Langmuir 1998, 14, 5775-5781
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Effect of Added Salts or Polyols on the Liquid Crystalline Structures of Polyoxyethylene-Type Nonionic Surfactants Tetsuro Iwanaga,*,† Masao Suzuki,‡ and Hironobu Kunieda§ Basic Research Laboratory, Noevir Company, Ltd., Okada-cho 112-1, Youkaichi-shi 527, Japan, Oleochemical Research Laboratory, NOF Corporation, Ohama-cho 1-56, Amagasaki 660, Japan, and Graduate School of Engineering, Yokohama National University, Tokiwadai 79-5, Hodogaya-ku, Yokohama 240, Japan Received March 18, 1998. In Final Form: July 17, 1998 The effect of added salts (NaCl, Na2SO4, and NaSCN) or polyols (glycerin, 1,3-butanediol, ethylene glycol, and poly(ethylene glycol) 400) on liquid crystalline structures of polyoxyethylene-type nonionic surfactants was investigated by means of small-angle X-ray scattering (SAXS). The effective cross-sectional areas of the lipophilic parts of aggregates, as, in both hexagonal and lamellar phases decreases upon addition of salts, which lower a cloud point in a dilute aqueous nonionic surfactant solutions. On the other hand, if added salt raises the cloud point, the as increases. The similar results were obtained in the case of adding polyols. Since the as mainly depends on the EO-chain length, the above results are the direct evidence that the hydration or dehydration of the EO-chain is affected by these additives which causes the change in the as in surfactant self-organizing structures. The effect of polyols on the three-phase behavior in water/heptaethylene glycol dodecyl ether (C12EO7)/heptane system was also investigated. Since 1,3-butanediol largely affects the HLB temperature, a considerable amount of the 1,3-butanediol is incorporated in the surfactant aggregates whereas the three-phase temperature is almost unchanged in ethylene glycol and poly(ethylene glycol) 400 systems. Hence, it is considered that the as value in 1,3-butanediol system is less accurate than those in ethylene glycol and poly(ethylene glycol) 400 systems.
Introduction It is well-known that solubilities of nonionic compounds are influenced in the presence of inorganic salts. Most of the inorganic salts decrease the solubilities of organic solutes (salting-out phenomenon) in water, while some of them (NaI, NaClO4, NaSCN) have an opposite action (salting-in effect).1 The anions can be classified into the so-called Hofmeister series2 according to their salting-out strengths at a given molar concentration: SO42- > HPO42> F- > Cl- > Br- > NO3- > I- > ClO4- > SCN-. The effect of the cation is usually smaller than that of the anion. Two mechanisms have been proposed to explain the Hofmeister series behavior. According to one line of thought, salts affect the “solvent property” of water; the salts on the left-hand side of the Hofmeister series are considered to be “structure-makers” while those on the right-hand side are “structure-breakers”.3 In an alternative interpretation, the salting-in and salting-out phenomena are directly related to adsorption and desorption of ions to the hydrophilic parts of the organic compounds.4 The controversy between two mechanisms has not been solved yet.5,6 Maclay reported that the lowering of the cloud point in a dilute aqueous nonionic surfactant solution by electrolyte is a linear function of the ionic strength.7 The effect of * To whom correspondence should be addressed. † Noevir Co., Ltd. ‡ NOF Corporation. § Yokohama National University. (1) Colins, K. D.; Washabaugh, M. W. Q. Rev. Biophys. 1985, 18, 323. (2) Hofmeister, F. Arch. Exp. Pathol. Pharmakol. 1888, 24, 247. (3) Franks, F. In WatersA Comprehensive Treatise; Franks, F., Ed.; Plenum Press: New York, 1973; Vol. 2, p 1. (4) Hall, D. G. J. Chem. Soc., Faraday Trans. 2 1974, 70, 1526. (5) Kabalnov, A.; Olsson, U.; Wennerstro¨m, H. J. Phys. Chem. 1995, 99, 6220. (6) Alexandridis, P.; Holzwarth, J. F. Langmuir 1997, 13, 6074. (7) Maclay, W. N., J. Colloid Sci. 1956, 11, 272.
