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Structure of Polystyrene Sulfonate/Surfactant Mixtures at Air-Water Interfaces and their Role as Building Blocks for Macroscopic Foam Felix Schulze-Zachau, and Björn Braunschweig Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00400 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017
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Structure of Polystyrene Sulfonate/Surfactant Mixtures at Air-Water Interfaces and their Role as Building Blocks for Macroscopic Foam Felix Schulze-Zachau1,2 and Björn Braunschweig*1
1
Institute of Physical Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstrasse 28/30, 48149 Münster, Germany
2
Erlangen Graduate School in Advanced Optical Technologies (SAOT), Friedrich-AlexanderUniversität Erlangen-Nürnberg (FAU), Paul-Gordan-Strasse 6, 91052 Erlangen, Germany
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ABSTRACT Air/water interfaces were modified by oppositely charged poly(sodium 4-styrenesulfonate) (NaPSS) and hexadecyltrimethylammonium bromide (CTAB) polyelectrolyte/surfactant mixtures and were studied on a molecular level with vibrational sum-frequency generation (SFG), tensiometry, surface dilatational rheology and ellipsometry. In order to deduce structure property relations, our results on the interfacial molecular structure and lateral interactions of PSS-/CTA+ complexes were compared to the stability and structure of macroscopic foam as well as to bulk properties. For that, the CTAB concentration was fixed to 0.1 mM, while the NaPSS concentration was varied. At NaPSS monomer concentrations 0.1 mM result in a significant decrease in surface pressure and a complete loss in foamability. However, SFG and surface dilatational rheology provide strong evidence for the existence of PSS-/CTA+ complexes at the interface. At polyelectrolyte concentrations >10 mM, air-water interfaces are dominated by an excess of free PSS- polyelectrolytes and small amounts of PSS-/CTA+ complexes which, how-
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ever, provide higher foam stabilities compared to CTAB free foams. The foam structure undergoes a transition from wet to polyhedral foams during the collapse. KEYWORDS Structure-property relation, micro to macro, surfactant, polyelectrolyte, interface chemistry, colloids
INTRODUCTION Foams are of great importance in many applications such as lightweight construction and heat insulation as well as in in food or cosmetic products.1–4 Mixtures of oppositely charged polyelectrolytes and surfactants also play a major role in cosmetics, pharmaceuticals, detergents, and mineral processing. Because liquid-gas interfaces are a major hierarchical element in foams, it is essential to understand the relation between molecular building blocks at gas-liquid interfaces and macroscopic foam properties such as foamability and stability via structure-property relationships. The latter can be used to predict and to tailor foam properties in a targeted way. Both coverage and structure formation of molecules at the interface are major factors that can dominate the stability of foam.5 That is because the latter determine the molecular interactions at the interface and the energy demand to create gas-liquid interfaces, which are both driving forces that determine bubble coalescence, foam drainage and Ostwald ripening.6,7 Changing interactions between molecules at the interface from repulsive to attractive regions is therefore one possibility to change interfacial and thus foam properties. This can be achieved by suppressing repulsive electrostatic interactions in a way that other interactions such as hydrophobic or van der Waals interactions become dominant. A possible way to realize the latter are for-
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mulations with positively charged surfactants and negatively charged polyelectrolytes in a concentration range where a point of zero net charge is being crossed at the interface. Surface and bulk behavior of oppositely charged polyelectrolyte/surfactant mixtures have been previously discussed in the literature,8–16 where it was shown that the polyelectrolyte/surfactant ratio can have a substantial influence on foam stability.12,17 Formulations with mixtures of tetradecyltrimethylammonium bromide (MTAB) and poly(acrylamidomethylpropanesulfonate) sodium salt (PAMPS) showed the highest stability of foam films when an enhanced synergistic adsorption of both species at the interface is observed which was at monomer concentrations of 0.1 mM and 1 mM for MTAB and PAMPS, respectively.9 Surface