Mixed Surfactant Nanoassemblies Formed

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Anisometric Polyelectrolyte/Mixed Surfactant Nanoassemblies Formed by the Association of Poly(diallyldimethylammonium Chloride) with Sodium Dodecyl Sulfate and Dodecyl Maltoside Beatrice Plazzotta, Edit Fegyver, Robert Meszaros, and Jan Skov Pedersen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01280 • Publication Date (Web): 09 Jun 2015 Downloaded from http://pubs.acs.org on June 21, 2015

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Anisometric Polyelectrolyte/Mixed Surfactant Nanoassemblies Formed by the Association of Poly(diallyldimethylammonium Chloride) with Sodium Dodecyl Sulfate and Dodecyl Maltoside Beatrice Plazzotta1, Edit Fegyver2, RóbertMészáros*2,3 and Jan Skov Pedersen*1 1

Aarhus University, Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark

2

Laboratory of Interfaces and Nanosized Systems, Institute of Chemistry, Eötvös Loránd University, 1117 Budapest, Pázmány Péter sétány 1/A, Hungary 3

Department of Chemistry, University J. Selyeho, 945 01 Komárno, Slovakia * [email protected] and [email protected]

Abstract The soluble complexes of oppositely charged macromolecules and amphiphiles, formed in the one-phase concentration range, are usually described on the basis of the beads on a string model assuming sphere-like bound surfactant micelles. However, around and above the charge neutralization ionic surfactant-to polyion ratio, a variety of ordered structures of the precipitates and large polyion/surfactant aggregates have been reported for the different systems which are difficult to connect to globular-like surfactant self-assembly units. In the present paper we have demonstrated through SAXS measurements that the structure of precipitates

and

that

of

the

soluble

polyion/mixed

surfactant

complexes

of

poly(diallyldimethylammonium chloride) (PDADMAC), sodium dodecyl sulfat (SDS) and dodecyl-maltoside (DDM) are strongly correlated. Specifically, the SDS binds to the PDADMAC molecules in the form of small cylindrical surfactant micelles even at very low SDS-to-PDADMAC ratios. In this way, these anisometric surfactant self-assemblies formed at polyelectrolyte excess, mimic the basic building units of the hexagonal structure of the

1 ACS Paragon Plus Environment

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PDADMAC/SDS precipitate and/or suspensions formed at charge equivalence or at higher SDS-to-PDADMAC ratios. The presence of DDM reduces the cmc and cac for the system, but does not alter significantly the structure of the complexes neither in the one- phase, nor in the two- phase region. The only exception is for samples at SDS-to-PDADMAC ratios close to charge neutralization and high concentration of DDM where the precipitate forms a multiphasic or distorted hexagonal structure. Keywords, Polyelectrolyte, Surfactant, SAXS, Anisometric, Hexagonal 1. Introduction Over the past few decades extensive and comprehensive studies have been performed to explore the basic features of oppositely charged polyelectrolyte/surfactant complexation.1-3 This is partly because of the interest in nanostructured self-assemblies4-5 and partly due to the variety of pharmaceutical, personal care and other applications based on these systems.6-8 In addition, the characteristics of polyelectrolyte/surfactant association can be used to understand the interaction between biomacromolecules and surfactants.9-11 Because of the combined hydrophobic and electrostatic driving force of polyelectrolyte/surfactant association, the oppositely charged amphiphile binds onto the polyelectrolyte chains at the critical aggregation concentration (cac) which is well below the critical micelle concentration (cmc) of the pure surfactant.1,12,13 With increasing surfactant concentration associative phase separation takes place and a solid phase concentrated in the macromolecules and surfactants and a dilute solution are observable.1,14 At even higher concentration of the ionic amphiphile, the system may form a thermodynamically

stable

solution

or

a

colloidal

dispersion

of

overcharged

polyelectrolyte/surfactant nanoparticles depending on the molecular weight and chemical nature of the polyion.3,15 There are also special block ionomers or double hydrophilic block copolymers


