J Solution Chem (2006) 35:951–968 DOI 10.1007/s10953-006-9041-1 ORIGINAL PAPER
Influence of the Alkyl Tail Length on the Anionic Surfactant-PVP Interaction Anna Maria Tedeschi · Elena Busi · Riccardo Basosi · Luigi Paduano · Gerardino D’Errico
Received: 4 November 2005 / Accepted: 1 January 2006 / Published online: 27 June 2006 C Springer Science+Business Media, Inc. 2006
Abstract The effect of the surfactant tail length on the interaction between sodium alkylsulfates (Cn OS, n = 6, 8, 10, 12) and poly(vinylpyrrolidone) (PVP) in aqueous solution has been investigated by electron paramagnetic resonance (EPR) spectroscopy employing TEMPO-choline (TC) as a spin probe. Experimental evidence show that all of the considered surfactants molecularly interact with PVP. However, the cooperative behavior of the surfactant molecules when self-aggregating onto the polymer strongly increases with the surfactant tail length. In fact, in the case of C6 OS, the TC EPR parameters indicate that surfactant monomers randomly associate with the polymer chain. In the case of C8 OS, formation of surfactant-polymer clusters occurs simultaneously to free micellization. In the case of C12 OS and C10 OS, the nitrogen isotropic hyperfine coupling constant of TC () shows that formation of surfactant-polymer clusters occurs. The correlation time (τ C ) of the nitroxide in the same systems shows that electrostatic repulsion among the clusters, formed on the PVP macromolecules, favors a broadening of the polymer coil and a stiffening of its chain. The average number of surfactant molecules participating in each cluster adsorbed onto the polymer, as determined by fluorescence quenching measurements, is much higher for C12 OS than for C10 OS. Keywords Micellization · Polymer-surfactant interaction · Electron paramagnetic resonance · Spin-probe · Fluorescence quenching 1. Introduction Surfactant-polymer interactions in aqueous solution have become a field of intense research interest in recent decades [1, 2], because of their importance from technological [3–5] as A. M. Tedeschi · L. Paduano · G. D’Errico () Universit`a di Napoli “Federico II”, Dipartimento di Chimica, Via Cintia, I-80126 Napoli, Italy; CSGI (Consorzio per lo Sviluppo dei Sistemi a Grande Interfase) e-mail:
[email protected] E. Busi · R. Basosi Universit`a di Siena, Dipartimento di Chimica, Via Aldo Moro, I-53100 Siena, Italy Springer
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well as basic viewpoints. The scientific interest in surfactant-polymer systems is due to the multiplicity of possible combinations that provide different intermolecular interactions. The nature and the entity of the interactions depend on the molecular structure of the two components. The most evident, and probably the most studied factor influencing the surfactant-polymer interaction, is the presence of net charges, either on the molecules of one component or on both. A large number of papers have been published concerning aqueous mixtures of ionic surfactants and oppositely charged polyelectrolytes [6–10]. The interactions between anionic surfactants and nonionic polymers have also been characterized [11–17]. In particular, PVP is reported to be highly reactive towards all anionic surfactants. In recent years, other factors, less evident but not less effective, have begun to be studied. As an example, some of us have investigated the effect of the charge density distribution of the surfactant headgroup as determined by its molecular structure, using a large number of experimental techniques. It has been found that, whereas sodium decylsulfate [CH3 (CH2 )9 OSO3 Na, C10 OS] interacts with PVP, this is not the case for sodium decylsulfonate [CH3 (CH2 )9 SO3 Na, C10 S] [18–22]. The experimental evidence has been related to the fact that the charge density of the decylsulfonate’s headgroup is partially delocalized on the tail. In contrast, the presence of an oxygen atom bridging the headgroup and the tail of decylsulfate opposes the spreading out of the charge density of the headgroup onto the alkyl chain [23]. In turn, the high charge density on the micellar surface favors the ion-dipole interaction between the headgroups of C10 OS and the N ---- C == O group of the pyrrolidonic rings of PVP, which, together with the hydrophobic interaction, stabilizes the surfactant-polymer association. In order to provide evidence for the role of the hydrophobic interaction on the aggregation behavior of sodium alkylsulfate + PVP mixtures, in the present work we investigate the ternary systems water + Cn OS + PVP, where n = 6, 8, 12. In particular, our focus is on surfactants with short tails that self-aggregate at higher concentrations and provide the opportunity for fundamental analysis of the mechanism for surfactant-polymer interactions. The microscopic characterization of these systems was studied by electron paramagnetic resonance (EPR) spectroscopy. The system water + C10 OS + PVP was analyzed through the same technique in a previous study [19]. EPR spectroscopy requires the introduction, into the system, of a molecular probe, whose spectroscopic parameters are directly related to the properties of the micro-environment in which it is embedded [24–32]. In particular, we have used the molecule 4-(N, N-dimethyl-N-(2-hydroxyethyl)) ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl chloride (TEMPO-choline, TC, see Fig. 1). This probe was previously employed for an experimental investigation of the self-aggregation process of sodium
Fig. 1 The TEMPO-choline molecule
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alkylsulfate [Cn OS, where n = 6, 8, 10] [19]. Because of the positive charge, a fraction of the TC cations condenses onto the aggregates formed by the anionic surfactants, by acting as a counterion, with the NO moiety fitting into the outer shell of the hydrophobic core. The choice of using a spin-probe acting as a counterion to study these systems is due to the fact that surfactant-polymer interactions involve, in many cases, the surface of the surfactant aggregates, i.e., the site where the spin-probe is located. A quantitative analysis of the values assumed by the TC EPR parameters in a given mixture can be done by a method described in a previous work [33], in which the factors affecting the TC EPR spectrum were quantitatively analyzed both experimentally and by an ab initio computational method. Furthermore, the aggregation numbers of the Cn OS micelles and those of the Cn OS-PVP clusters (if present) have been determined by fluorescence quenching measurements, using pyrene as a probe and dodecyl pyridinium chloride as a quencher. This report is organized as follows. First, the experimental results for the binary system water + C12 OS, not investigated by us in our previous works, are briefly presented. The results for the ternary systems water + Cn OS + PVP are then discussed individually for each surfactant. Finally, the results for all the ternary systems are compared, allowing us to analyze in detail the effect of the surfactant alkyl tail length on the sodium alkylsulfates + PVP interaction.
