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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Effect of Dilution on the Nonequilibrium Polyelectrolyte/Surfactant Association Krisztina Bali, Zsofia Judit Varga, Attila Kardos, Imre Varga, Tíbor Gilányi, Attila Domján, András Wacha, Attila Bóta, Judith Mihály, and Róbert Mészáros Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03255 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018

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Effect of Dilution on the Nonequilibrium Polyelectrolyte/Surfactant Association Krisztina Bali1, Zsófia Varga1, Attila Kardos2, Imre Varga1, Tibor Gilányi1, Attila Domján3, András Wacha4, Attila Bóta4, Judith Mihály4 and Róbert Mészáros1,2* 1

Laboratory of Interfaces and Nanosized Systems, Institute of Chemistry, ELTE Eötvös Loránd University, H-1117 Budapest, Pázmány Péter sétány 1/A, Hungary 2

3

Department of Chemistry, University J. Selyeho, 945 01 Komárno, Slovakia

NMR Research Laboratory, Research Centre for Natural Sciences, Hungarian Academy of Sciences, H-1117 Budapest, Magyar tudósok körútja 2, Hungary 4

Biological Nanochemistry Research Group, Institute of Materials and Environmental

Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, 1117 Budapest Magyar tudósok körútja 2, Hungary * Phone: +36-1-372-2500/1906, e-mail:[email protected] Polyelectrolyte(PE)/surfactant(S) mixtures play a distinguished role in the efficacy of shampoos and toiletries, primarily due to the deposition of PE/S precipitates on the hair surface upon dilution of the formulations. The classical interpretation of this phenomenon is a simple composition change during which the system enters into the two-phase region. Recent studies, however, indicated that the phase properties of PE/S mixtures could be strongly affected by the applied solution preparation protocols. In the present work, we aimed at studying the impact of dilution

on

the

nonequilibrium

aggregate

formation

in

the

sodium

poly(styrenesulfonate)(NaPSS)/dodecyltrimethylammonium bromide (DTAB)/NaCl system. Mixtures prepared with hundredfold dilution of concentrated NaPSS/DTAB/NaCl solutions in water were compared with those ones made by rapid mixing of dilute NaPSS/NaCl and DTAB/NaCl solutions. The study revealed that the phase-separation concentration range as well as the composition, morphology and visual appearance of the precipitates was remarkably different in the two cases. These observations clearly demonstrate that the dilution/deposition

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process is also related to the nonequilibrium phase properties of PE/S systems, which can be used to modulate the efficiency of various commercial applications.

Introduction Oppositely charged polyelectrolyte(PE)/surfactant(S) mixtures have been intensively investigated1-3 due to their frequent usage in a diversity of new applications4-6. These systems, in particular, play a crucial role in the efficacy of shampoos, due to the deposition of PE/S aggregates with other additives on the hair fibers during the dilution of the formulations.7-9 These adsorbed complexes could largely improve hair conditions, such as softness, shine and combability. The explanation of this dilution-deposition process is first given by Goddard et al. on the basis of the phase diagrams of PE/S systems. The authors described the impact of dilution as a concentration change leading to associative phase separation.10,11 Refining this concept further, Piculell and coworkers revealed that upon dilution with water, a concentrated clear solution of sodium polyacrylate and hexadecyltrimethylammonium bromide separated into a dilute aqueous phase and a solid phase of the oppositely charged components. With further addition of water the precipitate became even more concentrated in PE/S complexes due to the gain in the entropy of the released counterions of the surfactant and that of the polyion.12 Other research groups investigated directly the deposition process from PE/S mixtures upon dilution with water on different surface substrates.13-18 Lindman and coworkers reported the adsorption of precipitated cationic cellulose derivatives/sodium dodecyl sulfate (SDS) complexes on various silica surfaces upon rinsing the PE/S adsorbed layer formed at surfactant excess with the medium.13,14 Piculell et al. revealed a robust relationship between bulk phase separation and surface deposition. The authors also indicated the importance of the initial composition of PE/S mixtures and that of the hydrophobic nature of the polyions.15,16 Similar dilution-deposition phenomenon was also observable if these systems contained silicone oil


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compounds.17 The viscoelastic nature of the adsorbed polyelectrolyte/surfactant layer was also found to be significantly affected upon the dilution process as observed by Dhopatkar et al.18 The commercial cosmetic formulations usually also contain various electrolyte additives to enhance the delivery of the required benefits.7-9 In line with this requirement, Miyake et al. systematically investigated the deposition process from concentrated PE/S/salt mixtures upon dilution with water. The authors demonstrated that the deposited precipitates were packed densely at low salt concentrations while they had a mesh-like structure at higher ionic strengths.19,20 The common feature of the above studies is that their observations are interpreted on the basis of equilibrium PE/S phase properties. However, recent investigations revealed a pronounced role of trapped aggregates in the bulk21-25 and surface26-28 features of these systems at low ionic strengths. Namely, depending on the applied solution preparation methods, precipitation or the formation of charge stabilized dispersions can also be observed in the same composition range. The addition of inert electrolyte suppresses these nonequilibrium effects due to the destabilization of the colloidal dispersions as well as through largely reducing the amount of surfactants bound onto the polyions.29 In principle, PE/S/salt solutions could also be trapped in nonequilibrium states upon dilution in water due to the large concentration gradients present at the beginning of the mixing procedure. This may affect the properties of the formed bulk aggregates and their deposition on a given surface. Although the effect of different experimental pathways (such as the order of addition of the solution components, the rate of mixing, the nature of homogenization or the application of multistep preparation protocols) on the PE/S association have been thoroughly investigated,21-28 the impact of dilution in water of concentrated polyelectrolyte/surfactant/salt solutions on the nonequilibrium properties of the system has not been studied systematically yet. In order to explore this phenomenon, we aimed at comparing the characteristics of mixtures


