Langmuir 1994,10, 57-60
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Fluorescence Probe Studies of Poly(acry1ic acid)-Cationic Surfactant Interactions following High Shear Ling-Siu Choi and Oh-Kil Kim' Naval Research Laboratory, Washington, D.C. 20375-5320 Received March 26,1993. I n Final Form: July 20,1993@ Polarity and rigidity (induced by shear)in the local environment of poly(acry1icacid),PAA, in association with a cationic surfactant (alkyltrimethylammonium bromide) in aqueous solution were studied by fluorescence techniques using pyrene and auramine 0 as probes. The polarity along the PAA is strongly affected by the alkyl tail length of the surfactant. The criticalassociation concentrations(cac's) of polymerbound surfactants are more than 2 orders of magnitude less than their critical micellar concentrations. The local polarity and cac's are also affected by molecular weight and concentration of PAA; higher molecular weight PAA significantly lowers the polarity, leading to low cac's, whereas higher concentration of PAA tends to increase the cac, after passing through a minimum, suggesting a growing influence of interchain aggregation (of the bound surfactants) with increasing PAA concentrations. Chain rigidity resulting from the shear-inducedself-association(throughinterchain H-bonding)of PAA increases noticeably at low surfactant concentrationsbut it decreases sharplyat higher surfactant concentrations. This suggests that the interchain H-bonding of PAA (inducedby shear) is facilitated in a low hydrophobic environment but it undergoes disruption in a highly hydrophobic one. Such behavior is strongly pH dependent with an increased rigidity at a relatively low pH.
Introduction One of the interesting recent topics in polymer solution properties is the hydrophobic microdomain formation1-8 that occurs when ionic polymers undergo electrostatic interactions with oppositely charged surfactants, starting at a concentration far below their critical micelle concentration (cmc). Sych a surfactant binging to the ionic polymers is a highly cooperative process and is most often attributed to the contribution of hydrophobic interaction between bound surfactants. A similar phenomenon also occurs with either ionic or nonionic polymers containing hydrophobic residuesP1l e.g., polysoaps.lJ2 Earlier, we that an ultrahigh molecular weight (UHMW) poly(acry1ic acid), PAA, exhibited an unusual self-association behavior predominantly at pH 6-8 in very dilute aqueous solution (1.9 X 10-4 M) under a high shear rotating disk flow; drag reduction (DR) of the PAA decreased sharply with increasing shear without any turbidity in the s01ution.l~We concluded that this change is not due to chain degradation but a conformational transition resultingfrom intermolecular association through 0 Abstract published in Advance ACS Abstracts, November 15, 1993. (1) Binana-Limbele. W.: Zana. R. Macromolecules 1987.20. 1331. (2jChandar, Prem;'Somasundaran,P.; Turro, N. J. Macromolecules 1988.21.960. --(3) McGlade, M. J.;Randall, F. J.;Tehewekdjian,N. Macromolecules 1987,20,1782. (4) Ab&, E. B.; Scaiano, J. C. J. Am. Chem. SOC.1984,106,6274. (6) Dubin, P.L.; Th6, S.S.; Gan, L. M.; Chew, C. H. Macromolecules 1990,23,2500. (6) Gas, Z.; Wasyliihen, R. E.; Kwak, J. C. T. Macromolecules 1989, 22.2544. --- -- - (7) Skerjanc,J.;Kagej, K.;Vesnaver, G.J.Phys. Chem. 1988,92,6382. (8) Witte, F. M.; Engberts, B. F. N. J. Org. Chem. 1988,53,3085. (9) Tumo, N. J.; Kuo, P. L. hngmuir 1986,2, 438. (10) Wang, Y.; Winnik, M. A. Langmuir 1990,6,1437. (11) Zao, C.-L.; Winnik, M. A.; Riess, G.; Croucher, M. D. Langmuir 1990,6,514. (12) Barbieri, B.; Straw, U. P. Macromolecules 1985, 18, 411. (13) (a) Kim, 0.-K.; Long, T.; Brown, F. Polym. Commun. 1986,27, 71. (b)Kim, 0.-K.;Choi, L. 5.;Long, T.;McGrath, K.;Armistead, J. P.; Yoon, T. H. Macromolecules 1993,26,379. (14) Polymer drag reduction (DR)commonlyincreaseswith increasing shear rate up to a certain point after which DR decreases mainly due to shear degradation. However, in our w e the decreasing DR is not due to shear degradation (see, ref 13b) because the DR recovers completely after addition of minute amounta of salta (see,ref 17).
