Interfacial study on the interaction between hydrophobic nanoparticles and ionic surfactants

Colloids and Surfaces A: Physicochem. Eng. Aspects 488 (2016) 20–27

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Interfacial study on the interaction between hydrophobic nanoparticles and ionic surfactants Lei Jiang a,∗ , Songyan Li b , Wenyang Yu a , Jiqian Wang a , Qian Sun b , Zhaomin Li b,∗∗ a b

Heavy Oil State Laboratory and Center for Bioengineering and Biotechnology, China University of Petroleum (East China), Qingdao 266580, China College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266580, China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Interactions between hydrophobic nanoparticles and ionic surfacants including ionic liquid are studied. • The mechanism of nanoparticle affecting air–water surface properties is proposed. • The findings may shed light on further use of hydrophobic nanoparticles in industry.

a r t i c l e

i n f o

Article history: Received 7 August 2015 Received in revised form 6 October 2015 Accepted 7 October 2015 Available online 22 October 2015 Keywords: Hydrophobic nanoparticles Ionic surfactants Surface viscoelasticity Premicelles Dynamic surface tension Ionic liquid

a b s t r a c t The use of hydrophobic nanoparticles to manipulate the foam stability has attracted great interests recently. In this study the interactive behavior is investigated between the hydrophobic nanoparticles with the ionic surfactants including ordinarily charged and ionic liquid ones. The concentration of nanoparticles is varied and the surfactant concentration is fixed at below the critical micelle concentration, where the air–water surface property is sensitive to the state of surfactants in the bulk solution. The static and dynamic surface tension, zeta potential and dilational viscoelasticity of the mixed solution are measured at various conditions. It is observed that both surface tension and magnitude of zeta potential increase with nanoparticle addition, suggesting that the nanoparticles are attracting surfactant monomers in competition with the air–water surface and the premicelles in the bulk solution. The dynamic changes of surface tension and dilational viscoelasticity are monitored with changes of nanoparticle concentration. The possible assembly of surfactants with nanoparticles and the consequential effect are discussed with a proposed mechanism. Understanding the interaction and synergy between hydrophobic nanoparticles with ionic surfactants, in particular ionic liquid surfactant, may shed light on the use of such nanoparticles in the industrial practices such as enhanced oil recoveries. © 2015 Elsevier B.V. All rights reserved.

1. Introduction ∗ Corresponding author. ∗∗ Corresponding author. E-mail addresses: [email protected] (L. Jiang), [email protected] (Z. Li). http://dx.doi.org/10.1016/j.colsurfa.2015.10.007 0927-7757/© 2015 Elsevier B.V. All rights reserved.

Nanoparticles have long been used as the foam thickener and stabilizer in foods, cosmetics and enhanced oil recoveries [1–4]. The behavior of nanoparticles at the air–water interface is known

L. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 488 (2016) 20–27

to depend on their hydrophobicity and may behave similarly to the surfactant molecules [5,6]. Dickinson et al. found that the stability of foams can be significantly improved in the absence of surfactants by using only partly-hydrophobic nanoparticles [7]. Stocco et al. observed that hydrophobic nanoparticles can be used to increase the lifetime of foams up to several days without any surface active molecules [8]. The stabilizing effect of such nanoparticles was reported to be related to their degree of hydrophobicity which was characterized by contact angles [9]. Furthermore Hydrophobic nanoparticles can affect the deformations of bubbles from shear and compression stresses, which directly influences the rheology and stability of surfactant-containing foams and emulsions [10–12]. This may be due to the formation of a rigid nanoparticle skin on the surface of the bubble or droplet. The presence of such shell can effectively prevent the droplet and bubble from coalescence and coarsening in the fluid dispersion [8,13]. However in the industrial practices, the hydrophobic nanoparticles or particulates are rarely used alone in the foam or emulsion systems [14,15]. Solid particles have been incorporated in the surfactant-stabilized foam for many years, for example during the flotation of coal processing and waste water treatment [16]. In processed food or cosmetics products, the ingredient formulas often contain surfactants or surface active molecules such as sodium caseinate, whey protein and charged polysaccharide [14,15,17]. In the tertiary or enhance oil recovery techniques the inject complex fluids usually consist of steam, surfactant, foam etc. [1–3,18]. In comparison with the numerous literature on research related to the systems containing hydrophilic nanoparticles and surfactants [19–21], there are only a few reports on the interactions and influence of hydrophobic nanoparticles on the surfactants, let alone some uncommon types such as ionic liquid [22,23]. Hunter et al. [22] have studied the interactions between the non-ionic surfactant TX100 with the hydrophobic silica nanoparticles. They proposed that the surfactants may augment on the particle surface to form a Langmuir monolayer with their head-groups facing to the solution. An increment of 20% foam stability was observed with increasing surfactant concentration and fixed nanoparticle concentration. This is probably the result of the enhanced interfacial elasticity. Sun et al. [23] investigated on the interaction between the hydrophobic nanoparticles with sodium dodecylsulfate (SDS) and tested their combined effect on stabilizing the foam injection in the micromodel and sandpack experiments. It was found that the hydrophobic nanoparticles are effective additives to help flush oil from the pores and they can stabilize foams in the in-field practice [23]. Zargartalebi et al. [24] have found that the surfactant adsorption on the rock surface was reduced by nanoparticle addition, including hydrophilic and hydrophobic nanoparticles. These studies suggest that the hydrophobic nanoparticles could be potentially advantageous in a wide industrial applications because of the environment-friendly and temperature-stable characteristics. However little is known about how they interplay, their arrangements at the air–water surface or their dependence on the nanoparticle concentration. Obtaining these knowledge could underpin the wide utilization and function of the hydrophobic nanoparticles in the future. Generally speaking, the interaction of nanoparticles with ionic surfactants including ionic liquid is more complex than those with the non-ionic surfactants because more types of forces are involved including double layer repulsion, counterion condensation effect, etc. [25]. In practice such as in the petroleum industry, the employed surfactants are often ionic surfactants [26]. Recently the potential to use ionic liquid types of surfactant has attracted strong interests from users because that they can overcome the high temperature decomposition [27–29] and demonstrate good resistance to oxidation [30]. Further investigation has been reported on the desirable properties of ionic liquids in the chemical flooding of

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enhanced oil recovery practices [30–32]. It suggests that ionic liquid surfactant may be a promising replacement of conventional ionic surfactants in extreme conditions such as underground oil drilling. However, to the best of our knowledge, there is no study on ionic liquid’s interaction with hydrophobic nanoparticles and the understanding of interaction mechanism is limited. In this study three types of ionic surfactants, with similar level of critical micelle concentration (cmc), are mixed with hydrophobic nanoparticles respectively. The first two are common surfactants, the cationic hexadecyltrimethylammonium bromide (CTAB) and the anionic sodium dodecylbenzenesulfonate (SDBS), and the third one is an ionic liquid, hexadecylpyridinium chloride (CPC). A surfactant concentration below cmc is used where the air–water surface is very sensitive to the state of surfactants [33]. The dynamic changes of surface tension, tension in equilibrium, zeta potential, and interfacial viscoelasticity are all investigated. The results are analyzed to reveal how the ionic surfactant arrangement adapts around the hydrophobic nanoparticles and the influence of surfactant charges on the arrangement of monomer at the air–water surface. Understanding their interaction mechanism may enable us to correlate the surface activity of the hydrophobic nanoparticles with their performance in stabilizing foam and emulsion which often occurs in the industrial processes. 2. Material and methods All the chemicals including CTAB, SDBS, CPC were supplied by Sigma (USA) with purity of >99.8 wt%. The hydrophobic silica nanoparticles (H18, HDK grade Wacker-Chemie, Germany) as previously used [23] are ∼12 nm in diameter with a specific surface area of ∼200 m2 /g. The use and characterization of the same hydrophobic nanoparticles have been previously reported [23,34]. With a contact angle of 132◦ (data not shown) and 80% of surface hydrophobized, the density of remained silanol on surface is <0.5 per nm2 [34]. All the experiments were conducted at 25 ◦ C. All the glassware were cleaned in piranha solution (H2 O2 :H2 SO4 = 3:7) and rinsed thoroughly before the measurements. 2.1. Preparation of the surfactant/nanoparticle mixed solution In order to fully mix the hydrophobic nanoparticles in the surfactant solution, the mixing method from Cervantes Martinez et al. [35] are employed in this study. Briefly, the nanoparticle powders were wetted using 99% ethanol and then diluted with Milli-Q water (18.2 M cm, TOCs ≤ 4 ppb). After the addition of surfactants, the solution was turbo-mixed (homogenizer, CT18, IKA, Germany) for 30 min, stirred for over 24 h and sonicated for 30 mins before each measurement to break any aggregation. The ethanol was then removed by several centrifugation-washing cycles. 2.2. Characterization of the surfactant/nanoparticle mixed solution The air–water surface tension was measured by the Wilhelmy plate method using a surface tensiometer (K20 easydyne, KRUSS GMBH, Germany). The particle distribution and zeta potential were measured using the dynamic light scattering technique (Nano ZS90, Malvern instruments Ltd., UK). The dynamic changes of surface tension and interfacial viscoelasticity were measured using an interfacial rheometer (TRACKER, TECLIS, France). The principle for measuring the interfacial viscoelasticity has been described in details previously [36]. A 6 ␮l volume bubble was blown into the solution and the surface tension was recorded from the moment when the blowing was complete until the surface reached a stable state. The bubble was then forced to expand and contract in

