- Email: [email protected]
water reverse microemulsion
Colloids and Surfaces A: Physicochemical and Engineering Aspects 168 (2000) 109 – 113 www.elsevier.nl/locate/colsurfa
Solubilization of poly(ethylene oxide) in sodium dodecyl sulfonate/octane/butanol/water reverse microemulsion Zhenshan Hou a, Zhiping Li a,*, Hanqing Wang b b
a Xingjiang Institute of Chemistry, Chinese Academy of Sciences, Urumqi 830011, Xingjiang, P.R. China Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, Lanzhou, P.R. China
Received 23 February 1998; accepted 19 August 1999
Abstract We investigated the effect of addition of poly(ethylene oxide)(PEO) to the sodium dodecyl sulfonate(AS)/octane/ butanol/water system on phase behavior by methods of electrical conductivity and EPR spectra for a water/oil microemulsion of these components. When PEO is added to such a w/o microemulsion, the interaction between droplets of microemulsion decreases. PEO molecules are mainly located in the region between the head groups of the surfactant, inducing a slight reorganization of the interface which partly expels the probe into higher polarity regions and forms droplets with a tighter interface layer. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrical conductivity; EPR spectra; Polarity
1. Introduction Most work on water-soluble polymers on surfactant aggregation is carried out in aqueous solution, whereas the interaction between w/o microemulsions and water-soluble polymers is mainly concerned with the size of droplets and the interaction between droplets [1,2]. To understand similar w/o dispersions with macromolecules of biological interest, one must study more primitive systems, for example, those between simple polymers and w/o microemulsion [3]. In addition, as water-soluble polymers serve with surfactants and microemulsion as underground injecting fluids to * Corresponding author. E-mail address: [email protected] (Z. Li)
enhance oil recovery [4], so the studies of the phase behavior and physico-chemical properties of microemulsion-forming systems in the presence of polymers are important [5]. We report results about the effect of high molecular weight additives, namely synthetic polymers, on the phase behavior and microstructure of w/o microemulsions. Poly(ethylene oxide) is chosen as water-soluble polymer to dissolve in the water core of droplets [6,7]. The interaction between w/o microemulsion and polymer is studied by various methods, including electrical conductivity [8], FT-IR [9], dynamic light-scattering measurements etc., [10]. Lianos et al. [11] have studied the effect of poly(ethylene oxide) on the cyclohexane/pentanol/ sodium dodecyl sulfonate/water w/o microemul-
0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 4 0 5 - 7
110
Z. Hou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 168 (2000) 109–113
sion; the results indicate that the polymer induces reorganization of the dispersing phase so that aggregates grow in size. In contrast, according to the data of Suarez et al. [12], obtained by time-resolved fluorescence quenching, addition of watersoluble polymer to the microemulsion decreases the interdroplet attractive interaction and the droplet size, whereas an increased droplet size is observed for an oil-soluble polymer, such as polybutadiene. Thus, an opposite conclusion was obtained even for the same system. In this current work, we use the EPR technique, combined with conductivity measurements, to probe the effect of PEO on microstructure of w/o the microemulsion, such as the locus of the polymer solubilized in a microemulsion and the variation of micropolarity due to the addition of PEO so that structural changes induced by the polymer can be better understood.
2. Experimental section Materials: Octane (Fluka) and 1-butanol(A.R grade, Beijing Chemical Reagent Co) were dried with 4A, molecular sieve before use. The surfactant sodium dodecyl sulfonate (AS) (Beijing
Fig. 1. Partial phase diagram of the AS/octane/1-butanol/water system in the presence of PEO at 30°C. M: isotropic phase, L: liquid crystalline phase. Solid and dashed lines represent pure water or 4.0 g 100 ml − 1 PEO solution used, respectively. The dotted line corresponds to the system diluted with water.
