Wetting behavior of polyoxyethylene-type nonionic surfactant with multi-branched chains on solid surfaces

Abstract

Wetting of solid surfaces by surfactants is a fundamental phenomenon exploited in various applications, including cleaning, coating, dispersion, and adhesion, and in electronics materials. In this study, the interfacial properties and wettability of a polyoxyethylene (EO)-type nonionic surfactant with multi-branched chains (bC₇-bC₉EO_15.8) were systematically investigated and compared with those of corresponding linear double-(C₈-C₈EO_16.2) and single-chain (C_16.8EO_15.5) surfactants. Although bC₇-bC₉EO_15.8 exhibited a higher critical micelle concentration (CMC), it significantly reduced the surface tension to 26.3–27.0 mN m⁻¹, exhibiting excellent surface activity due to its methyl-rich branched structure. Additionally, contact angle measurements revealed that bC₇-bC₉EO_15.8 facilitated rapid spreading on hydrophobic surfaces even at low concentrations, with significantly lower contact angles on glass surfaces than those of its linear-type counterparts. This behavior was attributed to initial EO-chain adsorption followed by hydrophobic-chain reorientation, possibly involving micelle adsorption or partial bilayer formation at the solid/liquid interface. Moreover, bC₇-bC₉EO_15.8 consistently exhibited the highest wetting free energy across all substrates, regardless of surface polarity. These findings highlight the key role of branched hydrophobic chains in enhancing wettability.

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Introduction

The phenomenon of wetting on solid surfaces originates from differences in interfacial energies between solids and liquids, and is fundamental to various industrial applications, such as cleaning, floatation, dispersion, coagulation, dyeing, and waterproofing.^1–3 Wetting behavior is strongly influenced by chemical factors such as the molecular structure and interfacial interactions, as well as physical factors such as the surface roughness and charge, and is also highly affected by the adsorption behaviors of surfactants at solid/liquid interfaces. Wettability control through surfactant addition is widely applied in detergency,^4,5 powder dispersion,^6–9 penetration,¹⁰ coal dust removal and anthracite suppression,¹¹ oil recovery,¹² soil washing,^13,14 seed germination enhancement, and agricultural irrigation,^15,16 all of which are based on the polarity correlations between the surfactants and solid surfaces. Previous studies have investigated surfactant–surface interactions over a range of surface energies; that is, for low-(hydrophobic)^17,18 to high-energy (hydrophilic) surfaces.^19,20

Polyoxyethylene (EO) alkyl ether-type nonionic surfactants are widely applied because of their safety, low irritation, and nontoxicity, along with their excellent detergency, emulsification, dispersion, and penetration. EO-type nonionic surfactants are widely employed in various products, such as detergents and cosmetics, because their hydrophilic–lipophilic balance (HLB)²¹ can be easily changed by altering both the alkyl- and EO-chain lengths.^1,22

Double-chain surfactants, which have two hydrophobic groups and one hydrophilic group, exhibit slightly higher critical micelle concentrations (CMC), lower surface tensions, and superior penetration abilities than conventional single-chain surfactants.^10,23,24 Owing to their curvatures, which are similar to those of the phospholipids that comprise biological membranes, double-chain surfactants readily form bilayer structures and can assemble to vesicles or lamellar liquid crystals in aqueous solutions.^25–29 Cationic double-chain surfactants such as dioctadecyldimethylammonium chloride adsorb as monolayers on hydrophilic surfaces via electrostatic interaction with negatively charged surfaces;¹⁸ however, such adsorption is thermodynamically unfavorable because the alkyl chains are exposed to water. In contrast, nonionic double-chain surfactants adsorb as micelles rather than monomers on hydrophilic surfaces.^30,31 These findings suggest that wettability by nonionic surfactants may depend on micellization behavior; however, this relationship is not yet fully understood.

Multi-branched alkyl-chain surfactants exhibit lower surface tensions and superior penetration and emulsifying abilities than linear-type surfactants with the same total carbon number in the alkyl chain.^32–35 In particular, surfactants with methyl-rich branched chains exhibit enhanced adsorption at the air/liquid interface and improved micelle-forming abilities in aqueous solutions;³⁵ these properties may support control of the interfacial phenomena. To date, however, studies on wettability have focused on conventional single-chain surfactant systems,^18,19 additive-containing systems,¹⁶ and mixed surfactants.¹⁷ Detailed investigations of the influence of the molecular structure of a single surfactant on wettability remain limited, especially for EO-type nonionic surfactants with multi-branched double chains.