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surface tension (mN/m)

70

60

50

CTAB SDBS CPC

40

30

0.9mM

1E-4

1E-3

0.01

0.1

1.2mM

1

10

concentration (mM) Fig. 1. The surface tension changes with surfactant concentration for the solution of cationic CTAB (black), anionic SDBS (red) and cationic ionic liquid CPC (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

a sinusoidal periodic oscillation at 1 Hz frequency and 1 ␮l amplitude change. During the oscillation the images of the bubble were captured every 0.08 s and the contour of each image was post processed to calculate the corresponding surface modulus E as Eq. (1) where A and  are the surface area and surface tension of the bubble [37]. E=

d d (lnA)

(1)

3. Results and discussion 3.1. Characterization of the surfactant solution The cmcs of CTAB and SDBS surfactant samples are first determined from the surface tension measurement in order to ensure the samples free of contaminations. Fig. 1 presents the surface tension

change with the surfactant concentration in the pure surfactant solution. From the curve the cmcs of all surfactant are determined to be cmcCTAB = 0.9 mM, cmcSDBS = 1.2 mM, and cmcCPC = 0.9 mM, well in agreement with the values reported in the previous literatures [38–43]. It’s widely known that at low concentration, surfactants tend to form a monolayer at the air–water surface to decrease the surface tension. The higher the surfactant concentration is, the higher number of surfactant molecules in the monolayer is until the surface reaches the saturation and the micellization occurs in the solution [33]. Fig. 2 shows the surface viscoelasticity modulus E of CTAB, SDBS and CPC solution from 0.001 mM up to 10 mM (∼10 × cmc). The bubble, shown in Fig. 2(b) inset, was expanded and contracted at constant frequency 1HZ. The state of the monolayer affects the surface viscoelasticity and hence surface modulus which is an indicator of the layer’s strength against bubble deformation [38,44]. The result shows that the surface modulus E reaches the maximum at cp ∼0.5 mM, meaning that at 0.5 mM the air–water interface presents the strongest response to the surface deformation. At this point the air–water surface is likely to have a surface coverage of surfactant monolayer that is very sensitive to the system change [38,44]. Any condition change in solution, for example the afterward addition of hydrophobic nanoparticles, may trigger the change of surfactant state at surfaces and hence E changes. Therefore in this study surfactant concentration at cp = 0.5 mM is used to investigate the changes of surface properties at various levels of hydrophobic nanoparticles. In the mean time, the zeta potential for 0.5 mM CTAB, SDBS, and CPC is measured to be 20, −30 and 15 mV respectively. The zeta potential usually refers to the charges of all particles in solutions. For a solution with surfactant only, these charges are possibly from the premicelles in the solution, which have been reported to appear at 40–67% of cmc in the surfactant solution [45–48]. In this case 0.5 mM is about 56% of cmcCTAB , cmcCPC and 42% of cmcSDBS . The premicelles have been reported to be in similar shapes and structures with the ordinary micelles, except that they have fewer monomers.

