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PolyampholyteSurfactant Film Tuning in Reverse Microemulsions Mabya Fechner and Joachim Koetz* Institut f€ur Chemie, Universit€at Potsdam, Karl-Liebknecht-Strasse 24-25, Haus 25, 14476 Potsdam (Golm), Germany
bS Supporting Information ABSTRACT: The pH-dependent influence of two different strongly alternating copolymers [poly(N,N0 -diallyl-N,N0 -dimethylammonium-alt-N-phenylmaleamic carboxylate) (PalPh) and poly(N,N0 -diallyl-N,N0 -dimethylammonium-alt-3,5-bis(carboxyphenyl) maleamic carboxylate) (PalPhBisCarb)] based on N,N0 -diallyl-N, N0 -dimethylammonium chloride and maleamic acid derivatives on the phase behavior of a water-in-oil (w/o) microemulsion system made from toluenepentanol (1:1) and sodium dodecyl sulfate was investigated. It was shown that the optically clear phase range can be extended after incorporation of these copolymers, leading to an increased water solubilization capacity. Additionally, the required amount of surfactant to establish a clear w/o microemulsion depends on the pH value, which means the hydrophobicity of the copolymers. Conductivity measurements show that dropletdroplet interactions in the w/o microemulsion are decreased at acidic but increased at alkaline pH in the presence of the copolymers. From differenctial scanning calorimetry measurements one can further conclude that these results are in agreement with a change of the position of the copolymer in the interfacial region of the surfactant film. The more hydrophobic PalPh can be directly incorporated into the surfactant film, whereas the phenyl groups of PalPhBisCarb flip into the water core by increasing the pH value.
1. INTRODUCTION Microemulsions—a fascinating type of self-organized soft matter—consisting of two immiscible components, namely, oil and water, are stabilized by a third component, the surfactant, and often also by a cosurfactant.16 The key role of the surfactant is the formation and stabilization of the interface between the continuous and dispersed phases. One of the most interesting features of microemulsions is the spontaneous formation and thermodynamic stability. Furthermore, microemulsions reveal a high structure diversity, including the formation of different structures reaching from spherical over elongated droplets to bicontinuous structures. In this context one can differentiate between different types of microemulsions, i.e., (i) water-in-oil (w/o), (ii) oil-in-water (o/w), and (iii) bicontinuous microemulsions. The size of the dispersed droplets of the former two types covers a range from 2 to 20 nm. Another special feature of microemulsions is their high solubilization capacity for organic and inorganic compounds, which is why they are commonly used as templates for the formation of conducting,79 semiconducting,10,11 and magnetic12,13 nanoparticles. Since the size and the shape of the produced particles depend on the size and the composition of the microemulsion droplets, ionic and nonionic polymers can be added to tune the properties of the microemulsion. In this context it was shown that polymers can influence the droplet size,1416 the dropletdroplet interactions,1719 and the stability20 and rigidity2123 of the surfactant film in w/o microemulsion droplets due to polymersurfactant interactions. Furthermore, the adsorbed ionic and nonionic polymers can stabilize the synthesized nanoparticles during the solvent r 2011 American Chemical Society
evaporation and redispersion process.24 Recently, it has been shown that the crystal growth process can be hindered and simultaneously influenced by using oligosaccharide-modified poly(ethyleneimine) (PEI),25 ending up with spherical coreshell particles that reveal a narrow size distribution. Investigations using polyampholytes as stabilizing agents have shown that the position of the polymer strongly depends on the pH value and therefore on the conformation of the polyampholyte.26 By adding poly(N,N0 -diallyl-N, N0 -dimethylammonium-alt-maleamic carboxylate) to a w/o microemulsion system at pH 4 and 9, different chain conformations, i.e., an extended structure below the isoelectric point (pH , IEP) and a coiled structure above the isoelectric point (pH . IEP), can be observed since this polyampholyte behaves like a polycation at low pH values and a zwitterion at high pH values. Thus, the interactions with an anionic surfactant, e.g., sodium dodecyl sulfate (SDS), can be directly tuned by changing the pH value of the aqueous polymer solution. Polymersurfactant interactions which play a key-role in the area of the L2-phase range strongly depend on the experimental conditions, such as the ionic strength, temperature, and pH value. To identify those interactions, different methods can be used, e.g., conductometry2729 micro differential scanning calorimetry (micro-DSC),27 pulsed-field-gradient self-diffusion nuclear magnetic resonance (PFG-NMR) measurements,26 rheology,27,30,31 dynamic light scattering (DLS),27,32,33 electromotive Received: March 1, 2011 Revised: March 17, 2011 Published: April 04, 2011 5316
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Scheme 1. Polyampholytes PalPh and PalPhBisCarb below and above the IEP
Figure 1. Partial phase diagram of the L2-phase of the quasi-ternary system toluenepentanol (1:1)/SDS/water in the absence (- 3 -) and presence of PalPh at pH 5 (solid line) and pH 8.6 (dashed line) and 25 or 40 °C, including the imaginary dilution line at a surfactant:oil ratio of ω = 22.3:77.7 and points AC.
