Highly efficient oil-fouling and foam removal achieved by surfactant mixed systems

Abstract

Excessive usage of surfactants in daily life and industry and their undesirable high foamability have caused serious environmental pollution and economic loss. Improving cleaning efficiency and reducing foam stability concurrently is a delicate strategy but a challenging task. Herein, we mixed the most widely used surfactant sodium dodecyl sulfate (SDS) with cyclic amines (C_nN, n = 6, 8, 12), by which the self-assembly ability of SDS at the air/water interface and in bulk is significantly enhanced, while spherical micelles, vesicles and wormlike micelles are formed at appropriate total surfactant concentration (C_T) and molar fraction of SDS (X_SDS). Especially around X_SDS = 0.50 and above critical micellar concentration (CMC), the stronger self-assembly ability leads to a higher contact angle of machine oil on stainless-steel plates and lower oil–water interfacial tension in C_nN/SDS solution, thus the oil-fouling removal efficiency of C_nN/SDS solutions is remarkably improved. Meanwhile, the foamability and foam stability dramatically decline at smaller X_SDS and slightly above CMC, attributed to the rapid molecular migration from liquid film of foams to the bulk between the films when the limited surfactant molecules in the films prefer to aggregate in bulk. As a result, C₈N/SDS exhibits the best oil cleaning and lowest foaming simultaneously at low X_SDS and just above the CMC. This study opens an efficient avenue to eliminate the contradiction between cleaning ability and foamability, thereby obtaining a high-efficiency and low-foam detergent.

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Introduction

Amphiphilicity of surfactants endows them with unique surface activity and self-assembly ability in aqueous solution, thus they are widely applied in many fields, such as consumer household cleaning products,^1–4 industrial cleaning,^5–7 food industry,^8,9 agriculture,^10–14 drug delivery,^15–18etc. Particularly as detergents, the global surfactant market turnover reached USD 42.1 billion in 2020 according to the data from Markets and Markets™.¹⁹ Moreover, the global cleaning products industry is projected to reach USD 61.6 billion by 2026.²⁰ Excessive consumption of surfactants causes environment chemical exposure and huge economic loss. With regard to this issue, many researchers have devoted their efforts in developing a wide range of novel surfactants by introducing more functional groups,^21–24 increasing the oligomerization degree,^25–28 and adjusting the length of the spacer or alkyl chain^29–31 so as to optimize the cleaning performance of surfactants and reduce their dosage by altering the static/dynamic surface tension,^32–35 adsorption kinetics,^36–39 aggregation ability,^40–42etc. In parallel, mixing surfactants with additives (inorganic/organic salts, other surfactants, polymers, etc.) is also an effective way to adjust the physicochemical properties of surfactants at the surface/interface and in solution,^43–49 while a large number of commercial additives offer much more optionality, convenience and practicability. Hence, introducing appropriate additives into commonly used surfactants is undoubtedly an economic and convenient approach to obtain highly efficient surfactant systems and achieve sustainable development.

The generation of liquid foams is at the heart of numerous natural, technical, or scientific processes related to surfactants. In many cases, they have very useful properties for practical applications, such as cleaning, foam flotation, food processing and firefighting. However, foams are not always desirable. The unwanted foams can obstruct gas transport and render the process of interest ineffective with significant cost implications. Kister et al.^50,51 made a survey on causes of malfunctions in industry, and they found that the existence of foam in surfactant solutions in distillation processes was a major reason for 900 investigated cases of column malfunctions. The accumulation of impurities that stabilize the foam on the sea also produces foams and destroys the normal ecological environment.^52–54 Coincidently, those aforementioned physicochemical properties facilitating the cleaning efficiency, such as low surface tension, strong aggregation ability and fast adsorption kinetics, often result in the strong foamability and foam stability in surfactant solution.^55–58 Therefore, the development of surfactant systems with high cleaning efficiency but adjustable foaming ability in different conditions encounters great technological hurdles although it is urgently desired in multiple fields.

The length and topological structure of alkyl chains play important roles in physicochemical properties of surfactants at surface/interface and in solution. Cyclic amines with bulky alkyl chains may form a larger hydrophobic domain in bulk but create looser arrangement at the air/water interface.^59,60 Their special structure makes it greatly possible for cyclic amine-containing systems to achieve high cleaning efficiency and low foam concurrently. Bearing in mind the structural advantages, we designed mixed systems composed of cyclic amines with various sizes of the cycloalkane ring (C_nN, n = 6, 8, 12) and widely used sodium dodecyl sulfate (SDS) at pH = 7.0. We found that the mixed solutions (C_nN/SDS) show lower CMC and surface tension and form various assemblies at different molar fractions of SDS (X_SDS) and total concentrations (C_T), driven by the electrostatic and hydrophobic interactions between SDS and cyclic amines. With the balance of the self-assembly of surfactants at the liquid film of foam and in bulk between the films, the oil cleaning efficiency of C₈N/SDS is much higher than that of the other two systems, whereas its foamability and foam stability are remarkably weakened. This work offers a feasible method for effectively removing oil with low foamability and foam stability at low surfactant concentrations, thereby potentially reducing surfactant usage and promoting sustainable development of environments.

