Sodium Dodecyl Sulfate–Ethoxylated Polyethylenimine Adsorption at

Article pubs.acs.org/Langmuir

Sodium Dodecyl Sulfate−Ethoxylated Polyethylenimine Adsorption at the Air−Water Interface: How the Nature of Ethoxylation Affects the Pattern of Adsorption Stephen N. Batchelor,† Ian Tucker,† Jordan T. Petkov,† Jeffrey Penfold,*,‡,§ and Robert K. Thomas§ †

Unilever Research and Development Laboratory, Port Sunlight, Quarry Road East, Bebington, Wirral CH62 4ZD, UK STFC, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK § Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 2JD, UK ‡

S Supporting Information *

ABSTRACT: The strong interaction between ionic surfactants and polyelectrolytes of opposite charge results in enhanced surface adsorption at the air−water interface down to low surfactant concentrations and in some cases in the formation of ordered surface structures. A notable example which exhibits such properties is the mixture of polyethylenimine, PEI, and sodium dodecyl sulfate, SDS. However, the electrostatic interaction, around charge neutralization, between the surfactant and polymer often results in precipitation or coacervation. This can be mitigated for PEI−surfactant mixtures by ethoxylation of the PEI, but this can also result in a weaker surface interaction and a significant reduction in the adsorption. It is shown here that by localizing the ethoxylation of the PEI into discrete regions of the polymer precipitation upon the addition of SDS is suppressed, the strong surface interaction and enhanced adsorption of the polymer−surfactant mixture is retained. The adsorption of SDS in the presence of ethoxylated PEI is greatly enhanced at low SDS concentrations compared to the adsorption for pure SDS. The adsorption is equally pronounced at pH 7 and 10 and is largely independent of the degree of ethoxylation. Surface ordering, more than monolayer adsorption, is observed over a relatively narrow range of SDS concentrations and is most pronounced at pH 10 and for the polymers with the lower degree of ethoxylation. The results show that ethoxylated PEI’s reported here provide a suitable route to enhanced surfactant adsorption while retaining favorable solution properties in which precipitation effects are minimized.

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INTRODUCTION Polymer−surfactant mixtures are important elements of a wide range of applications, which include home and personal care products, cosmetics, foods, paints, and coating materials, and in drug delivery and other biomedical applications.1 In such applications, the polymers are used as viscosity modifiers, stabilizers, deposition aids, adsorption and self-assembly modifiers, and delivery vehicles. The strong interaction and complexation between polyelectrolytes and ionic surfactants give rise to a more complex range of surface and solution properties2 and hence have been a greater focus of recent studies. In this context poly(ethylenimine), PEI, is an important and attractive polyelectrolyte. It exists in linear and branched architectures and has the highest charge density of any polyelectrolyte. Furthermore, the charge density can be manipulated by pH,3 and the strength of the polyelectrolyte− surfactant interaction can also be modified by ethoxylation or the addition of other hydrophobic or hydrophilic moieties.4−6 In particular ethoxylation of PEI mitigates the occurrence of precipitation and phase separation that are prevalent in polyelectrolyte/ionic surfactant mixtures near charge neutralization.4,7 PEI and some differently modified PEI’s have been © 2014 American Chemical Society

widely investigated, and these properties are exploited in a diverse range of applications. They have been extensively exploited in polyelectrolyte layer by layer multilayer formation,8−11 where PEI forms an efficient initial charge reversal layer. PEI and differently modified PEI’s have been extensively investigated for the compaction of DNA, DNA delivery, in different gene therapy applications, and other biomedical applications.5,6,12−17 The broader applications of PEI and modified PEI’s frequently involve the interaction with different surfactants and notably the anionic surfactant sodium dodecyl sulfate, SDS. Hence, the interaction of PEI and ethoxylated PEI with SDS and its associated bulk solution structures have been extensively investigated.3,4,18−20 The significance of PEI as a polyelectrolyte with widespread applications has also stimulated a number of studies of the properties of their coadsorption with SDS at the air−water21,22 and the solid−liquid23,24 interfaces. In the context of the study reported here, Penfold et al.21 used neutron reflectivity, NR, to study the adsorption of SDS− Received: July 18, 2014 Revised: July 31, 2014 Published: July 31, 2014 9761

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Figure 1. Synthesis scheme for EPEIs.