force (EMF) measurements,27,3438 isothermal titration calorimetry (ITC),27,3942 surface tension measurements,27,4346 and fluorescence spectroscopy.27,4751 Nevertheless, systematic investigations on the role of pH in the hydrophobicity of the monomer units in polyampholytes are rather scarce. In ref 26 we have shown that the position of the polyampholyte inside the water droplets can be tuned by changing the pH. For that reason we synthesized two different polyampholytes with varying hydrophobicity, namely, poly(N,N0 -diallyl-N,N0 -dimethylammonium-alt-N-phenylmaleamic carboxylate) (PalPh) and poly(N,N0 -diallyl-N,N0 -dimethylammonium-alt-3,5-bis(carboxyphenyl)maleamic carboxylate) (PalPhBisCarb). By varying the surrounding pH, the phase behavior and the surfactant film rigidity of the pseudoternary w/o microemulsion system consisting of toluenepentanol (1:1)/SDS/10 wt % copolymer solution (PalPh, PalPhBisCarb) dependent on the pH value was investigated. One interesting feature of PalPh is that it is capable of inducing hydrophobic interactions between the phenylic side chain and the continuous phase, i.e., toluene. In contrast to PalPh, where the pH value has no great impact, the pH and thus the hydrophobicity of the copolymer will play a key role in the case of PalPhBisCarb containing three different carboxylic groups, two of them attached to the phenylic side chain. The concept of the present work was to tune the interfacial film due to surfactantpolyampholyte interactions by changing the pH value and to investigate changes in the microstructure of the polyampholyte-modified w/o microemulsions using microDSC and conductivity measurements. These investigations were done to improve the properties of a w/o microemulsion used as a template phase for the nanoparticle formation.9
2. EXPERIMENTAL SECTION 2.1. Materials. Toluene (>99%), pentanol (>99%) and SDS (>99%) were purchased from Fluka and used without further purification. Hydrochloric acid (37%, Merck) and sodium hydroxide pellets (>99%, VWR) were used as received. Water was purified with the Modulab PureOne water purification system (Continental).
2.2. Polymer Synthesis. The alternating maleamic acid copolymers PalPh and PalPhBisCarb were synthesized by a free radical polymerization of N,N0 -diallyl-N,N0 -dimethylammonium hydroxide (DADMAOH) and maleamic acid derivatives (MADs) according to refs 52 and 53. The structures of the synthesized polyampholytes below and above the IEP are shown in Scheme 1. The molecular weight of the investigated copolymers PalPh and PalPhBisCarb was determined by measuring the intrinsic viscosity and using the Kuhn-MarkHouwinkSakurada equation according to ref 54. 2.3. Phase Diagram. The isotropic phase ranges of the ternary systems were determined dependent on the temperature (25 or 40 °C) and the pH (pH < IEP, pH > IEP) by adding HCl or NaOH, respectively. For this purpose a pseudobinary oilcosurfactant/surfactant mixture was titrated with water or the corresponding 10 wt % aqueous polyampholyte solution (PalPh, PalPhBisCarb). After each addition the system was thermostated to establish equilibrium conditions, shaken, and/or treated by ultrasound for optical inspection of the isotropic phase range. 2.4. Conductivity Measurements. Percolation titrations were done using a microprocessor conductometer (LF 2000, WTW) equipped with a WTW electrode. The titration was carried out under thermostated conditions (25 °C) by adding water or the corresponding 10 wt % aqueous polymer solution to an oilcosurfactant/surfactant mixture (22.3:77.7) along a dilution line marked in Figure 1. 2.5. Micro-DSC. Calorimetric measurements were carried out in a temperature range between 20 and þ80 °C using a Setaram μ-DSC III device. The sample was subjected to two cooling and heating cycles at a fixed cooling and heating rate of 0.3 °C/min and kept frozen at 20 °C for a period of 5 h before the heating program was started again. All measurements were repeated a minimum of two times.
