Effect of Hydrophobic Modification of a Nonionic Cellulose Derivative

6460

J. Phys. Chem. B 1997, 101, 6460-6468

Effect of Hydrophobic Modification of a Nonionic Cellulose Derivative on the Interaction with Surfactants. Phase Behavior and Association Krister Thuresson* and Bjo1 rn Lindman Physical Chemistry 1, Chemical Center, UniVersity of Lund, P.O. Box 124, S-221 00 Lund, Sweden ReceiVed: January 13, 1997X

Phase separation studies have been carried out concerning the addition of different surfactants to systems containing either ethyl(hydroxyethyl)cellulose (EHEC) or the hydrophobically modified analogue (HM-EHEC). The polymer concentration was kept constant at 1 g polymer/100 g of water (1 w/w%), while the surfactant concentration was varied. In the polymer/sodium dodecyl sulfate (SDS) systems, which were studied in more detail, phase diagrams were obtained in the presence of various concentrations of inert salt. Moreover, the phase behavior of the HM-EHEC/SDS system showed no marked changes by purification of the surfactant or by contamination with decanol. It was shown that the SDS binding to EHEC was cooperative and could be described in the framework of a closed association model, while for HM-EHEC a two-step binding model had to be used to get a proper description of the binding. In the noncooperative part of the HM-EHEC binding isotherm, the binding showed similarities to the adsorption of SDS on a hydrophobic surface and could be described by a Langmuir adsorption model. From the binding isotherms for SDS, binding isotherms for ionic surfactants with varying chain length could be calculated from simple assumptions. These isotherms give a basis for the interpretation of the phase diagrams. For the ionic surfactants, the phase behavior could be rationalized by considering the polymer/surfactant complex to possess polyelectrolyte characteristics. The observations were discussed in terms of an attractive hydrophobic interaction and a repulsive electrostatic force. Addition of nonionic surfactants was discussed by observing that generally a pair of a nonionic polymer and a nonionic surfactant segregates. However, with HM-EHEC there is an extra attractive hydrophobic interaction due to the presence of polymer hydrophobic tails which can serve as nucleation sites for the surfactants.

Introduction Aqueous solutions of nonionic polymers are widely used in applications, with examples as latex paints and enhanced oil recovery and skin care products.1,2 In these applications, the aqueous solutions frequently also contain surfactants. However, it is not only from an industrial point of view interesting to study these mixtures, but it is also scientifically challenging to study how the surfactants interact with the polymers. In the last decade, a class of water-soluble polymers, referred to as hydrophobically modified polymers (HM-polymers), has found an increasing use. These polymers are known to self-associate via hydrophobic interactions, resulting in an increased effective molecular weight which makes them more effective as viscosifiers. The hydrophobic inter polymeric associations are, under certain conditions, considered to give a three-dimensional network with an infinite extension. The ability to hydrophobic association should also increase their field of use, with emulsification and surface modification as potential applications. In the process of self-association of the HM-polymers, surfactants are known to have a dramatic influence. For instance the viscosity is known to pass through a pronounced maximum at a certain surfactant to polymer ratio.3-5 At higher surfactant concentrations, the additional viscosifying effectsattributed to the self association of the hydrophobic tails grafted to the polymer backbonesis lost completely. The increase in viscosity on addition of surfactants has been referred to a decreased free energy of the polymer hydrophobic tails in the bound state on addition of surfactant,6 corresponding to a formation of mixed micelles that become more and more related to free surfactant * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 1, 1997.

S1089-5647(97)00205-8 CCC: $14.00

micelles as the binding process advances. Another suggestion to account for the viscosity increase is in terms of an increased number of interpolymer bonds at the expense of intrapolymer bonds, which increases the viscosity due to an increased number of elastically active chains in the solution.7,8 The increased viscosity on surfactant addition can of course also be a combination of the two given mechanisms. The decrease in viscosity has been attributed to the disruption of interpolymer bonds as the stoichiometry between micelles and polymer hydrophobic tails is changed.9 In this paper, we investigate the phase behavior of an hydrophobically modified ethyl(hydroxyethyl)cellulose polymer (HM-EHEC) on addition of different surfactants and comparisons are done with the nonmodified analogue. The hydrophobic modification is shown to give an important contribution to the observed phase behavior. To facilitate the understanding of the phase behavior we fitted the binding of SDS to a closed association model for EHEC, while for HM-EHEC a two-step binding isotherm was needed. In the noncooperative binding regime (at low SDS concentrations), the binding to the polymer showed to be reminiscent of the adsorption of SDS on a macroscopic hydrophobic surface. A substantial part of the discussion relating the phase behavior to a polymer/surfactant interaction can be used for interpretation of changes in rheological parameters on addition of surfactants, published in the accompanying paper. Experimental Section Materials. Both the unmodified and the hydrophobically modified polymer (HM-polymer) were manufactured by Akzo Nobel Surface Chemistry AB. They are ethyl(hydroxyethyl)cellulose ethers (EHECs) with the same molecular weight (Mw © 1997 American Chemical Society

Nonionic Cellulose Derivative

J. Phys. Chem. B, Vol. 101, No. 33, 1997 6461

Figure 1. The structure of the polymers in a schematic representation. For HM-EHEC, the hydrophobic side-group R is a branched nonylphenol.