Fig. 2. The surface modulus E at air–water surface changes with the concentration of ionic surfactant (a) CTAB, (b) SDBS, and (c) CPC. The inset in (b) is the image of a suspended bubble that was monitored to calculate the surface modulus.

L. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 488 (2016) 20–27

(b)

35%

25%

CTAB

30% 25%

number percentage

number percentage

(a)

0.001% 0.01% 0.1% 0.5% 1%

20% 15% 10% 5%

SDBS

20%

0.001% 0.01% 0.1% 0.5% 1%

15% 10% 5% 0%

0%

10

23

100

1000

10

10000

100

1000

10000

size (nm)

size (nm)

numble percentage

(c) 50% CPC

40%

0.001% 0.01% 0.1% 0.5% 1%

30% 20% 10% 0% 10

100

size (nm)

1000

10000

Fig. 3. The particle distribution of the surfactant/nanoparticle mixed solution at various nanoparticle concentrations. The surfactant concentration is fixed at 0.5 mM.

CTAB

20

CPC 0

zeta potential (mV)

30

-30

SDBS

-40 1E-3

0.01

0.1

1

nanoparticle concentration (%) Fig. 4. The change of zeta potential in the mixed surfactant/nanoparticle solution, containing 0.5 mM CTAB, SDBS, and CPC at various nanoparticle concentrations.

Each premicelle has a hydrophobic core from the monomer tails and a hydrophilic surface from the charged monomer heads, which underpins the zeta potential here [45–48]. 3.2. Static surface properties of the surfactant/nanoparticle solution After the characterization above, the nanoparticles are added to the surfactant solution and the particle distribution after thorough mixing is measured. Fig. 3 shows that most nanoparticles distribute as particles of size 60–100 nm. The distribution is relatively stable from 0.001% up to 1% nanoparticle concentration and no large aggregates are observed. But beyond 1%, the particle aggregation starts appearing (data not shown), therefore in this study, the nanoparticle concentration investigated is no more than 1%. The changes of zeta potential and surface tension with nanoparticle concentration are also measured. Fig. 4 shows that the magnitude of zeta potential keeps increasing when more nanoparticles are present in solution. With 1% nanoparticles, the zeta potential of CPC/nanoparticles solution has nearly doubled while for CTAB and SDBS the increase is 40–50%. The zeta potential data indicates the surface charges of all charged particles in solu-

tion, regardless of particle types. Its increase may result either from the total number increase of charged particles or from the charge increase per particle. For the 80% hydrophobized silica nanoparticles used in this study, the hydrophilic surface area is low and the corresponding charges are negligible in the aqueous solution. Hence it is unlikely that such charges could contribute to the number increase of the charged particles. Instead larger amount of nanoparticles may help to increase the charges per particle: before the nanoparticle mixing, the charges and measured zeta potential are mainly from the premicelles; after the mixing, the surfactants from premicelles can dissociate and relocate on nanoparticle surface, creating new surfactant-coated particle (SCP). The hydrophobic nanoparticle can intrinsically provide a larger and more hydrophobic core than the premicelles for the amphiphilic surfactants, hence induce higher adsorption. Each SCP, 60–100 nm size, may accommodate more monomers than a premicelle sterically. Therefore in the zeta potential measurement, the detected objects have in deed been gradually shifted from the charged premicelles to the more highly charged SCPs, resulting in the increase of zeta potential. The addition of hydrophobic nanoparticles may affect not only the surfactants in bulk solution, but also the arrangement of monomers at the air–water surface. The monomer arrangement will in turn directly impact on the equilibrium surface tension. Fig. 5 shows that, for all three types of surfactants, the surface tension of mixed solution increases with nanoparticle concentration. The surface tension is usually associated with the quantity of surface active molecules at the air–water surface and its increase usually implies the loss of such molecules at the interface [25]. Therefore the results suggest that when nanoparticles are added, surfactant monomers at the surface could be deprived from the interface and possibly participate in the adsorption to the nanoparticles to form SCPs. However, even with nanoparticle concentration as high as 1%, the surface tension never returns to 72 mV which is the surface tension for the surfactant-free solution. It implies that not all the monomers have been deprived from the air–water surface and there is always some surface active molecules left there resulting in the lower surface tension.