3. RESULTS AND DISCUSSION 3.1. Phase Behavior. 3.1.1. PalPh-Modified w/o Microemulsion. As seen in Figure 1 the copolymer PalPh can be successfully
incorporated into a w/o microemulsion system consisting of toluenepentanol (1:1)/SDS/water. The optically clear phase region, which can be attributed to the L2-phase, is extended toward the water-rich corner after the replacement of water by a 10 wt % PalPh solution at pH 5 and 8.6, whereas within this pH range no significant influence on the phase behavior can be observed. Furthermore, the addition of PalPh to the w/o microemulsion allows a diminution of the acquired amount of SDS (extension of 5317
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Figure 2. Partial phase diagram of the L2-phase of the quasi-ternary system toluenepentanol (1:1)/SDS/water in the absence (- 3 -) and presence of PalPhBisCarb at pH 4 (solid line) and pH 8.4 (dashed line) and 25 or 40 °C.
the phase range in the direction of the water axis). By increasing the temperature, a further widening of the phase region toward the water-rich corner can be detected that can be generally explained in terms of an increasing water solubilization capacity of the inverse swollen micelles combined with a lowering of the spontaneous curvature of the surfactant film. 3.1.2. PalPhBisCarb-Modified w/o Microemulsion. The results of the phase behavior obtained for the PalPhBisCarbmodified w/o microemulsion system at pH 4 and 8.4 are shown in Figure 2 (the phase ranges at 40 °C are also included). A strong increase in water solubilization capacity can be observed after incorporation of the polyampholyte PalPhBisCarb (10 wt %) at both investigated pH values, which is additionally more or less independent of the temperature. Moreover, the phase range is shifted toward the surfactant-rich corner for both pH values and temperatures. 3.1.3. Effect of the Type of Copolymer. By comparing the phase behavior of the two investigated polyampholytes PalPh and PalPhBisCarb, one can observe a higher water solubilization capacity for the PalPhBisCarb-modified system. Furthermore, there is a strong influence of the copolymer structure and charge state on the required amount of surfactant to build up a stable w/o microemulsion. At pH < IEP (where both copolymers act like a polycation) an extension of the phase range in the direction of the water axis can be observed for the PalPhmodifed system. This effect can be understood in terms of an incorporation of the polyampholyte into the surfactant layer due to noncovalent hydrophobic interactions. Since the phenylic side chains of PalPh are chemically similar to the continuous phase, i.e., toluene, they will orient in the direction toward the oil phase. Due to intermolecular hydrophobic interactions with SDS, pentanol, or toluene in addition to electrostatic interaction between the anionic head groups of the surfactant and the positively charged quaternary N-functions of the copolymer, one can expect an increased rigidity of the surfactant film, which is responsible for the higher water solubilization capacity and thus the widening of the L2-phase compared to those of the unmodified system. In the case of PalPhBisCarb, where two carboxylic groups are attached to the phenylic side chain, hydrophobic interactions will be of less importance, and the polymer is forced to be oriented more toward the inner core of the water droplets. The excess amount of free surfactant required to form the w/o microemulsion
