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Article

Adsorption and Dilational Viscoelasticity of Saponin at the β-Pinene/Water and Air/Water Interfaces

Natural Resources Canada, CanmetENERGY Devon, 1 Oil Patch Drive, Devon, AB T9G 1A8, Canada
Colloids Interfaces 2025, 9(5), 68; https://doi.org/10.3390/colloids9050068 (registering DOI)
Submission received: 30 July 2025 / Revised: 5 October 2025 / Accepted: 9 October 2025 / Published: 11 October 2025

Abstract

Understanding adsorption and interfacial properties of surface-active agents at interfaces is crucial to the formation and stability of colloidal systems such as emulsions and foams. In this work, interfacial tension and viscoelasticity of saponin at the β-pinene/water interface were studied using drop tensiometry and dilational rheology measurement. For comparison, saponin at the air/water interface was also evaluated. Both saponin and β-pinene are bio-based, eco-friendly, and abundant in plants, trees, and agricultural wastes. Results showed that dynamic interfacial tensions σ(t) of saponin adsorbed at β-pinene/water and air/water interfaces could be well described by the Ward and Tordai model, suggesting that the saponin adsorption kinetics at both interfaces are controlled by a kinetically limited mechanism. The equilibrium interfacial pressure πe data prior to critical micelle concentration (cmc) were adequately fitted by the Gibbs adsorption isotherm. At the β-pinene/water interface, a higher cmc and a larger area per molecule, but a lower πe, were observed compared to the air/water interface. Interestingly, the dilational moduli of saponin at β-pinene/water increased with increasing oscillating frequency, but with less significant frequency dependence than their counterparts at the air/water interface. The dilational moduli of saponin at β-pinene/water passed through a minimum with increasing saponin bulk concentration, while the air/water interface exhibited a strikingly different trend in terms of concentration dependence and a higher magnitude for the dilational moduli. The correlation between adsorption behaviors and dilational properties of saponin at the two interfaces is discussed. Fundamental knowledge gained from this study will be beneficial for the rational development of new biocompatible emulsions and foam products for more sustainable applications.

Graphical Abstract

1. Introduction

Developing bio-based formulas in emulsions and foams is essential for environmental sustainability and positive health and safety impacts [1,2]. Replacing the use of petroleum-based oils and surfactants with eco-friendly alternatives in commercial products is driven by legal regulations and increasing consciousness of consumers [3,4,5]. β-pinene and saponin are natural and biodegradable compounds with several attractive properties. β-pinene is a less volatile, abundant, yet underutilized monocyclic terpene present widely in forests and agricultural waste [6], and it is an economical, non-toxic compound, increasingly recognized as a green alternative for petroleum oils and hydrocarbon solvents [7]. Interestingly, β-pinene is water insoluble with an equivalent solubilizing power as toluene [8], while possessing a wide range of therapeutic advantages, including anti-inflammatory, antimicrobial, and antitumor properties [9]. Saponin is a natural surfactant composed of a hydrophobic aglycone structure with hydrophilic sugar residues, mainly extracted from plants [10]. With unique combination of interface and bio active properties, saponin emerges as a stabilizer in cosmetics, personal care products, beer, and soft drinks [11,12]; as a solubilizing agent for vitamins and minerals in food additives [13,14]; as an adjuvant in vaccines [15]; and as an essential ingredient in medicines for decreasing the cholesterol level in the bloodstream [16].
Despite significant publications concerning β-pinene and saponin separately [6,7,8,9,10,11,12,13,14,15,16], very few have been devoted to their combination. Herrmann and Wink demonstrated that significant synergistic cytotoxicity and biological activity occurred both in HeLa and Cos7 cells by combining pinene with saponin [17]. The surfactant properties of saponin, together with the environmental, health, safety, and pharmacological advantages of both β-pinene and saponin, make their combination attractive in producing new stable colloids for various applications. However, to our best knowledge, there still lacks research on utilizing the combination of these two substances in the formation and stabilization of emulsions and foams.
Understanding the adsorption properties of saponin and the surface rheology of adsorption layers at the liquid interface is crucial for predicting saponin-stabilized emulsions and foams [1,2,3,4,5,18,19,20,21]. While exploring saponin as an alternative to convenient surfactants such as sodium dodecyl sulfate (SDS), Yekeen et al. demonstrated that aqueous foams generated from the soapnut-extracted saponin were more stable than SDS-stabilized foams [18]. They ascribed the higher foam stability to the adsorption and aggregation of a dense saponin monolayer at the air/water interface lamellae, slowing down the rate of Oswald ripening and delaying the diffusion of gas through the foam film. Gonzalez and Sörensen showed that saponin produced a more stable foam than lecithin, commonly used in food systems [19]. The presence of sodium chloride, ethanol, and low pH did not have a negative effect on saponin foams. This was explained by the fact that the air/water interface formed by saponin behaved more like a viscoelastic solid, showing a higher yield stress and much higher storage moduli, compared with that by lecithin [19]. Golemanov et al. found that depending on their chemical structure, the shear viscoelastic response of saponins at the air/water interface could be different: Triterpenoid-type saponins exhibited a more solid-like viscoelastic response in contrast to steroid-type saponins, which showed a more liquid-like response to shear stress [3]. Despite some progress achieved in the study of saponin at the air/water interface, understanding the interfacial properties of saponin at the oil/water interface remains scarce.
In this work, we first use a drop-shape tensiometry to evaluate dynamic interfacial tensions of saponin at the β-pinene/water interface, from which the mechanisms governing saponin adsorption at the interface are discussed. Saponin at the air/water interface is also studied for comparison. Furthermore, we apply a sinusoidal oscillation method to measure dilational viscoelasticity of saponin adsorption layers at the β-pinene/water and air/water interfaces, attempting to provide useful information on the interfacial behavior of saponin at different interfaces for potential practical applications. Assessment of the impact of saponin and β-pinene contents on the formation and stability of eco-friendly foamulsions (emulsion foams) will be reported in a separate article.

