Titration Calorimetric Study of the Interaction between Ionic

were indifferent. However, changing the counterion to chloride or modifying the headgroup to the unsubstituted ammonium ion gave observable interactio...
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J. Phys. Chem. B 1998, 102, 9276-9283

Titration Calorimetric Study of the Interaction between Ionic Surfactants and Uncharged Polymers in Aqueous Solution Geng Wang and Gerd Olofsson* Thermochemistry, Center for Chemistry and Chemical Engineering, Lund UniVersity, P.O. Box 124, S-221 00 Lund, Sweden ReceiVed: May 22, 1998; In Final Form: August 18, 1998

The interaction between uncharged polymers (PEO, PPO, PVP, and EHEC) and ionic surfactants in dilute aqueous solution has been studied using isothermal titration microcalorimetry. The surfactants studied were lithium, sodium, and magnesium dodecyl sulfate, alkyltrimethylammonium halides (RTA+X- with R equal to C12, C14, and C16 and X either Br or Cl), and dodecylammonium chloride (DoAC). The critical aggregation concentration cac, the saturation concentration C2 and the amount of polymer-bound surfactant were derived from the calorimetric titration curves. The anionic surfactants interacted strongly with PEO while RTABr were indifferent. However, changing the counterion to chloride or modifying the headgroup to the unsubstituted ammonium ion gave observable interaction. The presence of extra salt lowered the cac and increased the C2 with a more pronounced effect for PPO and EHEC. An increase in temperature had no noticeable effect on cac and C2 although the shape of the calorimetric titration curves changed drastically. PPO 1000 showed both small molecule behavior and polymer character in the interaction with SDS. Due to the hydrophobicity of the backbone, PVP showed significant interaction with SDS well below the cac. PVP showed no affinity to the cationic RTAX surfactants.

Introduction Interaction between nonionic water-soluble polymers and ionic surfactants has attracted widespread research interest, and the topic is treated in several review articles.1-5 Among the number of systems studied, those involving nonionic cellulose ethers and ionic surfactants are attracting a special interest because of the unusual rheological and even thermal gelling properties.6-8 At room temperature, ethyl(hydroxyethyl)cellulose (EHEC) and small amounts of ionic surfactants behave as normal polymer solutions with expected rheologic characteristics, but an increase in temperature may result in an increase of viscosity or in some cases the formation of a stiff, clear gel.6,9 The large viscosity increase and gel formation take place at about the clouding temperature of the pure cellulose ether solution and appear reversible. The headgroup of the ionic surfactants and addition of extra electrolytes influence the phase behavior and gel property. The anomalous behavior of the cellulose ether solutions is considered a combination of the change of the hydrophobicity of the polymer with temperature and polymer-surfactant interaction, but a more detailed knowledge about the phenomenon is still lacking. Is any unusual driving force responsible for the thermally induced viscosity increase and gelation or is only a general polymer-surfactant interaction mechanism operating under special circumstances? To answer such questions, studies have been carried out on EHEC-surfactant systems using dynamic light scattering and rheology,10,11 fluorescence and self-diffusion,12 and other techniques to investigate, for instance, phase equilibria13 and surfactant aggregation.14 Most studies were made on solutions * Author to whom correspondence should be addressed at Thermochemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden. Fax: +46 46 222 8185. E-mail: [email protected].

containing 0.5 or 1.0 wt % cellulose ethers. We have studied the interaction between ionic surfactants and two EHEC samples of different hydrophobicity in more dilute solution using isothermal titration calorimetry.15 We found it of interest to compare different model systems with the cellulose ether systems to get a clearer picture of the interaction between ionic surfactants and uncharged polymers in dilute aqueous solution. Poly(ethylene oxide) (PEO) and poly(N-vinylpyrrolidone) (PVP) are water-soluble over large temperature and concentration ranges even at fairly high molar masses, while the solubility of the more hydrophobic poly(propylene oxide) (PPO), is limited to short-chain oligomers. PVP has a hydrocarbon backbone with polarizable pyrrolidone side groups. The heterochain PEO is usually considered hydrophilic. Systems of these polymers with various ionic surfactants have been extensively studied and a lot of information has been gained about polymer-surfactant interaction under varying conditions.1-5 However, fundamental questions about the influence of various polymers on surfactant aggregation and how the polymers are involved in aggregate formation are still to a large extent unanswered. In this work, we have used isothermal titration calorimetry to determine the critical aggregation concentration cac, the saturation concentration C2 and to derive the extent of binding of the lithium, sodium, and magnesium dodecyl sulfate salts, trimethylammonium halides (RTAX, R ) C12, C14, C16, and X ) Br, Cl), and dodecylammonium hydrochloride (DoAC), in various polymer solutions. From this and earlier work on the interaction between ionic surfactants and cellulose ethers by the same and other methods,8,15 we have found that (i) there is no indication of strengthened surfactant binding with increasing temperature, but there is an increase in the extent of binding in solutions with extra salt; (ii) the surfactants bind noncooperatively to hydrophobically modified cellulose ether and PVP at very low concentration; (iii) RTAX surfactants interact weakly

