Study of the Microcharacter of Ultrastable Aqueous Foam Stabilized by

Apr 15, 2013 - In this paper, ultrastable aqueous foam stabilized by a kind of flexible connecting bipolar-headed surfactant alkyl polyoxyethylene sul...
0 downloads 8 Views 650KB Size
Download PDF
Article pubs.acs.org/Langmuir

Study of the Microcharacter of Ultrastable Aqueous Foam Stabilized by a Kind of Flexible Connecting Bipolar-Headed Surfactant with Existence of Magnesium Ion Chunxiu Li, Ying Li,* Rui Yuan, and Weiqin Lv Key Laboratory of Colloid and Interface Chemistry of State Education Ministry, Shandong University, 27 Shanda Nanlu, Jinan, Shandong 250100, P. R. China S Supporting Information *

ABSTRACT: In this paper, ultrastable aqueous foam stabilized by a kind of flexible connecting bipolar-headed surfactant alkyl polyoxyethylene sulfate (AE3S) with coexisting Mg2+ was reported. Detailed molecular behaviors of AE3S in foam film with coexisting divalent cationic Ca2+ or Mg2+ were investigated by molecular dynamic simulation, comparing with the traditional surfactant sodium dodecyl sulfate (SDS), to find out how the microcharacter and array behavior of molecules in the foam film determined by molecular interaction effect the foam stability. It was found that the ultrastable foam film obtained by the cooperation of magnesium ions and AE3S was driven from two aspects: one is the favorable arrangement of surfactant molecules, and the other is the increase of capacity of foam films for resolutely holding water molecules deduced by a dipolar pair formed by the flexible connecting head groups of AE3S and hydrated Mg2+ via intermolecular coactions, both related to the presence of magnesium ions. Foam lamella stability measurement and foam decay method were both used to evaluate the stability of foam. Fourier transform infrared (FT-IR) was used to detect the composition variation of foam film in the drainage process; the vibration peak of OH for water molecule shifted from the 3390 cm−1 (being assigned to the bulk water integrated by hydrogen bonds) to 3685 cm−1 (being assigned to the vibration of isolated water molecules) for the ultrastable foam film after complete drainage, which agreed very well with the molecular simulation results.

■

molecules and penetration of gas through the foam film driven by the Laplace principle would constantly be strengthened, and foam usually collapses by the rupture of films. Stability of aqueous foam at high salt concentrations is also generally poor,19−21 such as in mineral water or seawater medium, and the interactions between inorganic ions and the hydrophilic groups of surfactants and polymers induce complicated variation of their properties; as a result, possessing stable foam under high salt concentration in convenient manner is still a great challenge, both from the theoretical and application aspects. In this paper, ultrastable aqueous foam stabilized by a type of bipolar-based surfactant alkyl polyoxyethylene sulfate AE3S with existence of inorganic salt MgCl2 was achieved. The system has great potential application in offshore oil production by foam flooding, or other occasion that seawater could be conveniently used as medium, during which the abundant magnesium ions in seawater could be taken into account in situ and utilized to form ultrastable aqueous foam. Besides, the dramatic enhancement of foam property by an ease action on a simple system provides a precious perfect model to study the structure− behavior−property relationship of aqueous foam.

INTRODUCTION Besides the application in traditional regions, such as, mineral floatation, detergent, and enhanced oil recovery,1−3 aqueous foam has got successful utilization in many other fields recently, such as photoelectricity resistance, infrared extinction or electromagnetic interference shielding,4−6 blast mitigation,7,8 and being used as a soft bionic template for controllable synthesis of functional nanoparticles.9−11 The formation and maintenance of the ultrastable overall static foam, sometime its half-life time would be requested to be days, are important and hard to be fulfilled. Most of the currently reported ultrastable aqueous foam systems are stabilized by solid particle or polymer with or without surfactants.12−17 Gonzenbach and coworkers16 have produced macroporous ceramics using particles instead of surfactants as stabilizers of the wet foams. They reported that the ultrastable foam with the inorganic particles or polymer particles are stabilized due to the strong particle attachment to the bubbles and controlled by the hydrophobicity. Fameau17 reported smart foams switching reversibly between ultrastable and unstable foams, by controlling the temperature. Reports about ultrastable foam stabilized only by surfactants are rare. At room temperature, foam stabilized only by surfactants could be very stable if Newton black foam film could be formed through drainage18 but is hard to be ultrastable. It is more difficult to be ultrastable under high temperature, such as 50 °C, as the quick diffusion of water © 2013 American Chemical Society

Received: December 17, 2012 Revised: April 12, 2013 Published: April 15, 2013 5418

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427

Langmuir

Article

Molecular dynamics simulation (MD) method has been used to give out molecular details of the foam film explaining the variation of the macrofoam stability.22,23 Interactions between head groups of surfactant and counter inorganic ions have been studied using MD, too.19,24 In this paper, the molecular behavior of surfactants in the foam film with or without multivalent courter ions was investigated by MD simulation, the effect of magnesium ion on adsorption behavior of AE3S at the air−water interface is found to be quite different from the traditional single head−single tail ones. Ultrastable mechanism of the foam was achieved by relating the MD results to macro foam stability, the molecular array behavior of all the substances in foam film has been analyzed, especially the states of water molecules, which was verified by FT-IR.

■

Figure 2. Scheme of the foam lamella stability measurement equipment.