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H - b ~ n d i n g . l ~This ~ J ~shear-induced association produces a stable supermolecular chain rigidity along the PAA chains that is reflected in the rise of auramine 0 (AuO) fluorescence16 and a sharp decrease in the viscosity.'& While without shearing, no AuO fluorescence is observable in the PAA solution, with shearing fluorescence appears and its intensity increases with increasing shear rate and shearing time up to 2 min, leveling off thereafter. The increase in AuO fluorescence in the sheared PAA solution is inversely proportional to the DR change during the shear.13b This indicates that the decrease in DR of PAA with shearing is closely related to the conformational transition of PAA into a supermolecular bundle of chains.13b Such a change in AuO fluorescence as well as DR remains unchanged for a t least 2 weeks after removal of the shear, indicative of a strong interchain association. However, the PAA association complex undergoes a disruption by the addition of a minute amount of salt to the shearing solution, resulting in the disappearance of AuO fluorescence and a full recovery of DR and the visc0sity.~3 Here we investigate how the PAA self-association induced by shear is affected by the presence of a competitive interaction with cationic Surfactants and how the hydrophobic interaction between the bound surfactants is altered by surfactant chain length, PAA molecular weight, and shear stress. Conformational transition in PAA under shear in the presence of the surfactant was monitored by fluorescence techniques with pyrene and AuO as probes for local polarity18and rigidityl9along the PAA chain, respectively.
Experimental Section Materials and Sample Preparation. An UHMW PAA (M,, = 8.0 X 106) was synthesizedlsb by a conventional free radical polymerization using ammonium persulfate as initiator. T w o (15) Choi, L.-S.; Yoon, T.-H.; Armistead, J. P.; McGrath, K.; Kim, 0.-K. Polym. Prepr. 1989,30 (2), 382. (16) Kim, 0.-K.;Choi, L A . ; Long, - T.; Yoon, T. H.Polym. Commun. 1988,29,168. (17) Kim, 0.-K.;Choi,L.-S. Makromol. Chem.,Makromol.Symp. 1990, 39, 203. (18) Oster, G . J.; Nishijima, Y. J . Am. Chem. SOC.1964, 78, 1581. (19) Thomas, J. K. Ace. Chem. Res. 1977,10,133.
This article not subject to U.S.Copyright. Published 1994 by the American Chemical Society
58 Langmuir, Vol. 10,No. 1, 1994
Choi and Kim
other PAAs were commercialproducts (M,, = 9 X l(r and 6 X 10s; Polysciences, Inc.). Cationic surfactants, alkyltrimethylammonium bromides ((2,-TMAB, where n is the chain length of the alkyl group),pyrene, and AuO were commercialproducts (Aldrich Chemical Co.) that were recrystallized twice from ethanol. A stock solution (1.90X 10-3 unit mol/L (M)of PAA Na salt was prepared by dissolution of the salt in deionized water with gentle stirring for 24h. PAA solutions for studies were made by diluting the stock solution with appropriate volumes of deioniqd water with (or without) the surfactants under gentle stirring for 1 h. A fresh stock solution was alwaysused within a day. The solution pH was adjusted, if necessary, by adding dilute hydrochloric acid or sodium hydroxide. Flow Processing. Shear flow was generated by a rotating disk flowapparatus.1" Aqueous solutionsof PAA (concentrations in the range of (1.9-9.5) X lo-'M,pH 8.0 (a= 0.7)) containing a cationic surfactant (Clo-, C12-, Cld-, or Cle-TMAB) at various concentrations (2X 1o-B to 4 X 106 M)were obtained by mixing a concentrated stock solution of PAA Na salt and surfactant solutions. Approximately 240 mL of the mixed solution was sheared by a rotating disk at a fixed shear rate, 47 s-I(2800rpm, corresponding to the Reynolds number, ca. 1 X 10s) for 4 min. Fluorescence Measurements. Samples for fluorescence measurements were made by mixing 0.2-mL aliquots of stock solutions of the probes with 50 mL of sheared (or unsheared) PAA sample solution with (without) the surfactant to give a fixed probe concentration which was maintained throughout the experiments: [pyrene] = 1.31 X 1O-eM and [AuO] = 7.64 X le7 M. The pyrene stock solution was made by dissolving pyrene in hot water to saturation, cooling it to room temperature (25"C), and filtering; the concentration of pyrene in the solution was M.20 Emission spectra of pyrene determined to be 6.53 X (hd.= 313 nm) and AuO (La= 366 nm) of the sheared (or unsheared) solutions were recorded with a Spex Fluorolog I1 fluorometer. For sheared solutions, samples were removed from the flow system immediately upon the cessation of shearing and mixed with the probe solution and their fluorescence was measured between 1 and 2 h thereafter. This is a valid process for the systems under study, since AuO fluorescence (due to the chain rigidity) of the sheared PAA remained unchanged for 2 weeks after removal of the shear, and fluorescence intensity observed for pyrene-containing systems fluctuated somewhat during the first 1 h after addition of pyrene to the sheared (or unsheared) solution but became stable for the next 24 h. The same procedure was repeated at least twice for each sample and found to be reproducible. The intensity ratios of pyrene (11/Z3) and AuO (Zllo)fluorescenceare used aa a measure of polarity and rigidity in the local environment surrounding the PPA, respectively, where ZIand 1 3 are the intensities of the first (372nm) and the third (383nm) peaks of pyrene fluorescence and Z and ZOare the intensities in the presence and absence of the surfactant, respectively.
