Interactions between Sodium Dodecyl Sulfate and Six Nonionic

Science Research Institute, University of Salford, Salford, M5 4WT, U.K., and Lever Faberge Europe, 32 Rue Jacques Ibert, 75858 Paris, France. Receive...
0 downloads 0 Views 207KB Size
Download PDF
2026

Langmuir 2003, 19, 2026-2033

Interactions between Sodium Dodecyl Sulfate and Six Nonionic Copolymers Containing 10 Mol % of Different Covalently Bonded Derivatives of Vinyl Acrylic Acid: Electromotive Force and Microcalorimetry Studies Y. Li,† R. Xu,‡ S. Couderc,† S. M. Ghoreishi, J. Warr,§ D. M. Bloor,‡ J. F. Holzwarth,*,† and E. Wyn-Jones*,†,‡ Fritz-Haber Institut der Max-Planck Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Dahlem, Germany, School of Chemical Sciences, Science Research Institute, University of Salford, Salford, M5 4WT, U.K., and Lever Faberge Europe, 32 Rue Jacques Ibert, 75858 Paris, France Received July 8, 2002. In Final Form: November 15, 2002 The binding of sodium dodecyl sulfate (SDS) to six random nonionic copolymers with the general chemical name polyvinyl(methyl imidazole-co-pyrrolidone-co-acrylate) were studied using electromotive force measurements (EMF) and isothermal titration calorimetry (ITC). In terms of their composition expressed in mole percent, each polymer contains 45 mol % methyl vinyl imidazole (MVI), 45 mol % vinyl pyrrolidone (VP), and 10 mol % of each of six different substituted acrylates. The purpose of the work was to investigate how subtle structural changes in the acrylate monomer affect the binding properties of SDS. The results showed significant differences in the binding behavior of the polymers, which are reflected in the determination of critical constants associated with SDS binding, like binding isotherms, the degree of sodium ion association to the bound SDS micelles, and the binding enthalpies as measured by ITC, especially in the early stages of binding. This opens the possibility of using ITC and EMF experiments which effectively measure the binding process as a mean of monitoring and characterizing subtle differences in structurally related macromolecules.

Introduction 1-6

that when neutral It is now generally accepted polymers interact with anionic surfactants such as sodium dodecyl sulfate (SDS), the resulting polymer/surfactant complex is a beadlike structure with segments of the polymer chain in direct contact with the micellar surface. The remaining parts of the polymer form loops or strands, which join other polymer bound surfactant micelles. In general terms the interactions which occur between polymers and surfactants are electrostatic and hydrophobic in nature. The results from many different experiments1-17 show that in terms of molecular recognition of a micellar surface * To whom correspondence should be addressed. Telephone: +49 30 84 13 55 16. Fax: +49 30 84 13 53 85. E-mail: Holzwarth@ fhi-berlin.mpg.de. † Fritz-Haber Institut der Max-Planck Gesellschaft. ‡ University of Salford. § Lever Faberge Europe. (1) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993. (2) Brackman, J. C.; Engberts, J. B. F. N. Che. Soc. Rev. 1993, 22, 85. (3) Polymer-Surfactant Systems; Kwak J. C. T., Ed.; Surfactant Science Series Vol. 77; Marcel Dekker: New York, 1998. (4) Hansson, P.; Lindman, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 604. (5) Linse, P.; Piculell, L.; Hansson, P. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series Vol. 77; Marcel Dekker: New York, 1998; pp 183-238. (6) (a) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2087. (b) WanBadhi, W. A.; Wan-Yunus, W. M. Z.; Bloor, D. M.; Hall, D. G.; WynJones, E. J. Chem. Soc., Faraday Trans. 1993, 89, 2737. (7) Li, Y.; McMillan, C. A.; Bloor, D. M.; Penfold, J.; Warr, J.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 7999. (8) Li, Y.; Xu, R.; Bloor, D. M.; Penfold, J.; Holzwarth, J. F.; WynJones, E. Langmuir 2000, 16, 8677.

the charged headgroups and the area between these headgroups, where parts of the hydrocarbon chains are exposed to water, provide two different binding sites for intermolecular attraction, which allow the micelles to recognize different segments of polymers. Specifically the charged headgroups of SDS offer sites for various types of electrostatic attraction with the oppositely charged polar moieties of the polymer. On the other hand the hydrocarbon parts in contact with water in the area between the headgroups of micelles will recognize hydrophobic parts of polymers, and mutual contact results in a reduction of the overall hydrocarbon contact with water.1-17 If a typical synthetic water-soluble polymer has sufficient flexibility, one can envisage a configuration allowing iondipole association between the positive end of the dipoles of the hydrophilic groups of the polymer and the anionic headgroups of the surfactants as well as contact between the hydrophobic parts of the polymer and the hydrocarbon areas of micelles which are temporarily exposed to water. (9) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 3093. (10) Ghoreishi, S. M.; Li, Y.; Holzwarth, J. F.; Khoshdel, E.; Warr, J.; Bloor, D. M.; Wyn-Jones, E. Langmuir 1999, 15, 1938. (11) Ghoreishi, S. M.; Fox, G. A.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 1999, 15, 5474, and references cited therein. (12) Ghoreishi, S. M.; Li, Y.; Bloor, D. M.; Warr, J.; Wyn-Jones, E. Langmuir 1999, 15, 4380. (13) Li, Y.; Ghoreishi, S. M.; Warr, J.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 1999, 15, 6326. (14) Li, Y.; Xu, R.; Bloor, D. M.; Holzwarth, J. F.; Wyn-Jones, E. Langmuir 2000, 16, 10515. (15) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Wyn-Jones, E.; Holzwarth, J. F. Langmuir 2001, 17, 183. (16) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Holzwarth, J. F.; WynJones, E. Langmuir 2001, 17, 5742. (17) Saito, S.; Anghel, D. F. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Surfactant Science Series Vol. 77; Marcel Dekker: New York, 1998; pp 357-408.