METHODOLOGY

used. The velocity of nitrogen gas flow was 75 mL/min and was stopped immediately when the foam volume reached 150 mL. The foam stability was estimated using the half-life time, t1/2, of the foam column volume. The temperature of the foam container was kept at 50 °C ± 0.1 °C with an interlayer water jacket for the foam decay measurement. Molecular Dynamics Simulation. Force Field and Parameters. In this paper, the MD simulation with a full atomistic force field was conducted. A SPC/E model33 was chosen to build the water box. Though the TIP5P or TIP5P-E model would be more specific for the hydrogen bonding calculations and there may be some uncertainty for the SPC/E model, it has generally been found to work well with the SPC/E model to describe the hydrogen bonding and hydration.22,24,34−37 The potential functions and atom interaction parameters were modeled with the compass force field.38,39 The total energy was given as the combination of valence terms and nonbond interactions, and the summation of energies are listed in eq 1:

Materials and Experiments. Alkyl polyoxyethylene ether sodium sulfate AE3S was provided by Shengli Oilfield of SINOPEC and used as given; the carbon number of the hydrophobic chain is 12. Sodium dodecyl sulfate (SDS, purity >99%) was purchased from SigmaAldrich. The chemical structures of the surfactants are shown in Figure 1. CaCl2 (A. R.) and MgCl2·6H2O (A. R.) were purchased from

Figure 1. Chemical structure of surfactants used in this study. Top, alkyl polyoxyethylene ether sodium sulfate AE3S; bottom, sodium dodecyl sulfate SDS. The atoms drawn as van der Waals spheres are shown as a small colored sphere: C, gray; H, white; O, red; S, yellow; Na+, violet.

E = E bonds + Eangles + Edihedrals + Ecross + E VDW + Eelec

(1)

The nonbonding interaction terms include an LJ-9-6 function for the van der Waals (vdW) term and a Coulombic function for an electrostatic interaction. The long-range electrostatic interactions were described by the Ewald method.40 A smart minimizer method was used to minimize the initial configuration energy. All the MD simulations were carried out under canonical ensemble (NVT), and the equations of motion were integrated using the Velocity−Verlet algorithm41 with a time step of 1.0 fs. The Nose−Hoover thermostat42,43 with a coupling constant of 0.1 ps was applied to control the temperature in accordance with the experiment. Simulation Model and Procedure. To construct a surfactant monolayer, 16 surfactant molecules of AE3S or SDS were arranged with spacing suitable for hexagonal close packing in an orthorhombic simulation box with periodic boundary conditions applied for all three spatial directions. Corresponding to the experimental results, the lattice constants were chosen to satisfy the surface area per molecule value for the saturated surfactant adsorption at the air−water interface at the critical micelle concentration. The surface area per molecule for AE3S is presumed to be 75 Å2, referring to the experimental value of the surface area per molecule of AES and AE2S at 50 °C, which are 61 and 68 Å2, respectively.44 The SDS surfactant monolayer was built correspondingly as well. Details of the simulating box are listed in Table 1. The water box with a 40 Å thickness was constructed, and the lateral scale was in accordance with the surfactant box. Right numbers of sodium ions as counterions of surfactants and inorganic salt ions were added randomly into the water box. For the salt solution containing CaCl2 or MgCl2, 16 divalent cations and 32 Cl− were added into the water layer, respectively, to simulate high salinity at about 1 M of divalent cation in aqueous solution. Water density was arranged to be 988 g/L, corresponding to the experimental density of water at 323 K. The sandwich model45,46 was used to describe the simulated foam system. Two surfactant monolayers were placed at the opposite site of

Tianjin Guangcheng Chemical Corp. Sea water was from Bohai, with a total salinity of 32082 ppm (Na+: 10638 ppm; Ca2+: 398 ppm; Mg2+: 1042 ppm). Fourier transform infrared spectroscopy FT-IR was used to detect the composition variation of foam film following the drainage process. The FT-IR spectroscopy was recorded on a Bruker Tensor 27 spectrophotometer, acquired in the frequency range of 4000−400 cm−1 at a resolution of 1 cm−1 with a total of 16 scans. First, a 135 mm long quartz tube with a 6.0 mm diameter was placed in the optical channel and was scanned as background. Then, a vertical foam film was formed in the tube by bubbling through a capillary and was scanned constantly as a function of time. FT-IR spectroscopy of the corresponding solutions and pure water was also measured. Foam Stability Determination. Evaluation of foam stability is a problem in a mix because the results vary according to the testing methods and conditions. To ensure the accuracy and comparability of the foam stability we measured, the foam decay method25,26 and foam lamella stability measuring method18,27−30 proposed by Gilanyi et al.31 were both used. Self-designed equipment for the latter method was shown in Figure 2. Actually, besides the external factors, Li and his coworkers32 have proposed the strong dependence of foam stability on environmental humidity, so all the experiments were done under a saturated humidity, and the humidity of the foam film environment was controlled to be the same. The container was put into a transparent glass bowl water bath to keep the temperature at 50 °C ± 0.1 °C. Measurements are repeated more than five times, for some systems about 10 times, to ensure reliable statistics. For the foam decay measurement, a cylindrical glass tube (500 mm height and 34 mm i.d.) with a sintered glass frit at the bottom was 5419

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427

Langmuir

Article

Table 1. Details of Simulated Systems system AE3S

SDS

a = b (Å)

Lx (Å)

Ly (Å)