Chemical Reagent Co) was crystallized twice from ethanol–water mixture (ethanol–water: 9:1,v/v) and dried under vacuum. Poly(ethylene oxide) (Merck, Mw = 20 000, GC quality) was used as received. Spin probes were 5-Doxylstearic acid(5DNS) (Sigma Chemical Co) and 4-dodecanoyloxyl-2,2,6,6-tetramethyl-piperdine-1-oxyl(C12-Tem po) (a gift from Lanzhou University), the purity of the latter was analyzed and confirmed to satisfy all experimental requirements.
2.1. Methods Many samples over the pseudo-ternary diagram were prepared by weighing an appropriate amount of substance into graded tube (10 ml), and then 1-butanol, octane and water were added to the tube in proportion; the mixture was shaken then equilibrated at 30°C for several h. All concentrations are expressed as weight×lot%. A liquid crystal phase was identified on examination of texture with a polarizing optical microscope. In the following Cp represents the polymer concentration (g 100 ml − 1) in water, and Wo corresponds to the molar ratio of water to surfactant: Wo = [water]/[surfactant] All w/o microemulsions studied with conductivity and EPR measurements were made using AS as an ionic surfactant, 1-butanol as a cosurfactant and octane as an oil phase. The ratio of AS to 1-butanol was 3:7. In Fig. 1, the dotted line in the w/o microemulsion region displays our choice of preparing w/o microemulsions. The electrical conductivity measurements were carried out with a conductimeter type DDS-11D with platinum electrodes (Shanghai Leizi instrument factory, Shanghai, China). The solutions were contained in a double-walled glass tube thermostated at 30°C. For EPR measurements, the required amount of nitroxide in absolute ethanol was evaporated in a volumetric flask and the sample was then added to give a concentration of spin probes of 2.4× 10 − 4M. Precautions were taken to keep the EPR tube tightly closed to prevent the constituents evaporating.
Z. Hou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 168 (2000) 109–113
111
region of liquid crystalline phase (L phase) shows similar behavior. This means PEO molecules can also be dissolved in the liquid-crystalline phase. The effect of the polymer on microstructure of the L phase remains unclear.
3.2. Electrical conducti6ity measurements
Fig. 2. Variation of the electrical conductivity K with Wo for AS/octane/1-butanol/water w/o microemulsion with varied PEO concentrations.
EPR spectra were recorded at 30°C (Varian E-115 spectrometer) with 100 kHz field at X-band frequency.
3. Results and discussion
3.1. Phase beha6ior A part of the phase diagram of the pseudoternary system AS/ octane/1-butanol/water at 30°C is shown in Fig. 1, in which an isotropic microemulsion (M phase) and a liquid-crystalline phase are only displayed. The M phase is bound by a solid line or dashed line (for pure water and 4 g 100 ml − 1 PEO in aqueous solution, respectively). As shown in Fig. 1, the addition of PEO extends that the region of M phase extends from the AS/butanol/octane side to the water/butanol/ octane side, so more water is solubilized. The
Fig. 2 shows the variation of the electrical conductivity K as a function of water content (Wo) for AS/1-butanol/ octane/water w/o microemulsion containing PEO of various concentrations in the dispersing phase. In all microemulsions, the electrical percolation is present. The electrical percolation threshold (values of Wo at the onset of increased conductivity) increases as PEO is added to the microemulsion. Information on interaction between droplets in w/o microemulsion can be obtained from electrical conductivity measurements. For example, the appearance of electrical percolation indicates the existence of a strong attraction between droplets [13–15]. Fig. 2 shows that the addition of PEO induces a decreased attraction between droplets, as the PEO concentration increases. The conductivity decreases with increasing PEO concentration at constant Wo values. This result is consistent with the results of Suarez. et al. [12], who suggested that the observed decrease of droplet size leads to decreased overlap of interface film of droplets during collision of two droplets and in turn to decreased interdroplet attraction, as Cp increases; hence K decreases. In contrast, Lianos et al. [11] proposed that droplets form larger aggregates and fewer in number than the original droplets with added PEO, which decreases the mobility of polymer-containing aggregates, so larger and fewer aggregates give decreased conductivity at constant Wo. In order to verify which conclusion above is valid for our w/o microemulsion, we measured EPR spectra to investigate the effect of polymer additive on microviscosity and micropolarity of droplets. We use an EPR spin labelling of AS reversed microemulsion with spin probes differing in chemical structure to provide the information on the local polarity and viscosity. Because 5-DNS la-
112
Z. Hou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 168 (2000) 109–113
belled with carboxyl groups can act as cosurfactant and anchor at the interface [16,17], and partially dissociated carboxyl group may enhance such a location, their nitroxide moieties are expected to probe the region within the polar groups of AS. C12-Tempo label, an uncharged probe, is more hydrophobic than 5-DNS. According to our data, the C12-Tempo probe is located at outer interface of the droplets or remains in the oil phase (octane and 1-butanol).