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droplets could be a result of additional ioninduced dipole interactions that can be established between the anionic surfactant head groups and the COOH groups attached to the phenylic side chain of the copolymer. However, at pH > IEP a different picture is obtained, where PalPh is acting as a polyampholyte and PalPhBisCarb as a polyanion. In terms of PalPh, where the repulsive interactions between the deprotonated and thus negatively charged COOH groups and the anionic surfactant seem to be overcompensated by hydrophobic interactions, the copolymer is supposed to be localized at the o/w interphase with an extended conformation in analogy to that at pH < IEP. However, since electrostatic interactions are weaker compared to those at pH < IEP, which is a result of intra- and intermolecular interactions between the deprotonated COOH groups and the quaternary N-functions, the copolymer is shifted more toward the core of the water droplets, leading to a lower surfactant film stability. Regarding the PalPhBisCarb-modified w/o microemulsion, the higher amount of surfactant required to build up a stable w/o microemulsion (compared to that at pH < IEP and for the unmodified system) could be due to a partial phase separation as a result of repulsive interactions between the negatively charged COOH groups and the anionic surfactant. Therefore, we assume that the COOH groups containing phenylic side chains will be orientated toward the core of the microemulsion droplets. A similar phase behavior was observed in ref 55, where a likewise shift of the L2-phase range was found after the addition of poly(dimethyldiallylammonium chloride) to a w/o microemulsion system. To verify pH-dependent differences in the position of the copolymer within the water droplets as well as the influence of electrostatic and hydrophobic interactions on the surfactant film stability, percolation titrations as well as micro-DSC measurements have been carried out. A lower surfactant film stability, which represents weaker electrostatic interactions with the surfactant as well as less hydrophobic interactions, can be directly correlated to the conductivity of the system, whereas the position of the copolymer should have an influence on the melting behavior of the water inside the reverse microemulsion droplets (i.e., free, interfacial, and bound water) that can be determined by micro-DSC measurements.5659 3.2. L2-Phase Characterization. 3.2.1. Conductivity Measurements. Conductivity measurements are a useful tool to characterize dynamic processes (structural changes of the microstructure) and dropletdroplet interactions in w/o microemulsions, which are influenced by additives such as ionic and nonionic polymers. Since the conductivity of the oil phase of w/o microemulsions is very low, phenomena such as percolation of the water droplets can be directly related to the conductivity achieved.20,60 For investigating the influence of pH-dependent copolymer addition on dropletdroplet interactions, conductivity measurements were done by adding equivalent volume increments of the aqueous 10 wt % PalPh or PalPhBisCarb solution along a dilution line (ω = 22.3:77.7) at pH < IEP or pH > IEP and 25 °C. 3.2.1.1. PalPh-Modified w/o Microemulsion. The results of percolation titration done at pH 5 and 8.6 in the absence and presence of PalPh are shown in Figure 3 (a combined figure is shown in the Supporting Information). While the conductivity in the unmodified system is increasesd more strongly with increasing water content, the conductivity in the PalPh-modified system is less at pH < IEP. This indicates that the polyampholyte hinders the dropletdroplet interactions. On the basis of this behavior, 5318
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Figure 3. Conductivity (κ) as a function of aqueous content at 25 °C along the dilution line ω = 22.3:77.7 in the absence (open symbols) and presence (filled symbols) of PalPh.
Figure 4. Conductivity (κ) as a function of aqueous content at 25 °C along the dilution line ω = 22.3:77.7 in the absence (open symbols) and presence (filled symbols) of PalPhBisCarb.