Lz (Å)

nwater

nNa+

nCa2+

nMg2+

nCl‑

9

31.176

36

160

32

9

31.176

36

160

1410 1340 1310 1410 1340 1310

0 16 0 0 16 0

0 0 16 0 0 16

0 32 32 0 32 32

no salt added with CaCl2 with MgCl2 no salt added with CaCl2 with MgCl2

32

Table 2. The Stability of Foam Column Evaluated by Foam Decay Measurement and the Life Time of Foam Film Determined Gilanyi Methoda foam stability

t1/2 (hour)

t* (min)

surfactant concentration

no salt added

1 M CaCl2

1 M MgCl2

sea water

no salt added

1 M CaCl2

1 M MgCl2

0.1 wt % SDS 0.1 wt % AE3S

9 11

no foaming 0.93

0.88 57

5 23

2.42 1.17

foam film collapses at once as it forms 4.35

3.68 70

a

t1/2, half-life times of foam column; t*, foam film life time. T = 50 °C ± 0.1 °C.

the water box to form a foam film model. The Z dimension was kept large enough to be 160 Å to avoid interactions between the periodic replicas. To avoid the possible molecule overlap of the initial built configurations, 20000 steps of minimized optimization were first run. Molecular dynamic simulation was then carried out starting with the minimized configuration. All simulations were equilibrated at a constant temperature (323 K) and volume (NVT). The systems equilibrated at about 1 ns. The MD simulations were carried out for 4 ns, and the results of the last 1 ns were used to analyze and evaluate the properties.

■

RESULTS AND DISCUSSION Outstanding Effect of Inorganic Salts on Stability of Aqueous Foam. Stabilities of foam column and foam lamellar film formed from solutions of surfactant SDS or AE3S were evaluated experimentally, and results were shown in Table 2. SDS is a kind of classical surfactant often used as a foam stabilizer, and the foam decay measurement showed that the half-life time of SDS foam without salt added got as high as 9 h but reduced to 0.88 h with the existence of MgCl2 and cannot even foam with 1 M CaCl2. The foam formed by the AE3S solution with no salt added was very stable; however, the halflife time of the foam decreased from 11 to 0.93 h with CaCl2 added, while it surprisingly increased to 57 h with added MgCl2. No foam collapse was observed even after 48 h of drainage at 50 °C, and the foam became cellular network constructed with ultrathin foam films (Figure 3). The results of foam film lifetime measurement coincided quite well with the foam decay measurement, and the foam film formed from the AE3S containing MgCl2 was far more stable than all of the rest. The aqueous foam column with 57 h half-life time at 50 °C should be undoubtedly ultrastable, which is not easy to get, let alone at high salt concentration. The amazing phenomenon drew our interest, not only because the dramatic enhancement of foam property by an ease action on such a simple system provided a perfect precious model system to study the structure−behavior−property relationship of foam but also the variation induced by Mg2+, where its abundant in seawater and biological systems is quite meaningful. The interesting phenomenon revealed a useful clue that AE3S might be a perfect candidate for the seawater-based foam stabilizer, which might be used in occasions where seawater could be conveniently used as a medium (e.g., foam flooding in offshore oil production). Actually, the foam column half-life time results listed in Table 2 proved the possibility.

Figure 3. Picture of ultrastable foam.

In order to probe the mechanisms of the ultrastable foam and the distinguished effect of divalent cations on foam stability formed by SDS and AE3S, molecular dynamic simulations of foam film were carried out. Array Behavior of Surfactant Molecules at the Gas/ Water Interface with or without Divalent Cations Coexisting. The equilibrated configurations of the investigated systems (Figure 4) showed that Na+ distributed closely to the surfactant headgroup when no salt was added, while parts of Na+ were replaced by the divalent ions when Ca2+ or Mg2+ existed. Surfactant molecules aggregated obviously for both SDS and AE3S with the existence of Ca2+, while there was no obvious array variation with Mg2+ existing. In order to clearly describe the orientation of surfactant molecules at the air/water interface, the tilt angle θ of the surfactant tail chain and headgroup (definition shown in Figure 5) was analyzed: 5420

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427

Langmuir

Article

illustrated in Figure 4b. There was no obvious aggregation in Figure 4c, and the tail orientation distribution in Figure 6a changed a little compared with the case of no salt added, which indicated that the interaction between the magnesium ions and SDS was weaker than with the calcium ions. For AE3S, the tailorientation distributions were narrow and the tilt angles of tails were mostly parallel to the interface with no salt added, which should be due to the decrease of the anchoring effect of the sulfate group induced by the flexible linking EO groups. The variation of maximum distribution probability of AE3S with CaCl2 added was similar to SDS. The tail-orientation distribution of AE3S molecules did not change with the addition of CaCl2, while it got very broad with the coexistence of MgCl2, corresponding to the looser association and unordered array shown in Figure 4f. The tilt angles of the hydrophilic group of AE3S changed a lot with the addition of CaCl2 or MgCl2, and the two complementary shoulder peaks disappeared. The location of maximum distribution probability shifted from 36° to 41° with CaCl2 added, while it shifted to 32° for existing MgCl2, revealing more vertical arrangement of the hydrophilic group. In accordance with the molecular information provided in Figure 6, the added Ca2+ and Mg2+ surely brought array behavior variations of head groups and tails of AE3S and SDS. The surfactant aggregation and less staggered arrangement of hydrophobic tails would make the foam film unstable at high salt concentration, which agrees with the reported results.19,24 Interaction between Surfactants and Counterions. To quantitatively describe the interaction degree between surfactant headgroup and the two types of divalent counterions, potential of mean force PMF24,47,48 is calculated, which was defined as