In this study, three EO-type nonionic surfactants—multi-branched type bC₇-bC₉EO_15.8 (where bC₇ and bC₉ represent branched alkyl chains with total carbon numbers of 7 and 9, respectively, and EO_15.8 is an EO chain with an average chain length of 15.8), linear double-chain type C₈-C₈EO_16.2 (where C₈ represents an octyl chain and EO_16.2 is an EO chain with an average chain length of 16.2), and linear single-chain type C_16.8EO_15.5 (where C_16.8 represents an alkyl chain with an average chain length of 16.8 and EO_15.5 is an EO chain with an average chain length of 15.5) (Fig. 1) are considered, and their surface-active properties at the air/liquid interface and wettability on solid surfaces such as polyethylene terephthalate (PET), polyurethane (PU), paraffin, and glass are investigated. Furthermore, the relationship between the hydrophobic structure and adsorption behavior at the air/liquid and solid/liquid interfaces is discussed.

Fig. 1 Structures of bC₇-bC₉EO_15.8, C₈-C₈EO_16.2, and C_16.8EO_15.5.

Materials and methods

Materials

The EO-type nonionic surfactants with multi-branched and linear double chains, bC₇-bC₉EO_15.8 (Fineoxocol 180-15EO) and C₈-C₈EO_16.2 (Fineoxocol 180T-15EO), respectively, were supplied by Nissan Chemical Corporation (Tokyo, Japan). The linear single-chain surfactant, C_16.8EO_15.5 (Nonion S-215), was obtained from NOF Corporation (Tokyo, Japan) and used as received. All surfactants were used without further purification. PET (Lumillor T60®; Toray Industries, Inc., Tokyo, Japan) and PU films (TG88-I, TPU, 100 μm thickness; Takeda Sangyo Co., Ltd, Tokyo, Japan) were used for the contact angle measurements. Parafilm M® (American National Can Co., Chicago, USA) and microscope glass slides (Matsunami Glass Ind., Ltd, Osaka, Japan) were used as substrates for the contact angle measurements. The HLB values of bC₇-bC₉EO_15.8, C₈-C₈EO_16.2, and C_16.8EO_15.5, were calculated as 14.8, 14.9, and 15.0, respectively, using Griffin's equation;^10,11 thus, the HLB characteristics were comparable.

Measurements

Aqueous solutions of the nonionic surfactant were prepared using ultrapure water (resistivity: 18.2 MΩ cm) from a Merck KGaA Direct-Q UV system (Darmstadt, Germany). The surface tension was measured at 25.0 ± 0.5 °C using temperature-controlled circulating water. The contact angle measurement was performed at room temperature (23 ± 1 °C).

Cloud point

The cloud points were determined from viscosity temperature profiles using a Brookfield LVDV2T viscometer (Middleboro, MA, USA). Clear 1.0 wt% aqueous solutions of the surfactants were prepared and placed in a refrigerator (∼5 °C) for a minimum of 24 h. The viscosity was measured at a contrast shear rate, while the solution was gradually heated at a rate of 1.2–1.5 °C min⁻¹. The cloud point was defined as the temperature at which the viscosity abruptly decreased in the viscosity–temperature plot. The solution temperature was controlled using temperature-controlled circulating water. The measurements were reproducible within ± 0.2 °C.

Equilibrium surface tension

The surface tension was measured using a Krüss K100C tensiometer (Hamburg, Germany) by the Wilhelmy plate technique. The surface excess concentration (Γ, mol m⁻²) and the occupied area per molecule (A) of each nonionic surfactant at the air/solution interface were calculated using the classic Gibbs adsorption isotherm equations: Γ = −(1/iRT)(dγ/dln C) and A = 1/(N_AΓ), where C, R, T, and N_A are the surfactant concentration, gas constant (8.31 J K⁻¹ mol⁻¹), absolute temperature, and Avogadro number, respectively. For the nonionic surfactants, the value of i was taken to be unity, corresponding to the number of possible species.

Contact angle

The contact angle was measured using a Krüss DSA30 drop shape analyzer (Hamburg, Germany) by the Sessile drop method.³⁶ A 2.0 μL droplet of each surfactant solution was placed on the solid substrate and the contact angle was recorded every 1 s for a period of 180 s at room temperature. The measurements were repeated for a minimum of five times at different substrate positions. The relationship between the contact angle (θ) and the interfacial tension is described by Young's equation:

γ_S = γ_L cos θ + γ_SL (1)

where γ_S and γ_L are the surface tensions of the solid and liquid, respectively, and γ_SL is the interfacial tension at the solid/liquid interface.