Figure 2. Structure of ethoxylated PEI (PEI-EOn). (a) EPEI formed by reaction of n moles ethylene oxide per NH, which produces an even distribution of ethoxylation; (b) ethoxylation scheme used in this study.

complexation between the SDS and PEI. They showed that this was largely independent of pH. At low pH, where the polymer is highly charged, this is due to the strong electrostatic

PEI mixtures at the air−water interface. They demonstrated that pronounced adsorption of SDS occurred down to low SDS concentrations due to the strong surface interaction and 9762

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ethylene oxide groups being evenly distributed among the primary and secondary nitrogens by replacement of the substitutable hydrogens (as shown in Figure 2a). The ethoxylated PEIs synthesized here have an uneven distribution of ethylene oxide groups, with the 3 EO12 groups capped with a methyl group at random secondary nitrogen positions in the ratios listed above (Figure 2b). All of the NR measurements were made in null reflecting water, nrw (92 mol % H2O/8 mol % D2O, with a scattering length density of 0.0, and a neutron refractive index of 1.0, the same as air). The Teflon troughs and glassware used for the measurements and sample preparation were cleaned in alkali detergent (Decon90) and rinsed in UHQ water. The measurements were all made at a temperature of 25 °C. All of the prepared solutions were clear and showed no visual evidence of phase separation or precipitation. Neutron Reflectivity. The NR measurements were made at the air−water interface on the INTER reflectometer at the ISIS neutron source.33 The reflectivity R(Q) was measured over a wave vector transfer, Q, (where Q = (4π sin θ)/λ) range of 0.03 to 0.5 Å−1 using an angle of incidence, θ, of 2.3° and neutron wavelengths, λ, from 0.5 to 15 Å. The reflectivity, R(Q), was calibrated with respect to the direct beam intensity and the reflection from a D2O surface. The measurements were made in sealed Teflon troughs at 25 °C with sample volumes ∼25 mL. The data acquisition for each neutron reflectivity profile took ∼20−30 min. Repeated measurements were made until the reflectivity showed no change with time, and this was typically ∼2−3 h. Hence, the data presented represent steady state or equilibrium structures. In the kinematic approximation, 34 the reflectivity is related to the square of the Fourier transform of the scattering length density profile, ρ(z), normal to the surface (ρ(z) = ∑ini(z)bi, ni(z), and bi are the number density and neutron scattering length of the ith component, and ρ(z) is related to the neutron refractive index, n(z), and n(z) = 1 − λ2ρ(z)/2π). By manipulation of ρ(z) through deuterium labeling (H and D have different scattering lengths, −3.7 × 10−6 Å for H and 6.67 × 10−5 Å for D), the NR profile can directly provide information about the amount adsorbed at the interface and the structure of the adsorbed layer. This approach has been extensively exploited for a range of surfactant34 and polymer− surfactant35 systems. The data in the Q range ∼0.35 to 0.5 Å−1 reaches a flat background of ∼7 × 10−6 (which is included in the modeling) and although the data are measured out to 0.5 Å−1, they are only plotted up to 0.35 Å−1. Measurements Made. All of the NR measurements were made in nrw at pH 7 and 10. The polymer concentration was 20 ppm for all of the measurements and for the three different polymers, P1, P3, and P10. The measurements were made for SDS concentrations from 4 × 10−6 to 0.01 M in the absence of electrolyte and for SDS/P3 in the presence of 0.1 M NaCl. The pH was adjusted by the addition of NaOH and HCl, and hence, in the absence or presence of 0.1 M NaCl the ionic strength is not significantly altered as a result of the change in pH.

