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Kinetics of Adsorption Layer Formation in Solutions of Polyacid/Surfactant Complexes Alexey G. Bykov,† Shi-Yow Lin,† Giuseppe Loglio,‡ Reinhard Miller,§ and Boris A. Noskov*,| Chemical Engineering Department, National Taiwan UniVersity of Science and Technology, 43 Keelung Road, Section 4, Taipei, 106 Taiwan., Dipartimento di Chimica Organica, UniVersita degli Studi di Firenze, Via della Lastruccia 13, 50019 Sesto Fiorentino, Firenze, Italy, MPI fu¨r Kolloid- and Grenzfla¨chenforschung, Forschungcampus Golm, D14476 Golm, Germany, and Chemical Faculty, St. Petersburg State UniVersity, UniVersitetsky pr. 2, 198904 St. Petersburg, Russia ReceiVed: NoVember 28, 2008; ReVised Manuscript ReceiVed: February 10, 2009
The dynamic dilational elasticity of adsorption layers of the complexes between polyacids and dodecyltrimethyl ammonium bromide were measured by the oscillating ring method as a function of surface age and surfactant concentration. The concentration dependencies of surface tension and dilational surface elasticity are similar to the corresponding dependencies of other polyelectrolyte/surfactant solutions. At the same time, the kinetic dependencies of the dynamic surface elasticity differ significantly from the results for other systems and exhibit in respective concentration ranges one or two local maxima. The first maximum and the associated distortion of the harmonical surface tension oscillations are caused by an aggregate formation in the surface layer. The second maximum occurs simultaneously with a multilayer formation at the liquid surface, which probably results in a new relaxation mechanism of surface stresses. The hydrophobic interactions influence the height and position of the second elasticity maximum, which is more pronounced for complexes of DTAB with the more hydrophobic polymethacrylic acid as compared to the poly(acrylic acid), and the surface tension values in the plateau region. Introduction Mixed solutions of polyelectrolytes and oppositely charged surfactants have a lot of different applications, for example, in the tertiary oil recovery and pharmaceutical industry.1,2 For many years the main efforts in the studies of these systems have been directed to the complex formation in the solution’s bulk.1-7 Only recently a few modern techniques have been applied to the adsorption layers of polyelectrolytes/surfactant complexes. For example, ellipsometry, neutron reflectivity, and Brewster angle microscopy allowed the estimation of the surface layer structure.8-10 Almost all obtained results are related to surface properties at or near the equilibrium. Although one usually deals in industry with nonequilibrium systems, the adsorption layer formation of polyelectrolyte/surfactant complexes at the liquid/ gas interface has been investigated to a lesser extent. Recently, measurements of the kinetic dependencies of the dynamic surface elasticity and surface tension have allowed one to obtain new information on conformational transitions in the adsorption layer of the complexes of conventional surfactants with synthetic11-14 and natural polyelectrolytes (polyampholytes).15 A model of the surface structure, which takes into account hydrophobic interactions between different parts of the polyelectrolyte and surfactant molecules, was proposed to explain peculiarities of the dynamic surface properties at low concentrations.11 It was also shown that drastic structural changes occurred in the surface layer of polyelectrolyte/ surfactant solutions in a narrow surfactant concentration range.11-13 These changes were connected with the aggregate formation in the surface layer in agreement with recent ellipsometric * To whom correspondence should be addressed. † National Taiwan University of Science and Technology. ‡ Universita degli Studi di Firenze. § MPI fu¨r Kolloid- and Grenzfla¨chenforschung. | St. Petersburg State University.