added salts, acid, and alkali on the phase inversion temperature (PIT) of emulsions and the cloud points of surfactant solutions were also studied by Shinoda et al.8 For example, both the PIT of emulsion and the cloud point of an aqueous solution of polyoxyethylene (9.7) nonylphenyl ether (5 wt % per system) are depressed about 14 °C in the presence of 6 wt % of sodium chloride in water. Kahlweit et al.9-11 extensively studied on the phase behavior of multicomponent systems including wateroil-amphiphile-electrolyte. Recently, Kabalnov et al. reported that the salts effects on equilibrium of nonionic microemulsions are driven by a weak adsorption/depletion of ions at the surfactant monolayer.5 Although there have been many investigations about the effect of additives on physicochemical properties in dilute aqueous nonionic surfactant solutions such as critical micelle concentrations and cloud points, few studies have been carried out on the effect of added salts on the structures of liquid crystals in concentrated systems. The details structures of liquid crystals can be analyzed by means of small-angle X-ray scattering (SAXS). Hence, the effect of added salt on the structures could be understood on a molecular level. It is known that polyols such as glycerin, which were widely used as humectants in cosmetics and pharmaceuticals, also influence the physicochemical properties such as a cloud point, etc.12,13 In this context, we investigated the effect of added inorganic salts and polyols on the liquid crystalline structures by means of SAXS. (8) Shinoda, K.; Takeda, H. J. Colloid Interface Sci. 1970, 32, 642. (9) Kahlweit, M. J. Colloid Interface Sci. 1982, 90, 197. (10) Kahlweit, M.; Lessner, E.; Strey, R. J. Phys. Chem. 1984, 88, 1937. (11) Kahlweit, M.; Strey, R.; Haase, D. J. Phys. Chem. 1985, 89, 163. (12) Sagitani, H.; Ikeda, Y.; Ogo, Y. J. Jpn. Oil Chem. Soc. 1984, 33, 156 (in Japanese). (13) Sagitani, H.; Hirai, Y., Nabeta, K.; Nagai, M. J. Jpn. Oil Chem. Soc. 1986, 35, 102.
S0743-7463(98)00315-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/04/1998
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Figure 1. Phase diagram of a water/salt/C12EO7 system as a function of temperature. The volume fraction of lipophilic part of surfactant is fixed at 0.20. H1 is the hexagonal liquid crystal phase. I is an isotropic solution phase. II is the region which the water or salts solution coexisits with the surfactant liquid. Key: (a) NaCl; (b) Na2SO4; (c) NaSCN.
Experimental Section Materials. Homogeneous heptaethylene glycol dodecyl ether (C12EO7) was obtained from Nikko Chemicals Co., Japan. Pure poly(oxyethylene) oleyl ethers, POlE(5.1) and POlE(10.7), were obtained from NOF Co., Japan. The purity of the oleyl group is greater than 99.7%. However, the EO chains of POlEs have normal distribution. Hexaethylene glycol oleyl ether (POlE(6)) was obtained by mixing the above POlEs. Sodium chloride (NaCl), sodium sulfate (Na2SO4), and sodium thiocynate (NaSCN) were obtained from Tokyo Kasei Kogyo Co. Glycerin (Gly), 1,3butanediol (1,3-BD), ethylene glycol (EG), and poly(ethylene glycol) 400 (PEG400) were obtained from Tokyo Kasei Kogyo Co. Extra-pure grade heptane was obtained from Tokyo Kasei Kogyo Co. These materials were used without further purification. Doubly distilled water was used for all experiments. Procedure To Prepare Samples. Water or aqueous salt solution was added to surfactant in ampules having a narrow constriction. Homogeneity was attained using a vortex mixer at 90 °C and repeated centrifugation through the narrow constriction. Evaluation of the Three-Phase Body. Various amounts of water, oil, and surfactant were sealed in ampules. A series of ampules kept in a thermostat were well shaken and left at constant temperature from several hours to 1 week depending on the stability of emulsions. Phase equilibria were determined by observing respective phases as functions of composition and temperature. Measurement of Small-Angle X-ray Scattering. Interlayer spacing of the liquid crystal was determined by smallangle X-ray scattering (SAXS), performed on a small-angle scattering goniometer with an 18 kW Rigaku Denki rotating anode goniometer (RINT-2500) at about 25 °C. The samples of liquid crystals were enclosed in plastic films for measurement (Mylar seal method). The measurements were performed at 50 kV, 300 mA. The type of liquid crystals was determined by SAXS. For example, the SAXS peak ratios of the lamellar and hexagonal phases are 1:1/2:1/3 and 1:1/x3:1/2, respectively. The liquid crystals were also identified by means of a polarizing microscope. Molar Volume of Surfactants. The molar volumes of each functional groups were measured in the previous paper.14 We assume that the molar volume of surfactant, VS, is the sum of each groups:
VS ) VL + nVEO + VOH
(1)
where VL, VEO, and VOH are the molar volumes of the lipophilic (14) Kunieda, H.; Shigeta, K.; Ozawa, K.; Suzuki, M. J. Phys. Chem. B 1997, 101, 7952.