Figure 4. Equilibrated configuration snapshots at the end of simulations (a) SDS, no salt, (b) SDS with CaCl2, (c) SDS with MgCl2, (d) AE3S, no salt, (e) AE3S with CaCl2, and (f) AE3S with MgCl2. For clarity, the surfactants and the cations (Na+, Ca2+, and Mg2+) are drawn as van der Waals spheres, shown as small colored sphere and water molecules drawn in line style. The atom coloring scheme is C, gray; H, white; O, red; S, yellow; Cl−, blue; Na+, violet; Ca2+, deep green; Mg2+, light green.

cos θ = lz /l

(2)

PMF = − kBT ln g (r )

(3)

The detailed results for values of free energy at the contact minimum (CM), the solvent-separated minimum (SSM), and the dissociation barrier (BARR) for the formation and separation of −SO4−Ca2+/Mg2+ dipolar pairs are listed in Table 3. ΔE+ = EBARR − ESSM is the binding energy barrier and ΔE− = EBARR − ECM is the dissociation energy barrier. In this paper, a parameter K was defined as a ratio of ΔE+ and ΔE− to reflect the relative tendency of binding and dissociation, which is more comparable for different substances with the similar binding and dissociation energy barrier. K < 1 means binding is easier happening and dissociation is harder processing, while it is the opposite for K > 1 The order of values of the K parameter is KSDS (−SO4−−Ca2+) (0.69) < KAE3S (−SO4−−Ca2+) (0.77) < KSDS (−SO4−−Mg2+) (0.98) < KAE3S (−SO4−−Mg2+) (1.04), representing the binding tendency for the calcium ion with the −SO4− group of SDS the strongest, the −SO4− (SDS)−Ca2+ dipolar pair easy to form and difficult to dissociate, while the −SO4− (AE3S)−Mg2+ binding tendency the weakest and the only one with the K value larger than 1. The stronger interaction between Ca2+ and −SO4− (SDS) than −SO4− (AE3S) and interaction between −SO4− and Mg2+ coincides with the results in Figures 4 and 6; it also explains the relatively high g(r)S−Ca2+ intensity in Figure S1 of the Supporting Information. The association between divalent counterions and head groups of surfactant with opposite charges is a dynamic process. Bound cationic ions may transform constantly between different states, such as being bound or being dissociated. In

Figure 5. Definition of the tilt angles of the tail and headgroup of AE3S with respect to the Z direction.

lz is the projection of the hydrophobic tail chain or headgroup in direction of the surface normal, and l is the length of total hydrophobic chain or the hydrophilic headgroup. As shown in Figure 6, the tail orientation of SDS has the maximum distribution probability around θ = 35° with no salt added, while the tail-orientation distribution got narrower and tended to be more vertical to the surface with the addition of CaCl2, indicating the more compact arrangement of the surfactants and being more vertical to the interface, which inferred the strong interaction between Ca2+ and the sulfate group of the surfactant. The extending tendency of the SDS tail along the Z direction corresponded to the distinct aggregation 5421

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427

Langmuir

Article

Figure 6. Tilt angle orientation distributions with respect to the Z direction of the surfactant hydrophobic tail for (a) SDS and (b) AE3S and (c) headgroup of AE3S.

Table 3. Values of the SSM, CM, and Energy Barriers of Binding−Dissociation Process of Cationic Ions and Sulfate Groups of SDS and AE3Sa system

dipolar pair

CM

BARR

SSM

ΔE+

ΔE−

K

SDS

−SO4−−Ca2+ −SO4−−Mg2+ −SO4−−Ca2+ −SO4−−Mg2+

−14.26 −7.59 −13.33 −7.52

10.48 18.27 10.79 21.7

−6.64 −7.04 −6.05 −8.66

17.12 25.31 16.84 28.96

24.72 25.86 24.12 27.82

0.69 0.98 0.77 1.04

AE3S a

The energy unit is kJ/mol.

this paper, dynamic binding and dissociation behavior were described by calculating the mean square displacement (MSD) of the bound ions and groups and the definition is 2 MSD = | r(⃗ t ) − r(0) ⃗ |

The bound divalent ions and sulfate were relatively defined, and the bound or unbound state could be identified by the MSDs deviation of the initially bound groups. As shown in Figure 7, the motion of calcium ions and the sulfate group of surfactant are synchronous almost all the time, and if they are

(4) 5422

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427

Langmuir

Article

Figure 7. Mean square displacement (MSD) of the bound divalent counterions and sulfate group of surfactant (sulfur atom represented). Sulfur atom (□); bound Ca2+ (○); and Mg2+ (△). (a and b) SDS systems; (c and d) AE3S systems.

Figure 8. Mean-square displacement of water molecules present in the first hydration shell of the sulfate group (a) SDS and (b) AE3S systems.

surfactant. Though the obvious deviation between −SO4− of AE3S and the magnesium ion happens, the synchronous moving indicates the still-existing interaction. Through the above discussion, definite interaction between the magnesium ion and sulfate can be concluded and the interaction degree corresponds to the foam stability. The too strong an interaction between calcium ions and −SO4− made

divided, they would start binding again quickly in 50 ps; this illustrates the stable association. For magnesium ions, they are slightly departed from the −SO4− groups of SDS but are obviously separated with −SO4− groups of AE3S. This is in high accordance with the K parameter results that the binding tendency is strong for the calcium ion with SDS and AE3S, while it is the weakest for the magnesium ion with the AE3S 5423