2. Materials and Methods

2.1. Materials

The surfactant Quillaja saponin, obtained from Millipore Sigma, Toronto, Canada (Cat. No. 84510, CAS number 8047-15-2), was used as received. A concentrated stock solution of the surfactant was acquired by dissolving the saponin in ultrapure deionized (DI) water (18.2 MΩ·cm resistivity). For all the experiments, an aqueous solution of the desired surfactant concentration was prepared by diluting the stock solution. β-pinene with purity of over 98.5% was supplied by Fisher Scientific (Ottawa, ON, Canada) and used as received. The chemical structures of Quillaja saponin and β-pinene are displayed in the Supporting Information (Figure S1), and their main physical properties were given elsewhere [2,22].

2.2. Measurement of Dynamic Interfacial Tension

An automated Teclis Tracker (previously IT Concept, Lyon, France) with axisymmetric drop shape analysis technique was used to measure dynamic interfacial tensions (σ(t)) of saponin at β-pinene/water and air/water interfaces, from which adsorption kinetics and isotherms of the surfactant at interfaces were derived [23,24]. Detail of its measurement procedure was described elsewhere [24]. Shortly, a 250 μL syringe with an attached U-shaped stainless-steel needle was prefilled with β-pinene. The syringe was mounted on a syringe holder and positioned above a cuvette that contained approximately 25 mL of saponin aqueous solution. The syringe was lowered such that the needle tip was immersed in the cuvette’s aqueous phase. A preset sample volume (about 12.5 μL) of a β-pinene oil drop was expelled to the tip of the needle (see Figure 1a). After the visual alignment, measurement started immediately once a fresh drop was generated. The images of the drop shape were captured with a high-speed camera across time, and then fitted to the Young–Laplace formula, from which σ(t) for the pinene/water interface was calculated. Similarly, σ(t) for the air/water interface was also determined using the air bubble dispensed from the tip of the needle.
The cuvette, syringe, and needle in contact with the sample were carefully cleaned to avoid any contamination. Prior to the experiment, the instrumental calibration was first conducted using a standard calibrated hemisphere to ensure the accurate size, good focus, and saturation of the image captured on the camera. Adjustment in alignment was further made using a test drop, such that the drop was in line with the optics and camera of the instrument to obtain clear visualization on the computer screen. The correct calibration and the cleanness of the instrument system were checked by measuring surface tension of ultrapure DI water, which was stable over time with values of approximately 72.0 mN/m at 20 °C (see Figure 1b).

