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Effect of Added Surfactant on Temperature-Induced Gelation of Emulsions Cristina Alava and Brian R. Saunders* Manchester Materials Science Centre, UMIST and the University of Manchester, Grosvenor Street, Manchester M1 7HS, U.K. Received December 16, 2003. In Final Form: February 3, 2004 This paper involves an investigation of the effect of added ionic surfactant on the temperature-induced gelation of oil-in-water (O/W) emulsions stabilized by a responsive copolymer. The oil phase used in this study is 1-bromohexadecane. The copolymer is poly(NIPAM-co-PEGMa) (NIPAM and PEGMa are N-isopropylacrylamide and poly(ethylene glycol) methacrylate, respectively). The lower critical solution temperature for the copolymer was 39.5 °C. The ionic surfactant used in this work was sodium dodecylbenzenesulfonate (NaDBS). The critical association concentration for NaDBS and poly(NIPAMco-PEGMa) was measured at 0.30 mM using fluorescence measurements (pyrene was the probe molecule). Gelation temperatures were measured for the O/W emulsions to establish the effect of added NaDBS and copolymer concentration (Cp) on the gelation temperature (Tgel). The strength of the gels was measured using dynamic oscillatory measurements. These measurements allowed the shear modulus of the gel at Tgel to be estimated as 100 Pa. A theoretical model based on transient network theory was developed that predicts the dependence of Tgel on Cp. The study revealed that NaDBS has two effects on the overall cross-link density of the emulsion gels: it contributes a source of cross-linking via micellar cross-links and also decreases the proportion of transient cross-links due to electrostatic repulsion.
Introduction Emulsions are dispersions of one immiscible liquid in another. They may be oil-in-water (O/W) or water-in-oil (W/O). Emulsions are widely used in a number of industries such as the cosmetics, cleaning, and food industries. They have been the subject of a number of reviews.1-3 It is usually the case that there is a significant density difference between the dispersed and continuous phases for emulsions. This leads to creaming for O/W emulsions (or sedimentation for W/O) which in turn accelerates droplet coalescence and phase separation. Ostwald ripening is also a problem for submicrometer droplets. An ideal emulsion for stability would have monodisperse submicrometer droplets. A monodisperse droplet size would reduce emulsion instability as a result of Laplace pressure differences (Ostwald ripening). Important progress toward this goal has been made by Umbanhowar et al.4 They have developed a technique based on the flow of a surfactant-containing phase past the end of a capillary through which dispersed phase is extruded. The droplet size range was restricted to 2-200 µm, which means that creaming would be a potential problem for emulsion stability. Creaming could be suppressed altogether if it were possible to reversibly stop droplet motion during emulsion storage. In our previous work we have shown that responsive copolymers can be used to produce O/W emulsions that reversibly gel upon heating.5,6 One of the criteria for such a method to be viable for improving emulsion stability of industrial dispersions is that temperature-induced gelation must (1) Pays, K.; Mabille, C.; Schmitt, V.; Leal-Calderon, F.; Bibette, J. J. Dispersion Sci. Technol. 2002, 23, 175. (2) Dickinson, E. Food Hydrocolloids 2002, 17, 25. (3) Aveyard, R.; Binks, B. P.; Clint, J. H. Adv. Colloids Interface Sci. 2003, 100, 503. (4) Umbanhowar, P. B.; Prasad, V.; Weitz, D. A. Langmuir 2000, 16, 347. (5) Koh, A.; Saunders, B. R. Chem. Commun. 2000, 24, 2461. (6) Koh, A.; Prestidge, C.; Ametov, I.; Saunders: B. R. Phys. Chem. Chem. Phys. 2002, 4, 96.