Fig. 3. Isotropic hyperfine splitting parameters AN and relational period tc as a function of water content Wo. Circles and squares correspond to presence and absence of PEO, respectively. Spin label: 5-DNS.
Fig. 4. Isotropic hyperfine splitting parameters AN and the relational period tc as a function of water content Wo. Circles and squares correspond to presence and absence of PEO, respectively. Spin label: C12-Tempo.
In the system investigated, the spectra of two spinning probes in microemulsions that are nearly isotropic are characterized through the rotational time given by [18]: tc = 6.5 ×10 − 10DHo[(ho/h − 1)1/2 + (ho/h + 1)1/2 − 2] (s) in which tc is the rotational correlation period, ho, h + 1 and h − 1 are the peak-to-peak heights of the central line, the low-field and high-field line, respectively, and DHo is the central line peak to peak width (Gauss). The tc values give the microviscosity and the hyperfine splitting parameter AN is a measure of the polarity of the spin label environment. Figs. 3 and 4 show that AN and tc vary with water (or 4 g 100 ml − 1 PEO in aqueous solution) content for 5-DNS and C12-Tempo, respectively (spectra not shown). For the 5-DNS label, in the absence of PEO AN increases; whereas tc passes a maximum value and then monotonically decreases upon continued addition of water, both approaching, but not reaching the corresponding values in bulk water (AN = 15.88 G, tc = 1.66× 10 − 10 s). All data indicate that the probe became incorporated in the water core of the micellar aggregate, and the local environment monitored with the probe became less viscous and more polar with increasing proportion of solubilized water. In the presence of PEO (Fig. 3), AN and tc display a behavior similar to the above results. At the same Wo, larger tc is found compared with the system not containing PEO, whereas AN increases slightly at the same Wo on addition of PEO. Hence the amplitude of motion of the probe decreases. From Table 1, at constant Wo values (Wo = 20.12) tc increases significantly, whereas AN increases slightly as a function of Cp PEO concentration in aqueous solution increases. Thus PEO molecules bind between the head groups, and decrease repulsion between the head groups, which induces closer packing of alkyl chains. In addition, the introduction of PEO in the water pool of the droplets may decrease the size of the droplet, which restricts motion of the probe molecules, and so microviscosity increases. The
Z. Hou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 168 (2000) 109–113 Table 1 EPR Parameters of spin probe 5-DNS in w/o microemulsiona Cp (g 100 ml−1)
AN (G)
tc (×10−10 s)
0.0 1.0 2.5 5.0 7.5 10.0
14.97 15.00 15.00 15.02 15.04 15.04
3.35 3.72 4.02 4.05 4.04 4.03
a
All samples have Wo = 20.12.