10.1021/la0206155 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/06/2003

Interactions between SDS and Nonionic Copolymers

One of the consequences implicit in a binding process leading to polymer interaction with the micellar surface is the release of bound water from the micellar surface and also the hydrated hydrophobic groups of the polymer and possibly counterions if they are involved. This displacement of water was highlighted by Nagarajan and also by Ruckenstein in theoretical treatments.18a-c In surfactants which contain bulky headgroups, e.g. cationic and nonionic surfactants, these authors assume there is less penetration of water into the area between headgroups and consequently their interaction with polymers should be very weak or almost non-existent. It is now accepted that anionic surfactants bind strongly to most neutral polymers whereas cationic surfactants show only affinity toward polymers which are surface active and contain hydrophobic groups. Attractive interactions between anionic micelles and neutral polymers occur via electrostatic and hydrophobic mechanisms. On the other hand the prime factor which promotes binding between cationic surfactants and surface active polymers is an enhancement of the hydrophobic effect via a reduction of the micelle water surface tension. The details of these interactions are, however, the subject of much speculation1-4,20-23 and are likely to remain so for some time. Overall it has been shown convincingly that sodium dodecyl sulfate has different affinity for different neutral polymers,1-17 and there are both quantitative and semiquantitative indicators to assess the extent of binding. Electrostatic attraction between the charged micellar headgroup and the polymer effectively reduce the headgroup repulsion in the bound micelle and together with the enhancement of the hydrophobic effect contribute to a reduction in the bound micellar free energy, which results in the promotion of polymer bound micelles at lower surfactant concentration than in pure surfactant solutions. Indeed, when binding occurs, the surfactant concentration at the onset of binding, traditionally1 denoted T1(Conset) and often referred to as the critical aggregation concentration (CAC), is used as a marker to measure the strength of the binding; i.e., the lower the value of T1(Conset) the stronger is the binding.1-6 Another useful critical parameter is the maximum capacity of a polymer to bind surfactants which can be measured at the point when the polymer is saturated with bound aggregated surfactant.9-12 The relative extent of binding can be measured by comparing binding isotherms under the same conditions and, in addition, the amount of counterions associated with the polymer bound surfactant aggregates very often expressed as the degree of counterion binding.6b,19 Addition of salt to the system can reduce the micellar headgroup repulsion but will also influence the electrostatic attraction between the negative micellar surface and the polymer. Finally the enthalpy associated with the formation of a polymer/surfactant complex is very sensitive to the nature of the polymer as well as the surfactant and the ionic strength of the solution. For example the sign of the enthalpy change (∆Hi) (negative for attraction and positive for repulsion) can, in favorable circumstances, help to identify the type of charge interaction in surfactant/ (18) (a) Nagarajan, R. Colloid Surf. 1985, 13, 1. (b) Ruckenstein, E.; Huber, G.; Hoffmann, H. Langmuir 1987, 3, 382. (c) Griffiths, P. C.; Roe, J. A.; Jenkins, R. L.; Reeve, J.; Cheung, A. Y. F.; Hall, D. G.; Pitt, A. R.; Howe, A. M. Langmuir 2000, 16, 9983. (19) Palepu, R.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1990, 86, 1535. (20) Chari, K. J. Colloid Interface Sci. 1992, 151, 294. (21) Xia, J. L.; Dubin, P. L.; Kim, Y. S. J. Phys. Chem. 1992, 96, 6805. (22) Wang, Y. L.; Han, B. X.; Yan, H.; Cooke, D. J.; Lu, J. R.; Thomas, R. K. Langmuir 1998, 14, 6054. (23) Purcell, I. P.; Lu, J. R.; Thomas, R. K.; Howe, A. M.; Penfold J. Langmuir 1998, 14, 1637.

Langmuir, Vol. 19, No. 6, 2003 2027 Chart 1. Structure of the Polymer Containing 45 mol % Methyl Vinyl Imidazole (MVI), 45 mol % Vinyl Pyrrolidone (VP), and 10 mol % of Each of Six Different Substituted Acrylates (the Names in Brackets for the Varying Monomers Corresponds to a Methyl Group on the Backbone)