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427

Langmuir

Article

the foam unstable by the relatively strong binding tendency to the surfactant, which leads to the surfactant aggregation. Further analysis shown in Figure S2 of the Supporting Information validates the definite interaction between the EO groups and the divalent ions, which means that the contribution of EO groups in AE3S, acting as a flexible connection between hydrophilic −SO4− groups and hydrophobic tails, to the equilibrium composition of foam film could not be neglected. It is critical for AE3S to get the ultrastable foam with the presence of MgCl2. Mechanism of Effect of Molecular Array Behavior on Stability of Foam. Widened tail orientation distribution of AE3S with the presence of Mg2+ (Figure 6b) can enhance the foam film elasticity and viscosity,23 which are important factors in determining the resistance capacity under disturbance. The decreased average headgroup tilt angle (Figure 6c) infers a strong water-holding capacity of the foam film, slow water drainage favors the macrofoam stability too. Foam film has a multilayer structure with the water phase clamped by bilayers of adsorbed surfactants, and stable NBF with nanometers thickness could be formed by draining out the water, for some types of surfactants under suitable salt concentrations.18,20,49,50 Mobility of water molecules in the foam film, corresponding to the drainage process, also has enormous implications for the foam stability. Hydrated water is constrained and hard to be drained out or lost. The mean square displacements of the water molecules around the first hydration shell are shown in Figure 8. The results illustrate that water molecules are more restricted with the presence of divalent ions than those with Na+. As divalent ions interact strongly with charged surfactant head groups, the divalent ions get close to the head groups and even penetrate into their hydration shells, sharing hydrated water molecules with each other (Figure 9). So, the enhancement of the hydration ability of the surfactant is achieved by the stronger hydration force of divalent ions than the surfactant headgroup and its original sodium counterion (Figure S3 of the Supporting Information). Figure 8b shows that the water molecules in foam film stabilized by AE3S coexisted with Mg2+ are restricted most seriously, and the number of restricted water molecules by Mg2+ is the largest, corresponding to the ultrastability of the AE3S−Mg2+ foam. But, it is not the whole story. Various investigations22,51−55 have reported that enormously strong repulsive forces, far stronger than the electric repulsion, existed between the charged or uncharged interfaces separated by small distances. For NBF, that cannot be explained by the DLVO theory, shortrange repulsive force was also found to be the main stabilizing force.56 The short-range force, so-called hydration force, was verified, attributing to the water structure closed to the charge interface different from that in the bulk.22,57 Faraudo and his co-workers22,57,58 observed the direct relation of the strong repulsive short-range force between the surfactant layers to the anomalous dielectric response of water near the charged surfactant layers. Thus, the hydrated water plays a vital role in the stability of the NBF. RDFs of water around the surfactant headgroup (Figure 10) show that water distribution intensity of the first hydration shell for the surfactant headgroup increased with the existence of Ca2+ and Mg2+, which can be explained by cohydration of surfactant hydrophilic group and counterions in Figure 9. The contrast-increased height rule of g(r)water−sulfate with existing

Figure 9. Co-hydration of counterion and surfactant headgroup. The atom coloring scheme is C, gray; H, white; S, yellow; Na+, violet; divalent ion, green; oxygen atom of water in the first hydration shell by the surfactant headgroup, red; water hydrated by the divalent counterion, pink; water shared by the counterion and surfactant first hydration shell, blue.

Mg2+ and Ca2+ of AE3S systems compared to the SDS systems corresponds to the enhanced hydration force between the bilayers of the foam film, which definitely enhances the stability of foam films. It could be concluded that the ultrastable foam was costabilized by AE3S and Mg2+ and benefited a lot from the increased hydration force. However, for systems containing Ca2+, its strong binding tendency with the sulfate group of surfactant made the foam film unstable, both due to the outlet of the hydrated water molecules and the clustering of the surfactant molecules at the air−water interface, which induce the collapse of foam before drainage to the NBF thickness. Foam stabilized by SDS with Mg2+ was unstable for similar reasons. For the AE3S system containing Mg2+, the combining of the EO group of AE3S surfactant to Mg2+, which has a strong hydration ability, increased distribution of hydrated Mg2+ around the surfactant sulfate group (Figure S1 of the Supporting Information) and resulted in the drastic increase of the amount of hydrated water molecules (Figure 10). FT-IR Detection of the Foam Film. IR spectra was used to experimentally detect the microstate of molecules in the foam films, especially the water, as the infrared (IR) absorption of the OH covalent bond is highly active and sensitive to the molecular environment. The IR results for AE3S systems were shown in Figure 11. The absorption band around 1640 cm−1 and the region from 3300 to 3700 cm−1 all correspond to 5424

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427

Langmuir

Article

Figure 10. Radical distribution of water around the sulfate group (the sulfur atom was chosen to represent). (a) SDS and (b) AE3S systems.

