Structural Investigation on the Poly ... - ACS Publications

9772

J. Phys. Chem. B 2004, 108, 9772-9779

Structural Investigation on the Poly(vinylpyrrolidone)-Water System in the Presence of Sodium Decyl Sulfate and Sodium Decanesulfonate: A Small Angle Neutron Scattering Study Gaetano Mangiapia,† Debora Berti,‡ Piero Baglioni,‡ Jose` Teixeira,§ and Luigi Paduano*,† Dipartimento di Chimica, Complesso UniVersitario di M. S. Angelo, UniVersita` degli Studi di Napoli “Federico II”, Via Cinthia, 80126 Napoli (NA), Italy, Dipartimento di Chimica and CSGI, UniVersita` degli Studi di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy, and Laboratoire Le´ on Brillouin (CEA-CNRS), CEA-Saclay, 91191 Gif-sur-YVette Cedex, France ReceiVed: February 2, 2004; In Final Form: April 6, 2004

Small angle neutron scattering (SANS) has been used to study the interaction of sodium decyl sulfate (C10OS) and sodium decanesulfonate (C10S) with poly(vinylpyrrolidone) (PVP). The measurements were performed on aqueous solutions of C10OS and C10S in the presence and absence of PVP. In all ternary systems the polymer concentration was kept constant at 1 wt %, and the surfactant molal concentration ranged from 0.002 to 0.150 mol kg-1. The analysis of the collected SANS profiles confirms the absence of any appreciable interaction between PVP and C10S in contrast with what observed for the PVP-C10OS-D2O system. In this latter system, in fact, a clear interaction peak in the scattering pattern appears well below the cmc of the surfactant, which suggests a complex formation between PVP and C10OS. The preferential interaction of poly(vinylpyrrolidone) toward sodium decyl sulfate with respect to sodium decanesulfonate reflects the difference in the chemical structure of the two surfactants that produces meaningful differences in the values of their cmc and aggregation number, as shown by the results here presented.

Introduction The large interest on polymer-surfactant systems has been driven by many current or possible applications in pharmaceutical formulations, personal care products, food products, industrial detergents, paints, and coating, but it has also been inspired by fundamental interest in intermolecular interactions.1,2 Since the first reports by Jones, Saito, Arai, Shirahama, and Cabane, a large number of publications and several reviews on this field has been published.3-12 A structural model for the polymersurfactant complex was proposed by Shirahama6 and by Cabane8-12 and was confirmed by NMR and neutron scattering experiments.8,13,14 In dilute solutions, the complex is viewed as composed of a series of micelle-like clusters bound to the polymer and connected by strands of the same polymer molecule, resembling a necklace of beads, the so-called “necklace model”. Although early on the complex formation was considered due to purely electrostatic forces between the surfactant ions and the opposite charged sites on the polymer, it is now accepted that a subtle balance of hydrophilic, hydrophobic, and ionic interactions governs the aggregation between polymer and surfactant.15,16 In the case of nonionic polymers, the theoretical efforts to highlight the driving forces responsible of the polymer-surfactant complexation have allowed us to propose two models that differ for the position of the polymer segments on the micelle. The first, proposed by Nagarajan, assumes that * Corresponding author. E-mail: [email protected]. Tel: +39 081 674229. Fax: +39 081 674090. † Universita ` degli Studi di Napoli “Federico II”. ‡ Universita ` degli Studi di Firenze. § CEA-CNRS.

polymer segments penetrate in the hydrophilic shell of the micelles and shield partially the contact area between the micellar hydrocarbon core and water.17 The second has been proposed by Ruckenstein et al. and assumes that polymer adsorption at the micelle surface produces a change in the microenvironment surrounding the micelle.18 More recently, Nikas and Blankschtein have presented a molecular-thermodynamic theory of complexation of nonionic polymers and surfactants considering the effect of solvent quality, polymer hydrophobicity, and flexibility and specific interactions between the polymer segments and surfactant hydrophilic moieties.19 In general, it is customary to classify polymer-surfactant interactions according to polymer or surfactant charge and with respect to surfactant concentration. Concerning this latter point, three characteristic surfactant concentrations can be defined: T1, T1*, T2. The first, known also as the critical aggregation concentration (cac), signals the concentration at which polymer and surfactant bind cooperatively to form necklace complexes. T1 is usually smaller than the cmc of the polymer-free surfactant solution. T1* corresponds to the polymer saturation point whereas above T2 free micelles form and coexist with polymersurfactant complexes.15 Pioneering works between the 50s and the 70s on the interaction between synthetic polymer and surfactants were focused on a variety of nonionic polymers such as poly(ethylene oxide) (PEO), methylcellullose (MeC), poly(vinyl alcohol)/ acetate (PVOH/Ac), poly(propylene oxide) (PPO), and poly(vinylpyrrolidone) (PVP).8-12,20 This latter polymer has been considered a model polymer suitable for fundamental studies. It is similar to proteins because it is composed of polar and

