New Perspective on the Cliff Edge Peak in the ... - ACS Publications

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New Perspective on the Cliff Edge Peak in the Surface Tension of Oppositely Charged Polyelectrolyte/Surfactant Mixtures Richard A. Campbell,*,† Anna Angus-Smyth,†,‡ Marianna Yanez Arteta,†,§ Katrin Tonigold,§ Tommy Nylander,§ and Imre Varga*,^ †

Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France, ‡Department of Chemistry, Durham University, South Road, DH1 3LE, United Kingdom, §Department of Physical Chemistry, Lund University, P.O. Box 124, S-221 00 otv€ os Lor and University, Budapest 112, P.O. Box 32, H-1518 Hungary Lund, Sweden, and ^Institute of Chemistry, E€

ABSTRACT We present how dramatically the nonequilibrium nature of an oppositely charged polyelectrolyte/surfactant mixture can affect the interfacial properties. We show for the first time that the cliff edge peak in the surface tension of the poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate system is produced as a direct result of depletion of surface-active material from the bulk solution due to a slow precipitation process in the phase separation region. Simple illustrations are given of how to control the production of the peak, to eliminate the feature for equivalent aged solutions through the use of different sample handling methods, and even to change its characteristics at short surface ages. The potential to tune nonequilibrium, steady-state interfacial properties for such strongly associating systems is clearly demonstrated. We propose that our findings in general may be applicable to a broad range of mixtures containing surfactants and oppositely charged macromolecules such as polymers, proteins, and DNA. SECTION Macromolecules, Soft Matter

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result, the origin of such a feature has generated some interesting recent discussion. Staples et al. identified the cliff edge peak for the poly(diallyldimethylammonium chloride)/sodium dodecyl sulfate (Pdadmac/SDS) system in 2002.11 They noted that the peak coincides with a surfactant concentration range where aggregation and phase separation occur but stated that this could not explain the feature and attributed the discontinuity in the experimental data to competitive formation of bulk complexes that are surface-active and those that are not. Several investigations followed, and three research groups interpreted the Pdadmac/SDS system from different perspectives. Bell et al. formulated12,13 and Penfold et al. illustrated the application14 of an equilibrium thermodynamic model, based on the previous hypothesis of competitive formation of different bulk P/S complexes.11,15 Through studying the adsorption kinetics on an overflowing cylinder, Campbell et al. commented that compared with Bell's model, there is at least one additional species present in the system, namely, macroscopic polymer/surfactant aggregates.16 Noskov et al. interpreted the interfacial properties of solutions in the absence of added inert electrolyte in terms of slow microparticle formation at the interface and suggested that aggregate formation

queous mixtures of oppositely charged polyelectrolytes and surfactants play a major role in a wide range of industrial applications, from drug delivery to detergency to use in everyday foodstuffs.1 Since such applications involve massive quantities of raw materials worldwide, there are strong commercial as well as environmental motivations for the development of more efficient formulations.2,3 Strongly associating polyelectrolyte/surfactant (P/S) mixtures exhibit rich phase behavior in the bulk solution and are known to produce kinetically trapped aggregates upon mixing of the components,4,5 as well as interesting physical properties at interfaces.6 However, many oppositely charged macromolecular systems are used far from equilibrium, and therefore, it is surprising how little work has been carried out on the impact of the sample history on their interfacial properties.7-10 The focus of this Letter is to rationalize the cliff edge peak that has been observed in surface tension data of oppositely charged P/S mixtures in the context of the bulk phase behavior. In solutions of constant bulk polymer concentration, there can be a dramatic rise in the surface tension as the bulk surfactant concentration is increased up to a value close to the point of stoichiometric mixing of the oppositely charged components. Given that there is only a differential relation between the surface tension of a solution, its surface excess, and bulk chemical potential, a direct interpretation of the adsorbed amount cannot be drawn from the observed surface tension values (see Supporting Information for further details). As a

r 2010 American Chemical Society

Received Date: August 18, 2010 Accepted Date: September 23, 2010 Published on Web Date: September 29, 2010

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DOI: 10.1021/jz101179f |J. Phys. Chem. Lett. 2010, 1, 3021–3026

