Complex Formations between Surfactants and Polyelectrolytes of the

Jul 7, 2017 - The mechanism of complex formation between surfactants and polyelectrolytes with the same charge on the water surface was investigated u...
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Complex Formations between Surfactants and Polyelectrolytes of the Same Charge on a Water Surface Amirhossein Mafi,†,‡ Dan Hu,† and Keng C. Chou*,† †

Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada

‡

ABSTRACT: The mechanism of complex formation between surfactants and polyelectrolytes with the same charge on the water surface was investigated using molecular dynamics simulations and phase-sensitive sum-frequency generation vibrational spectroscopy. Although complex formation between highly charged surfactants and polyelectrolytes of the same charge is generally expected to be prohibited by the electrostatic repulsive force, our study shows that it is possible to form thermodynamically stable complexes when excess ions are present in the solution. We found that anionic partially hydrolyzed polyacrylamide (HPAM) could interact with anionic sodium dodecyl sulfate (SDS) on a water surface in the presence of salts. With excess Na+ ions in the solution, the charge screening effect allows HPAM to weakly interact with SDS via hydrogen bonds. In the presence of divalent Ca2+ ions, the surfactant and the polymer are strongly coupled by forming Ca2+ ion bridges and hydrogen bonds. Our calculation shows that the presence of Ca2+ ions creates a steep binding energy of ∼30 kJ/mol near the water surface. These results were qualitatively verified using phase-sensitive sum-frequency generation vibrational spectroscopy.

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INTRODUCTION The interaction between surfactants and polyelectrolytes has been extensively studied because of their broad applications in pharmaceuticals, personal care products, and many industrial processes.1−7,51 The interaction leads to the formation of surfactant−polymer complexes, which modify the interfacial properties. Investigations of surfactant−polyelectrolyte interactions are motivated in part by seeking new types of surfactant−polymer complexes with novel interfacial or dispersion properties. In the past several decades, the interactions between oppositely charged surfactants and polyelectrolytes have been thoroughly studied.8 On the other hand, few studies have been reported on the interaction between surfactants and polyelectrolytes of the same charge. Generally, surfactants and polyelectrolytes of the same charge do not form complexes because of the long-range electrostatic repulsion force. It has been reported that amphiphilic polyelectrolytes in bulk solutions may form complexes with surfactants of the same charge via hydrophobic interactions.9−14 In these cases, the hydrophobic interaction may overcome the electrostatic repulsion and distort the polymer backbone such that the polymers form complexes with surfactants of the same charge. On a water surface, the hydrophobic interaction between surfactants and polyelectrolytes of the same charge diminishes because surfactants are known to have their hydrophilic groups pointing toward the liquid water. Therefore, complex formation via the aforementioned hydrophobic interaction becomes difficult on a water surface. It has been reported that cationic © 2017 American Chemical Society

polyethylenimine (PEI) and cationic alkyltrimethylammonium bromide (CTAB) form ordered mesostructured films on a water surface.15 However, the films were not thermodynamically stable because they showed a loss of structure from their neutron reflectometry profiles with time. The authors concluded that the dominant interaction between the polymer and surfactant is a neutral/cationic interaction, where the dipole on the amine groups of PEI interacts with the charged CTAB ammonium groups. Here we study the interaction between surfactants and polymers of the same charge on a water surface via a different mechanism: ion-bridged ionic/ionic interactions and hydrogen bonds. Divalent cations, such as Ca2+, may lead to a charge reversal of a charged surface.16,17 It has been reported that the addition of Ca2+ results in the binding of an anionic surfactant onto an anionic surface. On the other hand, monovalent ions (Na+) do not show any evidence of such adsorption.18 The current study investigates whether excess ions, such as Na+ and Ca2+, present in the solution can neutralize the charge of surfactants and allow polymers of the same charge to approach the surface and form a complex with the surfactant on the water’s surface. We used anionic surfactant sodium dodecyl sulfate (SDS), as shown in Figure 1a, and anionic polymer partially hydrolyzed polyacrylamide (HPAM), as shown in Figure 1b, to explore these cation-bridged anionic/anionic Received: April 12, 2017 Revised: June 5, 2017 Published: July 7, 2017 7940