results.8,12,16 The appearance of a heterogeneous surface layer structure was accompanied by strong changes of the surface tension oscillations measured by the oscillating drop method.12 The contribution of higher harmonics increased strongly in the range of heterogeneous surface layers. Although the surface visco-elasticity of all investigated polyelectrolyte/surfactant solutions displayed some general features, one could observe significant differences in the absolute values of the dynamic surface elasticity and in the rate of relaxation processes in the surface layer. In this work, the same approach is applied to solutions of the complexes between of dodecyltrimethyl ammonium bromide (DTAB) with polymethacrylic (PMA) and polyacrylic (PAA) acids. The main aim is to trace the influence of hydrophobic interactions on the peculiarities of the visco-elasticity of polyelectrolyte/surfactant adsorption layers and the kinetics of their formation. Note that the two polyacids differ significantly in the number of hydrophobic groups. At pH 9.2, the polyacids behave as strong polyelectrolytes in aqueous solutions, and one can compare the obtained results with the data for previously investigated systems. On the other hand, by neutron reflectometry Zhang et al. have determined recently the adsorption layer thickness of PAA/DTAB solutions as a function of surfactant concentration.17 Another aim consists in a more careful investigation of the distortion of induced harmonical surface tension oscillations and its connection with the formation of aggregates in the surface layer. To this aim the oscillating ring method18 is applied, which ensures more homogeneous surface dilation as compared with the oscillating drop method.19 Experimental Section The oscillating ring method as described by Kokelaar et al.18 was used to measure the dynamic dilational surface elasticity.
10.1021/jp810471y CCC: $40.75 2009 American Chemical Society Published on Web 03/19/2009
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The internal surface of a glass ring was roughened to ensure complete wetting by the solution under investigation. The ring touched the liquid surface inside a Petri glass dish and the ring axis was perpendicular to the liquid surface. The rotational motion of an electromotor was transformed into the translational motion of the ring by a special device. In operation the ring moved up and down along its axis at a low frequency and induced the corresponding motion of the liquid meniscus at the ring surface. This resulted in harmonic oscillations of the liquid surface area inside the ring. The amplitude of surface area oscillations was 7.4% and the frequency about 0.1 Hz in most of the experiments. The corresponding surface tension oscillations were measured by the Wilhelmy plate method using a roughened glass plate. The dynamic surface elasticity was determined from the oscillations of the surface tension γ(t) and surface area A(t). Both periodical functions of time γ(t) ) γ(0) + δγ(t) and A(t) ) A(0) + δA(t) can be represented in a complex form, and the complex surface elasticity ε at the given angular frequency ω can be determined as the ratio
ε(ω) ) εre + iεim ) δy/δ ln A
(1)
The main advantage of the oscillating ring method as compared with other methods consisted in the almost pure dilational deformation and minimization of shear effects.18 DTAB (Sigma-Aldrich) was purified by double recrystallization from an ethanol and ethyl acetate mixture before use. The 35% aqueous solution of sodium salt of poly(acrylic acid) with the molecular weight of 6 × 104 and poly(methacrylic acid) with the molecular weight of 105 (both from Polysciences, Germany) were used as received. Sodium chloride from Merck was purified from organic impurities by heating in an oven up to 800 °C. All solutions were prepared in fresh triple-distilled water. All PAA/DTAB and PMA/DTAB solutions were freshly prepared by mixing the solutions of individual components in water at given concentrations. The pH of aqueous solutions was adjusted to 9.2 by addition of NaOH. All measurements were carried out at 20 ( 0.5 °C. Results and Discussions The measurements of all surface properties were performed at constant NaPAA (cp ) 0.005 wt) and NaPMA (cp ) 0.0063 wt) contents, respectively, corresponding to the same monomer molar concentration of 5.3 × 10-4 M. The dissociation degree of monomers R can be estimated roughly from the dissociation constant of a monomer pKa using the following relation5
pKa ) -pH + 1g
R 1-R
(2)
According to eq 2,the degree of ionization of PAA (pKa 4.2) and PMA (pKa 4.5) are close to unity at pH 9.2. Note that this equation does not take into account two important effects that complicate the precise estimation of the dissociation degree. The Manning condensation of counertions6 decreases, and the second effect caused by surfactant/polyelectrolyte complex formation7 increases the dissociation degree. These two effects influence the dissociation degree in opposite directions. However, PAA and PMA behave as strong polyelectrolytes at pH 9.2 and only the first effect can be important. The concentration dependencies of the surface tension and the real and imaginary components of the dynamic surface
Figure 1. The dependences of the surface tension (circles), real (solid squares), and imaginary (open squares) parts of the dynamic surface elasticity of PAA/DTAB solutions at pH ) 9.2 and at cp ) 0.005 wt % on the surfactant concentration. Lines are guides for the eye.