2.3. Measurement of Dilational Viscoelasticity

Dilational viscoelasticity is characterized by the interfacial tension response for a small relative change in interfacial area. For the linear range, it could be given as
ε = d σ d ( l n A ) = A 0 d σ d A = σ ( A / A 0 )
where ε is the dilational total modulus, ∆σ is the amplitude for interfacial tension, ∆A is the amplitude for the drop area, and A0 is the initial area of the drop. ε consists of two components: the elastic modulus (εr) accounting for the energy stored in the system, and the viscous modulus (εi) reflecting the energy dissipated in the relaxation process. The relations of the two contributions to the total modulus could be distinguished by introducing the phase angle (ϕ) as
ε r = ε c o s ϕ
ε i = ε   s i n ϕ
To study dilational viscoelasticity, a low-deformation sinusoidal oscillation technique was used after the drop was equilibrated [25,26,27]. Applying a frequency sweep mode, the drop was subjected to a harmonic oscillation with a chosen amplitude and frequency. The area amplitude over the drop initial area (∆A/A0) was maintained at 5%. The frequency was in the range of 0.01 to 0.2 Hz. Oscillation yielded sinusoidal changes not only in the surface area but also in the drop shape. The changes in drop shape were recorded. By fitting them to the Young–Laplace equation for every video frame, the changes in interfacial tension were obtained accordingly as a function of time. A typical example of the collected data is shown in Figure 2. As seen, the data contained some scattered points and a minor drift of interfacial tension with time, due to the inertial effect and the finite speed of the motor. Using the Tracker’s internal algorithms, those scatters could be filtered out, and the raw data were smoothed to create a model sinusoid curve on the area or interfacial tension. Applying the Fourier transformation, the amplitude of each variable and the phase angle (ϕ) between the interfacial tension and surface area oscillations were calculated, from which interfacial dilational moduli would be determined. Oscillation frequency and surfactant concentration were the two variables in this study, as their impacts could reflect the viscoelastic properties of the interfacial film in a dilational rheology measurement.
All measurements were performed at 20 ± 0.5 °C. Ultrapure DI water was used for the preparation of the saponin aqueous solutions, and no salts, acids, or bases were added. For interfacial tension measurements, at least duplicated runs were conducted to ensure good reproducibility with less than 2% of standard deviation. The dilational viscoelasticity test at each condition was repeated at least five times to obtain an average and credible result.