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parameters provided by Aqvist33 were used for NaCl and CaCl2. The behavior of water molecules was described by the extended singlepoint charge (SPC/E) model.34 Although OPLS-AA was originally designed to work with the TIP3P water model, the combination of the SPC/E water model with the OPLS-AA force field has been applied successfully to study aqueous interfaces.28,35,36,48 Because SDS on the water surface orients interfacial water molecules,37 a minimum thickness of 5 nm is required to obtain a bulklike water region. Therefore, a simulation box of 5 nm × 5 nm × 10 nm with two water surfaces was created. Two oligomers of HPAM were initially placed within 2 nm from the water surface on each side. The simulation box was filled with 8067 water molecules. The surface coverage of SDS was fixed at its critical micelle concentration with a surface coverage of 0.44 nm2/molecule, which was equivalent to 57 SDS molecules on each side of the water surface. The SDS was randomly distributed on the water surface using the PACKMOL package38 such that the hydrophilic headgroups were pointing toward the liquid water. To maintain the electrical neutrality of the system, 122 water molecules were randomly substituted by 122 Na+ ions (114 from SDS and 8 from HPAM). Two vapor regions with a thickness of 25 nm were created on each side of the simulation box. To examine the effect of excess salts, 60 and 90 water molecules were randomly replaced by 30 NaCl and 30 CaCl2, respectively. The steepest-descent energy minimization (10 000 steps) was used to correct the position of each atom before the simulation. All simulations were performed using the GROMACS 5.1.2 GPU computation algorithm30−32 in the canonical ensemble. The temperature of water and the nonwater groups was maintained at 293 K independently using the V-rescale39 thermostat with a temperature constant of 0.1 ps. The OH bonds of water were constrained by the SETTLE algorithm40 to enable a simulation time step of 2 fs. The rest of the bonds were constrained using the P-LINCS algorithm30 with a LINCS order of 4. The Lennard-Jones interactions were truncated with a cutoff radius of 2.1 nm. Unlike-atom interactions were computed using the geometric combination rule, for which the OPLSAA was designed. Periodic boundary conditions were applied to all three directions. The cutoff radius for the Coulomb potential was 2.1 nm. The long-range Coulomb interaction was treated with the particle mesh Ewald (PME)41 algorithm with a grid spacing of 0.16 nm. The simulations were carried out for 40 ns, but 30 ns was needed to reach equilibrium. Therefore, the trajectories of the final 10 ns were used for the analysis presented in the current study. The visualization was prepared using VMD 1.9.2.42 Enhanced Sampling Method. Typical MD simulations suffer from a lack of sampling all possible metastable states, which a system may have. The transition between metastable states often requires overcoming high energy barriers, which rarely happens in a typical nanosecond or even microsecond simulation. To overcome this limitation, we carried out well-tempered metadynamics simulations43 in which an external Gaussian potential favorably biases the collective variables (CV) to release the system from the local minima and explore all possible states during the simulations. The CV considered in our simulations is the distance between the z component of the center of mass of the four acrylic acid groups in HPAM to the z component of the center of mass of all S atoms of SDS. This distance was biased separately for each interface. Biased potential V(S, t) is the sum of the Gaussians deposited in the CV space

Figure 1. Molecular structures of SDS (a) and HPAM (b) in aqueous solutions.