occur in the presence of added surfactant. In this paper the effect of surfactant on temperature-induced emulsion gelation is investigated. In our previous work poly(NIPAM-co-PEGMa) (NIPAM and PEGMa are N-isopropylacrylamide and poly(ethylene glycol) methacrylate) was used as a responsive copolymer.5,6 The copolymer contained 85 mol % NIPAM. The NIPAM portion conferred the temperature-responsiveness. Poly(NIPAM) exhibits a lower critical solution temperature7 in water at 32 °C. The copolymer adsorbs to the oil-water interface and is believed to contribute a steric barrier to attractive droplet-droplet interactions at temperatures less than the LCST. (Turbidity versus temperature measurements8 show that the LCST of poly(NIPAM-co-PEGMa) is ca. 37 °C in D2O.) When the temperature exceeds the LCST, the interdroplet interactions become strongly adhesive and bridging flocculation occurs.5,6 This results in reversible, temperature-induced emulsion gelation. It is important to note that the temperature-induced emulsion gelation occurred at polymer concentrations well below that necessary for the parent copolymer solutions to undergo temperatureinduced gelation. This implicates the droplet interface in the gelation process. Given that attractive droplet-droplet interactions are responsible for temperature-induced emulsion gelation, then one would expect that any process that prevents such interactions would impede gel formation. Addition of ionic surfactant is expected to oppose the close approach neighboring droplets. The present study will investigate this process. The temperature-induced gelation of O/W emulsions has been reported earlier by Dickinson et al.9 as well as Scherlund et al.10 Dickinson et al. studied the effect of temperature on the rheological properties of tetradecane(7) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352. (8) Koh, A.; Heenan, R. K.; Saunders, B. R. Phys. Chem. Chem. Phys. 2003, 5, 2417. (9) Dickinson, E.; Casanova, H. Food Hydrocolloids 1999, 13, 285. (10) Scherlund, M.; Malmsten, M.; Brodin, A. Int. J. Pharm. 1998, 173, 103.
10.1021/la036371l CCC: $27.50 © 2004 American Chemical Society Published on Web 03/16/2004
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in-water emulsions stabilized by sodium caseinate. It was reported that in order for temperature-induced gelation to occur, CaCl2 had to be present. Moreover, reversible gelation required a narrow window of calcium concentration to be maintained. The gelation temperature was pH dependent. Scherlund et al.10 reported temperatureinduced emulsion gelation for oil-in-water emulsions stablized by Lutrol, EO99PO65EO99. The emulsions required surfactant to be added in order to achieve good storage stability at room temperature. Moreover, the concentration of Lutrol was about 14 wt %. This concentration exceeded the gelation concentration for the parent copolymer solutions; i.e., it was sufficiently high that aqueous solutions of Lutrol exhibited temperature-induced gelation in the absence of droplets. In the present study, a much simpler system is examined. Here, we investigate the temperature-induced gelation of 1-bromohexadecanein-water emulsions that are stabilized by poly(NIPAMco-PEGMa). These three-component systems do not require additives to enhance stability or induce gelation. The copolymer concentration is much less than that of the critical gelation concentration for the parent copolymer solution. The simplicity of our systems makes it easier to study the effects of additives on gelation. In this paper the effect of added surfactant on temperature-induced gelation is studied. The formation of emulsion gels through temperature increase has been previously attributed by our group to temperature-induced transient network formation across the oil-water interface of neighboring droplets.6 Transient network formation is a well-established mechanism11 that has been successful in explaining gelation of hydrophobically modified polymer solutions. The poly(NIPAM) segments of the copolymer studied here, poly(NIPAMco-PEGMa), are believed to become strongly hydrophobic above the lower critical solution temperature (LCST). Below this temperature the polymer chains are weakly hydrophobic. This view is supported by neutron reflectivity data12 that show that poly(NIPAM) is adsorbed at the air-water interface at below the LCST and that the extent of adsorption increases considerably above the LCST. Poly(NIPAM-co-PEGMa) belongs to the new class of copolymers that have been termed thermoassociative polymers.13 The effect of added surfactant on polymers in solution has received considerable interest.14-18 It is well-known that addition of ionic surfactants increases the LCST of poly(NIPAM). This is a consequence of increased intersegment repulsion due to repulsive electrostatic interactions. It is also well-known that addition of ionic surfactant to polymer solutions increases the solution viscosity. A maximum as a function of added surfactant has been reported by Piculell et al.18 The viscosity maximum was attributed to two opposing effects of (11) Tanaka, F.; Edwards, S. F. Macromolecules 1992, 25, 1516. (12) Richardson, R. M.; Pelton, R.; Cosgrove, T.; Zhang, J. Macromolecules 2000, 33, 6269. (13) L’Alloret, F.; Maroy, P.; Hourdet, D.; Audebert, R. Rev. De L’Institut Franc. Du Petrole 1997, 52, 117. (14) Durand, A.; Hourdet, D. Polymer 2000, 41, 545. (15) Cosgrove, T.; Mears, S. J.; Obey, T. M.; Thompson, L.; Wesley, R. D. Colloids Surf., A 1999, 149, 329. (16) Ananthapadmanabhan, K. P. Surfactant Solutions: Adsorption and Aggregation Properties. In Interactions of surfactants with polymers and proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993; Chapter 2, p 5. (17) Goddard, E. D. Polymer-Surfactant Interaction. Part I. Uncharged Water-Soluble Polymers and Charged Surfactant. In Interactions of surfactants with polymers and proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993; Chapter 4, p 123. (18) Piculell, L.; Egermayer, M.; Sjostrom, J. Langmuir 2003, 19, 3643.
