J . Phys. Chem. 1993,97, 11332-1 1338
11332
Clouding and Diffusion of Nonionic Surfactants in Agarose Gels and Solutions M. H. G. M. Penders,’ S. Nilsson,? L. Piculell, and B. Lindman Physical Chemistry 1, Chemical Center, University of Lund, Box 124, S-221 00 Lund, Sweden Received: June 8, 1993; I n Final Form: August 9, 1993’
Light transmittance and N M R self-diffusion measurements were carried out on nonionic micellar systems of hexaethylene (C&) and octaethylene (C&) glycol mono-n-dodecyl ether in gels and solutions of agarose with and without NaSCN. The influence of agarose on the clouding behavior and the diffusion of C12Es and C12E6 micelles as well as the effect of Cl2E8 and C12E6 on the gelation behavior of agarose was studied. The light transmittance measurements show that on increasing the agarose or surfactant concentration, the cloudpoint of the surfactant on cooling is decreased, indicating an incompatibility between dissolved agarose chains and surfactant (segregative phase behavior) as also demonstrated directly, whereas the gelation temperature of agarose remains practically unchanged. The cloud-point of the surfactant on heating in agarose gels is much less affected, indicating a less repulsive interaction between agarose in the gel state and surfactant. This hysteresis in clouding of the surfactant becomes more apparent at higher concentrations. The observed selfdiffusion of Cl2E8 and C12E6 in agarose/ 1.O M NaSCN systems is faster in the gel (more open structure) than in the solution state. The self-diffusion of surfactant in the presence of agarose is lowered compared to the polymer-free case due to the obstruction effect. The presence of NaSCN (1 .O M) does not affect the selfdiffusion of C12Es but causes an increase of the self-diffusion coefficient of C&6, compared to the salt-free case. In salt-free solutions of C12E6 the micelles display an increase in size at increasing temperatures, whereas on addition of NaSCN (1 .O M) no significant micellar growth is observed.
I. Introduction Interactions between water-soluble polymers and surfactants have been under study for more than two decades, and several reviews have been Most of these studies deal with aqueous nonionic polymer + ionic surfactant systems (e.g., refs 6-19) or with charged polymer oppositely charged surfactant mixtures in water, see refs 2b, 4,2&26. Of all polymer/surfactant systems the interaction between the nonionic polymer poly(ethylene oxide) PEO and the anionic surfactant sodium dodecyl sulfate SDS is most thoroughly st~died.~-l3,2~-2~ In this system, like in many other polymer/surfactant systems, cooperative binding of SDS micelles to the PEO polymer has been observed. The structure of this PEO/SDS complex has been described by Cabane et al. in terms of a “pearl-necklace” s t r ~ c t u r e . ~ - l ~ Recently there has been a growing interest in studying polymersurfactant interactions in aqueous nonionic polymer + nonionic surfactant ~ystems.3”3~Nonionic surfactants are often found to be indifferent to nonionic polymers.”,3 If the hydrophobicity of the polymer is sufficiently large, however, surfactant binding to the polymer may take place because of hydrophobic interactions. Interactions between nonionic polymers and nonionic surfactants have been observed by Brackman et al.30 for the poly(propy1ene oxide) (PPO)/n-octyl thioglucoside (OTG) system, Winnik31for (hydroxypropyl) cellulose (HPC)/n-octyl-8-D-glucopyranoside (OG) and HPC/OTG systems. Wormuth33 for systems of PEO and ethoxylated alcohols, and Zhang et al.34for systems of ethyl(hydroxyethy1)cellulose (EHEC) and UCON-50HB660 (UCON), a linear random copolymer of ethylene and propylene oxide, in combination with nonionic surfactants, tetraethylene glycol monon-dodecyl ether (C12E4) and octaethylene glycol mono-n-dodecyl ether (CIZEB). In the latter case an associative type of phase separation was found for both EHEC and UCON in combination with Cl2E8 and C12E4. This means that one phase is concentrated in both polymer and surfactant and the other one is solvent-rich. The same type of phase behavior is usually observed in systems
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t Present address: Rogaland Research, Prof. Olav Hanssensvag 15, Box 2503 Ullandhaug, 4004 Stavanger, Norway. *Abstract published in Advance ACS Abstracts, October 1, 1993.