interactions. SDS is an anionic surfactant used in many industrial processes and consumer products. Anionic HPAM, a copolymer of poly(acrylamide) (PAM) and poly(acrylic acid) (PAA),19 is widely used for enhanced oil recovery23 and water treatment.21,22,47 HPAM and SDS are also commonly being used as chemical flooding agents to reduce the mobility of the aqueous phase and the interfacial tension between water and oil.23,24 It has been reported that the addition of SDS decreases the viscosity of HPAM solutions.25 However, the mechanism for the decrease in viscosity is not fully understood. Samanta et al.26 suggested that SDS affects the viscosity of HPAM solutions through a charge-shielding mechanism, but no details of the mechanism were described. Previous studies have shown that HPAM does not form complexes with SDS.25,11 Only anionic polymers with a very pronounced hydrophobic nature, such as poly(1-decene-co-maleic acid) and poly(l-octadeceneco-maleic acid), are able to form complexes with SDS in the bulk solution.9 To gain a molecular-level understanding of how ions affect the interaction between the anionic surfactant and the anionic polymer, we carried out a combined study using molecular dynamics (MD) simulation and phase-sensitive sum-frequency generation (SFG) vibrational spectroscopy. As expected, the MD simulation shows that the presence of SDS on the water surface pushes HPAM into the bulk liquid. However, in the presence of excess Na+ ions, the charge-screening effect allows HPAM to approach the water surface and weakly interact with the SDS via hydrogen bonding. The addition of divalent cation Ca2+ induces a much stronger interaction between SDS and HPAM by forming ion bridges and hydrogen bonds. The results of the MD simulation were confirmed by the SFG spectra of the water surface. We observed that introducing excess Na+ ions produced only a small change in the SFG spectrum of the water surface, indicating a minor change in the ordering of surface water molecules. On the other hand, Ca2+ ions significantly decreased the SFG intensity and disturbed the ordering of surface water molecules.

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τf

d (−V (S , T )/kBΔT ) − ∑i = 1

V (S , t ) ∑ W0τ e

MATERIALS AND METHODS

τ=1

MD Simulations. We employed full atomistic MD simulations to study the interaction between SDS and HPAM in the presence of NaCl and CaCl2. The all-atom optimized potentials for liquid simulation (OPLS-AA)27 were used for SDS and HPAM. The parameters reported by Li et al.28 were used to describe the intramolecular and intermolecular interactions of SDS molecules. HPAM molecules consisting of 20 monomers with an amine-tocarboxylate functional groups ratio of 4:1 were used in the simulation. The HPAM was parametrized using the OPLS-AA force field,27,29 which was provided in the GROMACS 5.1.230−32 database. The

e

(Si(R ) − Si(R(t )))2 2σi 2

(1)

where W0 refers to the initial Gaussian amplitude, kB is the Boltzmann constant, and τ is the time step between bias depositions. The first exponential factor on the right-hand side of eq 1 scales the amplitude to prevent overfilling the energy landscape. The value of ΔT controls the decay rate of the amplitude in the CV space and is scaled by the bias factor γ = (T + ΔT)/T, where T is the system temperature. In the second exponential term, σi is the width of the Gaussian for the ith CV. Si(R) represents the ith CV function. A bias factor of 20 was used. The bias was deposited with a Gaussian width of 0.05 nm, an initial 7941

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Langmuir Gaussian amplitude of 2.0 kJ/mol, and a deposition period of 0.4 ps. The well-tempered metadynamics simulations were carried out using PLUMED 2.44 To expedite the metadynamics convergence, we put two harmonic potentials with the following form for the distance between the HPAM and the SDS to limit the region of the phase space accessible during the run

Vκ = κ(z − a)2

(2)