Figure 11. FT-IR spectra of foam film formed by the AE3S solution containing (a) 1 M CaCl2 and (b) 1 M MgCl2.

surfactant bulk phase solutions with or without salt (Figure S5 of the Supporting Information). The results verified the existence of the abundant water molecules trapped by Mg2+ and the dipolar pair of −SO4−− Mg2+, which would be very hard to lose. The bonded, isolated water molecules help to stabilize the foam film not only by enhancing the short-range repulsion force, which supports the NBF, but also by restraining the water lost, which is critical for aqueous foam stability. It was obvious that the participation of hydrated Mg2+ in the molecular assembly of the foam film was a key adjustment factor on the formation of the optimum compact interface layer which guaranteed the ultrahigh stability of the foam. The FT-IR results correspond perfectly with the simulation results.

vibration modes of OH (Figure S4 of the Supporting Information). The weak peak around 2350 cm−1 corresponds to the characteristic absorption peaks for CO2. The other peaks belong to IR absorption of the surfactant molecules. As shown in Figure 11, along with the drainage process, the light transmittance decreases and the absorption peak of water (i.e., 3390 cm−1) becomes weak and even tends to disappear after about 10 min. The 3390 cm−1 peak is assigned to the hydrogen-bonded water molecules in the bulk phase. So the amount of water molecules in the foam film decreases along the drainage process, which happened very quickly. The absorption band of the OH stretch shifted from 3390 to 3666 cm−1 after about 10 min for the AE3S foam film with Mg2+ existing; the 3390 cm−1 peak disappeared, while the 3666 cm−1 peak was very notable and sharp. The peak keeps shifting, following the drainage process, and a 3685 cm−1 absorption peak was gradually observed. The 3626 cm−1 peak was very weak for the AE3S foam film with the existence of Ca2+ and remained unshifted after a 10 min water drainage. The high-frequency peaks 3626 cm−1 and 3685 ± 3 cm−1 are due to the inner hydroxyl group of water molecules bonding to the Ca2+ or Mg2+ ions by a complexation interaction, which are isolated and different with the hydrogen-bonded water molecules in the bulk phase.59 There is no such absorption peak in FT-IR spectra of SDS and AE3S foam films without salt (Figure S4 of the Supporting Information) and all the

■

CONCLUSION Ultrastable aqueous foam was obtained by the cooperation of hydrated magnesium ions and AE3S. Microscopic descriptions of foam film were given by molecular dynamic simulation. The favorable arrangements of surfactant molecules, such as wideorientation distribution of surfactant tail and small tilt angle of head groups, ensure the increase of both the elasticity and viscosity of foam film which benefit the foam stability. Interaction of surfactant headgroups with Mg2+ or Ca2+ were investigated. A strong interaction between the surfactant headgroup and cation inorganic ions is adverse for foam stability by binding to the surfactant and making the surfactant 5425

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427

Langmuir

Article

aggregate. The interaction between Mg2+ and headgroup of AE3S has been found to not be very strong but proper, and a dipolar pair formed by the flexible connecting head groups of AE3S and hydrated Mg2+ via intermolecular coactions increased the capacity of foam films for the resolute holding of water molecules for the favor of foam stability. The increasing amount of bonded water molecules helps to stabilize the foam film not only by enhancing the short-range repulsion force, which supports the NBF, but also by restraining the water lost, which is critical for foam stability. So participation of hydrated Mg2+ in the molecular assembly of the foam film was a key adjustment factor on the formation of the optimum compact interface layer, which guaranteed the ultra high stability of the foam. The FT-IR results verified the existing hydrated water molecules trapped by Mg2+ or dipolar pair in the foam film stabilized by AE3S with MgCl2, which agreed very well with the molecular simulation results.