0022-3654/93/2091-11332$04.00/0
of oppositely charged polymer/surfactant systems in water22-26 because of electrostatic interactions. For dextran/pentaethylene glycol mono-n-dodecylether (C12E5)and dextran/C~zEssystems, however, a segregative type of phase separation was found.35 In this case one of the two phases is surfactant-rich and the other one polymer-rich. Studies of polymer-surfactant interactions and diffusion of micellar systems in physical (polymer) gels, however, are rather scarce,36especially in the case of nonionic polymers and nonionic surfactants. They are of great importance, though, in many industrial applications and are potentially interesting, e.g., in pharmaceutical formulations. In the present paper we report the study of nonionic gelling polymer/nonionic clouding surfactant mixtures: agarose/C12E8 and agarose/hexaethylene glycol mono-n-dodecyl ether (C12E6) systems with and without added sodium thiocyanate (NaSCN). Agarose, an uncharged polysaccharide, in water forms a gel composed of a network of aggregated double helices, at temperatures below 43 OC on cooling. On heating, however, the gel melts at about 80 OC presumably because of stabilization of the helices through aggregation.3’-39 The large thermal hysteresis in agarose facilitates comparison of mixtures in solution with mixtures in the gel at the same temperature. Due to the combination of a gelling polymer and clouding surfactants, it is possible to probe both the influence of agarose on the clouding behavior of C12E8 and C&6 and the effect of C12E8 and Cl& on the gelation behavior (gel setting/melting point) of agarose. These phenomena were studied at different temperatures with light transmittance. With FT-pulsed field gradient spin-echo (PGSE) 1H-NMR the transport properties (self-diffusion) of C&8 and C&6 in agarose gels and solutions vs temperature, with and without NaSCN were measured, which also provides information about the polymer network. The transmittance and diffusion results vs temperature for the agarose/nonionic surfactant systems are compared with those of K-carrageenan/nonionic surfactant mixtures. The diffusion of C12EB and C12E6in K-carrageenangels and solutions has recently been studied in more detail by Johansson et al.36 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 43, 1993 11333
Clouding and Diffusion of Surfactants in Agarose 11. Experimental Section
11.1. Materials. Agarose (type VIII, for isoelectric focusing, No. A-4905) and K+ K-carrageenan (from Eucheuma cottonii, type 111)wereobtained from Sigma (St Louis, Missouri). Agarose was used without further purification. The K-carrageenan was purified as described e1sewhere.N Inorganic salts (KC1 and NaSCN) were of analytical grade. The agarose and K-carrageenan solutions were prepared by dissolving the polysaccharides in the appropriate solvent (water or salt solution) in sealed glass tubes, which were heated in boiling water with occasional shaking. Hexaethyleneglycol monodocecyl ether (C12Es)and octaethylene glycol monodocecyl ether (C12E8) of high quality were purchased from Nikko Chemicals, Tokyo, Japan, and were used without further purification. For the preparation of the samples, Millipore water was used in the case of the light transmittance measurements and D20 (99.8%purity, supplied by Merckor Dr. Glaser AG Basel) in the case of the NMR self-diffusion studies. All solutions were prepared by weight. 11.2. Methods. Light transmittance measurements were carried out with a 5 cm path length cell in a Hitachi PerkinElmer (Model 124) double beam spectrophotometer. The temperature was controlled by the circulation of thermostatically regulated water through the jacketed cell. The transmittance (in %T) of the samples was recorded vs temperature on cooling and heating with a rate of 0.3 OC/min. From the transmittance vs temperature curves the cloud-points of the surfactant on heating and cooling as well as the gel and melting temperatures of agarose were determined. The onset of a sharp decrease in transmittance due to the clouding of the surfactant was taken as the cloud-point. The change in transmittance related to the gelation or "melting" process of agarose was usually more gradual. The gelation temperatures, T,, were determined from the intersection of straight lines extrapolated from the higher temperature part ("constant" high transmittance) and the lower temperature part (large decrease in transmittance). A similar extrapolation procedure was used for the determination of the melting points, T,, of agarose. 1H NMR self-diffusion measurements were performed on a JEOL FX-60 spectrometer, operating at 60 MHz, using the FTPGSE technique, as described in more detail by Stilbs.41 With this technique one uses a 90'-7-1 80°-7-echo pulse sequence, with two added rectangular magnetic field gradient pulses of magnitude G,separation time A and duration time 6. The echo amplitude at time 27 is given by42 A(27) = A(0) exp[-27/T2 - y2G2Da2(A- 6 / 3 ) ]
(1)
where T2 is the transverse relaxation time and y the magnetogyric ratio for the proton. The self-diffusion coefficients D were determined by measuring the echo amplitude A as a function of 6 keeping G and A fixed. For all the experiments A = 140 ms and G = 16.7 mT/m or 40.0 mT/m, depending on the size of the diffusion coefficient. The temperature control during the experiments was within 0.5 OC. 111. Results and Discussion
III. 1. Light Transmittance. ZZZ.1 .I. AqueousAgarose/C& Mixtures. Transmittance vs temperature curves were measured for aqueous agarose/Cl& mixtures. The results are presented in Figures 1 and 2. Figure la represents the curve for an aqueous agarose (1 wt %) sample in the absence of On cooling, the transmittance (about 40%) of the agarose solution stays practically constant over a large temperature region from 85 to 45 OC (see Figure la). Lowering the temperature further results in a rapid decrease in transmittance (or increase in turbidity) due to the association of agarose helices (gelation process). The gelation temperature for agarose was found to be 41.0 OC. This
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Figure 1. Transmittance (in 46T) vs temperature at a wavelength of 600 nm for (a) agarose (1 wt 46) in water. (b) C ~ ~ (1 E wt B 96) in water. (c) agarose (1 wt 96) + CllE8 (1 wt 46) inwater. Arrows indicate thedircction of temperature change. T, represents the gel-point of agarose. T, the melting point of agarose, TOcthe cloud-point of the surfactant on cooling, Td the cloud-point of the surfactant on heating, and Td the cloud-point of C12E8 in absence of agarose.