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increasing surfactant concentration: an increase in the lifetime of mixed micellar cross-links and a decrease in the number density of cross-links.18 This study involves addition of an anionic surfactant to a three-phase system consisting of oil droplets, copolymer, and water. An anionic surfactant is used because these types of surfactants normally cause the most pronounced changes to polymer conformation. There are several possibilities for the interaction of added surfactant within a gel forming a polymer-stabilized emulsion. The surfactant may (i) oppose temperature-induced gelation by increasing the repulsive interactions between droplets, (ii) increase the LCST of the responsive polymer through association, (iii) increase the strength of gelation by formation of micellar cross-links, (iv) displace polymer from the droplet interface, and (v) provide combinations of these interactions. The principal objective of this study is to determine the mechanism by which added surfactant affects temperature-induced emulsion gelation. Materials and Methods Materials. NIPAM (97%), PEGMa (Mn ) 360 g mol-1), 1-bromohexadecane (97%), azobis(isobutyronitrile) (98%), and sodium dodecylbenzenesulfonate (NaDBS) were purchased from Aldrich and used as received. Water was of Milli-Q quality. Copolymer Synthesis. The synthesis for poly(NIPAM-coPEGMa) has been described elsewhere.5,6 It is based on the method reported by Cardenas-Valera et al.19 for graft copolymers containing PEO macromonomers. Brief details are given in the following. PEGMa (1.7 g) and tert-butyl alcohol (t-BuOH, 38 mL) were added to a reaction flask. The solution was degassed using nitrogen and stirred. While a temperature of 80 °C was maintained, NIPAM (3.27 g) and AIBN (0.03 g) in t-BuOH (13 mL) were added at a constant rate over a period of 1 h. The polymerization was continued for a further 5 h. A white solid precipitant was recovered by precipitation using diethyl ether and then extensively dialyzed using Milli-Q water. Emulsion Preparation. The emulsions were prepared by mixing a known quantity of 1-bromohexadecane (BrHD) and poly(NIPAM-co-PEGMa) solution followed by high-speed shearing at 9400 rpm for 5 min using a Silverson L4R mixer. Typically, the volume fraction of BrHD was 0.30. The NaDBS was added to the as-made emulsions in the form of surfactant solutions followed by gentle mixing in the cases where the effect of added surfactant was investigated. Physical Measurements. Gel phase chromatography (GPC) data were supplied by RAPRA technologies (Shropshire, U.K.) using DMF as the eluent. The LCST measurements were made using visual observation of the temperature at which a 1 wt % solution became turbid. Fluorescence measurements were performed using a Fluorlog-3 FL3-22 instrument. The excitation wavelength was 330 nm. The copolymer concentration used for these experiments was 0.1 wt %. Pyrene was used as the fluorescent probe. The ratio of the intensities of the first and third emission peaks (I1/I3) was used to study micellization and association with the copolymer. The I1/I3 ratio is known to decrease when pyrene moves from a hydrophilic to hydrophobic environment.20,21 Droplet sizes were obtained using an Olympus AH2 optical microscope. Gelation temperature measurements were performed using a tube inversion method. Emulsions were placed in capillaries (internal diameter of 0.55 cm), and these were equilibrated in a temperature-controlled water bath. The temperature at which the fluid ceased flowing upon tube inversion was taken as the gelation temperature, Tgel. Rheology measurements were performed using a Rheometrics RMS-800. Parallel plates were used with a 50 mm diameter and a gap of 0.3 mm. A frequency range of 0.01-15.9 Hz was employed. The strain was 10%. (19) Cardenas-Valera, A. E.; Bailey, A. I.; Doroskowski, A. Colloids Surf., A 1995, 96, 53. (20) Liu, F.; Frere, Y.; Francois, J. Polym. 2001, 42, 2969. (21) Chee, C. K.; Rimmer, S.; Shaw, D. A.; Soutar, I.; Swanson, L. Macromolecules 2001, 34, 7544.