elasticity measured five hours after the surface formation are shown in Figure 1. Note that this time is not enough to reach the adsorption equilibrium at (c < 3 mM) (cf. kinetic dependencies below). However, longer measurements were difficult due to the influence of solution evaporation. Nevertheless, the obtained results are close to the surface tension data reported earlier and also display two specific points where the concentration dependence changes abruptly.17 Moreover, qualitatively the same concentration dependencies of the surface tension isotherms were also obtained for other polyelectrolyte/surfactant solutions.8,10,20-22 After a drop at c < 0.15 mM, the surface tension reaches a plateau (Figure 1). The DTAB concentration, where the derivative of the surface tension isotherm undergoes a discontinuity, is usually associated with the critical aggregation concentration (CAC).20 However, the reliability of the CAC determination from the surface tension data is under question because for some polyelectrolyte/surfactant solutions measurements with a surfactant selective electrode lead to much lower CAC values.21 The end of the plateau at c ≈ 2 mM can be attributed to the saturation of polyelectrolyte chains by surfactant molecules and the onset of the increase of free surfactant in the solution bulk.22 The surface tension data indicate that one can divide the whole concentration range into the following four zones: c e 0.15 mM where the surface tension decreases from the value of pure water down to about 44 mN/m in the plateau range, the concentration range of the plateau at 0.15 mM < c < 2 mM, the range of further surface tension decrease at 2 mM < c < 15 mM, and the concentration range where the surface tension coincides approximately with the value of micellar DTAB solutions c > 15 mM. The real part of the dynamic surface elasticity also takes different characteristic values in these four concentration zones (Figure 1). The surface elasticity is very high (up to ∼50 mN/ m) at low concentrations and drops to low values at the end of zone 1. The first two points in Figure 1 probably correspond to strong deviations from the equilibrium at the early steps of adsorption. In zone 2, the dynamic surface elasticity is low (e6 mN/m) and goes through a small local maximum in zone 3. Finally, the surface elasticity is close to zero in zone 4 in agreement with the results for pure DTAB solutions. The imaginary part of the dynamic surface elasticity is much smaller than the real part in zone 1 where the adsorption layer is elastic. The two parts become comparable in other zones where the adsorption layer is visco-elastic. The surface elasticity data in Figure 1 resemble the corresponding recent results for some other polyelectrolyte/surfactant systems where the formation of microparticles in the surface layer has been discovered.11-13 This
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Figure 2. The dependence of the surface tension (circles), real (solid squares), and imaginary (open squares) parts of the dynamic surface elasticity of PMA/DTAB solutions at pH ) 9.2 and cp ) 0.0063 wt % on the surfactant concentration. Lines are guides for the eye.
similarity facilitates the interpretation of the data for PAA/DTAB solutions to a certain extent. However, the kinetic dependencies of the dynamic surface elasticity proved to be more complicated than for the earlier studied systems (cf. below) thereby indicating a different mechanism of the adsorption layer formation. These dependencies change strongly at the transitions between the four concentration ranges and will be considered separately in different zones below. The concentration dependencies of the surface tension and dynamic surface elasticity of PMA/DTAB solutions are qualitatively the same as in the case of PAA/DTAB solutions (Figure 2). One can distinguish between the same four concentration ranges with the characteristic surface properties. The transitions between the four zones occur at almost the same concentrations. The main discrepancies between the two systems consist in the absolute values of the dynamic surface elasticity, which is lower for PMA/DTAB solutions in zone 1 but significantly higher in zone 3. The surface pressure in the plateau region (zone 2) is higher for PAA/DTAB solutions. Besides, one can observe at least two local maxima in zone 3 of Figure 3. This is caused by significant deviations from the