one can assume an increased surfactant film stability after addition of PalPh (pH < IEP) due to electrostatic and hydrophobic interactions. Those results are in good agreement with the phase diagrams, where an extension in the direction of the water axis allows a decrease of the required amount of surfactant. This can be understood in terms of a penetration of the phenylic side chains into the interphase, which increases the droplet film stability and thus decreases the amount of surfactant needed to form a clear w/o microemulsion phase. Turning our attention toward the results obtained at pH > IEP, one can see clearly that the conductivity in the unmodified system remains nearly constant until about 30% aqueous content, followed by a slight increase in slope, due to the onset of droplet percolation. However, the conductivity of the PalPh-modified system increases comparatively more strongly. This indeed reveals that dropletdroplet interactions become stronger or even attractive after the incorporation of the copolymer PalPh. Taking the phase behavior into account, this increase in dropletdroplet interactions has to be a result of weaker electrostatic and hydrophobic interactions, which can be understood in terms of a shift of
the copolymer from the interphase toward the inner core of the water droplets. 3.2.1.2. PalPhBisCarb-Modified w/o Microemulsion. Percolation titration at 25 °C was also employed for the PalPhBisCarbmodified w/o microemulsion system at pH 4 and 8.4. As seen in Figure 4 (a combined figure is shown in the Supporting Information) the conductivity in the copolymer-modified system is increased less at pH < IEP compared to that in the unmodified system and even less compared to that in the PalPh-modified system at low pH. Thus, dropletdroplet interactions are hindered after the addition of a 10 wt % copolymer solution due to copolymersurfactant interactions that increase the surfactant film rigidity. Similar results were observed for another copolymermodified w/o microemulsion system where the polyampholyte was found at the interphase as a result of electrostatic interactions with the anionic surfactant SDS.26 Recently, molecular dynamics simulations have shown that the experimentally found shift of the percolation boundary is accompanied by a more rigid and ordered surfactant film due to the formation of a palisade layer.61 However, hydrophobic interactions which are assumed to be established 5319
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Table 1. Composition of Points A, B, and C
a
point
SDS/:oil:watera (w/w/w)
A
20:70:10
B
16:54:30
C
13:47:40
Or aqueous polyampholyte solution.
additionally in the PalPh-modified system seem not to be of relevance. A qualitative explanation could be that the high hydrophilicity of the carboxylic groups that are attached to the phenylic side chain avoids a strong penetration of the phenylic groups into the interphase. Nevertheless, the very low values of observed conductivity furthermore imply the surfactant film to be very stable. Including the results from the phase diagrams, where a higher amount of surfactant was required, the higher film stability has to be a result of additionally occuring ioninduced dipole interactions between the COOH groups of the copolymer and the anionic head groups of the surfactant. Regarding the results of percolation titration obtained at pH > IEP, one can observe an effect similar to that observed for the PalPhmodified system at alkaline pH. After the addition of PalPhBisCarb to the w/o microemulsion the conductivity is strongly increased due to stronger or even attractive dropletdroplet interactions. The weaker surfactant film stability involved can be explained by repulsive interactions between the polyanion and the anionic surfactant SDS, leading to an orientation of the negatively charged COOH groups attached to the phenylic side chains and thus a shift of the whole molecule toward the inner core of the water droplets. Therefore, neither hydrophobic nor ioninduced dipole interactions will be of relevance for this system. Moreover, the distance between the copolymer and the surfactant and the existence of intra- and intermolecular electrostatic interactions between the deprotonated and thus negatively charged COOH groups and the quaternary N-functions of the copolymer lead to weaker electrostatic interactions compared to those at pH < IEP, which also lowers the stability of the surfactant film. 3.2.2. Differential Scanning Calorimetry. In general, microDSC is a widely used method to investigate the melting behavior of frozen water in aqueous systems, e.g., in w/o microemulsions. Depending on the melting point, water can differ among free (melting point around 0 °C), interfacial (melting point between 0 and 10 °C), and bound (melting point below 10 °C). However, the transition temperatures and the different types of water are still in discussion.5962 Furthermore, it is well-known that the bulk water peak is shifted to lower temperatures after the addition of a polyelectrolyte to a w/o microemulsion system.62 To obtain more detailed information on the pH-dependent position of added polyampholyte, micro-DSC measurements were performed in points AC (compare Table 1) following a dilution line marked in Figure 1. 3.2.2.1. PalPh-Modified w/o Microemulsion. The results of the micro-DSC measurements, where the heat flow was recorded as a function of the temperature at pH 5 and 8.6, are summarized in Figure 5. At the lowest water content (point A) no significant influence of added polyampholyte on the melting behavior of the water can be detected for both pH values. The main peak with a maximum of about 7.5 °C (pH < IEP) and 9.1 °C (pH > IEP) can be related to interfacial water. That behavior indeed clarifies that the microemulsion droplets are very small, and only one type of water can be observed. By increasing the water content further
Figure 5. Heat flow as a function of temperature at points A, B, and C under addition of PalPh at pH 5 (solid line) and pH 8.6 (dashed line).