excess measurements revealed complex changes in the surface adsorption behavior when changing the PAMPS concentration, showing that there is a depletion of both molecules from the air-water interface when the PAMPS concentration is increased to 3 mM which also corresponds to a lower foam film stability.9 Tensiometry of PAMPS/DTAB (dodecyltrimethylammonium bromide) mixtures showed lower surface tensions when co-adsorption of the weakly surface active polyanion and the cationic surfactants took place and a highly surface active complex at the interface was proposed while binding in the bulk was negligible.18 Screening of net charges in mixed layers of polyelectrolyte and surfactant is shown to reduce the stability of foam films and is caused by a reduction of electrostatic repulsion between the surfaces19,20 which stabilize foam films. However, the stability of macroscopic foams is shown to have a maximum when net charges are screened.21 Here, the major dynamic effects that are crucial for foam stabilization are Ostwald ripening, foam drainage and bubble coalescence.7 Bubble coalescence and Ostwald ripening are inherently controlled by the properties of the ubiquitous air-water interfaces, while drainage is effected by the bubble size distribution and the bulk viscoelasticity.22–24
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Nevertheless, the exact nature of the adsorption mechanism and the influence of the charging in the system are yet not fully understood. Petkova et al.17 showed that strongly interacting systems like cationic surfactant and anionic polyelectrolytes provide enhanced foam stability, but reduced foam capacity. However, clear synergistic effects on surface tension lowering and/or foam capacity were observed independent of the nature of surfactants and polyelectrolytes (anionic, cationic, nonionic).17,25,26 Consequently, the stability of macroscopic foams from mixtures of polyelectrolytes and surfactants can be enhanced not only by charge screening, but also by other effects that demand further studies. In order to understand the properties and the composition of the interface, it is equally important to understand the phase behavior of the bulk system. Meszaros et al.27 and later Abraham et al.8,28 have shown that oppositely charged polyelectrolyte/surfactant systems might be far from an equilibrium state even if the surface appears to be in a (local) equilibrium. Investigations of such mixtures in the bulk have provided evidence that in concentration regions around the point of zero net charge, the oppositely charged components and their complexes tend to form aggregates and phase separation in the bulk is taking place due to a lack of colloidal stability. Resolving interfacial properties is important, however, the latter also shows that it is equally important to address possible aggregate formation and precipitation in the bulk solution over long time periods. The effects of the colloidal stability of oppositely charged polyelectrolyte/surfactant mixtures on the surface properties have been widely discussed in the literature. Here, major contributions come from Campbell and co-workers.29–33 In our work, we report on a unique combination of interface specific techniques such as ellipsometry, SFG spectroscopy and tensiometry as well as macroscopic foaming experiments to investigate the structure-property relationship between surfactant/polyelectrolyte modified air-
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water interfaces and macroscopic foam. Vibrational sum-frequency generation (SFG) spectroscopy has been previously applied to study interfaces of polymers34, surfactant35 and proteins,36 however, only a few studies exist that address polyelectrolyte/surfactant mixtures37,38 and none of these show specific bands of each component in different spectral regions. By addressing both solvent and adsorbate specific bands, we gain detailed information on the molecular composition, charging state and structure of the interfacial layer.
MATERIALS AND METHODS Sample preparation Hexadecyltrimethylammonium bromide (CTA+/Br-, CTAB; purity >99%) was purchased from G-Biosciences and was used as received. Deuterated CTAB (purity >99 %, degree of deuteration >99.2 %) was purchased from CDN Isotopes and used as received. Poly(sodium 4styrenesulfonate) (Na+/PSS-, NaPSS) with an average molecular weight of about 70,000 g/mol was purchased from Sigma-Aldrich and was used as received. Stock solutions were prepared by dissolving the necessary amounts of surfactant and polyelectrolyte powders in ultrapure water (18.2 MΩcm; total oxidizable carbon