polyelectrolyte systems. In addition a stepwise change of ∆H with addition of surfactant is an indication of a cooperative change, e.g., micelle formation. The amount of surfactant binding information can be obtained from surfactant selective electrodes via the electromotive force (EMF)24,25 and the enthalpy of binding from isothermal titration calorimetry (ITC).7-16 In this investigation, we examined if there are any specific changes in the above binding characteristics of SDS to a nonionic copolymer when the composition of the polymer is changed in a systematic way. The base macromolecule that we used is a 50/50 mol % copolymer of methyl vinyl imidazole (MVI) and vinyl pyrrolidone (VP). To this polymer six different substituted vinyl acrylates (VA) were covalently bonded so that the resulting six copolymers, named polyvinyl(methyl imidazole-co-pyrrolidone-co-acrylate), have the composition MVI/VP/VA in the ratio 45/45/10 mol % (see Chart 1). The structure of these acrylates together with other details of the polymers are listed in Table 1. Since each polymer contains the same amount of the monomers MVI and VP, our main objective was to examine how subtle changes in the third monomer which is present in 10 mol % of the polymer chain affects the binding behavior. As a result of previous binding experiments it has been established that poly(vinyl imidazole),12,13 poly(vinyl pyrrolidone),1,6b,20 and poly(ethylene oxide)1,21,22 interact strongly with SDS. Therefore we have every reason to believe that all the VA monomers in Table 1 will bind SDS. Recently13 we showed that the related copolymer MVI/VP/AA (where AA represents acrylic acid) also binds SDS and the change in the amount of AA in the copolymer can be used to manipulate the binding. In our present case, the EMF of a dodecyl sulfate ion (DS-) selective electrode was measured relative to both a standard Brand a Na+ ion electrode. The results were analyzed to produce binding isotherms which measure the following:6b,19 (i) the amount of SDS bound to the polymer; (ii) the amount of Na+ counterions which are associated with the polymer bound SDS micelles. The ITC method measures the enthalpy per injection, ∆Hi, as SDS is titrated into an aqueous solution with and without the polymer. When binding takes place, the ∆Hi’s are different in comparison to the reference with no polymer. In theory the measured enthalpies are related to the equilibrium concentrations during bindings however, the stage has not yet been reached when full binding isotherms can be evaluated from ITC data.26-29 (24) Davidson, C. S. Ph.D. Thesis, University of Aberdeen, U.K., 1983. (25) Xu, R.; Bloor, D. M. Langmuir 2000, 16, 9555.

2028

Langmuir, Vol. 19, No. 6, 2003

Table 1. Structure and Molecular Mass (MM) of the Vinyl Acrylate Derivative (VA) for MVI/VP/VA Polymers

Li et al.

absence of added salt to ensure that the maximum possible contribution due to the electrostatic attraction between the polymer and SDS is reflected in the experimental parameters monitored. The salt effect is left to another investigation because of its complexity. Experimental Section

On the positive side the ITC method has a distinct advantage over other more conventional methods to study binding in that the magnitude of the measured enthalpies are very sensitive to the binding process. Very small changes in binding enthalpies can be measured at polymer concentrations which are almost an order of magnitude lower than those required by other experimental methods to receive a measurable effect. Nevertheless it should be kept in mind that ITC monitors all changes in ∆Hi associated with the injection of concentrated SDS solutions into a solution with and without the polymer. To compare like with like, we have calculated the molecular mass of each polymer in terms of one “molar unit” containing 45 mol % MVI (molecular mass ) 108), 45 mol % VP (molecular mass ) 111), and 10 mol % of the substituted vinyl acrylates and adjusted the weights (w/v %) of each sample so that they all contain the same molar concentration expressed in terms of this “molar unit”. Because of the different sensitivities of the EMF and ITC methods, the concentration of the polymers used in the ITC experiments is normally less than that used in the EMF measurements, with the exception of MVI/VP/PEG(6000). The measurements have been carried out in the (26) Olofsson, G.; Wang, G. In Polymer Surfactant Systems; Surfactant Science Series, Vol. 77; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; pp 193-238. (27) Blandamer, M. J.; Briggs, B.; Cullis, P. M.; Irlam, K. D.; Engberts, J. B. F. N.; Kevelam, J. J. Chem. Soc., Faraday Trans. 1998, 94, 259. (28) Torn, L. H.; de Keizer, A.; Koopal, L. K.; Lyklema, J. Colloids Surf., A 1999, 160, 237. (29) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276.

Materials. The compositions of the polymers in Table 1 are predetermined with the starting materials in the synthesis and to our knowledge the samples are pure in the sense that they do not contain significant impurities. We have no information available on the distribution of the three monomers along the polymer chains and expect that each polymer molecule contains individual monomers distributed in a random fashion as well as in blocks of various sizes. We also expect that there is an element of polydispersity in both the large EO blocks of the VA unit and the whole polymers and some uncertainty in the exact mole percent composition. The polymers were supplied by BASF and have an approximate molecular mass of 44 000 with an uncertainty of 10%. The SDS was synthesized using the method described by Davidson.24 Both EMF and ITC experiments were carried out in a similar way such that concentrated surfactant solutions containing a known concentration of polymer are titrated into an aqueous solution containing the same amount of polymer. The respective EMF and ∆Hi values are then plotted as a function of surfactant concentration for solutions with and without the polymersthe latter being the control experiment. In both EMF and ITC experiments we ensure that the same concentration of polymer used in the titrated sample is included in the injectant. This guaranteed that during the titrating process the concentration of polymer was always constant. This practice is not always followed by other groups applying ITC,26-29 because it often (especially using neutral polymers) makes little difference to the overall results when using ITC to monitor and compare surfactant binding processes. These matters will be discussed in a later section. All measurements presented here were carried out at 25 °C. EMF Experiments. Monomer surfactant concentrations were measured using a new coated wire SDS selective electrode25 whose EMF is measured relative to a Ag/AgBr electrode. First the EMF of the surfactant electrode relative to the bromide electrode was measured at increasing surfactant concentrations well into the micellar range. At concentrations below the critical micelle concentration (CMC) all the surfactant is in the monomeric form (DS-) and the respective EMF’s yield good Nernstian responses (see Figure 1a,b) according to the following equation:

RT ln(m1) F

EDS-/Br- ) E0DS-/Br- -

(1)

0 where EDS -/Br- is a constant and m1 refers to the monomer concentration of DS-. In eq 1, DS- and Br- have the same charge and the ratio of their activity coefficients may be assumed to be near unity. Typical EMF plots are shown in Figure 1a,b. At the CMC of SDS a sharp minimum is observed because the EMF of the cell monitors the monomer concentration of DS-, m1, which decreases with increasing surfactant concentration in the micellar range. The experiment was then repeated by measuring the relative EMF’s in the presence of a constant amount of each polymer. The existence of a binding process is clearly demonstrated in the EMF experiments shown in Figure 1a,b. In all cases the EMF’s with and without the polymer are initially the same and diverge at T1(Conset), which represents the onset of the binding. As the binding proceeds, the EMF’s remain different before merging again at T2(Csat.), which signals the end of the binding due to the polymer becoming fully saturated with bound SDS. The T1(Conset) and T2(Csat.) values estimated from the EMF and ITC data are listed in Table 2. To compare EMF and ITC results under the same conditions, we performed an ITC experiment for the polymer MVI/VP/PEG(6000) at 2.16% (w/v). The T2(Csat.) values in both cases are very close. In the process of estimating these values, it is clear that pinpointing the exact position of T1(Conset) and T2(Csat.) is not a 100% exact exercise.

Interactions between SDS and Nonionic Copolymers

Langmuir, Vol. 19, No. 6, 2003 2029 m2 is obtained, γ( can then be evaluated from

log γ( )

-AxI 1 + xI

(4)

where I, the ionic strength, is defined as

1 I ) (m1 + C3 + m2) 2

(5)

and A is the Debye-Hu¨ckel constant. In this calculation of an effective ionic strength the concentration of the micelles is not included. Once γ( is known, it is then inserted in eq 3 and a new value of γ( is evaluated, which in turn leads to a new estimate of γ( via eq 4. This procedure is then repeated until γ( and m2 converge. Therefore the values of m1 and m2 become known at each surfactant concentration C1 in the binding region. In Figure 4, a plot of the Na+ binding isotherms expressed as the amount of Na+ ions bound as a function of total SDS concentration for the different polymers is shown. At any surfactant concentration where binding occurs, the following mass balance equation (eq 6) holds

m2 ) m1 + C3 + R(C1 - m1)

Figure 1. Plot of the EMF of the SDS electrode (reference Br-) as a function of total SDS concentration in NaBr (10-4 mol dm-3) for the two copolymers MVI/VP/HEA (a) and MVI/VP/ MPEG(350) (b): (O) pure SDS; ([) SDS + polymer. T1(Conset) represents the onset of binding of SDS on the polymer and T2(Csat.) the saturation of the polymer by micellar SDS. Polymer concentrations as indicated in the figure. T ) 25 °C. In the presence of polymer, binding starts when the EMF of the cell begins to deviate from Nernstian behavior. Once this occurs the monomer concentration of SDS, m1, can be evaluated at each total surfactant concentration, C1, which is known. In the absence of free micelles, the amount of bound surfactant, Γ, is given by:

Γ ) C1 - m1

(2)

Typical binding isotherms in which C1 - m1 (or Γ) is plotted against m1 are shown in Figure 2. As can easily be seen from Figure 2, the SDS monomer concentration is not constant above T1(Conset); it increases until T2(Csat) is reached and decreases above T2(Csat.). In the second EMF experiment, the EMF, EDS-/Na+, of the SDSselective electrode was measured relative to a commercial Na+ ion electrode in the presence and absence of the polymer. The EMF of this cell is given by

2RT ln(γ(m11/2m21/2) F

EDS-/Na+ ) E0DS-/Na+ -

(3)

0 where EDS -/Na+ is a constant, γ( is the mean activity coefficient, and m2 refers to the free counterion concentration. In Figure 3 a typical plot of EDS-/Na+ against (C1C2)1/2 is shown. C1 is the total surfactant concentration, and C2 ) (C1 + C3) the total sodium ion concentration with C3 being the added NaBr concentration. The plot in Figure 3 displays Nernstian behavior until T1(Conset) is reached, after which there is a distinct break. Clearly at surfactant concentrations exceeding T1(Conset) the EMF of this cell monitors γ((m1m2)1/2. When binding occurs, m1 follows from eq 1 and the unknowns in eq 3 are γ( and the free counterion concentration, m2. At each total surfactant concentration these values are evaluated using the following iterative procedure. First, an initial estimate of m2 is obtained by assuming γ( ) 1 and substituting m1 instead of C1 in eq 1. Once an estimate of