equilibrium at c < 3 mM and nonmonotonous kinetic dependences of the dynamic surface elasticity in zone 3 (cf. below). Note that the local maxima in zone 3 are almost comparable with the first one in zone 1 (Figure 2). Such behavior has never been observed earlier. Zone 1. The surface tension of pure PAA and PMA solutions at the given concentrations is identical to that of pure water. The surface tension of polyacid/DTAB solutions deviate from the value for pure water already at c < 10-2 mM, that is, at concentrations more than 1 order of magnitude lower than the corresponding concentrations of pure surfactant solutions, thereby indicating the formation of polyelectrolyte/surfactant complexes of high surface activity at extremely low concentrations (Figures 1 and 2). The surface tension decreases monotonically during five hours after the surface formation and does not reach equilibrium values (Figure 3a). The rate of surface tension change is close to that of polydiallyldimethylammonium chloride/sodium dodecyl sulfate (PDAC/SDS) solutions12 and significantly slower than the rate for polyvinylpyridinium chloride/SDS (PVPmCl/SDS) solutions13 in the same concentration range. This discrepancy can be explained if one takes into account that PVPmCl/SDS solutions were prepared at pH 2 and consequently correspond to higher ionic strengths than the polyacid/DTAB solutions at pH 9.2. The electrostatic adsorption barrier is the main cause of the slow adsorption rate of charged macromolecules and surfactant/polyelectrolyte complexes. The
Bykov et al. first adsorbed macromolecules repulse other charged molecules in the solution and thereby slow down the adsorption process. The increase of the solution’s ionic strength results in a partial shielding of electrostatic forces leading to strong acceleration of the adsorption.23,24 The adsorption barrier decreases and the adsorption kinetics becomes determined by the diffusion from the bulk phase to the surface. The adsorption kinetics in Figure 4a,b show that the addition of NaCl leads to the acceleration of surface tension and dynamic surface elasticity changes. In 0.1 M NaCl, the main changes of surface properties correspond to the first few minutes after surface formation and then the surface elasticity remains almost constant, in agreement with the diffusion controlled adsorption mechanism.25 At the same time, the rate of surface tension change is not a monotonous function of surface age (Figure 3a). In the DTAB concentration range from 0.04 to 0.1 mM the adsorption rate first decreases and then increases again after about one hour. Approximately the same effect was also observed for PDAC/ SDS solutions;12 however, the intermediate time range of slow surface tension changes was much longer and led to an apparent local maximum of the surface tension isotherm if the surface age did not exceed approximately four hours. On the other hand, this effect cannot be the consequence of electrostatic repulsive forces23 and indicates that the adsorption process consists of two steps at least.12 It can also serve as an indirect evidence of the adsorption layer heterogeneity that is probably due to the hydrophobic interactions between uncharged polyelectrolyte segments and hydrocarbon tails of the surfactant, between the segments themselves, and probably between the surfactant tails.11 The dynamic surface elasticity increases monotonously to high values (Figure 3b) indicating the formation of a rigid adsorption layer structure, which is characteristic for other polyelectrolyte/surfactant solution in this concentration range.11-13 The surface elasticity drops abruptly with increasing the solution’s ionic strength (Figure 4b) and approaches values typical for solutions of nonionic polymers ( 0.8 mM). The interpretation of the experimental results becomes easier if one takes into account the changes of the shape of surface tension oscillations, which arise simultaneously with the local maxima of the kinetic dependencies (c > 0.15 mM for both systems). The surface tension oscillations at low concentrations (zone 1) and during the initial rise of the surface elasticity for more concentrated solutions (Figure 5b) were almost purely
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Figure 6. Surface tension oscillations for PAA/DTAB solutions at a frequency of 0.075 Hz cp ) 0.005 wt % and c ) 0.004 M after 15 (black curve), 19 (red curve), and 24 min (green curve).
