Article pubs.acs.org/Langmuir
Effect of Polymer Molecular Weight and Solution pH on the Surface Properties of Sodium Dodecylsulfate-Poly(Ethyleneimine) Mixtures Silvia S. Halacheva,† Jeff Penfold,†,‡,* Robert K. Thomas,† and John R. P. Webster‡ †
Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, United Kingdom ISIS Facility, STFC, Rutherford Appleton Laboratory, Chilton, Didcot, OXON, United Kingdom
‡
S Supporting Information *
ABSTRACT: The effect of polymer molecular weight and solution pH on the surface properties of the anionic surfactant sodium dodecylsulfate, SDS, and a range of small linear poly(ethyleneimine), PEI, polyelectrolytes of different molecular weights has been studied by surface tension, ST, and neutron reflectivity, NR, at the air−solution interface. The strong SDS−PEI interaction gives rise to a complex pattern of ST behavior which depends significantly on solution pH and PEI molecular weight. The ST data correlate broadly with the more direct determination of the surface adsorption and surface structure obtained using NR. At pH 3, 7, and 10, the strong SDS−PEI interaction results in a pronounced SDS adsorption at relatively low SDS and PEI concentrations, and is largely independent of pH and PEI molecular weight (for PEI molecular weights on the order of 320, 640, and 2000 Da). At pH 7 and 10, there are combinations of SDS and PEI concentrations for which surface multilayer structures form. For the PEI molecular weights of 320 and 640 Da, these surface multilayer structures are most well-developed at pH 10 and less so at pH 7. At the molecular weight of 2000 Da, they are poorly developed at both pH 7 and 10. This evolution in the surface structure with molecular weight is consistent with previous studies,1 where for a molecular weight of 25 000 Da no multilayer structures were observed for the linear PEI. The results show the importance with increasing polymer molecular weight of the entropic contribution due to the polymer flexibility in control of the surface multilayer formation.
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INTRODUCTION Mixtures of polyelectrolytes with ionic surfactants of opposite charge are extensively used in a wide range of applications, which include detergent formulations, cosmetics, foods, paints, coatings, and in lubrication.2,3 As a consequence, their surface and solution properties have been extensively studied, and the combination of techniques such as surface tension, ST, and neutron reflectivity, NR, has provided significant progress toward a detailed understanding of many aspects of their adsorption behavior.4,5 In many of these applications and potential applications, the ability to form surface multilayer structures offers many attractive benefits: in the delivery of therapeutic agents or perfumes, in antibacterial and anticoagulation applications, in the control of cell adhesion and growth, and in soft and biolubrication, such as hair and fabric conditioning.6−10 Such surface multilayer structures are encountered in concentrated surfactant solutions11,12 and in systems designed primarily for encapsulation.13,14 Indeed, layerby-layer, LbL, formation of surface multilayer structures by the sequential adsorption of polyelectrolytes and surfactants of opposite charge has been extensively studied and exploited.6,7,15 More recently, it has been demonstrated that spontaneous surface multilayer formation can be induced in ionic surfactants by the addition of multivalent counterions, such as Ca2+ and Al3+,16,17 or by the addition of some polyelectrolytes.1,4 Such surface multilayer structures can be induced to form at © 2012 American Chemical Society
relatively low surfactant concentrations. This feature, coupled with the ability to control the structures with the addition of a cosurfactant and by manipulating the surfactant or polyelectrolyte architecture, makes them particularly attractive for a wide range of potential applications. In this context, PEI, which is the focus of this paper, is a particularly important polyelectrolyte, and the different forms of PEI have been extensively exploited in a range of technological and biological applications.18 Some different aspects of the ability of PEI to promote complex surface structures have also been reported.1,19−21 A wide range of strongly interacting polyelectrolyte− surfactant mixtures have now been studied by ST and NR,4 and two distinct patterns of behavior have emerged. These can be explained broadly in terms of the formation of surface and solution polyelectrolyte−surfactant complexes, and result in a different ST response and a different surface structure. In all polyelectrolyte−surfactant mixtures, there is an initial sharp decrease in the ST at relatively low surfactant concentrations (≪ cmc) due to the strong polyelectrolyte−surfactant interaction. For mixtures, such as poly(styrenesulfonate), PSS, and the alkyltrimethylammonium bromide cationic surfactant, Received: June 15, 2012 Revised: August 8, 2012 Published: September 28, 2012 14909
dx.doi.org/10.1021/la302444b | Langmuir 2012, 28, 14909−14916
Langmuir
Article
Figure 1. Structure of the LPEIs used in this study.
dipole interaction between the sulfate headgroup and the amine nitrogen, and the hydrophobic inter-alkyl chain interaction between bound SDS molecules. The other important feature of the PEI−SDS interaction is that surface multilayer formation is observed for these low MW oligoamines and for the much larger MW branched PEIs,1,19,20 but only monolayer adsorption was observed for the higher MW linear PEI.1 More recently, Halacheva et al.23 showed the importance of polymer architecture of the oligoamines in the formation of the surface multilayer structures. They observed that the branched oligoamines formed surface multilayer structures more readily than the linear ones. This was attributed to the lower charge density, different charge distribution, and more rigid structure of the branched oligoamines than for the linear oligoamines. The results summarized in refs 1, 21, and 23 strongly suggest that MW has an important impact upon the surface structure in SDS/PEI mixtures, where at low MW, multilayer structures were observed, but at the higher MW (25 000 Da),1 only monolayer structures were observed. The focus of this paper is specifically to explore that MW dependence in more detail and to identify and quantify the factors which determine the transition from monolayer to multilayer structures at the interface. Hence, we report on the use of the combination of ST and NR measurements on the surface properties of SDS in combination with different small liner PEIs (MW ∼ 320 to 2000 Da) at different solution pH values (at pH 3, 7, and 10).