3. Results and Discussion

3.1. Adsorption Kinetics

The adsorption of saponin at the interfaces was characterized by dynamic interfacial tension (σ(t)) data. The symbol points in Figure 3a show σ(t) data experimentally collected at the β-pinene/water interface. At a fixed saponin concentration, σ(t) decreased monotonically with increased aging time (t). The tension dropped very rapidly after a short period of time in the beginning, and then gradually decelerated as the tension approached its equilibrium value. The slope of initial σ(t) decreased and the time required to achieve equilibrium depended on the saponin concentration. The more concentrated the aqueous saponin solution, the steeper the initial slope and the sooner the interfacial tension reached equilibrium. A similar feature was found for σ(t) of saponin at the air/water interface shown in Figure 3b.
To probe the dynamics of saponin adsorption, we first attempted to apply the Langmuir kinetic model and an additional empirical model based on a diffusion-controlled adsorption process [28,29]. We found that neither of these two models can provide a satisfying fit to our experimental σ(t) data for all but the smallest saponin concentration. This suggests that the adsorption kinetics of saponin at β-pinene/water and air/water interfaces may not be solely dictated by diffusion, and other mechanisms may play a more important role. In this context, the theory of Ward and Tordai, proposing a possible kinetically limited adsorption, is implemented [30,31,32,33]. Its asymptotic forms interpreting σ(t) data from the early (t → 0) and late (t → ∞) stages of adsorption are given as [30,31]
t 0                                                     σ t = σ 0 2 R T C D 0 t π 1 2
t                                             σ t = σ e + R T Γ 2 C 4 D t π 1 2
where σ0 and σe are interfacial tensions at initial (t = 0) and equilibrium (t = ∞), respectively; universal gas constant R = 8.314 J mol−1 K−1,T = 293.15 K for tested temperature; C is the surfactant concentration in bulk, and surface excess concentration Γ (which related to the area of one surfactant molecule As and Avogadro’s number NA, Γ   =   1 N A A s ); D0 and D are the diffusion coefficients, respectively, at the early and late stages of adsorption. To verify the fits of Equation (3), values of σ(t) are plotted against t 1 2 over the early time regime and against t 1 2 over the late time stage, respectively. The linear relations for the two plots are observed for every set of data in a general trend. By fitting straight lines to σ(t) vs. t 1 2 at an early time and σ(t) vs. t 1 2 at a late time, using a least-squares functional minimization method, the intercepts and slopes of the two plots could be readily obtained.
The solid lines in Figure 3a show the theoretical σ(t) values calculated from Equation (3). They agreed with the experimental σ(t) values for every set of data points at the β-pinene/water interface in our system of study, suggesting the excellent fit of the Ward and Tordai model. Such a reasonable agreement between the model and experimental σ(t) values was also seen for the air/water interface in all the saponin concentrations, illustrated in Figure 3b.
Using Equation (3), the diffusivity of saponin (D0 and D) could be calculated from the slopes of σ(t) vs. t 1 2 at early time and σ(t) vs. t 1 2 at a late time, respectively. Their relative magnitudes would offer an insight into the adsorption kinetics. For the purely diffusion-controlled adsorption kinetics, one should find D 0     D . Figure 4 shows the D0 and D for the adsorption of saponin at different C. At either interface, both D0 and D values decreased with increasing C. For an identical C, D was significantly less than D0 in most cases except for the lowest C. The gap between D0 and D at the air/water interface was more apparent than that at the β-pinene/water interface.
Only for the smallest C, D was almost equivalent to D0 within an experimental error, proposing that the diffusion-controlled adsorption kinetics dominate only under low coverage of either interface by saponin. Over increasing adsorption time at a higher C, the adsorption rate would become significantly slower, as indicated by D0 < D. This suggests that with increasing C, the adsorption kinetics switches from a diffusion-controlled to a kinetically limited process at a later adsorption time. Similar results have been demonstrated by several researchers for different interface-active species (IAS), particularly nanoparticles [34,35,36,37]. Kutuzov et al. (2007) studied the σ(t) of tri-n-octylphosphine oxide stabilized CdSe nanoparticles in the size range 2.3 to 6 nm adsorbed at the toluene/water interface. They showed that at the lowest concentration, D0 and D equally reached the diffusion coefficient limit as predicted by the Stokes–Einstein equation, but D decreased more rapidly than D0 when the nanoparticle concentration was increased [34]. Ferdous et al. also found D0 > D by at least an order of magnitude for all but the smallest nanoparticle concentration, while studying adsorption of alkanethiol-capped gold nanoparticles at the hexane/water interface [35]. The authors have attributed such departure of D from D0 to the existence of an energy barrier to adsorption with increasing interfacial coverage [34,35,36,37]. The origin of the energy barrier is traced to the interactions in the sub-layer between adsorbing IAS and those desorbed from the interface; such interactions increase as the interface transitions from an IAS-free state (t → 0) to an IAS-covered one (t → ∞). The energy barrier for kinetically limited adsorption, Ea, is computed according to the equation proposed by Liggieri et al. [38,39]:
D = D f e x p E a k B T
Df is a free diffusion coefficient, often treated as the diffusion coefficient predicted by the Stokes–Einstein equation [34,35]. In the absence of an energy barrier to adsorption, Df could be fairly estimated from the D0 value at the smallest surfactant concentration [34,35,36,37,38,39]. Using Equation (4), the Ea values of saponin adsorption at the β-pinene/water and air/water interfaces at different C values were calculated. Results are plotted in Figure 5. The general trend observed in the plots is that Ea at either interface increased rapidly in the beginning, then relaxed with the increase in C. Meanwhile, Ea for saponin adsorption at the air/water interface was relatively higher than that at the oil/water interface in most cases at the same C. Ea values were about 2.1 to 6.9 kBT for the air/water interface and about 1.3 to 5.0 kBT for the β-pinene/water interface for all but the smallest C tested in this study. The literature has shown that depending on the type and size of the IAS, a wide spectrum of the adsorption energy barriers from negligible to several orders of magnitude kBT [31,34,35,36,37,38,39,40,41,42]. For the adsorption of most petroleum-based surfactants and polymers at oil/water and air/water interfaces, the energy barrier Ea was found to be less than 3kBT [31,37,38,39,40,41]; for nanoparticle adsorption, Ea was 10 kBT or much greater [34,35,36,42].
It is emphasized that the above-described diffusion coefficients and energy barriers were based on our simplified assumptions that the surface activity of micelles was negligible and their adsorption to the interface was close to zero. In fact, for the surfactant concentrations well above cmc (particularly C > 10 cmc), a significant impact of micelles on surfactant adsorption dynamics often occurs at a timescale up to 1 s (even in the order of micron-second). A modified form of diffusion coefficients accounting for the influence of micelles with the concentration well above the cmc needs to be considered [43,44]. Unfortunately, the technical limitation of our available instrument prevented us from measuring dynamic interfacial tensions on the sub-second timescale.
It is also worthwhile noting that saponin contains a wide range of complex molecules and may undertake different forms of structural relaxation at interfaces. A strong dependence of D on saponin concentration indicates a significant role of the adsorption energy barrier or the presence of other relaxation processes, such as reconformation or reorientation of adsorbed molecules [45,46]. Intrinsic relaxation within the adsorption layer may need to be considered along with the diffusion-controlled and barrier-limited adsorption mechanisms, but it is beyond the scope of this study.