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Figure 1. Variation of the LCST for poly(NIPAM-co-PEGMa) with the ratio of added surfactant to polymer repeat units.
Results Copolymer Characterization. The reactivity ratios for NIPAM and PEGMa (rNIPAM and rPEGMa) were determined from compositions of the copolymers formed at low conversion from a series of copolymerizations performed using known comonomer compositions. The full details of the procedure and results will be published elsewhere. The analysis yielded rNIPAM ) 1.2 and rPEGMa ) 0.14. The low value for rPEGMa is due to steric constraints for the monomer reacting with other PEGMa active centers. Simulations (to appear elsewhere) indicate that NIPAM is readily incorporated into the copolymer chains at high mole fractions when the conversion is low. However, these simulations also suggest that about 10 wt % of the total polymer formed will be poly(NIPAM) homopolymer. NMR data (not shown) revealed an average mole fraction of NIPAM within the copolymer of FNIPAM ) 0.85. The GPC data showed a biomodal distribution with a number average molar mass (Mn) of 63 000 g mol-1 and a polydispersity of 5.5. Other preparation methods for poly(NIPAM-co-PEGMa) were tested including dual feed and batch copolymerization in water. The dual feed method gave very low polydispersities (ca. 1.5). However, for all the other methods tested the copolymer was not able to sabilize the emulsion at room temperature and give temperatureinduced emulsion gelation. Our tentative conclusion from this is that a range of copolymer chains with differing extents of NIPAM is beneficial for achieving temperatureinduced emulsion gelation. More work in this aspect is planned. The poly(NIPAM-co-PEGMa) copolymer with Mn ) 63 000 g mol-1 was used for the remainder of this study. The LCST of poly(NIPAM-co-PEGMa) in the absence of added surfactant was measured as 39.5 ( 0.5 °C. This value is about 7 °C higher than the LCST reported7 for poly(NIPAM) in H2O. This is attributed to the presence of the hydrophilic PEGMa units. Effect of Added Surfactant on the Lower Critical Solution Temperature. In this study the ratio of the number of molecules of surfactant added to the number of copolymer repeat units (Rs,pru) is used to quantify the proportion of surfactant present
Rs,pru ) Mpru
( ) [s] Cp
(1)
where Mpru is the molar mass of the copolymer repeat unit. The symbols [s] and Cp represent the concentrations of surfactant (mol m-3) and copolymer (kg m-3), respectively. Our 1H NMR data (not shown) indicate a value for Mpru of 0.15 kg mol-1. Figure 1 shows the variation of the
Figure 2. Variation of the ratio I1/I3 ratio with NaDBS concentration. The aqueous solutions contained only NaDBS ([) or poly(NIPAM-co-PEGMa) and NaDBS (]).
LCST (TLCST) as a function of Rs,pru. These data clearly indicate that TLCST is an increasing function of Rs,pru. This is attributed to an increased intersegment repulsion upon association of the surfactant with the copolymer chains. The data were fitted with the following second-order polynomial in order to allow TLCST (°C) to be calculated for a given value of Rs,pru
TLCST ) 489.8Rs,pru2 + 271.2Rs,pru + 39.3
(2)
The data shown in Figure 1 can be compared with those reported for related systems. Idziak et al.22 measured the effect of added sodium dodecyl sulfate on the LCST of poly(diethylacrylamide). From their data it can be shown that an 8 °C increase of the LCST occurs when Rs,pru (calculated for their system using eq 1) is 0.09. This compares to a 28 °C increase in the LCST for poly(NIPAMco-PEGMa). Although the trends with added surfactant are similar, the exact cause for the difference in the LCSTs cannot be determined with certainty because of the differences in the polymer and surfactants used for each study. The micellization of NaDBS and its association with poly(NIPAM-co-PEGMa) was investigated using fluorescence measurements. Pyrene was used as the probe molecule. The ratio of the intensities of the first and third fluorescent peaks for pyrene provides a measure of the effective polarity of the portion of micelle where pyrene is located. Figure 2 shows the variation of I1/I3 for pyrene with [s]. The figure shows data measured in the presence of NaDBS and also in the presence of poly(NIPAM-coPEGMa) and NaDBS. These data represent micellization and association measurements. The ratio of I1/I3 decreases from 1.65 when the pyrene becomes solubilized within the hydrophobic centers of NaDBS micelles. The point of maximum gradient for each curve is used to estimate the critical concentrations. From these data the cmc for NaDBS is 1.5 mM. The critical association concentration (cac) for NaDBS in the presence of poly(NIPAM-coPEGMa) is 0.3 mM. The critical micelle concentration (cmc) reported here is comparable to the value of 1.6 mM reported by Taylor.23 The cac is much lower than the cmc. However, this is usually observed for polymer-surfactant aggregates.17 From eq 1 and the concentration of copolymer used during the experiments (Cp), it can be shown that Rs,pru ∼ 0.05 at the cac. This low value indicates that more than one copolymer chain is involved in the polymersurfactant complexes. (22) Idziak, I.; Avoce, D.; Lessard, D.; Gravel, D.; Zhu, X. X. Macromolecules 1999, 32, 1280. (23) Taylor, P. Colloid Polym. Sci. 1996, 274, 1061.