where κ is the wall energy constant and 1500 kJ mol−1 nm−1 was used, z is the z component of Cartesian coordinates, and a is the position of the wall. In this study, we set the wall at the distance of 6 nm between the center of mass of the four acrylic acid groups in HPAM and the z component of the center of mass of all S atoms of SDS for each surface. To start the well-tempered metadynamics, the last trajectory of the typical MD simulation was used as the initial configuration for each system. The temperature of water and nonwater groups was maintained at 293 K independently using the V-rescale39 thermostat with a temperature constant of 0.1 ps. The OH bonds of water were constrained by the SETTLE algorithm40 to enable a simulation time step of 2 fs. The rest of the bonds were constrained using the PLINCS algorithm30 with a LINCS order of 4. The Lennard-Jones cutoff radius was 1.0 nm, where the interaction was smoothly reduced to 0 after 0.9 nm. The unlike-atom interactions were computed using the geometric combination rule. Periodic boundary conditions were applied in all three directions. The cutoff radius for the Coulomb potential was 1.0 nm. The long-range Coulomb interaction was treated with the particle mesh Ewald (PME)41 algorithm with a grid spacing of 0.16 nm.41 The simulations were carried out for 100 ns, where the convergence was attained. Finally, to calculate the binding free energy between HPAM and SDS, the reweighting algorithm45 developed by Tiwary and Parrinello was used. Materials and Sample Preparation. HPAM (average molecular weight ∼520 000 g/mol with 80 wt % acrylamide), NaCl (>99%), CaCl2 (>99%), and SDS (>99%) were purchased from Sigma-Aldrich. Water with a resistivity >18.2 MΩ·cm was obtained from a Millipore system. A 0.008 M SDS solution was made by dissolving SDS in pure water. A solution with 0.008 M SDS and 10−8 M HPAM was made by adding 0.016 M SDS solution to an equal volume of 2 × 10−8 M HPAM solution under vigorous stirring. Mixtures of SDS, HPAM, and the salts were made by mixing an equal volume of the SDS/salt solution and the HPAM solution. All solutions were freshly prepared right before spectroscopic measurement. All experiments were performed at 20 °C and 1 atm. Phase-Sensitive SFG Setup. A femtosecond Ti-sapphire laser (120 fs, 800 nm, 1 kHz, and 1 mJ/pulse) was used to pump an optical parametric amplifier to generate a femtosecond IR beam. The broadband IR beam and a narrow-band picosecond 800 nm beam were aligned collinearly.46,54 The incident angle was 60°. A reference SFG was obtained by focusing the IR and picosecond 800 nm beams into a 50-μm-thick quartz crystal. The IR, 800 nm, and reference SFG beams were then focused again on the sample. The reference SFG and the SFG generated at the sample went through a time delay, a polarizer, a band-pass filter, a lens, and a monochromator, and then the interference pattern was recorded with a camera. The polarization combination used in this study was SSP (s-polarized SFG, s-polarized 800 nm, and p-polarized IR). The energies of the 800 nm and IR beam were ∼10 and ∼3 μJ/pulse, respectively. Spectra presented in the article were acquired over a period of 20 min.

Figure 2. Simulation box with dimensions of 5.0 × 5.0 × 10.0 nm3 filled with ∼8000 water molecules, 114 SDS, 2 HPAM, and 122 Na+. (a) No excess ions. (b) Thirty excess NaCl. (c) Thirty excess CaCl2. Color codes for atom types: red (oxygen), green (carbon), blue (nitrogen), yellow (sulfur), white (hydrogen), magenta (Na+), orange (Cl−), black (Ca2+), and cyan (water).