(point B), a splitting into a free (peak maximum 0.9 °C), an interfacial (peak maximum 8.0 °C) and a bound (peak maximum 11.4 °C) water peak is observed. The position of the peak maxima and the distribution are more or less the same in point C with the highest water content. It can be concluded that the interfacial water peak observed at point A (lowest water content) is split into free, interfacial, and bound water with increasing water content, whereas the interfacial water peak is still dominant. This indeed indicates that the polyampholyte is localized at the interphase, which is in good agreement with the conclusions drawn concerning the phase and percolation behavior. When the same experiments were carried out at pH > IEP, the interfacial water peak detected at low water content became smaller with increasing amount of water and was furthermore split into bound and free water at points B and C. Since there is quite less interfacial water detectable, it can be concluded that the copolymer penetration into the surfactant film is hindered compared to that at pH < IEP, which is a result of additional inter- and intramolecular electrostatic interactions. Furthermore, the huge amount of free water (at points B and C) supports the results of percolation titration, where stronger dropletdroplet interactions were observed. 3.2.2.2. PalPhBisCarb-Modified w/o Microemulsion. The heat flow curves of micro-DSC measurements obtained at pH 4 and 8.4 are summarized in Figure 6. At point A where the water content is the lowest only interfacial water can be detected for both investigated pH values (peak maximum 7.7 °C) in analogy with the PalPh-modified system. At point B a splitting into free (peak maximum: pH < IEP, 0.9 °C; pH > IEP, 1.4 °C), interfacial (peak maximum: pH < IEP, 6.7 °C; pH > IEP, 8.1 °C), and bound (peak maximum: pH < IEP, 10.2 °C; pH > IEP, 10.9 °C) water is observed. The distributions of the different types of water detected for the two investigated pH values only show marginal differences regarding the amount of free water. However, related to the amount of interfacial and bound water, opposite allocations within the different pH values can be observed. While at 5320
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Figure 6. Heat flow as a function of temperature at points A, B, and C under addition of PalPhBisCarb at pH 4 (solid line) and pH 8.4 (dashed line).
Scheme 2. Schematic Representation of the Location of the Copolymer PalPh within the w/o Microemulsion Droplets Dependent on pH
Scheme 3. Schematic Representation of the Location of the Copolymer PalPhBisCarb within the w/o Microemulsion Droplets Dependent on pH
pH < IEP the amount of interfacial water is lower, and the bound water becomes dominant, the opposite effect is observed at pH > IEP.
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By increasing the water content (point C), the bound water peak at pH < IEP is lowered and at the same time the amount of interfacial water is increased, so that primarily free (peak maximum: pH < IEP, 1.4 °C; pH > IEP, 0.2 °C) and interfacial (peak maximum: pH < IEP, 7.9 °C; pH > IEP, 8.0 °C) water can be detected. A similar melting behavior was found for another copolymer-modfied w/o microemulsion system, where the polymer was assumed to be localized at the surfactant/oil interphase at pH < IEP, leading to free and interfacial water, whereas at high pH the copolymer was localized in the inner core of the water droplets, leading to primarily bound and free water at high water content.28 Transferring this knowledge to the PalPhBisCarb-modified system, where an extended conformation with opposite charge states (polycation, polyanion) can be assumed at pH < IEP and pH > IEP, the results of micro-DSC measurements can be interpreted in terms of a pH-dependent penetration of the polymer into the surfactant film. At low pH, where predominatly free and interfacial water can be found, the coplymer is localized at the interphase, while at high pH, where bound and free water is primarily existent, the copolymer has to be more orientated toward the core of the water droplets. Furthermore, the higher amount of free water found at high pH can be explained by increasing water droplet size (decreasing number of droplets) as a result of the hydration of the deprotonated COOH groups that are oriented toward the droplet core.