interaction. At high pH, where the polymer is essentially neutral, this was attributed to the combination of an ion−dipole interaction between the SDS sulfate group and the amine nitrogen, and the hydrophobic attraction between neighboring surfactant alkyl chains. This supposition has been more recently reinforced by SDS/oligoamine26 and SDS/polyamine27 studies. The polymer molecular weight (MW) and architecture have a profound effect. For the linear PEI, only monolayer adsorption was observed, whereas for the branched PEI, surface ordering, which is dependent on MW, pH, and polymer/surfactant ratio, occurs. Penfold et al.7 showed that for the ethoxylated PEI (PEI-EO 7 ) with SDS the adsorption arises from the competition between the relative surface activities of the two components. In that study, the PEI was ethoxylated using ethylene oxide which builds ethoxylated chains with on average 7 (CH2CH2O) per N-H; so PEI-EO7 refers to 7 mol of ethylene oxide reacted per mol of N-H groups in the PEI so that the PEI is effectively covered in ethylene glycol chains. The weaker interaction between the ethoxylated PEI and SDS results in a reduced surfactant adsorption, and the adsorption is now most pronounced at low pH. This pattern of adsorption is similar to that observed in weakly interacting polymer− surfactant mixtures.28,29 In addition to this, Zhang et al.25 studied the effect of the degree of ethoxylation on the PEI/SDS adsorption at the air−water interface. For degrees of ethoxylation with < EO3, i.e., short chains covering the PEI, the adsorption is similar to that of PEI/SDS mixtures, whereas for a degree of ethoxylation ≥EO3, the weaker interaction and adsorption observed by Penfold et al.7 for PEI-EO7 was found. Thus, increasing the length of the polyethylene glycol chains by increasing ethoxylation results in a transition from strong surface polymer/surfactant behavior to weak polymer/ surfactant behavior. Although higher levels of PEI ethoxylation mitigates the unfavorable precipitation and phase separation in solution, the weaker interaction results in less favorable surface properties. The focus of this article is to demonstrate how a different ethoxylated PEI architecture can retain those favorable solution properties while promoting enhanced surface adsorption from polymer/surfactant mixtures.

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MATERIALS AND METHODS

Materials. The deuterium labeled SDS, d-SDS (C12D25SO4Na), was synthesized at the Oxford Isotope Facility30 as described previously and purified prior to use by recrystallization from ethanol.31 The chemical purity of the surfactant was verified by surface tension, where no minimum was observed at the critical micellar concentration (cmc), and from the adsorption value above the cmc, measured by NR. The ethoxylated PEI was derived from a branched PEI with a MW of 1300 Da, as supplied by Sigma-Aldrich. Three different ethoxylated PEIs, EPEI, were synthesized, by the reaction of monotosylate methyl terminate PEG (polyethylene glycol) with PEI, described in detail elsewhere.32 The monotosylate methyl terminate PEG was TosO(CH2CH2O)12CH3, i.e., with an average of 12 ethoxy groups in the chain. Three polymers were synthesized with PEI to polymer chain ratios of 1:1, 1:3, and 1:10, named P1, P3, and P10, respectively. This synthetic route provided distinctly different EPEI architecture to the reaction with ethylene oxide as single long ethylene glycol may be bound to the PEI, as illustrated below in Figure 1 for a smaller PEI. Here, 12 mol of ethylene oxide provides reaction at all the N-H groups then starts to form longer chains, whereas the tosylate route provides one chain of 12 (CH2CH2O) units attached to a single PEI N. The principle is further illustrated in Figure 2, where an ethylene oxide EPEI is compared to P3 on a PEI with a MW of ∼1300. The usual synthetic route for producing ethoxylated PEI results in the