polymers approach the surface and interact with the SDS layer on the water surface. In the MD simulations, the weak association between HPAM and SDS in the presence of excess Na+ often results in a nonsymmetrical structure between the upper and lower water surfaces in Figure 2b because the bonds between SDS and HAPM may break and form repeatedly. To address this issue and to test whether the complex is thermodynamically stable, we calculated the binding free energy between HPAM and SDS using well-tempered metadynamics43 as described in the Enhanced Sampling Method section. The binding free energy profile for the salt-free system (Figure 3a) shows that the repulsive electrostatic force dominates the interaction between SDS and HPAM. In this case, HPAM needs to overcome an energy barrier of ∼20 kJ/mol to reach the averaged position of the S atoms in SDS, at which the x axis is defined as 0 in Figure 3. Therefore, both HPAM polymers prefer to stay in the bulk as indicated in Figure 2a. Figure 3b shows that the presence of excess Na+ significantly reduces the repulsive force and creates a small and gradual attractive potential for HPAM. The relatively weak potential explains why it is rare to have both HPAM polymers attached to the surface. In contrast, Figure 3c shows that the presence of Ca2+ creates a steep potential well of ∼30 kJ/mol, which is higher than typical hydrogen bonds. Therefore, the Ca2+-bridged HPAM-SDS complex, shown in Figure 2c, is thermodynamically stable. To better understand the nature of the surfactant−polymer interaction, Figure 4 shows the time-averaged number density profiles of the −SO4 groups in SDS, the H atoms in the NH2 groups of HPAM (HP), the O atoms in the CO2 groups of HPAM (OP), and the intrinsic Na+ ions, which come with SDS and HPAM. Even without excess ions (Figure 4a), a significant number of intrinsic Na+ ions accumulate near the surface, but they do not allow HPAM to overcome the strong electrostatic repulsion force. This result is consistent with the previous experimental observation that HPAM and SDS do not associate with each other in pure water.25 Figure 4b shows that the addition of NaCl to the solution results in more Na+ ions accumulating at the surface. The charge screening produced by the additional Na+ ions allows the polymers to approach the

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RESULTS AND DISCUSSION The snapshots of the MD simulation without excess ions, with excess Na+, and with excess Ca2+ are shown in Figure 2. Without excess ions (Figure 2a), HPAM is not able to overcome the repulsive forces from the surfactants and stay in the bulk water. Excess Na+ ions in the solution led to a weak association between SDS and one of the HPAM polymers (Figure 2b). In the presence of Ca2+ (Figure 2c), both HPAM 7942

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Figure 3. Calculated binding free energy as a function of distance between the z component of the center of mass of the four acrylic acid groups in HPAM to the z component of the center of mass of the S atoms in all SDS for the systems with no excess ions (a), 30 excess NaCl (b), and 30 excess CaCl2 (c). The red and blue curves represent the results from each water surface in the simulation box. The negative distance refers to states in which the position of the center of mass of the acrylic acid groups in HPAM is higher than the center of mass of the S atoms.

Figure 4. Time-averaged number density profiles of the −SO4− groups in SDS, the H atoms in the NH2 groups of HPAM (HP), the O atoms in the CO2 groups of HPAM (OP), Na+, Cl−, and Ca2+. (a) No excess ions. (b) Thirty excess NaCl. (c) Thirty excess CaCl2. The values of HP, OP, Na+, Cl−, and Ca2+ are magnified by 5, 20, 5, 10, and 5 times, respectively for better visibility.

surface. In the presence of excess Ca2+ (Figure 4c), nearly all Ca2+ ions are located near the surface, and the distributions of the surfactant and the polymers become overlapped near the surface. A detailed analysis of the simulation data reveals that the complex formation between the surfactants and the polymers in the presence of Ca2+ ions is dominated by the Ca2+-bridged interaction between the OP in HPAM and the ionic O atoms (Oi) of SDS. Figure 5a shows the radial distribution function of Oi with respect to OP in the presence of excess Na+ and Ca2+ ions. It is clear that Ca2+ ions induce a distinguishable layered structure between OP and Oi (red curve in Figure 5a) whereas Na+ ions allow only OP and Oi to approach each other (blue curve in Figure 4a). Figure 5b shows that the ester O atoms of SDS (Oe) are not directly involved in the interaction with the polymer because Oe mostly resides in the second layer. Our simulation indicates that hydrogen bond formation plays an important role in the interaction between HPAM and SDS. Hydrogen bonds may form between the O atoms of SDS (both