10.1021/jp9823446 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/27/1998

Interaction of Ionic Surfactants and Uncharged Polymers or not at all with PEO and PVP but strongly with PPO and EHEC; (iv) SDS interacts with cellulose ethers through both the ethyl and ethylene oxide groups, while the trimethylammonium surfactants mainly interact with the alkyl segments; (v) change of the headgroup of dodecyltrimethylammonium chloride to the smaller one in the unsubstituted dodecylammonium chloride results in an observable interaction between the cationic surfactant and ethylene oxide groups. Experimental Section Materials. Dodecylammonium hydrochloride (DoAC) was prepared by dissolving dodecylamine (Fluka AG, purity > 99.6% as determined by acidimetric titration) in warm 1% aqueous hydrochloric acid. The product was recrystallized three times from water (Krafft temperature 23 °C) and freeze-dried and then recrystallized twice from 99.6% ethanol. The final product was dried under reduced pressure at 65 °C. Magnesium dodecyl sulfate [Mg(DS)2] was prepared by adding MgCl2 solution (20% excess of stoichiometry) to a sodium dodecyl sulfate (SDS) solution. The precipitated Mg(DS)2 was recrystallized and washed three times with water till it was free from MgCl2. The precipitate was dried under vacuum to constant mass. Sodium dodecyl sulfate (SDS) (BDH, 99%), lithium dodecyl sulfate (LDS) (Aldrich), dodecyltrimethylammonium bromide (DoTAB) (Aldrich, 99%), tetradecyltrimethylammonium bromide and chloride (TTAB and TTAC, respectively) (Sigma, 99%), hexadecyltrimethylammonium bromide (CTAB), (Sigma, 99%), and hexadecyltrimethylammonium chloride (CTAC) (Tokyo Chemical Industry, 95%) were used as received. Values of the critical micelle concentration (cmc) and the enthalpy of micelle formation ∆Hmic for the surfactant samples used are summarized and compared with literature values in Table 1 in ref 15. Samples of poly(ethylene oxide) (PEO), with nominal molar masses of 4 000 (PEO 4k), 8 000 (PEO 8k), 20 000 (PEO 20k), 100 000 (PEO 100k), and 1 500 000 (PEO 1.5m) (Aldrich) were used without further treatment. Poly(Nvinylpyrrolidone) (PVP) with an average molar mass of 20 000 (PVP 20k) and 40 000 (PVP 40k) (Aldrich, special grade) and poly(propylene oxide) (PPO) with molar mass of 1 000 (PPO 1k) (Aldrich) were used as received. Experimental Method. The calorimetric measurements were made using the commercial microcalorimetric measuring channel of the 2277 TAM Thermal Activity Monitor system (Thermometric AB, Ja¨rfa¨lla, Sweden) in a home-built highprecision thermostatic bath.16 The calorimetric titration experiments consisted of series of consecutive additions of concentrated surfactant solution to the calorimetric vessel initially containing 2.7 g of polymer solution. In some of the experiments concentrated polymer solution was added to surfactant solution in the vessel. Comparison experiments starting with pure water in the calorimetric vessel were made for all titrant solutions. The liquid samples were added in portions of 7-15 mg from a gastight Hamilton syringe through a thin stainless steel capillary tube. A microprocessor-controlled motor-driven syringe drive was used for the injections. The fast titration procedure17 was used with 6 min between each injection. Results and Discussion We have made measurements of additions of concentrated surfactant solutions (e.g., 10.00 wt % SDS) to water and polymer solutions (e.g., 0.100% PEO) at 25 and 35 °C. During a titration series of dilution in water of concentrated solution containing surfactant in micellar form, three different situations arise. In the experiments where the final concentration is below the cmc,

J. Phys. Chem. B, Vol. 102, No. 46, 1998 9277 TABLE 1: Critical Aggregation Concentration (cac), Saturation Concentration (C2), and the Extent of Binding of SDS Derived from the Calorimetric Titration Experiments at 25 °C; Results of LDS and Mg(DS)2 Are Also Included polymer (wt %) 0.10 PEO 4k 0.10 PEO 8k 0.10 PEO 20k 0.10 PEO 100k 0.10 PEO 1.5m 0.10 PEO 8k (35 °C)a 0.10 PVP 40k 0.50 PVP 40k 0.05 PPO 1k 0.10 PPO 1k 0.20 PPO 1k 0.25 E230Gb 0.25 CST 103b Mg(DS)2 to 0.10 PEO 8k (35 °C)c LDS to 0.10 PEO 8kd

extent of binding cac C2 (mmol/kg) (mmol/kg) (mmol/g of polymer) In Watera 4.2 ( 0.1 24 ( 1 4.2 ( 0.1 18 ( 1 4.2 ( 0.1 18 ( 1 4.2 ( 0.1 18 ( 1 4.2 ( 0.1 18 ( 1 4.2 ( 0.2 18 ( 1 13 ( 1 35 ( 1 2.7 ( 0.1 15 ( 1 2.2 ( 0.1 25 ( 2 1.8 ( 0.1 3.8 20.5 1.6 20.5 1.9 ( 0.05 24 ( 1 3.9 ( 0.1