chain, the oxyethylene unit, and the end-hydroxyl group, respectively, and n is the number of the oxyethylene units. The VL is 215 cm3 mol-1 for dodecyl group and 309 cm3 mol-1 for oleyl group, respectively. VEO and VOH are 38.8 and 8.8 cm3 mol-1, respectively. These values are used to calculate the volume fraction of lipophilic chain, φL, in the system
φL )
VL ML + nMEO + MOH 1 - WS VS + FW WS
(
)(
)
(2)
where FW is the density of water or salt (or polyol) solution and WS is the weight fraction of surfactant. ML, MEO, and MOH are the molecular weights of each functional groups. As described later, the φL was used to calculate the effective cross-sectional area of surfactant molecule and the radius of the hydrophobic part of the cylindrical micelles in the H1 phase using the SAXS date. In the present study, the volume fractions of lipophilic chains for C12EO7 and POlE(6) are fixed at 0.20 and 0.25, respectively, at which C12EO7 forms a hexagonal liquid crystal whereas POlE(6) forms a lamellar liquid crystal.
Results and Discussion Phase Behavior of C12EO7 in Aqueous Salt Solutions. In a water/C12EO7 system,11 the sequence of mesophase is changed Wm-H1-V1-LR-Om with increasing surfactant concentration at 25 °C. Then, the mesophase for φL ) 0.20 corresponds to the hexagonal liquid crystal phase. The maximum melting point of this H1 phase is at about 50 °C. The phase diagrams of a water/C12EO7 system were determined as a function of a salt concentration in water and are shown in Figure 1. The volume fraction of the lipophilic part of surfactant in the system, φL, is fixed at 0.20, and the weight percent of salt in water is plotted horizontally. In the absence of salt, the melting temperature of the hexagonal liquid crystal (H1) and the cloud point at φL ) 0.20 are about 49 and 87 °C, respectively. In addition of NaCl or Na2SO4, both melting temperature and cloud point decrease with increasing NaCl or Na2SO4 concentrations as is shown in parts a and b of Figure 1 because of the dehydration of the EO chain by the salting-out phenomena. On the other hand, in the presence of NaSCN, the melting temperature gradually raises with increasing salinity, but then, decreases at high concentration and the liquid crystal
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Figure 2. Phase diagram of a water/polyol/C12EO7 system as a function of temperature. The volume fraction of lipophilic part of surfactant is fixed at 0.20. H1 is the hexagonal liquid crystal phase. I is an isotropic solution phase. II is the region which the water or polyols solution coexisits with the surfactant liquid. Key: (a) Gly; (b) 1,3-BD; (c) EG; (d) PEG400.