Oi and Oe) and the H atoms in the NH2 group of HPAM (HP). Figure 6 illustrates the radial distribution functions of Oi and Oe with respect to HP in the presence of Ca2+ and Na+. Overall, Ca2+ promotes hydrogen bond formation more than Na+ does. In Figure 6, narrower peaks within 1 nm originate from the neighboring H atoms of the polymer (NH2 groups), and the broad peak above 1 nm originates from the O atoms in the neighboring SDS. To quantitatively study the mechanism of complex formation between SDS and HPAM, the number of ion bridges was calculated. It was assumed that an ion bridge (OP-ion-Oi or OPion-Oe) is established if an ion (either Na+ or Ca2+) resides within a cutoff distance from the C atom in the CO2 groups of HPAM (Rcutoff(ion−C)) and from the S atom of SDS headgroups (Rcutoff(ion−S)). The cutoff distances were determined by the first minimum in the radial distribution function: Rcutoff(Ca−C) = 0.45 nm, Rcutoff(Na−C) = 0.40 nm, Rcutoff(Ca−S) = 0.44 nm, Rcutoff(Na−C) = 0.40 nm, and Rcutoff(Na−S) = 0.43 nm. The numbers of various types of ion 7943

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Table 1. Number of Ion Bridges and Hydrogen Bonds for Different Systems in Terms of the Number of Excess Ions mediate ions

ion bridges (per ns)

hydrogen bonds (per ns)

Na+ (no salts) Na+ (with NaCl) Na+ (with CaCl2) Ca2+ (with CaCl2)

0 7 364 193

0 2568 3239

NaCl to the system allows a small number of Na+ ions to participate in the formation of ion bridges between SDS and HPAM. On the other hand, the presence of Ca2+ significantly facilitates complex formation between SDS and HPAM. When Ca2+ ion bridges are formed between SDS and HPAM, Na+ ions further stabilized the SDS-HPAM complex as the number of Na+ ion bridges increases significantly. To quantitatively study hydrogen bond formation between SDS and HPAM, the number of hydrogen bonds was calculated using geometric criteria. A hydrogen bond is formed between HP and Oi (or Oe) if the distance between the N atom of NH2 groups of HPAM and Oi (or Oe) is less than the cutoff distance Rcutoff(N−Oi) = 0.46 nm, Rcutoff(N−Oe) = 0.60 nm, which was derived from the first minimum in the radial distribution function. Additionally, the angle ∠H...N...O should be less than 30°, which is commonly used to identify hydrogen bonding.49 The number of hydrogen bonds between SDS and HPAM is presented in Table 1. With the addition of NaCl, a significant number of hydrogen bonds are formed, although very few Na+ bridges are present, indicating that hydrogen bonds are the dominating interaction between SDS and HPAM with the addition of NaCl. Although a greater number of hydrogen bonds are formed in the presence of Ca2+, the stronger electrostatic interaction of Ca2+ ions plays a more important role in linking SDS and HAPM. Phase-sensitive SFG vibrational spectra were collected at air/ water interfaces to verify the MD simulation results. SFG has been known for its high surface sensitivity because under the electric-dipole approximation the second-order optical process is forbidden in a centrosymmetric medium such as bulk water. Although traditional SFG vibrational spectroscopy measures only the amplitude of the second-order nonlinear optical susceptibility |χ(2)|, phase-sensitive SFG also measures the phase of χ(2).20,50,52 For the OH-symmetric stretch of the water surface, the imaginary part of χ(2), Im(χ(2)), can be positive or negative, depending on the sign of the OH projection with respect to the surface normal: a positive peak indicates water molecules with the hydrogen pointing toward the air (up), and a negative peak indicates the OHs pointing toward the bulk (down).53 Figure 7 illustrates the Im(χ(2)) of a water surface with (a) SDS only, (b) SDS + HPAM, (c) SDS + HPAM + excess Na+, (d) SDS + HPAM + Ca2+, and (e) SDS + Ca2+. As shown in Figure 7, because of the negative charge on SDS, SDS (magenta curve) produces an ordered water structure with the water’s OHs pointing up (a positive SFG peak). This observation is in agreement with previous studies revealing that anionic surfactants lead to the flipping of water molecules at the surface.53 Adding HPAM with or without excess Na+ does not significantly alter the SFG spectrum. However, the presence of Ca2+ dramatically changes the SFG spectrum of water. The decrease in SFG intensity suggests that the ordering of water is significantly disturbed in the presence of Ca2+. Although Ca2+ ions alone perturb the hydrogen network of surface water molecules (black curve), the addition of HPAM