Figure 7. The kinetic dependencies of the real part of the dynamic surface elasticity (squares) and THD (circles) for PAA/DTAB solutions at cp ) 0.005 wt % and c ) 2 × 10-3 (black), 4 × 10-3 (red).
harmonic indicating that the main relaxation processes in the system are in the linear domain. This behavior breaks abruptly when the surface elasticity reaches the maximal value (Figure 6). Although the surface area oscillated always harmonically at a given frequency, the surface tension oscillations contained a significant contribution of higher harmonics and the system became nonlinear in the time range beyond the local maximum of the surface elasticity. Figure 6 shows as an example the consecutive changes of the shape of surface tension oscillations for PAA/DTAB solutions at c ) 4 mM as a function of surface age t and Figure 7 shows the changes of the total harmonic distortion (THD), which characterizes the deviation from pure sinusoidal oscillations28 for two DTAB concentrations. When the surface tension decreased down to about 44 mN/m upon surface compression, it changed only a little after that for a few seconds until a certain moment during the surface expansion when it began to increase again. At t ≈ 24 min the surface tension was almost constant during about half a period of the oscillations. The THD was low at low surface ages when the surface tension did not reach the value of 44 mN/m and abruptly increased after that (Figure 7). The subsequent decrease of the amplitude of surface tension oscillations and consequently of the dynamic surface elasticity decreased the contribution of higher harmonics again and the THD went through a local maximum. The same changes of the surface tension oscillations occurred also for PMA/DTAB solutions but the surface tension stopped to decrease at a different value (47.5 mN/m). These observations show that the curves of the dynamic surface elasticity in Figures 1, 2, and 5b in the range beyond the local maximum depend on
Bykov et al.
Figure 8. Changes of the adsorption layer structure at the transitions from zone 1 to zone 2 and zone 3.
the amplitude of the surface area oscillations. The surface elasticity maximum would become higher, shift to higher concentrations or surface ages at a smaller amplitude of surface area oscillations and the surface elasticity drop can be more abrupt. Direct measurements confirmed this conclusion. Therefore Figures 1, 2, and 5b show only rough approximate values of the dynamic surface elasticity in zone 2. One can also assume that the decrease of the surface area amplitude would lead to a decrease of the effective surface elasticity in zone 2. Significant deviations of the surface tension oscillations from a pure harmonic shape have been also discovered recently by means of the oscillating drop method for solutions of PDMDAAC/SDS in a narrow concentration range where macroscopic aggregates were visible in the bulk solution.12 This effect was explained by the mass exchange between the adsorption layer on one hand and the visible macroscopic aggregates or microparticles discovered by ellipsometry at the surface on the other hand.12 The results for polyacid/DTAB solutions show that large aggregates near the interface are not a precondition of the observed deviations of the surface tension oscillations from the sinusoidal form. Note that some discrepancies between the results of the given work and the earlier study12 can be connected with differences in the experimental techniques, that is, oscillating drop and oscillating ring methods. It is known that the oscillations of the surface in the oscillating drop method are not pure dilational and can also depend on the surface shear viscosity.29 The oscillations of a liquid meniscus inside a glass ring are closer to the model of pure dilational oscillations. Although the transparency of polyacid/DTAB solutions at concentrations corresponding to the distortion of surface tension oscillations exclude the influence of large aggregates, microparticles at the surface can induce the observed effects. The strong drop of the dynamic surface elasticity to lower values with increasing surfactant concentration does really indicate the formation of microparticles in the adsorption layer as was shown by ellipsometry.8,12 This drop exactly corresponds to the beginning of the distortion of surface tension oscillations in the systems under investigation and the obtained results give additional support to the earlier proposed interpretation.12 The surface tension decrease upon compression to the critical value (44 and 47.5 mN/m for PAA/DTAB and PMA/DTAB solutions, correspondingly) induces the formation of aggregates in the adsorption layer. Figure 8 depicts an approximate adsorption layer structure corresponding respectively to zones 1, 2 and 3.
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Figure 9. The kinetic dependencies of the surface tension (a) and real part of the dynamic surface elasticity (b) for PAA/DTAB solutions at cp ) 0.005 wt % and c ) 0.002 (squares), 0.004 (circles), 0.005 (triangles), and 0.0089 M (diamonds).
Figure 10. The kinetic dependencies of the surface tension (a) and real part of the dynamic surface elasticity (b) for PMA/DTAB solutions at cp ) 0.0063 wt % and c ) 0.002 (squares), 0.003 (circles), 0.004 (triangles), and 0.005 M (diamonds).