10.1021/jp0495388 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/09/2004

SANS of PVP-Water with C10OS and C10S

J. Phys. Chem. B, Vol. 108, No. 28, 2004 9773

Figure 1. Surface tension measurements performed on the following systems: (b) C10OS-H2O; (O) PVP (1 wt %)-C10OS-D2O.

Figure 2. Surface tension measurements performed on the following systems: (9) C10S-H2O; (0) PVP (1 wt %)-C10S-D2O.

nonpolar groups and it has the ability to bind reversibly a variety of organic and inorganic solutes. In their review of 1973, Breuer and Robb presented a “reactivity index”, assigned to nonionic polymers, on the basis of their affinity to anionic surfactants.21 The “reactivity” followed the sequence PVOH < PEO < MeC < PVAC < PPO ≈ PVP. Although the authors generically refer to “anionic surfactants”, in most cases, experiments on these polymers have been performed using as anionic surfactant sodium dodecyl sulfate (SDS), a surfactant of widespread use.2 The relatively high reactivity of PVP toward anionic surfactants was explained in terms of the slight effective positive charge present on the pyrrolidone group. There are several papers that claim complex formation between PVP and surfactants of the alkyl sulfate series.4,20 To our knowledge, experimental studies on systems containing PVP and alkanesulfonates, excluding two recent papers of some of the authors, are absent in the literature.22,23 However, despite this lack, and due to the chemical similitude between the alkanesulfonate and the alkyl sulfate surfactants, PVP is assumed to be the most efficient nonionic polymer in interaction with all anionic surfactants, no matter their kind of polar head. The authors in previous studies have experimentally shown that alkanesulfonate surfactants do not interact with PVP.22,23 As an example, in Figures 1 and 2 surface tension measurements γ performed on C10OS-H2O, C10S-H2O, PVP(1 wt %)-C10OS-H2O, and PVP(1 wt %)C10S-H2O systems have been reported.22 Analysis of the figures (detailed discussed elsewhere22) shows a quite different profile of the surface tension for C10OS-H2O and PVPC10OS-H2O systems, corresponding to a different behavior in solution. On the other hand, the overlapped trend is revealed for C10S-H2O and PVP-C10S-H2O systems, confirming the absence of any appreciable interaction between C10S and the PVP. This suggests an underlying subtler mechanism. In this report we present a small angle neutron scattering study on aqueous solutions of C10OS and C10S in the presence and in the absence of PVP. The measurements have been carried out in a wide range of surfactant concentrations whereas PVP concentration has been kept constant at 1 wt % for ternary samples, to have a good comparison among the systems. The aim of this paper is to confirm through a structural characterization a preferential interaction of the PVP toward the sodium decyl sulfate with respect to the sodium decanesulfonate, as suggested by previous

studies performed with different techniques and to highlight differences between the two surfactants, even in pure surfactant-water solutions. Experimental Section Materials. Sodium decanesulfonate C10S, sodium decyl sulfate C10OS (both with stated purity >99%), and poly(vinylpyrrolidone) (PVP) (average molecular weight 24000 amu) were purchased from Fluka. The molar masses of the surfactants C10S and C10OS used were 244.33 and 260.32 g mol-1, respectively. All solutes were reagent grade and were used without further purification. Details on all the molecules used are reported in Chart 1. All solutions were prepared by weight with D2O (Aldrich, stated purity 99.8%, molar mass 20.03 g mol-1) to enhance the scattering contrast and minimize incoherent background from hydrogen atoms. The deuterated sodium decyl sulfate DC10OS (molar mass ) 281.11 mol kg-1) was purchased from CDN Isotopes. Small Angle Neutron Scattering. SANS measurements were performed at 25 °C on the PAXE spectrometer at Laboratoire Le´on Brillouin, Saclay, France. Neutrons with an average wavelength of 7.3 Å and wavelength spread ∆λ/λ < 10% were used. Neutrons scattered from the samples were revealed by a two-dimensional arrays detector at two different sample-todetector distances, 1.05 and 5.0 m. These configurations allowed collecting the scattered neutrons in a range of moment transferred Q between 0.01 and 0.3 Å-1. Samples, prepared by weight using as solvent D2O, were contained in 1 mm path length quartz cells, and measurement times ranged between 20 min and 2 h. The data were then corrected for background, empty cell, and solvent contributions and then reduced to scattering cross sections (in absolute units cm-1), following the standard procedure.24 The errors affecting experimental SANS data are within 1% of the scattering cross section. The error bars have not reported in the figures because they would have made less clear the trends of the fitting curves. Method of Data Analysis The general scattering cross section, containing information about shape, size, and interactions of monodisperse scattering bodies, is given by25,26