Figure 5. Radial distribution functions between the O atoms in the CO2 groups of HPAM (OP) and the O atoms in SDS: (a) ionic O atoms (Oi) and (b) ester O atoms (Oe). OP is the reference point of the radial distribution functions.

Figure 6. Radial distribution functions between the H atoms in the NH2 group of HPAM (HP) and (a) the ionic O atoms (Oi) and (b) the ester O atoms (Oe) of SDS. HP is the reference point of the radial distribution functions.

bridges are presented in Table 1. Without salts in the solution, the intrinsic Na+ ions from SDS and HPAM do not produce any ion bridges between SDS and HPAM. The addition of 7944

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(5) Rai, K.; Johns, R. T.; Delshad, M.; Lake, L. W.; Goudarzi, A. OilRecovery Predictions for Surfactant Polymer Flooding. J. Pet. Sci. Eng. 2013, 112, 341−350. (6) Noskov, B. A. Dilational Surface Rheology of Polymer and Polymer/Surfactant Solutions. Curr. Opin. Colloid Interface Sci. 2010, 15, 229−236. (7) Taylor, D. J. F.; Thomas, R. K.; Penfold, J. Polymer/Surfactant Interactions at the Air/Water Interface. Adv. Colloid Interface Sci. 2007, 132, 69−110. (8) Miyake, M. Recent Progress of the Characterization of Oppositely Charged Polymer/Surfactant Complex in Dilution Deposition System. Adv. Colloid Interface Sci. 2017, 239, 146−157. (9) Mcglade, M. J.; Randall, F. J.; Tcheurekdjian, N. Fluorescence Probe Studies of Aqueous-Solution Interaction between Sodium Dodecyl-Sulfate and Anionic Polyelectrolytes. Macromolecules 1987, 20, 1782−1786. (10) Mcglade, M. J.; Olufs, J. L. Adsorption of Sodium DodecylSulfate onto Alpha-Olefin Maleic-Acid Copolymers in AqueousSolutions. Macromolecules 1988, 21, 2346−2349. (11) Iliopoulos, I.; Wang, T. K.; Audebert, R. Viscometric Evidence of Interactions between Hydrophobically Modified Poly(Sodium Acrylate) and Sodium Dodecyl-Sulfate. Langmuir 1991, 7, 617−619. (12) Zana, R.; Kaplun, A.; Talmon, Y. Microstructural Aspects of Polysoap Sodium Dodecyl-Sulfate Interactions. Langmuir 1993, 9, 1948−1950. (13) Bakeev, K. N.; Ponomarenko, E. A.; Shishanova, T. V.; Tirrell, D. A.; Zezin, A. B.; Kabanov, V. A. Complexation of Amphiphilic Polyelectrolytes with Surfactants of the Same Charge in Water Solutions. Macromolecules 1995, 28, 2886−2892. (14) Bai, G.; Catita, J. A. M.; Nichifor, M.; Bastos, M. Microcalorimetric Evidence of Hydrophobic Interactions between Hydrophobically Modified Cationic Polysaccharides and Surfactants of the Same Charge. J. Phys. Chem. B 2007, 111, 11453−11462. (15) O’Driscoll, B. M. D.; Milsom, E.; Fernandez-Martin, C.; White, L.; Roser, S. J.; Edler, K. J. Thin Films of Polyethylenimine and Alkyltrimethylammonium Bromides at the Air/Water Interface. Macromolecules 2005, 38, 8785−8794. (16) Kekicheff, P.; Marcelja, S.; Senden, T. J.; Shubin, V. E. Charge Reversal Seen in Electrical Double-Layer Interaction of Surfaces Immersed in 2−1 Calcium Electrolyte. J. Chem. Phys. 1993, 99, 6098− 6113. (17) Liu, J.; Xu, Z.; Masliyah, J. Studies on Bitumen−Silica Interaction in Aqueous Solutions by Atomic Force Microscopy. Langmuir 2003, 19, 3911−3920. (18) Wang, X. F.; Lee, S. Y.; Miller, K.; Welbourn, R.; Stocker, I.; Clarke, S.; Casford, M.; Gutfreund, P.; Skoda, M. W. A. Cation Bridging Studied by Specular Neutron Reflection. Langmuir 2013, 29, 5520−5527. (19) Morgan, S. E.; Mccormick, C. L. Water-Soluble Polymers in Enhanced Oil-Recovery. Prog. Polym. Sci. 1990, 15, 103−145. (20) Stiopkin, I. V.; Weeraman, C.; Pieniazek, P. A.; Shalhout, F. Y.; Skinner, J. L.; Benderskii, A. V. Hydrogen Bonding at the Water Surface Revealed by Isotopic Dilution Spectroscopy. Nature 2011, 474, 192−195. (21) Zhao, P.; Gao, B. Y.; Yue, Q. Y.; Kong, J. J.; Shon, H. K.; Liu, P.; Gao, Y. Explore the Forward Osmosis Performance Using Hydrolyzed Polyacrylamide as Draw Solute for Dye Wastewater Reclamation in the Long-Term Process. Chem. Eng. J. 2015, 273, 316−324. (22) Sen, G.; Ghosh, S.; Jha, U.; Pal, S. Hydrolyzed Polyacrylamide Grafted Carboxymethylstarch (Hyd. Cms-G-Pam): An Efficient Flocculant for the Treatment of Textile Industry Wastewater. Chem. Eng. J. 2011, 171, 495−501. (23) Wever, D. A. Z.; Picchioni, F.; Broekhuis, A. A. Polymers for Enhanced Oil Recovery: A Paradigm for Structure-Property Relationship in Aqueous Solution. Prog. Polym. Sci. 2011, 36, 1558−1628. (24) Cheraghian, G. An Experimental Study of Surfactant Polymer for Enhanced Heavy Oil Recovery Using a Glass Micromodel by Adding Nanoclay. Petrol. Pet. Sci. Technol. 2015, 33, 1410−1417.