2. Experimental section Pyrene (Sigma product, purity > 99%), TC (Molecular Probes product) and dodecyl pyridinium chloride (Aldrich product, purity > 98%) were used without further purification. Sodium hexylsulfate (C6 OS, 98% purity, Acros product), sodium octylsulfate (C8 OS, 98% purity), sodium decylsulfate (C10 OS, 98% purity), sodium dodecylsulfate (C12 OS, 98% purity) and poly(vinylpyrrolidone) (PVP, average molecular weight 24000 corresponding to an average polymerization degree of 220, 99% purity), all Sigma products, were dried under vacuum in a desiccator containing P2 O5 and were used without further purification. All solutions were prepared by weight. 2.1. EPR measurements For the system water + C12 OS, a stock aqueous solution of TC (1.0 × 10−4 mol·kg−1 ) was prepared by weight and used as a solvent for preparing the samples at various surfactant molalities. This TC concentration is low enough to exclude spin-exchange among the molecular radicals. For the systems water + Cn OS + PVP, an aqueous stock solution containing 1.0 × 10−4 mol·kg−1 TC and 1% w/w polymer was prepared and used as a solvent for preparing samples at various surfactant molalities. The samples were saturated with nitrogen and successively sealed in 1.00 mm i.d. quartz capillaries. EPR spectra were obtained by employing a Bruker ELEXYS e500 X-band spectrometer. The instrument parameters were as follows: modulation amplitude 0.16 G to avoid signal over modulation, time constant 1.28 ms, receiver gain 60 dB and microwave power 2 mW (20 dB) to prevent saturation effects. All measurements were performed at 25 ◦ C. EPR simulations were performed using computer software freely available on the anonymous FTP server at the Illinois EPR Research Center [34], but which was modified by us to include routines for automatic least-squares fitting (Simplex and Levenberg-Marquardt methods). The resulting program for the simulation of the EPR spectra is available upon Springer
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request from the authors. As was well described in previous papers [35, 36], the simulation program takes as input the rigid-limit A and g matrices and a starting vector consisting of the rotational diffusion tensor. The program then constructs the stochastic Liouville superoperator computes the matrix elements of the super-operator and tri-diagonalizes it with the Lanczos algorithm. The tridiagonal matrix is then solved for the stochastic Liouville equation to obtain the simulated spectrum [36]. The simulated and experimental spectra are compared and the parameters are varied until a good fit is obtained. When the quality of the fit is good enough, non-linear least-squares routines are employed to produce the best-fit spectrum. In the case of anisotropic rotational tumbling of the paramagnetic species, the perpendicular and parallel components of the rotational diffusion tensor can be separately optimized. The simulation program also allows the calculation of linear combinations of two simulated spectra that is a useful tool when different radical species are simultaneously present in the system. The simulated and experimental spectra were compared and the input parameters were varied until a good fit was obtained. By this procedure the TC EPR parameters were estimated with a mean error of 0.02 G for and about 10% for τ C . For all the simulated spectra, no anisotropy of TC rotational tumbling was found. 2.2. Fluorescence quenching measurements The Cn OS aggregation numbers both in water and in PVP (1% w/w) aqueous solutions were determined by studying the reduction of the pyrene fluorescence due to the presence of dodecyl pyridinium chloride acting as quencher [37]. The variation of the pyrene fluorescence spectrum was investigated with a Jasco Spectrofluorimeter model FP-750 using an excitation wavelength of 290 nm. For the binary systems water + Cn OS, an aqueous solution that was formed by mixing the surfactant (at a composition well above its cmc), the quencher and pyrene (2 × 10−6 mol·dm−3 ), was initially put into the spectrofluorimetric couvette. Weighed amounts of a solution containing the same components, at the same composition, except without the quencher, were subsequently added with the pyrene fluorescence spectrum being registered after each addition. With the assumptions based on the Tachiya model [38], a simple relation is obtained between the total fluorescence intensity at a fixed wavelength with and without quencher, I and I0 , respectively, and the molar concentration of the quencher, [Q]: I = I0 exp(−[Q]/[M])
(1)
where [M] is the concentration of the micelles. The surfactant stoichiometric concentrations, C0 , and [M] are related by the equation [M] = [C0 − Cmon ]/N M
(2)