dΣ ) Nb( dΩ

∑ibi - VbFs)2P(Q) S(Q) + (dΩ)inc dΣ

(1)

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CHART 1: Bond Structure of Sodium Decyl Sulfate, Sodium Decanesulfonate, and Poly(vinylpyrrolidone)

where Nb is the number density of scattering bodies, Vb is the volume of each scattering body, ∑ibi is the sum of the scattering lengths over the atoms constituting the body, and Fs is the solvent scattering length density. P(Q) and S(Q) are the form and structure factors, respectively, and (dΣ/dΩ)inc is the incoherent scattering cross section. The form factor contains information on the shape of the scattering objects, whereas the structure factor accounts for interparticle correlations and is normally observed for concentrated or charged systems. In the next sections the expressions for P(Q) and S(Q) are reported according to the models used to analyze the SANS data collected on the systems studied in this paper. PVP-D2O. SANS data of the PVP-D2O system have been analyzed by assuming that PVP is a Gaussian polymer coil in dilute condition (S(Q) = 1), through the Debye equation27

x - 1 + exp(-x)

P(Q) ) 2

2

x

(2)

with

h g2 x ) Q2R

∫01|F(Q,µ)|2 dµ

S(Q) ) 1 +

|

∫01|F(Q,µ)|2 dµ|2 (SMM(Q) - 1) ∫01|F(Q,µ)|2 dµ

(4)

whereas S(Q), the orientationally averaged interparticle structure

(5)

In the above equations F(Q,µ) is the angle-dependent form factor for ellipsoidal two-shell micelles where µ is the direction cosine between the direction of the symmetry axis of the ellipsoid and the Q B vector

3j1(u2) 3j1(u1) + (1 - f) u1 u2

F(Q,µ) ) f

(6)

with

u1 ) Qxµ2a2 + (1 - µ2)b2

(7)

u2 ) Qxµ2(a + d)2 + (1 - µ2)(b + d)2

(8)

(3)

where R h g is the average gyration radius of the polymer. C10OS-D2O; C10S-D2O. Structural parameters of micelles in C10OS-D2O and C10S-D2O systems have established modeling the micellar aggregates as prolate ellipsoids and analyzing scattering data through eq 1, where the number density of scattering bodies has been imposed to be Nb ) (c - cmc) × LA/Nagg, with c the stoichiometric surfactant concentration, Nagg the aggregation number of the micelles, and LA the Avogadro constant. The form factor P(Q) can be written as26

P(Q) )

factor, is given by26

f)

Vcore(F1 - F2)

∑

(9) bi - VbFs

i

where j1 is the Bessel function of first order, Vcore is the micelle core volume, and a, b, and d are the long and short semiaxes of the ellipsoidal core (containing the hydrophobic tail of the surfactant) and the thickness of the hydrophilic shell (formed by the polar heads, counterions and hydration water molecules), respectively. F1 and F2 are the scattering length densities of the core and shell. SMM(Q) has been calculated by solving the Ornstein-Zernike equation for the pair correlation function within the nonadditive radius mean sphere approximation (NARMMSA) closure that yields analytical solutions.28-30 The above-

SANS of PVP-Water with C10OS and C10S

J. Phys. Chem. B, Vol. 108, No. 28, 2004 9775

Figure 3. Scattering cross sections obtained for the solution of PVPD2O at 1 wt % of the polymer. The solid line is the fitting curve; see text.

