, the aS is increased and the H1-LR transition takes place as shown in Figure 7. A similar tendency was observed in the C12EO3 -mxylene system, but we did not investigate the phase transition of the LR phase. Discussion
For the H2 phase (Figure 8c), we use the following equations
rH )
Figure 9. Schematic representation of “penetration” and “swelling” effects of oil in the H1 phase.
}
2VL 1 - φL - φO rRH φL
1/2
d
(8)
(9)
where rRH is the radius of a cylinder of the hydrophilic part. We assumed that infinitely long cylindrical rods are packed in the H2 phase. In the decane systems, the effective crosssectional area is only slightly increased upon addition of oil. This is in good agreement with the previous result.28 Since aS is not largely changed, penetration of decane molecules into the surfactant palisade layer is rather small compared with m-xylene molecules. However, the LR-H2 transition takes place in the C12EO3 system, whereas the H1-I1 transition is observed in the C12EO7 system. Since only one SAXS peak is observed for the I1 phase in the present system, we could not analyze the details of the structure. However, the I1 phase is considered to be a discontinuous-type cubic phase judging from the previous results.28 We assumed that spherical micelles are packed in face- or body-centered-cubic array in the I1 phase. Then, the effective cross-sectional area, aS, in the spherical micelle at the H1-I1 transition point in Figure 4 was calculated using the extrapolated value of the interlayer spacing. The aS
H1-I1 and H1-Lr Transitions in the C12EO7 System. As described before, there are two possibilities of oil solubilization in a liquid crystal. On one hand, oil molecules penetrate into the surfactant palisade layers and tend to increase the effective cross-sectional area, aS (penetration effect). On the other hand, oil molecules are also solubilized inside the core of surfactant aggregates (swelling effect). Both effects on the H1 structure are schematically shown in Figure 9. Judging from the present data, the penetration power of m-xylene is considerably larger than that of decane. We consider the contribution of both effects on the phase transition from H1 to I1 or H1 to LR in the C12EO7 systems. We define “penetration” to mean the effect of oil to expand the aS without increasing the volume of aggregates, whereas “swelling” is the effect of oil to increase the volume of the lipophilic part of aggregates without expanding aS. It seems that a radius or thickness of a cylinder or lamella would be changed due to the change in the net hydrocarbon chain length of surfactant even in the case of penetration, but such an effect is neglected for simplification. Suppose that oil molecules are completely penetrated into the palisade layer and would never be solubilized in the core of the surfactant aggregates. In this case, as a first approximation, the radius of the H1 phase, rH, is regarded as constant, and eq 4 is rewritten as the following equation.
836 J. Phys. Chem. B, Vol. 102, No. 5, 1998
d)
(
x3π 1 2 φL + φO
)
Kunieda et al.
1/2
rH
for complete penetration (10)
On the other hand, it is assumed that oil molecules are solubilized only in the core of aggregates and would never penetrate into the palisade layer as also shown in Figure 9. In this case, aS would be constant and the following is obtained by combining eqs 4 and 5:
d ) (2x3π)
1/2
VL (φL + φO)1/2 aS φL
for complete swelling (11)
It is known that the aS is not affected by the hydrocarbon chain length in the case of ionic surfactant homologs,30 and the aS in the H1 system depends on the EO chain length of nonionic surfactant systems.25 For this reason, the constant aS approximation for complete “swelling” is considered to be reasonable. According to eqs 10 and 11, we calculated the change in d as a function of φL + φO for the two extreme cases, and the results are shown in Figure 10. The experimental values for the interlayer spacings in decane, m-xylene, SH244, and dodecanol systems are shown in Figure 10. In the calculation, the constant values of rH and VL/aS are obtained from the measured d in the absence of oil. The change in d in real H1 systems should be observed between two extreme cases calculated by eqs 10 and 11 because oil molecules are distributed between the core and the palisade layer. It is clear from Figure 10 that decane molecules are mainly swollen in the core of surfactant cylinders whereas m-xylene molecules are distributed between the palisade layer and the core. In the case of decane or SH244, the swelling effect is mainly acted. Since the total surface areas of aggregates are almost constant, the surfactant aggregates must minimize the surface area when decane solubilization is increased. As a result, the cylinder-sphere transition takes place. It is considered that the H1-LR transition takes place since the penetration effect of m-xylene is larger than the swelling effect. Dodecanol molecules, first, tend to penetrate in the palisade layer, but they are also solubilized in the core of aggregates with increasing solubilization. Lr-H2 Transition in the C12EO3 System. In the case of C12EO3, a lamellar liquid crystal is formed in the absence of oil, and the layer-reverse cylinder transition takes place upon addition of decane as shown in Figure 1. Concerning the surfactant curvature, the tendency is opposite to the result in the decane-C12EO7 system. In the LR phase, we can also consider the penetration and swelling effect of oil. If oil penetrates into the surfactant layer and would never be solubilized in the core of the aggregates, we can rewrite eq 6 as follows:
d ) 2rLR/(φL + φO)
complete penetration
(12)
where rLR is constant in this case. On the other hand, if oil molecules are solubilized only in the core of the aggregates and would never penetrate into the surfactant palisade layer, we can obtain the following equation by combining eqs 6 and 7,
d ) 2VL/aSφL
complete swelling
(13)
where aS is constant in this case. We calculated the change in interlayer spacing for the two extreme cases by using rLR and
Figure 10. Correlation between the interlayer spacing and the swelling or penetration effects of oil in the H1 phase: 9, decane; 0, SH244; b, m-xylene; O, dodecanol.