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which form soluble complexes with the oppositely charged surfactant molecules even at charge neutralization and at surfactant excess.16-21 In this case, the complexes consist of a hydrophobic core containing the charged blocks and surfactant micelles and large shell of hydrophilic blocks. The structure of water-insoluble stoichiometric homopolyelectrolyte/surfactant precipitates have been thoroughly investigated and a diversity of more or less ordered structures involving cubic, lamellar and hexagonal phases was reported.22-27 The degree of ordering and the type of structures were found to depend on a number of factors such as the alkyl chain length of the surfactant, the ionic strength, and the molecular architecture and charge density of the polyions.22-27 At ionic surfactant excess, depending on the applied solution preparation protocols, precipitates and/or nonequilibrium suspension of homopolyelectrolyte/amphiphile aggregates can be prepared over a wide concentration range.28,29 In this latter composition range, nanostructures similar to those of the stoichiometric precipitate23,30,31 were observed for the colloidal dispersion of overcharged polyelectrolyte/surfactant aggregates. In contrast to the variety of the structures detected in the two-phase concentration region, much less information is known about the structural features of soluble polyelectrolyte/surfactant nanoassemblies in the one-phase composition region of polyelectrolyte excess. At low ionic surfactant-to-polyelectrolyte ratios the amount of bound amphiphilic molecules was considered to be too low to study the structural characteristics of the formed complexes.32 In spite of this lack of detailed structural information, the beads on a string (or necklace) model is generally used for the description of the oppositely charged polymer/surfactant complexes at surfactant concentrations above the cac.13,33,34 In this model, globular ionic micelles bind onto the polyion which is wrapped around the micelles. While this model is well-established in the case of uncharged polymer/surfactant systems1, it is not straightforward to see the interrelation of this type of selfassemblies with the variety of long-range ordered structures of the precipitates or aggregates formed


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at larger ionic surfactant-to-polyelectrolyte ratios. To our knowledge, there is only one study which probed the thermodynamically stable solutions of individual polyion/surfactant complexes via SANS measurements32 (i.e. sufficiently low surfactant-to-polyelectrolyte ratios were used in order to avoid the presence of nonequilibrium aggregates at polyelectrolyte excess30,31, 35 with similar structure than that of the stoichiometric precipitate). In this study, Bergström and coworkers investigated the mixtures of sodium dodecyl sulfate (SDS) with copolymers of uncharged acrylamide (AM) and cationic {3-(2-methylpropionamido)propyl}trimethylammonium chloride (MAPTAC) monomers over a wide charge density of AM-MAPTAC polyelectrolytes.32 The authors have shown that in the low turbidity concentration region of polyelectrolyte excess prolate or rodshaped micelles are bound onto the AM-MAPTAC molecules. On the other hand, two-dimensional (2D) hexagonal lattice of close-packed cylindrical micelles were suggested for the structure of precipitate formed at or close to charge neutralization.32 In the present paper, we focus on the aqueous mixtures of poly(diallyldimethylammonium chloride) (PDADMAC) and SDS. While the different aspects of PDADMAC/SDS association was thoroughly investigated, the connection between the structures of polyion/surfactant nanoassemblies formed at polyelectrolyte excess and that of the precipitates is far from being understood. To this end, we utilize the highly sensitive SAXS technique, which is suitable to follow both the dynamic and equilibrium characteristics of surfactant self-assemblies. Through SAXS, turbidity and electrophoretic mobility measurements we demonstrate that the structure of PDADMAC/SDS precipitates and that of the soluble complexes are correlated. Additionally, we extended the study to the case of systems containing a third component, such as nonionic amphiphiles, to see how this can impact the architecture of the assemblies. Although, the different aspects of the complexation between polyelectrolytes and mixtures of ionic and nonionic surfactants have been thoroughly investigated,36 the impact of the nonionic additives on the


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structure of polyion/ionic surfactant assemblies is relatively unexplored. For example, Dubin and coworkers found, that the ordered structure of PDADMAC/SDS precipitates and aggregates becomes disordered via the addition of the nonionic Triton X-100 surfactant.37 In contrast, Janiak et al. found that the addition of penta(ethylene glycol) monododecyl ether (C12E5) to the stoichiometric complex salt of hexadecyltrimethylammonium-polyacrylate could lead to another type of liquid crystalline structure than that of the polyion/ionic surfactant salt formed in the absence of the uncharged additive.38 Also, polyelectrolyte/ionic surfactant systems with nonionic amphiphilic additives are far more favorable and superior in household and cosmetic applications compared to the oppositely charged polyion/surfactant mixtures and their study may help the formulation of new products.1 We have chosen dodecyl maltoside (DDM) as uncharged additive, which is a promising candidate in environmental applications due to its biodegradable nature.39 It was possible to see that the addition of this component is not influencing significantly the structure of the complexes, but that it reduces the cmc and cac of SDS by forming mixed micellar structure containing both surfactants. 2. Experimental 2.1 Materials Poly(diallyldimethylammonium chloride) (PDADMAC) with a mean molecular weight between 400 and 500 kDa was purchased in the form of a 20 wt% aqueous solution. The stock solution was purified by Sartorius StedimVivaflow 50 (regenerated cellulose based) filter membrane with a cutoff molecular weight of 100 kDa. The critical overlap concentration of the polymer chains, C*, was estimated from the radius of gyration40 and is approximately 900 ppm. This was calculated using a gyration radius of 60 nm.41 The concentration of PDADMAC in the samples was 1000 ppm, so approximately the value of C*.