One can assume that the local surface concentration of the adsorbed complexes does not change at further compression as a result of the mass exchange between the aggregates at the surface and an almost two-dimensional adsorption layer between them. The size and/or the number of aggregates increase at the compression but the local surface concentration and hence the surface tension remains almost constant. The adsorption layer expansion leads to a mass transfer in the opposite direction, and the surface tension does not change until the disappearance of aggregates or until their internal structure hinders their dissolution. In this case, the surface tension begins to increase. The visco-elasticity of the adsorption layer gives additional evidence of relaxation processes with finite rate in the surface layer (Figures 1 and 2). The rate of mass exchange between the microaggregates and the surrounding adsorption layer is probably comparable with the rate of surface expansion and contraction. Another relaxation process is possible in the system if the microparticles in the surface layer can be desorbed at a critical surface tension and adsorbed again upon surface expansion. The obtained results show that the formation of a loose surface structure is not necessary to explain the surface elasticity drop as was assumed for the earlier investigated systems.11,13 The microparticle formation and the rate of mass exchange between the aggregates and adsorption layer have probably the main influence on the dynamic surface properties of polyelectrolyte/surfactant solutions. In this case, the structure of the twodimensional film between the aggregates can change only slightly during the surface elasticity drop. The beginning of the plateau of the surface tension isotherm (Figures 1 and 2) corresponds approximately to the abrupt decrease of the surface
elasticity and thereby can be connected with the formation of aggregates in the surface layer. It is noteworthy that aggregate formation in PAA/DTAB and PMA/DTAB adsorption layers occurs at almost the same DTAB bulk concentration, which is several times lower than the molar concentration of polyacid monomers and corresponds to transparent solutions. This allows us to rule out the possibility of a significant number of aggregates in the bulk phase. Zone 3. The kinetic dependencies of the surface tension are almost monotonous in this concentration range but display a plateau after a strong initial drop (Figures 9a and 10a). Beyond the plateau range the surface tension decreases again approaching the equilibrium values. The length of the plateau decreases with the surfactant concentration. The dynamic surface elasticity as a function of surface age displays two local maxima (Figures 9b and 10b) corresponding approximately to the beginning and the end of the surface tension plateaus (Figures 9a and 10a). To the best of our knowledge these are the first observations of the time dependencies of surface elasticity showing two local maxima for solutions containing complexes between synthetic polyelectrolytes and surfactants. Note that the surface age of 300 min corresponds approximately to the range of the second local maximum of the surface elasticity kinetic dependencies for PMA/DTAB solutions at 1.5 < c < 4 mM. This results in the local maximum of the concentration dependence of the dynamic surface elasticity in this concentration range (Figure 2). The first maximum in the dynamic surface elasticity arises when the surface tension reaches ∼44 mN/m for PAA/DTAB solutions and ∼47.5 mN/m for PMA/DTAB solutions, respectively, and is accompanied by the appearance of nonharmonic