where Cmon is the concentration of the free monomer and N M is the mean aggregation number of the micelles. By using Eqs. (1) and (2), and making the reasonable hypothesis that Cmon coincides with the critical micellar concentration (Cmon ∼ = c.m.c.), then N M /(C0 – cmc) is the slope of the straight line obtained by relating ln(I0 /I) as a function of [Q]. Thus, the fit of parameters of Equations (1) and (2) to the experimental ln(I0 /I) values allows the determination of the mean aggregation number, N M . In the case of the ternary systems water + Cn OS + PVP, solutions were prepared by the same procedure employed for the binary systems, but with the further restriction that they have the same PVP weight per cent (1% w/w) in the titrating and titrated solutions. Equations (1) and (2) can also Springer
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Fig. 2 Experimental TC EPR spectra in C12 OS aqueous solutions at 25 ◦ C: (a) m C12 OS = 0.0031 mol·kg−1 ; (b) m C12 OS = 0.0081 mol·kg−1 ; (c) m C12 OS = 0.0280 mol·kg−1
be used to determine the surfactant aggregation number in the surfactant-polymer clusters, NC , by replacing the c.m.c. with the critical association concentration (c.a.c.). Since all the considered solutions were sufficiently dilute, the surfactant molalities were used instead of the molar concentrations. 3. Results and discussion 3.1. Aqueous solutions of sodium dodecylsulfate The TC spectrum was recorded in C12 OS aqueous solutions over the surfactant molality range (0.001 mol·kg−1 < m C12 OS < 0.028 mol·kg−1 ), including the critical micellar concentration (c.m.c. ≈ 0.008 mol·kg−1 , reference 39). Some examples of the TC spectra in C12 OS solutions are shown in Fig. 2. In very dilute solutions (m C12 OS ≤ 0.005 mol·kg−1 ) the spectrum shows a typical three-line signal with narrow line shapes, see Fig. 2a. These spectra can be easily simulated by assuming the presence of only one species, i.e., TC in the aqueous medium. In this concentration range, the and τ C values are almost constant, see Fig. 3a and b. In mixtures at a C12 OS molality well above its c.m.c. (m C12 OS ≥ 0.014 mol·kg−1 ), the TC spectrum also shows a three-line signal with narrow line shapes, which can be easily simulated by assuming that only one species is present in the system, see Fig. 2c. In this concentration range both and τ C tend to assume constant values ( M = 16.08 G and τCM = 1.4 × 10−10 s), see Fig. 3a and b. These values indicate that in these mixtures, TC Springer
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Fig. 3 a Nitrogen hyperfine coupling constant, , of TC in aqueous (•) and micellar ( ) environments in C12 OS aqueous solutions as a function of the C12 OS molality at 25 ◦ C. b Correlation time, τ C , of TC in aqueous () and micellar () environments in C12 OS aqueous solutions as a function of the C12 OS molality at 25 ◦ C. c Percentages of TC in aqueous (♦) and micellar () environments in C12 OS aqueous solutions as a function of C12 OS molality at 25 ◦ C. Solid lines are guides for the eyes
experiences a local environment less polar than in water. Furthermore, the spin-probe motion is much more restricted. Consequently, it is reasonable to assume that, in this concentration range, the species responsible for the EPR spectrum is TC condensed onto the surface of the C12 OS micelles. It is interesting to note that the M value is higher than the value found for TC in a variety of organic solvents including methanol and ethanol [33]. This means that the NO moiety of the micelle-bound TC molecules is still partially hydrated. Also, in mixtures with C12 OS molalities close to its c.m.c. (0.005 < m C12 OS < 0.014 mol·kg−1 ), the TC spectrum apparently shows a three-line signal. However, in this concentration range, the line shape is deformed; this is particularly evident for the line with m I = –1, which is clearly asymmetric, see Fig. 2b. This experimental evidence suggests that the spectrum results from the superimposition of two EPR signals that result from two paramagnetic species being present in the system, i.e., TC ions in the aqueous phase (i.e., a dilute solution of C12 OS monomers) and TC ions bound to C12 OS supramolecular Springer
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aggregates. Consequently, the simulation of the experimental spectra has been performed by generating the simulated spectra of these two species. These were afterwards linearly combined, yielding a spectrum that was then compared with the experimental one. Interestingly, a linear combination of the simulations of TC spectra in very dilute solution and in very concentrated solution (e.g., simulations of spectra a and c in Fig. 2), keeping the relative weight in the linear combination as the only adjustable parameter, did not lead to a good simulation of the experimental spectrum. Consequently, the spectroscopic parameters of the two species were also adjusted through an iterative procedure until good agreement was obtained between the simulated and experimental spectra. An example of this procedure is shown in Fig. 4. In particular, the agreement between the experimental and the simulated spectrum was significantly improved by optimizing the EPR parameters of the TC molecules