CnTAB, this is followed by a broad plateau in the surface tension with a relatively low value between that initial decrease and the cmc. At some point on that plateau, the adsorbed layer at the interface changes from a monolayer to a bilayer/ multilayer. This transition in the surface structure occurs close to the charge neutralization point, where the solutions are often cloudy. Recent studies on oligoarene sulfonate/surfactant mixtures22 have shown that this low surface tension and surface multilayer formation in the coacervation/precipitation region can be likened to a two-phase wetting transition. That is, the dense coacervate or precipitate phase is highly surface active, has a lower surface tension, and preferentially wets the air−water interface. The other limiting ST behavior is epitomized by the poly(dimethyldiallylammonium bromide), polydmdaac,/SDS mixture, where after the initial decrease in ST at low surfactant concentrations there is a peak in the surface tension before the cmc and in the region close to charge neutralization where the solutions are again cloudy. This is associated with precipitation, which results in the surface being partially depleted of polymer−surfactant in favor of solution or precipitate complex formation. The PEI−SDS mixtures represent a class of polyelectrolyte− surfactant surface behavior which is intermediate between those two extremes, embraces aspects of both of the patterns of behavior described above, and is in general more complex. The variations of ST and surface structure for PEI−SDS mixtures depend upon solution pH, MW, and the polymer architecture (branched or linear).1,19,20 A notable feature of the PEI−SDS behavior is that the SDS−PEI interaction is strong at low pH when the PEI is a highly charged polyelectrolyte and also at high pH when it is essentially a neutral polymer. Similar observations were made by Winnik et al.24,25 and Sherstenin et al.26 who reported evidence for electrostatic and hydrophobic interactions in the binding of SDS to PEI. This is also consistent with the observations of Edler et al.27 on the interaction between PEI and ionic surfactants with similar charge to the PEI. Penfold et al.21 have studied the adsorption of SDS with a range of oligoamines (ethylenediamine to pentaethylenehexamine) and ascribed the nature of the interaction at high pH as due to a combination of the ion−
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EXPERIMENTAL DETAILS
Materials. The ST and NR measurements were made at 25 °C. The ST measurements were made in H2O and the NR measurements in null reflecting water, nrw (92 mol % H2O/8 mol % D2O mixture). The solution pH was adjusted by the addition of hydrochloric acid, HCl, and sodium hydroxide, NaOH, and measured using an Oakton Acron Series 6 pH Meter. The HCl (32%) was purchased from Fisher Scientific and the NaOH from Sigma Aldrich (≥98%, pellets). Deuterium oxide (Sigma Aldrich) and high-purity water (Elga Ultrapure) were used throughout. The deuterated sodium dodecyl sulfate (d-SDS) was obtained from the Oxford Isotope Facility28 and purified before use by recrystallization from ethanol/acetone. The purity of the d-SDS was verified by ST measurements by the absence of a minimum in the ST at the cmc and from the NR measurements in 14910
dx.doi.org/10.1021/la302444b | Langmuir 2012, 28, 14909−14916
Langmuir
Article
The typical error in the area/molecule is of the order ±2 Å2 (for an area per molecule of 50 Å2).32 There are combinations of LPEI/SDS concentrations and solution pH where the reflectivity is consistent with a more complex surface structure. In such cases, there is a first-order Bragg peak visible, and this is consistent with the formation of a surface multilayer structure. Using the kinematic approximation, the reflectivity can be written as33,34
the absence of PEI. The glassware and Teflon trough used for the measurements and sample preparations were cleaned in alkali detergent (Decon 90) and rinsed thoroughly in high-purity water before use. A two-step process23 was used for the synthesis of the linear PEIs (LPEI6, LPEI12, and LPEI40, with approximate MWs of 320, 640, and 2000 Da). First, poly(2-ethyl-2-oxazoline) (EtOx6, EtOx12, EtOx40) precursors were synthesized, followed by an acidic hydrolysis step to afford their corresponding LPEIn products. All PEtOx polymers were characterized by GPC and 1H NMR. GPC analyses gave monomodal distributions with polydispersity indices (PDI) ranging from 1.1 to 1.3. The degree of polymerization (DP) of each PEtOx sample was estimated from its 1H NMR spectrum (CDCl3). The experimental DPs were in good agreement with the theoretical values and with those calculated from the feed. The LPEI products were obtained by acidic hydrolysis and 1H NMR spectra showed the degree of hydrolysis to be >95%. The DPs of the resulting LPEIs were confirmed as being equivalent to that of their corresponding PEtOx precursors, and their structural formulas are shown in Figure 1. Surface Tension. The ST measurements were made on a Kruss K10 maximum pull digital tensiometer with a platinum−iridium ring using the du Nouy ring method. Before each measurement, the ring was rinsed in high-purity water (Elga Ultrapure) and dried in a Bunsen burner flame. The tensiometer was calibrated using a ST value for high-purity water of ∼72 mN/m at 25 ± 0.2 °C. Repeated measurements were made until equilibrium was observed and the variation in the ST was