3.2. Adsorption Isotherms

Using Equation (3b), equilibrium interfacial tension (σe) could be obtained by fitting a straight line to σ(t) vs. t 1 2 data at a late time and reading σe as the intercept for each tested C. For more convenience, σe is converted to interfacial pressure at equilibrium (πe) by subtracting the σe value for each C from the value for the corresponding neat (surfactant-free) β-pinene/water or air/water interface. As seen in Figure 1b, the tension values for neat β-pinene/water and air/water interfaces were approximately 17.0 and 72.0 mN/m, respectively, at 20 °C. A semilogarithmic plot of the πe vs. C is shown in Figure 6.
At the same time, C, a lower πe (lower σe) value, was seen for saponin adsorbed at the β-pinene/water interface than its adsorption at the air/water interface. This is possibly attributed to intermolecular attraction between β-pinene and water. β-pinene oil molecules, while less attracted to water than cohesive forces between water molecules, are still more attracted to water than air molecules are. The stronger attraction between β-pinene and water molecules at their interface can provide a more balanced force distribution than the air/water interface. This reduced imbalance of forces leads to a lower net inward pull on the molecules at the β-pinene/water interface, resulting in lower interfacial pressure and tension. Furthermore, the saponin structure at the β-pinene/water interface is less dense (as shown in Table 1), also contributing to a lower interfacial pressure compared to the air/water interface. Figure 6 also shows a linear increase in πe at either interface when represented against log C for the examined range before the critical micelle concentration (cmc); the Gibbs isotherm equation thereby applies:
Γ = 1 2.303 n R T d π e d l o g C
n is a constant that depends on the number of species constituting the surfactant adsorbed at the interface. For saponin (a non-ionic surfactant), n = 1 [47].
The slope for each set of πe against log C before cmc in Figure 6 was used to calculate surface excess Γ, assumed to be the limiting adsorption amount Γ. Accordingly, cmc was determined from the breakpoint concentration in the two linear regressions at either interface. The values Γ and cmc for the two interfaces are listed in Table 1, together with surface area per molecule As. This shows that the saponin adsorption at β-pinene/water interface had a larger As than the air/water interface, suggesting that the packing of saponin molecules at the β-pinene/water interface was less dense compared to the air/water interface.
From molecular modeling calculations, Stanimirova et al. [2] determined that the triterpenoid hydrophobic skeleton of Quillaja saponin occupies around 0.75 nm2 when the molecules lie parallel to the surface (lay-on configuration). As seen in Table 1, the As for the air/water interface is 0.63 nm2 per molecule, suggesting that the adsorption layers of saponin are in a condensed state as the approximately perpendicular orientation of molecules with respect to the air/water interface [47,48,49,50]. However, for the β-pinene/water interface, the saponin molecules are probably oriented in the loosely packed, lay-on configuration, as suggested by a larger area (2.93 nm2) per molecule, and the β-pinene solvent may play a significant solvation effect—facilitating large empty spaces among the saponin molecules but also decreasing their intermolecular interaction [47,48,49,50]. A more detailed structural discussion is outside the scope of this study.
It should be noted that β-pinene may contain some impurities, but these impurities were absent from highly interface-active components and would not pose an impact on the dynamic and equilibrium interfacial tension outcomes. Please refer to Figure 1b and Supporting Information, Figure S2, for the confirmation. Furthermore, β-pinene is a non-polar, water-immiscible solvent. There is no mutual dissolution of β-pinene and water. This was verified by measuring surface tensions of β-pinene saturated DI water and water saturated β-pinene, which were stable over time with values of approximately 72.0 and 26.7 mN/m, respectively, at 20 °C and equivalent to the counterparts of original pure water and β-pinene.
Table 1 also shows a higher cmc for the saponin adsorption at the β-pinene/water interface than at the air/water interface. Such a difference suggests the presence of oil-soluble and interface-active components or impurities in the saponin sample. While saponin is largely a water-soluble surfactant, during the extraction process, the saponin sample may contain a small portion of the components or impurities that can dissolve in oil like β-pinene [2,47,48,49,50]. These components or impurities do not remain at the β-pinene/water interface because of their higher hydrophobicity but are adsorbed at the air/water interface as they cannot “dissolve” in air, thus increasing the cmc for β-pinene/water interface. Due to their small contents, the diffusion relaxation of these interface active components is rather slow, causing a further decrease in the surface tension at the air/water interface after a long relaxation time and a more apparent gap between the diffusion coefficient D0 and D at the air/water interface as described above. Quantifying the small content of these oil-soluble components in the saponin sample needs to be addressed in a future study.