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Figure 3. Effect of added surfactant concentration on the gelation temperature for emulsions stabilized by poly(NIPAMco-PEGMa). The copolymer concentrations used in the aqueous phase were 30 (∆), 35 ([), 40 (]), and 50 kg m-3 (9). For a given curve, the temperatures above and below the line correspond to the gel and fluid states, respectively.
Figure 5. Plots of G′(ω) vs oscillatory frequency (a) and tan δ vs oscillatory frequency (b) for an emulsion prepared using Cp ) 35 kg m-3. No surfactant was added. The data were recorded at 30 (]), 39.5 ([), and 47 °C (0). The curves are fits to the data using the Maxwell model. The values for the fitting parameters (G and τm) for each temperature were 30 °C (4.3 Pa and 7.5 s), 39.5 °C (21 Pa and 10.5 s), and 47 °C (136 Pa and 9.5 s). Figure 4. The variation of the gelation temperatures for emulsions prepared using Cp ) 30 (b) and 50 kg m-3 (9) and the LCSTs calculated for copolymer solutions with Cp ) 30 (O) and 50 kg m-3 (0). The values for TLCST were calculated using eqs 1 and 2.
Gelation Temperature vs Copolymer Concentration Diagrams. Figure 3 shows the effect of added surfactant on the gelation temperature for BrHD-in-water emulsions stabilized by poly(NIPAM-co-PEGMa). The data were obtained using different Cp values. The emulsion is a gel or fluid, respectively, above and below each curve. The droplet diameter for the emulsions decreased with increasing Cp used during preparation. In an investigation into the effect of droplet size on Tgel the droplet size was varied using shear rate control using Cp ) 35 kg m-3. The number length mean diameters varied from 10 to 150 µm. For the latter system the stirring rate was reduced to 600 rpm (from 9400 rpm for the 10 µm emulsion). The value for Tgel in each case was the same. These experiments showed that droplet size (in the range 10-150 µm) has no effect on Tgel for these emulsions. The droplets are required for gelation to be observed at these values of Cp, as demonstrated by our previous work.5,6 It follows that the differences in Tgel observed in Figure 3 depend primarily on the concentration of polymer added to the aqueous phase at fixed volume fraction of oil phase. In an effort to investigate the relationship between Tgel and the LCST, values for TLCST were calculated from eqs 1 and 2 using Cp ) 30 and 50 kg m-3. The data are shown in Figure 4 together with Tgel data for emulsions prepared using the same values for Cp. These data show clearly that the cause for the increase in Tgel for each emulsion upon increasing [s] is not an increase in the LCST. A more likely cause for this behavior is electrostatic repulsion
between neighboring droplets. This is discussed in more detail below. Rheological Studies. Dynamic rheological measurements were performed on the emulsion in the absence of added surfactant in the vicinity of Tgel. The data appear in Figure 5. The purpose of these measurements was to determine the shear modulus, G, in the vicinity of Tgel. For these samples, Tgel is 44 °C. The data show that G′(ω) is greater than G′′(ω) (i.e., tan δ < 1) at all temperatures. Furthermore, tan δ ()G′′(ω)/G′(ω)) appears to become independent of frequency when ω g 0.2 rad s-1. This indicates that a gel is present at each temperature but that it becomes much stronger above the LCST. It can be shown that the majority of the copolymer present in these emulsions at room temperature will be present in the solution if monolayer adsorption at the oil-water interface is assumed. It follows that the emulsion at less than the LCST is subject to depletion flocculation. A depletioninduced gel forms which would be consistent with the tan δ data shown in Figure 5b. The low values for G′ at 30 °C and visual observation of the ease of fluid flow show that the gel strength at T < TLCST is quite weak. In an effort to estimate G from the G′(ω) vs ω data, a Maxwell model of the following form was used to fit the data