Experimental section

Materials

Cyclohexylamine (C₆N, >99.5%) was purchased from Energy Chemical, cyclooctylamine (C₈N, >97%) was purchased from Acros Organics, cyclododecylamine (C₁₂N, >99%) was purchased from Leyan Chemical, sodium dodecyl sulfate (SDS, >99%) was purchased from Sigma-Aldrich, and hydrochloric acid (HCl, 36%) was purchased from Beijing Chemical Works. 20^# machine oil was purchased from Kunlun Lubricant Company. Deionized water (18.2 MΩ cm) from Milli-Q equipment was used in all the experiments.

Surface tension measurement

The surface tension measurements of the C_nN/SDS (n = 6, 8, 12) mixed systems at different molar fractions of SDS (X_SDS) were carried out with a Wilhelmy plate method on the DCAT21 tensiometer (DataPhysics Co., Germany). The length and width of the plate were 19.90 and 0.20 mm, respectively. The standard error of surface tension is ∼0.03 mN m⁻¹, and the test temperature was 25.00 ± 0.01 °C controlled by using a thermostat. Each measure was tested at least three times.

Turbidity measurement

The turbidity values of the C_nN/SDS (n = 6, 8, 12) mixtures at different C_T and X_SDS values, reported as 100 − % T, were measured at 450 nm using a Shimadzu UV-vis spectrophotometer (model UV-2800) with a water-circulating thermostat at 25.0 ± 0.01 °C. All the measured values were corrected by taking the turbidity of Milli-Q water as the controller.

Dynamic light scattering (DLS)

The size distribution of C_nN/SDS (n = 6, 8, 12) mixtures at C_T = 20 mM with X_SDS from 0 to 1.00 was measured using a Malvern ZetaSizer Nano ZS Instrument (ZEN3600, Malvern Instruments, Worcestershire, UK) at a scattering angle of 173° equipped with a 4 mW He–Ne laser (λ = 632.8 nm) and a thermosetting chamber at 25.0 ± 0.1 °C.

Cryogenic transmission electron microscopy (cryo-TEM)

5 μL C₆N/SDS solutions at X_SDS = 0.10, C₈N/SDS solutions at X_SDS = 0.20, 0.60 and C₁₂N/SDS solutions at X_SDS = 0.80 (C_T = 20 mM) were loaded onto a carbon-coated holey TEM grid. The excessive solution was sucked away by filter paper leaving a thin liquid film on the grid. After a few seconds, the grid was quickly vitrified by plunging into liquid ethane (cooled by liquid nitrogen) at −183 °C. The vitrified sample was transferred to a cryogenic sample holder and examined with a Themis 300 TEM (Thermo Fisher Scientific, America) at about −174 °C. The images were recorded on a Gatan charge-coupled device (CCD) camera in the minimal electron dose mode.

Interfacial tension measurement

The interfacial tension between surfactant mixtures and machine oil was measured by a TX500™ Spinning Drop Interface Tensiometer (Model TX500C). 1 mL C_nN/SDS (n = 6, 8, 12) mixtures at C_T = 5 mM with X_SDS from 0 to 1.00 were injected into a quartz glass tube as an external phase and 10 μL of machine oil was injected into liquid solution as an internal phase. Oil drop formations at different rotation rates were recorded by a CCD camera, and interfacial tension was calculated by measuring the length and width of the oil drop and using Vonnegut and Bashford–Adams fitting. All the measurements were tested at 25.0 ± 0.1 °C.

Contact angle measurement

C_nN/SDS (n = 6, 8, 12) mixture solutions at C_T = 5 mM with X_SDS from 0 to 1.00 were poured into a cuboid glass case. The stainless-steel plate was stuck to the bottom of a floating foam broad on the liquid solution. The machine oil droplet was squeezed out through a crooked needle and floated to the plate. The contact angle was recorded by a digital camera and fitted by Image J software. Each measure was tested at least three times.

Cleaning measurement

Oil cleaning efficiency was determined by weighing the metage of the weight gap during cleaning progress. The weight of the stainless-steel plate was M₀. Each 50 μL of machine oil was daubed onto stainless-steel plates evenly. Stainless-steel plates with oil were weighed as M₁ and then soaked into 10 mL liquid solution at different ratios of SDS. After ultrasonic vibration in an ultrasonic cleaning tank with an ultrasonic power of 120 W at the temperature of 25 °C for 10 min, the oil on the stainless-steel plates was removed to different extent and weighed as M₂. All the measures were tested at least three times. The oil cleaning efficiency was calculated by using the following equation,