Fig. 4 a Experimental TC EPR spectrum in a C12 OS aqueous solution (m = 0.0081 mol·kg−1 ) at 25 ◦ C. b Simulation of the TC EPR spectrum in an aqueous environment. c Simulation of the TC EPR spectrum in a micellar environment. d Simulation of the TC EPR spectrum in a C12 OS aqueous solution (m = 0.0081 mol·kg−1 ), obtained as a combination of spectrum b (40%) and c (60%) Springer
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bound to the C12 OS aggregates. This evidence indicates that these aggregates are not exactly equivalent to the micelles present in the molality range m C12 OS ≥ 0.014 mol·kg−1 . No significant improvement in the spectra simulation was observed by changing the parameters of TC in the aqueous medium. The EPR parameters obtained for the two species are shown in Fig. 3a and b. The weights of the two spectra in the linear combination that lead to a good simulation of the experimental spectrum (see Fig. 4) correspond to the relative fraction of the two TC species in the mixture and are shown in Fig. 3c. It is interesting to observe that the superimposition of two EPR spectra begins at a C12 OS molality lower than the c.m.c. (0.008 mol·kg−1 ). However, at m C12 OS = 0.006 mol·kg−1 , the value of the micellebound species is very high and close to that measured in water, so it seems reasonable to ascribe this signal to TC molecules bound to pre-micellar aggregates rather than to true micelles. At the c.m.c., the value of the micelle-bound TC assumes a value only slightly higher than that measured in concentrated micellar solutions, indicating the formation of micellar aggregates. With a further increase of the surfactant concentration, the value decreases and stabilizes at the M value, suggesting that a progressive enlargement of the micellar aggregates occurs, whose structure increases and becomes less penetrable by water molecules. In the m C12 OS range close to the c.m.c., τ C for the micelle-bound TC molecules seems to decrease. However, these τ C values are not completely reliable because in this concentration range the micelle concentration is of the same order of magnitude as the spin probe concentration, thus making possible the dissolution of more than one spin probe molecule in a single micellar aggregate with consequential spin exchange and broadening of the EPR lines [32]. From a quantitative point of view, the partition of TC between aqueous and micellar pseudo-phases can be described in terms of the distribution coefficient defined as [28, 40]: M nM TC n K dM = W W (3) n TC n where nW is the number of moles of water in the system and nM is the number of moles of micellized surfactant (in our case, on the basis of 1 kg of water, nW = 55.5 mol and n M M = m C12 OS – c.m.c.); n W TC and n TC denote the number of moles of TC in the aqueous and in the micellar medium, respectively, and KdM can be easily computed from the relative W M fraction of the two species, shown in Fig. 3c., considering that the sum n TC = n TC + n TC −4 = 1.0 × 10 mol is the total number of moles of TC in the system. The average value of Kd M obtained for the partitioning of TC in C12 OS micelles (for m C12 OS > 0.008 mol·kg−1 ) is (3.1 ± 1.2) × 105 . It is interesting to compare the behavior of TC in C12 OS solutions to that in the micellar solutions of other sodium alkylsulfates with shorter tail lengths. First of all, in the case of Cn OS (n = 6, 8, 10), the TC EPR spectrum consists of a three-line signal with narrow line shapes over the whole surfactant concentration range with no evidence of spectra superimposition [19]. This experimental evidence indicates that for TC in C12 OS solutions, the chemical exchange rate of the species between the different environments, ν C E , is very low with respect to the separation between their EPR signals, ω. In contrast, in the case of Cn OS (n = 6, 8, 10), the presence of a single signal in the EPR spectrum indicates that ν C E >> ω [41]. In this connection, we tried to obtain quantitative information on the TC exchange kinetics in C12 OS micellar solutions by simulating the experimental spectra using the program described by Rockenbauer and Korecz [42]. This program was used in the past by Rizzi et al. [28], who were able to determine the exchange time of some β-phosphorilated cyclic Springer
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Table 1 Variation of the EPR parameters of TC (1 × 10−4 mol·kg−1 ) due to Cn OS self-aggregation in water and the TC partition coefficient at 25 ◦ C
C6 OSd C8 OSd C10 OSd C12 OS
M /Ga
τCM ·1011 /sb
KdM c
16.14 ± 0.03 16.13 ± 0.03 16.07 ± 0.01 16.08 ± 0.02
10 10 13 14
110 ± 20 390 ± 50 3700 ± 300 (3.1 ± 1.2) × 105
a Nitrogen
isotropic hyperfine constant of TC solubilized in micelles. correlation time in concentrated micellar solutions. coefficient of TC between aqueous and micellar pseudo-phases. d From reference [19]. b TC
c Distribution