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surface tension γ (mN/m)

60

alkyl chain so that its adsorption to the air–water surface as well as the equilibration is more slowly [50]. In the pure SDBS solution in Fig. 7(b), the surface tension takes ∼25 s to reach the equilibrium ( ≤ ± 0.2 mN/m over 20 s), while only 0.001% nanoparticles can increase the stabilization time to over 200 s. When the nanoparticle concentration increases, the time that’s required to reach the equilibrium keeps increasing. As Langevin et al. [51] reported for a diffusion controlled adsorption, the surface tension variation  may be closely related with the diffusion time t as in Eqs. (2) and (3)

SDBS CTAB CPC

55

50

45

1E-4

1E-3

0.01

0.1

nanoparticle concentration (%)

1

Fig. 5. The change of surface tension in the mixed solution, containing 0.5 mM CTAB, SDBS and CPC with various nanoparticle concentrations.

3.3. Dynamic surface properties of surfactant/nanoparticle solution

surface modulus E (mN/m)

Having measured the static surface tension of surfactant/nanoparticle system at the equilibrium state, the dilational viscoelasticity and dynamic surface tension as a function of nanoparticle concentration are studied with the bubble method. Fig. 6 shows that the surface modulus E keeps increasing with nanoparticle concentration, for all three types of surfactants including ionic liquid. It means that the strength of the air–water surface is getting stronger. This may be due to the increased number of nanoparticles at the surface which serves as an armed sheath to resist the deformation [49]. It is also noted that at 0.5% nanoparticle concentration, the surface modulus E of CTAB, SDBS and CPC is in the range of 40–60 mN/m, much higher than the E value (max 20 mN/m [22]) of nonionic surfactant TX 100 at similar conditions. It means that ionic surfactant is more effective in enhancing surface strength than nonionic surfactant when they are mixed with hydrophobic nanoparticles. This is probably because that, as the main component of surface sheath, the SCPs from ionic surfactants are more evenly dispersed or ordered due to the compacted charges from monomers. A more ordered SCP arrangement at the surface can improve the sheath’s overall stability and therefore higher surface strength. Fig. 7 presents the dynamic change of surface tension when a bubble is blown in the mixed surfactant/nanoparticle solution. Over a period of time, the surface tension  gradually decays to the equilibrium value. It appears that the time it takes for the air–water surface to stabilize varies with the nanoparticle concentration. Such difference is more distinct with SDBS than CTAB and CPC, possibly because that SDBS has a shorter and hence less hydrophobic

70 60 50 40 30

CTAB SDBS CPC

20 10

1E-4

1E-3

0.01

0.1

1

nanoparticle concentration (%)

Fig. 6. The influence of nanoparticle concentration on surface modulus E of the mixed solution, containing 0.5 mM CTAB, SDBS and CPC with various nanoparticle concentrations.

 = t − ∞

k=

RT c

2



RT = c

2



7 −1/2 = kt 12Dt

7 12Dt

(2)

(3)