≈ 100 000) and degrees of ethyl and hydroxyethyl substitutions, DSethyl ) 0.6-0.7 and MSEO ) 1.8, respectively. The DS and MS values, which correspond to the average number of ethyl and hydroxyethyl groups per repeating anhydroglucose unit of the polymer, were, together with the Mw, values all given by the manufacturer. The hydrophobic modification consists of a low number of branched nonylphenol groups grafted to the polymer backbone, and the degree of substitution (1.7 mol %) was measured by UV absorbance at 275 nm using phenol in aqueous solution as a reference. In a 1 w/w% polymer solution (1 g polymer/100 g of water) this value corresponds to 0.64 mm, distributed, on average, as 6.5 HM-groups/polymer chain. Figure 1 displays a schematic picture of the chemical structures of the polymers. Before use the polymers were purified as described elsewhere10 and stored in a dessicator. The surfactants (sodium dodecyl sulfate, SDS, n-alkyltrimethylammonium chloride, CnTAC, and oligoethylene glycol monododecyl ether, C12En) were of high quality, and they were in most cases used as received without any further purification. SDS was in one experiment subject to purification by recrystallization from water several times by decreasing the temperature to below the Krafft temperature. Samples were prepared by weighing aliquots from stock solutions and mixing by turning end over end at room temperature for several days, or by mixing with a magnetic stirrer if the solutions were highly viscous. To all samples, water of Millipore quality was used. Cloud Point Measurements. Cloud point temperatures Tcp were measured in sealed glass tubes. The solutions were equilibrated at each temperature in a jacketed glass cell, and the phase separation temperature was measured with visual detection in runs with both increasing and decreasing temperature.10 These determinations were usually within (1 °C, and the reported cloud point temperatures are mean values of the two determinations. However, the phase diagram based on HM-EHEC and C12E5 has considerably larger uncertainties in the determinations (see vide infra Figure 7c). Results and Discussion To better understand the phase behavior of the polymers on addition of surfactants, we start by discussing the effect of an added cosolute in general. The phase separation seen in a binary EHEC/water solution, at an increased temperature, has been attributed to a less favorable interaction between the polymer

and the solvent. This behavior is opposite from what is usually seen (i.e., for EHEC the solubility decreases with an increased temperature). The temperature where the solution phase separates can be referred to as the cloud point Tcp because of the concomitant scattering of light. In a model by Karlstro¨m based on a Flory-Huggins approach, the less favorable interaction is attributed to conformational changes of the EHEC molecule with an increased temperature.11,12 On addition of a third species to the polymer/water solution, a phase separation (and a modulation of Tcp) can have different origins, but we will concentrate on the hydrophobic attractive interactions and the electrostatic repulsive forces when discussing the addition of an ionic surfactant. Zhang et al.13 showed that a cosolute that strongly prefers to interact with either the polymer or the water favors a phase separation. The first promotes an associative phase separation, while the latter induces a segregative phase separation. Otherwise the addition favors a one-phase (1Φ) behavior. A simplified picture is that the addition of a cosolute that strongly interacts with the polymer via an association in some sense transfers its properties to the complex and in connection herewith affects the solubility of the polymer or more correctly, the solubility of the complex is different. For instance, an ionic surfactant gives the complex a polyelectrolyte behavior, which can act to increase the solubility compared to the polymer itself. Different mechanisms can be responsible for an elimination of the hydrophilic contribution of added surfactant; in particular, for ionic surfactants, the solubility naturally decreases at salt addition, especially at low surfactant concentrations as in this regime the polymer complex is comparable with a polyelectrolyte having a low charge density. In most cases the binding of a surfactant to a polymer is cooperative resulting in micelles of a rather high aggregation number.14,15 (The aggregation number is though frequently smaller than in nondisturbed micelles.) The cooperativity and the high aggregation number suggest that the mixed micelles are related to free micelles, and it can be assumed that the surfactant hydrocarbon tails effectively are out of water exposure. Therefore, when a surfactant binds to EHEC, the Tcp is expected to increase. However, at addition of surfactants there are also contributions to the polymer solubility that favor a phase separation. The interaction with surfactant micelles raises constraints and therefore decreases the conformational

6462 J. Phys. Chem. B, Vol. 101, No. 33, 1997

Thuresson and Lindman

Figure 2. Tcp (O) as a function of concentration for an EHEC sample (batch E230G) that has been degraded by sonication. Tcp of the unsonicated EHEC at a concentration of 1 wt % is 62 °C.19

entropy of the polymer molecules. In the process of surfactant binding, the polymer molecules are also brought closer together, which can be seen as an increased polymer/polymer attraction (which is a first step toward a phase separation). Finally, following the association the weight of the polymer/surfactant complex increases as each complex can contain several mixed micelles and polymer molecules. An increased molecular weight is reflected as a decreased entropy of mixing (cf. an increased polymerization degree). However, the latter effect is probably of minor significance as an increased polymer molecular weight rapidly loses its importance when the polymer has a large number of repeating units. This follows for instance from the Flory-Huggins expression,16

(

∆Amix ) RT φ1 ln φ1 +

)

φ2 ln φ2 + χφ1φ2 σ

(1)

where the φ1 and φ2 denote the volume fractions of solvent and polymer, respectively. χ is the interaction parameter between solvent and polymer and σ is the polymerization degree of the polymer. It has also been verified experimentally that Tcp does not show any noticeable change for EHEC samples that are degraded to lower molecular weights (as indicated by a lower viscosity). The degradation was performed either by a chemical procedure17 or by sonication for 12 h.18 The former was carried out with an EHEC sample with an initial molecular weight of approximately Mw ≈ 50 000 and the latter with an EHEC sample (batch E230G) with a somewhat higher initial molecular weight, Mw ≈ 100 000. Intrinsic viscosity, following degradation with the sonication procedure, indicated that the molecular weight was reduced by a factor of ca. 20. Tcp as a function of polymer concentration for that sample is given in Figure 2. In the literature a, Tcp of 62 °C is reported for the unsonicated EHEC sample in a 1 wt % solution,19 in agreement with the Tcp found for the sonicated sample. The net contribution to the polymer solubility at surfactant addition is a balance between the mentioned opposing contributions. At low concentration of ionic surfactant, where the complex is expected to behave as a weak polyelectrolyte, the solubility may decrease, while at higher concentration, where the charge density is increased the solubility, is promoted. The situation with the hydrophobically modified polymer is entirely different because, in the solution, hydrophobic nucleation sites exist (i.e., the polymer hydrophobic tails) at which