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prepared by hundredfold dilution of concentrated PE/S/salt solutions in water, with those ones where the same final compositions were attained by rapid mixing of dilute polyelectrolyte and surfactant solutions at constant low NaCl concentrations. For this aim, we chose the sodium poly(styrenesulfonate)

(NaPSS)/dodecyltrimethylammonium

bromide

(DTAB)/sodium

chloride (NaCl) system due to the good solubility of the surfactant and polyelectrolyte and that of their mixtures in highly concentrated NaCl solutions. In addition, the equilibrium phase properties, structures and surfactant binding isotherms30-37 as well as the nonequilibrium behavior27,38 has been thoroughly investigated in similar NaPSS/cationic surfactant mixtures. Electrophoretic mobility, turbidity, UV spectroscopy, ATR-FTIR, SAXS and surfactant binding measurements were used to monitor the charge, size and composition as well as the structure of the formed nanoassemblies. We will demonstrate that the dilution process indeed affects these characteristics of the NaPSS/DTAB complexes as well as the visual appearance, stickiness and morphology of the formed precipitates.

Experimental Section Materials High molecular mass (1 MDa) poly(sodium 4-styrenesulfonate) (NaPSS) and sodium-chloride (NaCl) were purchased from Sigma-Aldrich. Dodecyltrimethylammonium bromide (DTAB, Sigma-Aldrich) was recrystallized twice in an 80 wt% acetone/20 wt% ethanol mixture. The cmc of DTAB in water was found to be 15.2 mM at 25°C from conductivity measurements. Ultrapure water (MilliQ) was used for the solution preparation. The concentrated NaPSS/DTAB/NaCl mixtures were prepared by dissolving appropriate amount of solid polymer and surfactant in 0.6 M NaCl solution.


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Solution Preparation Methods The final analytical concentration of NaPSS and NaCl was fixed at cNaPSS=2.4 mM (in monomer concentration) and at cNaCl=6 mM, and the following mixing protocols were applied: ”1:1 mixing”: NaPSS and DTAB solutions were prepared in the presence of 6 mM NaCl. Equal volumes of these salt containing surfactant (5 ml) and polyelectrolyte (5 ml) solutions (at double of their intended final analytical concentrations) were mixed quickly under continuous stirring with a magnetic stirrer (1800 rpm). “Hundredfold dilution”: 100 l of the concentrated NaPSS/DTAB/NaCl solution was added to 9.9 ml water under continuous stirring. The final NaPSS, DTAB and NaCl concentrations were the same as in the case of the “1:1 mixing” method. UV Absorbance and Turbidity Measurements The formation of soluble NaPSS/DTAB complexes was monitored by observing changes in the UV absorption spectra of NaPSS at 25 °C. A Perkin Elmer Lambda 1050 UV/VIS/NIR spectrophotometer was used to record the spectra in the 250-280 nm wavelength intervals (in a quartz cuvette with a path length of 1.00 cm). The turbidity of the mixtures was also determined at 25.0 ± 0.1 °C from the transmittance (T) measured at 400 nm using the same spectrophotometer. The turbidity is given as (100-T%) and the measurements were carried out immediately or 24 hours after solution preparation. In the case of the transparent systems, the turbidity measurements were repeated after one week and no deviations were observed within experimental error. Electrophoretic Mobility Measurements The mean electrophoretic mobility (u) of the samples was determined at 25.0 ± 0.1 °C using a Malvern Zetasizer Nano ZSP instrument. The apparatus utilizes the M3-PALS technique to derive the mean velocity of the PE/S complexes (vE) at a given electric field strength (E), from the measured frequency shift of the scattered light due to the moving particles. The mean