4. CONCLUSIONS Our results have revealed that both investigated copolymers (PalPh, PalPhBisCarb) can be incorporated into a w/o microemulsion consisting of toluenepentanol (1:1)/SDS/water without a macroscopic phase separation. The clear isotropic phase range is futhermore extended in the direction of the waterrich corner, indicating a strong boosting effect of the copolymers which is more or less independent of the pH value but depends on the copolymer type. The solubilization capacity of water inside reverse swollen micelles was found to depend on many factors, including the rigidity of the surfactant film, which depends upon the size of the polar head groups of the surfactant and the alkyl chain, the droplet composition (addition of polymers), the oil type, the ionic strength, the nature and valence of the counterions, and the temperature. An increased temperature only has an impact on the PalPhmodified system and can be explained in terms of a lowering of the spontaneous curvature of the surfactant film. The less required amount of SDS also identified for the PalPh-modified system reveals that the copolymer seems to behave like a surfaceactive agent localized at the oil/water interphase. The opposite effect found for the PalPhBisCarb-modified system suggests that a higher amount of surfactant is needed to stabilize the water droplets. Conductivity measurements have revealed that percolation phenomena still exist after copolymer incorporation, whereas weaker dropletdroplet interactions were found at pH , IEP and stronger ones at pH . IEP. Thus, the surfactant film is better stabilized at low pH, where intra- and intermolecular electrostatic interactions between the COOH groups and the quaternary N-functions of the copolymer can be excluded. Thus, the increased surfactant film stability can be directly traced back to hydrophobic interactions of the phenylic side chain in the PalPhmodified system. However, the penetration of PalPhBisCarb into the interphase will be less due to the two COOH groups that are attached to the phenylic side chains, which decreases the 5321
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Langmuir hydrophobicity compared to that in the PalPh-modified system. Due to the very low conductivity, detected for PalPhBisCarb at pH , IEP, the stability of the interphase has to be a result of ioninduced dipole interactions. The increase in dropletdroplet interactions at pH . IEP can be understood in terms of a lower penetration of PalPh and PalPhBisCarb into the surfactant film due to weaker (PalPh) or even repulsive (PalPhBisCarb) electrostatic interactions with the anionic surfactant SDS, for which reason both polymers are more shifted to the inner core of the water droplets. Furthermore, the negatively charged COOH groups attached to the phenylic side chain (PalPhBisCarb) will be twisted toward the droplet core. Those conclusions are confirmed by micro-DSC measurements, where interfacial water was detected at pH < IEP, where the copolymers are more located at the interphase, and bound water was detected at pH > IEP, where both copolymers are displaced more toward the water droplet core. Combining the results of all investigations, one can assume the picture illustrated in Scheme 2 with a penetration of the polymer PalPh into the interphase and the formation of a palisade layer stabilized by hydrophobic and electrostatic interactions at pH , IEP. At pH . IEP the electrostatic interactions are diminished, and the polymer is shifted/orientated more toward the core of the water droplets, resulting in a lower stabilization of the w/o microemulsion droplets. Regarding the droplet assembly of the PalPhBisCarb-modified system (compare Scheme 3), we can conclude that the high surfactant film stability that was detected at pH , IEP can be traced back to ioninduced dipole interactions enhanced by electrostatic interactions, overcompensating the influence of hydrophobic interactions (found for PalPh at pH , IEP). At pH . IEP the anionic copolymer becomes more hydrophilic, and the negatively charged COOH groups attached to the phenylic side chain will be oriented toward the inner core of the water droplets. Because of additional repulsive interactions with the anionic surfactant SDS, the phenyl groups of the polyampholyte flip into the water core. At the same time the droplet radius will be increased as a result of the hydration of the deprotonated carboxylic groups, which increases the interphase and thus the amount of surfactant required to establish a clear microemulsion phase. To understand the nature of copolymersurfactant interactions in more detail, further investigations in highly diluted aqueous systems using EMF, ITC, and surface tension measurements have also been performed. Those investigations are useful to differentiate between electrostatic and hydrophobic interactions and to understand the influence of pH from the thermodynamic point of view, i.e., enthalpy and entropy change, the number of binding sites, and the association constant.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figure S1 showing the conductivity (κ) as a function of aqueous content at 25 °C along the dilution line ω = 22.3/77.7 in the absence (open symbols) and presence (filled symbols) of PalPh and PalPhBisCarb above and below the IEP. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: þ49-331-9775220. Fax: þ49-331-9775054.
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