■

(9) Chen, B. D.; Cilliers, J. J.; Davey, R. J.; Garside, J.; Woodburn, E. T. Templated Nucleation in a Dynamic Environment: Crystallization in Foam Lamellae. J. Am. Chem. Soc. 1998, 120, 1625−1626. (10) Mandal, S.; Arumugam, S. K.; Adyanthaya, S. D.; Pasricha, R.; Sastry, M. Use of Aqueous Foams for the Synthesis of Gold Nanoparticles of Variable Morphology. J. Mater. Chem. 2004, 14, 43−47. (11) Shankar, S. S.; Patil, U. S.; Prasad, B. L. V.; Sastry, M. Liquid Foam as a Template for the Synthesis of Iron Oxyhydroxide Nanoparticles. Langmuir 2004, 20, 8853−8857. (12) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Stabilization of Foams with Inorganic Colloidal Particles. Langmuir 2006, 22, 10983−10988. (13) Binks, B. P.; Horozov, T. S. Aqueous Foams Stabilized Solely by Silica Nanoparticles. Angew. Chem. 2005, 117, 3788−3791. (14) Fujii, S.; Ryan, A. J.; Armes, S. P. Long-Range Structural Order, Moiré Patterns, and Iridescence in Latex-Stabilized Foams. J. Am. Chem. Soc. 2006, 128, 7882−7886. (15) Alargova, R. G.; Warhadpande, D. S.; Paunov, V. N.; Velev, O. D. Foam Super Stabilization by Polymer Microrods. Langmuir 2004, 20, 10371−10374. (16) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Macroporous Ceramics From Particle-Stabilized Wet Foams. J. Am. Ceram. Soc. 2007, 90, 16−22. (17) Fameau, A. L.; Saint-Jalmes, A.; Cousin, F.; Houssou, B. H.; Novales, B.; Navailles, L.; Nallet, F.; Gaillard, C.; Boué, F.; Douliez, J. P. Smart Foams: Switching Reversibly between Ultrastable and Unstable Foams. Angew. Chem., Int. Ed. 2011, 50, 8264−8269. (18) Evers, L. J.; Shulepov, S. Y.; Frens, G. Rupture of Thin Liquid Films from Newtonian and Viscoelastic Liquids. Faraday Discuss. 1996, 104, 335−344. (19) Yang, W. H.; Yang, Y. X. Molecular Dynamics Study of the Influence of Calcium Ions on Foam Stability. J. Phys. Chem. B 2010, 114, 10066−10074. (20) Schelero, N.; Hedicke, G.; Linse, P.; Klitzing, R. v. Effects of Counterions and Co-ions on Foam Films Stabilized by Anionic Dodecyl Sulfate. J. Phys. Chem. B 2010, 114, 15523−15529. (21) Zhang, H.; Miller, C. A.; Garrett, P. R.; Raney, K. H. Defoaming Effect of Calcium Soap. J. Colloid Interface Sci. 2004, 279, 539−547. (22) Faraudo, J.; Bresme, F. Anomalous Dielectric Behavior of Water in Ionic Newton Black Films. Phys. Rev. Lett. 2004, 92, 236102. (23) Hu, X.; Li, Y.; He, X.; Li, C.; Li, Z.; Cao, C.; Xin, X.; Somasundaran, P. Structure Behavior Property Relationship Study of Surfactants as Foam Stabilizers Explored by Experimental and Molecular Simulation Approaches. J. Phys. Chem. B 2012, 116, 160− 167. (24) Yan, H.; Guo, X. L.; Yuan, S. L.; Liu, C. B. Molecular Dynamics Study of the Effect of Calcium Ions on the Monolayer of SDC and SDSn Surfactants at the Vapor/Liquid Interface. Langmuir 2011, 27, 5762−5771. (25) Koehler, S. A.; Hilgenfeldt, S.; Stone, H. A. A Generalized View of Foam Drainage: Experiment and Theory. Langmuir 2000, 16, 6327−6341. (26) Bhattacharyya, A.; Monroy, F.; Langevin, D.; Argillier, J. F. Surface Rheology and Foam Stability of Mixed Surfactant-Polyelectrolyte Solutions. Langmuir 2000, 16, 8727−8732. (27) Stoyanov, S. D.; Paunov, V. N.; Basheva, E. S.; Ivanov, B.; Mehreteab, A.; Broze, G. Motion of the Front between Thick and Thin Film: Hydrodynamic Theory and Experiment with Vertical Foam Films. Langmuir 1997, 13, 1400−1407. (28) Golemanov, K.; Denkov, N. D.; Tcholakova, S.; Vethamuthu, M.; Lips, A. Surfactant Mixtures for Control of Bubble Surface Mobility in Foam Studies. Langmuir 2008, 24, 9956−9961. (29) Tan, S. N.; Yang, Y.; Horn, R. G. Thinning of a Vertical FreeDraining Aqueous Film Incorporating Colloidal Particles. Langmuir 2010, 26 (1), 63−73. (30) Koelsch, P.; Motschmann, H. Relating Foam Lamella Stability and Ssurface Dilational Rheology. Langmuir 2005, 21, 6265−6269.

ASSOCIATED CONTENT

S Supporting Information *

Radical distribution functions of the sulfate group and divalent cations; EO group interaction with the divalent cations; hydration of the surfactant counter ions; FT-IR results of the AE3S and SDS with or without salt solutions and pure water; FT-IR results of the foam film stabilized by the AE3S and SDS only. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The funding from National Science Fund of China (Grant 21173134) and the National Municipal Science and Technology Project (Grant 2008ZX05011-002) is gratefully acknowledged.

■

REFERENCES

(1) Paulson, O.; Pugh, R. J. Flotation of Inherently Hydrophobic Particles in Aqueous Solutions of Inorganic Electrolytes. Langmuir 1996, 12 (20), 4808−4813. (2) Binks, B. P. Particles as Surfactants Similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21−41. (3) Farajzadeh, R.; Andrianov, A.; Zitha, P. L. Investigation of Immiscible and Miscible Foam for Enhancing Oil Recovery. Ind. Eng. Chem. Res. 2010, 49 (4), 1910−1919. (4) Philip, W. Aerosol Influences on Marine Atmosphere Surface Layer Optics. Proc. SPIE 1998, 23, 102−107. (5) Seki, N. Baek-Me1ting of a Horizonta1 C1oudy ice Layer with Radiative Heating. Trans. ASME, Ser. C 1979, 10, 90−95. (6) Yang, Y.; Gupta, M. C. Novel Carbon Nanotube−Polystyrene Foam Composites for Electromagnetic Interference Shielding. Nano Lett. 2005, 5 (11), 2131−2134. (7) Ball, G. J.; East, R. A. Shock and Blast Attenuation by Aqueous Foam Barriers: Influences of Barrier Geometry. Shock Waves 1999, 9, 37−47. (8) Ma, G. W.; Ye, Z. Q. Analysis of Foam Claddings for Blast Alleviation. International Journal of Impact Engineering 2007, 34, 60− 70. 5426