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mobility values are given from the 𝑢𝜁 = 𝑣𝐸 ⁄𝐸 relationship. The mobility of samples was measured directly after their preparation. However, above a certain DTAB concentration (i.e. in the precipitation composition range), prior the measurements, the mixtures was filtered through 450 nm pore-sized cellulose-acetate membrane to remove the remaining macroscopic solid precipitates. For the “hundredfold dilution” method this was necessary already from ~2.5 mM DTAB, whereas in the case of “1:1 mixing” it was possible to measure the mobility of the NaPSS/DTAB mixtures or dispersions without filtering up to 4.0 mM DTAB (at constant 2.4 mM final NaPSS concentration for both mixing methods). In order to make feasible comparison between the two mixing methods, the above-mentioned filtering procedure was applied in the whole investigated composition range prior to the mobility measurements. However, control experiments were also performed for “1:1 mixing” where the mobility data of the filtered and unfiltered samples were found to be the same within experimental error up to 4.0 mM DTAB. Characterization of the Supernatants (Density, Na+ and PSS- Concentration Measurements) The chosen samples of NaPSS/DTAB mixtures were centrifuged immediately after their preparation for 2 hours at 362 000 g with a Beckman Coulter Optima XPN-100 Ultracentrifuge at 25°C. The density of the supernatant samples was determined by the oscillation-type Anton Paar DMA 60 density meter with DMA 602 measurement cell at 25.00 ± 0.01 °C. The density values were calculated from the 𝜌1 − 𝜌2 = 𝑘(𝑇12 − 𝑇22 ) expression, where and are the density, T1 and T2 are the period time of the sample and reference, respectively. The constant k was derived by measuring two samples with known density (water and air). Instead of the absolute density, the relative density with respect to water 𝜌𝑟𝑒𝑙 = 𝜌𝑠𝑎𝑚𝑝𝑙𝑒 ⁄𝜌𝑤𝑎𝑡𝑒𝑟 was used throughout the paper. The sodium ion concentration was determined by a Jenway PFP7 flame photometer using calibration curves with NaCl solutions. The PSS content of the supernatants was found to be negligible over the investigated composition range (between 3 and 6 mM DTAB) according to UV spectra measurements. 6 ACS Paragon Plus Environment

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Small-Angle X-ray Scattering (SAXS) Small-angle X-ray scattering experiments were carried out on the CREDO instrument.39 The freshly prepared samples were transferred into borosilicate capillaries of approx. 1.5 mm outer diameter and 0.01 mm wall thickness. The sample prepared by the “hundredfold dilution” method has been taken out from its solution using a thin glass rod and pressed into the capillary already containing a small amount of the solvent. The one-to-one mixed sample was left for a couple of minutes to settle, and after removing the supernatant the slightly sedimented part of the sample has been transferred into the capillary with a pipette. The capillaries were sealed by gluing glass plugs into their open ends using a quick-setting two-component epoxy resin, and they were subsequently put into the sample holder block in the vacuum chamber of the instrument. Cu Kα X-rays were generated using a GeniX3D Cu ULD integrated X-ray beam delivery system equipped with a FOX3D parabolic multilayer mirror (Xenocs SA, Sassenage, France). The beam size at the sample has been limited to a circle of approx. 1 mm diameter with three pinholes between the X-ray source and the sample stage. Scattered X-rays were detected using a Pilatus-300k CMOS hybrid pixel detector (Dectris Ltd, Baden, Switzerland) situated 408 mm downstream from the sample. The angular range covered by this setup was between 0.26 and 6.6 nm-1 in terms of the scattering variable q=4πsinθ/λ (where 2θ is the scattering angle and λ=0.154 nm is the X-ray wavelength). Scattering patterns were recorded in several short exposures, a technique which allowed us to filter out images affected by systematic errors such as external radiations and to assess the stability of the nanostructure of the samples throughout the whole duration of the experiment. Scattering patterns were corrected and calibrated using the on-line data reduction routine implemented in the control software of the instrument. Finally, one-dimensional scattering curves were calculated by azimuthally averaging the scattering patterns.


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Attenuated Total Reflection - Fourier Transform Infrared Spectroscopy (ATR-FTIR) IR spectra were recorded by means of a Varian 2000 (Scimitar Series) FTIR spectrometer (Varian Inc, US) equipped with an MCT (Mercury-Cadmium-Telluride) detector and with a ‘Golden Gate’ single reflection diamond ATR (Attenuated Total Reflection) accessory. The freshly prepared NaPSS/DTAB/NaCl precipitates were placed immediately on the diamond ATR surface and pressed with a sapphire anvil to ensure perfect contact between the sample and the ATR element. For spectrum acquisition 64 scans with a spectral resolution of 2 cm -1 were applied. Determination of Binding Isotherms NaPSS/DTAB systems were prepared at 500 mgdm-3 NaPSS (which is equivalent to 2.4 mM monomer styrene sulfonate concentration) by the 1:1 mixing method up to the charge neutralization surfactant concentration in the presence of 100 and 600 mM NaCl concentration in D2O. The mixtures were left to stand for 24 hours at room temperature and then they were centrifuged at 362 000 g with a Beckman Coulter Optima XPN-100 Ultracentrifuge for 2 hours at 25 C°. Then the supernatant was filtered through 0.1 m membrane filter before the NMR measurements. Qualitative solution state NMR spectra were obtained by a Varian NMR System spectrometer operating at the 1H frequency of 400 MHz. All the measurements were carried out at 25 ºC with 4 s of acquisition and 20 s repetition time. Samples were thermally equilibrated for 30 minutes before the acquisition. The equilibrium free DTAB concentration was determined from the integral of the CH2 proton signal of the surfactant alkyl chains and that of the internal standard of glycine. The binding degree of DTAB on NaPSS was calculated on the basis of the following equation: =(cDTABce,DTAB)/cNaPSS , where cDTAB and ce,DTAB are the analytical and equilibrium DTAB concentrations, respectively, whereas cNaPSS denotes the monomer concentration of NaPSS.