18 ( 1 10 ( 1 10 ( 1 10 ( 1 10 ( 1 10 ( 1 5 ( 0.5 6 ( 0.2 15 ( 1 19 ( 1 5.4 5.4 23 ( 1

20 ( 1

14 ( 1

In 0.10 M NaCle 0.10 PEO 4k e0.8 30 ( 2 0.10 PEO 8k 0.8 21 ( 2 0.10 PEO 8k (35 °C) e0.8 21 ( 2 0.10 PPO 1k e0.8 38 ( 2 0.25 E230G e0.8 25 ( 2 0.25 CST 103 e0.8 45 ( 2

29 ( 2 20 ( 2 20 ( 2 38 ( 2 10 ( 1 18 ( 2

a Cmc of SDS in water is 8.4 ( 0.1 and 8.2 ( 0.1 mmol kg-1 at 25 and 35 °C. b From ref 15 c Cmc of Mg(DS)2 in water is 2.1 ( 0.05 mmol kg-1 at 35 °C d Cmc of LDS in water is 7.7 ( 0.1 mmol kg-1 at 25 °C. e Cmc of SDS in 0.1 M NaCl is 1.4 ( 0.1 mmol kg-1 at 25 and 35 °C.

the added micelles break up to give monomers in solution. The observed enthalpy change ∆Hobs includes contributions from dilution of the micelles in the concentrated titrant solution, demicellization, and interaction between monomers. In experiments with the final concentration in the micellization region, part of the injected micelles demicellize. When the final concentration is above the cmc, the added micelles are only diluted and ∆Hobs is the enthalpy of dilution of the concentrated surfactant solution. The cmc can be determined, for instance, as the concentration at the crossing point of the extrapolated linear premicellar region and the linear rise in the demicellization region. The ∆Hmic, can be derived from the difference between the linear pre- and postmicellar regions extrapolated to the cmc. The difference between the titration curve in water and in polymer solution can be ascribed to surfactant-polymer interactions. The onset of aggregation of surfactant in the presence of polymer is characterized by a critical aggregation concentration (cac). Free micelles are considered to start to form at a second critical concentration denoted C2. We have also made titration series with the addition of concentrated PEO and PVP solutions to SDS solutions (e.g., 0.100 mol kg-1) with concentrations well above the cmc. When the concentrated polymer solution is added to SDS solution, the micelles are in large excess in the beginning of the titration. As more polymer, is added, more of the surfactant is bound to the polymer and at some stage no free micelles exist. Thus the titration curves develops reversibly to the addition of concentrated SDS to dilute polymer solution. PEO and Ionic Surfactants PEO-SDS. Results of the calorimetric titration of 0.1 wt % (0.0227 mol repeat unit per kg) solution of PEO 1.5m with

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Figure 1. Calorimetric titration curves (a) from addition of 10.00 wt % SDS to 0.10 wt % PEO 1.5m solution (b) and dilution in water (O) at 25 °C; the inset shows the derived differential curves; (b) addition of 10.00 wt % SDS to 0.100 wt % PEO 8k at 25 °C (9) and 35 °C (b). Open symbols denote dilution of SDS in water at the corresponding temperature; (c) addition of 20.00 wt % PEO 8k to 0.100 mol kg-1 SDS (b) and water (O), and 10.00 wt % PEO 8k to 0.050 mol kg-1 SDS (9) and water (0) at 25 °C.