Figure 11. Correlation between the interlayer spacing and the swelling or penetration effects of oil in the LR phase: 9, decane; 0, SH244; b, m-xylene; O, dodecanol.
aS values in the absence of oil. The result is shown in Figure 11. The changes in interlayer spacing for four kinds of oils are also shown in Figure 11. Although we do not know the details of the decane distribution between the surfactant palisade layer and the core in the LR phase, it is considered that the penetration effect is larger than the former C12EO7 system because the experimental curve is considerably separated from the curve for complete “swelling”. Hence, in this case, the LR-H2 transition takes place due to the penetration effect of decane. In the real systems, the “penetration” and “swelling” of oil take place simultaneously. Even if the same oil is added, the “penetration”/ ”swelling” ratio depends on the HLB of surfactant. Penetration and Swelling Effects of Oil. When oil molecules penetrate into the surfactant palisade layers, oil molecules tends to expand the effective cross-sectional area. Since the surface area of aggregates, aS, is increased from the equilibrium state in the absence of oil and the surface energy would increase, the contraction force would act to shrink the surface area. The packing parameter for the relationship between the shape of self-organizing structure and the curvature of surfactant molecular layer in a surfactant-water system is represented by
Effect of Oil on the Surfactant Molecular Curvatures
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the packing equation31
(
)
VL 1 l2 l 1 + + )1aSl 2 R1 R2 3R1R2
(14)
where l is the effective lipophilic chain length of surfactant and R1 and R2 are two principal curvatures of the surface. The packing parameter is 1/3 for spherical micelles in the I1 phase, 1/2 for the H1 phase, and 1 for the LR phase. Hence, the packing parameter increases when a self-organizing structure is changed from normal (or hydrophilic) type to a lipophilic one. Although the VL is originally the volume of the lipophilic part of the surfactant in the water-surfactant system, it increases due to the penetration of oil molecules into the lipophilic chain. Hence, to minimize the increase in aS while oil is penetrated, the change of the shape of self-organizing structure to the one with a large packing parameter is energetically favorable. For this reason, the spontaneous curvature of the surfactant layer would be negative (or less positive), and a phase transition to a more lipophilic self-organizing structure takes place. On the other hand, when oil molecules do not penetrate but are swollen in the core of surfactant aggregates, the effective cross-sectional area is practically constant upon addition of the oil. Since the volume of the lipophilic part is increased, the surfactant mean curvature [) (1/R1 + 1/R2)/2] tends to be more positive in order to keep the total surface area constant because the ratio of total surface area to the volume becomes minimum in a spherical shape. It is known that long-chain hydrocarbon oil increases the cloud temperature in an aqueous polyoxyethylene-type nonionic surfactant solution whereas an aromatic hydrocarbon oil decreases it.32 The increase in cloud temperature means that the surfactant forms a more hydrophilic spherelike micelle due to the “swelling” effect whereas the curvature of surfactant layer would be less positive due to the “penetration” effect in the case of lowering the cloud temperature. A similar result is obtained in a dilute aqueous micellar system.33 Even if the same oil is added, the penetration effect is increased in the lipophilic or short EO chain surfactant system as shown in Figures 10 and 11. The radius of a cylinder in the H1 phase is close to the hydrocarbon chain length of surfactant in the extended form, whereas the half thickness of the lipophilic part in the LR phase is considerably short.1,10 Hence, it is considered that the surfactant palisade layer in the H1 phase is more rigid, and the penetration tendency becomes small. Structures of Microemulsions. There are at least four kinds of isotropic fluid phases to be identified as microemulsions solubilizing water and/or oil in a water/surfactant/oil system.19,34 The oil-swollen aqueous micellar solution phase (Wm) and the water-swollen reverse micellar solution phase (Om) are wellknown. Apart from these phases, two bicontinuous isotropic phases, D1 and D2, solubilize water or oil. As described before, the D2 phase is called L3 phase in which the curvature of the surfactant layer is slightly negative, and it forms in a waterrich region. On the other hand, the D1 phase is considered to be a counter structure to the D2 phase, and it forms in an oilrich region although the curvature of the surfactant layer is considered to be slightly positive. These four phases are from time to time merged in a space of temperature and compositions, and it is not easy to distinguish each phase.19,34 In decane and squalane systems, the microemulsion phase originates from an aqueous micellar solution phase, and a single D2 phase is narrow; the solubilization is not very large as shown in Figures 1 and 3. On the other hand, the D2 phase solubilizes