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Results and Discussion NaPSS/DTAB association at different NaCl concentrations One of the most important effects of the dilution process in water is related to the electrolyte concentration gradients present in the beginning of the mixing procedure. Since these inhomogeneities could crucially affect the amount of locally bound surfactant ions on the polyions, we start our discussion with the presentation of the DTAB binding isotherms on NaPSS at different NaCl concentrations. The earlier NaPSS/DTAB binding studies30-32 based on the application of surfactant ion selective electrodes. However, the determination of the free surfactant concentration, and thus the bound amount of DTAB was limited to a binding degree of 0.6 presumably due to the interaction between the NaPSS and/or the NaPSS/surfactant aggregates with electrode membranes.32 This fact inspired us for the determination of the bound amount of DTAB up to the charge neutralization of the PSS molecules (i.e. up to =1) utilizing NMR technique for the measurement of free surfactant concentration after separation of the NaPSS/DTAB complexes from the mixtures. In Figure 1, the measured binding isotherms of DTAB are shown in the presence of 100 mM and 600 mM NaCl. For the sake of comparison, the binding data of Almgren et al32 without added salt is also plotted in the graph. As indicated in Figure 1, the data approximately fall onto the same master curve in the investigated surfactant concentration range regardless of the applied salt concentration. However, the accessible equilibrium free surfactant concentration range and thus, the maximum available amount of bound DTAB decreases with increasing NaCl concentration. This finding is attributable to the salt concentration dependence of the cmc (2.6, 7.8 and 15.5 mM DTAB in 600, 100 and 0 mM NaCl, respectively40), which represents an approximate upper limit for the chemical potential of the surfactant. We note that the bulk cmcs of pure DTAB solutions at the given NaCl concentrations (indicated by horizontal lines in Figure 1) give good estimates for that critical equilibrium free surfactant


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concentration, where polymer free micelles also appear in addition to the NaPSS/DTAB complexes. Because of the largely reduced cmc in 600 mM NaCl, the charges of PSS molecules can only be partially compensated by DTAB binding, and therefore precipitation is not observable in this case. This finding is consistent with earlier results on NaPSS/cationic surfactant mixtures, which revealed that the associative phase separation was completely suppressed via the addition of a large amount of NaCl.29,30

1,5

water ref 32 100 mM NaCl 600 mM NaCl

cmc

cmc

cmc

1,0



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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0,5

0,0 0,01

0,1

1 ce,DTAB / mM

10

.

Figure 1. The bound amount of DTAB (normalized to the amount of NaPSS monomers, ) as a function of the equilibrium concentration of the free surfactant molecules (ce,DTAB) in 100 mM and 600 mM NaCl solution determined from NMR measurements. The binding isotherm data in pure water were taken from ref 32. The bulk cmcs of pure DTAB (which estimate those critical equilibrium free surfactant concentrations, where polymer free micelles appear in addition to the NaPSS/DTAB assemblies at the given NaCl concentrations) are also shown in the graph by horizontal lines.


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Impact of dilution on the complexation and phase properties We aimed at studying systematically the effects of dilution in water from thermodynamically stable concentrated solutions of NaPSS, DTAB and NaCl. The mixtures were prepared through the application of two considerably different mixing methods. The final concentrations of NaPSS (2.4 mM) and NaCl (6 mM) were kept constant, whereas the concentration of DTAB varied from 1 mM to 6 mM. During the first mixing method (“1:1 mixing”), NaPSS and DTAB solutions (both containing 6 mM NaCl) were mixed rapidly in equal volumes. In the case of the second mixing method (“hundredfold dilution”), largely concentrated NaPSS/DTAB/NaCl solutions (50 gdm-3 NaPSS (240 mM in monomer concentration), 600 mM NaCl with DTAB concentrations between 100 and 600 mM) were prepared at first. Next, these solutions were diluted in water hundredfold resulting in the same final compositions than that of the first mixing method. In Figure 2, the turbidity and the mean electrophoretic mobility of the mixtures were compared for the two types of mixing protocol. As expected, the charges of NaPSS compensated gradually as a consequence of the cationic surfactant binding resulting in decreasing (absolute) mobility values and increasing turbidity and/or precipitation. However, there are remarkable differences in the observed data over an intermediate surfactant concentration range (~ 2.5 - 4.0 mM DTAB). First of all, the precipitation composition range is significantly wider in the case of the largely diluted samples, whereas for the “1:1 mixing” method, mixtures with gradually increasing turbidity are observable instead of precipitation. Second, the mobility data are slightly lower for the “hundredfold dilution” mixing protocol over the mentioned DTAB concentration range. In this composition regime, the standard error of the measured u values are enhanced – especially for the samples made by “hundredfold dilution” – compared to the lower or higher DTAB concentrations, where the two preparation methods lead to similar mobility values.


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a.)