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RESULTS AND DISCUSSION Some typical NR data are shown in Figure 3a for SDS with the EPEI P3 at pH 10 and for SDS concentrations from 4 × 10−6 to 10−4 M. The data are analyzed using the simplest model consistent with the data and fitted by a least-squares algorithm using calculated reflectivities based on the exact optical matrix method.36,37 This provides a thickness, d, and scattering length density, ρ, per layer. Assuming that the reflectivity is dominated by the deuterium labeled SDS and that the contribution from the polymer is relatively small,21,25 the amount of SDS in each layer is then given by A = ∑b/dρ, where ∑b is the scattering length of the deuterium labeled SDS (2.763 × 10−3 Å) and A is the area/ molecule. The adsorbed amount is then estimated from Γ = 1/ NaA, where Na is Avogadro’s number. The typical statistical/ systematic error associated with an area/molecule ∼50 Å2 is ∼ ±2 Å2,34 and the errors quoted herein (in Table 1 and Tables 9763

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35 26 34

0.5 0.5 1.0

176 256 78

0.9 0.7 2.1

3.1 3.9 5.9 5.1 7.8 4.5 4.5 3.1 3.9 5.0 4.4 5.7 4.5 4.5 54 43 33 38 29 37 37 4.1 4.5 3.5 4.8 4.3 4.5 4.7 17 15 22 15 22 17 16 0.004 0.01 0.04 0.1 0.4 1.0 1.0.0

Γtotal (±0.2 × 10−10 mol cm−2) SDS concentration (mM)

d1 (±1 Å) ρ1 (±0.2 × 10−6 Å−2)

A1 (±2 Å2)

Γ1 (±0.2 × 10−10 mol cm−2)

d2(±1 Å) ρ2(±0.2 × 10−6 Å−2)

A2(±2 Å2)

Γ2 (±0.2 × 10−10 mol cm−2)

3.0 3.8 4.2 4.5 3.8 4.3 4.4 55 44 39 37 43 39 38 4.2 4.6 4.8 4.6 4.5 5.0 4.8 (b) pH 10 15 14 14 16 15 14 12 0.004 0.01 0.04 0.1 0.4 1.0 1.0.0

ρ (±0.2 × 10−6 Å−2) d (±1 Å)

S1 and S2 in the Supporting Information) are on that basis. There will be a finite contribution from the EPEI at the interface, which can only be estimated approximately here. The ∑b for the EPEI is typically ≤2 × 10−4 Å/monomer. Assuming a surface composition of one SDS molecule per EPEI monomer, this results in a 7% contribution of the EPEI to the adsorbed amount. The results for SDS/PEI-EO1,25 where the use of deuterium labeled PEI-EO1 to directly determine the amount of EPEI at the interface was possible, gave a maximum contribution of the polymer at the interface of ∼10%. The key model parameters for the data in Figure 3a are summarized in Table 1, along with a complete set of parameters for the SDS concentrations measured with the polymer P3 at pH 7 and 10. At pH 7, the NR data are consistent with a simple monolayer with an average thickness of ∼15 Å and an area/molecule from ∼40 to 55 Å2. At pH 10, the variation in the adsorption with SDS concentration is more complex. At low, ≤10−5 M, and at higher, ≥10−3 M, SDS concentrations, the adsorption is in the form of a monolayer. This is shown in Figure 3a where the NR data at SDS concentrations of 4 × 10−6 and 10−5 M have a

SDS concentration (mM)

Figure 3. (a) Neutron reflectivity for 20 ppm P3/SDS at pH 10 at the air−nrw interface; see the key for details of the different curves which are shifted vertically (by a factor of 2, 8, and 32 for the data at 10−5, 4 × 10−5, and 10−4 M, respectively) for clarity. (b) Neutron reflectivity data for 10−5 and 4 × 10−5 M SDS plotted as RQ4 (a background of 7 × 10−6 was subtracted from the reflectivity data). The solid lines are model calculations as described in the main text and for the model parameters summarized in Table 1

(a) pH 7

Table 1. Key Model Parameters from Analysis of NR Data for SDS/P3 at (a) pH 7 and (b) pH 10