local polarity felt by the probe increases due to a slight reorganization of the interface, which partly expels the probe into a more polar environment. For the C12-Tempo spinning label, tc and AN display similar behavior to that of the 5-DNS probe with increasing Wo (Fig. 4), but with increasing Wo, tc and AN change little, and both of them are so small that they are near those in octane –butanol (1:1, mass ratio; AN =15.6 G, tc =2.05 × 10 − 11 s) and differ much from those in water; AN = 17.3, tc = 5.22× 10 − 10 s). Therefore, the probe is located in the outer interface of the droplets or in the oil phase. At the same Wo, both AN and tc change little upon introduction of PEO. Regarding the location of C12-Tempo in droplets of reversed microemulsion, we suggest that the polymer is mainly solubilized in the water core, mostly between headgroups of surfactants, not penetrating into the hydropholibic chains close to the outer interface of droplets or remaining in oil phase. Such a process excludes the possibility that polymer can interpenetrate the interfacial layer to connect different droplets, which has been suggested by Meier [19]. According to the EPR spectra, the interfacial layer of droplets becomes tighter on addition of PEO. Hence interpenetration of the interfacial layer of two colliding droplets is decreased and thus decreases attraction between droplets; this is consistent with decreased electrical conductivity due to polymer addition as shown in Fig. 2.
4. Conclusion In the system investigated, the addition of PEO
113
has a significant influence on phase behavior. Solubilized water is increased in both w/o microemulsion and liquid-crystalline phases. Conductivity measurements indicate that added PEO increases the percolation threshold; and this effect becomes more pronounced as Cp increases, and electrical conductivity decreases at constant Wo with added PEO. EPR spectra indicate that the interface layer reorganizes slightly and becomes closer. All above results indicates that the interaction between droplets in w/o microemulsion decreases when PEO is added to a w/o microemulsion. We plan to confirm whether this phenomenon is also valid for non-ionic surfactant systems.
References [1] D. Schubel, G. Ilgenfritz, Langmuir 13 (1997) 4246. [2] D. Papoutsi, P. Lianos, W. Brown, Langmuir 9 (1993) 663. [3] L. Schlicht, J.H. Spilgies, F. Runge, S. Lipens, S Boye, D. Schubel, G. Ilgenfritz, Biophys. Chem. 58 (1998) 39. [4] D.O. Shah, Macro- and Microemulsions theory and applications, American Chemical Society, Washinton, DC, 1985, p. 150. [5] A.M. Bellocq, Prog. Coll. Polym. Sci. 105 (1997) 290. [6] A.M. Bellocq, Langmuir 14 (1998) 3730. [7] M-J. Suarez, H. Levy, J. Lang, J. Phys. Chem. 87 (1993) 9808. [8] M.L. Das, P.K. Bhattacharya, S.P. Moulik, Langmuir 7 (1991) 636. [9] C. Gonzalez-Blanco, L.J. Rodriguez, M.M. Velazquez, Langmuir 13 (1997) 1938. [10] D. Papoutsi, P. Lianos, W. Brown, Langmuir 10 (1994) 3402. [11] P. Lianos, S. Modes, G. Staikos, W. Brown, Langmuir 8 (1992) 1054. [12] M-J. Suarez, J. Lang, J. Phys. Chem. 99 (1995) 4626. [13] J. Lang, N. Lalem, R. Zana, Coll. Surf. 68 (1992) 199. [14] R.T. Hamilton, J.F. Billman, E.W. Kaler, Langmuir 6 (1990) 1696. [15] B. Antalek, A.J. Williams, J. Texter, Y. Feldman, N. Garti, Coll. Surf., 128 (1997). [16] G. Haering, P.L. Luisi, H. Hauser, J. Phys. Chem. 92 (1988) 3574. [17] N.S. Kommareddi, V.T. John, Y.Y. Waguespack, G.L. McPherson, J. Phys. Chem. 97 (1993) 5752. [18] H. Caldararu, A. Caragheorgheopol, M. Vasilescu, H. Dragutan, H. Lemmetyinen, J. Phys. Chem. 98 (1994) 5320. [19] W. Meier, Langmuir 12 (1996) 1188.