Figure 4. Scattering cross sections measured on the C10OS-D2O system at the following concentrations of surfactant: (4) c ) 0.0201 mol kg-1; (b) c ) 0.0500 mol kg-1; (O) c ) 0.0701 mol kg-1; ([) c ) 0.150 mol kg-1. Solid lines are the fitting curves; see text.

reported is the well-known theory first applied to the SANS data of charged micelle system by Hayter and Penfold.28-30

concentration between 0.02 and 0.15 mol kg-1. Although at m ) 0.02 mol kg-1 the scattering profile is quite flat, as the surfactant concentration is raised, above the cmc value (0.025 mol kg-1; see Figure 1),32 dΣ/dΩ shows an increasing single broad maximum. This behavior is characteristic of charged micellar systems where significant correlations between the micelles are present. To gain structural information on the C10OS micelles, eq 1 was fitted to the scattering cross section distributions taking into account eqs 1 and 4-9 with Nagg, d, and z (the actual micelle charge) as unknown parameters.33 The length of the short axis, b, has been taken equal to the length of a fully extended alkyl chain, according to Tanford equation34

Results The SANS experiments were carried out on binary and ternary systems at different surfactant concentrations following the results obtained through the surface tension measurements shown in Figures 1 and 2. As inspection of the figures reveals, the γ curve of the ternary system containing C10OS crosses over the surface tension curve of the corresponding surfactantwater system at cC10OS ∼ 0.006 mol kg-1 and at cC10OS ∼ 0.09 mol kg-1, namely, at T1 and T2, whereas for C10S, Figure 2 shows that the presence of PVP in the system does not change substantially the behavior of the surfactant concentration dependence of the surface tension with respect to that of the C10S-water system. In fact, as the surfactant concentration is raised, the γ values of the ternary systems overlap to that of the binary ones, so that the only break point detectable corresponds to the cmc of the C10S-water system. On this basis SANS measurements were performed on the binary systems PVP-D2O, with 1 wt % polymer, C10OS-D2O, and C10S-D2O at about m ) 0.02, 0.05, 0.07, and 0.15 mol kg-1 and on the ternary systems in the presence of PVP. In these latter cases the concentration of the polymer was kept constant in all measurements (1 wt %), whereas the surfactant molal concentration ranged between 0.002 and 0.15 mol kg-1. In particular, for the PVP-C10OS-D2O system, measurements were performed at 0.015, 0.02, 0.03, 0.05, and 0.07 mol kg-1, namely, in the range where PVP and C10OS interact; see Figure 1. In the last system, where decyl sulfate has been replaced by the corresponding deuterated one, measurements were carried out at 0.07 and 0.15 mol kg-1. The deuterated surfactant, in D2O, gives a negligible contribution to the scattering by itself so that changes in the conformation of the polymer due to the presence of the surfactant can be highlighted. PVP-D2O. In Figure 3 the experimental SANS profile of 1 wt % PVP in D2O along with the fitting curve of scattering law for a Gaussian coil (see eqs 1-3) is reported. The Debye function fits to the experimental data quite well over the range of scattering angles, and the value of the fitting parameter, R hg ) 62 ( 3 Å, agrees well with those for PVP of the same molecular weight, present in the literature.31 C10OS-D2O. Figure 4 shows the scattering cross section distribution, dΣ/dΩ vs Q, of C10OS in D2O in the range of

b ) 2.765 + 1.265(nc - 1) Å

(10)

Hence the a value has been computed assuming a close-packed core:

4 Vcore ) NaggVchain ) πab2 3

(11)

where Vchain is the volume of the alkyl chain given by33

Vchain ) 54.3 + 24.8(nc - 1) Å

(12)

In eqs 11 and 12 nc represents the number of carbon atoms in the alkyl chain. In the computation of the structure factor, micelles have been considered spherical with an equivalent radius R )

x(a+d)(b+d)2. 3

As shown in Figure 4, there is a satisfying agreement between experimental and calculated SANS data and the parameters extracted are listed in Table 2. In the explored concentration range, there is no meaningful micellar growth, because the micellar form factor does not depend on surfactant concentration. The differences observed in the experimental scattering patterns are therefore caused by an increase in correlation, due to the increased volume fraction of charged scattering objects. C10S-D2O. The scattering cross sections collected for C10S-D2O, at the same concentrations of the C10OS-D2O system, along with the model fit for micelles described in previous section, are shown in Figure 5. As observed for the

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Figure 5. Scattering cross sections measured on C10S-D2O system at the following concentrations of surfactant: (4) c ) 0.0202 mol kg-1; (b) c ) 0.0500 mol kg-1; (O) c ) 0.0701 mol kg-1; ([) c ) 0.150 mol kg-1. Solid lines are the fitting curves; see text.