a large amount of water in the m-xylene system as shown in Figure 2. Microemulsion is an isotropic fluid phase containing surfactant aggregates in which oil or water is solubilized. However, the structure of the microemulsion phase would be different depending on the nature of the oil. Conclusions The effect of added oil on the structures of liquid crystals in polyoxyethylene dodecyl ether (C12EOn)-water systems were investigated by means of phase study and small-angle X-ray scattering at 25 °C. When decane is added to the hydrophilic C12EO7 system, the H1-I1 transition takes place whereas the LR-H2 transition occurs in the C12EO3 system. Hence, the effect of added oil on the curvature of the surfactant layer is opposite. The curvature tends to be positive in the hydrophilic surfactant system whereas it becomes negative in the lipophilic surfactant system. For this reason, the change in the curvature becomes drastic in the presence of oil, compared with a binary water-surfactant system. There are two kinds of oil effects: penetration and swelling effects. If oil molecules penetrate into the surfactant palisade layers, the effective cross-sectional area of surfactant molecule tends to expand and the curvature becomes negative or less positive. If oil molecules are solubilized in the core of aggregates and swell the volume, the cross-sectional area tends to remain constant and the curvature becomes positive in order to keep the total surface area of aggregates. In a hydrophilic surfactant system, the H1-I1 phase transition takes place due to the “swelling” effect upon addition of decane. On the other hand, the LR-H2 transition occurs mainly due to the “penetration” effect. The penetration tendency is very large for dodecanol and aromatic hydrocarbons such as m-xylene, whereas the swelling tendency is large for saturated hydrocarbons such as decane and squalane. References and Notes (1) Clunie, J. S.; Goodman, J. F.; Symons, P. C. Trans. Faraday Soc. 1969, 65, 287. (2) Shinoda, K. J. Colloid Interface Sci. 1970, 34, 278. (3) Saito, H. Nihon Kagaku Zasshi, 1971, 92, 223. (4) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (5) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (6) Strey, R.; Schomacker, R.; Roux, D.; Nallet, F.; Olsson U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253. (7) Shigeta, K.; Suzuki, M.; Kunieda, H. Prog. Colloid Polym. Sci., in press. (8) Karlstroem, G. J. Phys. Chem. 1984, 88, 4769. (9) Matsuura, H.; Fukuhara, K. J. Mol. Struct. 1985, 126, 251. (10) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952. (11) Winsor, P. A. Chem. ReV. 1968, 68, 1. (12) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Surfactant Science Series Vol. 30; Marcel Dekker: New York, 1988. (13) Laughlin, R. G. The Aqueous Phase BehaVior of Surfactants; Academic Press: San Diego, 1994; p 304. (14) Shinoda, K.; Kunieda, H. J. Colloid Interface Sci. 1972, 42, 381. (15) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (16) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1980, 75, 601. (17) Kunieda, H.; Friberg, S. E. Bull. Chem. Soc. Jpn. 1981, 54, 1010. (18) Kunieda, H.; Shinoda, K. Bull. Chem. Soc. Jpn. 1982, 55, 1777. (19) Kunieda, H.; Shinoda, K. J. Dispersion Sci. Technol. 1982, 3, 233, (20) Solans, C. Kunieda, H., Eds. Industrial Applications of Microemulsions; Marcel Dekker: New York, 1997. (21) Kunieda, H.; Shinoda, J. Jpn. Oil Chem. Soc. (Yukagaku) 1980, 29, 676. (22) Kunieda, H.; Aoki, R. Langmuir 1996, 12, 5796. (23) Evans, D. F.; Wennerstrom, H. The Colloidal Domain; VCH: New York, 1994; Chapter 11.
838 J. Phys. Chem. B, Vol. 102, No. 5, 1998 (24) Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I. AdV. Colloid Interface Sci. 1990, 33, 59. (25) Kunieda, H.; et al., to be published. (26) Fontell, K. Liquid Crystals & Plastic Crystals; Gray, G. W., Winsor, P. A., Eds.; John Wiley & Sons: New York, 1974; Vol. 2, Chapter 4. (27) Reference 23, Chapter 6. (28) Bouwstra, J. A.; Jousma, H.; van der Meulen, M. M.; Vijverberg, C. C.; Gooris, G. S.; Spies, F.; Junginger, H. E. Colloid Polym. Sci. 1989, 267, 531.
Kunieda et al. (29) Tanford, C. J. Phys. Chem. 1972, 76, 3020. (30) Gallot, B.; Skoulios, A. Kolloid Z. Z. Polym. 1966, 208, 37. (31) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (32) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Sons: New York, 1989; Chapter 4. (33) Hoffmann, H.; Ulbricht, W. J. Colloid Interface Sci. 1989, 129, 388. (34) Olsson, U.; Shinoda, K.; Lindman, B. J. Phys. Chem. 1986, 90, 4083.