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The sodium dodecyl sulfate (SDS) sample was recrystallized twice from a 1:1 benzene-ethanol mixture. The critical micelle concentration (cmc) of SDS after purification was 8.1 mM at 25.0 ± 0.1 ºC as determined from conductivity measurements. The n-dodecyl-β-D-maltoside (DDM) sugar surfactant was used without further purification. The cmc of DDM was found to be 0.16 mM, as determined from surface tension measurements. All the mentioned chemicals were provided by Sigma Aldrich. Ultrapure water (Milli-Q) was used for preparation of the solutions. 2.2 Solution Preparation The PDADMAC/SDS solutions were either prepared without added dodecyl maltoside or at constant concentrations of DDM (1 mM or 10 mM). The so-called rapid-mixing protocol was used for the preparation of the majority of mixtures.8 2 ml of SDS (or SDS/DDM) solution were added rapidly to 2 ml of PDADMAC (or PDADMAC/DDM) solution using an automatic pipette under continuous stirring with a magnetic stirrer at 2000 rpm. In the case of added dodecyl maltoside both the polyelectrolyte and the SDS solutions contained the nonionic surfactant at the same concentration. In some cases, a slow-mixing procedure7-8 was also applied during which an SDS (or SDS/DDM) solution was added slowly, drop by drop to a PDADMAC or (PDADMAC/DDM) solution of equal volume under continuous stirring with a magnetic stirrer at 2000 rpm. 2.3 Methods 2.3.1 Turbidity Measurements The turbidity was determined at 25.0 ± 0.1 °C from the transmittance (T) of the mixtures which was measured at 400 nm by an UV/vis (Perkin-Elmer Lambda 2) spectrophotometer. The turbidity is given as 100-T% and the measurements were carried out immediately after mixing and 1 week after the preparation of the mixtures. 2.3.2 Conductivity Measurements


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The electrical conductivity (G) of the SDS/DDM mixtures was measured at 25.0 ± 0.1 °C using a Radelkis conductometer. The conductivity values measured at a given composition immediately and 24 hours after the solution preparation were found to be identical within the experimental error. 2.3.3 Electrophoretic Mobility Measurements The mean electrophoretic mobility (uζ) of the PDADMAC/surfactant complexes was determined at 25.0 ± 0.1 °C immediately after solution preparation using a Malvern Zetasizer Nano Z instrument. The apparatus uses the M3-PALS technique which is a combination of laser Doppler velocimetry and phase analysis light scattering. In the case of concentrated surfactant solutions the detected uζ values are weighted averages of the mean mobility of the polyion/mixed surfactant complexes and that of the mixed bulk micelles. Therefore, in this concentration range only the sign of the mobility data is relevant and their absolute values need to be handled with care. 2.3.4 SAXS Measurements SAXS scattering curves were recorded using the lab instrument at Aarhus University56, which is a flux- and background optimized version of the NanoSTAR camera from Bruker AXS. The wavelength used is 1.54 Å (Copper rotating anode) and the setup uses home-built compact ‘scatterless’ slits in front of the sample. Liquid samples, both homogeneous and with a suspended precipitate, were measured in a quartz capillary of about 2 mm in diameter, loaded using a homebuild flow-through injection system. Solid hydrated samples were measured in a home-built sample holder where the solid is pressed between two Kapton windows. The acquisition time for the data was 1200 s for liquid samples and 300 s for solid samples. All samples were measured at 20 °C. Particle scattering is obtained from the data of the solution/mixture by subtracting the scattering from pure milliQ water from that of the sample. The initial data treatment was done using the SUPERSAXS program package (Oliveira, C.L.P. and Pedersen, J.S. Unpublished) using milliQ


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water as a standard15 for absolute scale. The intensity of scattered radiation is presented as a function of the magnitude of the scattering vector: =

4 sin (1)

where λ is the wavelength of the incident radiation and 2 is the scattering angle. The total measured intensity can be seen as the contribution of the normalized form factor P(q), which is determined by the shape and size of the particles, and the structure factor S(q), which arises when we have concentrated sample and thus interference between x-rays scattered from the different particles. If we considered a sample where the number concentration is N, the scattered intensity can be expressed as: ( ) = ( )() (2) 3. Modelling of SAXS data Samples containing surfactants without added polyelectrolytes were described using an ellipsoidal core-shell model. Single phase samples containing PDADMAC with surfactants were modeled as cylindrical micelles decorated by polyelectrolyte. Actually a “cylinders on a string” model with polyelectrolyte bridging between micelles was fitted to the data. However, the number of micelles per cluster came out to be close to one so the structures are single cylinders rather than cylinders on a string. An attempt to model the systems with a more classical “beads on a string” model55,56 was also done, but the results were not as satisfactory as for the cylinder model. Details for the cylindrical models are described here, while those for the “beads on a string” structure are available in section S2 of the Supporting Information (SI), together with a comparison between the fitting of the cylindrical and bead-like models (Figure S1, Table S1) . The samples containing a solid precipitate were described as a 2D hexagonal structure, with an added background of free micelles. 3.1 Ellipsoidal Core-Shell Micelle Models for mixed self-assemblies of SDS and DDM