3.3. Dilational Moduli

Figure 7 shows the effect of oscillation frequency ranging from 0.01 to 0.2 Hz on the dilational viscoelasticity for the two interfaces at a fixed saponin concentration (C). Note that only the C = 0.02 wt% is presented here, as a similar trend of frequency dependence of dilational moduli was observed for all C in this study.
The following features could be derived from Figure 7: ( i ) For both interfaces, the three moduli ( ε , ε r , ε i ) of the saponin adsorption layer appeared an increase with increasing frequency. This may be easy to understand: At low frequency, the interfacial layer has time to adsorb molecules upon expansion and desorb molecules upon compression, leading to little variations in interfacial tension and the small viscoelastic moduli. With an increasing frequency, there is no sufficient time for saponin molecules to diffuse and exchange between the interface and the bulk solution, therefore increasing the viscoelastic moduli. ( i i ) The total and elastic moduli of saponin at the air/water interface showed a much stronger dependency on frequency than those for β-pinene/water interface. As previous researchers demonstrated, the diffusion rate (or characteristic frequency of a relaxation process) cannot directly influence the magnitude of the modulus variation [51,52,53,54]. The stronger dependence of viscoelastic moduli on oscillation frequency at the air/water interface could be attributed to larger limiting elasticity (an equilibrium parameter) [53]. ( i i i ) For the β-pinene/water interface, the dependency of elastic modulus on frequency was almost equivalent to that of the viscous modulus, also indicated by little changes in phase angle with increasing frequency. In contrast, for the air/water interface, the increase in elastic modulus is faster than its viscous component with the increase in frequency, and the phase angle becomes smaller with higher frequency.
To gain more information on the relaxation mechanism of surfactant molecules at the interface under compression/expansion of area, the slope of a dual-logarithmic plot of the total dilatational modulus against the oscillating frequency was calculated. These slopes are plotted as a function of the surface pressure at equilibrium in Figure 8. As seen, the slope varies between about 0.1 and 0.4 for either the β-pinene/water or air/water interface. Especially for those interfacial pressures higher than πe,cmc, the slopes remain lower than 0.3, suggesting that the dilational rheological response of saponin adsorption at either the β-pinene/water or air/water interface appears not to be diffusion-controlled for high saponin surfactant concentrations. Previous studies have shown that for purely diffusion-controlled matter exchange, the slopes approach 0.5 at high interfacial pressures [54,55,56].
Figure 9 shows the impact of the saponin bulk concentration (C) on dilational viscoelasticity at the β-pinene/water interface. The oscillation frequency was fixed at 0.02 Hz and 0.2 Hz, respectively. At the same frequency, the three moduli ( ε , ε r , ε i ) of saponin at the oil/water interface decreased initially and then increased with the increase in C. The concentrations corresponding to minimum viscoelastic moduli were in the range of 0.02 to 0.03 wt%, approximately half the value of cmc at the β-pinene/water interface. However, the phase angle (ϕ) increased steadily with increasing C up to 0.02 wt%, after which ϕ did not change much in our studied range. Generally, the increase in C affects dilational moduli in two opposing ways [57,58]. On one aspect, the diffusion of surfactant molecules between the bulk and the interface is enhanced with increasing C, leading to reduced interfacial tension (σ) gradient and a decrease in dilational moduli. On the other hand, the increase in interfacial concentration could increase the intermolecular steric repulsion (higher adsorption energy barrier), thereby producing a higher σ gradient and an increase in the dilational moduli. Results in Figure 9 suggest the switch of a dominant mechanism governing dilational viscoelasticity at the β-pinene/water interface with C. At low C, molecular diffusion is the primary factor controlling dilational rheology, whereas intermolecular steric repulsion plays a more pivotal role at a higher C; therefore, dilational moduli run through a minimum with increasing C.
An alternative possibility for the minimum modulus shown in Figure 9 could be a signature of an aggregation transition [59,60]. Due to lateral attraction, the adsorbed saponin molecules may form aggregates (or 2D condensed phase domains) at the β-pinene/water interface beyond critical interface coverage. At low C, where the aggregates are rare and small, the dilational modulus is decreased, since variations in the adsorption are compensated by the exchange of molecules between the condensed and the fluidic states [59]. With higher C and progressing adsorption, in which a sufficient number of aggregates are formed and larger in size, the modulus would be increased [59].
Figure 10 shows the dilational viscoelasticity of saponin at the air/water interface as a function of C. It is evident that the air/water interface exhibited a much different magnitude and trend of viscoelastic moduli with C compared with the β-pinene/water interface described above. First, at the equal C and oscillating frequency, the values of three moduli ( ε , ε r , ε i ) of saponin adsorption at the air/water interface were 2 to 12 times higher than their counterparts at the β-pinene/water interface for all but the smallest C . This could be mainly attributed to the formation of a more rigid, solid-like structure of saponin adsorption at the air/water interface [58,61,62]. As shown in Table 1, the tighter packing (larger surface excess Γ , and smaller surface area As) of saponin molecules at the air/water interface would yield a more robust, stiffer interfacial film within the adsorption layer, leading to greater resistance to area dilation/compression and therefore larger viscoelastic moduli. Conversely, at the β-pinene/water interface, molecules of the oil phase could produce the salvation effect and enlarge the distance between the adsorbed saponin molecules as indicated by a larger surface area As, leading to a more flexible, liquid-like film that is prone to interfacial deformation and lower dilational viscoelasticity. The presence of oil-dissoluble interface-active components or impurities of saponin adsorbed at the air/water interface rather than the β-pinene/water interface is another contributing factor in producing higher viscoelastic moduli at the air/water interface.
Also referring to Figure 10, at the same oscillating frequency, the general trends of total and elastic moduli at the air/water interface increased continuously with C, in contrast to their counterparts at the β-pinene/water interface. This indicates that over the whole C of this study, intermolecular steric repulsion is the dominant factor in determining the dilational elasticity at the air/water interface. The higher surfactant surface concentration (Γ) and the formation of a more tightly packed adsorption layer at the air/water interface would also hinder the diffusion ability of the surfactant molecules between the bulk and the interface, making the diffusion mechanism a less significant contributing factor to total and elastic moduli at the air/water interface. On the other hand, the dilational viscosity at the air/water interface presented a maximum with increasing C. It is generally believed that dilational viscosity reflects the summation of the various microscopic relaxation processes at and near the interface, along with diffusion and intermolecular steric repulsion. In this case, the relaxation (or rearrangement) of saponin molecules may be as important as the intermolecular steric repulsion, but in an opposite aspect. With increasing C, the saponin molecules would switch from being disorderly packed to being more tightly and orderly packed at the interface. The rearrangement process for more orderly packed molecules would become faster when the interface is disturbed, leading to a decrease in dilational viscosity. However, the formation of a stiffer, solid-like interfacial film at a higher concentration would yield an increase in dilational viscosity. As a result, the viscous moduli went through a maximum with increasing C.