Oil cleaning efficiency% = [(M₁ − M₂)/(M₁ − M₀)] × 100

Foaming and defoaming measurement

The foaming and defoaming ability of various surfactant mixtures were tested by two methods. One is the air blowing method using a dynamic foam analyzer DFA 100 (KRÜSS, Germany). N₂ was continuously blown into 50 mL analyte liquor at the bottom of the container until the foam height did not increase, then the highest foam height was recorded as foaming ability. The state change of foams with time was recorded with a CCD camera, and bubble counts and half-time were recorded as defoaming ability. The other is the visual inspection method, i.e., shaking 10 mL C_nN/SDS solutions of 5 mM at different X_SDS or fixed X_SDS = 0.10 with various C_T for 30 s. Following that, we recorded the change in foam height with aging and obtained the equilibrium foam height after resting for 2 h. The relative foam height is termed as the ratio of the measured foam height to the vial height.

Results and discussion

Self-assembly of C_nN/SDS mixtures at the air–water interface

The surface activity and the onset of micellization of SDS with a series of cyclic amines (C_nN, n = 6, 8, 12, Fig. 1A₁–A₃) were determined by surface tension measurements. Fig. 1B₁–B₃ show the surface tension curves for the C_nN/SDS mixtures as a function of the total concentration (C_T) at a fixed C_nN/SDS molar fraction (X_SDS = C_SDS/(C_SDS + C_CnN), from 0.00 to 1.00). For comparison, the surface tension curves of pure SDS and C_nN aqueous solution are also included. Correspondingly, all the CMC and the surface tension at CMC (γ_CMC) of C₆N/SDS, C₈N/SDS and C₁₂N/SDS are summarized in Fig. 1C₁–C₃ to clearly show the influence of X_SDS on CMC and γ_CMC.

Fig. 1 Chemical structures and self-assembly behaviors of C_nN/SDS mixtures at the air/water interface. (A₁–A₃) Chemical structures of C_nN (n = 6, 8, 12). (B₁–B₃) Surface tension of (B₁) C₆N/SDS, (B₂) C₈N/SDS and (B₃) C₁₂N/SDS mixtures at 25.00 ± 0.01 °C. (C₁–C₃) The variation of CMC and γ_CMC values of (C₁) C₆N/SDS, (C₂) C₈N/SDS and (C₃) C₁₂N/SDS mixtures as a function of X_SDS.

Obviously, the surface tension curves exhibit broadly similar profiles for all the C_nN/SDS mixtures regardless of the sizes of the cycloalkane ring and the X_SDS values. The CMC value of SDS without C_nN derived from the breakpoint is 8.0 mM and the surface tension at CMC (γ_CMC) is 38 mN m⁻¹, which are consistent with previously reported values. For C_nN solution alone, the surface tension curves of C₆N and C₈N display no inflection point in the concentration range studied (<50 mM) and just show a slight decline to ∼55 mN m⁻¹, whereas the CMC and γ_CMC values of C₁₂N are 28 mM and 47 mN m⁻¹, indicating that the three cyclic amines possess a very weak surface activity. As expected, the molecular arrangement at the air/water interface of cyclic amines with a bulky alkyl chain is much looser than that of the alkyl amine with the same carbon number, e.g., CMC and γ_CMC values of N-dodecylamine are 2.6 mM and 30 mN m⁻¹ (Fig. S1†).

Upon mixing the two components, the CMC and γ_CMC values decrease significantly to lower than those of individual components, attributed to the strong electrostatic attraction between the cationic headgroup of protonated C_nN and the anionic sulfate of SDS and the hydrophobic interaction of the alkyl ring of amines with the alkyl chain of SDS. Even at X_SDS = 0.01, the CMC values have already reduced to 27.8, 11.6 and 1.1 mM for C₆N/SDS, C₈N/SDS and C₁₂N/SDS mixtures, respectively, whereas the γ_CMC values declined to ∼30 mN m⁻¹, nearly 8.0 mN m⁻¹ lower than that of the pure SDS solution. When X_SDS approaches 0.5 from 0 or 1.0, the CMC value is steadily decreased to the lowest one, i.e., 4.0, 2.0 and 0.4 mM for C₆N/SDS, C₈N/SDS and C₁₂N/SDS, respectively, while the γ_CMC values are maintained at low values (∼30 mN m⁻¹) and there is almost no further decrease with varying X_SDS. Thus, the variation of CMC and γ_CMC values displays similar trends to X_SDS changing from 0 to 1.0, i.e., decreasing first and then reaching a plateau and increasing rapidly again. It is worth noting that the surface tension curves of the mixed solutions at high X_SDS show an upward end with increasing C_T, which is attributed to the significant excess SDS, thus causing the increase in surface tension to reach the γ_CMC value of SDS itself. These results show that the binding affinity becomes stronger when the molar ratio of C_nN and SDS is close to 1 : 1, and the strong binding between the two components possibly induces the remarkable deformation or the transformation of the molecular configuration of C_nN,^60–62 thereby the molecular packing is significantly compacted at the air/water interface. However, the surface tension curves of the C₁₂N/SDS mixtures at 0.20 < X_SDS < 0.70 are excluded because the precipitation takes place in such a wide range of X_SDS, which is different from the C₆N/SDS and C₈N/SDS mixtures with smaller cycloalkane rings. Despite these, we can still observe that the ability to reduce CMC is enhanced from C₆N/SDS to C₁₂N/SDS, demonstrating that the larger alkyl ring leads to the stronger hydrophobic interaction of the cyclic amines with SDS.