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in solution has only a minor influence on the surface properties, except within a narrow range of compositions where the dynamic surface elasticity is affected.17 Cliff edge peaks or even more modest rises in surface tension isotherms can be found in a range of experimental studies on oppositely charged P/S systems.8,18-24 The feature was also commented on three decades ago for a homopolypeptide/surfactant mixture25 and more recently has been observed in protein/ surfactant26 and DNA/surfactant mixtures,27 which hints at its widespread nature and its relevance to biology as well as soft matter systems. A logical starting point for examining the relationship between the surface properties of Pdadmac/SDS solutions and the slow precipitation process in the bulk solution is samples measured immediately after mixing the polyelectrolyte and surfactant, for which the dispersion of P/S complexes formed is highest (fresh-mixed samples; Figure 1A and B). These solutions are transparent at low bulk SDS concentrations (cSDS), indicating that the extent of aggregation of P/S complexes is low. High turbidity for compositions around charge neutralization of the complexes (see discussion of Figure 2 below) indicates the existence of a dispersion of aggregates in solution immediately after mixing. These observations are typical of the associative phase separation of polyelectrolytes and oppositely charged surfactants.4,8,28-30 The surface tension in the measured range is practically constant at ∼35 mN/m and is almost independent of both the surface age (between 1 and 30 min) and the amount of surfactant in the system, that is, the cliff edge peak in the data is completely missing. These results suggest that in all of these solutions, there is enough surface-active material to quickly form an adsorption layer at the air/water interface regardless of the composition of the complexes and aggregates formed. Next, we consider the extreme opposite bulk state with a minimal degree of dispersion of P/S complexes, where the mixed samples were aged for 3 days while allowing any precipitate formed to settle out of solution (aged-settled samples; Figure 1B and C). For the solution compositions which had exhibited the highest degree of aggregation when fresh-mixed, a two-phase system exists after 3 days, with a clear solution phase and higher density white flocks settled out of solution under gravity. Precipitation occurs over a range of compositions around charge neutralization of the complexes (arrow d in Figure 1 at cSDS = 0.82 mM; see discussion of Figure 2). In this region, the P/S complexes have low electrostatic repulsion resulting from their reduced surface charge density; therefore, they aggregate and precipitate over time due to their lack of colloidal stability.4,28 The cliff edge peak in the surface tension data has now been produced, with its maximum at the low-cSDS edge of the precipitation region (arrow b in Figure 1 at cSDS = 0.55 mM), which is close to but does not match two other compositions of note. First, there is a narrow region at lower cSDS where kinetically trapped, nonequilibrium aggregates formed as a result of high concentration gradients during mixing having long colloidal stability (arrow a in Figure 1 at cSDS = 0.48 mM). Second, there is the point of stoichiometric mixing at higher cSDS where the number of moles of monomer equals the number of moles of dodecyl sulfate ions (arrow c in Figure 1 at cSDS = 0.62 mM).


Figure 1. Panels A, C, and E indicate the turbidity of fresh-mixed (blue regions), aged-settled (red regions), and aged-redispersed (green regions) samples, where a dark color indicates the optical density at wavelength 450 nm, OD450 > 0.4, a light color indicates 0.04 < OD450 < 0.4, and white indicates OD450 < 0.04. Panels B and D show corresponding measurements of the surface tension, γ, for surface ages of 1 (open symbols) and 30 min (closed symbols). The gray shaded areas in panels B and D indicate the compositions where bulk precipitation occurs for the aged solutions. The compositions indicated by arrows are (a) a point on the low-cSDS side of charge neutralization where nonequilibrium aggregates with long colloidal stability form turbid aged solutions, (b) the low-cSDS edge of the precipitation region, (c) the point of stoichiometric mixing, and (d) the point of charge neutralization of the complexes.

In order to see if the precipitation and settling processes which coincide with the cliff edge peak result in marked depletion of surface-active material from the bulk solution, we performed gravimetric analysis. Table 1 shows the amount of precipitate in the two solution compositions marked by arrows a and c (just before and close to the top of the cliff edge peak) prepared with two different sample handling methods (fresh-mixed and aged-settled). In each case, solutions were filtered through a micrometer-scale membrane to reveal a minimum estimation of the amount of material that had precipitated out of the solution phase, which is compared with the maximum amount of solid material that could possibly be generated for the solution. The minimum proportion of Pdadmac and SDS that had been depleted from the agedsettled solution with a high surface tension value close to the