4. Conclusions

In this work, the adsorption of saponin at the β-pinene/water and air/water interfaces was evaluated by dynamic interfacial tension σ(t) measurements using a drop shape tensiometer. We showed that for all the saponin concentrations, the experimental σ(t) data of saponin adsorbed at the two interfaces could be well fitted with the asymptotic forms of the Ward and Tordai model. The larger deviation between diffusion coefficients at the early and late stages with a higher saponin concentration suggests a switch of saponin adsorption kinetics, with increasing interfacial coverage, from diffusion-controlled to barrier-limited, at a later adsorption time. Saponin adsorption at the β-pinene/water interface had a relatively lower energy barrier than its adsorption at the air/water interface, with both barriers estimated to have a few molecular kinetic energy kBT. The equilibrium interfacial pressure πe data prior to critical micelle concentration (cmc) were reasonably described with the Gibbs adsorption isotherm. At the β-pinene/water interface, a higher cmc and a larger area per molecule, but a lower πe were observed compared to the air/water interface. Following the equilibration of the drop for measuring σ(t), dilational viscoelasticity of saponin adsorption layers at the two interfaces as a function of oscillating frequency and saponin concentration was measured by a sinusoidal oscillation method under small deformation. Results indicated that the dilational moduli of saponin at the β-pinene/water interface increased with increasing oscillating frequency, but the air/water interface displayed a stronger dependence of the moduli on frequency. Interestingly, the dilational moduli of saponin at the β-pinene/water interface went through a minimum value with increasing saponin concentration, while the viscoelasticity of the air/water interface showed a completely different trend of concentration dependence and a higher magnitude at the same saponin concentration. The findings in this study will contribute to our ongoing research avenues on developing biocompatible emulsions and oil-containing foams for many attractive applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/colloids9050068/s1, Figure S1: General chemical structure of (a) Quillaja saponin [2] and (b) β-pinene [22]; Figure S2: Dynamic interfacial tensions for β-pinene/water and air/water interfaces at 0.2 wt% saponin concentration.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