A (±2 Å2)

Γ (±0.2 × 10−10 mol cm−2)

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similar slope but slightly different adsorbed amounts and so different mean levels of adsorption. At SDS concentrations in the range 4 × 10−5 to 4 × 10−4 M, the adsorbed layer is best described as two layers. At the surfactant concentrations of 4 × 10−5 and 10−4 M in Figure 3a, the NR data have different slopes, and a weak interference fringe is visible in the data at high Q values (∼0.17 Å−1). This difference is illustrated more clearly in the RQ4 plots in Figure 3b for 10−5 M and 4 × 10−5 M SDS, and the interference fringe present at an SDS concentration of 4 × 10−5 M is now more evident. The structure of the surface in this region of SDS concentrations is described by two layers, consisting of an initial monolayer with a thickness of ∼20 Å adjacent to the air phase and a second thicker more dilute layer ∼32 Å adjacent to the solvent phase. This is broadly similar to the structure previously observed in SDS/PEI mixtures.21 The notable difference between the EPEI/SDS data presented here and the SDS/PEI data21 is that for the SDS/PEI mixtures there were regions of SDS concentration where the ordering at the surface was even more pronounced. That resulted in a trilayer structure, which is observed more widely in many of the strongly interacting polyelectrolyte/surfactant mixtures.35 In the region where the adsorption is more than a monolayer, the surface structure is best described as an initial monolayer with a dilute layer of small polymer/surfactant aggregates immediately adjacent to the initial monolayer. In some SDS/PEI,21 ethoxylated PEI/SDS,25 and oligoamines/polyamine/SDS26,27 mixtures, the surface structure evolved into a more extended and ordered layered structure. The extent and degree of ordering gives rise to pronounced Bragg scattering from the interface, and this is not observed here for the EPEI/SDS mixtures studied. The variation in the adsorbed amount of SDS with SDS concentration at a fixed EPEI concentration and for the three differently ethoxylated PEIs, P1, P3, and P10, is shown in Figure 4 and compared with the adsorption for SDS in the absence of polymer. The data in Figure 4a are measured at pH 7 and in Figure 4b at pH 10. At both pH 7 and pH 10, there is a substantial enhancement in the SDS adsorption down to very low SDS concentrations with the addition of each of the different EPEIs and which is broadly independent of the EPEI structure. At pH 7, the SDS concentration dependence of the adsorption is similar for P1 and P3. For P10, there is a narrow region where the adsorption is more than a monolayer. For all three polymers between SDS concentrations of 0.4 and 10 mM, there is a slight reduction in the adsorption, and this is most pronounced for P10. This suggests a slight depletion of the SDS from the surface as bulk aggregate formation becomes more favorable. At pH 10, the SDS adsorption is significantly enhanced down to relatively low SDS concentrations for all three polymers. There is now a range of SDS concentrations over which the adsorption is more than a monolayer (as discussed earlier for P3 at pH 7), and in these regions, the adsorption is further enhanced. This occurs at slightly different SDS concentrations for the three polymers and is more extensive for P3 and P10. Furthermore, compared to the adsorption at pH 7 there is no evidence of significant depletion in the region from 0.4 to 10 mM SDS concentrations. NR measurements were also made for SDS/P3 in 0.1 M NaCl at pH 7 and 10, and the variation in the amount of SDS adsorbed with SDS concentration is summarized in Figure 5. The equivalent adsorption in the absence of electrolyte and the SDS adsorption with/without electrolyte in the absence of

Figure 4. Variation in the adsorbed amount of SDS with SDS concentration with P1, P3, and P10 ethoxylated PEIs. (a) pH 7; (b) pH 10; see the key for details. The key model parameters for the NR data associated with these data are summarized in Tables 1 and S1 in the Supporting Information. The solid lines are for SDS alone at pH 7.