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This model was applied for samples containing only surfactants (SDS 1, 2.5, 4, 6, 8 and 16 mM with 1 or 10 mM DDM). The base for this model is a simple core-shell structure, with the core made of C12 chains, either from SDS or DDM, and the shell made up as a homogeneous mixture of the head groups of SDS (sodium sulfate) and DDM (maltose). For samples containing 1 mM DDM the interactions between micelles are negligible and only the form factor P(q) is considered, while for samples containing 10 mM DDM an effective hard-sphere structure factor is added. This is included assuming that the interactions are independent of the orientation of the micelles and given by an average minimum distance of the particles (decoupling approximation). Therefore, the intensity can be given as a function of the scattering vector as: ( ) = (() +< () > (S( ) − 1)) (3) where Nmicelles is the number density of micelles and is the orientationally averaged scattering amplitude. Since part of the surfactants is present as free molecules in solution, the number of micelles is not calculated from the total surfactant concentration, but from the effective concentration of surfactant in the micelles, as: =

!""#$ % (4) %&&

where NA is the Avogadro number, Ceffective is the concentration of surfactant in the micelles and NAgg is the aggregation number of the micelles. Those parameters are obtained as fit parameters. The general core-shell model of an ellipsoid of revolution is described by a form factor: 6 7 7 4 / ( ) = ' (Δ*+ ,#-# .#-# Φ1.#-# (2)34 578 9:;< 3 > 6 7 4 / 5 8 7 9AB 7 − (Δ*+ − Δ*-? ),-? @ 21.-? (2)34 C sin(2) D2 (5) 3 = 7


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The integration is performed over all orientations 2 of the micelles. The amplitude is equal to the integral without the square of the integrant. The parameters rcore and rtot are given by radii Rcore and Rtot as: .-? (2) = @-? (sin (2) + ,-? cos(2) ) (6) (sin (2) + ,#-# cos(2) ) (7) .#-# (2) = @#-#

where εcore is the eccentricity of the micellar core, εtot is the eccentricity of the whole micelle (the shell is considered to have a constant thickness, D) calculated as: ,#-# =

,-? @-? + J (8) @-? + J

Φ(x) in eq. (5) is the form factor amplitude of a sphere: 2(@ ) = 3

sin(@) − .LMN(@) (9) (@)

and σout and σin are the widths of the Gaussian distributions used to smear, respectively, the shellwater interface and the core-shell interfaces; this smearing is included due to the fact that the interfaces are not sharp, but diffuse and it is described using the exponential component e5

Q7 R7 7

.

∆ρshell and ∆ρcore in eq. (5) are the excess scattering length density of the shell and the core with respect to the water. The shell contains both SDS and DDM headgroups as well as water, and due to the water the volume of the shell Vshell is larger than the total volume of the headgroups Vheadtot. The shell excess scattering length density was calculated as: Δ*+ = Δ*+ST

U+ST #-# (10) U+

Δ*+ST = (*+ST − *-$ )@W+-X-Y (11)

where *+ST and *-$ are, respectively, the electron densities of the head and of the solvent. For the mixture, an average between electron density of SDS and DDM is considered. More details can be found in section S1 of the SI.


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The parameters directly obtained from the fit are the thickness of the outer shell of the micelle, the total aggregation number, the ratio between the radii of the ellipsoidal core (ε), the fraction of SDS present in the micelles and the total concentration of surfactants. The SDS fraction was kept fixed, to prevent it to assume values inconsistent with the sample composition. The fraction of SDS with associated counterions was fixed to 0.62. Variations in this value did not affect the fit significantly. In addition to the parameters obtained from the fit, the core radius was also calculated as: @=Z \

3%&& U[67 (12) 4,

where NAgg is the fitted aggregation number for the micelles, ε is the eccentricity of the ellipsoid and VC12 is the volume of a single C12 chain, which is assumed to be 352.4 Å3.57 3.2 Cylindrical model for complexes of SDS/DDM micelles and PDADMAC chains Compared to the ellipsoidal model we have to redefine the geometrical parameters of radius and

volume. The volume of the core, Vcore, is obtained from the aggregation number, NAgg, and the volume of a single C12 chain, VC12, as U-? = U[] %&& (13) For the length l of the cylindrical micelles, the core radius can be written as: @-? = Z

U-? (14) ^

The total radius (Rtot) will just be given by the core radius plus a constant shell thickness D. The volume of the shell can then be calculated as: U+ = ^@#-# − U-? (15)


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In the case of complexes, the contribution of the polymer to the electron density should be taken into account. The polymer interacts electrostatically with the headgroups of SDS and this means that the polymer is partly penetrating into the micellar structures (see section 4.3). Due to those considerations, the scattering length density of the shell has to be expressed as: Δ*+ =