1:1 mixing hundredfold dilution

cNaPSS = 2.4mM, cNaCl = 6 mM

80

100-T%

60 40 20 0 1

2

3

4

5

6

cDTAB / mM

b.) 1:1 mixing (filt.) hundredfold dil. (filt.) 1:1 mixing (unfilt.)

cNaPSS = 2.4 mM cNaCl = 6 mM

-1 -1

0

-8

2

u / 10 m V s

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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-2

-4

1

2

3

4

5

6

cDTAB / mM

Figure 2. a.) Turbidity and b.) mean electrophoretic mobility values of NaPSS/DTAB/NaCl samples prepared via different methods as a function of the analytical surfactant concentration (cDTAB). The red and blue shaded areas designate the formation of precipitates in the mixtures prepared by “1:1 mixing” and “hundredfold dilution”, respectively. The standard error of the data is denoted by error bars except when it is commensurate with the size of the symbols.


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As we discussed in the Experimental Section, prior to the mobility measurements the asprepared mixtures were filtered to remove macroscopic precipitates formed above a certain DTAB concentration (cDTAB > 2 mM for “hundredfold dilution” and cDTAB >4 mM for “1:1 mixing” ). This filtering procedure was extended to the transparent and one phase concentration range as well. Thus, in Figure 2b the mobility data of these filtered PSS/DTAB/NaCl mixtures were compared for the two mixing methods over the whole composition range. In the case of “1:1 mixing”, however, it was possible to determine the mobility of the suspensions formed directly after the preparation of the mixtures up to 4 mM DTAB (without the need of filtering). As shown in Figure 2b, the mobility values of the filtered and unfiltered samples are indistinguishable within experimental error for “1:1 mixing”. Therefore, the mixing dependent deviations in the mobility of the filtered samples over the intermediate DTAB composition range is likely to be attributable to the different size and charge distribution of the NaPSS/DTAB aggregates for the two mixing methods. In Figure 3, the photos of the systems prepared via the two kinds of mixing methods are shown. As indicated by the pictures, the mixtures look largely different for the two types of preparation protocol. Namely, the “1:1 mixing” method results in turbid mixtures or fluffy precipitated samples, while the “hundredfold dilution” method leads to rubber-like, sticky aggregates in the same composition range. It is important to note that these rubber-like precipitates were also formed temporarily upon dilution of the concentrated samples in the ~1.5-2.0 mM DTAB (final) concentration range, however, they redissolved within a day.


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1:1 mixing

hundredfold dilution

cDTAB Figure 3. Photos of the NaPSS/DTAB/NaCl mixtures prepared by the “1:1 mixing” (top) and “hundredfold dilution” (bottom) methods. The analytical concentration of DTAB increases from left to right (1, 2.5, 3, 4 and 6 mM). cNaPSS = 2.4 mM, cNaCl = 6 mM

Comparison of the structure of precipitates formed by the two mixing methods Because of the large deviations in the visual appearance of the precipitates, formed through the application of the two preparation methods, we analyzed their structure using IR and SAXS techniques. The ATR-FTIR spectra of the precipitates for the two mixing methods at one composition (at 2.4 mM NaPSS, 5 mM DTAB, and 6 mM NaCl final concentrations) are shown in Figure 4a, together with the spectra of the solid polyelectrolyte and surfactant samples. The fingerprint spectral region (1300-1000 cm-1) clearly indicates small but reproducible differences between the results of the two protocols. The largest deviation is observable in the S-O stretching band around 1180 cm-1 with a shoulder at 1214 cm-1. Better resolution is offered by creating the second derivative spectra (Fig 4b), where an additional local minimum at 1190 cm-1 appears, corresponding to a band component, too. The observed band splitting of the triple 14 ACS Paragon Plus Environment

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degenerated normal vibration mode for SO3- is attributed to the perturbation of the SO3- Td symmetry (towards C2v symmetry)41 by addition of DTAB. For “1:1 mixing”, the perturbing effect is more pronounced and the main peak position is shifted to smaller wavenumber compared to the precipitate formed by the other mixing method. These findings may suggest a more compact structure for the “1:1 mixing” precipitates.

1036

1127

1175

1215 1214

Absorbance / arb. units

1181

1179

hundredfold dilution 1:1 mixing NaPSS DTAB

1200

1009

a.)

1000

Wavenumber / cm

-1

b.) -4

4x10

hundredfold dilution 1:1 mixing

-4

2x10

0

1190

-4

-2x10

-4

1218

-4x10

-4

-6x10

1178

Second derivative / arb. units

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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-4

-8x10

-3

-1x10

1240

1220

1200

1180

Wavenumber / cm

1160

-1

Figure 4. a) The ATR-FTIR spectra of solid NaPSS and DTAB samples and that of the NaPSS/DTAB/NaCl precipitates prepared by the two different mixing methods with the final concentrations of cDTAB = 5 mM, cNaPSS = 2.4 mM and cNaCl = 6 mM at t = 25 °C. b) Second derivative spectra of the NaPSS/DTAB/NaCl precipitates.


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SAXS measurements were also performed for the two kinds of precipitates at the same composition as in Figure 4. The scattering curves of the 1:1 mixed and hundredfold diluted samples (shown in Fig. 5) are characterized by complex and broad small-angle diffraction peaks, which is the result of a non-lamellar structure. This finding is in line with the observations of Sitar et. al, who found a less-ordered structure for the poly(styrenesulfonate (PSS-)/dodecyltrimethylammonium(DTA+) complex salt in contrast to the well-defined hexagonal structure of the PSS-/hexadecyltrimethylammonium (CTA+) salt.35 The position of the first peak (q1) indicates the most prominent periodic repeat distance (d) in the sample by the relation d=2π/q1.