the surfactants can bind.6,20 The effect of an increased effective molecular weight at surfactant association is, therefore, expected to be even less important in a HM-EHEC solution because hydrophobic associations between different polymer molecules already exist. The addition of surfactants can thus only be expected to strengthen the already existing hydrophobic associations. To capture the important differences between HM-EHEC and the unmodified parent polymer (EHEC), a discussion reviewing the binding isotherms of SDS to both EHEC and HM-EHEC is informative.6,21 It was found that, at high surfactant concentrations, the binding to the two polymers is similar,21 while at low surfactant concentrations it is different.6 The binding isotherms have been determined by using two different techniques. At the lowest surfactant concentrations, a DS- sensitive electrode was used to directly determine the surfactant activity, while at higher surfactant concentrations, where the response of the electrode may be changed, the fraction of polymer bound surfactant molecules was calculated from self diffusion coefficients determined by NMR.6 In Figure 3a, the binding of SDS to EHEC is presented, and the figure indicates that the binding is cooperative. Here we will go a step further and represent the micellization process K1

NaggA 98 ANagg

(2)

in a closed association model.22,23

K1 )

p Nagg(1 - p)NaggctotNagg -1

(3)

p is the fraction of the total amount of added surfactant ctot that is bound in mixed micelles. Equation 3, which is developed for nonionic surfactants, assumes that the activity coefficients of both micelles and monomers are set equally to unity. From the fit of the data, Figure 3a, we obtain the equilibrium constant K1, which together with Nagg, gives a free energy of micellization ∆G0mixedmic that is reasonable (see Table 1). 0 ∆Gmixedmic ) -RT ln(NaggxK1)

(4)

The value is slightly lower than the value found for free SDS micelle formation ∆G0mic Table 1, which can be attributed to a decreased unfavorable surfactant head group repulsion following from a smaller aggregation number in the mixed micelles. (The contact with the polymer can be seen to screen the head group repulsion). The very low aggregation number found from the fit (Nagg ≈ 5.8) as compared with experimental aggregation numbers for the same system6 (Nagg ≈ 20-40) can partly be attributed to the nonideality of the solution, which in the fit is absorbed into this parameter.22 The low aggregation number can also, as pointed out by So¨derman and Guering,24 be due to the covariance in K1 and Nagg, which is a feature of the closed association model. Several different pairs of K1 and Nagg can almost equally well represent the experimental data. In the mentioned investigation24 an aggregation number of less than one-half (ca. 24) of that generally accepted (ca. 60) for SDS micelles was found from a fit to eq 3. For comparison, a fit of the binding data to EHEC with Nagg set to 28, as suggested by experimental determination,6 is included in Figure 3a. In an attempt to account for a distribution of critical aggregation concentrations (cac’s), the binding data of SDS to EHEC was fitted with different values of Nagg. The cac could be calculated with the aid of eq 5 from the K1 value found from the fit, and a measure of the agreement between experimental data and the fit was obtained from eq 6.24 The two lowest concentrations have been excluded as these points can not be represented by



K)

1 ) NaggxK1 cac

ERRSUM )

(pobs - pcalc)2

∑

p2obs

(5)

(6)

the closed association model used with the higher values of Nagg. The exclusion corresponds to an elimination of the lowest cac values. In Figure 3b it can be seen that a changed aggregation number gives small changes in the ERRSUM value, which suggests that the data presented here can be represented almost equally well with different K1 - Nagg pairs. The small changes in cac with Nagg indicate that a small heterogeneity in the polymer strongly affects the cooperativity of the binding. Probably this is a main reason for the very small apparent values of Nagg found from the fit. It was also found that ∆G0mixedmic showed only small changes as a function of different aggregation numbers. To describe the SDS binding to the hydrophobically modified polymer, a two-step binding model is needed. As has been pointed out, cac in HM-EHEC/SDS mixtures either is very low or non-existing.25 Furthermore, at low SDS concentrations, the binding to HM-EHEC is noncooperative.6 This type of binding cannot be described in the framework of a closed association model. However, in a previous publication it was recognized6 that at low SDS concentrations the binding isotherm to HMEHEC has a similar appearance as the binding to a hydrophobic macroscopic surface. This noncooperative binding can be represented by a Langmuir isotherm.23

p)

Figure 3. (a) Binding isotherm of SDS to EHEC.6 Open symbols refer to NMR data and filled symbols to data obtained from SDS activity measurements. The dotted line shows the best fit of the data to the closed association model eq 3 giving an aggregation number of Nagg ) 5.8, while the dashed line represents a fit of the data to the model with Nagg ) 28. The full line indicates the isotherm constructed from a 0 ∆Gmixedmic value obtained from the cac, eq 5, together with the aggregation number Nagg ) 5.8 obtained from the best fit. (b) The variation of the ERRSUM (O) from eq 6, the cac (0) from eq 5, and 0 (]) from eq 4, given by the best fit of eq 3 to -∆Gmixedmic experimental data given in Figure 3a, by using different aggregation numbers Nagg of the polymer-bound mixed micelles. (c) Binding isotherm of SDS to HM-EHEC.6 Open symbols refer to NMR data and filled symbols to data obtained from SDS activity measurements. In the cooperative region, the isotherm from EHEC with a 0 value obtained from the cac of EHEC is used. In the ∆Gmixedmic 0 noncooperative region the dotted line has used a ∆Gads,ref value obtained from the adsorption of SDS on a hydrophobic macroscopic surface26 together with a concentration of nucleation sites B given by fluorescence quenching.6 The full line describes the best fit of ∆G0ads with the concentration of association sites held constant.