polymer are also shown for comparison. At pH 7, the adsorption data with/without electrolyte are very similar, except that at the highest SDS concentration (10 mM) where there is a transition from monolayer adsorption to a more complex and extended surface structure represented by two layers. At pH 10, the adsorption in the presence of electrolyte is in the form of a monolayer, and there were no occurrences of more complex surface structures that were observed in the absence of electrolyte. At both pH values, the SDS adsorption in the presence of electrolyte is substantially enhanced compared to the adsorption in the absence of polymer. However, the effect of electrolyte on the EPEI/SDS mixtures is in contrast to that observed for SDS/PEI mixtures, where in the latter case the addition of electrolyte tended to greatly enhance the tendency toward the formation of more ordered surface structures.35 The NR data in Figure 6, for SDS/P3 in 0.1 M NaCl at pH 7, shows the comparsion between the data at 1.0 and 10.0 mM. At 1 mM SDS, the adsorption is in the form of a simple monolayer, whereas at 10 mM SDS, a pronounced interference fringe is now visible in the data. The key model parameters 9765

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from the analysis of the data in Figure 6 are summarized in Table 2. The surface structure at 10 mM SDS/P3 in 0.1 M NaCl is similar to the two-layer structures summarized in Table 1, except that the amount of SDS in the second thicker layer is significantly enhanced. It is similar to the amount adsorbed in the initial monolayer, and hence, the total adsorption is also substantially enhanced. The effect of electrolyte is opposite to what was observed for PEI/SDS,35 where enhanced surface ordering occurred over a wider SDS concentration range compared to the extent of the formation in the absence of electrolyte. However, there is no evidence for the SDS/P3 mixture of other than monolayer adsorption in the presence of electrolyte at pH 10. At pH 7, the data for 10 mM SDS shows pronounced surface ordering, and this could be associated with a shift in the region of ordered structure formation to higher SDS concentration. It has been previously shown for PEI/SDS21 and polyamine/ SDS26 mixtures that the strong interaction between the amine groups and the SDS resulted in substantially enhanced adsorption of the SDS at the air−water interface down to relatively low SDS concentrations. At low pH, this arises from the electrostatic attraction between the highly charged PEI and the oppositely charged SDS. The effects were equally pronounced at high pH where the polymer is essentially uncharged. Winnik et al.3,18 and others38−43 reported evidence of both electrostatic and hydrophobic interactions between PEI and surfactant. Jon and Chang44 described an additional interaction between the amine groups and SDS in the form of an ion−dipole interaction. Penfold et al.26 subsequently attributed the strong interaction at high pH as arising from a combination of the ion−dipole interaction between the sulfate headgroup and the amine nitrogen groups and a cooperative hydrophobic interaction between the alkyl chains of the attached surfactants. Ethoxylation of the PEI (PEI-EO7) resulted in a much weaker interaction between SDS and the polymer,7 as expected from PEO-SDS studies.28,38 This gave rise to competitive adsorption between the polymer and surfactant and no enhancement of the SDS adsorption compared to that in the absence of polymer. Furthermore, the nature of the interaction and hence the strength of the adsorption of the SDS was directly related to the charge on the polymer. That is, it was most pronounced at pH 3 and least pronounced at pH 10. In light of these observations, Zhang et al.24,25 explored the effect of the degree of ethoxylation on the PEI−SDS interaction and on the nature of the adsorption. For degrees of ethoxylation P3 > P1. This would imply that the form of ethoxylation of these PEIs does not disrupt the strong polymer−surfactant interaction but does inhibit the formation of the more extended surface structures. This will arise as a result of the presence of the ethylene oxide groups preventing the lateral growth of the surface domains associated with the more ordered structures. From the previous studies,21,35 the effect of added electrolyte on the adsorption behavior of PEI/SDS mixtures was to promote more extended regions (over a wider SDS concentration and pH range) over which ordered surface structures occurred. For the ethoxylated PEI studied here, the addition of electrolyte had little impact upon the enhanced adsorption at low SDS concentrations but importantly did not result in an extended range of SDS concentrations over which ordered surface structures occurred. It was previously postulated35 that for SDS/PEI mixtures the addition of electrolyte had the most profound effect on SDS−SDS headgroup interactions along the polymer chain and not on the PEI−SDS interactions. This would result in a closer surfactant packing and a stronger surfactant chain−chain hydrophobic attraction, and this would drive the transition toward more ordered surface structures. For PEI−SDS, it is assumed that this transition was sufficiently favorable that the changes associated with the addition of electrolyte was sufficient to promote the transition. However, the pattern of ethoxylation of the PEIs studied here, while retaining the strong SDS−polymer interaction, may be sufficient to make the monolayer to more ordered structure transition more remote and less sensitive to the addition of electrolyte. The final feature in the adsorption data that requires some further discussion is the adsorption in the region of the SDS cmc. There is a slight suppression of the adsorption at an SDS concentration of ∼1 mM, which is most pronounced at pH 7 and for the polymer P10. More pronounced depletion effects were observed recently for the PEI precursor PEtOx with SDS.45 This was attributed to the onset of the formation of solution polymer−surfactant aggregates, as was seen in other polymer−surfactant mixtures.35