Aggregation behavior of C_nN/SDS mixtures in bulk

To further understand the impact of the cyclic amines with different sizes of the cycloalkane ring on the self-assembly of SDS in bulk, the aggregation behaviors of C_nN/SDS mixtures were studied by turbidity measurement (Fig. 2A₁–A₃), dynamic light scattering (DLS, Fig. 2C) and cryogenic transmission electron microscopy (cryo-TEM, Fig. 2D₁–D₃). The different aggregate structures were observed with changing X_SDS and C_T as depicted in phase diagrams (Fig. 2B₁–B₃), which are derived from the turbidity curves of the C_nN/SDS aqueous solution as a function of C_T at various X_SDS values.

Fig. 2 Self-assembly of C_nN/SDS mixtures in bulk solution. (A₁–A₃) Turbidimetric curves of (A₁) C₆N/SDS, (A₂) C₈N/SDS and (A₃) C₁₂N/SDS at different X_SDS values measured by UV-vis spectroscopy. (B₁–B₃) Phase diagram of (B₁) C₆N/SDS, (B₂) C₈N/SDS and (B₃) C₁₂N/SDS mixtures. (C) The size variation of C_nN/SDS aggregates at C_T = 20 mM as a function of X_SDS. (D₁–D₃) Cryo-TEM images of the C_nN/SDS aggregates at C_T = 20 mM: (D₁) C₈N/SDS at X_SDS = 0.20, (D₂) C₈N/SDS at X_SDS = 0.60, and (D₃) C₁₂N/SDS at X_SDS = 0.80.

For C₆N/SDS, the turbidity value remains extremely low no matter which value X_SDS or C_T is. Taking the situation at C_T = 20 mM as a representative, the mean particle size remains below 20 nm. The broadly invariable turbidity and size as well as the lower CMC values (Fig. 1B₁) demonstrate that the addition of C₆N promotes the micellization of SDS, but only induces the formation of small spherical or rodlike micelles, which is confirmed by the cryo-TEM micrographs of C₆N/SDS aggregates at X_SDS = 0.20 and C_T = 20 mM (Fig. S2†).

In contrast, the self-assembly behavior of SDS is significantly affected by C₈N and C₁₂N, and undergoes different processes at various X_SDS and C_T. For the C₈N/SDS mixture, there is almost no change in the turbidity with C_T as X_SDS > 0.5, indicating that the small fraction of C₈N has a strong ability to accelerate the micellization of SDS but not enough to affect the molecular packing and the aggregate structures. At X_SDS > 0.5, the turbidity values increase with increasing C_T, and the largest value appears at X_SDS = 0.40. Fixing C_T = 20 mM, a weakly bluish solution was observed, and the C₈N/SDS mixed solution forms vesicles of a few tens of nanometers at X_SDS = 0.20, while the solution becomes transparent and forms wormlike micelles with a few millimeters long at X_SDS = 0.60, as evidenced by the results of DLS (Fig. 2C) and cryo-TEM (Fig. 2D₁ and D₂). Compared with the phase boundaries of C₈N/SDS and C₁₂N/SDS mixtures, they go through similar changing processes with increasing X_SDS at the same C_T, but the big differences take place at much lower X_SDS and C_T for the aggregate formation, the larger vesicles (>100 nm, Fig. 2D₃) and the wider precipitate region over the C_T range in C₁₂N/SDS.

In summary, the addition of C_nN only promotes the micellization of SDS for C₆N, while leads to an aggregate transition from small micelles to vesicles and then to wormlike or spherical micelles at a fixed C_T for C₈N without precipitation and for C₁₂N with a large range of precipitation. In all the three C_nN/SDS mixtures, the electrostatic interaction between the protonated amine and sulfate groups and the hydrophobic interaction among the alkyl chains of C_nN and SDS are the main driving forces for their self-assembly. However, taking a variety of conformations of cycloalkylamine in aqueous solution into account,^60–62 we speculate that both the size and conformation of the cycloalkane ring of C_nN may play a decisive role in the different aggregate formation and transition. Fig. 3 summarizes a simplified schematic diagram of aggregation behavior for the C_nN/SDS mixture in bulk with changing in X_SDS at a fixed C_T.