B K2(ctot - csurface) ctot K2(ctot - csurface) + 1

(7)

B in eq 7 is the concentration of binding sites for the surfactant, and csurface is the concentration of SDS bound to the polymer. In Figure 3c the fraction of bound surfactant as a function of added SDS in a 1 w/w% HM-EHEC solution is depicted. The closed association model for the cooperative part of the binding isotherm (at high surfactant concentrations) is taken to be the same as for EHEC, which probably is a reasonable assumption because at higher surfactant concentrations the effect of hydrophobic tails gradually fades away. The figure shows that the model closely describes the experimental data. To describe the binding with a Langmuir isotherm in the noncooperative region B is needed. This value can be estimated from fluorescence data presented in an earlier paper.6 The concentration of hydrophobic regions (nucleation sites) in the upper part of the noncooperative binding region (at 3 mm SDS) can be approximated to 0.092 mm. Furthermore, at 3 mm SDS ca. 11 surfactant molecules bind at each hydrophobic region. Altogether this gives B ≈ 1 mm for SDS at a HM-EHEC concentration of 1 w/w%. The equilibrium constant K2 is also needed to construct the Langmuir isotherm. The dotted line 0 has used a ∆Gads,ref value from the adsorption of SDS on a macroscopic hydrophobic surface determined by ellipsometry in our laboratory, Table 1.26 The full line has been constructed by fitting an equilibrium constant, which gives a reasonable ∆G0ads value. The concentration of binding sites B was held constant. The ∆G0 values indicate that the adsorption is more favorable at a polymer nucleation site than at the macroscopic surface. Furthermore, the free energy in the bound state is lower when the binding is noncooperative. (Compare the ∆G0ads value given by the Langmuir model with the ∆G0mixedmic value given by the closed association model.) It seems reasonable that the free energy is lower for a surfactant bound to a mixed



TABLE 1: Physical Data of the Aggregation Processes of the Surfactants. All ∆G0 Values are Given in Units of kT at 298 K 0 cmc (mm) cac (mm) from ∆Gmic from cmc from ref 36 Figs 4a and 5a and eqs 4 and 5

SDS C12TAC C16TAC C18TAC C12E5 C12E8

8.2 20 1.3 0.34 0.057 0.071

3.2 19 0.5 0.2

-8.8 -7.9 -10.7 -12.0

0 ∆Gmixedmic

0 ∆Gmixedmic from cac and eqs 4 and 5

-9.9 (eq 4) -9.0 (eq 8) -11.8 (eq 8) -13.1 (eq 8)

-9.8 -8.0 -11.6 -12.5

Figure 4. (a) The influence of SDS and SDS + NaCl on Tcp of EHEC. The full lines have only been drawn as a guidance for the eye. (b) The influence of SDS or SDS + decanol, and SDS + NaCl on Tcp of HM-EHEC. The full lines have only been drawn as a guidance for the eye. In the figure is included Tcp as a function of recrystallised SDS (.), as a function of SDS with 5.1 mol % decanol (+), and as a function of SDS with 9.9 mol% decanol (×).

micelle than to a hydrophobic planar surface. It also seems reasonable that the free energy is lower in the noncooperative part than in the cooperative part because the unfavorable surfactant head group repulsion becomes more important in the latter. Note that the data points describe a minima in the bound amount (at about 3 mm SDS), which is a strong evidence for the two step isotherm (see Figure 3c). We now return to the influence of surfactant binding on HM-EHEC solubility. The contributions favoring phase separation at surfactant binding (see above) are more pronounced with HM-EHEC. The solubility of the polymer/surfactant complex clearly decrease in the concentration regime where the surfactant binds noncooperatively (see Figure 4b). A parallel might be drawn to the decreased solubility of the polymer molecules following the grafting of hydrophobic tails in the conversion

Nagg

∆G0ads

B (mm)

5.8 (eq 3) 5.8 (eq 9) 10.4 (eq 9) 13.1 (eq 9)

-11.3 (eq 7) -10.4 (eq 8) -13.2 (eq 8) -14.5 (eq 8)

1.03 (ref 6) 1.03 (eq 9) 1.83 (eq 9) 2.32 (eq 9)

0 ∆Gads,ref from ref 26

-10.1

of EHEC to HM-EHEC.10 In this context it may also be valuable to draw attention to similarities with bridging flocculation.27 In the present system, the bridging flocculation mechanism suggests that the tendency to phase separation depends on the strength of the attraction between different polymer chains (which can be expected to depend on the level of surfactant binding). However, at some surfactant concentration the tendency to phase separation again decreases, because the electrostatic repulsion starts to be a dominating contribution to the solubility. This effect is expected to be important in the cooperative binding regime, because the mixed aggregates become more related to highly charged pure surfactant micelles,9 and the hydrophobic tails are effectively protected from water contact. Furthermore, at and above this surfactant concentration the attraction between different polymer molecules decreases due to the changed stoichiometry of the mixed micelles and the lowered connectivity of the network that follows.9,28 Generally, a nonionic surfactant shows no associative interaction with a nonionic polymer which is in contrast to the ionic surfactants. The difference is presumably due to a decreased electrostatic repulsion between surfactant head groups with the latter, which facilitates the surfactant assembly and the formation of micellar-like aggregates. Nonionic micelles are stable by themselves and the stability is not increased on association to a polymer. As an indication of the low tendency toward polymer/surfactant association, a mixture of a nonionic polymer and a nonionic surfactant often displays a segregative phase behavior (a phase separation into two phases, each enriched in one of the species),29,30 provided that no extra attractive forces, such as a hydrophobic interaction, exist. On the other hand, if the polymer has binding sites that are sufficiently hydrophobic, such as for HM-EHEC, the possibility for binding of nonionic surfactants increases markedly.31 (Compare also this with the different tendency to association of nonionic surfactants to EHEC polymers with different cloud points.19) With a nonionic surfactant and HM-EHEC, the attractive interaction is therefore related to that for an ionic surfactant, while the repulsive force rather has a short range steric origin. Phase diagrams for a number of surfactants will be rationalized along these lines. We will see that different ionic surfactants behave in a similar way, and we start by discussing the case of SDS, which was investigated in more detail than the other surfactants. Moreover, for SDS, Tcp as a function of surfactant concentration was determined at various concentrations of NaCl. On addition of SDS to EHEC solutions, Tcp passes through a shallow minimum at approximately 3 mm SDS (Figure 4a), while on addition of SDS to HM-EHEC solutions the minimum is more pronounced and located at a lower surfactant concentration, Figure 4b. Upon addition of NaCl to the polymer/ surfactant solutions, the two-phase (2Φ) regions increase in size for both polymers. All these observations are in line with the discussion above and, to reiterate the main arguments, can be understood by noting that the phase separation is controlled by a balance between repulsive electrostatic forces and attractive hydrophobic interactions.29,32 The decreased Tcp at low SDS concentrations can for EHEC be addressed an increased attrac-