10.00 wt % SDS solution at 25 °C are shown in Figure 1a, where the observed enthalpy changes ∆Hobs for each injection are plotted against the total SDS concentration. The corresponding curve from dilution of the SDS solution in water is included in the figure. At 25 °C, ∆Hmic for SDS is close to zero, so the dilution curve in water shows only a change of slope at the cmc. The titration curve in the PEO solution starts to deviate from the dilution curve above 3 mmol kg-1 SDS to give a pronounced endothermic peak followed by a broad

Wang and Olofsson shallow exothermic one. Then the curve joins the dilution curve at about 18 mmol kg-1. These characteristic concentrations are seen somewhat clearer in a plot of the incremental enthalpy changes against SDS concentration, [∆Hobs(k) - ∆Hobs(k - 1)]/ ∆m, where ∆Hobs(k) is the observed enthalpy change in the kth injection and ∆m is the change in molality (see inset in Figure 1a). We find it the easiest and most reproducible way to identify cac with the concentration for the maximum of the first peak in the differential plot which corresponds to the concentration for the inflection point in the leading edge of the endothermic peak in Figure 1a. We define C2 as the concentration where the slope of the differential curve becomes zero, within uncertainty limits, that is the concentration where the titration curve in the polymer solution joins the dilution curve in water. The extent of binding, that is the amount of surfactant bound to the polymer, was calculated from the total concentration of surfactant at C2 minus the concentration of monomeric surfactant assumed to be the same as in the polymer-free solution at the same concentration. Results of addition at 25 °C of 10.00 wt % SDS solution to 0.100 wt % PEO with molar masses varying between 4000 and 1.5 × 106 are summarized in Table 1. We find that for SDS-PEO cac is 4.2 ( 0.1 mmol kg-1 SDS independent of polymer chain lengths. Neither do we observe any significant variation of cac with PEO content between 0.010 and 1.00 wt %.18 Also the second critical concentration C2 is independent of chain length, 18 ( 1 mmol kg-1 SDS, except for PEO 4k which gives a value of 24 ( 1 mmol kg-1 SDS. The C2 value for PEO with molar mass of 8000 or larger is consistent with the value of 17 mmol kg-1 determined from ultrasound measurement.19 However, lower values have been reported by Witte and Engberts20 (13 mmol kg-1) and Cabane21 (14 mmol kg-1) with the same amount of PEO using conductivity and NMR, respectively. The PEO-SDS system is by far the most studied polymersurfactant system.2,21-24 In PEO solution, the lowering of the cmc of SDS to cac mainly arises from the solubilization of EO groups in the headgroup region of the micelles with a concomitant release of electrostatic repulsion. A secondary contribution could be that EO groups intermingling in the headgroup region will shield superficial hydrocarbon groups from direct contact with water and thus give a favorable surface-energy contribution.25 In a spherical micelle of, for instance, SDS, some unfavorable contacts remain between the hydrocarbon groups and water at the micellar surface.26-28 When polymers with not too short chain length (mw > 4000) are used, more than one surfactant aggregate will form on each chain, and as the aggregates are charged there will be electrostatic repulsion and the polymer will come to show (weak) polyelectrolyte behavior. The number of SDS monomers per cluster formed on the PEO chain at 20 °C was found to increase in a regular manner from about 30 just above the cac to close to 60 at the saturation concentration C2.29,30 The free micelles that start to form above C2 are considered to have a slightly higher aggregation number than the polymer-bound aggregates in equilibrium with them. A qualitative description of the calorimetric curve showing the enthalpies of dilution of SDS in 0.100% PEO 1.5m solution in Figure 1 could be as follows. In the first couple of injections the slightly more positive dilution enthalpy in the polymer solution compared to water indicates a weak endothermic interaction between PEO and SDS monomers. When the cac region is reached, aggregates start to form with SDS incorporating EO segments to give mixed micelles containing about 30 surfactant monomers. The enthalpy change for the transfer of EO groups from water to the dehydrated core