TABLE 1: Radius of Gyration of PVP, from SANS Data, in 1 wt % PVP-D2O and in PVP-C10OS-D2O and PVP-C10S-D2O Systems in the Range of Concentrations Where Surfactant Aggregates, Bound onto the Polymer, are Absent R hg (Å) PVP-D2O (1wt %) PVP-C10OS-D2O (mC10OS ) 0.002 mol kg-1) PVP-C10OS-D2O (mC10OS ) 0.01 mol kg-1) PVP-C10S-D2O (mC10S ) 0.002 mol kg-1) PVP-C10S-D2O (mC10S ) 0.01 mol kg-1)

62 ( 3 57 ( 3 60 ( 2 63 ( 1 60 ( 2

TABLE 2: Position and Magnitude of the Scattering Peaks Observed for the C10OS-D2O and C10S-D2O Binary Systems Q (Å-1)

m (mol kg-1)

(dΣ/dΩ)max (cm-1)

0.05 0.07 0.15

C10OS-D2O 0.060 0.070 0.090

0.35 0.67 2.10

0.05 0.07 0.15

C10S-D2O 0.050 0.064 0.085

0.18 0.46 1.62

decyl sulfate, when the imposed concentration of C10S is above its cmc (0.035 mol kg-1; see Figure 2),22,32,35 a broad maximum in the dΣ/dΩ profile, which strongly depends on concentration, appears. However, the comparison of Figures 4 and 5 highlights two main differences; see Table 2. At the same surfactant concentration the correlation peak appears at a lower Q value for C10S with respect to that observed for C10OS. Furthermore, for a given surfactant concentration, the scattered cross section measured for C10OS is higher than that of C10S (about 3040%), notwithstanding the similarity of the contrast. This difference is expected and reflects the higher number density of micelles for the system C10OS-D2O, due to its lower cmc with respect to C10S-D2O.

Figure 6. Scattering cross sections measured on PVP(1 wt %)C10OS-D2O system at the following concentrations of surfactant: (O) c ) 0.00991 mol kg-1; (9) c ) 0.00200 mol kg-1; (open left triangle) c ) 0.0150 mol kg-1; (4) c ) 0.0201 mol kg-1; (1) c ) 0.0310 mol kg-1; ([) c ) 0.0505 mol kg-1; (3) c ) 0.0700 mol kg-1; (solid left triangle) c ) 0.0900 mol kg-1; (open right triangle) c ) 0.150 mol kg-1.

In Table 3 the values of the fitting parameters obtained from the scattering data of C10S-D2O are reported. Again, the structural parameters are representative of the whole explored concentration range, where no appreciable variation of the micellar structure factor could be detected. Inspection of the table shows a sensible difference in the structural parameters of the micelles of the two surfactants. PVP-C10OS-D2O. The addition of PVP results in a marked change in the scattering of C10OS, as shown in Figure 6. The measurements were performed in the range of surfactant concentration between 0.002 and 0.15 mol kg-1, at constant concentration of PVP (1 wt %). Other experimental techniques, like surface tension (here reported), NMR chemical shift, and EPR measurements have allowed determining that in the range of C10OS concentration between ∼0.008 (T1) and ∼0.09 (T2) mol kg-1 the solution consists of aggregated surfactant bound to the polymer in equilibrium with surfactant monomer.22,35-37 It is worth pointing out that some differences in the T1 and T2 values have been observed among the various techniques used, as expected due to the different concentration dependence of the physicochemical properties monitored by the technique used. A reasonable T1 is more correctly 0.008 ( 0.002 mol kg-1 whereas T2 ) 0.09 ( 0.005 mol kg-1.22,35-37 The above values can be compared with those reported by Arai in an early paper (T1 ) 0.008 mol dm-3 and T2 ) 0.020 mol dm-3), although the system studied contained PVP (Mw ) 90 000 amu) at 0.10 wt % and in the presence of NaCl.38 The difference in the T2 value between that reported by us and Arai is essentially due to the different molecular weight and concentration of PVP used. At 0.09 mol kg-1 the surfactant reaches the concentration condition to form “free” micelles, and further addition of C10OS produces an increase of the number of micelles in the system.