Δ*+ST U+ST + Δ*_%[ _%[ U_%[ (16) U+

where NDAC is the number of DAC repeat units in a micelle, VDAC is its volume and ∆ρDAC is itd excess scattering length density with respect to the water, (see eq. 11). The form factor related to the cylinders is: = 7

() = ' `@#-# ^ a*+ [2 >

c] (@-d# sine) sin(^ cose/2) 5½8 79 7 :;< ]4 @-d# Nfge ^ cose/2

− (a*+ − a*-? )@-? ^[2

k6 (8lm:no pqrs) Y(8 tups/) 8lm:no pqrs

8 tups/

]4 5½9AB v sine De (18) 7

where the integral is performed over the all possible orientations α of the micelles. ∆ρshell is the excess scattering length density of the shell with respect to the water, ∆ρcore is the excess scattering length density of the core with respect to the water, σout is the width of the Gaussian distribution used to smear the shell-water interface, σin is the width of the Gaussian distribution used to smear the core-shell interfaces. The corresponding orientationally averaged scattering amplitude of the cylinder model is given by the integral in (22) without squaring the integrant. As mentioned, a model with the possibility of having the micelles in a “cylinder on a string” structure with PDADMAC was used (see SI eq. 3). The parameters directly obtained from the fit are the thickness of the outer shell of the micelle (D), the total aggregation number (NAgg), the number of DAC monomers per micelle (NDAC), the length


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of the cylinders (l), the number of micelles clustering together on the PDADMAC (nclust), the fraction of SDS in the micelles and the total effective concentration of surfactants in micelles. No endcap is considered for the cylinders, as this gives the best representation of the sharp minima. As the cylinders are relatively thin and in a dynamic equilibrium, the absence of endcaps does not hinder the physical significance of the model. A background of free micelles was also added to the model, using the experimental data for SDS/DDM without PDADMAC as a background. However, the presence/absence of such background was not improving the fit significantly, due to the quite similar scattering of the pure micelles and of the micelles with PDADMAC in the high q region.3.3 Precipitated Phase: Debye-Bueche-like model with a Lorentzian term describing ordered phase (Hexagonal 2D phase of cylindrical micelles wrapped by PDADMAC) The data from two-phase samples show an intense scattering at low q plus Bragg-like peaks in the high q region due to the ordering. The first contribution can be modeled with a Debye-Bueche term, while the Bragg Peaks can be described with two Lorentzian terms as ()X?X#S# = Lw^4_x _x () + Lw^4y-?] y-?] ()+ Lw^4y-? y-? () (19) while the low q scattering was described by Debye-Bueche-like term: _x () =

7 1 518l{ 3 4 (20) (1 + (z_x ) )

where ξDB is a correlation length and Rg is an additional parameter in the last factor which gives stronger decay of the term. Finally, the Lorentzian terms can be written as y-?] () = y-? () =

1

(21)

1 + 1( − ] )zy 3 1

1 + |1 − ] √33zy ~

(22)


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where q1 corresponds to the q value of the peak in the intensity vs. q plot and ξL is a correlation length. The square root of three gives the position of the second order peak in a hexagonal lattice. Most of the samples show also a significant contribution from free micelles or cylinders in solution. The total scattering can then be described as a sum of the scattering from the precipitate and scattering from the free micelles, as modeled in section 3.1, giving () = ()X?X#S# + () (23) Or as a sum of the scattering from the precipitate plus cylindrical complexes in solution, giving () = ()X?X#S# + ()YT? (24) It was noticed that some of the peaks were quite narrow and therefore they are expected to be influenced by the instrumental smearing function. This was taken into accounts in the fits. 58,59 4. Results and Discussion 4.1 The structure of DDM/SDS Micelles We start our discussion with the analysis of SDS/DDM interaction, since the micelles formed by those compounds are presents before the addition of PDADMAC and the formation of the complexes, as well as they are a part of the background in these samples. The scattering curves of SDS/DDM mixtures were fitted using the ellipsoidal core-shell model as discussed in section 3.1. Examples of fit are sown in Figure 1. The values of the fit parameters for the different samples can be found in Tables S2-S3 in the SI.


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-1

10

-2

10

I(q)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-3

10

-4

10

-2

10

-1

-1

q(Å )