Nonionic Cellulose Derivative tion between different polymer molecules and a decreased number available polymer conformations. For EM-EHEC, in addition to an increased attraction between different polymer chains the decreased Tcp can be due to the very low surfactant aggregation number of the mixed micelles and the concomitant exposure of the hydrophobic core. To prove that the decrease in Tcp of the HM-EHEC system at low SDS concentrations is not associated with decomposition products of the surfactant (compare the big initial drop in surface tension on addition of unpurified SDS to an aqueous solution23), Tcp measurements were repeated with purified SDS. The Tcp measurements were also repeated with SDS to which 5.1 and 9.9 mol % decanol had been added. (The decomposition products of SDS are dodecanol and hydrogen sulphate). These Tcp curves are included in Figure 4b, and as can be seen no marked changes can be observed on changes in the surfactant composition. These observations support the view that in the noncooperative binding regime6 it is the hydrophobic contribution from the surfactant itself, that gives rise to the decrease in Tcp (it is not an effect of an impure surfactant). At higher SDS concentrations, the electrostatic forces increase in strength and provide a repulsion between different surfactant nucleation sites (compare the swelling of a covalently bonded gel at an addition of an ionic surfactant33). At the same time the probability of finding two chains (or more) associated to the same micelle decreases. This picture is valid for both HM-EHEC and the unmodified parent polymer and is supported by rheology measurements; the viscosity shows a maximum at a certain (HM-)EHEC/SDS ratio and then decreases at further surfactant addition.5 As can be seen from the figures, the Tcp decrease for the HM-EHEC/SDS system is located at a lower surfactant concentration than for the EHEC/SDS system. This reflects that surfactants apparently bind already from the first addition,6 and is consistent with the suggested Langmuir isotherm at low surfactant concentrations. The drop in Tcp upon addition of salt is due to a screening of the electrostatic forces promoting the phase separation in analogy with a salting out of a weakly charged polyelectrolyte.20,32,34 The electrostatic force that opposes phase separation arises from counterion entropy of polymer bound surfactant, and upon salt addition the importance of this effect decreases. From Figure 4a it can also be seen that the cac in the EHEC solution decreases on addition of salt as Tcp starts to decrease from a lower SDS concentration with an increasing NaCl concentration. This seems reasonable as the electrostatic repulsion of SDS head groups is screened and aggregation promoted (compare the lowering of critical micellar concentration (cmc) on increasing the inert electrolyte concentration35). The interpretation of the phase behavior on addition of cationic surfactants is challenging. The length of the surfactant tail strongly influences the balance between the opposing forces that act in the solution. Starting with EHEC, Figure 5a shows that C12TAC decreases Tcp more than both C16TAC and C18TAC. To understand this we note that cac is lowered when the surfactant chain length is increased. The cac of the EHEC/ surfactant mixture follows the same trend as cmc for the pure surfactant systems,35 and in Table 1 the cmc’s of the used surfactants are tabulated.36 A decreased cac results in a lowered free surfactant concentration, which has the same effect as lowering the concentration of an inert electrolyte, cf. above. Thus, the screening of the electrostatic forces is decreased when the surfactant chain length is increased, and an increased electrostatic repulsion promotes a one-phase behavior. In Figure 5a is also shown that the upturn in the Tcp curves is in a monotonic manner shifted toward lower surfactant concentrations as the surfactant chain length is increased. This


Figure 5. (a) The influence of CnTAC on Tcp of EHEC. The full lines have only been drawn as a guidance for the eye. (b) The influence of CnTAC on Tcp of HM-EHEC. The full lines have only been drawn as a guidance for the eye.

reflects the lowering of the cac and decreased electrostatic screening from free surfactants. With longer surfactant hydrocarbon chains the tendency to phase separation is also decreased by an increased micellar size, which gives a stronger repulsion between different polymer bound micelles (due to a higher charge). With HM-EHEC, Figure 5b, the balance between the forces is more delicate. The decrease in Tcp is again largest upon addition of C12TAC, with the same explanation as for EHEC; the electrostatic screening is largest with C12TAC due to a nonnegligible free surfactant concentration. It may be noted that C12TAC decreases Tcp more than SDS both for EHEC, as suggested by the cac values (see Table 1), and for HM-EHEC. As discussed above, an increasing surfactant chain length results in a decreased free surfactant concentration which becomes quite small with C16TAC and C18TAC, especially in the presence of HM-EHEC. An interesting observation in Figure 5b is that on increasing the hydrocarbon chain length from 16 to 18 carbons, the monotonic change of the upturn in Tcp upon increasing the chain length as seen with EHEC is not reproduced with HM-EHEC. This can be explained by the presence of hydrophobic nucleation sites in the HM-EHEC solution and by the fact that surfactants can be assumed to distribute randomly over these sites.37 The noncooperative binding step results in the cooperative binding starting at higher concentrations, as is indicated by the following discussion.