Solubilization of poly(ethylene oxide) in sodium dodecyl sulfonate/octane/butanol/water reverse microemulsion Zhenshan Hou a, Zhiping Li a,*, Hanqing Wang b b
a Xingjiang Institute of Chemistry, Chinese Academy of Sciences, Urumqi 830011, Xingjiang, P.R. China Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, Lanzhou, P.R. China
Received 23 February 1998; accepted 19 August 1999
Abstract We investigated the effect of addition of poly(ethylene oxide)(PEO) to the sodium dodecyl sulfonate(AS)/octane/ butanol/water system on phase behavior by methods of electrical conductivity and EPR spectra for a water/oil microemulsion of these components. When PEO is added to such a w/o microemulsion, the interaction between droplets of microemulsion decreases. PEO molecules are mainly located in the region between the head groups of the surfactant, inducing a slight reorganization of the interface which partly expels the probe into higher polarity regions and forms droplets with a tighter interface layer. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrical conductivity; EPR spectra; Polarity
1. Introduction Most work on water-soluble polymers on surfactant aggregation is carried out in aqueous solution, whereas the interaction between w/o microemulsions and water-soluble polymers is mainly concerned with the size of droplets and the interaction between droplets [1,2]. To understand similar w/o dispersions with macromolecules of biological interest, one must study more primitive systems, for example, those between simple polymers and w/o microemulsion [3]. In addition, as water-soluble polymers serve with surfactants and microemulsion as underground injecting fluids to * Corresponding author. E-mail address: [email protected] (Z. Li)
enhance oil recovery [4], so the studies of the phase behavior and physico-chemical properties of microemulsion-forming systems in the presence of polymers are important [5]. We report results about the effect of high molecular weight additives, namely synthetic polymers, on the phase behavior and microstructure of w/o microemulsions. Poly(ethylene oxide) is chosen as water-soluble polymer to dissolve in the water core of droplets [6,7]. The interaction between w/o microemulsion and polymer is studied by various methods, including electrical conductivity [8], FT-IR [9], dynamic light-scattering measurements etc., [10]. Lianos et al. [11] have studied the effect of poly(ethylene oxide) on the cyclohexane/pentanol/ sodium dodecyl sulfonate/water w/o microemul-
0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 4 0 5 - 7
110
Z. Hou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 168 (2000) 109–113
sion; the results indicate that the polymer induces reorganization of the dispersing phase so that aggregates grow in size. In contrast, according to the data of Suarez et al. [12], obtained by time-resolved fluorescence quenching, addition of watersoluble polymer to the microemulsion decreases the interdroplet attractive interaction and the droplet size, whereas an increased droplet size is observed for an oil-soluble polymer, such as polybutadiene. Thus, an opposite conclusion was obtained even for the same system. In this current work, we use the EPR technique, combined with conductivity measurements, to probe the effect of PEO on microstructure of w/o the microemulsion, such as the locus of the polymer solubilized in a microemulsion and the variation of micropolarity due to the addition of PEO so that structural changes induced by the polymer can be better understood.
2. Experimental section Materials: Octane (Fluka) and 1-butanol(A.R grade, Beijing Chemical Reagent Co) were dried with 4A, molecular sieve before use. The surfactant sodium dodecyl sulfonate (AS) (Beijing
Fig. 1. Partial phase diagram of the AS/octane/1-butanol/water system in the presence of PEO at 30°C. M: isotropic phase, L: liquid crystalline phase. Solid and dashed lines represent pure water or 4.0 g 100 ml − 1 PEO solution used, respectively. The dotted line corresponds to the system diluted with water.