where  ∞ is the surface tension at equilibrium, c is the concentration of surfactant, R is the molar gas constant, T is the temperature, D is the diffusion coefficient and  is the surface coverage. Using  ∼ t data in Fig. 7, the relationship between  and t−1/2 can be plotted as in Fig. 8. It shows that the relationship between  and t−1/2 always appear to be linear. From the linear fit of  vs t−1/2 (solid lines), the slope k = /t−1/2 can be derived and plotted against the corresponding nanoparticle concentration, illustrated as insets in Fig. 8. At constant temperature T and surfactant concentration c, it can be seen from Eqs. (2) and (3) that the variable k is determined mainly by the ratio of  2 / D1/2 . Fig. 8 insets show that for all three types of ionic surfactants, k tends to increase then decrease with nanoparticle concentration. This may possibly be linked with the variation in surface coverage  and diffusion coefficient D that will be discussed later. 3.4. Possible mechanism on molecular arrangement at air–water surface The results above have shown the influence of hydrophobic nanoparticles on the static and dynamic surface properties of the ionic surfactant solution. The changes of surface properties with nanoparticle concentration are measured and analyzed. It is noted that a little differences among the three types of surfactants are observed. For example, SCPs formed from SDBS are of opposite charges to those from CTAB and CPC, because the coating monomers are oppositely charged; SDBS requires longer time than CTAB and CPC to reach equilibrium surface tension, probably because of its much shorter hydrophobic tail and hence less entropical drive toward the air–water surface [50]; CPC has the same length of alkyl line and cmc as CTAB, but with different charged heads. CPC premicelle is likely to have fewer monomers than CTAB one because the zeta potential is 15 and 20 mV for CPC and CTAB respectively; but once the monomers have all relocated on SCP surfaces at high nanoparticle concentration, the difference between CPC and CTAB SCPs is negligible as observed in Fig. 4; The surface modulus E of CPC mixed solution is always slightly higher than CTAB solution at all nanoparticle concentrations. This implies that the ionic liquid has a stronger surface strength at the air–water surface than the alkyltrimethylammonium salt, possibly because that it has higher ability to self-organize [52]. Despite the differences above, the overall trends of surface parameter changes are similar for these three types of ionic surfactants in Figs. 1–8. Even for ionic liquid, neutron scattering data has previously indicated that its physicochemical properties are dominated by the nature of the surfactant anion [53]. Therefore we have summarized the data and proposed a mechanism underlying the effect of nanoparticles on the general arrangement of ionic sur-

L. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 488 (2016) 20–27

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Fig. 7. The dynamic changes of surface tension for the mixed solution containing 0.5 mM CTAB, SDBS, and CPC with various nanoparticle concentrations.

factants at the air–water surface. The mechanism is schematically illustrated in Fig. 9. As discussed in section 3.1, for 0.5 mM CTAB, SDBS and CPC solution, it’s likely that there are already some premicelles that account for the detected charges in zeta potential measurements and the relatively high surface modulus in dilational viscoelasticity measurements [45–48]. Fig. 9(a) demonstrates that before the addition of nanoparticles, some monomers form a monolayer at

the air–water surface and others exist as premicelles in solution. When a little quantity of nanoparticles are added to the solution (concentration ≤ 0.01%), as in Fig. 9(b), the monomers in the premicelles start to dissociate and relocate to the nanoparticles surface to form SCPs. The monomers adsorb on the surface of SCPs with hydrophobic tails inward and hydrophilic heads outward, increasing SCPs’ overall hydrophilicity and dispersity in the solution. At low nanoparticle concentration, there may be enough monomers

Fig. 8. The changes of surface tension variation  with t−1/2 in the mixed nanoparticle solution with 0.5 mM (a) CTAB, (b) SDBS, and (c) CPC. The solid lines are the linear fit to the  vs t−1/2 data with the derived slope k. The insets illustrate the change of derived k with the nanoparticle concentration.

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L. Jiang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 488 (2016) 20–27

Fig. 9. The schematic description of mechanism underlying the arrangement of ionic surfactants and hydrophobic nanoparticles at the air–water surface. Graph (a–e) represent the scenarios with increasing nanoparticle concentration from none (a), to low (b), medium (c), high (d) and extra high (e).