Fig. 3 The schematic illustrations of the variation of the aggregate morphologies in the C_nN/SDS mixtures with increasing X_SDS. In C₆N/SDS, only spherical micelles are formed regardless of C_T and X_SDS. In C₈N/SDS, the aggregates transit from spherical micelles to vesicles, wormlike micelles and then to spherical micelles again by increasing X_SDS from 0 to 1.00. In C₁₂N/SDS, the precipitate is formed rich in C₁₂N because of the enhanced interaction between C₁₂N and SDS due to the distinctive molecular conformation of C₁₂N. With increasing X_SDS, the precipitate is redissolved and transfers to vesicles and spherical micelles.

The C₆N molecule has the smallest cycloalkane ring. It is not long enough to enlarge the hydrophobic area in the aggregate relative to the SDS itself, thereby the C₆N/SDS mixture only forms small micelles. In comparison, the C₈N molecule with a larger cycloalkane ring can produce stronger hydrophobic interaction with SDS and form a larger hydrophobic area. As such, the spontaneous curvature of the C₈N/SDS aggregates becomes slightly smaller, forming the wormlike micelles rich in SDS (X_SDS > 0.5) and vesicles rich in C₈N (X_SDS < 0.5). As for C₁₂N with a much larger cycloalkane ring, it forms precipitate with SDS in a large range, which could be attributed to the remarkably enhanced hydrophobic interaction by the unique C₁₂N ring. It was reported that the conformers of cyclooctane can be classified into three families, including boat–chair, chair–chair or crown, and boat–boat, while cyclododecane shows a square topology and has at least 100 possible conformations.⁶² Thus, the large hydrocarbon ring is flexible enough to optimize the molecular conformation and nest together, in which case the ion pairs formed by C₁₂N and SDS may be drawn closer with each other due to the tight entanglement between the C₁₂N rings, thereby the precipitate with compact microstructure is formed, especially with the excessive C₁₂N. For the same reason, a small amount of C₁₂N can lead to a significant change in the molecular packing of C₁₂N/SDS mixtures, thus the vesicles are formed at a large X_SDS (e.g., 0.65 < X_SDS < 0.85 at C_T = 20 mM). Hence, the cycloalkylamines with different sizes of rings lead to diverse aggregate structures, and the large chair conformation of C₁₂N displays the most obvious effect on the association with SDS.

Oil-fouling removal efficiency of C_nN/SDS

Surfactants that form aggregates with a strengthened hydrophobic domain and show strong ability to enhance surface activity should be beneficial to strip and solubilize the oil from substrates, so we selected C_nN/SDS with C_T = 5 mM, which is higher than their CMC but as low as possible, to test the oil-fouling removal efficiency. Stainless-steel plates covered with machine oil-fouling were immersed into the C_nN/SDS solutions, and ultrasonically treated for 10 min, it was observed that the homogenous emulsion with machine oil was entrapped in the interior, or the machine oil was separated from the plate and suspended in the solution. After that, the stainless-steel plates were taken out from the solutions to evaluate the oil-fouling removal efficiency by inspection and weighting methods.

With the visual approach to observe the oil-fouling efficiency, we took the photos of residual oil on stainless-steel plates under ultraviolet light (UV) based on the blue light character of oil-foiling under UV light (Fig. 4A). For both the C₆N/SDS and C₈N/SDS mixtures, the area of blue light on plates becomes smaller first and then enlarges again with the increase of X_SDS, indicating that the optimum oil-fouling removal result appears around X_SDS = 0.5. The C₁₂N/SDS mixture follows the same changing pattern except for the precipitate region. In parallel, the quantitative evaluation by the weighting method is summarized in Fig. 4B, also indicating that the X_SDS value for the highest oil-fouling removal efficiency is nearly 0.5. In addition, the C₈N/SDS system shows the wider X_SDS region (0.1 < X_SDS < 0.5) to reach high cleaning efficiency and the efficiency is better than that of C₆N/SDS. For C₁₂N/SDS, in the region without precipitation at larger X_SDS, it displays better cleaning performance than either C₆N/SDS or C₈N/SDS. That is, the C_nN/SDS mixture with a larger cycloalkylamine and stronger aggregation ability exhibits higher oil-fouling removal efficiency.

Fig. 4 Cleaning efficiency and mechanism of C_nN/SDS for removing machine oil contamination from stainless-steel plates. (A) Photos of stainless-steel plates contaminated with machine oil after ultrasonic cleaning by C_nN/SDS solutions at different X_SDS observed under the irradiation of UV light. The plates with machine oil contamination emit blue light under irradiation of UV light. (B) The oil-fouling cleaning efficiency of C_nN/SDS mixtures at different X_SDS by the weighting method. Oil cleaning efficiency% = [(M₁ − M₂)/(M₁ − M₀)] × 100. (C) Solid/liquid contact angle of machine oil droplets on the stainless-steel plates under water with C_nN/SDS. (D) Oil/water interfacial tension values for machine oil in the C_nN/SDS solutions. In all the experiments, C_T was fixed at 5 mM. (E) The possible mechanism of oil-fouling cleaning efficiency for single SDS solution and C_nN/SDS (n = 6, 8, 12) mixtures.