value is in accordance with what has been found before39 and remains practically unchanged on increasing or decreasing the rate of cooling. The gelation temperature (41.0 "C) lies close to the onset temperature of helix formation of agarose (43-44 "C) on cooling as can be seen from optical rotation measurements using polarimetry.39 The decrease in transmittance occurs over a broad temperature range (41-30 "C) and is rather gradual. On heating, the transmittance (ca.9%) of the sample stays constant between 30 and 70 O C , reflecting the well-known hysteresisin the agarose gel sol transition. On further heating, the transmittance of the sample increases. Above T, = 76.8 OC a large increase in transmittance can be seen. This phenomenon accompaniesthe 'melting" of the agarose gel network. Therefore T , can be considered as the gel melting temperature. T, remains practically unchanged on increasing or decreasing the rate of heating. The gelation of agarose is "thermoreversible" in the sense that the sol gel sol transition cycle (see Figure 1a) can be repeated many times by successive heating and cooling. The curve presented in Figure l a is in good agreement with turbidity measurements published before.39 In Figure l b the transmittance vs temperature curve for an aqueous 1 wt 7% C12E8 solution in the absence of agarose is
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11334 The Journal of Physical Chemistry, Vol. 97, No. 43, 1993
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- Cagarose(Wt "/.I Figure 2. Phase behavior of aqueous agarosc/ClzE8 samples. (a) Dependence of surfactant concentration: the agaroseconcentrationequals 1 wt 96. (b) Dependence of agarose concentration: the surfactant concentrationequals 1 wt 96. For an explanation of the legends see Figure 1.
presented. From this figure it follows that the surfactant C12E8 has a clouding temperature of 78.9 "C on heating, which is in accordance with the results given by Mitchell et al.43 The phase transition is sharp and shows no hysteresis, in contradistinction to the sol gel and gel sol transition in agarose (Figure la). Figure ICcontains the transmittance vs temperature curve for an aqueous 1 wt % agarose/l wt % Cl2E8 mixture. Comparing Figure ICwith Figure 1, a and b, the following observations can be made: The gel point (T,)of agarose remains practically unchanged in the presence of C1&. The gel melting point (T,) of the agarose/C1& sample is decreased compared to the surfactantfree case. The latter finding is rather surprising. The aggregation of agarose is believed to yield structures on many length scales." Evidently, Cl& destabilizes at least such structures that contribute to the scattering of visible light. At present, the mechanism behind this effect is unclear to us. The cloud-point of the surfactant on cooling ( TW)decreases in the presence of agarose coils from 78.9 to 69.7 "C due to an "incompatibility" effect (repulsive coil-micelle interactions). A similar result has been found before by Sjbberg et a1.,41-48who observed a depressionof the cloud-point for poly(ethy1ene glycol) solutions on the addition of low-weight saccharides or dextran. Their interpretation was that the effective interaction. between the saccharide and poly(ethy1ene glycol) is more repulsive ("incompatible") than the water-saccharide interaction. The consequence is that such a system separates into two phases of which one is poly(ethy1ene glycol)-rich and the other one saccharide-rich (segregative phase behavior). The segregation phenomenon has often been observed in ternary polymer 1/
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Penders et al. polymer2/water sy~tems~5.~5-5~ and more recently in polymer/ surfactant/water systems.33.35 In the case of the agarose/ClzEs/ H 2 0 system a surfactant-rich upper phase and a polymer-rich lower phase were indeed observed. In Figure ICa hysteresis in clouding of the surfactant can be inferred. The cloud-point of the surfactant on heating Tch (73.8 "C) is situated at a higher temperature than Too(69.7 "C) and approaches the cloud-point of the surfactant in the absence of agarose (78.9OC). Evidently, the interaction between the agarose coil and C12Es micelles is more repulsive than the gel-micelles interaction, since in the latter case there is not much agarose in solution. The discrepancybetween the cloud-pointof C12E8in the absence of agarose and Tch may be interpreted by considering the fact that above T, = 67.7 "C the gel starts to "melt" partially and more agarose molecules