region finally disappears (Figure 1c). The cloud point monotonically rises with increasing NaSCN concentration because of the hydration of EO chain promoted by the salting-in phenomena. The surfactant becomes relatively hydrophobic in NaCl and Na2SO4 systems whereas it becomes hydrophilic in NaSCN systems judging from the change in cloud temperature. However, the H1 phase disappears in all systems. Apart from the HLB factor of the surfactant, we have to consider an additional factor to form the H1 phase. Note that an aqueous salt solutions separates from the liquid crystal in the two-phase region after the single H1 phase disappears in the Na2SO4 system. Phase Behavior of C12EO7 in Aqueous Polyol Solutions. The phase diagrams of a water/C12EO7 system were also constructed as a function of polyol content, and the results are shown in Figure 2. The volume fraction of the lipophilic part of surfactant in the system, φL, is also fixed at 0.20. As is shown in Figure 2a, the cloud point gradually decreases with increasing glycerin (Gly) content. On the other hand, the
cloud point rises with increasing polyol content in 1,3butanediol (1,3-BD), ethylene glycol (EG), and polyethylne glycol 400 (PEG400) systems (Figure 2b-d). Therefore, Gly makes the surfactant hydrophobic, whereas 1,3-BD, EG, and PEG400 make it hydrophilic. These results are in good agreement with the previous data by Sagitani et al.12 The liquid crystal region disappears at the higher polyol content in all of the systems. It is well-known that the short-chain alcohols such as ethanol raises the cmc (critical micelle concentration) of surfactant and breaks the micelle structure.15,16 The liquid crystal would disappear in the same mechanism. Measurement of Interlayer Spacing by Means of SAXS. The effect of added salts or polyols on the interlayer spacing, d (nm), was measured by means of SAXS in C12(15) Ueda, M.; Urabata, T.; Katayama, A.; Kuroki, N. Colloid Polym. Sci. 1980, 258, 1202. (16) Megro, K.; Ueno, M.; Esumi, K. Nonionic Surfactants. Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; p 153.
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Figure 3. Effect of added salts and polyols on the change in interlayer spacings of liquid crystal in C12EO7 system. Key: (9) NaCl; (b) Na2SO4; (2) NaSCN; ([) Gly; (0) 1,3-BD, (O) EG; (4) PEG400.
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with polyols and the weight fraction of water + polyols in the system is always constant in their study. Since the densities of these polyols are higher than water, the volume fraction of the surfctant increases when the polyol content increases. It causes the reduction in interlayer spacing. On the other hand, the interlayer spacing increases upon addition of polyol even if the volume fraction of the hydrophilic part is kept constant. In their study, it is considered that both effects compensate each other. In the Na2SO4 system, first, the interlayer spacing increases as well as in NaCl system, and then, the d value drops to a lower value sharply. This inflection point corresponds to the boundary between the single H1 phase and two-phase region. We always keep the volume fraction of the hydrophobic part of surfactant constant during the replacement of water with polyols or salts. Hence, these changes in interlayer spacing are directly related to the changes in liquid crystalline structures by salts or polyols. In the following section, we analyze the liquid crystalline structures using the d value. Correlation between the Hydration of the EOChain and the Effective Cross-Sectional Area. According to Israelachivili,18 the surface free energy of surfactant molecule in aggregates, µsur, is represented by
µsur ) γA +
K A
(3)
where γ is the interfacial tension of the water-hydrocarbon chain of the surfactant, A is the interfacial area per one mole of surfactant in the aggregates, and K is a constant related to the repulsive force of hydrophilic group due to the hydration of the EO chain in the present systems. In equilibrium, the optimal interfacial area A0 is obtained by eq 4.
A0 )
xKγ
(4)
Hence, the change in A0 is directly related to the change in K, the hydration force of the EO-chain. Since γ is constant in the present case, the water-soluble additives influence the K and as a result, A0. Analysis of SAXS. The cylinders are packed in hexagonal array in the H1 phase as is shown in Figure 5(a). We assume that H1 phase consists of infinitely long cylinders. In this case, the radius of the cylinder, rH (nm), and the cross-sectional area of hydrocarbon core per surfactant molecule, as (nm2), can be calculated by the following equations: Figure 4. Effect of added salts and polyols on the change in interlayer spacings of liquid crystal in POlE(6) system. Key: (9) NaCl; (b) Na2SO4; (2) NaSCN; ([) Gly; (0) 1,3-BD; (O) EG; (4) PEG400.