Interaction of Ionic Surfactants and Uncharged Polymers would be positive, ∆Hdehyd ) 7 kJ [mol (-C2H4O-)]-1 at 25 °C31,32 so the observed enthalpy ∆Hobs becomes more positive as the cac is passed. ∆Hobs goes through a maximum and then drops to become exothermic relative to the dilution curve in water. The number of SDS monomers per micelle will increase as the total concentration increases and so will the fraction of dodecyl chains in the micellar core. The exothermic contribution to ∆Hobs could arise from the rehydration of EO segments that are expelled from the core in the reorganization of the mixed micelles. As the saturation concentration C2 is approached, the EO groups will be found in the outer part of the headgroup region where to a large extent they will stay hydrated. The cac and the maximum of the endothermic peak are almost independent of PEO concentration, while the peak height, the downward slope, the location and size of the exothermic peak (relative to the dilution curve), and C2 are directly related to the polymer content.15,18 Figure 1b shows the titration curves from addition of SDS to PEO solution at 25 and 35 °C. As the temperature increased, the first endothermic peak decreased and at the same time the broad exothermic peak preceding C2 faded away. Although the shape of the curves changed drastically, cac and C2 were not significantly affected by the temperature increase (see Table 1). The same temperature effect was seen for the SDS-E230G EHEC system which shows closely similar features.15 In the titration curves from addition of 10.00 wt % SDS to 0.250% E230G solution, the first endothermic peak almost vanished and the broad exthothermic peak preceding C2 disappeared when the temperature increased from 25 to 45 °C. Although the shape of the enthalpy curves changed drastically, cac and C2 were not significantly changed. An important contribution to the change of shape of the enthalpy curves is the increasingly more exothermic ∆Hmic shown by the jump in the SDS dilution curve in water (see Figure 1b). At 35 °C ∆Hmic is -5.1 kJ/mol, and at 45 °C it is -10.0 kJ/mol for SDS at the cmc.33 In addition, the change with temperature of the thermal effects of hydrophobic hydration of polymer segments or substitutents involved in the mixed surfactant aggregates may contribute to the change of shape similar to what was observed for the solubilization of pentanol in SDS micelles.26 There may also be contributions from changes in polymer-surfactant interaction with temperature.15 The aggregation number of the SDS aggregates decreases with increasing temperature, and the average number of SDS monomers per aggregate was found to be about 30% lower at 40 °C.30 Increasing temperature normally induces a higher degree of ionization and lower aggregation number.34 The aggregates become softer and more open to water penetration and EO groups associating with aggregates probably may retain the hydration water. Smaller enthalpic effects will be observed for such interaction. Addition of PEO to SDS Micelles. The titration curves of addition of concentrated PEO solution to 0.050 and 0.100 mol kg-1 SDS at 25 °C are shown in Figure 1c. The polymer content is expressed as molality of the repeat unit. In the initial phase extending up to a EO/SDS monomer ratio close to 1, ∆Hobs varied only little and was in fact quite close to the dilution enthalpy in water. According to the NMR distribution study of PEO in SDS micelles by Gao et al.,22 nearly 90% of the PEO should be incorporated into the SDS micelles in solutions where SDS is in large excess. The EO groups bound to the SDS micelles are probably located in the surface region and keep their hydrated state. When the molar ratio of EO monomer to SDS reaches 1, the interaction enthalpy starts to increase. We believe that the SDS micelles start to rearrange and change

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Figure 2. Addition of 2.50 wt % CTAC solution to 0.10 wt % solution of PEO 8k (b) in water (9) in 3 mM NaCl. Dilution of the CTAC (O) in water, (0) in 3 mM NaCl at 25 °C.

from full size aggregates with an aggregation number around 60 to smaller ones at this EO to SDS ratio and thereafter. The split into two exothermic peaks in the SDS-PEO titration curve at 25 °C in Figure 1b and the two well-resolved peaks in Figure 1c are only seen in curves using PEO 8k. The curve in Figure 1a is the typical curve when adding SDS to PEO solutions and when adding PEO 20k to SDS solution the first part of the curve looks the same as in Figure 1c but in place of a second peak a shoulder is seen. The reason for the particular shape of the curves with PEO 8k is unknown but may be related to possible aggregate-aggregate repulsion. We are using polydisperse samples and a significant fraction of PEO chains will carry on the average two SDS aggregates at C2.29,30 PEO-Cationic Surfactants. DoTAB, TTAB, and CTAB are indifferent to PEO at ambient temperatures.4,18 This difference in behavior between cationic and anionic surfactants toward PEO parallels the observed difference in behavior toward C4EOj (j ) 0-3).25 The variation in the number of EO groups was observed to be without measurable effect on the cmc of the cationic surfactant DoTAB which indicates that EO groups do not have a stabilizing effect on RTAB micelles. However, the cmc of SDS decreased regularly with increasing number of EO groups. This difference may at least in part stem from dissimilarities in size and hydration of the headgroups. The bulky RTA+ groups already shield most of the core from contact with water and have a hydrophobic character.4,18,35-37 Furthermore, steric repulsion between the headgroups and the polymer segments may produce an unfavorable contribution to the free energy of formation of polymer-bound micelles.38-40 However, the conditions may change if the counterion or the headgroup of the surfactants is changed. CTAC has a detectable interaction with PEO at 25 °C by titration methods (see Figure 2). The addition of CTAC to PEO in water shows a clear lowering of the cac and a large endothermic peak. Chloride ions bind more weakly to the CTA+ micellar surface than bromide ions, which leaves a higher positive charge density on the surface. Polymer segments may interpose between headgroups and reduce the electrostatic repulsion and lower the surface energy. We have also made a comparison between DoTAC and DoAC.15 The former shows no interaction with PEO but the latter does (see Figure 3). DoAC has a cac of 13.0 ( 0.2 mmol kg-1 in 0.1-0.2% PEO 8k compared to its cmc of 14.0 mmol kg-1 in water. The extent of binding is 22 ( 4 and 15.5 ( 2 mmol/g PEO in 0.1 and 0.2% PEO 8k, respectively. The headgroup of DoAC is smaller than in DoTAC and this probably leaves more space

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Figure 3. Addition of 10.00 wt % DoAC to 0.100 wt % PEO 8k in water (b) and in 0.1 M NaCl (9) at 25 °C. Open symbols denote dilution of DoAC in water and 0.1 M NaCl.