(6)

where C3 is the concentration of added salt and R is the degree of micelle dissociation. Isothermal Titration Calorimetry. The microcalorimeter used was the Microcal Omega ITC instrument. In the ITC experiment, one measures directly the energetics (enthalpy changes) associated with processes occurring at constant temperature. Experiments were carried out by first titrating micellar surfactants into water and then into an aqueous solution containing a known amount of polymer. An injection schedule (number of injections, volume of injection, and time between injections) is set up using interactive software, and all data are stored to a hard disk. During and after each injection, the heat released or absorbed as a result of the various processes occurring in solution is monitored for 4 min by the calorimeter. Here, we present the results of the ITC experiments in terms of the enthalpy change per injection (∆Hi) as a function of surfactant concentration. To cover the SDS concentration spanning the whole binding process, four different injection schedules were used. This allowed that sufficient overlap between each schedule was achieved and differences at low SDS concentrations are highlighted. Because the concentration of SDS in the injectant solution and also the number of injections for each schedule are different, it is not possible at present to match the corresponding ∆Hi’s for the different injection schedules in the overlapping regions as shown in Figure 5. The discontinuity in the individual ∆Hi curves at the border of each schedule is an inherent consequence for all ITC data which deal with the binding of surfactants to polymers. Attempts at adjusting the individual ∆Hi’s are not satisfactory, and at present we are not aware of a simple procedure where the data can be normalized in the overlapping range. On the positive side when dealing with surfactant binding data using ITC we have never found such results to be a handicap in dealing with the subsequent binding considerations. For comparison purposes we plotted all the data for each schedule on different graphs, i.e Figures 6-8 and 9. Here one can see that the experiments to measure T1(Conset) show injection conditions identical to the ones for T2(Csat.) evaluation. We now consider the ITC data in Figure 6, which cover SDS concentrations that span the T1(Conset) values found in the EMF experiments (with the exception of HEMA with a very low T1(Conset)). Strictly speaking, if no interaction takes place between SDS and the polymer below T1(Conset), then the corresponding ∆Hi’s with or without the polymer should be the same as indeed are the EMF data from the electrodes in Figures 1 and 3. In practice for most ITC work on polymer/surfactant binding, the ∆Hi’s with and without the polymer in the sample are often different below T1(Conset) and at T1(Conset). However for SDS and other anionic and cationic surfactants, we and others found that the onset of cooperative binding in the ITC experiment is invariably accompanied by a pronounced extremum in the

2030

Langmuir, Vol. 19, No. 6, 2003

Li et al.

Table 2. Binding Parameters of SDS to the Six Different Polymers at 25 °C EMF/ITCb Cpola polymer

(% (w/v))

MVI/VP/PEG(6000)

2.16 0.54 0.4 0.1 0.4 0.1 0.48 0.12 0.336 0.084 0.34 0.085

MVI/VP/PEG(300) MVI/VP/MPEG(350) MVI/VP/MPEG(550) MVI/VP/HEA(100) MVI/VP/HEMA(114)

103T

1 (or Conset) (mol dm-3)

0.5 0.4* 0.05 0.05* 0.20 0.15* 0.3 0.2* 0.05 0.05* 0.02 ∼0.02*

EMF 103T

SDSc

capacityd

bound binding 2 (or Csat.) (mol dm-3) (10-3 mol dm-3) (10-3 mol g-1) 93

90.5

90

ITC capacitye

binding 103T2 [bound SDS]/[polymer unit] (mol dm-3)

4.2

2.9

86

21.5

2.8

60

57

14.3

1.9

80

76.5

15.8

2.5

90

88

26.2

2.8

80

77.5

22.8

2.5

97 60 18 22 22 18 23

CPol is the concentration of the polymer. b The asterisk indicates T1 values from ITC. c Bound SDS ) T2 - m1(10-3 mol dm-3) with m1 being the free monomer SDS concentration at T2 obtained from EMF measurements. d Binding capacity ) (T2 - m1)/Cpol, Cpol being here the polymer concentration in g dm-3; here the binding capacity unit is 10-3 mol of SDS per g of polymer. e Binding capacity ) (T2 - m1)/Cpol, Cpol being here the polymer unit concentration in mol dm-3 (the molecular mass of one unit is the molecular mass of each of the monomers weighed by the mole percent of each monomer. For example, molecular mass of one MVI/VP/ PEG(6000) unit: 0.45 × (108) + 0.45 × (111) + 0.1 × (6000) ) 698.5). a

Figure 2. Binding isotherms showing a plot of the amount of bound SDS (C1 - m1 or Γ), as a function of SDS monomer concentration for the polymers (4) MVI/VP/ PEG(300), (×) MVI/ VP/PEG(6000), (/) MVI/VP/MPEG(350), (O) MVI/ VP/MPEG(550), ([) MVI/VP/HEA, and (0) MVI/VP/HEMA (the dashed line A-B is used to compare the binding of SDS on each polymer). Polymer concentrations are indicated in the diagram. T ) 25 °C.