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427

Langmuir

Article

(31) Gilanyi, T.; Stergiopulos, C.; Wolfram, E. Equilibrium Surface Tension of Aqueous Surfactant Solutions. Colloid Polym. Sci. 1976, 254, 1018. (32) Li, X.; Karakashev, S. I.; Evans, G. M.; Stevenson, P. Effect of Environmental Humidity on Static Foam Stability. Langmuir 2012, 28, 4060−4068. (33) Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. Effect of Support Pretreatments on Carbon-Supported Iron Particles. J. Phys. Chem. 1987, 91, 6269. (34) Skarmoutsos, I.; Guardia, E. Effect of the Local Hydrogen Bonding Network on the Reorientational and Translational Dynamics in Supercritical Water. J. Chem. Phys. 2010, 132, 074502. (35) Rosenfeld, D. E.; Schmuttenmaer, C. A. Dynamics of the Water Hydrogen Bond Network at Ionic, Nonionic, and Hydrophobic Interfaces in Nanopores and Reverse Micelles. J. Phys. Chem. B 2011, 115, 1021−1031. (36) Stephenson, B. C.; Goldsipe, A.; Beers, K. J.; Blankschtein, D. Quantifying the Hydrophobic Effect. 2. A Computer SimulationMolecular-Thermodynamic Model for the Micellization of Nonionic Surfactants in Aqueous Solution. J. Phys. Chem. B 2007, 111, 1025. (37) Yang, W. H.; Wu, R. L.; Kong, Bin.; Zhang, X. F.; Yang, X. Z. Molecular Dynamics Simulations of Film Rupture in Water/Surfactant Systems. J. Phys. Chem. B 2009, 113, 8332. (38) Sun, H.; Ren, P.; Fried, J. R. The COMPASS Force Field: Parameterization and Validation for Phosphazenes. Comput. Theor. Polym. Sci. 1998, 8, 229. (39) Sun, H. COMPASS: An ab Initio Force-Field Optimized for Condensed-Phase Applications-Overview with Details on Alkane and Benzene Compounds. J. Phys. Chem. B 1998, 102, 7338. (40) Ewald, P. Die Berechnung Optischer und Elektrostatischer Gitterpotentiale. Ann. Phys. 1921, 369, 253−287. (41) Verlet, L. Computer “Experiments” on Classical Fluids. I. Thermodynamical Properties of Lennard-Jones Molecules. Phys. Rev. 1967, 159, 98−103. (42) Nose, S. A Unified Formulation of the Constant Temperature Molecular Dynamics Methods. J. Chem. Phys. 1984, 81, 511−519. (43) Hoover, W. G. Canonical Dynamics: Equilibrium Phase-Space Distributions. Phys. Rev. A 1985, 31, 1695−1697. (44) Dahanayake, M.; Cohen, A. W.; Rosen, M. J. Relationship of Structure to Properties of Surfactants. 13. Surface and Thermodynamic Properties of Some Oxyethylenated Sulfates and Sulfonates. J. Phys. Chem. 1986, 90, 2413−2418. (45) Jang, S. S.; Goddard, W. A., III. Structures and Properties of Newton Black Films Characterized Using Molecular Dynamics Simulations. J. Phys. Chem. B 2006, 110, 7992−8001. (46) Bandyopadhyay, S.; Tarek, M.; Lynch, M. L.; Klein, M. L. Molecular Dynamics Study of the Poly(oxyethylene) Surfactant C12E2 and Water. Langmuir 2000, 16, 942−946. (47) Ghosh, T.; García, A. E.; Garde, S. Molecular Dynamics Simulations of Pressure Effects on Hydrophobic Interactions. J. Am. Chem. Soc. 2001, 123, 10997−11003. (48) Ghosh, T.; García, A. E.; Garde, S. Water-Mediated ThreeParticle Interactions Between Hydrophobic Solutes: Size, Pressure, and Salt Effects. J. Phys. Chem. B 2003, 107, 612−617. (49) Muruganathan, R. M.; Krustev, R.; Müller, H. J.; Möhwald, H. Foam Films Stabilized by Dodecyl Maltoside. 1. Film Thickness and Free Energy of Film Formation. Langmuir 2004, 20, 6352−6358. (50) Muruganathan, R. M.; Krustev, R.; Müller, H. J.; Möhwald, H. Foam Films Stabilized with Dodecyl Maltoside. 2. Film Stability and Gas Permeability. Langmuir 2006, 22, 7981−7985. (51) Israelachvili, J.; Wennerström, H. Role of Hydration and Water Structure in Biological and Colloidal Interactions. Nature 1996, 379, 219. (52) Deméa, B.; Zemb, T. Hydration Forces between Bilayers in the Presence of Dissolved or Surface-Linked Sugars. Curr. Opin. Colloid Interface Sci. 2011, 16, 584−591. (53) Petsev, D. N.; Vekilov, P. G. Evidence for Non-DLVO Hydration Interactions in Solutions of the Protein Apoferritin. Phys. Rev. Lett. 2000, 84, 1339−42.

(54) Valle-Delgado, J. J.; Molina-Bolívar, J. A.; Galisteo-González, F.; Gálvez-Ruiz, M. J.; Feiler, A.; Rutland, M. W. Existence of Hydration Forces in the Interaction between Apoferritin Molecules Adsorbed on Silica Surfaces. Langmuir 2005, 21, 9544−54. (55) Marčelja, S. Hydration Forces near Charged Interfaces in Terms of Effective Ion Potentials. Curr. Opin. Colloid Interface Sci. 2011, 16, 579−583. (56) Bresme, F.; Faraudo, J. Computer Simulation Studies of Newton Black Films. Langmuir 2004, 20, 5127−5137. (57) Stanley, C.; Rau, D. C. Evidence for Water Structuring Forces between Surfaces. Curr. Opin. Colloid Interface Sci. 2011, 16, 551−556. (58) Jordi, F. Origin of the Short-Range, Strong Repulsive Force between Ionic Surfactant Layers. Phys. Rev. Lett. 2005, 94, 077802. (59) Sufriadin; Idrus, A.; Pramumijoyo, S.; Warmada, I. W. Thermal and Infrared Studies of Garnierite from the Soroako Nickeliferous Laterite Deposit, Sulawesi, Indonesia. Indonesian Journal of Geology 2012, 7 (2), 77−85.

5427

dx.doi.org/10.1021/la4011373 | Langmuir 2013, 29, 5418−5427