Relative intensity / a.u.

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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hundredfold dilution 1:1 mixing

1.81

0,8

1.76

0,6 0,4 0,2 0,0

1

2

3

4

5

-1

q / nm

Figure 5. The small-angle scattering curves of NaPSS/DTAB/NaCl precipitates prepared by different methods. The final concentrations of the components were cDTAB = 5 mM, cNaPSS = 2.4 mM and cNaCl = 6 mM at t = 25 °C. The SAXS measurements were carried out on a homogenized turbid suspension of the “1:1 mixing” sample and on the compact precipitate part of the “hundredfold dilution” sample. Consequently, the latter sample exhibits higher intensity given in absolute units of scattering cross section.


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This gives d=3.49 and 3.58 nm for the “1:1 mixing” and “hundredfold dilution" samples, respectively, hinting that the latter mixing method leads to a slightly looser structure compared to the other one. This is further supported by the increasing width of the first reflection peak (from 0.22 to 0.31 nm-1), which corresponds to a less ordered structure in the hundredfold diluted sample. These findings indicate that the nanostructures of the aggregates formed via “hundredfold dilution” exhibits a more expressed amorphous character compared to the systems prepared by “1:1 mixing” in agreement with the ATR-FTIR measurements.

Impact of the mixing methods on the morphology of soluble NaPSS/DTAB complexes According to Figure 2, the applied preparation protocols do not affect the mean mobility and the turbidity of the mixtures in the one-phase region (i.e. below ≈ 2 mM DTAB) within experimental error. However, a sudden reduction in the local electrolyte concentration around the NaPSS/DTAB complexes upon dilution may cause changes in their structure which are unseen by the mobility and turbidity measurements. This phenomenon may be monitored through the UV absorbance spectra of the mixtures, since they are sensitive to the ionic strength dependent conformational changes of PSS molecules.42-44 In Figure 6a and b the UV spectra of the NaPSS/DTAB mixtures and that of the pure NaPSS solution in the presence of 6 mM NaCl were plotted immediately and 24 hours after preparation, respectively. As indicated by the Figure, the absorption reduces (hypochromic shift) and a red shift of the characteristic band positions is also observable as a consequence of the addition of the surfactant. These spectral changes may be connected to the variations in the ionic strengths between the samples with different DTAB concentrations. However, according to the binding isotherms in Figure 1, the maximum variation in the ionic strength (due to the differences in the free surfactant concentration) is estimated to be around 0.1 mM, which is a negligible effect in the presence of 6 mM NaCl.41-43


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a.) immediately after preparation

0,8

without DTAB

1.00 1.25 Hundredfold dilution

0,6

Absorbance

cDTAB / mM 1:1 mixing

PSS

without DTAB

1.00 1.25

0,4

0,2 250

260

270

280

/ nm

b.)

cDTAB / mM

24 hours after preparation

0,8

1:1 mixing

PSS

without DTAB

1.00 1.25 Hundredfold dilution

0,6

Absorbance

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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without DTAB

1.00 1.25

0,4

0,2 250

260

/ nm

270

280

Figure 6. UV spectra of NaPSS and soluble NaPSS/DTAB complexes prepared by the two mixing methods a.) immediately and b.) 24 hours after solution preparation. In Figure 6b the immediately measured spectrum of NaPSS after “1:1 mixing” is also indicated by black dashed line (replotted from Figure 6a). For the sake of clarity, the immediately measured UV spectra of the investigated NaPSS/DTAB samples are not replotted in Figure 6b, since they did not reveal any time dependence within experimental error. The final polyion and salt concentrations are equal to cNaPSS = 2.4 mM and cNaCl = 6 mM.

In recent work, Popov and coworkers reported very similar hypochromic and batochromic shifts due to the binding of TTAB (tetradecyltrimethylammonium bromide) on NaPSS at constant ionic strength.36 The authors clearly demonstrated via additional ESR measurements that these 18 ACS Paragon Plus Environment

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results are consistent with the surfactant binding induced conformational changes of the PSS molecules, possibly due to the stacking of the aromatic rings. The most remarkable features of Figure 6, however, are related to the mixing method dependent differences in the observed spectra of NaPSS and that of the NaPSS/DTAB complexes. For the bare polyelectrolyte a slight (but well-reproducible) hypochromic shift is induced by the “hundredfold dilution”, which completely disappears after one day, when both mixing methods result in the same spectra. This finding suggests that a more coiled conformation is formed upon “hundredfold dilution”, which relaxes back to its (quasi)-equilibrium state over a day. When surfactant is added to the polyion, similar small hypochromic shift is observable for the largely diluted samples. However, this shift remains unaltered over a day, in sharp contrast to the pure NaPSS solution. These spectral changes suggest, that the large concentration gradients upon “hundredfold dilution” lead to a frozen state of the NaPSS/DTAB complexes, possibly due to the more stacked arrangements of the aromatic parts of the PSS molecules, which could equilibrate much slower compared to the surfactant free polyion.