Results and Discussion Polarity of Surfactant-Modified PAA. The microenvironmental polarity (probed by pyrene) along the PAA is influenced strongly by alkyl tail length of the bound surfactant due to hydrophobic interaction that undergoes a subtle change by shearing. Some significant influences are also observed with molecular weight and concentration of PAA. 1. Effect of Surfactant Chain Length. When PAA interacts with a cationic surfactant, the hydrophobicity along the PAA (due to the bound surfactants) increases with increasing surfactant concentration, and a t a certain concentration a micelle-like structure begins to form that will be reflected in a sharp decrease in the polarity parameter (IdI3)of pyrene. This results from an increase of pyrene concentration solubilized in the hydrophobic microdomain created by the bound surfactants. Figure 1 shows the decrease in the micropolarity of the modified PAA, depending on the alkyl tail length of the surfactant; ~~
(20) May, W.E.;Wasik, S.P.;Freeman, D.H.Anal. Chem. 1978,50, 997.
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Figure 1. Fluorescence intensity ratio (Z1/Z3) of pyrene as a = 8 X 1Oe) function of surfactant concentration in PAA solution (1.9X lo-'M)with shear (- -) and without shear (-). The surfactant alkyl tails are CIS (01,CU (e),C12 (e),and CIO
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the polarity (reflected by = 1.6) of the PAA alone in solution decreases to 1.4 in the presence of 5 X 10-6 M Cu-TMAB, for example, while the same decrease in polarity requires the C12-TMAB to be 3.5 X 106M. The sharp transitions, defined as the critical association concentration (cac) by taking the inflection point,2JO in the fluorescenceparameter for CIS-and ClgTMAB systems appear at surfactant concentrations, 5.7 X 10-6 and 2.2 X 106 M, respectively, which are far below their cmc's (8.0 X 10-4and 1.3 X le2 M, respectively). A similar tendency has been observed in other polymer-surfactant complex Turro and co-workers reported2 a similar transition in 11/13 when a PAA was mixed with (212-TMAB, in that a cac appeared at around 2 X 10-4 M (without added salt which brings about normally a large increase in cac but pH effect insignificant2), and this is 1 order of magnitude higher than ours (2.2 X 106 M). Such a difference may result from the low concentration (1.9 X 10-4M) and/or high molecular weight (8X 10s) of the PAA in our system compared to the reported system2 (1.06 x le2 M in concentration and 4 X 104 in molecular weight). As illustrated in Figure 1, regardless of shearing, there is a growing tendency of polarity decrease (hydrophobicity) along the PAA chains with increasing surfactant concentrations and with increasing alkyl length (C12, cl4, and C16) of the surfactant tail. However, when a comparison is made among the surfactant systems with respect to the hydrophobicity change due to the shearing, a marked difference appears depending on the surfactant chain length; the microenvironment produced by the Cle- and C14-TMABs under shearing becomes less hydrophobic, more noticeably with the CI~-"I'MAB, while the C12-TMAB becomes more hydrophobic, whereas, Clo-TMAB is not affected by shearing and increasing concentration, suggesting that the hydrophobicity of Clo-alkyl chain is not strong enough to form a micellar association even a t high concentrations. These results can be interpreted to mean that the hydrophobic association of the Cl2-surfactant residues is intensified by shear while with longer alkyl residues ((314- and Cle-TMABs) it seems to be disrupted by the shear stress. With the longer chain surfactants, the onset of the hydrophobic association occurs a t a very low critical association concentration (cac) so that the resulting hydrophobic microdomains are loosely formed (compared to the Cu-TMAB),becoming vulnerable to the
PAA-Cationic Surfactant Interactions
Langmuir, Vol. 10, No. 1, 1994 59
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Figure 2. Molecular weight effect of PAA (1.9 X 10-4 M) on fluroescenceintensity ratio (Z1/I3)of pyrene as a function of ClzTMAB concentration. Molecular weights are ( 0 )6 X 109,(0) 9 X loC, and ( 0 )8 X 108.