G′(ω) ) G
(
(ωτm)2
)
1 + (ωτm)2
(3)
where τm is a characteristic decay time. Values of τm in the range of 7.5-10.5 s were determined from the fits to the data shown in Figure 5a. Although the fits for G′(ω) appear
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reasonable (Figure 5a), the calculated curves for tan δ do not represent good fits (Figure 5b). Presumably, a more complex model than the Maxwell model is appropriate. However, this is not required for the present analysis as the Maxwell model allows an adequate estimation of G for each temperature. This will be used below to calculate polymer functionality. Discussion Theoretical Model for Temperature-Induced Emulsion Gelation. What is the relationship expected between Tgel and Cp for temperature-induced gelation of the emulsion considered in this work? Green and Tobolsky24 considered the elasticity of a transient network formed by junctions that continually break and recombine due to Brownian motion. The high-frequency storage modulus, G∞, is given by the equation
G∞ ) νeffkT
(4)
where νeff is the number of elastically effective chains per unit volume. In the temperature responsive emulsions discussed here, a network is not considered to form until the temperature reaches a critical value, Tnet. This is the temperature of the onset of network formation via bridging flocculation of interfacial chains (and hence transient network formation). This is the reference temperature against which the relative temperature, T′ is measured. For a nonthermally associative, conventional transient network, the rate of junction breaking increases with temperature.11 However, for thermoassociative networks the rate of junction breakage would be expected to decrease rapidly above the LCST. To simplify the analysis, it is assumed that the rate of junction formation and disruption is invariant with temperature above Tnet. The following equations apply
T′ ) T - Tnet
(5)
G ) νeffkT′
(6)
According to eq 6 the shear modulus is zero at Tnet. The tube inversion measurements used in this study indicate gelation if G g Gcrit, where Gcrit is the minimum value for G at which the gel is able to support its weight upon tube inversion. There will be a temperature at which this is observed to occur, T′gel.
T′gel ) Gcrit/kνeff
(7)
The value for νeff will depend on the average functionality of the polymer chains, f, and the proportion of loops. Here, f represents the average number of intermolecular associations the polymer chain can make. By analogy with the functionality of monomers, f is assumed to take values of greater than or equal to unity. The polymer chains in this discussion reside at the O/W interface. (At T > TLCST the copolymer chains will aggregate and adsorb to the particle interface.) The local segment density is high, and the probability of loop formation is low. It follows that νeff is related to the number density of polymer chains (ν) by the following equation
νeff ) (f - 1)ν
Figure 6. Variation of the emulsion gelation temperature with the reciprocal of polymer concentration added to the continuous phase prior to emulsification. The concentrations of added surfactant were 0.16 mM (]), 0.32 mM (b), and 0.48 mM (0). Data obtained in the absence of added surfactant ([) are also shown.
to form, f g 2. It is assumed that νeff is independent of temperature for T′ > 0 and that the following equation applies
ν ) CpNA/M
where Cp is the polymer concentration (kg m-3) and M is the average molar mass (kg mol-1). Insertion of eqs 8 and 9 into eq 7 gives a useful equation.
T′gel )
GcritM
(24) Green, M. S.; Tobolsky, A. V. J. Phys. Chem. 1946, 14, 80.