10

Figure 1 Example of experimental SAXS data for samples containing 4 mM SDS with 1 mM DDM (full circles) and 4 mM SDS with 10 mM DDM (empty circles). The fitting curves (lines) are obtained using an ellipsoidal model. Samples at all concentrations of SDS and DDM can be fitted as oblate ellipsoids with a constant ε of 0.6, in good agreement with the structure of pure SDS and DDM micelles.59 Also, it was seen that the core radius of the micelles decreases as the SDS content increases. This finding may suggest a slightly different organization of the alkyl chains in the micelles due to the significantly different headgroups of the anionic and nonionic surfactant. Similar effect of the surfactant headgroup was also seen recently during the comparison of the adsorption properties of nonionic, cationic and anionic surfactants with dodecyl chains at the free aqueous interface.42 The aggregation numbers in the mixed micelles are reasonable and relatively close to the aggregation numbers of pure SDS (≅ 60)43,44 and pure DDM (≅120)45-46 micelles, at higher and lower SDS-to-DDM ratios, respectively. The comparison of the fitted total concentration values of surfactants in micelles with the analytical SDS and DDM concentrations indicates that 1 to 6 mM of free surfactant (probably SDS) is in equilibrium with the mixed micelles. The data of conductivity measurements in Figure S2 of the Supporting Information also reveal a reduction of the free SDS concentration via the addition of 1 and 10 mM DDM, respectively. It should be noted, that for the samples with 2.5 mM SDS + 10 mM


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DDM and with 4 mM SDS + 10 mM DDM the total concentration of surfactant in micelles was fixed to the content of the sample. For these two compositions, keeping the parameter free resulted in too high values, incompatible with the real sample concentration of the two amphiphiles. 4.2 PDADMAC/SDS association in the absence and presence of DDM -3

a.)

1000 mgdm PDADMAC 6

without DDM 1 mM DDM 10 mM DDM

uζ / 10-8m2s-1V-1

4 2 0 -2 -4 -6 0

3

6

9

12

15

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1000 mgdm PDADMAC 2φ 2φ

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Turbidity / (100-T%)

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turbid suspension

100 80 60 40 without DDM 1 mM DDM 10 mM DDM

20 0 0

3

6

9

12

15

cSDS / mM

Figure 2a.) Mean electrophoretic mobility (uζ) of the PDADMAC/SDS/DDM complexes and b.) the turbidity (100-T%) of the systems as a function of SDS concentration at cPDADMAC = 1000 mg⋅dm-3, in the presence of 0 mM (green ), 1 mM (red ) and 10 mM DDM (blue ▲). The blue solid and the blue striped boxes indicate the precipitated and the highly turbid composition ranges, respectively, at 10 mM DDM. For the sake of clarity, the very narrow precipitation concentration ranges, where gross precipitation occurs at 0 and 1 mM DDM, respectively, are not shown in Figure 2. The PDADMAC molecules do not interact with dodecyl maltoside47,48, however, they strongly associate with the oppositely charged SDS.31,48-52 This is shown in Figure 2a and b, where the mean


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electrophoretic mobility of polycation/surfactant assemblies and the turbidity of the mixtures, prepared by the rapid-mixing protocol, are plotted as a function of SDS concentration at 1000 mg·dm-3 PDADMAC. In the absence of the nonionic additive, soluble polycation/anionic surfactant complexes are formed at low SDS concentrations. Increasing the SDS-to-PDADMAC ratio,the net charge of the nanoassemblies is reduced, whereas the turbidity of the mixture increases until in the charge neutralization SDS concentration range is observable a phase separation. With a further increase of the anionic surfactant concentration, a charge reversal of the complexes takes place and a non-equilibrium suspension of negatively charged PDADMAC/SDS complexes is formed. This is due to the quick homogenization of the system, which ensures the formation of trapped large polyion/surfactant aggregates with kinetic stability. If instead of rapid-mixing the slow-mixing protocol is applied, a massive precipitation is observable in a wide concentration range of anionic surfactant excess. Similar deviations in the impact of slow- and rapid-mixing procedures have been found in a variety of polyelectrolyte/oppositely charged surfactant mixtures.28,29,53 As indicated by the reduced mobility data of Figure 2a at polyelectrolyte excess, the addition of dodecyl maltoside increases the binding of the anionic surfactant at low SDS concentrations. This shifts the onset of phase separation to lower SDS-to-PDADMAC ratios, especially in the case of 10 mM DDM as shown in Figure 2b. These findings are attributable to the synergic binding of SDS and dodecyl maltoside on the PDADMAC molecules as revealed recently in refs. 47 and 48 in the dilute SDS concentration range. At high SDS-to-PDADMAC ratios, a suspension of PDADMAC/mixed surfactant aggregates are formed, the turbidity of which is larger compared to the dispersion of PDADMAC/SDS nanophases in the absence of DDM. This observation is related to another effect of the nonionic additive, which becomes dominant at higher surfactant concentrations Namely, in this composition range, the formation of mixed micelles reduces the