Chemical Reagent Co) was crystallized twice from ethanol–water mixture (ethanol–water: 9:1,v/v) and dried under vacuum. Poly(ethylene oxide) (Merck, Mw = 20 000, GC quality) was used as received. Spin probes were 5-Doxylstearic acid(5DNS) (Sigma Chemical Co) and 4-dodecanoyloxyl-2,2,6,6-tetramethyl-piperdine-1-oxyl(C12-Tem po) (a gift from Lanzhou University), the purity of the latter was analyzed and confirmed to satisfy all experimental requirements.
2.1. Methods Many samples over the pseudo-ternary diagram were prepared by weighing an appropriate amount of substance into graded tube (10 ml), and then 1-butanol, octane and water were added to the tube in proportion; the mixture was shaken then equilibrated at 30°C for several h. All concentrations are expressed as weight×lot%. A liquid crystal phase was identified on examination of texture with a polarizing optical microscope. In the following Cp represents the polymer concentration (g 100 ml − 1) in water, and Wo corresponds to the molar ratio of water to surfactant: Wo = [water]/[surfactant] All w/o microemulsions studied with conductivity and EPR measurements were made using AS as an ionic surfactant, 1-butanol as a cosurfactant and octane as an oil phase. The ratio of AS to 1-butanol was 3:7. In Fig. 1, the dotted line in the w/o microemulsion region displays our choice of preparing w/o microemulsions. The electrical conductivity measurements were carried out with a conductimeter type DDS-11D with platinum electrodes (Shanghai Leizi instrument factory, Shanghai, China). The solutions were contained in a double-walled glass tube thermostated at 30°C. For EPR measurements, the required amount of nitroxide in absolute ethanol was evaporated in a volumetric flask and the sample was then added to give a concentration of spin probes of 2.4× 10 − 4M. Precautions were taken to keep the EPR tube tightly closed to prevent the constituents evaporating.
Z. Hou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 168 (2000) 109–113
111
region of liquid crystalline phase (L phase) shows similar behavior. This means PEO molecules can also be dissolved in the liquid-crystalline phase. The effect of the polymer on microstructure of the L phase remains unclear.
3.2. Electrical conducti6ity measurements
Fig. 2. Variation of the electrical conductivity K with Wo for AS/octane/1-butanol/water w/o microemulsion with varied PEO concentrations.
EPR spectra were recorded at 30°C (Varian E-115 spectrometer) with 100 kHz field at X-band frequency.
3. Results and discussion
3.1. Phase beha6ior A part of the phase diagram of the pseudoternary system AS/ octane/1-butanol/water at 30°C is shown in Fig. 1, in which an isotropic microemulsion (M phase) and a liquid-crystalline phase are only displayed. The M phase is bound by a solid line or dashed line (for pure water and 4 g 100 ml − 1 PEO in aqueous solution, respectively). As shown in Fig. 1, the addition of PEO extends that the region of M phase extends from the AS/butanol/octane side to the water/butanol/ octane side, so more water is solubilized. The
Fig. 2 shows the variation of the electrical conductivity K as a function of water content (Wo) for AS/1-butanol/ octane/water w/o microemulsion containing PEO of various concentrations in the dispersing phase. In all microemulsions, the electrical percolation is present. The electrical percolation threshold (values of Wo at the onset of increased conductivity) increases as PEO is added to the microemulsion. Information on interaction between droplets in w/o microemulsion can be obtained from electrical conductivity measurements. For example, the appearance of electrical percolation indicates the existence of a strong attraction between droplets [13–15]. Fig. 2 shows that the addition of PEO induces a decreased attraction between droplets, as the PEO concentration increases. The conductivity decreases with increasing PEO concentration at constant Wo values. This result is consistent with the results of Suarez. et al. [12], who suggested that the observed decrease of droplet size leads to decreased overlap of interface film of droplets during collision of two droplets and in turn to decreased interdroplet attraction, as Cp increases; hence K decreases. In contrast, Lianos et al. [11] proposed that droplets form larger aggregates and fewer in number than the original droplets with added PEO, which decreases the mobility of polymer-containing aggregates, so larger and fewer aggregates give decreased conductivity at constant Wo. In order to verify which conclusion above is valid for our w/o microemulsion, we measured EPR spectra to investigate the effect of polymer additive on microviscosity and micropolarity of droplets. We use an EPR spin labelling of AS reversed microemulsion with spin probes differing in chemical structure to provide the information on the local polarity and viscosity. Because 5-DNS la-
112
Z. Hou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 168 (2000) 109–113
belled with carboxyl groups can act as cosurfactant and anchor at the interface [16,17], and partially dissociated carboxyl group may enhance such a location, their nitroxide moieties are expected to probe the region within the polar groups of AS. C12-Tempo label, an uncharged probe, is more hydrophobic than 5-DNS. According to our data, the C12-Tempo probe is located at outer interface of the droplets or remains in the oil phase (octane and 1-butanol).