Figure 6. Calculated binding isotherms of different CnTAC:s to HM-EHEC. In the cooperative regime the full lines have used 0 values calculated from cac values, while the dotted lines ∆Gmixedmic 0 have used ∆Gmixedmic values obtained from eq 8. The circles indicates the surfactant concentrations where the cooperative binding becomes more important than the noncooperative binding.

The binding isotherm of C12TAC in Figure 6 was constructed by using the same number of adsorption sites in the Langmuir isotherm (in the noncooperative region at low C12TAC concentrations) as was used for the SDS isotherm. The equilibrium constant K2 in eq 7 was calculated by taking the ∆G0 value found from the fit of experimental binding data of SDS to the Langmuir isotherm, ∆G0ads obtained for SDS, and by shifting this value by the difference seen in ∆G0 values for the free micellization process of C12TAC, ∆G0mic(C12TAC) as compared to SDS, ∆G0mic(SDS),

∆G0ads(C12TAC) ) ∆G0ads(SDS) - [∆G0mic(SDS) - ∆G0mic(C12TAC)] (8) (The ∆G0mic values were calculated from the tabulated cmc values, Table 1, by using eq 5 in combination with eq 4, for cmc values rather than cac values.) To get the cooperative part of the binding isotherm, the K1 value in the closed association model eq 3 was extracted in analogy with the K2 value, using eq 8. In addition K2 was also calculated from the cac value of the surfactant in the presence of the unmodified parent polymer, see eq 5. The cac was taken as the surfactant concentration needed to get a cooperative binding to the EHEC molecule, and is determined from Figure 5a as the concentration were the Tcp of EHEC starts to increase. (This procedure gives rough estimates, but good agreement with direct measurements was obtained for SDS). ∆G0 and cac values are tabulated in Table 1. To construct the binding isotherms of C16TAC and C18TAC to HM-EHEC, the same scheme as for C12TAC was used with the extension that the aggregation number of surfactants in the mixed micelles in the closed association model and the number of binding sites in the Langmuir model were scaled with

N ∝ n2

(9)

where N is the surfactant aggregation number of the mixed micelle, or the number of binding sites, and n is the number of carbons in the surfactant tail. In the closed association model (at high surfactant concentrations) in Figure 6, the dotted line refers to ∆G0mixedmic values obtained from the “shifting and scaling procedure”, eqs 8 and 9, while the full lines refer to

∆G0mixedmic values obtained from experimental cac values, eqs 4, 5 and 9. It can be seen that the surfactant concentration where the cooperative binding is predicted to take over from the noncooperative one follows the same trend as was seen from the upturn in Tcp on addition of different CnTAC’s; the cooperative binding starts at the highest surfactant concentration with C12TAC, while C16TAC gives the lowest concentration. As for both C16TAC and C18TAC, the electrostatic screening is low (due to the low free surfactant concentration), the reason for the larger decrease in Tcp on addition of C18TAC, compared to that of C16TAC, is due to the larger number of surfactant molecules that are bound to HM-EHEC before the available binding sites are saturated (see eq 9) and the cooperative part of the binding can take over. Upon addition of the nonionic surfactants C12E5 and C12E8, a different behavior is observed. These surfactants, in analogy with the polymers and by an analogous mechanism, become less soluble at elevated temperatures and thus have a Tcp. In earlier investigations on related EHEC polymers,12,13 Tcp of the mixed system EHEC/C12E5 showed to be lower than Tcp for either polymer or surfactant. The same observation is made here, Figure 7a. The reason for the strong influence on Tcp is referred to the phase separation being segregative as usually seen in polymer mixtures where no specific attractive force between the different polymer species is present.29 A good description is offered by considering the micelles as a second polymer.30 In Figure 7b, the relative volumes of the two phases are shown, and the evolution with surfactant concentration suggests a segregative phase separation. Two quite large phases are in equilibrium with each other, and the relative volumes of them are to a high degree determined by the ratio of the dissolved components, with surfactant determining the volume of the top phase and polymer determining the volume of the bottom phase. (Usually in an associative phase separation a large top phase, which contains most of the solvent, is in equilibrium with a small bottom phase concentrated in both the solutes.) Earlier it has also been found that the same EHEC polymer did not interact with a microemulsion made up from C12E5 and decane31 and that a related (rather polar) EHEC polymer showed no association with C12E8 micelles,19 which further fortifies the suggestion. Upon an addition of C12E8 to the EHEC solution, no change in Tcp is seen below a very high concentration, ca. 225 mm. Again, this suggests that the polymer does not associate with the surfactant and that the phase separation is segregative. A key to understanding the strong decrease in Tcp on addition of C12E5, but not on addition of C12E8, to EHEC might be that the C12E5 micelles grow rapidly at an increasing temperature, which has been shown by NMR diffusion.38,39 (Actually, they are so large that it is difficult to determine micelle size with a standard fluorescence-quenching technique.30) The growth promotes the segregative phase separation, which follows from the FloryHuggins expression (see eq 1).29 Free micelles of C12E8, on the other hand, are known not to show any growth with increasing temperature.40,41 The decrease of Tcp above 225 mm is, however, possibly related to a micellar growth (micelles are generally known to grow in size at an increasing concentration). In an investigation by Piculell et al., it was suggested that there is a nonnegligible growth in a mixture of C12E8 and a poly(ethylene oxide) polymer already in the temperature interval 25-40 °C,30 which is far below the temperature where we observe a phase separation. On addition of the nonionic surfactants to a HM-EHEC solution, Figure 7c, we again have to bear in mind that the polymer hydrophobic tails can act as nucleation sites for the surfactants. The modulation of Tcp on addition of C12E8 can