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concentration of free SDS and therefore the charge of the polycation/mixed surfactant aggregates reducing their stability compared to the PDADMAC/SDS nanoparticle dispersions.46 4.3 The structure of soluble polyion/mixed surfactant complexes formed at PDADMAC excess In this section, the molecular architecture of the positively charged nanoassemblies formed at low SDS-to-PDADMAC ratios (equilibrium one-phase systems, see Figure 3 for an example of experimental data) is explored via modeling of SAXS scattering curves. Samples containing SDS and SDS/DDM mixtures with added PDADMAC were fitted using a cylindrical model (see 3.2). The fitted parameters are available in Table S4 of the SI. From the fits we can see that both in the absence and presence of the nonionic surfactant, there is formation of relatively short cylinders (L≅ 200 Å) with aggregation numbers between 200 and 250. Thus, these data qualitatively indicate that the basic structure of the nanoassemblies is not modified considerably via the addition of DDM to the complexes. In addition, the shell thickness of the bound micelles is increased whereas their core radius is decreased compared to that of the mixed micelles without PDADMAC. This finding can be rationalized assuming that PDAMAC chain may enter into the micelle and it is located at the boundary between core and shell. As was described in section 3.2, the polymer is considered to contribute only to the shell electron density; however, if it is located close to the outer part of the core, it would be more correct to include the polymers contribution in the model as distributed between core and shell. Since this is not done, the result is an underestimation of the core radius and an overestimation of the shell thickness. In addition to modeling the structure of the assemblies, the model allowed the possibility of clustering between cylinders on the same or different polymer strings. However, as mentioned the fit clearly indicated that the cylinders can be considered as almost not interacting as the “micelle per cluster” parameter is equal to one for all the samples analyzed.


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Also, the total surfactant concentration parameter had to be kept fixed to prevent it from assuming values higher than the nominal concentration of the sample. This suggests that all the SDS participates in the complexation.

-2

I(q)

10

10

-3

10

-4

10

-2

-1

10

q (Å-1)

Figure 3 Example of experimental SAXS data for sample containing PDADMAC with 2.5 mM SDS. The fitting curve (line) was obtained using a “cylinder” model. 4.4 The structure of polyion/mixed surfactant aggregates or precipitates formed at SDS excess At high SDS-to-PADMAC ratios (above 8 mM SDS concentration), suspension of large aggregates and precipitates are observable via the application of rapid-mixing protocol. In this case, the model used was one of cylinders organized in a hexagonal structure (see section 3.4 for details on the model). Examples of experimental data are shown in Figure 4a, while the parameters obtained from the fitting are shown in Table S5 of the SI. 10

1

10

0

2x10

-2

10

-2

b)

I(q)

a)

-1

10

0.10

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q(Å

-2

10

I(q)

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-1 )

-3

10

-4

10

-5

10

0.0

0.1

-1

0.2

q(Å )

0.3


0.4

Langmuir

Figure 4 a) Example of experimental SAXS data for sample containing PDADMAC and SDS as a solid precipitate in suspension. A peak is present at high q and it can be seen how its position in the sample containing 4 mM SDS (empty circles) is shifted towards lower q values than that in the 8 mM SDS sample (full circles). b) Enlargement of the low q peak of the sample containing 4 mM SDS. It is possible to notice that the peak is not symmetric The data clearly reveals that the PDADMAC/SDS precipitate or suspension can be described by the applied model which is in line with the SAXS and Cryo-TEM results of Nizri et al.31 Furthermore, the dodecyl maltoside molecules could be well fitted into the bound cylindrical SDS micelles without a significant variation of the spacing. The only exception is represented by the sample containing 4 mM SDS and 10 mM of DDM, in which case the spacing appears to be higher (46 Å rather than the 38 Å of the other samples). To clarify the reason for this, the solid was separated by ultracentrifugation and the wet precipitate was analyzed. The same procedure was also applied to the other two-phase systems, but unfortunately it was not always possible to retrieve enough precipitate to measure SAXS data. When available, data obtained at other compositions were successfully modeled with a hexagonal structure and the fits are presented in Figure 5. Fit parameters can be found in Table S6 of the SI.

100

10

I(q)

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1

0.1 0.01

-1

q(Å )

0.1

Figure 5 Experimental SAXS data for the wet precipitate of the sample containing PDADMAC with 16 mM SDS and 10 mM DDM. The smeared (blue line) and non-smeared (red line) fitting curves were obtained using the model for hexagonal 2D precipitates described in section 3.4.


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Data for the wet precipitate of the sample containing PDADMAC with 4 mM SDS and 10 mM DDM could not be modeled as a hexagonal structure. The asymmetry in the shape of the peak (Figure 4b) may indicate the presence of two superimposing peaks and hence the presence of two or more coexisting phases. Also, the distance between the first and the second peak is not compatible with a hexagonal structure, but rather with a distorted or multiphasic one. This may be due to the presence of the high fractions of DDM in the micelles, causing a partial distortion in the structure. For the centrifuged samples, the supernatant solutions were also analyzed by SAXS and the data (Figure S3 of the SI) revealed the presence of globular micelles similar to the ones observed in absence of polymers. A first treatment of the data suggests that only DDM is present, however, a more refined fit was not performed due to the lack of information on the supernatant’s composition, causing the number of parameters to be higher than what recommended for a trustable model. Both the suspensions and the wet precipitated samples were modeled including a background of free micelles. It was not possible to fit the data with a background of oblate micelles, such as the ones formed in absence of polyelectrolyte, but good fits were obtained both considering a background of cylindrical or prolate micelles, indicating that prior to centrifugation part of cylindrical complexes may still be present in solution. It can also be noted that the cylindrical structural units observed both in the pre- and postprecipitation SDS concentration range are in good agreement with the data of recent pyrene fluorescence measurements on PDADMAC/SDS/DDM mixtures.48 Specifically, it has been shown that the characteristic ratio of the first and third vibronic peak of the pyrene fluorescent spectra (I1/I3) is roughly constant over a wide SDS-to-PDADMAC ratio range both in the absence and presence of DDM. The measured I1/I3 values indicated that the microenvironment of the pyrene probe −solubilized in the PDADMAC/SDS/DDM mixtures− is an intermediate between the aqueous medium and the hydrophobic core of the polyelectrolyte-free mixed surfactant assemblies