Fig. 3. Isotropic hyperfine splitting parameters AN and relational period tc as a function of water content Wo. Circles and squares correspond to presence and absence of PEO, respectively. Spin label: 5-DNS.
Fig. 4. Isotropic hyperfine splitting parameters AN and the relational period tc as a function of water content Wo. Circles and squares correspond to presence and absence of PEO, respectively. Spin label: C12-Tempo.
In the system investigated, the spectra of two spinning probes in microemulsions that are nearly isotropic are characterized through the rotational time given by [18]: tc = 6.5 ×10 − 10DHo[(ho/h − 1)1/2 + (ho/h + 1)1/2 − 2] (s) in which tc is the rotational correlation period, ho, h + 1 and h − 1 are the peak-to-peak heights of the central line, the low-field and high-field line, respectively, and DHo is the central line peak to peak width (Gauss). The tc values give the microviscosity and the hyperfine splitting parameter AN is a measure of the polarity of the spin label environment. Figs. 3 and 4 show that AN and tc vary with water (or 4 g 100 ml − 1 PEO in aqueous solution) content for 5-DNS and C12-Tempo, respectively (spectra not shown). For the 5-DNS label, in the absence of PEO AN increases; whereas tc passes a maximum value and then monotonically decreases upon continued addition of water, both approaching, but not reaching the corresponding values in bulk water (AN = 15.88 G, tc = 1.66× 10 − 10 s). All data indicate that the probe became incorporated in the water core of the micellar aggregate, and the local environment monitored with the probe became less viscous and more polar with increasing proportion of solubilized water. In the presence of PEO (Fig. 3), AN and tc display a behavior similar to the above results. At the same Wo, larger tc is found compared with the system not containing PEO, whereas AN increases slightly at the same Wo on addition of PEO. Hence the amplitude of motion of the probe decreases. From Table 1, at constant Wo values (Wo = 20.12) tc increases significantly, whereas AN increases slightly as a function of Cp PEO concentration in aqueous solution increases. Thus PEO molecules bind between the head groups, and decrease repulsion between the head groups, which induces closer packing of alkyl chains. In addition, the introduction of PEO in the water pool of the droplets may decrease the size of the droplet, which restricts motion of the probe molecules, and so microviscosity increases. The
Z. Hou et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 168 (2000) 109–113 Table 1 EPR Parameters of spin probe 5-DNS in w/o microemulsiona Cp (g 100 ml−1)
AN (G)
tc (×10−10 s)
0.0 1.0 2.5 5.0 7.5 10.0
14.97 15.00 15.00 15.02 15.04 15.04
3.35 3.72 4.02 4.05 4.04 4.03
a
All samples have Wo = 20.12.