in the sample will be present in the coil state. If there are enough coils present, the sample becomes cloudy again: this takes place at Tch = 73.8 OC. The hysteresis effect is only observed if during the measurement on cooling an aggregated network of agarose molecules is formed (gelation). If, however, during the whole measurementon cooling and heating the temperature stays above T, and the agarose remains in the coil state, Tch becomes equal to Tw. The observed effects can be influenced by changing the agarose and/or Cl&, concentration (see Figure 2). By increasing the Cl& concentration and keeping the agarose concentration at 1 wt % (Figure 2a), the cloud-point of the surfactant on cooling (TW)is lowered due to the incompatibility of the agarose coil/ Cl& couple. (This is in contradistinction to the behavior in the binary C12E8/H20 system, where between 0.25 and 10 wt % of surfactant concentration there is a slight increase (1-2 "C) of the cloud-point of C12E8.43.51)The cloud-point of the surfactant on heating stays practically unchanged. The observed decrease in cloud-point of the surfactant on cooling in the presence of agarose coils may be interpreted in terms of energetic and/or entropic contributions. In the former case, short-range pair interactions between sugar units of agarose, surfactant headgroups, and water molecules (the rparameters in the parlance of the Flory-Huggins theory) play an important role. In the latter case the observed effect of agarose on the surfactant clouding can be explained in terms of the phenomenon commonly referred to as "depletion flocculation" in the context of colloid stability. Even in the absence of effectively repulsive short-range interactions, the polymer segment density decreases near the surface of an interpenetrable particle (in this case, the micelle), owing to the loss in configurational entropy experienced by a polymer close to a surface. This depletion gives rise to a net attraction between the micellar particles. The gel temperature Tg does not depend on the surfactant concentration. This indicates that no adsorption of the surfactant to the polymer takes place, which supports the segregation type of phase behavior in the agarose/C1& system. For an effective repulsion between agarose coils and surfactant one might expect an increase in Tg,as the unfavorable agarosesurfactant contacts become fewer on helix formation of agarose. Previousexperiments on low molecular additives have shown, however, that the typical magnitude of this effect is of the order of 1-2 "C/M added cosolute.52 As the concentration of surfactant EO groups (the groups significantly exposed to contact with agarose) was on the order of 0.7 M or less, the small effect of the surfactant on T8 is thus expected. By increasing the agarose concentration and keeping the surfactant concentration at 1 wt % (Figure 2b), the cloud-point of C12E8 on cooling is lowered very drastically, caused again by the incompatibility of the agarose/C12Es couple. Tch stays practically unchanged, indicating that the effect of aggregated agarose on the clouding is quite small. The gel temperature of the agarose/CI& mixtures does not depend muchon theagarose
Clouding and Diffusion of Surfactants in Agarose
The Journal of Physical Chemistry, Vol. 97,No.43, 1993 11335
TABLE I: Influence of NaSCN (1.0 M)on the Celation of Agarose (1 wt %) and on the Clouding of C& (1 wt %). The Symbols Are Explained in the Text system TJ0C TJ0C T&IoC agarose in water 41.0 76.8 agarose in 1.0 M NaSCN 20.5 65.3 C& in water 51.8 C12E6 in 1.0 M NaSCN 69.9 concentration. This phenomenon has been found before for aqueous agarose samples in the absence of s~rfactant.~8.53 The aqueous agarose/C12E8system can be compared with the dextran/clzE8 system,'s which displays the same type of phase behavior as the analogous dextran/PEO mixture.4548 The interactions between polysaccharide and C12E8 are repulsive in both systems, which results in segregative phase behavior. In both systems the tendency toward segregation becomes stronger at increasing polymer and/or surfactant concentration. On addition of dextran (1 wt %) to the surfactant (1 wt %) the cloudpoint of C12E8 is decreased from 78.9 to 75.0 "C and no hysteresis in clouding is observed. The cloud-point depression is thus less than that in the case of agarose. III.l.2. Agarose/C12& Mixtures in 1.0 M NaSCN. C&6 (1 wt %) in water has a clouding temperature of 51.8 "C (see Table I). In the presence of agarose (1 wt %) the cloud-pointon cooling is lowered (