Wang and Olofsson the beginning of the titration, there is a fairly strong endothermic interaction between SDS monomers and the polymer chain. A more detailed study at 25 °C of the low concentration region18 showed no clear break in the titration curve indicating the cac but rather a steady increase to a well-defined maximum at 3.5 mmol kg-1 SDS. The location of the maximum is independent of polymer content in our experiments. The value for the maximum is higher than the reported value of cac, e.g., 2 mmol L-1 by surface tension measurements29,41 or 2.6 mmol L-1 from conductometric experiments.42 Thus, there is no clear indication of the cac in the titration curves at 25 °C. After the maximum, ∆Hobs drops to a pronounced minimum relative to the dilution curve in water. The extension of the broad exothermic peak is proportional to the polymer concentration. The endothermic peaks are smaller and the exothermic ones larger for PVP than for PEO 1.5m. At 35 °C, the endothermic peak has disappeared and there is a sudden drop in ∆Hobs at 2 mmol/kg which is the expected cac.29 In general, an increase in temperature has a profound influence on the shape of the enthalpy curves. The titration curves resulting from additions of 20.00% PVP solution to 0.100 mol kg-1 SDS and water at 25 °C are shown in Figure 4b. At the beginning of the titration, SDS micelles are in large excess and all added polymer interacts with the micelles. The initial part of the curve of PVP addition to SDS lies well below the dilution curve in water, indicating an exothermic interaction between PVP and SDS micelles amounting to -1.05 kJ mol-1 of PVP monomer at 25 °C. As in the titration of PEO (see Figure 1b) there is then a steep increase in ∆Hobs to a maximum but in the PVP system the peak is broad and ∆Hobs decreases only slowly. The steep rise occurs at 0.1 mol kg-1 PVP monomer which corresponds to a monomer-toSDS ratio of about 1. Addition of PVP to 0.050 mol kg-1 SDS gave a similar curve with a steep rise at the same monomerto-SDS ratio.18 PVP is a special polymer with a hydrocarbon backbone and polarizable pyrrolidone sidegroups. SDS monomers can adsorb on PVP with the hydrophobic part resting on the backbone and the headgroups can interact either with the surrounding water or pyrrolidone rings. In the initial phase below cac, surfactant monomers may accumulate alongside the polymer and give an endothermic pairwise interaction. We have also added CTAB to PVP solutions. Although CTAB has four more methylene groups than SDS, only a slight endothermic shift of ∆Hobs before the cmc is seen in the titration curve while the aggregation process is not affected by the presence of PVP. PPO and Surfactants

Figure 4. Addition of (a) 10.00 wt % SDS to 0.50 wt % PVP (b) and water (O) at 25 °C and to 1.00% PVP (9) and water (0) at 35 °C; (b) 20.00 wt % PVP in 0.100 mol kg-1 SDS (b) and water (O) at 25 °C.

for water penetration in the surface area. Binding of PEO segments to the micellar surface may reduce the surface energy of the DoAC micelle by the effects mentioned above. PVP and Ionic Surfactants PVP-SDS. Titration curves from the addition of 10.00 wt % SDS to 0.500 wt % PVP 40k at 25 °C and to 1.00% solution at 35 °C are shown in Figure 4a. Values of C2 at 25 °C from 0.100 and 0.50 wt % PVP solutions are given in Table 1. In

SDS-PPO. Titration curves from addition of 10.00 wt % SDS to 0.050, 0.10, and 0.20% solution of PPO at 25 °C are shown in Figure 5. Values for cac, C2, and the extent of binding were derived from differential plots and are given in Table 1. The cac decreases and the height of the endothermic peak increases with increasing PPO content. As can be seen, the curves have the same general features as SDS-PEO 1.5m curves, i.e., pronounced endothermic peaks followed by broad shallow exothermic peaks (relative to the dilution curve in water), only they are shallower than in PEO solutions. Normally PPO 1k is not considered as a real polymer but rather as an oligomer since on the average it contains only 14 monomer units. Our titration experiments show that it does behave as polymer in the sense that the titration curves in PPO solutions after a maximum crosses the dilution curve at a certain point and then, after going through a minimum, goes back to merge with the dilution curve. Titration curves for small

Interaction of Ionic Surfactants and Uncharged Polymers

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Figure 5. Addition of 10.00 wt % SDS to 0.050 (2), 0.100 (9), and 0.200 (b) wt % PPO 1k and dilution in water (O) at 25 °C.