from the premicelle reservoir to coat SCPs. Hence at this stage most of the surfactants are still on the air–water surface, the monolayer remains intact and the surface coverage  change is little. Meanwhile, the diffusion coefficient D for the Brownian motion in the dilute dispersion is decreased, partly due to the replacement of premicelles by the larger and slower SCPs, partly due to the increased crowdedness. Thus from Eq. (3) the reduced D results in the increase of parameter k. This may be the reason for k rise at nanoparticle concentration ≤ 0.01% in the insets of Fig. 8. When the nanoparticle concentration exceeds 0.01%, the condition may become more complicated. It’s possible that at this stage SCPs have occupied all the surfactant monomers from premicelles and they start to recruit monomers from the air–water surface. As in Fig. 9(c), when nanoparticle concentration increases, the surfactants originally at the air–water surface are gradually removed and shifted to adsorb on SCPs, resulting in the decrease of both surface coverage  and diffusion coefficient D. As in Eq. (3), the impact of surface coverage  is relatively larger than diffusion coefficient D on k change because  has an exponential factor of 2 and for D the factor is 0.5. The exact quantitative changes of  and D are not known, but it’s possible that the effect of  exceeds that of D, resulting in the total reduction of k at the nanoparticle concentration over 0.01% in the insets of Fig. 8. The mechanism may also be used to explain the surface tension increase in Fig. 5. With the total number of monomers unchanged, the addition of hydrophobic nanoparticles triggers the competition between the nanoparticles and the air–water surface for monomers. With 80% surface hydrophobized, the nanoparticles are likely to have a stronger attraction for the monomers. The monomers move from the air–water surface to adsorb on the nanoparticle or SCP surface. The loss of monomers results in the increase of surface tension ␥, as in Fig. 5. However it is noted that ␥ can only reach up to ∼60 mN/m and it never recover back to 72 mN/m, as previously reported for the mixed solution with CTAB and hydrophilic silica nanoparticles [54]. It is likely that even if at high nanoparticle concentration, there are still some surfactants left at the air–water surface. As illustrated in Fig. 9(d) and (e), it is possible that at high nanoparticle concentration (concentration ≥ 0.1%), some SCPs start moving to the air–water surface, exposing parts of the bare area to the air and parts of the surfactantcoated hydrophilic boundary to the solution. This could release some monomers back to the air–water surface, hence reducing the surface tension. Such assembly in Fig 9(d) may also be viewed as the islands of SCPs surrounded by surfactant spacers, as Wolert et al. reported for latex and polymer dispersant [55]. Fig. 9(d) and (e) illustrate the scenarios when more islands get closer and they tend to aggregate which is entropically favorable. The aggregation could eventually form a network and the network serves as an ‘armed

sheath’ to resist any surface deformation. Consequently it results in the increase of surface modulus E as in Fig. 6. Generally speaking, the hydrophobic nanoparticles have dual effect on the surface properties in the ionic surfactant solution at concentration below cmc: on the one side they deplete the monomers, thus reducing the surface tension; on the other side they gradually aggregate and move to the air–water surface to form a protective network, thus increasing the surface strength. The nanoparticles act synergistically with the surfactants to form SCPs and their interaction adapts accordingly with the nanoparticle concentration. Similar adsorption has been reported for the non-ionic surfactant on other hydrophobic nanoparticles although no significant reduction in surface tension was observed [22]. With increasing concentration, the SCPs progressively move from solution to the surface surrounded by the charged surfactants. At high concentration they may create an interconnected structure with a strong steric integrity across the air–water surface. This structure may be effective in producing stable foams and emulsions and thus shed light on the improvement of foam stabilization using new surfactant materials such as ionic liquid. Furthermore the performance of ionic liquid CPC is similar to the conventional counterpart CTAB, implying that it could be an effective replacement of classical surfactants. The mechanism and knowledge may be potentially useful for the control of surfactant dispersion which is crucial for the foam performance.

4. Conclusions The behavior of hydrophobic nanoparticles are becoming more important in the industrial application, often together with the presence of ionic surfactants. In this study three types of ionic surfactants are employed including two common ionic surfactants and one ionic liquid with similar cmcs. The static and dynamic surface tension, the zeta potential and the dilational viscoelasticity of the surfactant/nanoparticle mixed solution are measured in order to understand the interplay between the ionic surfactants and nanoparticles. The results show that both surface tension and zeta potential increase with nanoparticle concentration, probably due to the depletion of surfactant monomers from the air–water surface. The surface modulus which defines the dilational viscoelasticity and surface strength also rises with the nanoparticle increase. It seem reasonable that the coated SCPs move toward the surface to form a connected network and shell against the surface deformation. A mechanism on how the ionic surfactants interact with the nanoparticles spanning a wide range of nanoparticle concentration is proposed to explain for their synergistic effect on the surface parameters. This knowledge will be beneficial for the use

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