Figure 7. (a) The influence of C12En on Tcp of EHEC, (O, 0). The full lines have only been drawn as a guidance for the eye. Tcp of the binary systems C12En/water has been incorporated in the figure, (b, 9). The Tcp values for C12E5 are reproduced from the literature.43 The variation of cmc of C12E5 as a function of temperature is schematically given, and the cmc value at 23 °C is indicated (×). (b) The relative volume of the top and bottom phases for EHEC/C12E5 mixtures at 25 °C. Note that the concentration axis not is to scale. (c) The influence of C12En on Tcp of HM-EHEC, (O, 0). The full lines have only been drawn as a guidance for the eye. Tcp of the binary systems C12En/water has been incorporated in the figure, (b, 9). The Tcp values for C12E5 are reproduced from the literature.43 The variation of cmc of C12E5 as a function of temperature is schematically given, and the cmc value at 23 °C is indicated (×). (d) The relative volumes of the top and bottom phases for HM-EHEC/C12E5 mixtures at 25 °C. Note that the concentration axis not is to scale.

be explained in similar terms as for an ionic surfactant, with the distinction that the hydrophilic contribution has a different origin and is steric rather than electrostatic. From viscosity measurements, C12E8 is known to associate with HM-EHEC,28 and the initial decrease in Tcp can be addressed to the binding of surfactant molecules to the preformed micelles. At higher concentrations, where the micelles are dominated by surfactant, the attraction between different polymer chains decreases and Tcp increases (cf. the discussion with ionic surfactants). In an earlier investigation it was found that HM-EHEC can be incorporated into a microemulsion based on C12E5 and decane.31 This was interpreted as an association between the microemulsion film and the hydrophobic tails of HM-EHEC. It, therefore, seems reasonable that, at low C12E5 concentration, the segregative phase separation is disfavored. However, the polymer hydrophobic tails quickly become saturated and at higher C12E5 concentrations the HM-EHEC solution behaves similarly as the EHEC solution from a judgement of Tcp. On the other hand, the evaluation of the relative phase volumes, which is depicted in Figure 7d, indicates that with HM-EHEC the phase behavior is more complex than that in the EHEC/C12E5 mixtures. The relative volume of the top phase passes through a maximum

and at some point a macroscopic phase separation is not easily obtained although the mixture is cloudy. It should be noted that a full understanding of the phase behavior is not accessible without a full determination of the three-component phase diagrams.10 Summary and Conclusions The modulation of the phase separation temperature Tcp of EHEC on addition of ionic surfactants can be rationalized in terms of an attractive hydrophobic interaction and an electrostatic repulsion. Provided that the surfactant is ionic, the polymer/surfactant complex possesses features of a polyelectrolyte (i.e., a phase separation is promoted by an addition of an inert electrolyte). The effect of a longer surfactant hydrocarbon chain length turns up as a decreased critical aggregation concentration (cac), in analogy with a decreased cmc on increasing the surfactant tail length. Due to the decreased cac, the free surfactant concentration in the polymer solution decreases, which causes a decreased tendency to “self-salting out”. To understand the effect of surfactants on the phase behavior of the hydrophobically modified analogue HM-EHEC, the

6468 J. Phys. Chem. B, Vol. 101, No. 33, 1997 binding of surfactants to the nucleation sites originating from the polymer hydrophobic tails had to be rationalized. It was shown that the binding of sodium dodecyl sulfate, SDS, to the unmodified parent polymer EHEC could be described by a closed association model, while the binding to HM-EHEC had to be described by a two-step binding isotherm. At high surfactant concentrations, where the surfactant binding is cooperative, the binding to HM-EHEC is akin to the binding to EHEC, while at low surfactant concentrations, where the binding is noncooperative, a Langmuir isotherm gives an appropriate description. This emerged from the observation that in the noncooperative regime the isotherm is similar to an adsorption isotherm on a macroscopic surface. Generally the binding of an ionic surfactant to a nonionic polymer is attributed decreased electrostatic repulsive forces,42 often in combination with a hydrophobic attraction. If the polymer has a homogeneous structure the polymer/surfactant interaction is described by a distinct cac and a highly cooperative binding process. A blocky structure of the polymer molecule (i.e. some parts are more hydrophobic than the rest of the polymer backbone) naturally favors the surfactant binding and renders a distribution of cac’s. This results in a surfactant binding which is less cooperative, and it shows up as an apparent lower aggregation number in the closed association model. With a hydrophobically modified (HM) polymer, which is the ultimate example of a blocky polymer structure, the binding to the hydrophobic moieties is noncooperative. We note that the unmodified EHEC molecule is a copolymer with a substitution of side chains with varying polarity, and it is expected that the surfactant binding to EHEC is marked by this blockiness. EHEC can, therefore, be regarded as a polymer with a weak hydrophobic modification, to which the surfactant binding has a cooperativity inbetween that of the binding to a homogeneous polymer and to a HM-polymer (cf. Figures 3a,c). The description of the binding isotherms in the two-step model with HM-EHEC allowed us to construct calculated binding isotherms of ionic surfactants with different chain lengths. These gave information on the surfactant aggregation with HM-EHEC, which could be related to observed differences in phase diagrams between EHEC and HM-EHEC. On addition of nonionic surfactants, most of the features observed could be understood by observing that generally there is no tendency toward association between polymer and surfactant unless there is an extra attractive force. The nucleation sites, originating from the polymer hydrophobic tails, in the HM-EHEC solution provide such an attraction. Acknowledgment. The National Bord for Industrial and Technical Development (NUTEK) and Akzo Nobel Surface Chemistry AB are greatly acknowledged for financial support. Akzo Nobel is also acknowledged for supplying the polymers. Ingegerd Lind is gratefully acknowledged for skillful technical assistance. References and Notes (1) Glass, J. E., Ed. Polymers in Aqueous Media; American Chemical Society: Washington, DC, 1989; Vol. 223. (2) Goddard, E. D., Ananthapadmanabhan, K. P., Eds. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, 1993. (3) Gelman, R. A. Hydrophobically modified hydroxyethylcellulose. TAPPI International Dissolving Pulps Conference, Geneva, Switzerland, 1987. (4) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304-1310.