5670 J. Phys. Chem. C, Vol. 113, No. 14, 2009 surface tension oscillations (Figure 7). Therefore it has the same nature as the single maximum in zone 2 and is connected with the microparticle formation when the surface tension reaches a critical value (cf. above). The subsequent regions of approximately constant surface tension and dynamic surface elasticity correspond to a microheterogeneous adsorption layer with a number of aggregates. One can assume that the adsorption of polyelectrolyte/surfactant complexes continues further leading to the increase of the number and/or size of the aggregates at the surface. The aggregates begin to interact and the dynamic surface elasticity increases again during the next step of adsorption (Figures 9b and 10b). Simultaneously, the surface tension oscillations begin to approach a more harmonic form. Note that the application of the neutron reflection method to PAA/DTAB solutions at pH 9.2 discovered that the thickness of the adsorption layer increased almost two times at the transition between zones 2 and 3.17 Zhang et al. explained these results by formation of a multilayer structure.17 These findings allow interpretation of the second maximum in the kinetic dependencies of surface elasticity (Figures 9b and 10b). Some surface aggregates in zone 3 can be displaced into the sublayer at increased concentration and, consequently, the interactions between them (Figure 8). The mass exchange between different layers becomes possible and the dynamic surface elasticity decreases again. The simultaneous conformational transitions in the proximal region of the surface layer lead to a further surface tension decrease from 44 to 42 mN/m for PAA/DTAB solutions and from 47.5 to 43 mN/m for PMA/DTAB solutions. Both maxima shift to lower surface ages with increasing surfactant concentration at the expense of adsorption acceleration. For c > ∼ 5 mM, the applied experimental method allows a detection of the second local maximum only. The obtained results indicate a similarity between the aggregate displacement into the sublayer and the protein displacement from a monolayer studied by Mackie et al.30,31 Although the whole process of the polyelectrolyte/surfactant adsorption film formation is closer to the model of simultaneous protein and surfactant adsorption,32 one can probably describe some its steps by the “orogenic” model of Mackie et al., which assumes the formation of a thicker layer before the complete protein transition from the monolayer into the bulk phase.30,31 The equilibrium multilayer structure corresponds to a dynamic surface elasticity of PMA/DTAB solutions (∼18 mN/m) much higher than for PAA/DTAB solutions (∼7 mN/m). This leads to the higher local maximum in the surface elasticity concentration dependency in zone 3 (Figures 1 and 2). This difference in the surface elasticities is obviously a consequence of different structures. One can speculate that the more hydrophobic PMA leads to stronger hydrophobic interactions between the segments and between the segments and surfactant tails. As a result the PMA/DTAB adsorption layer structure becomes more rigid as compared to PAA/DTAB adsorption layer. This leads to the higher dynamic surface elasticity in zone 3 and lower surface pressure corresponding to the second local maximum for PMA/ DTAB solutions. Zone 4. When the bulk DTAB concentration exceeds the CMC of the pure surfactant solutions the surface properties of mixed PAA/DTAB and PMA/DTAB solutions are determined by the excess of DTAB molecules in the surface layer. In this case the dynamic surface elasticity approaches zero at the applied frequencies and the surface tension coincides with value for surfactant micellar solutions. The same results have been also obtained for earlier investigated systems.11-14
Bykov et al. Conclusion The concentration dependencies of the dynamic surface elasticity of PAA/DTAB and PMA/DTAB solutions agree qualitatively with the corresponding results for the polyelectrolyte/surfactant solutions investigated previously. At the same time a quantitative agreement was not achieved and in some concentration ranges the kinetic dependencies of the dynamic surface elasticity differed significantly from the previous results. The interpretation of the obtained results is possible if one takes into account a microparticle formation in the adsorption layer. It was shown that the mass transfer between microaggregates and the surrounding layer influences strongly the surface tension oscillations and, consequently, the effective dynamic surface elasticity leading to a local maximum of the kinetic dependence. In a narrow concentration range the kinetic dependencies of the dynamic surface elasticity have two local maxima. These features have not been observed earlier for