local polarity felt by the probe increases due to a slight reorganization of the interface, which partly expels the probe into a more polar environment. For the C12-Tempo spinning label, tc and AN display similar behavior to that of the 5-DNS probe with increasing Wo (Fig. 4), but with increasing Wo, tc and AN change little, and both of them are so small that they are near those in octane –butanol (1:1, mass ratio; AN =15.6 G, tc =2.05 × 10 − 11 s) and differ much from those in water; AN = 17.3, tc = 5.22× 10 − 10 s). Therefore, the probe is located in the outer interface of the droplets or in the oil phase. At the same Wo, both AN and tc change little upon introduction of PEO. Regarding the location of C12-Tempo in droplets of reversed microemulsion, we suggest that the polymer is mainly solubilized in the water core, mostly between headgroups of surfactants, not penetrating into the hydropholibic chains close to the outer interface of droplets or remaining in oil phase. Such a process excludes the possibility that polymer can interpenetrate the interfacial layer to connect different droplets, which has been suggested by Meier [19]. According to the EPR spectra, the interfacial layer of droplets becomes tighter on addition of PEO. Hence interpenetration of the interfacial layer of two colliding droplets is decreased and thus decreases attraction between droplets; this is consistent with decreased electrical conductivity due to polymer addition as shown in Fig. 2.
4. Conclusion In the system investigated, the addition of PEO
113
has a significant influence on phase behavior. Solubilized water is increased in both w/o microemulsion and liquid-crystalline phases. Conductivity measurements indicate that added PEO increases the percolation threshold; and this effect becomes more pronounced as Cp increases, and electrical conductivity decreases at constant Wo with added PEO. EPR spectra indicate that the interface layer reorganizes slightly and becomes closer. All above results indicates that the interaction between droplets in w/o microemulsion decreases when PEO is added to a w/o microemulsion. We plan to confirm whether this phenomenon is also valid for non-ionic surfactant systems.
References [1] D. Schubel, G. Ilgenfritz, Langmuir 13 (1997) 4246. [2] D. Papoutsi, P. Lianos, W. Brown, Langmuir 9 (1993) 663. [3] L. Schlicht, J.H. Spilgies, F. Runge, S. Lipens, S Boye, D. Schubel, G. Ilgenfritz, Biophys. Chem. 58 (1998) 39. [4] D.O. Shah, Macro- and Microemulsions theory and applications, American Chemical Society, Washinton, DC, 1985, p. 150. [5] A.M. Bellocq, Prog. Coll. Polym. Sci. 105 (1997) 290. [6] A.M. Bellocq, Langmuir 14 (1998) 3730. [7] M-J. Suarez, H. Levy, J. Lang, J. Phys. Chem. 87 (1993) 9808. [8] M.L. Das, P.K. Bhattacharya, S.P. Moulik, Langmuir 7 (1991) 636. [9] C. Gonzalez-Blanco, L.J. Rodriguez, M.M. Velazquez, Langmuir 13 (1997) 1938. [10] D. Papoutsi, P. Lianos, W. Brown, Langmuir 10 (1994) 3402. [11] P. Lianos, S. Modes, G. Staikos, W. Brown, Langmuir 8 (1992) 1054. [12] M-J. Suarez, J. Lang, J. Phys. Chem. 99 (1995) 4626. [13] J. Lang, N. Lalem, R. Zana, Coll. Surf. 68 (1992) 199. [14] R.T. Hamilton, J.F. Billman, E.W. Kaler, Langmuir 6 (1990) 1696. [15] B. Antalek, A.J. Williams, J. Texter, Y. Feldman, N. Garti, Coll. Surf., 128 (1997). [16] G. Haering, P.L. Luisi, H. Hauser, J. Phys. Chem. 92 (1988) 3574. [17] N.S. Kommareddi, V.T. John, Y.Y. Waguespack, G.L. McPherson, J. Phys. Chem. 97 (1993) 5752. [18] H. Caldararu, A. Caragheorgheopol, M. Vasilescu, H. Dragutan, H. Lemmetyinen, J. Phys. Chem. 98 (1994) 5320. [19] W. Meier, Langmuir 12 (1996) 1188.