L a n g m u i r 1995,11, 2312-2313
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Polymer/Surfactant Interactions. The Use of Isothermal Titration Calorimetry and emf Measurements in the Sodium Dodecyl Sulfate/ Poly(N-vinylpyrrolidone)System
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D. M. Bloor,* J. F. Holzwarth,+and E. Wyn-Jones Division of Chemical Sciences, University of Salford, Salford M 5 4WT,U.K.
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Received January 23, 1995. I n Final Form: March 27, 1995
Introduction As a result of their potential applications in controlling the colloidal properties of many formulations of industrial importance, the properties of polymer/surfactant mixtures in aqueous solution have been the subject of many fundamental Most of these studies have utilized a diverse range of experimental techniques and have focused on the binding of the surfactant to the polymer. As pointed out by R ~ b b one , ~ of the most fundamental pieces of information concerning these studies is the binding isotherm, which essentially is a quantitative measure of the extent of interaction between the surfactant and the polymer. Unfortunately this vital information is unavailable for the majority of systems, and the result in many cases has led t o ambiguous conclusions being drawn from more indirect experiments. As a result of the development of versatile surfactant selective electrodes, we have shown that emf studies using these electrodes on polymer surfactant systems can lead to the determination of the binding isotherm and other properties of the ~ y s t e m . ~In - ~principle, the electrode monitors the monomer concentration of surfactant m1 in a surfactant/polymer mixture and when binding of surfactant to polymer takes place the amount of bound surfactant is given by (C1 - ml) a t a total surfactant concentration C1. The binding isotherm is usually displayed as a plot of (C1 - mJC, against ml, where C, is the polymer concentration. Previously reported emf data on the binding of sodium dodecyl sulfate (SDS)to polyW-vinylpyrrolidone) (PVPF7 in NaBr [ l x mol dm-31 is shown in Figure l a . In this diagram, the emf of the SDS electrode (referenced to a bromide electrode) is plotted as a function of SDS concentration in the presence and absence of the polymer. Historically two critical concentrations associated with the binding process can be deduced from this information. The onset of binding corresponds to the surfactant concentration, TI,a t which the emf of the surfactant plus polymer deviates from the emf data for the surfactant alone. As binding proceeds, the two sets of emf data diverge and then eventually merge again a t a surfactant t Fritz-Haber Institut der Max Planck Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany. (1)Goddard, E. D. Colloid Surf: 1986,19,255and references quoted therein. (2) Goddard, E. D. J. SOC.Cosmetic Chem. 1990,41, 23. (3) Robb, I. P.Anionic Surfactants, Surfactant Science Series; LuassesReynders, Ed.; Marcel Dekker: New York, 1981; Vol. 11,Chapter 4, p 109. (4) Painter, D. M.; Bloor, D. M.; Takisawa, N.; Hall, D. G.; WynJones, E. J. Chem. SOC.,Faraday Trans. 1 1988, 84, 2087. ( 5 ) Takisawa, N.; Brown, P.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J . Chem. SOC.,Faraday Trans. 1989, 85, 2099. ( 6 ) Wan-Badhi, W. A,; Wan Yunis, W. M. Z.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1993, 89, 2737. (7) Wan-Badhi, W. A. Ph.D. Thesis, University of Salford, 1993. (8) Hoffman, H.; Haber, G. Colloids Surf. 1989, 40, 181.
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Total SDS Conc. [mM] Figure 1. SDS/PVP [1%w/vl system (298 K) in NaBr l0.l mM1 as a functionoftotal SDSconcentration: (a)surfactantelectrode emf data where the PVP concentrationis (A) 0% w/v or (e)1% w/v; (b) SDS monomer concentration (ml);(c) total enthalpy change (A&) per mole of injectant where the injectant is (A) pure SDS into water or ( 0 )PVP + SDS into PW solution.