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It is interesting to note that the maximum of the short-lived peak in the surface tension produced after redispersion of surface-active material occurs at a shifted bulk composition compared with that of the cliff edge peak observed for aged-settled solutions. The decrease in surface tension with increasing surface age is also slowest for this sample, which indicates that there is a minimum in the amount of surfaceactive material redispersed for this composition. Figure 2 shows the electrophoretic mobility, uζ, of complexes from fresh-mixed Pdadmac/SDS solutions as a function of cSDS. The variation of uζ(cSDS) follows a similar trend as that in previous studies;4,31 with increasing cSDS, the net positive charge of the complexes decreases, and then, the complexes pass neutrality (uζ = 0) before charge reversal occurs with further increasing cSDS. We infer therefore that redispersion of surface-active material through a light mechanical stress on the settled precipitate is least effective for neutral Pdadmac/ SDS complexes, which have the lowest colloidal stability. The striking nonequilibrium nature of the Pdadmac/SDS system is evident from the very different surface tensions that can be produced for equivalent aged solutions subjected to changed sample handling methods. Our approach allows us to control the surface properties such that we can decide whether or not the cliff edge peak is produced and even change its characteristics at short surface ages. It should be noted that previous explanations of the peak based on the assumption of chemical equilibrium are not consistent with these findings. The hypothesis of competitive formation of different bulk solution complexes by Staples et al.,11 which is formulated in the thermodynamic model by Bell et al.,12-14 assumes that 100% of the polymer and surfactant molecules remain in the solution phase. This assumption is in contradiction with our elimination of the peak simply by applying a light mechanical stress to the settled precipitate of equivalent aged samples, which, together with our striking gravimetric data, highlights the true nonequilibrium nature of the Pdadmac/SDS system. Furthermore, while Noskov et al. revealed the process of microparticle formation at the interface (for solutions without added inert electrolyte which limits scope for a direct comparison with our work here),17 we note that they did not link the slow adsorption in the phase separation region to depletion of surface-active material from solution. Our experimental work has now shown conclusively that there is marked depletion of surface-active material from the solution phase due to slow bulk precipitation, which in turn causes the production of the surface tension peak. With this greater understanding of the underlying reason behind the cliff edge peak, we now turn our attention to two simple tests designed to see if we can control the surface properties of Pdadmac/SDS solutions with respect to the settling and redispersion mechanisms. First, given that the cliff edge feature only occurs after surface-active material has been slowly depleted from the bulk solution, we sought to characterize the time scale of solution ages corresponding to the development of the peak (i.e., the transition of samples from fresh-mixed to agedsettled). Figure 3A shows the surface tension evolution of several samples measured at a bulk composition corresponding to close to the top of the cliff edge peak (cSDS = 0.62 mM)

Figure 2. Electrophoretic mobility, uζ, of complexes in freshmixed Pdadmac/SDS samples as a function of cSDS. Measurements were recorded using the M3-PALS technique. The curve is a guide to the eye only. Further information can be found in the Supporting Information. Table 1. Gravimetric Analysis of Four Pdadmac/SDS Solutionsa methodology

cSDS (mM)

surface tension (mN/m)

dry mass (mg)

minimum total loss (%)

fresh-mixed fresh-mixed

0.48 0.62

35 35

3 24

3 23

aged-settled

0.48

35

31

26

aged-settled

0.62

58

103

85

a

Gravimetric analysis of four Pdadmac/SDS solutions: cSDS = 0.48 mM and 0.62 mM each for fresh-mixed and aged-settled samples. The solutions were filtered through a 1.2 μm cellulose acetate membrane to reveal the minimum proportion of Pdadmac and SDS lost from the solution phase. Further information can be found in the Supporting Information.

top of the cliff edge peak is a striking 85%. The other measurements, all with low surface tension values, registered less than one-third of this amount of solid material. These results show that the cliff edge peak is produced at the low-cSDS edge of the precipitation region after extensive depletion of surface-active material from the solution phase has occurred. To evaluate whether the cliff edge peak is coincident with or a consequence of the slow precipitation process, we redispersed surface-active material back into the solution phase of equivalent aged samples by exerting a light mechanical stress on the settled precipitate (aged-redispersed samples; Figure 1D and E). The turbidity is practically unaffected by the redispersion process, but the peak in the surface tension exhibits dramatic changes compared with that of the aged-settled solutions. A relatively short-lived peak with different characteristics is seen before the surface tension decreases to ∼35 mN/m over a period of 30 min. These observations are consistent with adsorption kinetics that are limited by mass transport, on the assumption that the redispersion process only releases a small fraction of the surfaceactive material back into the solution phase. As we have shown that it is possible to switch off the cliff edge peak simply through liberating surface-active material into the solution phase by agitating the solid material, we conclude that the peak is produced as a direct result of the slow precipitation process, which depletes the solution phase of surface-active material.


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scales simply by varying the amount of surface-active material redispersed into the bulk solution through using a different sample handling method. It is also a vivid reminder of the nonequilibrium nature of the system; with consistent experimental protocols, one may measure reproducible interfacial properties that have reached a reasonable steady state, but in fact, they may or may not be close to the eventual equilibrium state. The equilibrium nature of the system can be confirmed after steady state is reached only if samples of different histories exhibit identical physical properties. To summarize our findings, by systematically controlling the bulk properties of Pdadmac/SDS solutions, we have shown that the cliff edge peak in the surface tension is produced as a direct result of depletion of surface-active material from solution due to a slow precipitation process in the phase separation region. We have outlined for the first time how to influence the production or elimination of the peak for equivalent aged samples simply by changing the sample handling methods used. The high surface tension values indicative of the peak can be produced for solutions aged long enough so that precipitate has settled out of solution; yet, the surface tension can be lowered simply through the redispersion of surface-active material by exerting a light mechanical stress on the settled solid material. The fact that equivalent aged samples can yield such different interfacial properties on practical time scales demonstrates the pronounced nonequilibrium nature of the system. It follows that our findings may apply to a broad range of oppositely charged macromolecular systems that exhibit rich bulk phase behavior in soft matter and biology, such as mixtures of surfactant with polymers, proteins, or DNA.