TABLE 3: Structural Parameters Obtained from Fitting the C10OS-D2O and C10S-D2O Spectra and the Scattering Length Densities of the Core and Shell of Micelles

C10OS-D2O C10S-D2O

Nagg

z

a (Å)

b (Å)

d (Å)

Fcore (Å-2)

Fshell (Å-2)

50 ( 3 39 ( 4

25 ( 3 18 ( 2

17.0 15.0

14.0 14.0

4.5 (1 3.2 ( 0.7

-4.4 × 10-7 -4.4 × 10-7

4.4 × 10-6 3.9 × 10-6

SANS of PVP-Water with C10OS and C10S

Figure 7. Comparison of scattering cross sections measured on the following systems: (2) PVP(1 wt %)-D2O; (]) C10OS (c ) 0.0200 mol kg-1)-D2O; (O) PVP(1 wt %)-C10OS (c ) 0.0150 mol kg-1)D2O; (b) PVP(1 wt %)-C10OS (c ) 0.0200 mol kg-1)-D2O; (3) PVP(1 wt %)-C10OS (c ) 0.0310 mol kg-1)-D2O. The solid line is the fitting curve; see text.

As Figure 6 shows, at low surfactant concentrations, between 0.002 and 0.01 mol kg-1, dΣ/dΩ is a monotonically decreasing function of Q and the Debye function fits the scattering profiles quite well over the range of scattering angles. The values of R hg determined from eqs 1 and 2 are 57 ( 3 and 60 ( 2 Å, respectively, nearly identical to that obtained above for the pure PVP-water system (62 ( 3 Å). This suggests that at these concentrations, either pure micelles or significant numbers of micelle-like aggregates bound on the polymer are absent. As the C10OS concentration is further raised, a marked change in the scattering profile is observed. Each curve shows a maximum whose position is a strong function of concentration; the peak moves to higher Q with increasing concentration. Such a rising peak appears in the scattering profile even when the concentration of the surfactant is quite low (at 0.015 mol kg-1 it looks like a shoulder) and it is seen to be absent for the corresponding C10OS-D2O system. Indeed, no scattering maxima have been detected in the binary system up to 0.03 mol kg-1 C10OS concentration, and in general, in the ternary system the peaks appear at lower Q values than in C10OSD2O. For comparison, the scattering of the samples of C10OSD2O, in the presence and absence of PVP and along with dΣ/ dΩ of PVP-D2O, are reported in Figure 7. Inspection of the figure suggests that the scattering profile in the ternary system in the range of concentration 0.015-0.031 mol kg-1, namely, below the surfactant cmc, is due to the presence of polymerbound micelles. PVP-C10S-D2O. SANS measurements on PVP-C10SD2O system have been performed at the same concentration of the system containing C10OS, the scattering profiles are reported in Figure 8. The samples at 0.002 and 0.01 mol kg-1 show scattering profiles similar to those obtained for the corresponding C10OS samples. Once again, the Debye function fits well dΣ/dΩ vs Q distributions and the extracted values of R h g, 63 ( 1 and 60 ( 2 Å, are substantially equivalent to those obtained for the system containing C10OS at the same concentration of the surfactant; see Table 1. Increasing the surfactant concentration yields a rising peak, as observed for C10OS. However, there is a sensible difference between the two systems: for C10S there is no evidence of an interaction peak in the scattering profile for

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Figure 8. Scattering cross sections measured on PVP(1 wt %)-C10SD2O system at the following concentrations of surfactant: (b) c ) 0.00210 mol kg-1; (1) c ) 0.0101 mol kg-1; (0) c ) 0.0200 mol kg-1; (2) c ) 0.0502 mol kg-1; (3) c ) 0.0707 mol kg-1; ([) c ) 0.0902 mol kg-1; (]) c ) 0.151 mol kg-1.

concentrations below the cmc (0.035 mol kg-1), different from what is observed for C10OS. Such a peak appears in the PVPC10S-D2O system at the same concentration of the corresponding C10S-D2O system. In other words, the behavior of the decyl sulfate seems unaffected by the presence of PVP. Discussion The analysis of the SANS data collected in this report allows several considerations even in the case of the binary systems despite the chemical similarity of the two surfactants under study. C10OS-D2O, C10S-D2O. The behavior of the two surfactants in aqueous solution is somewhat different as well as the structure of the corresponding micelles. Beyond the difference in the cmc values of the two surfactants (the C10S cmc is higher than that of C10OS), quite evident are the differences in the characteristics of the pure micelles obtained by the fitting procedure of the scattering profiles reported in Table 2. This feature is unexpected considering the close similarity of the chemical structures of the surfactant monomers. The presence of an extra oxygen atom in the polar head thus produces a noticeable effect on the hydrophobic-hydrophilic balance that leads to the formation of aggregates. The aggregation numbers of micelles are sensibly different, ∼40 for C10S and ∼50 for C10OS. These values, which in the explored concentration range are substantially constant for each surfactant, are in reasonable agreement with those reported in the literature. For the alkanesulfonate surfactant series the micelle aggregation numbers along the series have been found to follow39