To further understand the role of cycloalkylamine size in the oil-fouling removal efficiency of C_nN/SDS, the cleaning process is investigated by oil–water interfacial tension and contact angles (θ) of oil on plate under solution (Fig. 4C and D). Normally, cleaning by surfactants is realized through the de-wetting mechanism,⁶³ for which the contact angles of oil drops on substrates under surfactant solutions are very large. Herein, the contact angles of machine oil drops on the plates in the C_nN/SDS solutions at the representative X_SDS show a similar changing trend with the oil-fouling cleaning performance, i.e., good cleaning performance corresponds to a large contact angle. The oil-fouling tends to separate from the stainless-steel substrates at the medium X_SDS (Fig. 4C). Therefore, the de-wetting occurs around X_SDS = 0.5. Meanwhile, the interface tension between the oil and C_nN/SDS solutions becomes smaller than that of either C_nN or SDS, which is beneficial to the formation of O/W emulsion (Fig. 4D). And the O/W interfacial tension curves as a function of X_SDS display a concave edge, showing a contrary changing trend of cleaning efficiency curves. It means that both the de-wetting and emulsion are enhanced at the medium X_SDS, which facilitates the oil-fouling removal and impedes the separated oil to stick back on the substrates. Moreover, C₈N/SDS shows 0.5 mN m⁻¹ lower interfacial tension than C₆N/SDS at X_SDS < 0.50, whereas C₁₂N/SDS shows the minimum interfacial tension at X_SDS > 0.50. As a result, the oil-fouling is readily dispersed into the C_nN/SDS solutions due to de-wetting and emulsion, and the highly efficient cleaning performance is obtained for C₈N/SDS at X_SDS < 0.50 and C₁₂N/SDS at X_SDS > 0.50.

Inspired by the Young's equation, we try to figure out the reasons for the various cleaning ability of C_nN/SDS solutions in the view of equilibrium of forces. Herein, the interfacial tensions of solid/water, oil/water and solid/oil are defined as γ₁, γ₂ and γ₃, respectively. The three parameters fit the equation, γ₁ = γ₂ cos θ + γ₃ (Fig. 4E). The contact angle of liquid on solid is proportional to the interfacial tension and energy, so according to the contact angles of C_nN/SDS (n = 6, 8, 12) solutions on the stainless-steel plates (Fig. S3†), the γ₁ values for pure SDS (γ₁⁰), C₆N/SDS (γ₁^a) and C₈N/SDS (γ₁^b) at X_SDS < 0.5 are in order of γ₁⁰ > γ₁^a ≈ γ₁^b, while for C₆N/SDS (γ₁^A), C₈N/SDS (γ₁^B) and C₈N/SDS (γ₁^C) when X_SDS > 0.5, the order is γ₁^A > γ₁^B > γ₁^C. Meanwhile, γ₂⁰ > γ₂^a > γ₂^b at X_SDS < 0.5 and γ₂^A > γ₂^B > γ₂^C at X_SDS > 0.5 are obtained from Fig. 4D. Combining with the order of contact angle of machine oil droplets on the plates in the C_nN/SDS solution (Fig. 4C), i.e., θ⁰ < θ^a < θ^b at X_SDS < 0.5 and θ^A < θ^B < θ^C at X_SDS > 0.5, we can observe that γ₃⁰ ≤ γ₃^a < γ₃^b at X_SDS < 0.5 and γ₃^A < γ₂^B < γ₂^C at X_SDS > 0.5. The results mean that the oil droplet has a stronger trend to retract inward and escape from the plate in C₈N/SDS at X_SDS < 0.5 or in C₁₂N/SDS at X_SDS > 0.5. All the experimental results and theoretical derivations provide insights into the cleaning efficiency of C_nN/SDS, from which the oil-foiling removal performance could be predicted by the oil/water interfacial tension and solid/liquid contact angle of oil droplets in surfactant solutions.

Foaming and defoaming test

For surfactant applications in cosmetics, detergents, food, etc., strong foaming ability is normally needed. However, in industrial cleaning, petroleum refining and other large-scale applications, strong foamability and foam stability often lead to environmental hazards, wastewater treatment difficulty and economic loss. Therefore, surfactant systems with strong oil cleaning ability but weak foamability and foam stability are of importance in these fields.