Figure 3. Plot of the EMF of the SDS electrode (reference Na+), EDS-/Br-, as a function of (C1C2)1/2 for (O) pure SDS and (2) SDS + MVI/VP/MPEG(550). C1 is the total surfactant concentration, and C2 ) (C1 + C3) the total sodium ion concentration with C3 being the added NaBr concentration. Polymer concentration: 0.48% (w/v). T ) 25 °C. ∆Hi.8-17,26-29 If the electrostatic interaction is attractive as in our case, a minimum is observed. The onset of the minimum corresponds to T1(Conset), which can also be found from EMF experiments. In the present ITC experiments (see Figure 6) with

Figure 4. Plot of the Na+ binding isotherm expressed as the amount of Na+ ions bound as a function of total SDS concentration for the polymers (4) MVI/VP/PEG(300), (×) MVI/VP/ PEG(6000), (/) MVI/VP/MPEG(350), (O) MVI/VP/MPEG(550), ([) MVI/VP/HEA, and (0) MVI/VP/HEMA. Polymer concentrations are indicated in the diagram. T ) 25 °C.

Figure 5. Plot of ∆Hi in ITC experiments as a function of total SDS concentration for the system SDS + MVI/VP/HEMA using three injection schedules as indicated [(9) 10 mM SDS, (×)100 mM SDS, (2) 1 M SDS]. Polymer concentration: 0.085% (w/v). T ) 25 °C. the polymer in the sample cell and the injected solutions, full minima of ∆Hi’s are detected for MPEG550, MPEG350, PEG300, and HEA, but we did not observe the full minima in ∆Hi’s after the onset of binding for PEG6000 and HEMA. We observed positive and constant values of ∆Hi for PEG6000 below 0.3 mM SDS (T1(Conset)) in the sample. This effect was due to the dilution of the injected 10 mM SDS- polymer aggregate which resulted

Interactions between SDS and Nonionic Copolymers

Figure 6. Plot of ∆Hi in ITC experiments as a function of total SDS concentration for (4) SDS + MVI/VP/PEG(300), (×) SDS + MVI/VP/PEG(6000), (/) SDS + MVI/VP/MPEG(350), (O) SDS + MVI/VP/MPEG(550), ([) SDS + MVI/VP/HEA, and (0) SDS + MVI/VP/HEMA. Polymer concentrations are indicated in the diagram. A 10 mM SDS/polymer solution is titrated into the samples of polymers at 25 °C.

Figure 7. Plot of ∆Hi in ITC experiments as a function of total SDS concentration for (b) SDS in water, (4) SDS + MVI/VP/ PEG(300), (×) SDS + MVI/VP/PEG(6000), (/) SDS + MVI/VP/ MPEG(350), (O) SDS + MVI/VP/MPEG(550), ([) SDS + MVI/ VP/HEA, and (0) SDS + MVI/VP/HEMA. A 100 mM SDS/ polymer solution is titrated into the samples of polymers at 25 °C. in dissociation of SDS from the polymer. As soon as binding in the sample was initiated, the ∆Hi values decreased and finally became negative, indicating attractive electrostatic interaction. The reason why we cannot observe the onset of binding for HEMA (decreasing ∆Hi at low SDS concentrations) is due to the low T1(Conset) value of 0.02 mM. Experiments at the limits of ITC sensitivity around 0.01 mM SDS (not included in Figure 6) showed that there are less negative values of ∆H indicating T1(Conset) between 0.01 and 0.02 mM SDS. Since T1(Conset) is known to be independent of polymer concentration, we can use the data evaluated from EMF and compare those with the onset of binding in the ITC experiments (see Table 2). In Figure 8, the ∆Hi’s with and without the polymer are substantially different between each individual binding processes and always merge for each polymer at its T2(Csat.) when binding is complete.8-17 Keeping in mind that the estimation of T2(Csat.) is not an exact exercise, the values from ITC in Figure 8 are also summarized in Table 2. The intermediate binding range in Figure 7 just demonstrates that the largest differences in ∆Hi between the polymers occur at low SDS concentrations because of low ionic strength.

Discussion The specific interactions which occur between SDS and the polymer components PVI, PVP, and PEO are primarily electrostatic attraction in the sense that all the polymers bind strongly to SDS whereas the binding is much less

Langmuir, Vol. 19, No. 6, 2003 2031

Figure 8. Plot of ∆Hi in ITC experiments as a function of total SDS concentration for (b) SDS in water, (4) SDS + MVI/VP/ PEG(300), (×) SDS + MVI/VP/PEG(6000), (/) SDS + MVI/VP/ MPEG(350), (O) SDS + MVI/VP/MPEG(550), ([) SDS + MVI/ VP/HEA, and (0) SDS + MVI/VP/HEMA. A 1000 mM SDS/ polymer solution is titrated into the samples of polymers at 25 °C.

Figure 9. Plot of ∆Hi in ITC experiments as a function of total SDS concentration for (4) SDS + MVI/VP/PEG(300), (×) SDS + MVI/VP/PEG(6000), (/) SDS + MVI/VP/MPEG(350), (O) SDS + MVI/VP/MPEG(550), ([) SDS + MVI/VP/HEA, and (0) SDS + MVI/VP/HEMA. A 20 mM SDS/polymer solution is titrated into the samples of polymers at T ) 25 °C. All polymers have the same concentration: 0.336% (w/v). T ) 25 °C.