concentration T2 after which further addition of SDS gives emf s that are the same. T2 is generally regarded1$2a s the surfactant concentration a t which the polymer is fully saturated with bound surfactant after which any further surfactant that is added form "free" micelles in solution. Whereas this is certainly the explanation for the binding of SDS to PVP it is possible, in certain cases, for "free" micelles to form in solution before the polymer becomes fully saturated with bound s u r f a ~ t a n t . ~This J ~ type of behavior cannot be detected by the surfactant electrode since it basically measures the quantity (C1 - ml) and cannot distinguish whether this quantity refers to bound surfactant or "free" micelles or more likely both forms of aggregated surfactant. In a n attempt to gain a better understanding of the behavior of the solution properties of polymer/surfactant systems especially with respect to the occurrence of various species in solution, we report here our preliminary results involving the assessment of isothermal titration calorimetry (ITC) as a method to study surfactant/polymer interactions. We are aware of only two other previous publications concerning the use of the calorimetric technique in the investigation of a polyelectrolyte/surfactant systemll and also a polymer/nonionic surfactant system.12 In this work we report our ITC measurements for the PVPISDS system and compare the results with our previous electrode data.6,7 (9) Ruckenstein, E.; Nagarajan, R. J. Phys. Chem. 1975, 79, 2622.
(10)Nagarajan, R.; Ruckenstein, E. J. Colloid Interface Sci. 1983, 91, 500. (11)Skerjanc, J.;Kogej, K.; Vesnaver, G. J. Phys. Chem. 1988,92, 6382. (12) Brackman, J. C.; van Os, N. M; Engberts, J. B. F. N. Langmuir 1988, 4, 1266.
0743-7463/95/2411-2312$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 6, 1995 2313
Notes
Experimental Section Materials. The SDSused in this work was synthesised as previously reported6and the PVP (Sigma, MW = 40 000) sample was purified using ultrafiltration techniques. All measurements were made a t 298 K. Isothermal Titration Calorimetry (ITC). The calorimeter used in this work was the Omega ITC instrument (Microcal, Inc.). In the ITC experiment one measures directly the enthalpy changes associated with processes occurring a t constant temperature. Experiments were carried out by injection of a concentrated SDS solution containing PVP into a sample solution containing PVP a t the same concentration as the injectant. An injection schedule (number of injections, volume for injection, and time between injections) is set up using interactive software, and this schedule is automatically carried out with all data stored to disk. After each addition, the rate of energy change per injection is first plotted against injection number, and this is considered as the raw data. For each injection the rate is integrated to give the total enthalpy change per injection (mi)which is plotted against surfactant concentration. In the present work, it is the shape of the enthalpy profile a s a function of total surfactant concentration that we wish to consider in relation to the electrode data of polymer/surfactant interactions. For SDS in PVP (1% w/v) this is shown in Figure ICwhere we have also inserted the position ofthe TI and T2values found from the electrode work. The following features are noteworthy. (1)There is a pronounced maximum in the enthalpy profile at TI which corresponds to the onset of SDSbinding to PVP. Below T 1the measured M i refers to the enthalpy of dissociation of the micelles in the injectant plus a dilution effect. (2) With the addition of more surfactant, at concentrations greater than T1 the enthalpy profile passes through a minimum corresponding to the binding region. (3) T2 occurs a t a minimum M i after which the measured value increases and approaches the values for micellar SDS alone (Figure IC).
In summary, the most important characteristics of the enthalpy profile are a pronounced maximum a t T I and a minimum at T2 (in the binding region) with AHi finally reaching a value similar to that for free micelles. A further hitherto unexplained but previously observed behavior in these system^^-^ is the dependence of the monomer surfactant concentration (ml)with overall surfactant concentration as shown in Figure lb. In the present work ml as measured by the surfactant electrode reaches a maximum at T2. This clearly shows that in the binding region (TI T2) the binding of surfactant in the form of aggregates to the polymer is a cooperative process associated with a n increase in the monomer concentration (ml). On the other hand, a t T2 the polymer becomes fully saturated with bound surfactant aggregates and free micelles are formed. Micellization is a well-known aggregation process, where for ionic surfactants ml decreases with increasing surfactant c~ncentration.~-'Thus the maximum in ml must be associated with the formation of free micelles a t T2. It is also noteworthy that this occurs a t a total SDS concentration for which the minimum in the enthalpy profile corresponds to the onset of free micelle formation (Figure IC). Finally, it must be emphasized that the concentration a t which free micelles form occurs over a narrow range of surfactant concentrations analogous to the CMC of small chain length surfactants. In summary, the ITC method shows great potential in characterizing the critical concentrations occurring in polymer/surfactant interactions. In particular the technique is also suitable for nonionic systems where other methods cannot be used. It allows a n insight into the energetics of the various interactions occurring which will in the future become the subject of further investigations.
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Acknowledgment. We thank the British Council for a research grant awarded under the British-German Academic Research Collaboration Programme. LA9500484