shear stress. Thus, the conformational states of the surfactant-modified macromolecules in a high shear flow are determined by the balance of two competing factors, namely,shear stress and molecular association. The effect of the bound surfactants on the shear-inducedlocal rigidity along the PAA chain is discussed later. 2. Effect of P A A Molecular Weight. The molecular weight effect of PAA on the local polarity upon the addition of Cu-TMAB is shown in Figure 2. When compared at a fixed PAA concentration, the decrease in the polarity parameter becomes steeper with increasing molecular weight of PAA, corresponding to a larger decrease of the cac and a lower molar ratio of the surfactant to PAA. This indicates that higher molecular weight PAA, namely, an increasing number of binding sites, facilitates the cooperative interaction with surfactants such that a locally high concentration of bound surfactants results, leading to the predominant intramolecular association. In an analogy, if the PAA molecular weight is extremely low, the cac would approach the surfactant cmc. On the other hand, when the PAA concentration is increased at a fixed surfactant concentration, it is likely to occur that the intramolecular contribution to the surfactant association becomes smaller and, instead, the intermolecular contribution may become appreciable. 3. Effect of P A A Concentration. The concentration effect of PAA was indeed observed as shown in Figure 3. Interestingly, the polarity changes critically depending on the PAA concentration; when the PAA concentration is doubled to 3.8 X 10-4 M from 1.9 X 10-4 M, the polarity parameter, 11/13, at any given concentration of CWTMAB, decreases sharply but it adversely increases as the polymer concentration is further increased. It is difficult to interpret this result based on intramolecular association, because doubling the PAA concentration at a fixed surfactant concentration means statistically a decrease of the bound surfactant density by half. This is an unfavorable condition for the intramolecular association. However, the occurrence of such a significant decrease in the polarity (Figure 3) clearly suggests that the association of bound surfactants is favored by doubling the PAA concentration. The only reasonable interpretation of this is a possible involvement of intermolecular association of
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Figure 3. Concentration effect of PAA (fin= 9 X l(r) on fluorescenceintensity ratio (z~/Is)of pyrene as a function of TMAB concentration. PAA concentrationsare (0)1.9 X l W M, ( 0 )3.8 X 10-4 M, ( 0 )5.7 X lo-' M, and (e) 9.5 X lo-' M.