Interpretation of the effect of dilution on the nonequilibrium aggregate formation The previous sections clearly revealed that the application of the two mixing methods leads to remarkable deviations in the morphology, stickiness and visual appearance of the formed NaPSS/DTAB assemblies. These findings are likely to be attributable to the preparation protocol dependent compositions of the formed dilute aqueous solution and concentrated solid phases. In order to test the possible variations in the composition of the solution phase, density, sodium ion and PSS- concentration measurements of the supernatant of NaPSS/DTAB/NaCl mixtures (which were centrifuged immediately after their preparation) were carried out. The concentration of the PSS- ions was found to be negligible via UV measurements at each compositions for both mixing methods. The density as well as the sodium ion concentration


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data are plotted in Figure 7a and b, respectively, in the function of the analytical DTAB concentration for the two mixing protocols. a.)

cNaPSS = 2.4 mM, cNaCl = 6 mM

1,00055


rel

1,00050

1,00045

1,00040

3

4

5

6

cDTAB / mM

b.) cNaPSS = 2.4 mM


12

cNaCl = 6 mM

10

cNa+/mM

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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total sodium ion concentration

8 6 4

2

3

4

5

6

7

cDTAB / mM

Figure 7. a.) The relative density (normalized to the density of water, rel) and b.) the concentration of sodium ions in the supernatant of the centrifuged samples as a function of the analytical surfactant concentration for the two preparation protocols at 25.0°C.

According to the graphs, both the measured density as well as the sodium ion concentration is lower for the supernatants of the mixtures prepared by the "hundredfold dilution" protocol. These results suggest that the precipitates formed upon the dilution method are more concentrated in the counterions compared to the ones produced by “1:1 mixing” at the same analytical concentrations of the components. 20 ACS Paragon Plus Environment

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These deviations in the composition of the precipitates (aggregates) are closely related to the considerably different concentration gradients present in the beginning of the two mixing methods. The major difference is the huge jump in the ionic strength as well as in the NaPSS and DTAB concentrations developed upon “hundredfold dilution” in contrast to “1:1 mixing”, where the NaCl concentration is constant and much lower concentration gradients are present for the polymer and the surfactant as well. In order to assess whether the impact of the ionic strength jump is superior compared to the effects of the NaPSS and DTAB concentration gradients during the application of the “hundredfold dilution” method, comparison with similar dilution experiments at constant low ionic strength (i.e. in 6 mM NaCl) would be useful. However, thermodynamically stable solutions (i.e equilibrium one-phase systems), which are hundredfold more concentrated in the polymer and that of the surfactant as compared to the phase separation concentration range of these components in Figs 2-5, cannot be prepared in 6 mM NaCl. This is due to the low ionic strength, which is not enough to suppress the associative phase separation contrary to the PSS/DTAB mixtures prepared in 600 mM NaCl. For instance, upon mixing of concentrated NaPSS and DTAB solutions (both containing 6 mM NaCl) in equal volumes, the system separates into a gel-like and an aqueous phase (see Figure S1 of the Supporting Information, SI). Similar two-phase systems are formed, when solid NaPSS and DTAB samples are mixed under continuous stirring with 6 mM NaCl solution over the same composition regime (i.e. to be comparable with the concentration range of Figs 2-5 after hundredfold dilution in 6 mM NaCl) as shown in Figure S2. In fact, the formation of a gel-like phase in contact with a solution phase was already observable from low DTAB-to-NaPSS ratios in 6 mM NaCl. These latter mixtures conserved the two-phase state (i.e. the gel did not dissolve) after their hundredfold dilution in 6 mM NaCl even outside the phase-separation concentration range in Figure 2. For example, upon hundredfold dilution


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in 6 mM NaCl of the two-phase system with 240 mM PSS, 200 mM DTAB and 6 mM NaCl initial composition, no redissolution of the soft concentrated phase was observed although the final analytical concentrations (2.4 mM PSS, 2 mM DTAB and 6 mM NaCl) would indicate an equilibrium one phase system according to Figure 2. Similar results were also observed at even lower DTAB-to-PSS ratios as well (see Figure S3 of the SI). These latter findings clearly indicate that the dissolution of the gels is largely hindered due to the slow exchange processes between the soft concentrated and the dilute solution phase. It is also likely that this kinetic barrier leads to a trapped ion distribution and therefore to a compositional heterogeneity within the gel-like phase (i.e. it could cause a significant variation in the amount of surfactant ions bound to the individual PSS chains). The above-mentioned results suggest that the impact of the initial concentration gradients of the added salt and that of the polymer and surfactant are coupled and mutually responsible for the observed deviations in the properties of the NaPSS/DTAB aggregates formed via the application of the two mixing methods. In the light of the previous results and reasoning, the various processes which may occur upon dilution of concentrated NaPSS/DTAB/salt formulations in water can be summarized as follows. The huge NaCl concentration gradients present during the homogenization of the mixtures have at least two important consequences. According to the binding isotherms in Figure 1, it can be expected, that the cmc and thus the maximum bound amount of DTAB on PSS increases with reducing NaCl concentration. This will lead to decreasing solubility of the NaPSS/DTAB complexes and increasing dispersion forces acting between them. Another important effect of the decreasing ionic strength upon dilution is the increasing persistence length (i.e. stiffness) of the PSS molecules. On the other hand, with decreasing ionic strength the large local polyion and surfactant concentrations could lead to phase separation even at compositions outside of the two-phase