( )
1 C (f - 1)R p
(10)
Equation 10 does not consider the effect of droplet size. As noted above, droplet size variation in the range of 10150 µm did not affect T′gel. Equation 10 does not consider the volume fraction of the emulsion, φo. In the present work this is not important because φo is fixed at 0.3. An implicit assumption in deriving eq 10 is that all of the polymer chains are part of the transient network. The key prediction from eq 10 is that Tgel ()Tnet + T′gel) is proportional to 1/Cp. Figure 6 shows the variation of Tgel with 1/Cp. The data were obtained from Figure 3 using selected values for [s]. The data clearly show linearity. This is support for the suitability of eq 10 as a model equation for describing emulsion gelation. The data from Figure 6 can be extrapolated to 1/Cp to yield Tnet. This allows calculation of T′gel ()Tgel - Tnet) for each value of Cp. Figure 7 shows a plot of T′gel against 1/Cp. These data more clearly show linearity. The gradient for each curve provides valuable information about the relative changes in the proportion of cross-linking present upon addition of surfactant. The data shown in Figure 7 allow the calculation of a variable Reff(s,0), which is defined as the ratio of the number density effective chains in the presence of added surfactant (νeff(s)) to that in the absence of added surfactant, νeff(0). (The subscripts s and 0 indicate the presence and absence, respectively, of surfactant.) The value for Reff(s,0) can be calculated from the gradients of the T′gel vs 1/Cp data using the following equation
(8)
It can be seen from eq 8 that there will be no elastically effective chains if f ) 1 (i.e., νeff ) 0). Thus, for a network
(9)
Reff(s,0) )
( (
dT′gel(0)
) )
d(1/Cp(0)) dT′gel(s)
d(1/Cp(s))
(11)
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Figure 7. Variation of T′gel with the reciprocal of polymer concentration. T′gel is the gelation temperature relative to the temperature at which the network forms, Tnet. The concentrations of added surfactant were 0.16 mM (]), 0.32 mM (b), and 0.48 mM (0). Data obtained in the absence of added surfactant ([) are also shown.
Figure 8. The effect of added surfactant concentration on (]) the network formation temperature, Tnet, and ([)Reff(s,0) (see text).
Equation 11 shows that Reff(s,0) can be determined from the ratio of the gradients from Figure 7. Figure 8 shows the variation of Tnet and Reff(s,0) with added NaDBS concentration. The data show that added surfactant leads to a decrease in both Tnet and Reff(s,0). What is the physical significance of Tnet and Reff(s,0) and the changes of these parameters observed? These will be considered in turn. The parameter Tnet is an extrapolated, intercept value (via eq 10), which corresponds to the temperature for Tgel obtained when Cp reaches the highest value possible, which would correspond to the polymer melt. For the systems without added surfactant it corresponds to TLCST. It is very interesting that Tnet is lower than TLCST (39.5 °C) when surfactant is present (Figure 8) and that the difference between these two values becomes increasingly pronounced as [s] increases. To determine whether the values for Tnet that were lower than TLCST were experimentally accessible, the gelation temperature for an emulsion prepared using Cp ) 100 kg m-3 and containing [s] ) 0.48 mM was measured. For comparison, the value for Tgel for the same emulsion without any added surfactant was also measured. The new data are shown in Figure 9. In both cases the values for Tgel were 42.5 °C. This value is expected for the emulsion without added surfactant from Figure 6. However, a Tgel of 38 °C was expected for the emulsion containing [s] ) 0.48 mM. The reason for this discrepancy is that the contribution from cross-linking due to temperatureinduced gelation must vanish at temperatures less than TLCST. However, the extrapolation method used to get Tnet
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Figure 9. Variation of Tgel with 1/Cp for emulsions containing no added surfactant (0) and 0.48 mM NaDBS (b). The data are taken from Figure 6 except that new points at Cp ) 100 kg m-3 were also measured (indicated with arrow). Table 1. Average Functionality of Poly(NIPAM-co-PEGMa) Chains in Gelled O/W Emulsions as a Function of Added NaDBS Concentration [s]/mM
Tnet/°C
{dT′gel/d(1/Cp)}/ (kg m-3)
f
0 0.16 0.32 0.48
40.2 37.8 35.1 29.8
140 275 415 755
6.4 3.8 2.8 2.0