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Figure 4. Critical association concentration (cac) of C12-TMAB and molar ratio of the surfactant to PAA residue at cac as a function of PAA (M,= 9 x 10.') concentration. the bound surfactants. Figure 4 is a plot based on the cac's (taken as inflection points) of the bound surfatants in Figure 3, which gives a vivid picture of the concentration effect of PAA on cac's of the bound Clz-TMAB and stoichiometry of the Clz-TMAB/PAA complexes at cac. The cac of the bound surfactants increases gradually with increasing PAA concentrations except for an initial drop at a PAA concentration, ca. 4 X 10-4 M. It is therefore anticipated that the cac of the bound surfactant may approach close to 10-4 M when PAA concentration is increased to 1X 10-2 M. However, the cause of the initial drop in the cac is not clear. On the other hand, the molar ratio of Cu-TMABto PAA residue at cac sharply decreases until the PAA concentration reaches around 4 X lo-" M where the cac drop occurs, while it increases gradually with a further increase in the PAA concentration; the molar ratio drops to ca. 0.09 from ca. 0.19 when the PAA concentration is doubled from 1.9 X lo-' to 3.8 X lo-' M. Such a finding that lower surfactant concentration (relative
Choi and Kim
60 Langmuir, Vol. 10, No. 1, 1994 2.0
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Figure 5. Fluorescence intensity ratio (ZlZo) of auramine 0 as a function of surfactant concentration in 1.9 X 104 M PAA = 8 X 1Oe) solution under shear. Surfactant alkyl tails are C16 (01, C I(e), ~ and CIO( 0 )at pH 8.0 and cl6 (e)at pH 6.8. to PAA concentration) gives rise to lowered polarity, namely, an enhancedhydrophobicityalong the PAA,seems to suggest that intermolecular aggregation of the bound surfactants is the dominant interaction over intramolecular association under the condition. When the PAA concentration is further increased beyond that optimum, it is likely to occur that ther aggregation efficiency decreases due to the growing difficulty of mutual accessibility between the bound surfactants so that the aggregation requires a higher surfactant concentration, resulting in a steady increase of the cac and polarity as well. In this regard, the cac drop is an indication of transition in the mode of bound surfactant interactions. Although there is no doubt about the occurrence of such a tendency, a slight error range is unavoidable in determining the cac’s (from the plot of Figure 3) due to the somewhat broad association transition exhibited by the bound surfactant. Hydrophobe-Assisted PAA Rigidity. Shear-induced local rigidity along the PAA chains occurs as a result of a supermolecular self-association of PAA through interchain H-bonding.13b If the rigidity occurs in the surfactantmodified PAA (M,= 8.0 X lo6) under shear, it may be attributed to the same origin. As shown in Figure 5, the relative intensity of rigidity parameter, 1/10,of AuO fluorescence indicates that the shear-induced self-association of the PAA is facilitated at low surfactant concentrations; with 5 X 10-6 M Cl6-TMAB at pH 8.0, the local rigidity along the PAA chain is increased about 50 7% relative to the PAA alone. Such enhancement of the rigidity in the modified-PAA is higher with longer alkyl tail of the bound surfactant. Since the PAA selfassociation is based on interchain H-bonding, it can be said that H-bonding is assisted by the interchain hydrophobic association among the elongated PAA molecules, suggesting an important role of the bound surfactant residues. With a further increase beyond a critical surfactant concentration (where1/10= l.O), referred to as the critical shear deformation concentration (csdc), 1.8 X 106 M for Cl6- and 1.4 X 1o-S M for C12-TMABs (Figure 51, the rigidity of the PAA sharply decreases. It is likely, therefore that the morphological state of the surfactantmodified PAA below the csdc is nearly domain-free, but at concentrations above csdc, microdomains develop through the intra- and intermolecular associations of the bound surfactants. As a result, an ordered bundle of PAA chains is disrupted by the formation of microdomains which act as a steric barrier to the shear-assisted selfassociation. However, the Clo-TAB has insufficient associative force even at a high concentration to interfere with the PAA self-association. Incidentally, this suggests
(a,,
that AuO fluorescence is a valid probe for microenvironmental rigidity but not for hydrophobicity (beyond the CSDC in Figure 5). It is worthwhile to note the pH effect on the rigidity in the sheared PAA modified with Cl6-TMAB, in that at pH 5.8, 1/10 (relative rigidity) increases almost linearly with increasing surfactant concentration, reaching a maximum of ca. 1.1X 10-5 M,while at pH 8.0, it comes to a maximum at a somewhat low surfactant concentration (ca. 6 X 10-6 M). Such differences in the position and the extent of the rigidity maximum seem to be related to the sensitivity of interchain H-bonding of PAA toward the binding activity of the surfactant (to PAA). At pH 8.0 (a = 0.71, for example, there are more binding sites (in PAA) available for the surfactants and, at the same time, this is the best condition for the PAA self-association (without surfactants) by shear. It is therefore, anticipated that these two concurrent interactions are very competitive so that the interchain H-bonding is limited to occur only in a low surfactant concentration range (csdc, the interchain Hbonding is interrupted, mainly due to the steric interference from the association of the bound surfactants, while if the bindings are separated from each other at a distance (at concentrations