aminoxyl radicals between C12 OS micelles and the surrounding aqueous medium. However, at present, we have not succeeded in obtaining reliable values of the exchange time of TC. In particular, some attempts to simulate the spectra by the Rockenbauer program resulted in magnetic and dynamic parameters of TC that, in our opinion, are not completely consistent with a realistic physical description of the system. Indeed, spectra simulations obtained by using the Freed program [34–36], through the procedure discussed above, gave more satisfactory results. In Table 1, the KdM , M and τCM values obtained for TC in C12 OS aqueous solutions are compared with those obtained for shorter Cn OS molecules (n = 6, 8, 10). It appears that KdM strongly increases with the value of n. The TC-micelle interaction is mainly determined by the electrostatic interaction between the TC ammonium group and the anionic micellar surface [33]. The charge density of this surface increases with the surfactant tail length, resulting in a larger fraction of the counterions being condensed on it and at a slower chemical exchange rate for TC between the aqueous and micellar environments. However, the concurrent decrement of M and increment of τCM are very limited, suggesting that the properties of the solubilization site in the considered systems are only slightly affected. 3.2. Alkylsulfate-poly(vinylpyrrolidone) aqueous mixtures In all the water + Cn OS + PVP(1% w/w) mixtures, the TC spectrum shows a three-line signal with narrow line shapes, which can be easily simulated by assuming that only one species is present in the system. The and τ C values of TC in the mixtures of water + C6 OS + PVP, determined as a function of the surfactant molality, are shown in Fig. 5a. In these curves only one break-point is observed, corresponding to the onset of surfactant micellization, which occurs at a C6 OS molality equal to the c.m.c. value in the binary system (0.04 mol·kg−1 ). The main effect of PVP can be detected in the premicellar composition range; in fact, although the and τ C values are constant for the binary system, in the ternary system the values of slightly decrease and those of τ C increase. This evidence suggests that some random interactions occur between the surfactant monomers and the polymer chains. This enhances the hydrophobic and the electrolyte behavior of the polymer, thus favoring the interaction with TC. Figure 5a also shows that the τ C trend for the ternary system stabilizes at a higher value above the c.m.c. with respect to the trend observed for the binary system, because of the viscosity increase in the system due to the presence of the polymer chains. Also for the water + C8 OS + PVP system, the and τ C values of TC, plotted as a function of the surfactant molality, exhibit only one break-point corresponding to the onset of surfactant micellization, see Fig. 5b. However, in the presence of PVP, the C8 OS micellization Springer
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Fig. 5 The nitrogen hyperfine coupling constant, , and correlation time, τ C , of TC in: a C6 OS-PVP, and b C8 OS-PVP (1% w/w) aqueous solutions as a function of the surfactant molality at 25 ◦ C. Dashed lines are guides for the eyes. The continuous lines show the trend of the EPR parameters in the absence of PVP (taken from [19])
occurs at a molality slightly lower than the c.m.c. value in the binary system. Overall, in this system no clear evidence can be detected for surfactant-polymer cluster formation. However, the presence of the polymer seems to enhance the surfactant micellization. The system water + C10 OS + PVP was studied in a previous work [19]. The results are qualitatively similar to those discussed just below for the system water + C12 OS + PVP. The and τ C values of TC for the system, water + C12 OS + PVP, plotted as a function of the surfactant molality, are shown in Fig. 6. The trend clearly shows two break-points: the first slope change is due to the onset of the formation of surfactantpolymer aggregates (critical aggregation concentration, c.a.c. ≈ 0.002 mol·kg−1 ), whereas the second slope change occurs at the surfactant molality at which free micelles form (c2 ≈ 0.020 mol·kg−1 ). This trend provides clear evidence for surfactant-polymer interactions. In the concentration range of c.a.c. < m C12 OS < c2 , the TC partitions between the aqueous pseudo-phase and the clusters. This process can be described in terms of a distribution coefficient defined as:
K dC Springer
n CTC n C = W W n TC n
(4)
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Fig. 6 a The nitrogen hyperfine coupling constant, , and b the correlation time, τ C , of TC in C12 OSPVP (1% w/w) aqueous solutions as a function of C12 OS molality at 25 ◦ C. In the case of , the continuous line represents the fitting of the parameters of Eqs. (5) and (6) to the experimental data; in the case of τ C the line is only a guide for the eyes
where n CTC denotes the number of moles of TC solubilized in the clusters and nC is the number of moles of surfactant participating in the clusters (in our case, nC = m C12 OS – c.a.c.). The KdC value, as well as that of the TC nitrogen coupling constant in the clusters, C , can be determined by fitting the parameters of the relation [19] =