facile with cationic surfactants, and the latter is almost not existent when compared under the same conditions. The negative values of ∆Hi are a strong indication for this, but the exact details of these interactions are not very well understood. In PVI it is generally accepted that there is a surplus positive charge on the imidazole ring because of its properties as a weak base.13 In PVP both attractive electrostatic and favorable hydrophobic forces have been invoked1,7b,23 although the exact origin of the former is clearly a matter of speculation.20 Finally the system SDS/PEO is the most widely studied polymer/ surfactant system and has maintained the interest of many different investigators for over 30 years. Many studies1,21,22 were performed, but there is still a debate concerning the exact nature of the attraction between PEO and bound SDS micelles. It can be argued that the nature of the driving force is electrostatic attraction in the sense that PEO does not interact with cationic micelles. Several different explanations for the origins of PEO/micellar SDS interactions were proposed21,22 including direct attractive interactions between SDS micelles and the oxonium ion on the PEO chain or that the Na+ counterion coordinates to the PEO providing a site to bind anionic micelles. In

2032

Langmuir, Vol. 19, No. 6, 2003

addition the removal of bound water17 from the micellar surface or bound hydrated counterions is important, or alternatively the EO segment can replace bound hydrated counterions on the micellar surface. In the present work we found that SDS binds strongly to all the polymers investigated and the binding data in Table 2, and the binding isotherms in Figure 2 show that there are significant differences between the binding characteristics of the six polymers. Table 2 gathers the values of T1(Conset), the onset of SDS binding on the polymer, the values of T2(Csat.), the saturation of the polymer by SDS aggregates, and the binding capacity corresponding to the ratio between the polymer bound SDS concentration and the polymer concentration. The T1(Conset) values obtained from EMF measurements (related to the free SDS monomer concentration), and ITC experiments agree well if specific effects occurring in ITC are taken into account. In particular the T1(Conset) values for the PEG300, HEA, and HEMA polymers are very low at 0.05 × 10-5 and 0.02 × 10-5 mol dm-3, respectively; these basically represent the CMC of the polymer bound SDS micelles compared to the value of 8 × 10-3 mol dm-3 measured for pure SDS. Indeed as far as we are aware, these T1(Conset) values are the lowest that have been recorded so far for the interaction between SDS and a nonionic polymer. The T1(Conset) values for the remaining polymers are in the region (0.2-0.5) × 10-3 mol dm-3. The T2(Csat.) values are given for both EMF and ITC data. The concentrations used for ITC experiments were lower due to the higher sensitivity of this method. For comparison purpose, we performed the same SDS titration at the same MVI/VP/PEG(6000) concentration (2.16% (w/v)) for both methods and the values agree well. The other interesting binding parameter is the binding capacity, which allows us to compare the different polymers. We defined two kinds of binding capacities depending on whether the polymer concentration (Cpol) is expressed in g dm-3 or mol dm-3. The differences in the values of binding capacities expressed in moles of SDS per gram of polymer reflect the differences in the molecular mass of the varying monomers during the binding process. The binding capacities in moles of SDS per mole of polymer unit are quite similar for almost all the polymers. We can notice that the binding capacities of the short HEA and HEMA chains are bigger in comparison with the other polymers. The comparison of the binding capacities is not sufficient to classify these polymers. So we decided to compare the binding isotherms for the six polymers represented in Figure 2. There are significant differences shown between the six polymers. For example if we consider the horizontal line A-B in Figure 2, then the total amount of SDS required to produce the same uptake of bound SDS by each polymer increases as we go from left to right. Therefore under these conditions the extent of binding for the polymers in question follows the order:

HEMA > PEG(6000) > HEA > MPEG(350) > MPEG(550) > PEG(300) Although there is some crossover in the binding isotherms at other SDS concentrations, this method of assessing the binding is very instructive for a comparison. We now turn our attention to the counterion data presented in Figures 3 and 4. During these experiments we found that the Na+ ion selective electrode was not performing as well as one would expect under our experimental conditions. Therefore we believe that the binding isotherms from the Na+ ion electrode in Figure 4 can only be regarded as semiquantitative. These

Li et al.

Figure 10. Plot of the degree of micelle dissociation, R, as a function of total SDS concentration in the binding region below T2(Csat.), the saturation of polymer by SDS micelles) for (4) SDS + MVI/VP/PEG(300), (×) SDS + MVI/VP/PEG(6000), (/) SDS + MVI/VP/MPEG(350), (O) SDS + MVI/VP/MPEG(550), ([) SDS + MVI/VP/HEA, and (0) SDS + MVI/VP/HEMA. Polymer concentrations are indicated in the figure. T ) 25 °C.