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boundary (with nonuniform distributions of the surfactant ions bound onto the individual macroions). These reasoning also explain the more extended precipitation composition range and the lower electrophoretic mobility values observed for the “hundredfold dilution” method. In addition, because of the huge ionic strength jump, the diluted samples could conserve more stacked, nonequilibrium conformations of the polyion as suggested by the UV absorbance results in Figure 6. In contrast, upon “1:1 mixing”, the polymer chains remain stiffer due to the constant low ionic strength. Therefore, the PSS molecules may be organized into a more compact structure than during the "hundredfold dilution" method, where the aggregation of the complexes leads to a more amorphous state due to the trapped conformation of the polymer chains in agreement with the SAXS and ATR-FTIR results in Figs 4 and 5. Finally, according to Figure 7 more counterions remain trapped in the concentrated phases formed upon “hundredfold dilution” compared to the precipitates produced by “1:1 mixing”. To assess the impact of the locally increased concentration of the compounds, we also studied the structure of the precipitate formed at the same DTAB-to-NaPSS monomer ratios as in Figs 4 and 5, but at 10 times higher final NaCl (60 mM), NaPSS (24 mM) and DTAB (50 mM) concentrations. The results of the SAXS measurements (see Figure S4 of the SI) revealed higher characteristic distance (d  3.70 nm) and further broadening of the first peak compared to the SAXS curves in Figure 5. This finding suggests that the higher concentrations of the solution components and the higher ionic strength lead to more amorphous structure of the PE/S aggregates possibly due to the more stacked conformation of the PSS chains. One may argue that the observed phenomenon is a specific effect due to the characteristics of the NaPSS/DTAB systems. Indeed, the association between PSS and cationic surfactants differs remarkably compared to the systems of other hydrophilic polyions and oppositely charged amphiphiles.30-37 In addition to the electrostatic and hydrophobic interactions governing generally the PE/S assembly formation, there is an additional interaction between the benzene


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ring of PSS and that of the surfactant ions resulting in very low cac values compared to hydrophilic polyions with similar linear charge densities.30-32,37 The shape of the binding isotherms is also different compared to hydrophilic polyion/amphiphile systems. Above a binding degree 0.5, the initial steeply increasing part is followed by a slowly increasing noncooperative part in accordance with Figure 1.30-32,37 This unusual feature of the isotherms is attributed to a structural transition according to the studies of Sitar35, Popov36 and their coworkers. However, in spite of the unique features of NaPSS/cationic surfactant systems, the large dilution may cause similar nonequilibrium phase separation in a variety of other PE/S systems. The changes in the appearance, structure and stickiness of the formed precipitates are likely to be dependent on the chemistry of the applied polyions and surfactants with a special importance of the hydrophobic nature of the polymers. Further studies are in progress to explore the role of polyelectrolyte chemistry in the development of trapped states upon dilution.

Summary and Conclusions We have shown that upon hundredfold dilution of a concentrated NaPSS/DTAB/NaCl solution in water, a solid phase separates out, which is different from the precipitate formed at the same final composition as a result of mixing salt containing dilute polyion and surfactant solutions in equal volumes. Specifically, a less hydrated but more sticky and amorphous solid phase formed in the former case due to the higher local concentration of the ionic compounds and thus the nonequilibrium conformation of the PSS molecules. Furthermore, the precipitation concentration range is also larger compared to the “1:1 mixing” method. These results clearly demonstrate that during the dilution/deposition processes of shampoos, not only the equilibrium phase properties but the nonequilibrium aggregate formation and phase separation could also play an important role. Therefore, the present study may contribute to the development of more efficient personal care applications.


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ACKNOWLEDGEMENTS SUPPORTED MINISTRY

OF

BY THE

ÚNKP-17-3 NEW NATIONAL EXCELLENCE PROGRAM

OF THE

HUMAN CAPACITIES. This publication is the partial result of the Research &

Development Operational Programme for the project "Modernization and Improvement of Technical Infrastructure for Research and Development of J. Selye University in the Fields of Nanotechnology and Intelligent Space", ITMS 26210120042, co-funded by the European Regional Development Fund. A part of this work was also supported by the Hungarian Scientific Research Fund (OTKA K 108646), which is gratefully acknowledged. The CREDO research instrument has been funded jointly by Gedeon Richter Plc, the Hungarian National Scientific Research Fund (grant No. CNK 810520) and the Central Hungarian Operative Program of the National Research, Development and Innovation Fund (KMOP-1.1.2-07/12008-0002)

Supporting Information Available: Additional experimental information, graphs and photos as discussed in the text. This information is available free of charge via the Internet at http://pubs.acs.org.


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"Table of contents"

Effect of Dilution on the Nonequilibrium Polyelectrolyte/Surfactant Association Krisztina Bali1, Zsófia Varga1, Attila Kardos1,2, Imre Varga1,2, Tibor Gilányi1, Attila Domján3, András Wacha4, Attila Bóta4, Judith Mihály4 and Róbert Mészáros1,2

1:1 mixing

100x dilution

vs


Surfactant

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