Figure 3. (A) Kinetic data of the surface tension, γ, of the supernatant of settled samples at different solution ages, (a) fresh, (b) aged 3 h, (c) aged 8 h, (d) aged 1 day, (e) aged 2 days, and (f) aged 3 days. (B) Corresponding data for samples aged for 3 days after different extents of mechanical stress exerted on the settled precipitate, (f) no agitation, (g) 1, (h) 2, (i) 5, and (j) 10 mL of supernatant expelled toward the precipitate and (k) a single flask inversion. Panel A shows the transition from fresh-mixed to aged-settled solutions, and panel B shows the transition from aged-settled to aged-redispersed solutions; therefore, in each panel, the curves marked f are the same data. Solutions were measured at stoiciometric mixing of the components (arrow c in Figure 1) with a bulk composition of 100 ppm Pdadmac, 0.62 mM SDS, and 0.1 M NaCl.

EXPERIMENTAL METHODS Our experimental methodology is based principally on surface tension measurements at room temperature of fresh-mixed, aged-settled, and aged-redispersed solutions, which involved different controlled protocols. All solutions were prepared using a standard mixing6 approach and consisted of 100 mL of 100 ppm Pdadmac, 0.2-4 mM SDS, and 0.1 M NaCl in pure water. For every solution, the solution age is zero the moment that the components are mixed, and the surface age is zero the moment that a fresh surface is created by aspiration. The mixed solutions were transferred at once either to the measuring dish (fresh-mixed samples) or to a volumetric flask for storage. The aged solutions were left to equilibrate for 3 days, and then, either supernatant was transferred to the dish with minimal disturbance to any settled material (aged-settled samples) or the flask was inverted once to redisperse the settled material before solution was transferred to the dish (aged-redispersed samples). In every case, 35 mL of solution was transferred to the dish by a glass pipet, and the solution was cleaned by aspiration for 5 s just before measurements to leave 25 mL. The surface cleaning procedure served to remove trapped aggregates from the surface and to create a fresh adsorption layer, which allowed details of the adsorption kinetics to be revealed. The surface tension values presented with a surface age of 30 min agree to within 10% with examples of steady-state data recorded with

for a range of solution ages ranging from fresh to 3 days. It can be seen that for solutions aged up to at least 8 h, the surface tension reaches low values within a few minutes. The longer the solutions are left to age, the higher the measured surface tension values become, and after a few days, the cliff edge peak is fully formed. This observation is consistent with the production of the peak when the amount of surface-active material in the solution phase drops below a threshold. Second, to gain insight into the redispersion process, we varied the extent of mechanical stress exerted on the settled precipitate of six aged samples for solutions of the same age and bulk composition as those used in the first test above (i.e., the transition of samples from aged-settled to agedredispersed). We applied a gentle and controlled agitation by drawing up a set volume of liquid using an automatic pipet and then injecting the liquid back into the solution a given number of times. Figure 3B shows the surface tension kinetics for different degrees of agitation to the precipitate. The larger the degree of mechanical stress exerted on the solid material, the lower the surface tension of the system at the start of the measurement, and the faster the low surface tension was achieved. This simple demonstration shows that one can control the surface tension value measured on practical time


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a surface age of up to 1 day, assuming that agitation of settled precipitate during sample handling was minimized for the aged-settled solutions. Further information about the experimental methodology used, including details about measurements recorded using UV-vis spectroscopy, electrophoretic mobility, and gravimetric analysis, can be found in the Supporting Information.

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SUPPORTING INFORMATION AVAILABLE Further experimental methods and surface tension information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: campbell@ ill.eu (R.A.C.); [email protected] (I.V.).

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ACKNOWLEDGMENT This work was supported within the 6th and 7th European Community RTD Framework Program by the Marie Curie fellowships SOCON and PE-NANOSTRUCTURES, the Marie Curie European Reintegration Grant, PE-NANOCOMPLEXES (PERG02-GA-2007-2249), the Hungarian Scientific Research Fund OTKA H-07A 74230, the Swedish Foundation for Strategic Research, and the Linnaeus grant “Organizing Molecular Matter” (OMM). I.V. is a Bolyai Janos fellow of the Hungarian Academy of Sciences, which is gratefully acknowledged. We thank Colin Bain for helpful discussions.

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New Perspective on the Cliff Edge Peak in the ... - ACS Publications

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