(via eq 10) assumes that this contribution remains constant at all temperatures. Consequently, the values for Tnet shown in Figure 8 (and Table 1) are nominal values. What these values reveal is the additional cross-linking contribution that occurs from added surfactant in addition to the temperature-induced transient cross-linking component. This contribution of cross-linking from the surfactant is not dependent on Cp and must be ascribed to micellar cross-links. There is strong support in the literature for the presence of this general form of crosslinking.18 The effect of added surfactant on Reff(s,0) is also of interest. This term reflects cross-linking from attractive polymerpolymer interactions. Added surfactant clearly reduces the contribution of this type of cross-linking, which is a temperature-induced transient contribution to the total cross-linking present. The most logical way in which added ionic surfactant could reduce the cross-link density and hence νeff is through electrostatic repulsion. It is easy to test for such an occurrence. This was done by measuring Tgel for an emulsion prepared using Cp ) 40 kg m-3 containing [s] ) 0.72 mM in the presence of 0.1 M NaCl. The value for Tgel was considerably reduced from the value of 58 °C expected from Figure 3. Gelation was observed at 53 °C. This indicates a much higher f and Reff(s,0). Thus, added surfactant reduces the extent of temperatureinduced hydrophobic association. The most likely way in which this occurs is through increased electrostatic repulsion between neighboring droplets. An additional explanation is that added surfactant decreases the proportion of copolymer that adsorbs at the droplet surface at temperatures greater than TLCST. As a final check of the theory and its applicability values for f were calculated and compared to what is expected intuitively. It can be seen from eq 10 that f can be calculated provided Gcrit is known. This is simply the shear modulus at Tgel. The rheological data from Figure 5a and the Maxwell model ( eq 3) allow this to be estimated. The data provided values for G on either side of the Tgel. Using the values obtained and linear interpolation, a value of G )
Effect of Surfactant on Gelation
Langmuir, Vol. 20, No. 8, 2004 3113
100 Pa was estimated for Tgel ) 44 °C. The data shown in Table 1 give f values calculated using Gcrit ) 100 Pa and M ) 63 kg mol-1 (i.e., the number average molar mass). The high polydispersity for the copolymer makes it impossible to identify unique values for f. However, these values can be considered minimum f values. That these values are greater than or equal to the minimum value of 2.0 expected for network formation is pleasing. Further work is required using lower polydispersity copolymers in order to more rigorously test the theory for temperatureinduced gelation given above. This is planned. Conclusions This study has considered the effect of added NaDBS on the temperature-induced gelation of O/W emulsions. In the first part of the study the interaction between NaDBS and the copolymer, poly(NIPAM-co-PEGMa) was investigated. It was found that NaDBS associates with the copolymer in solution at a cac of equal to ca. 20% of the cmc. Added NaDBS is very effective in increasing the gelation temperature for the emulsions. However, this effect is not due to an increase of the LCST. A simple theory based on transient network theory was presented which explains the variation of the emulsion gelation temperature with copolymer concentration rather well. The gelation temperature can be predicted using a rearranged form of eq 10
Tgel ) Tnet +
GcritM
( )
1 (f - 1)R Cp
(12)
This equation provides experimentally accessible values for emulsions containing added surfactant provided the predicted Tgel is greater than or equal to the value that would be obtained for the emulsion in the absence of added
surfactant (at the same value of Cp). The theory provides some important insights into the effect of added surfactant on temperature-induced gelation of dispersions. The principal objective of this study was to determine the mechanism by which added surfactant affects temperature-induced emulsion gelation. Added ionic surfactant affects gelation in the following ways. (a) Added surfactant introduces micellar cross-links. This form of cross-linking is not dependent on the copolymer concentration. (b) Added surfactant reduces the effectiveness of interchain cross-links due to increased electrostatic repulsion. This contribution to cross-linking is dependent on the copolymer concentration. From these conclusions it can be suggested that addition of nonionic surfactants to responsive emulsions should be less effective at increasing the gelation temperature (cf. ionic surfactants). This is because the repulsion between particles and neighboring segments will not be as effective for nonionic surfactants. Thus, temperature-induced emulsion gelation should be more tolerant to higher concentrations of added nonionic surfactants. However, the contribution to cross-linking from micellar cross-links will still be effective. Another prediction is that under conditions where the interparticle separation cannot be altered and the temperature is just below the gelation temperature for a responsive dispersion, addition of surfactant will lead to a decrease in the gelation temperature resulting in gel formation at that temperature. Acknowledgment. Financial support for this project from the EPSRC (Fast Stream Grant) is gratefully acknowledged by B.R.S. We also thank Professor P. O’Brien (School of Chemistry, University of Manchester) for access to the fluorescence spectrometer. LA036371L