(C − W )K dC K dC +
nW nC
+ W
(5)
using the experimental data for molality range, c.a.c. < m C12 OS < c2 . Here W is the TC coupling constant in water (W = 16.84 G). By this procedure, we obtained the values KdC = (1.1 ± 0.2) × 105 and C = 16.76 ± 0.02 G. In the surfactant molality range, m C12 OS > c2 , the following relation, which describes the TC partitioning among water, clusters and micelles, represents the data well [19]:
nW = 1 − TC n TC
nM M nC M Kd M n K dC n C
C + KKddCM 1+
+
W n TC W n TC
(6) Springer
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where W n TC nW = W C n TC n + n K dC + n M K dM
(7)
By assuming that nC = c2 – c.a.c. and n M = m C12 OS – c2 , and using the KdC and C values previously estimated, we obtain Kd M = (2.7 ± 0.3) × 105 and M = 16.04 ± 0.02 G. It is worth noting that the values of KdM and M seem to be unaffected, i.e., free micelles are unperturbed by the presence of the polymer. Our results show that the value of C is higher than M , indicating that TC experiences a more polar environment in the C12 OS + PVP clusters than in the C12 OS micelles. In contrast, KdC is of the same order of magnitude as KdM , showing that the electrostatic interaction between TC and the aggregate surface, which is the driving force of the solubilization process, is similar for both the clusters and the micelles. It is interesting to note that in the system, water + C12 OS + PVP, the experimental spectra do not show any superposition of signals due to TC species being embedded in different solubilization sites. As discussed above, this suggests that the exchange rate of TC among aqueous solution, surfactant-polymer clusters and surfactant micelles is faster than the separation between their EPR signals [41]. This result is reasonable in the case of the clusters, which are smaller and looser than the micelles. However, it is quite surprising in the case of micelles since, as discussed in the previous section, slow exchange was found to take place between the aqueous solution and micelles for the binary system, water + C12 OS. One hypothesis is that spectra superposition in the ternary system becomes less detectable in the presence of the clusters, provided that the value of C is intermediate between W and M . If this is true, then the use of Eq. (6), which is based on fast exchange [19], would be only a realistic approximation. In this connection, it is worth noting that the spectra simulated by assuming that only one species is present in the system, using averaged EPR parameters, are in very good agreement with the experimental ones. This evidence, combined with the fair agreement between the KdM and M values obtained for the ternary system with those obtained for the binary system, for which two superposed spectra were separately simulated, leads us to conclude that the results obtained by using Eq. (6) for the ternary system are sufficiently reliable. In Fig. 6b, the τ C values of TC are plotted versus the C12 OS molality. Their trend shows only an abrupt slope change corresponding to the c.a.c.. In fact, in the composition range, c.a.c. < m C12 OS < c2 , τ C , in contrast to , does not tend towards a plateau but continuously increases. This evidence indicates that the motion of the clusters to which TC molecules are bound is increasingly reduced by a gradual broadening of the polymer coil and by a stiffening of its flexible chain due to the electrostatic repulsions between the clusters formed on the same macromolecule. From this point of view, adding C12 OS to PVP is similar to electrostatically charging of a neutral polymer [43]. With further increases of m C12 OS , a maximum is reached above which the τ C value slightly decreases and stabilizes at τCM . This decrease is due to the shift of TC from the surfactantpolymer clusters to the micelles and shows that the motion of the spin-probe in the clusters is slower than in the micelles, because the polymer reduces the dynamics of the aggregate. The surfactant aggregation numbers in the ternary mixtures, water + Cn OS + PVP, determined by the fluorescence quenching approach, are summarized in Table 2, where they are compared with those obtained for the binary mixtures, water + Cn OS, preliminary determined. In the case of C6 OS and C8 OS, the surfactant aggregation number in the presence of PVP (1% w/w) was measured by the fluorescence quenching method, at a Springer
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Table 2 Critical compositions of Cn OS in water and in PVP aqueous solutions (1% w/w), surfactant aggregation numbers and surfactant-polymer association factor [51] at 25 ◦ C
C6 OS C6 OS-PVP C8 OS C8 OS-PVP C10 OS C10 OS-PVP C12 OS C12 OS-PVP
c.m.c./mol·kg−1a
N Mb
0.4 0.4 0.08
12 ± 3 12 ± 4 34 ± 3 28 ± 5 50 ± 5 47 ± 5 65 ± 4 62 ± 5
0.03 0.008
c.a.c./mol·kg−1c
c2 /mol·kg−1d
NCe
Af
−
−
−
−
0.055g
−
−
0.65
0.012
0.040
10 ± 3
0.40
0.002
0.020
30 ± 6
0.25
a Critical