This research was funded by the government of Canada’s interdepartmental Program of Energy Research and Development (PERD). © His Majesty the King in Right of Canada, as represented by the Minister of Natural Resources, 2025.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. (a) Typical drop images acquired during the interfacial tension measurements. (b) Data of dynamic interfacial tension (σ(t)) for β-pinene oil drop and the air bubble in pure deionized water, and σ(t) for the air bubble in β-pinene.
Figure 1. (a) Typical drop images acquired during the interfacial tension measurements. (b) Data of dynamic interfacial tension (σ(t)) for β-pinene oil drop and the air bubble in pure deionized water, and σ(t) for the air bubble in β-pinene.
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Figure 2. Example showing the change in interfacial tension (left axis, black points) in response to sinusoidal oscillation in drop surface area (right axis, blue points) at a frequency of 0.02 Hz.
Figure 2. Example showing the change in interfacial tension (left axis, black points) in response to sinusoidal oscillation in drop surface area (right axis, blue points) at a frequency of 0.02 Hz.
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Figure 3. Dynamic interfacial tensions at different saponin concentrations for the (a) β-pinene/water interface and (b) air/water interface. Lines are the fits of Ward and Tordai model using Equation (3).
Figure 3. Dynamic interfacial tensions at different saponin concentrations for the (a) β-pinene/water interface and (b) air/water interface. Lines are the fits of Ward and Tordai model using Equation (3).
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Figure 4. Diffusion coefficients at early (D0) and late (D) stages of adsorption as a function of saponin concentration (C) for the (a) β-pinene/water interface and (b) air/water interface.
Figure 4. Diffusion coefficients at early (D0) and late (D) stages of adsorption as a function of saponin concentration (C) for the (a) β-pinene/water interface and (b) air/water interface.
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Figure 5. Adsorption energy barrier (Ea) for β-pinene/water and air/water interfaces as a function of saponin concentration (C).
Figure 5. Adsorption energy barrier (Ea) for β-pinene/water and air/water interfaces as a function of saponin concentration (C).
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Figure 6. Interfacial pressure (πe) as a function of saponin concentration for the β-pinene oil/water and air/water interfaces.
Figure 6. Interfacial pressure (πe) as a function of saponin concentration for the β-pinene oil/water and air/water interfaces.
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Figure 7. Dilational viscoelasticity of saponin adsorption at β-pinene/water and air/water interfaces as a function of oscillation frequency at a fixed saponin concentration of 0.02 wt%: (a) total modulus; (b) phase angle; (c) elastic modulus; (d) viscous modulus.
Figure 7. Dilational viscoelasticity of saponin adsorption at β-pinene/water and air/water interfaces as a function of oscillation frequency at a fixed saponin concentration of 0.02 wt%: (a) total modulus; (b) phase angle; (c) elastic modulus; (d) viscous modulus.
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Figure 8. Slope of a dual-logarithmic plot of total modulus against oscillation frequency as a function of equilibrium interfacial pressure for the following: (a) β-pinene oil/water interface and (b) air/water interface. The dashed lines show the respective surface pressures at the cmc (πe,cmc).
Figure 8. Slope of a dual-logarithmic plot of total modulus against oscillation frequency as a function of equilibrium interfacial pressure for the following: (a) β-pinene oil/water interface and (b) air/water interface. The dashed lines show the respective surface pressures at the cmc (πe,cmc).
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Figure 9. Dilational viscoelasticity of saponin adsorption at the β-pinene/water interface as a function of saponin concentration: (a) total modulus; (b) phase angle; (c) elastic modulus; (d) viscous modulus. Oscillation frequency was fixed at 0.02 Hz or 0.2 Hz, respectively.
Figure 9. Dilational viscoelasticity of saponin adsorption at the β-pinene/water interface as a function of saponin concentration: (a) total modulus; (b) phase angle; (c) elastic modulus; (d) viscous modulus. Oscillation frequency was fixed at 0.02 Hz or 0.2 Hz, respectively.
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Figure 10. Dilational viscoelasticity of saponin adsorption at the air/water interface as a function of saponin concentration: (a) total modulus; (b) phase angle; (c) elastic modulus; (d) viscous modulus. Oscillation frequency was fixed at 0.02 Hz or 0.2 Hz, respectively.
Figure 10. Dilational viscoelasticity of saponin adsorption at the air/water interface as a function of saponin concentration: (a) total modulus; (b) phase angle; (c) elastic modulus; (d) viscous modulus. Oscillation frequency was fixed at 0.02 Hz or 0.2 Hz, respectively.
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Table 1. Values of the cmc, surface excess Γ, and surface area As for saponin adsorption at the β-pinene/water and air/water interfaces.
Table 1. Values of the cmc, surface excess Γ, and surface area As for saponin adsorption at the β-pinene/water and air/water interfaces.
InterfacecmcΓAs
wt%mMμmol/m2nm2/molecule
pinene/water0.0500.410.572.93
air/water0.0150.122.64 0.63
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Lin, F. Adsorption and Dilational Viscoelasticity of Saponin at the β-Pinene/Water and Air/Water Interfaces. Colloids Interfaces 2025, 9, 68. https://doi.org/10.3390/colloids9050068

AMA Style

Lin F. Adsorption and Dilational Viscoelasticity of Saponin at the β-Pinene/Water and Air/Water Interfaces. Colloids and Interfaces. 2025; 9(5):68. https://doi.org/10.3390/colloids9050068

Chicago/Turabian Style

Lin, Feng. 2025. "Adsorption and Dilational Viscoelasticity of Saponin at the β-Pinene/Water and Air/Water Interfaces" Colloids and Interfaces 9, no. 5: 68. https://doi.org/10.3390/colloids9050068

APA Style

Lin, F. (2025). Adsorption and Dilational Viscoelasticity of Saponin at the β-Pinene/Water and Air/Water Interfaces. Colloids and Interfaces, 9(5), 68. https://doi.org/10.3390/colloids9050068

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