EO7/water and POlE(6)/water systems. The results are shown in Figures 3 and 4. The volume fraction of lipophilic part of POlE(6) is fixed at 0.25, at which a lamellar liquid crystal (LR) phase is formed. In both H1 (C12EO7) and LR (POlE(6)) phases, the interlayer spacing increases when NaCl or Gly is added. On the other hand, the d values decrease in the case where NaSCN or 1,3-BD is added. Sagitani et al. also measured the interlayer spacing in the LR phase at a fixed surfactant/water weight ratio (60/40) in a commercial POlE(6) system.12 In their results, the interlayer spacing is almost unchanged upon additions of Gly or PEG400, which is different from our results. Apart from the problem of using impure POlE(6), water is replaced
rH )
(
)
2 φL x3π
as )
2vL rH
1/2
d
(5)
(6)
Here d is an measured interlayer spacing, φL is the volume fraction of lipophilic part in system and vL is the volume of lipophilic part per one surfactant molecule. vL is 0.36 nm3 for C12EO7 and 0.51 nm3 for POlE(6).14,17 The as corresponds to A0/Avogadro’s constant. Concerning the lamellar liquid crystal (Figure 5b), as and the half-thickness of hydrocarbon part of the bilayer, (17) Huang, K. L.; Shigeta, K.; Kunieda, H. Prog. Colloid Polym. Sci., in press. (18) Israelachvili, J. W.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
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Figure 5. Interlayer spacing, d, rH, and rLR in the H1 and LR phases.
Figure 7. Effect of added salts and polyols on the change in effective cross-sectional areas per one surfactant molecule in POlE(6) system. Key (9) NaCl; (b) Na2SO4; (2) NaSCN; ([) Gly; (0) 1,3-BD; (O) EG; (4) PEG400.
Figure 6. Effect of added salts and polyols on the change in effective cross-sectional areas per one surfactant molecule in C12EO7 system. Key: (9) NaCl; (b) Na2SO4; (2) NaSCN; ([) Gly; (0) 1,3-BD; (O EG; (4) PEG400.
dLR (nm), can be calculated by the following equations:
d dLR ) φL 2 as )
vL dLR
(7) (8)
The values of as in both H1 and LR phases are shown in Figures 6 and 7, respectively. The cross-sectional area decreases when NaCl or Gly is added, whereas as increases upon addition of NaSCN or 1,3-BD. It is known that as is mainly dependent on the EO-chain length in polyoxyethylene-type nonionic surfactant systems.14,17 In other words, as is the same for the nonionic surfactants having the same EO chain, even if the hydrocarbon-chain length is different. Therefore, the as increases due to the increases in hydrophilicity of the EO chain upon addition of NaSCN or 1,3-BD and vice versa. In a water-polyoxyethylene-type nonionic surfactant system, the H1-LR phase transition occurs when
Figure 8. Effect of added salts and polyols on the change in the radius of lipophilic core in the hexagonal liquid crystal phase. Key: (9) NaCl; (b) Na2SO4; (2) NaSCN; ([) Gly; (0) 1,3-BD; (O) EG; (4) PEG400.
the cross-sectional area is around 0.40-0.47 nm2 for C12EO7 at φEO/φS ) 0.45. In the present study, the crosssectional area in H1 phase for C12EO7 system is 0.51 nm2 in the absence of additives. The radius of cylinder, rH, for hexagonal liquid is calculated according to eq 5, and the results are shown in Figure 8. The rH value corresponds to the effective hydrocarbon chain length of surfactant in the H1 phase. And, rH is inversely proportional to as. In the C12EO7 systems, rH increases upon additions of Na2SO4 or NaCl, whereas it decreases upon additions of NaSCN or 1,3-BD. The radius of cylinder of the hydrocarbon core in the H1 phase approaches the alkyl-chain length in the extended form at the phase boundary between the H1 and the LR phase.14 The most extended
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Figure 9. Phase diagram of a water/C12EO7/polyol/heptane system as a function of temperature. The water/oil and C12EO7/polyol ratios are both 50/50 (wt/wt). I, II, and III are the one-, two-, and three-phase regions, respectively. “LC present” is a region including a liquid crystal: (a) none; (b) 1,3-BD; (c) EG; (d) PEG400.