micellar concentration. aggregation number in free micelles. aggregation concentration. d Concentration at which free surfactant micelles form in the presence of the polymer. e Surfactant aggregation number in surfactant-polymer clusters. f Surfactant-polymer association factor [51]. g For discussion whether this is value is a c.a.c. or a c.m.c., see the text. b Surfactant c Critical
surfactant concentration equal to three times the c.m.c. In the case of C10 OS and C12 OS, the measurements were performed at two surfactant concentrations: the first is slightly below c2 , i.e., a concentration at which only surfactant-polymer clusters are present, so that NC is determined; the second at four times c2 , i.e., a concentration at which surfactant-polymer clusters are negligible and the obtained values are the aggregation numbers of the free micelles, N M . For the binary systems, the Cn OS aggregation number, N M , was measured at a surfactant concentration equal to three times the c.m.c. Inspection of Table 2 shows that, for all considered surfactants, N M is almost unperturbed by the presence of the polymer. For both C10 OS and C12 OS, NC is significantly lower than the aggregation number of free micelles. 3.3. Effect of surfactant tail length on the Cn OS -PVP interaction Various literature papers are concerned with the problem of the influence of the surfactant tail length on the surfactant-polymer interaction [44–49]. In most of them, surfactants with long tail lengths (n > 10) have been used, so the only systems that have been considered are those in which the interaction is easily detectable, thus limiting the discussion to the strength of this interaction. In the present work, surfactants with relatively short tail lengths have been investigated. This allows us to monitor how the surfactant-polymer interaction occurs. Summarizing, it can be said that the C6 OS monomers randomly interact with the PVP chains, but no surfactant-polymer cluster forms and the surfactant-free micellization starts at a critical concentration that practically coincides with that observed in the absence of the polymer. In the case of C8 OS, surfactant self-aggregation is enhanced by the polymer and occurs at a significantly lower surfactant concentration; however, no clear evidence was obtained for the formation of surfactant-polymer clusters. The C10 OS and C12 OS molecules clearly interact with PVP, forming surfactant-polymer clusters at a surfactant concentration, c.a.c., that is lower than the corresponding c.m.c. in water. The logarithm of the c.m.c. and c.a.c. values are reported in Fig. 7 as a function of the surfactant tail length. Note that, for the water + C8 OS + PVP system, the concentration at which surfactant self-aggregation starts is reported, even if it is not evident whether or not this involves polymer chains. Inspection of Fig. 7 shows that the logarithms of both critical Springer
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Fig. 7 Natural logarithm of the critical micellar concentration (◦), c.m.c., in the water + Cn OS system and the critical surfactant-polymer aggregation concentration (•), c.a.c., in the water + Cn OS + PVP system as a function of the surfactant alkyl tail length at 25 ◦ C
concentrations vary linearly with n. Intersection of the two trends occurs at an n value slightly higher than six. This means that in the case of C6 OS, formation of surfactant-polymer clusters does not take place because it would have to start at a surfactant concentration higher than the c.m.c. In other words, free micellization is energetically favored with respect to cluster formation onto PVP chains. Interestingly, the logarithm of the concentration at which C8 OS starts to aggregate in the presence of PVP is in perfect agreement with the linear trend of the ln(c.a.c.). This could indicate that, in the case of C8 OS, cluster formation occurs but is difficult to detect because free micellization starts at a surfactant concentration only slightly higher. The almost contemporaneous formation of the C8 OS + PVP clusters and the C8 OS free micelles can probably explain why the average C8 OS aggregation number is slightly lower in the presence of PVP. In this connection, De Lisi et al. [50] showed by thermodynamic considerations that contemporary surfactant-polymer aggregation and free micellization is possible if the aggregation number of the clusters bounded to the polymer is sufficiently lower than that for the micelles. Recently, Diamant and Andelman [51] presented a theoretical description of surfactantpolymer interactions, based on the effect that small associating molecules, such as surfactants, have on the polymer chain statistics. This model is particularly well suited for polymers with a flexible chain, which is the case for PVP. Furthermore, in comparison with previous, more sophisticated, models of such systems [52, 53], the number of adjustable parameters is reduced. In particular, in the case of weak interactions, the strength of the surfactant-polymer association is quantified by the factor A defined as: c.a.c. = A(c.m.c.)