1.0 10.0

d2(±1 Å) ρ2(±0.2 × 10−6 Å−2) Γ1 (±0.2 × 10−10 mol cm−2) A1 (±2 Å2) d1 (±1 Å) ρ1 (±0.2 × 10−6 Å−2) SDS concentration (mM)

Table 2. Key Model Parameters from Analysis of NR Data for SDS/P3/0.1 M NaCl/pH 7

A2(±2 Å2)

Γ2 (±0.2 × 10−10 mol cm−2)

Γtotal (±0.2 × 10−10 mol cm−2)

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systems in biomedical applications. J. Colloid Interface Sci. 2014, 418, 300−310. (7) Penfold, J.; Taylor, D. J. F.; Thomas, R. K.; Tucker, I.; Thompson, L. J. Adsorption of polymer-surfactant mixtures at the airwater interface: ethoxylated polyethyleneimine and sodium dodecyl sulfate. Langmuir 2003, 19, 7740−7745. (8) Decher, G. Fuzzy nano-assemblies: towards layered polymeric multi-composites. Science 1997, 277, 1232−1237. (9) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. In-situ determination of the structural properties of initially deposited polyelectrolyte multilayers. Langmuir 2000, 16, 1249−1255. (10) Glinel, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Influence of polyelectrolyte charge density on the formation of multilayers of strong polyelectrolytes at low ionic strength. Langmuir 2002, 18, 1408−1412. (11) Steitz, R.; Jaeger, W.; v. Klitzing, R. Influence of charge density and ionic strength on the multilayer formation of strong polyelectrolytes. Langmuir 2001, 17, 4471−4474. (12) Perevyazko, I. Y.; Bauer, M.; Darlov, G. M.; Hoeppener, S.; Schubert, S.; Fischer, D.; Schubert, U. S. Polylectrolyte complexes of DNA and linear PEI: formation, composition and properties. Langmuir 2012, 28, 16167−16176. (13) Kim, S.; Choi, J. S.; Jang, H. S.; Juh, H.; Park, J. Hydrophobic modification of polyethyleneimine for Gene transfectant. Bull. Korean Chem. Soc. 2001, 22, 1068−1075. (14) Liu, C.; Liu, F.; Feng, L.; Li, M.; Zhang, J.; Zhang, N. The targeted co-delivery of DNA and doxorubicin to tumor cells via multifunctional PEI-PEG based nano-particles. Biomaterials 2013, 34, 2547−2564. (15) Victor, A. B. DNA condensation. Curr. Opin. Struct. Biol. 1996, 6, 334−341. (16) Godbey, W. T.; Wu, K.; Mikos, A. G. Poly(ethyleneimine) and its role in gene delivery. J. Controlled Release 1990, 60, 149−160. (17) Godbey, W. T.; Wu, K. K.; Mikos, A. G. Size matters, molecular weight affects the efficiency of poly(ethyleneimine) as a gene delivery vehicle. J. Biomed. Mater. Res. 1999, 45, 268−275. (18) Winnik, M. A.; Bystryak, S. M.; Siddiqui, J. Interaction of pyrene labeled poly(ethyleneimine) with sodium dodecyl sulfate in aqueous solution. Macromolecules 1999, 32, 624−632. (19) Meszaros, R.; Thompson, L.; Bos, M.; Varga, I.; Gilanyi, T. Interaction of sodium dodecyl sulfate with polyethyleneimine: surfactant-induced polymer solution colloid dispersion transition. Langmuir 2003, 19, 609−615. (20) Li, Y.; Xu, R.; Couderc, S.; Bloor, D. M.; Warr, J.; Penfold, J.; Holzwarth, J. F.; Wyn-Jones, E. Structure of the complexes formed between sodium dodecyl sulfate and a charged and uncharged ethoxylated polyethyleneimine, small angle scattering, electromotive force and isothermal calorimetry measurements. Langmuir 2001, 17, 