Figure 7. Im(χ (2)) spectra of air/water interfaces in the OH regions for various aqueous solutions of 0.008 M SDS (magenta), 0.008 M SDS + 1 × 10−8 M HPAM (green), 0.008 M SDS + 1 × 10−8 M HPAM + 0.1 M excess NaCl (blue), 0.008 M SDS + 1 × 10−8 M HPAM + 0.1 M excess CaCl2 (red), and 0.008 M SDS + 0.1 M excess CaCl2 (black).

further disrupts the ordering of water molecules, which is consistent with MD simulations suggesting that SDS and HPAM form stable complexes at the surface via ion bridges and hydrogen bonds.

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CONCLUSIONS We investigated the interaction between surfactants and polymers of the same charge at air/water interfaces using MD simulation and SFG. The results indicated that excess Na+ in the system allowed the polymer to approach the surface and interact with the surfactant via hydrogen bonding. In the presence of Ca2+ ions, HPAM and SDS interact via forming Ca2+ ion bridges and hydrogen bonds. These results are qualitatively consistent with the observed SFG spectra.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Keng C. Chou: 0000-0002-8782-5253 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Sciences and Engineering Research Council of Canada. A.M. was partially supported by the Four Year Doctoral Fellowship Program at the University of British Columbia.

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REFERENCES

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DOI: 10.1021/acs.langmuir.7b01246 Langmuir 2017, 33, 7940−7946

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DOI: 10.1021/acs.langmuir.7b01246 Langmuir 2017, 33, 7940−7946