(8)
It is evident that A ≤ 1 and its value decreases with an increasing strength of interaction. The A values for the systems, water + Cn OS + PVP [n = 8, 10, 12], are reported in Table 2. It can be seen that the values of A decrease with increasing n. Reasons for this are: i) increasing the tail length promotes the hydrophobically driven self-aggregation of surfactant monomers randomly interacting with the polymer chain; ii) the surfactant-polymer interaction is favored Springer
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by the hydrophobic interaction between the surfactant tail and the polymer backbone; and iii) self-aggregation of surfactant molecules with long tails occurs at low concentration, i.e., in mixtures in which the ionic strength due to free monomers is low. This strengthens the ion-dipole interaction between the Cn OS headgroups and the PVP pyrrolidonic group that is slightly positively charged. It is interesting to compare the properties of systems for which surfactant-polymer cluster formation has been ascertained. The quantity (c2 – c.a.c.)/(NC × mPVP ), where mPVP is the molality of the polymer, represents the maximum number of clusters per polymer chain. From our data we compute that the values are ∼6 for C10 OS and ∼2 for C12 OS. The ratio between the maximum number of clusters per polymer chain and the average polymerization degree of our PVP sample represents the “polymer binding site”, i.e., the minimum number of vinyl pyrrolidone repeating units onto which a single surfactant aggregate is bound. From our data it is possible to compute that the value is ∼40 for C10 OS and ∼100 for C12 OS. This is related to the larger aggregation number of the C12 OS clusters, which require a longer polymer chain to be “wrapped”. Furthermore, the stronger electrostatic repulsion among the clusters disfavors formation of clusters too close to each other. EPR measurements confirm this interpretation: the τCM value is much higher for the system, water + C12 OS + PVP than for the system, water + C10 OS + PVP, i.e., the viscosity increase due to the presence of surfactant-polymer clusters is higher for the former system. This experimental evidence indicates a more prominant broadening of the polymer coil, related to the electrostatic repulsions between the surfactant cluster on the same macromolecule. Inspection of Table 3 shows that the KdC value is much higher for C12 OS than for C10 OS. This evidence indicates that the density of charged surfactant headgroups on the surface of surfactant-polymer clusters is much higher in the former case, favoring the condensation of TC molecules that act as counterions. Interestingly, the C value is higher for C12 OS than for C10 OS, indicating that the local environment experienced by TC molecules condensed on the clusters’ surfaces is more hydrophilic. It is not easy to understand this evidence. Recently, an NMR-noesy investigation has provided evidence that the alkylsulfatePVP interaction involves the methylene group of the surfactant alkyl tail that is closer to the headgroup [20]. Consequently, in the Cn OS + PVP clusters, the surfactants’ headgroups are exposed to the aqueous medium; the polymer being located right below the external hydrophilic layer. Once the ammonium moiety of the TC molecules electrostatically interacts with the surfactant headgroups, the insertion of the nitroxide moiety into the cluster core is obstructed by the presence of the polymer. Within this framework, one could hypothesize that the higher C value for C12 OS indicates that the C12 OS + PVP clusters present a larger fraction of their surface to interact with the polymer than with C10 OS, which tends to reduce the electrostatic repulsion among the surfactant headgroups. Consequently the C12 OS + PVP clusters present a less penetrable core.
4. Conclusions In the present work the associative behavior between the uncharged polymer PVP and the anionic surfactants Cn OS [n = 6, 8, 10, 12] in aqueous solution has been analyzed by using TC as an EPR spin probe, focusing on the effect of the surfactant tail length on the surfactant-polymer interaction. The high-charge density on the alkylsulfates’ headgroup favors its ion-dipole interaction with the N ---- C == O group of the pyrrolidonic rings of PVP [20, 23], probably enhanced through a synergistic effect by some hydrophobic interaction between the surfactant tail Springer
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Table 3 Variation of the EPR parameters of TC (1 × 10−4 mol·kg−1 ) due to surfactant self-aggregation for C10 OSa and C12 OS in water and in PVP aqueous solutions (1% w/w) at 25 ◦ C
C10 OS C10 OS-PVP C12 OS C12 OS-PVP
C /Gb
KdC c
M /Gd
Kd M e
τCM × 1011 /s f
– 16.44 ± 0.06 – 16.76 ± 0.02
– 3000 ± 900 – (1.1 ± 0.2) × 105
16.07 ± 0.01 16.09 ± 0.05 16.08 ± 0.02 16.04 ± 0.02
3700 ± 300 5200 ± 800 (3.1 ± 1.2) × 105 (2.7 ± 0.3) × 105
13.0 13.9 14.0 24.0
a From
reference [19]. isotropic hyperfine constant of TC solubilized in surfactant-polymer clusters. c Distribution coefficient of TC between aqueous and cluster pseudo-phases. d Nitrogen isotropic hyperfine constant of TC solubilized in micelles. e Distribution coefficient of TC between aqueous and micellar pseudo-phases. f TC correlation time in concentrated micellar solution. b Nitrogen
and the polymer backbone. At very low surfactant concentrations the surfactant monomers randomly associate with the polymer chain. This is particularly evident for short-tailed surfactants, such as C6 OS and C8 OS, for which cooperative surfactant self-aggregation occurs at concentrations high enough to allow a thorough investigation of the premicellar concentration range. The crowding of the surfactant molecules close to the polymer chain favors hydrophobic interactions among their tails, eventually inducing their self-aggregation. In the resulting surfactant-polymer clusters, the polymer chain wraps up in the surfactant aggregates, partially shielding the contact area between the aggregate’s hydrophobic core and water. However, a minimum surfactant tail length is required in order to promote this process. In fact, only for C10 OS and C12 OS is formation of surfactant-polymer clusters found experimentally. In the case of C6 OS, surfactant-polymer clusters do not form and only surfactant micellization occurs. With respect to the interaction with PVP, C8 OS seems to be a ‘borderline’ surfactant, in the sense that some cluster formation occurs at a surfactant concentration only slightly lower than that at which free micellization begins. Acknowledgments We thank Prof. L. Costantino and Prof. O. Ortona for their helpful comments.
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