Herein, to intuitively observe the foaming and defoaming ability of C_nN/SDS systems with great oil cleaning ability, we firstly shake 10 mL C_nN/SDS solutions of 5 mM for 30 s and record the change in foam height with aging (Fig. S4†). In parallel, we gain the variation of foam volume (V_{foam max}), foam state, foam height, and bubble counts with aging by using a dynamic foam analyzer (Fig. 5 and S5†). We compare foam state against aging for the C₆N/SDS and C₈N/SDS mixtures at fixed X_SDS = 0.10 mM and C_T = 5.0 mM with the visual inspection method (inset) and air blowing method using a dynamic foam analyzer (Fig. S6†), and apparently the visual and quantitative results are consistent with each other. C_nN (n = 6, 8, 12) alone almost has no ability to form liquid foams. With the addition of C_nN, the foamability characterized by the maximum foam volume (V_{foam max}, Fig. 5B) gradually becomes weaker with decreasing X_SDS, and the C₈N/SDS system displays the smallest foam volume when X_SDS < 0.50, especially at X_SDS = 0.10. Although the values of the maximum foam volume do not show significant differences, the differences in the foamability and foam stability are manifested in the changes of the bubble size, the foam heights and bubble count with time. Obviously, the low foamability of C₈N/SDS is further verified by the bigger bubbles formed at X_SDS = 0.10 and 30 s (Fig. 5A and S5†), following which the bubbles begin to break at 1500 s when the foams in pure SDS, C₆N/SDS (X_SDS = 0.10, 0.40, 0.50, 0.80), C₈N/SDS (X_SDS = 0.30, 0.40, 0.80) and C₁₂N/SDS (C₁₂N/SDS = 0.80) are still very stable. Correspondingly, the C₆N/SDS system shows the similar changing speed in foam height against aging compared with SDS, the C₁₂N/SDS system defoams also quite slowly, but the foam height of C₈N/SDS decreases 3–5 times faster than that of SDS itself (Fig. 5C). The bubble count of C_nN/SDS also confirms that the C₈N/SDS system shows the fastest defoaming speed (Fig. 5D). All the above results demonstrate that the C₈N/SDS mixture at X_SDS < 0.50 can effectively reduce the foamability and enhance the defoam ability, perfectly corresponding to the condition for the high oil-fouling removal efficiency.

Fig. 5 Foamability and foam stability of C_nN/SDS (n = 6, 8, 12). (A) Images of foam state with aging recorded by a CCD camera for 5 mM SDS, 5 mM C₆N/SDS at X_SDS = 0.10, 5 mM C₈N/SDS at X_SDS = 0.10, and 5 mM C₁₂N/SDS at X_SDS = 0.80. The maximum foam volume after gas admission (B), the variation of foam height against aging (C) and the change of bubble count against aging (D) for C_nN/SDS mixtures at different X_SDS. (E) The possible mechanism of the decay process for foams in single SDS solution and C_nN/SDS (n = 6, 8, 12) mixtures.

We speculate that there are two factors affecting the foam stability and defoaming ability of C_nN/SDS mixed systems: surface tension and dynamic molecular exchange between the bulk phase and air/water interface. The low surface tension manifests that the surfactant molecules prefer packing at the air/water interface, which contributes to the formation and stabilization of foams. However, all the C_nN/SDS systems reduce the surface tension of SDS from 38 mN m⁻¹ to 30 mN m⁻¹, thus the foamability and foam stability should be improved in principle. Actually, only the C₁₂N/SDS system obeys this prediction, while the C₆N/SDS and C₈N/SDS systems exhibit similar or even weaker foamability and foaming stability than SDS. We turn back to analyze the C_T value we selected. In order to compare the foams, we selected the same C_T (5 mM), which is below the CMC of SDS (∼8.0 mM) and C₆N/SDS (∼6.0 mM), or much larger than that of C₁₂N/SDS (∼0.40 mM), but only slightly higher than the CMC value of C₈N/SDS (∼3.0 mM). In general, there are three different mechanisms governing the lifetime of foam: (i) foam drainage caused by gravity, (ii) coarsening caused by the transfer of gas between bubbles generated by the capillary pressure differences, and (iii) bubble coalescence caused by the rupture of liquid films between neighbouring bubbles.⁶⁴ For the present systems, it is proposed that the dynamic molecular exchange between the bulk phase and air/water interface can accelerate the enlargement and destruction of bubbles, which can perfectly explain the anomalous phenomenon in the foam (Fig. 5E). Given that the selected C_T is close to but below the CMC for SDS and C₆N/SDS, the surfactant molecules are enough to promote the adsorption saturation at the air/water interface but still have no ability to form micelles, in which case the molecules locating in the liquid film are relatively stable, resulting in the low defoaming ability. However, this C_T is much higher than the CMC of C₁₂N/SDS, which indicates that the absorption saturation has been completely reached at the air/water interface, and the number of molecules in the C₁₂N/SDS aqueous solution is large enough to form stable micelles, thereby the molecules at the air/water interface do not exhibit the tendency to enter the bulk solution, leading to the stronger foam stability. In comparison, the C_T is slightly higher than the CMC value of C₈N/SDS. In the foaming process of C₈N/SDS, more surfactant molecules join the air/water interface, while the real surfactant concentration in bulk is just high enough for micellization. The unstable bubble films promote the fusion of neighbour bubbles, and the surfactant molecules prefer to return to the micelles in bulk solution from the bubble films, i.e., the air/water interface, leading to the enlargement and destruction of bubbles under this condition.