5657−5665. (21) Penfold, J.; Tucker, I.; Thomas, R. K.; Zhang, J. Adsorption of polyelectrolyte/surfactant mixtures at the air-solution interface: polyethyleneimine/sodium dodecyl sulfate. Langmuir 2005, 21, 10061−10072. (22) Angus-Smyth, A.; Bain, C. D.; Varga, I.; Campbell, R. A. Effects of bulk aggregation on PEI-SDS monolayers at the dynamic air-liquid interface: depletion due to precipitation versus enrichment by a convection/spreading mechanism. Soft Matter 2013, 9, 6103−6117. (23) Angelescu, D. G.; Nylander, T.; Picullel, L.; Linse, P.; Lindman, B.; Tropsch, J.; Detering, J. Adsorption of branched-linear polyethyleneimine-ethylene oxide conjugate on hydrophilic silica investigated by ellipsometry and Monte Carlo simulations. Langmuir 2011, 27, 9961−9971. (24) Zhang, X.; Taylor, D.; Thomas, R. K.; Penfold, J.; Tucker, I. Modifying the adsorption properties of anionic surfactants onto hydrophilic silica using the pH dependence of the polyelectrolyte PEI, ethoxylated PEI and polyamines. Langmuir 2011, 27, 3569−3577. (25) Zhang, X. L.; Taylor, D. J. F.; Thomas, R. K.; Penfold, J. Adsorption of polyelectrolyte/surfactant mixtures at the air-water

CONCLUSIONS The strong interaction between SDS and PEI results in a substantial enhancement in the adsorption of SDS at the air− water interface over a wide pH range and down to very low SDS concentrations. Also, depending upon pH, polymer architecture, and polymer/surfactant concentrations, it results in the formation of more ordered surface structures. However, around the point of charge neutralization, precipitation or coacervation often occurs; and these less favorable solution properties often detract from the application of the more favorable surface properties. Ethoxylation of the PEI, in which the ethylene oxide groups are evenly distributed among the primary and secondary nitrogens of the amine groups of the PEI, will produce more favorable solution properties and inhibit precipitation. However, this usually results in a weaker polymer−surfactant interaction and a surface behavior more closely aligned to the weak neutral polymer/surfactant surface properties. It has been demonstrated here how a differently structured ethoxylation of the PEI results in favorable solution properties with SDS and enhanced surface adsorption due to the strong PEI−SDS interaction. Furthermore, and more broadly, the results demonstrate the importance of the polymer geometry and structure on the pattern of surfactant adsorption.

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ASSOCIATED CONTENT

S Supporting Information *

Parameters for NR data for the other polymers and for the data in NaCl. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: jeff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The provision of beam time on the INTER instrument at ISIS is acknowledged. The invaluable scientific and technical assistance of the Instrument Scientists and support staff is gratefully recognized.

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REFERENCES

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