Nagg ) (9 ( 1) + (1.26 ( 0.05)(nc - 5)2

5 e nc e 11 (13)

where nc is the numbers of carbons in the alkyl chain. In contrast, for the alkyl sulfate series, the aggregation number of the micelle increases with nc according to40

Nagg ) (10 ( 2) + (5.2 ( 1)(nc - 6) + (1 ( 0.2)(nc - 6)2 6 e nc e 16 (14) Thus, for the same length of the hydrophobic tail, eqs 13 and

9778 J. Phys. Chem. B, Vol. 108, No. 28, 2004 14 give for the aggregation number of C10OS and C10S a value of 47 ( 9 and 40 ( 2, respectively. Again for C10S a smaller value of the aggregation number is found with respect to the alkyl sulfate ones. The differences reported above reflect the slight dissimilarity in the chemical structure of the two surfactants, which seems to play a significant rule in defining their physicochemical properties. In the C10S system, as a result of the absence of an oxygen atom in the polar head, it is likely that the hydration shell covers a larger part of the apolar chain, which “shortens” the hydrophobic tail with respect to that of the equivalent sulfate surfactant. This, in turn, leads to a reduction of the hydrophobic driving force of the micellization process of the decanesulfonate. Theoretical calculations performed by Huibers have shown that the carbon next to the hydrophilic head of C10OS bears a partial positive charge.41 In contrast, when the headgroup is SO3-, the same position bears a partial negative charge. Due to this difference Huibers suggested that the headgroup charge repulsion influences a self-assembly process, which reflects on the cmc and on the micelle aggregation number for the two different surfactant classes we are dealing with. The above arguments seem validated by the results here obtained. PVP-C10OS-D2O, PVP-C10S-D2O. With regard to the systems containing PVP, the overall picture of the SANS profiles collected on the system PVP-C10S-water suggest the absence of an interaction between the polymer and the surfactant. In fact, the scattering cross section distributions of all solutions with up to 0.03 mol kg-1 of C10S are quite well fitted with the Debye equation for the Gaussian coil, and in all cases the gyration radius R h g is about 60 ( 3 Å, and this value are nearly identical to that obtained for PVP-D2O. Such evidence suggests that in this range of concentration neither micelle-like cluster bound to the polymer nor free pure micelle is present in the system. A further increase of the C10S concentration above 0.035 mol kg-1, i.e., above its cmc, leads to the formation of pure micelle, as reflected in the rising peak observed in SANS profile; see Figure 8. In contrast, in the PVP-C10OS-water system there is clear evidence of a cooperative association between the polymer and the surfactant. In fact, as shown in Figure 7, even at low C10OS concentrations (0.015 mol kg-1), a rising correlation peak is present in the dΣ/dΩ profile for the system containing PVP that is absent in the corresponding C10OS-D2O binary system. The formation of a PVP-C10OS complex has been confirmed by a series of measurements performed on samples of PVP (1 wt %)-D2O where deuterated sodium decyl sulfate (DC10OS) was used. Substitution of H-C10OS with D-C10OS, shown in Figure 9, renders micellar aggregates invisible to neutrons (due to the small contrast between the surfactant and the solvent scattering density lengths), as can be deduced from scattering profile of the corresponding D-C10OS/D2O samples, also reported in the figure. SANS spectra of the deuterated ternary systems show a dependence on surfactant concentration, despite the fact that surfactant aggregates cannot be detected. The contrast matching experiment has been performed at two different surfactant concentrations, 0.07 and 0.15 mol kg-1, respectively, below and above T2. In fact, once the hydrogenated surfactant is replaced with the corresponding deuterated one, the scattering amplitude is sensibly reduced, revealing a close polymer-surfactant interaction. What is observed is a surfactant-induced change in polymer conformation, caused by the presence of charged micellar clusters along its chain, which in turn gives rise to a correlation

Mangiapia et al.