Thuresson and Lindman (5) Nystro¨m, B.; Thuresson, K.; Lindman, B. Langmuir 1995, 11, 1994-2002. (6) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909-4918. (7) Annable, T.; Buscall, R.; Ettelaie, R.; Shepherd, P.; Whittlestone, D. Langmuir 1994, 10, 1060-1070. (8) Semenov, A. N.; Joanny, J.-F.; Khokhlov, A. R. Macromolecules 1995, 28, 1066-1075. (9) Piculell, L.; Thuresson, K.; Ericsson, O. Faraday Discuss. 101, 1995, 307-318. (10) Thuresson, K.; Karlstro¨m, G.; Lindman, B. J. Phys. Chem. 1995, 99, 3823-3831. (11) Karlstro¨m, G. J. Phys. Chem. 1985, 89, 4962-4964. (12) Karlstro¨m, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94, 5005-5015. (13) Zhang, K.; Karlstro¨m, G.; Lindman, B. Colloids Surf., A 1992, 67, 147-155. (14) Zana, R.; Binana-Limbele´, W.; Kamenka, N.; Lindman, B. J. Phys. Chem. 1992, 96, 5461-5465. (15) Kamenka, N.; Burgaud, I.; Zana, R.; Lindman, B. J. Phys. Chem. 1994, 98, 6785-6789. (16) Flory, P. J. Principles of Polymer Chemistry, 13th ed.; Cornell University Press: Ithaca, NY, 1953. (17) Johansson, H. O. Private communication. (18) Zhang, K.; Lindman, B. Unpublished results. (19) Zhang, K.; Jonstro¨mer, M.; Lindman, B. J. Phys. Chem. 1994, 98, 2459-2463. (20) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201-9209. (21) Thuresson, K.; Nystro¨m, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730-3736. (22) Desnoyers, J. E.; Perron, G. Thermodynamic Methods. In Surfactant Solutions. New Methods of InVestigations; Zana, R., Ed.; Marcel Dekker: New York, 1987; Vol. 22, pp 1-55. (23) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain: where Physics, Chemistry, Biology and Technology Meet, 1st ed.; VCH Publishers: New York, 1994. (24) So¨derman, O.; Guering, P. Colloid Polym. Sci. 1987, 265, 76-82. (25) Thuresson, K.; Nilsson, S.; Lindman, B. Effects on Phase Behaviour and Viscosity of Hydrophobic Modification of a Nonionic Cellulose Ether. Influence of cosolutes. In Cellucon-93; Kennedy, J. F., Phillips, G. O., Williams, P. A., Eds.; Woodhead Publishing, Ltd: Lund, Sweden, 1993; pp 323-329. (26) Tiberg, F. Unpublished results. (27) Russel, W. B. Macroscopic Consequences of Polymer-Particle Interactions. In Colloid-Polymer Interactions. Particulate, Amphiphilic, and Biological Surfaces; Dubin, P., Tong, P., Eds.; American Chemical Society: Washington, DC, 1993; p Chapter 1. (28) Thuresson, K.; Lindman, B.; Nystro¨m, B. J Phys. Chem. B 1997, 101, 6450-6459. (29) Piculell, L.; Lindman, B. AdV. Colloid Interface Sci. 1992, 41, 149178. (30) Piculell, L.; Bergfeldt, K.; Gerdes, S. J. Phys. Chem. 1996, 100, 3675-3679. (31) Kabalnov, A.; Olsson, U.; Thuresson, K.; Wennerstro¨m, H. Langmuir 1994, 10, 4509-4513. (32) Lindman, B.; Carlsson, A.; Karlstro¨m, G.; Malmsten, M. AdV. Colloid Interface Sci. 1990, 32, 183-203. (33) Ericsson, O.; Piculell, L. Submitted. for publication. (34) Guillemet, F.; Piculell, L.; Nilsson, S.; Lindman, B. Phase Behaviour of Mixtures of Associative Polyelectrolyte with Oppositely Charged Surfactant. In Cellucon -93; Kennedy, J. F., Phillips, G. O., Williams, P. A., Eds.; Woodhead Publishing, Ltd: Lund, Sweden, 1993; pp 287-293. (35) Lindman, B.; Wennerstro¨m, H. Micelles. Amphiphile aggregation in Aqueous Solution. In Topics Current Chemistry, 1980; Vol. 87; pp 1-83. (36) Mukerjee, P.; Mysels, K. J. CMC of Aqueous Surfactant Systems; National Standards Reference Data Series; U.S. National Bureau of Standards, U.S. Government Printing Office: Washington, DC, 1971. (37) Magny, B.; Iliopoulos, I.; Zana, R.; Audebert, R. Langmuir 1994, 10, 3180-3187. (38) Nilsson, P.-G.; Wennerstro¨m, H.; Lindman, B. J. Phys. Chem. 1983, 87, 1377-1385. (39) Lindman, B.; Wennerstro¨m, H. J. Phys. Chem. 1991, 95, 60536054. (40) Jonstro¨mer, M.; Jo¨nsson, B.; Lindman, B. J. Phys. Chem. 1991, 95, 3293-3300. (41) Anthony, O.; Zana, R. Langmuir 1994, 10, 4048-4052. (42) Cabane, B.; Duplessix, R. J. Phys. 1982, 43, 1529-1542. (43) Strey, R.; Schoma¨cker, R.; Roux, D.; Nallet, F.; Olsson, U. J. Chem. Soc., Faraday Trans. 1990, 86, 2253-2261.

Effect of Hydrophobic Modification of a Nonionic Cellulose Derivative

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