isotherms are however encouraging as they are sufficiently different for the six polymers to suggest that the composition of the polymer matters in counterion binding. This is not unreasonable since the polymers interact with the micellar surface. Because of the problems with the Na+ electrode and the fact that m1 and m2 are very small numbers at the early stages of binding, we have not used eq 6 to determine the degree of micelle dissociation, R, in this region. We did however use the electrodes to check the value of R for SDS micelles well above its CMC ()8 × 10-3 mol dm-3) and found that R ) 0.22 is in good agreement with previous data. As a result we evaluated R at concentrations approaching T2(Csat.) for the bound SDS micelles on each polymer, and the results are shown in Figure 10. These data provide further evidence that the structure of the polymers influences the R values. Finally we compare the ITC data for the polymers which are displayed as independent plots for each injection schedule in Figures 6-8. It must be emphasized here that we are comparing enthalpies and not free energies, and as such any changes are not necessarily associated with the amount of bound SDS because dilution effects can be involved. The most important aspect concerning ITC is the apparent sensitivity of the technique. At first sight the experimental data look complicated; however the following general features emerge: (1) At a concentration well below T1(Conset) and up to the total added SDS concentrations of 10-3 mol dm-3, there are significant differences between the corresponding enthalpies per injection for each polymer. (2) As the binding process proceeds above 2 mM SDS, the ∆Hi for all the polymers except the MVI/VP/PEG(6000) sample seem to get very close. This behavior is not unreasonable since the early stages of binding involve the formation of polymer bound micelles which in turn means that the polymer must interact with the micellar surface. Thus the differences in the enthalpies for the different polymers are reflecting the attractive electrostatic affinity of SDS for the different polymers. As the binding proceeds, the binding mechanism is then dominated by the growth of the bound SDS micellar aggregates. This is a process which primarily involves the surfactant self-aggregation and as such is expected to produce similar enthalpies under the same conditions. Finally for all the polymers the ∆Hi’s with and without the polymer merge at the T2(Csat.) values listed in Table 2. At this stage, we need to introduce a word of caution with respect to the way polymer concentrations are expressed in binding

Interactions between SDS and Nonionic Copolymers

studies. In Figures 6-8 the polymer concentrations were worked out on the basis that the number of molar units of each polymer, as defined earlier, are equal. This means that on a molar basis the data for MVI/VP/PEG(6000) stand out because of the big poly(ethylene oxide) (PEO) block which in turn means the weight percent (w/v) is 0.54 compared to 0.08-0.1% (w/v) for the other polymers. In past binding studies it has been normal practice to express the concentration of the polymer as weight percent (w/v) and compare data on this weight percent basis. When this is done for the present polymers, the nature of Figure 6 changes significantly as shown in Figure 9. Here all the polymers, with the exception of MVI/VP/HEA and MVI/ VP/HEMA, behave almost alike. This implies that differences in structure for these various copolymers are manifest more clearly in ITC when similar molar concentrations are compared. As stated previously there is sometimes a difference between the ∆Hi’s of corresponding injections with and without each polymer at T1(Conset) and SDS concentrations below T1(Conset); only some of those differences can be explained (see paragraph on ITC). In theory one would expect these ∆Hi values to be close because no binding interaction between SDS and the polymer is taking places the corresponding EMF data clearly show that there is no uptake of SDS by the polymer below T1(Conset). In practice the corresponding ∆Hi’s refer to enthalpies of the same injectant into water and a polymer solution respectively and, when no interaction takes place between SDS and the polymer, the enthalpies should be very similar. It is a fact, however, that these ∆Hi’s can be different at T1(Conset) and below T1(Conset). This is not a new observation7-9,11-15,26,27 in ITC experiments, and indeed differences in other physical parameters below T1(Conset) were observed between SDS/water and SDS/nonionic polymer systems. For example, in a recent publication Sorci and Reed30 used an automated dilution technique to monitor changes in viscosity and light scattering by titrating SDS into poly(vinylpyrrolidone) (PVP). These authors claim that binding interaction occurs between SDS and PVP below T1(Conset). Our extensive measurements on this system6b,8 using electrodes showed that the EMF of the electrode with and without PVP is the same at all concentrations below T1(Conset), showing that no uptake of SDS by the polymer (30) Sorci, G. A.; Reed, W. F. Langmuir 2002, 18, 353.

Langmuir, Vol. 19, No. 6, 2003 2033

is taking place. The only comment we wish to make at present is that the titration techniques applying ITC, viscosity, or light scattering measure the sum of several parameters of a system. These techniques are nonspecific in terms of the various components in solution since they refer to the bulk properties. On the other hand, the SDS selective electrode is specific to the monomer of the dodecyl sulfate anion. We now consider the mechanism of the process leading to the formation of polymer/surfactant complexes. The most plausible explanation is that of Holmberg et al.31 who proposed the formation of micelles at concentrations lower than the CMC of the pure surfactant by assuming that the initial step in the surfactant/polymer association consists of a redistribution of monomeric surfactant with preference for the polymer coil regions over the bulk solution. As a result, the local surfactant concentrations in the coil regions reach values of the order of normal CMC values long before the bulk concentration reaches the CMC. This leads to the formation of micelles which interact with the polymer to further reduce their free energy so that they remain stable in comparison to the less favorable bulk conditions of the solution. Conclusion Important differences occur in the experimental data associated with SDS specific EMF and very sensitive ITC experiments on the binding of SDS to a polymer in which small structural changes were introduced. For example, by carefully choosing the conditions, it is possible to get a handle on the ability of polymers to lower the CMC of bound SDS aggregates and also on the relative binding abilities of the polymers. In addition, at SDS concentrations spanning T1(Conset) to T2(Csat.), there are large changes in interaction enthalpies as reflected in the ITC data. In terms of practical applications this latter observation together with the inherent versatility and sensitivity of ITC means that surfactant binding studies could be developed as a fingerprinting method to differentiate between structurally similar natural products derived from gums exudates which were collected from a family of closely related species of trees. LA0206155 (31) Holmberg, C.; Nilsson, S.; Sundelof, L. O. Langmuir 1997, 13, 1392.