molecules cannot reach the dilution curve since, when new surfactant micelles are added, the solute molecules will redistribute to give mixed micelles of equal composition.21 There will be an enthalpy difference between the titration and the dilution curves in water until the small molecules are infinitely diluted by the micelles.26 RTAB-PPO. Results of additions of 20.00 wt % DoTAB, 10.00 wt % TTAB, and 2.50% CTAB to 0.10 and 0.30 wt % PPO 1k at 25 °C by titration calorimetry are summarized in Figure 6a-c; 20.00 wt % TTAB to 0.50 wt % PPO 1k is also included in Figure 6b. As in the PPO-SDS systems, the cac and the position of the endothermic peaks in the curves vary with the concentration of PPO. It can be noted that the shallow exothermic peak seen in the SDS curve is lacking in the RTAB curves and that the curves in the polymer solutions approach the dilution curve in water from above. Due to this shape, the saturation concentration C2 was difficult to derive precisely. Values for cmc, cac in polymer solution, C2, and the extent of binding for RTAX are given in Table 2. The significant decrease in the aggregation concentration and the sizable peaks in the titration curves indicate a strong interaction between RTAB and PPO. The interaction strengthens with increasing alkyl chain length of the surfactant. Effect of Counterions Results of addition of 10.00 wt % LDS to 0.10 wt % PEO 8k and water at 25 °C are shown in Figure 7 and Table 1. The curve of LDS in PEO 8k solution has the same features as SDS in PEO 8k.18 The decrease of cmc to cac is about the same while C2 is slightly higher for LDS in the same amount of PEO (see Table 1). The endothermic peak after cac of LDS is larger than for SDS which reflects the larger and endothermic ∆Hmic for LDS of 1.5 kJ/mol compared to -0.2 kJ/mol for SDS. On the whole, the change of lithium for sodium as counterion has only minor effects on the interaction with PEO. The titration curves from the addition of 10.00 wt % Mg(DS)2 solution to 0.10 wt % solutions of PEO 8k and water at 35 °C are shown in Figure 8a. The cmc of Mg(DS)2 in water at 35 °C is 2.1 ( 0.1 mmol kg-1 and ∆Hmic is -5.0 ( 10.2 kJ mol-1. It is noteworthy that although the cmc of the divalent salt is much lower than for SDS, the values of ∆Hmic are nearly the same. Thus, the micellization enthalpy is dominated by the enthalpy contribution from the alkyl chain and not significantly influenced by the counterion. The Mg(DS)2 titration curve in PEO shows the same features as the curve obtained when adding SDS to PEO in 0.1 mol kg-1 NaCl (see Figure 8b). (∆Hmic for

Figure 6. Titration of (a) 20.00 wt % DoTAB, (b) 10.00 wt % TTAB, and (c) 2.50 wt % CTAB in 0.100 (9), 0.30(b), and 0.50(2) [only for (b)] wt % PPO 1k and dilution in water (O) at 25 °C.

SDS in 0.1 mol kg-1 NaCl is -6.6 ( 0.2 kJ/mol at 35 °C.) The presence of PEO has little effect on the cmc, but the curves in the polymer solution show deviation from the water curve in the region just above the cmc, i.e., both of them begin with endothermic shoulders followed by two shallow exothermic peaks. The extent of the shoulder and the two exothermic peaks (relative to the dilution curve in water) depends on the polymer concentration. Derived values of cac, C2, etc., are summarized in Table 1. Divalent magnesium ions have a similar effect on the interaction with PEO to that of monovalent sodium ions at a higher concentration. The stronger binding of divalent

9282 J. Phys. Chem. B, Vol. 102, No. 46, 1998

Wang and Olofsson

TABLE 2: Critical Micelle Concentration (cmc) of Cationic Surfactants, Critical Aggregation Concentration (cac), Saturation Concentration (C2), and the Extent of Binding of Surfactants Derived from the Calorimetric Titration Experiments at 25 ˚C surfactant DoTAB TTAB CTAB DoAC CTAC

polymer (wt %)

cmc (mmol/kg)

cac (mmol/kg)

0.10 PPO 1k 0.30 PPO 1k 0.10 PPO 1k 0.30 PPO 1k 0.50 PPO 1k 0.10 PPO 1k 0.30 PPO 1k 0.10 PEO 8k 0.20 PEO 8k 0.10 PPO 1k 0.10 PEO 8k 1.00 PVP 40k

14.5

12.4 ( 0.2 11.6 ( 0.2 2.5 ( 0.1 2.3 ( 0.1 1.8 ( 0.1 0.48 ( 0.2 0.33 ( 0.2 13.0 ( 0.2 13.0 ( 0.2 8.0 ( 0.2 1.14 ( 0.05

3.5 0.90 14.0 1.25

C2 (mmol/kg)

extent of binding (mmol/g polymer)