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over a wide composition range (which included both the soluble complexes observed at polyelectrolyte excess, and the charge stabilized colloidal dispersion region at higher SDS-toPDADMAC ratios). This latter finding can be likely attributable to the compact cylindrical mixed surfactantassembly units attached to the PDADMAC, whose interior may not be completely accessible for the pyrene molecules. In this case the pyrene molecules can rather be found in the palisade layer of the bound micelles and not in their hydrophobic core.54 Conclusions In the case of oppositely charged macromolecule/amphiphile systems, a variety of ordered macroscopic and mesoscopic structures of the precipitates or large polyion/surfactant aggregates have been reported around the charge neutralization concentration range or in the presence of ionic surfactant excess. In the one-phase dilute concentration range of polyelectrolyte excess, however, the oppositely charged polymer/surfactant complexes are generally described on the basis of the beads on a string model involving globular-like surfactant’s self-assemblies which are similar to the free surfactant micelles. In the present paper we have demonstrated that the SDS rather binds to the PDADMAC molecules in the form of small cylindrical surfactant micelles, even at low SDS-toPDADMAC ratios (i.e. at less than 20 percent of bound surfactant compared to the charges of the polyion). These anisometric surfactant self-assemblies formed at polyelectrolyte excess mimic the basic building units of the hexagonal structure of the PDADMAC/SDS precipitate and/or suspensions formed at charge equivalence or at higher SDS-to-PDADMAC ratios. The addition of dodecyl maltoside does not lead to significant changes in the structure of PDADMAC/SDS complexes or their aggregates. In the absence of the polycation, ellipsoidal mixed micelles of SDS and DDM are observed. When the polyelectrolyte is added, cylinder-like mixed surfactant self-assemblies

bind to the polymer, until at charge neutralization they undergo an

associative phase separation, leading to a hexagonal phase precipitate. The presence of DDM


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reduces the cmc and cac for the system, but does not alter significantly the structure of the complexes neither in the one-phase, nor in the two-phase region. The only exception is for samples at SDS-to-PDADMAC ratios close to charge neutralization and high concentration of DDM where the precipitate forms a multiphasic or distorted hexagonal structure. These observations are in good agreement with recent pyrene fluorescence measurements on PDADMAC/SDS/DDM mixtures48 which indicated an intermediate micropolarity of the solubilized pyrene molecules between an aqueous and an apolar medium over a wide composition range. This latter finding is related to the compact cylindrical mixed surfactant-assembly units, formed either at low or at high SDS-to-PDADMAC ratios, whose interior may not be accessible completely for the pyrene molecules. Due to this, one can infer that pyrene is situated in the outer shell of the mixed surfactant-assemblies. Finally, the comparison of our study with earlier investigations reveals that the structure of the polyion/mixed surfactant complexes crucially depends on the type of the nonionic surfactant. For instance, in contrast to dodecyl maltoside, the addition of Triton X-100 to the PDAMAC/SDS system results in the formation of disordered polycation/SDS/Triton X-100 surfactant aggregates.38This deviation in the observed structure of the polyion/mixed surfactant nanoparticles is likely to be related to the different headgroups and polydispersity of the nonionic additives. Acknowledgements This work was supported by the Hungarian Scientific Research Fund (OTKA K 108646) as well as by the NanoS3 – 290251 ITN and the COST Action CM1101 project of the European Commission, which is gratefully acknowledged. Supporting Information Available: Additional experimental information and graphs as discussed in the text. This information is available free of charge via the Internet at http://pubs.acs.org.


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Langmuir

"table of contents"

Anisometric Polyelectrolyte/Mixed Surfactant Nanoassemblies Formed by the Association of Poly(diallyldimethylammonium Chloride) with Sodium Dodecyl Sulfate and Dodecyl Maltoside

Beatrice Plazzotta, Edit Fegyver, RóbertMészáros* and Jan Skov Pedersen*

SDS + DDM SDS

PDADMAC


Mixed Surfactant Nanoassemblies Formed

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