Figure 9. Scattering cross sections measured on PVP(1 wt %)C10OS-D2O system at the following concentrations of surfactant [(b) c ) 0.0701 mol kg-1; ([) c ) 0.150 mol kg-1], on PVP(1 wt %)D-C10OS-D2O system at the following concentrations of surfactant [(O) c ) 0.0708 mol kg-1; (]) c ) 0.151 mol kg-1], and on D-C10OS-D2O at the following concentrations of surfactant [(1) c ) 0.0690 mol kg-1; (0) c ) 0.150 mol kg-1].

peak in the SANS profile. When the scattering from the polymer and hydrogenated surfactant is measured, all components are visible. The correlation peak observed arises from the intermicellar electrostatic interaction. Use of the deuterated C10OS surfactant means that this component does not contribute to the scattering, and the data appear similar to that obtained from the hydrogenated surfactant because the polymer is saturated by the surfactant micelles. Wesley et al., studying the complex poly(2-(dimethylamino)ethyl methacrylate)-SDS, have reached the same conclusion, once they replaced the hydrogenated SDS with the deuterated one in the system.42 As in the case of Wesley, the scattering pattern evolution observed passing from 0.07 to 0.150 mol kg-1 (Figure 9) can be attributed to the saturation of the polymer by C10OS. In other words, the polymer saturation point, namely T1*, is reached at concentration above 0.07 mol kg-1. Thus above this concentration the binding of C10OS still proceeds, generating, in turn, a further saturation of the polymer. This is, probably, reflected in the significant increase of the scattering cross section profile, as observed in the spectra collected in the presence of deuterated C10OS, shown in Figure 9. In the saturation condition, some of the authors of the present paper in a previous work36 have determined from fluorescence quenching and intradiffusion coefficients measurements that the number of surfactant aggregates on the PVP chain is ∼10. This is in substantial agreement with the results, recently, reported by Li et al. that have investigated the effect of a nonionic surfactant (hexaethyleneglycol mono-n-dodecyl ether) on the complex formed between PVP and sodium dodecyl sulfate. They found, concerning the complex, that when the PVP (Mw ) 360 000 amu) is saturated with SDS micelles, the complex is a beadlike structure in which the polymer wrapped about 25 surfactant aggregates.43 Finally, a change of the PVP conformation, as suggested by the SANS data collected on the system containing the deuterated surfactant, has been also observed through viscosity measurements on four poly(vinylpyrrolidone) fractions in the molecular weight range from 2.75 × 104 to 3.5 × 105 amu, raising the C10OS concentration.44

SANS of PVP-Water with C10OS and C10S Conclusion This paper has provided a comparison between sodium decanesulfonate and sodium decyl sulfate with respect to their interaction with poly(vinylpyrrolidone) in water, obtained with the SANS technique. The analysis of the experimental results has highlighted differences in the behavior of these two surfactants even in binary aqueous solution. Despite their close chemical similarity, both the values of the cmc and the aggregation number of the pure micelles of decyl sulfate and decanesulfonate are shown to be sensibly different. Due to this evidence it is possible to suggest that for decanesulfonate the driving force of the micellization process is weaker with respect to that of the decyl sulfate. Even more remarkable is the difference between these two surfactants with respect to the interaction toward PVP; in fact, C10S does not show any detectable interaction with the polymer, different from the behavior of C10OS. This is in contrast with the wide conviction that all anionic surfactants form complexes with PVP. Indeed, the present work, as well as a previous one in which structural detail on the PVP-C10OS complex has been gained by NMR,36 provides strong a basis to suggest that the interaction between PVP and the alkyl sulfate surfactants is a multifaceted mechanism. That probably originates from the structural differences of the two surfactants that lead to an opposite partial charge on the carbons next to the hydrophilic head of the surfactant. Thus, in particular for the PVP-C10OS system, our evidence suggests that the interaction takes place through both electrostatic and hydrophobic contributions. The former consists of the electrostatic attraction between the surfactant headgroup and the PVP nitrogen plus the additional attractive interaction between the negatively charged PVP oxygen (as suggested by the mesomer N+dCsO- form) and the electron poor carbon next to the headgroup. The latter involves the alkyl chain moiety of the surfactant and the methylene groups belonging to the pyrrolidone rings of the polymer. It is likely that the partial positive charge localized on the carbon next to the sulfonate group, as evaluated by Huibers,41 is responsible for the lack of interaction between PVP and the alkanesulfonate. Acknowledgment. This research was carried on with the financial support of Italian MURST. We thank LLB for provision of beam time. References and Notes (1) Goddard, E. D.; Ananthapadmanabhan, K. P. Applications of Polymer-Surfactant Systems. Polymer-Surfactant Systems; Surfactant Science Series Vol. 77; Kwak, Jan C. T., Ed.; Marcel Dekker: New York and Basel, 1998; Chapter 2, pp 21-64. (2) Taber, J. J. Pure Appl. Chem. 1980, 52, 1323-1347. (3) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36-42. (4) Jones, M. N. J. Colloid Interface Sci. 1968, 26, 532-533.

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