48 ( 3

37.5 ( 3

22 ( 1

20 ( 1

34 ( 4 42 ( 4 42 ( 2 4.0 ( 0.2 2.2 ( 0.1

22 ( 4 15.5 ( 2 31 ( 2 3.3 ( 0.2 0.13 ( 0.01

Figure 7. Addition of 10.00 wt % LDS to 0.100 wt % PEO 8k (b) and water (o) at 25 °C

counterions to surfactant surfaces gives a screening effect similar to an increased concentration of monovalent ions. Effect of Extra Salt Table 1 shows the cac and C2 of SDS in 0.100% PEO 4k and 8k, 0.100% PPO 1k, 0.250% EHEC E230G, and CST 103 solutions containing 0.1 mol kg-1 NaCl. In solutions with extra electrolyte the cac decreased and C2 increased and accordingly the amount of SDS bound to the polymer increased. In other words, the same amount of polymer can accommodate more SDS in the presence of extra salt. Cabane and Duplessix43 found in their small angle neutron scattering study of the PEO-SDS system that the smallest distance between neighboring SDS aggregates along a PEO chain is sensitive to the ionic strength: it expands from 60 Å in 0.8 M NaBr to 90 Å in water. Their interpretation was that electrostatic repulsion between neighboring aggregates dominates and restricts the variation in density of individual aggregates within the SDS-PEO assembly. Most PEO segments are stretched out between these aggregates. At higher ionic strength the electrostatic repulsion between individual aggregates is screened and one polymer chain can accommodate more surfactant aggregates and C2 increases. The increase is more pronounced for hydrophobic polymers such as PPO or EHEC CST 103 than for more hydrophilic ones such as PEO (see Table 1). In weakly interacting systems the addition of extra salt can obliterate the signs of interaction. Titration curves of 2.50 wt % CTAC in 0.10% PEO 8k in water and in NaCl solutions are shown in Figure 2. The interaction peak seen in polymerwater solution disappeared in the presence of 3 mmol kg-1 NaCl.

Figure 8. Titration of (a) 10.00 wt % Mg(DS)2 to 0.100 wt % PEO 8k (b) and water (O), and (b) 10.00% SDS to 0.100 wt % PEO 8k with 0.10 M NaCl (b) and to 0.10 M NaCl (O) at 35 °C.

The same thing happened for DoAC-PEO as can been seen in Figure 3. The extra salt lowers the cmc and the polymer has no further stablizing effect on the systems. Depending on the strength of the interaction between the cationic surfactant and the polymer, the vanishing of the observed interaction happened at different salt concentrations for different systems. Conclusions Enthalpic titration curves from the addition of surfactants to aqueous polymer solutions give information about the influence of polymers on the aggregation of surfactants. The curves show the same basic features for different systems. The concentration for the start of aggregation, cac, can be derived as well as the

Interaction of Ionic Surfactants and Uncharged Polymers concentration where the influence of the polymer ceases, C2. The curves also show if the binding of surfactant to polymer is cooperative or not. From measurements at varying temperatures, the effect of temperature on the aggregation behavior can be inferred. The size of the headgroup, the polarity of counterions, and the presence of extra salt influnce the interaction between polymers and surfactants. Extra salt increases the binding of SDS and the increase was larger the more hydrophobic the polymer. Increase in temperature had little effect on cac and C2 but changed the shape of the titration curves indicating changes in the thermal effects of interaction. Due to the short chain length, PPO 1k showed properties intermediate between a low molar mass solute and a polymer. Strong preaggregation behavior indicates a special attraction between PVP and SDS monomers. The titration curve for Mg(DS)2 in PEO solution is similar to the one for SDS in the presence of 0.1 mol kg-1 NaCl at the same temperature. In both cases, increased shielding gave a lower cac and an increase in the amount of polymer-bound surfactant. Acknowledgment. This work was supported by The Swedish National Board for Technical and Industrial Development (NUTEK). References and Notes (1) Robb, I. D. In Anionic SurfactantssPhysical Chemistry of Surfactant Action; Lucassen-Reynders, E. H., Ed.; Surfactant Science Series 11; Dekker: New York, 1981; p 255. (2) Goddard, E. D. Colloids Surf. 1986, 19, 255. (3) Saito, S. In Nonionic Surfactants: Physical Chemistry; Schick, M. J., Ed.; Surfactant Science Series 23; Dekker: New York, 1989; p 881. (4) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants: Physical Chemistry; Rubingh, D. N.; Holland, P. M., Ed.; Surfactant Science Series 37; Dekker: New York, 1991; p 189. (5) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, D. E., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, 1993; p 203. (6) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Colloids Surf. 1990, 47, 147. (7) Thuresson, K.; Nystro¨m, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (8) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909. (9) Thuresson, K. Ph.D. Thesis, Lund University, Lund, Sweden, 1996. (10) Nystro¨m, B.; Walderhaug, H.; Hansen, F. K. Langmuir 1995, 11, 750.

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