In brief, it is concluded that high oil cleaning efficiency and low foam stability of surfactant mixtures are achieved at an appropriate concentration, i.e., above CMC, but not too large at a proper X_SDS. Inspired by the present results, we tested oil-fouling removal efficiency and foam height with the C₆N/SDS and C₈N/SDS mixtures after vortexing for 30 s and resting for 2 h at X_SDS = 0.10 and various C_T. For the given X_SDS at 0.10, we found that the cleaning efficiency becomes higher with the increase of C_T and is significantly enhanced especially above CMC. However, there is a minimum foam height just above CMC, after which the height becomes larger again (Fig. 6A and B, S7 and S8†). The CMC of C₆N/SDS is larger than that of C₈N/SDS, so the minimum appears at the larger concentration for C₆N/SDS, i.e., at ∼7.5 mM for C₆N/SDS and ∼6.0 mM for C₈N/SDS. Combining with the continuous enhancement of cleaning efficiency and the minimum foam height above CMC, it can be speculated that selecting the concentration slightly above CMC at low X_SDS should be beneficial to obtain a high-efficiency and low-foam cleaning system. Therefore, the concentration of 5 mM used above meets the requirements for C₈N/SDS, resulting in good cleaning and deforming performance at X_SDS < 0.50. Due to the weaker self-assembly ability of C₆N/SDS at the air/water interface and in bulk, its performance in cleaning and defoaming efficiency cannot be better than that of C₈N/SDS.

Fig. 6 The cleaning and foaming performance of C_nN/SDS (n = 6, 8) at X_SDS = 0.10 and different C_T. (A) The oil-fouling cleaning efficiency by the weighting method and (B) foam height after vortexing for 30 s and standing for 2 h for C_nN/SDS (n = 6, 8) at X_SDS = 0.10 and different C_T as marked in the plots.

Conclusions

In this work, we designed mixed surfactant systems composed of SDS and C_nN (n = 6, 8, 12) and studied their oil-fouling and foam removal ability. With addition of C_nN, the surface activity and aggregation ability of SDS are significantly enhanced, and the degree of enhancement is increased with the larger size of the cycloalkane ring because the hydrophobic interaction becomes stronger. As a result, only spherical micelles form in C₆N/SDS mixtures, while vesicles, spherical micelles and wormlike micelles form in the C₈N/SDS and C₁₂N/SDS mixtures. However, the flexible and adjustable cycloalkane ring of C₁₂N leads to the changeable conformation transformation and facilitates entanglement with each other tightly, thereby precipitation takes place in a large concentration range in C₁₂N/SDS. Except for the region of precipitate, the oil cleaning efficiency of C_nN/SDS shows a similar changing trend against X_SDS at a fixed C_T and the optimum condition mainly appears around X_SDS = 0.50. Meanwhile, the stronger aggregation ability induces the higher cleaning efficiency with increasing cycloalkane ring size and C_T. As a result, the best cleaning performance is achieved by C₈N/SDS in a broader X_SDS region, especially at low X_SDS. In parallel, the changing trend of foam stability is broadly consistent with cleaning efficiency against X_SDS for a given C_T, i.e., the strong defoaming ability appears at X_SDS < 0.5, which is attributed to the low surface activity of C_nN. But it is totally different with changing C_T for a fixed X_SDS, i.e., there is an obvious decline of foam ability slightly above CMC, which is driven by the migration of the limited molecules from the liquid film of foams to bulk between films for micellization. Due to the stronger self-assembly ability of C₈N/SDS than C₆N/SDS, the highly efficient foam removal is realized by C₈N/SDS at smaller C_T. It can be concluded that the highly efficient oil-fouling and foam removal can be achieved concurrently in a surfactant system mixing with an additive of relatively high solubilization and low surface activity like cyclic amines as we expected, which may form a larger hydrophobic domain in bulk through molecular conformation transition but create looser arrangement at the air/water interface because of bulky alkyl rings. This could also be realized in a single surfactant solution with such properties like star-shaped oligomeric surfactants. As such, the paradoxical self-assembly ability in bulk and at the air/water interface could induce a different changing tendency of cleaning and foaming performance against molar fraction or total concentration, providing a possible condition to achieve such a pair of contradictory properties. Thus, this work offers a simple approach obtain a high-efficiency and low-foam detergent with a low usage, meeting the requirements of some applications and supporting the sustainable development of the environment.

Author contributions

Z. Z., Y. F., and Y. W. designed the research. Y. F. and Y. W. supervised the experimental work. Z. Z. performed most of the experiments. J. Y. performed cryo-TEM experiments on the C_nN/SDS aggregates. T. W. and Z. Z. performed the experiments on cleaning measurements together. All authors discussed and contributed to the interpretation of the data. Z. Z. wrote the original manuscript, and Y. F. and Y. W. edited the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the National Key R&D Program of China (2021YFA0716700), National Natural Science Foundation of China (21972149 and 21988102) and the Beijing National Laboratory for Molecular Sciences (BNLMS).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S7. See DOI: https://doi.org/10.1039/d3lf00145h

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