polyelectrolyte/ surfactant solutions. The second local maximum arises when the adsorption layers thickness increases abruptly and can be explained by the increase of interactions between the microparticles in the adsorption layer, and the subsequent formation of the second layer of particles. In this concentration range the hydrophobic interactions in the surface layer influence strongly the dynamic surface elasticity leading to higher values for PMA/ DTAB solutions than for PAA/DTAB solutions. The surface properties of polyacid/DTAB solutions approach the properties of micellar solutions of the pure surfactant at high DTAB concentrations. Acknowledgment. We are grateful to the referee for attracting our attention to refs 30 and 31 and to Mrs. A. Latnikova for the discussion of the surface elasticity data. The work was financially supported by the Russian Foundation of Basic Research (RFFI No. 08-03-00207_a) and the National Taiwan University of Science and Technology (Project NTUST-2007R-05). References and Notes (1) Goddard, E. D.; Ananthapadmanabhan K. P. Interactions of Surfactants with Polymers and Proteins; Chemical Rubber Company Press: Boca Raton, FL, 1993. (2) Goddard, E. D.; Phillips, T. C.; Hannan, R. B. J. Soc. Cosmet. Chem. 1977, 26, 461. (3) Goddard, E. D. J. Am. Oil Chem. Soc. 1994, 71, 1. (4) Kogej, K.; Theunissen, E.; Reynaers, H. Langmuir 2002, 18, 8799. (5) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and Polymers in Aqueous Solutions; Wiley: New York, 1998. (6) Kiefer, J.; Somasundaran, P. Langmuir 1993, 9, 1187. (7) Fundin, J.; Hansson, P.; Brown, W.; Lidegran, I. Macromolecules 1997, 30, 1118. (8) Monteux, C.; Williams, C. E.; Meunier, J.; Anthony, O.; Bergeron, V. Langmuir 2004, 20, 57. (9) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. Langmuir 2002, 18, 4748. (10) Jain, N. J.; Albouy, P. A.; Langevin, D. Langmuir 2003, 19, 5680. (11) Noskov, B. A.; Loglio, G.; Miller, R. J. Phys. Chem. B 2004, 108, 18615. (12) Noskov, B. A.; Grigoriev, D. O.; Lin, S.-Y.; Loglio, G.; Miller, R. Langmuir 2007, 23, 9641. (13) Noskov, B. A.; Bykov, A. G.; Lin, S.-Y.; Loglio, G.; Miller, R. Colloids Surf., A 2008, 322, 71. (14) Noskov, B. A.; Loglio, G.; Lin, S.-Y.; Miller, R. J. Colloid Interface Sci. 2006, 301, 386. (15) Latnikova, A. V.; Lin, S.-Y.; Loglio, G.; Miller, R.; Noskov, B. A. J. Phys. Chem. C 2008, 112, 6126. (16) Monteux, C.; Williams, C. E.; Bergeron, V. Langmuir 2004, 20, 5367. (17) Zhang, J.; Thomas, R. K.; Penfold, J. Soft Matter 2005, 1, 310. (18) Kokelaar, J. J.; Prins, A.; Gee, M. J. Colloid Interface Sci. 1991, 146, 507.
Solutions of Polyacid/Surfactant Complexes (19) Rippner Blomqvist, B.; Ridout, M. J.; Mackie, A. R.; Wa¨rnheim, T.; Claesson, P. M.; Wilde, P. Langmuir 2004, 20, 10150. (20) Penfold, J.; Thomas, R. K.; Taylor, D. J. F. Curr. Opin. Colloid Interface Sci. 2006, 11, 337. (21) Monteux, C.; Llauro, M.; Baigl, D.; Williams, C. E.; Anthony, O.; Bergeron, V. Langmuir 2004, 20, 5358. (22) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. AdV. Colloid Interface Sci. 2007, 132, 69. (23) Cohen-Stuart, M. A.; Hoogendam, C. W.; de Keizer, A. J. Phys.: Condens. Matter 1997, 9, 7767. (24) Noskov, B. A.; Bilibin, A.; Yu.; Lezov, A. V.; Loglio, G.; Filippov, S. K.; Zorin, I. M.; Miller, R. Colloids Surf., A 2007, 298, 115. (25) Noskov, B. A. AdV. Colloid Interface Sci. 1996, 69, 63.
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5671 (26) Noskov, B. A.; Akentiev, A. V.; Bilibin, A. Yu.; Zorin, I. M.; Miller, R. AdV. Colloid Interface Sci. 2003, 104, 245. (27) Noskov, B. A.; Akentiev, A. V.; Loglio, G.; Miller, R. J. Phys. Chem. B 2000, 104, 7923. (28) Loglio, G.; Pandolfini, P.; Miller, R.; Makievski, A. V.; Kraegel, J.; Ravera, F.; Noskov, B. A. Colloids Surf., A 2005, 261, 57. (29) Yeung, A.; Zhang, L. Langmuir 2006, 22, 693. (30) Mackie, A. R.; Gunning, A. P.; Ridout, M. J.; Wilde, P. J.; Patino, J. R. Biomacromolecules 2001, 2, 1001. (31) Mackie, A. R.; Gunning, A. P.; Ridout, M. J.; Wilde, P. J.; Morris, V. J. Langmuir 2001, 17, 6593. (32) Kotsmar, Cs.; Kraegel, J.; Kovalchuk, V. I.; Aksenenko, E. V.; Fainerman, V. B.; Miller, R. J. Phys. Chem. B 2009, 113, 103.
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