chain length is 1.67 nm for dodecyl group according to the Tanford’s equations.19 In the H1 phase (Figure 8), rH is still shorter than the above extended value. Hence, the H1-LR transition does not take place even if as decreases upon addition of Na2SO4 and other compounds. Phase Behavior of Water/C12EO7/Polyol/Heptane Systems. It is reported that diols such as 1,3-BD act as cosurfactant, and surfactant and diol form mixed micelles.12 To confirm this, the phase diagram of water/ C12EO7/polyol/heptane systems was constructed at constant mixing ratio of surfactant (C12EO7/polyol ) 1/1) as a function of temperature. The weight ratio of water/ heptane is fixed at 1/1. These results are shown in Figure 9. Generally, in a nonionic surfactant/water/oil system, a surfactant forms an aqueous micelle and dissolves mainly in water at lower temperature, whereas it forms a reversed micelle and dissolves mainly in oil at higher temperature. The type of emulsion also inverts from O/W to W/O with the increase in temperature.20 At the transition temper(19) Tanford, C. J. Phys. Chem. 1972, 76, 3020.
ature, there is a three-phase region consisting of water, oil, and surfactant phases. This temperature was termed the phase-inversion temperature (PIT) of the emulsion or the HLB temperature.21 The HLB temperature in a water/C12EO7/heptane system is 61 °C and is constant and independent of the composition as is shown in Figure 9a. If the three-phase body changes by the addition of polyols, these polyols would be incorporated in the surfactant layer of microemulsion. The HLB temperature in the EG system is almost unchanged and that in the PEG400 system slightly decreases at high surfactant + polyol concentration as is shown in parts c and d of Figure 9. Hence, these polyols mainly act as a cosolvent in water. In other words, they are mainly soluble in water and change the nature of the water phase. When as values are calculated by eq 6, we assume that only surfactant molecules form aggregates in liquid crystal. In the EG and PEG400 systems, the calculation of as is rather accurate because the polyols are (20) Kunieda, H.; Shinoda, K. Bull. Chem. Jpn. 1982, 55, 1777. (21) Shinoda, K.; Saito, H. J. Colloid Interface Sci. 1968, 26, 701.
Effect of Added Salts or Polyols
regarded not to be incorporated with surfactant molecules. On the other hand, in the 1,3-BD system, the three-phase temperature rises from the original HLB temperature in the dilute region with increasing surfactant + 1,3-BD content. It means that a considerable amount of 1,3-BD forms a mixed aggregate with surfactant molecules. Hence, the as values for 1,3-BD system in Figures 6 and 7 are not as accurate as those in the EG and PEG400 systems. Conclusion The effect of added salts or polyols on liquid crystalline structure of polyoxyethylene-type nonionic surfactant was studied by means of small-angle X-ray scattering (SAXS). It is considered that NaCl, Na2SO4, and Gly have a saltingout function and make the surfactant hydrophobic, and NaSCN and 1,3-BD have a salting-in effect and make the surfactant hydrophilic according to the change in cloud temperature. We measured the change in interlayer spacing in the H1 phase (C12EO7 system) and in the LR phase (POlE(6) system) upon addition of these inorganic
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salts and polyols. The volume fraction of the hydrophobic part of the surfactant in the system is always kept constant. Hence, the change in as upon addition of these additives is directly related to their effect on the hydrophilicity of the EO chain because the as values in the H1 and LR phases depend on the EO-chain length. The as always increases when the salting-in additives are mixed with water whereas the opposite tendency is obtained upon additions of the salting-out additives. The effect of added polyol on the three-phase (HLB) temperature was also investigated in a water/C12EO7/heptane system. Judging from the change in three-phase temperature, it is concluded that EG and PEG400 act as cosolvents, and they are practically not incorporated with surfactant aggregates whereas a considerable amount of 1,3-BD is incorporated in the surfactant layer in liquid crystals. Hence, the as in the 1,3-BD obtained from the SAXS data is not as accurate as that for the former polyol system. LA980315G