Results and discussion

Contact angle

The cloud points of the EO-type nonionic surfactants with multi-branched chains (bC₇-bC₉EO_15.8), linear double chains (C₈-C₈EO_16.2), and a linear single chain (C_16.8EO_15.5) were determined from the viscosity–temperature relationships of their 1.0 wt% aqueous solutions (Fig. S1). For all surfactants, the viscosity gradually decreased with increasing temperature, while the solutions remained visually clear. At specific temperatures, which were defined as the cloud points, the viscosities of bC₇-bC₉EO_15.8 and C₈-C₈EO_16.2 abruptly decreased; this behavior coincides with the onset of cloudiness in the solution. The cloud points were 73.0 and 68.3 °C for bC₇-bC₉EO_15.8 and C₈-C₈EO_16.2, respectively, both of which were consistent with visual observation. In contrast, the viscosity of the C_16.8EO_15.5 solution continued to decrease gradually beyond these temperatures, and the solution remained transparent up to 80 °C; therefore, this result indicates that the solution cloud point exceeded 80 °C. The cloud points of bC₇-bC₉EO_15.8 and for C₈-C₈EO_16.2 were significantly lower than that of C_16.8EO_15.5.

This trend suggests that the EO-chain hydration state was closely related to the cloud point, which is consistent with the findings of Schott.³⁷ Specifically, when the EO chains were located within the hydrophobic moiety, as opposed to being terminal, there were fewer associated water molecules and the cloud point was lower. These results indicate that the EO-chain linkage position and hydrophobic-group structure significantly affected the cloud point. However, such differences cannot be sufficiently explained using conventional parameters representing the overall hydrophilic/hydrophobic balance. In fact, the three EO-type nonionic surfactants examined in this study had similar HLB values (14.8–15.0), indicating comparable hydrophilic/lipophilic balances. Moreover, there were no substantial differences in their CMC values.³⁶ However, clear differences were observed in their cloud points; in particular, the lowest cloud point was observed for C₈-C₈EO_16.2. These results suggest that common metrics such as the HLB and CMC do no predict cloud point behavior, with molecular features such as the branched structure and the number of hydrophobic chains playing a more dominant role.

Previously, we reported the cloud points of structurally homogeneous EO-type surfactants; namely, bC₇-bC₉EO₁₂, C₈-C₈EO₁₂, and C₁₈EO₁₂, the cloud points of which were 43.2, 46.4, and 80.1 °C, respectively.³⁵ These findings demonstrated that increased hydrophobic chain complexity reduces water solubility. Interestingly, in the present study, the cloud point of bC₇-bC₉EO_15.8, which had a distribution of EO chain lengths, exceeded that of C₈-C₈EO_16.2. This behavior contrasts with the trend previously observed for homogeneous EO-type surfactants (bC₇-bC₉EO₁₂ and C₈-C₈EO₁₂), implying that the EO-chain length distribution and branched structure could influence the cloud point in a distinct manner. Cloud-point phenomena in nonionic surfactants are generally attributed to increased hydrophobicity due to dehydration of the EO chain, which in turn, alters the micelle aggregation numbers and structures.^37–39 In particular, a markedly low cloud point was observed for C₈-C₈EO_16.2, which was likely due to strong hydrophobic interactions between its two symmetric linear alkyl chains. These interactions reduce water solubility and promote the formation of larger or more densely packed aggregates, causing earlier phase separation. Surfactants with branched and/or multiple hydrophobic chains can influence micelle structures because of their bulky molecular configurations. Branched chain-type surfactants, in particular, tend to enhance the packing density within the micelle core, thereby promoting more compact micelles. Conversely, linear double-chain surfactants may induce transitions to higher-order structures–such as rod-like micelles and lamellar liquid crystals–because of the strong hydrophobic interactions occurring between the chains within the micelle core. Therefore, the observed differences in cloud points are attributed not only to the EO-chain hydration states but also to variations in the aggregate structures and densities, which are governed by the hydrophobic-group architectures.

Surface tension

Fig. 2 shows the surface tension as a function of concentration for each of the EO-type nonionic surfactants considered in this work: the multi-branched bC₇-bC₉EO_15.8, linear double-chain C₈-C₈EO_16.2, and linear single-chain C_16.8EO_15.5. In all cases, the surface tension decreased with increasing surfactant concentration, reached a distinct breakpoint that was regarded as the CMC, and remained constant. Table 1 lists the CMC values, surface tension at the CMC (γ_CMC), A, Γ, adsorption efficiency (pC₂₀), effectiveness of the adsorption and micellization process (CMC/C₂₀), and standard free energies of the surface adsorption and micelle formation ( and , respectively) determined from the surface tension curves.

Fig. 2 Variations in surface tension with concentration for EO-type nonionic surfactants.

Table 1 Physicochemical parameters obtained from surface tension curves of nonionic surfactants

Surfactant	CMC/mmol dm^−3	γ CMC/mN m^−1	Γ × 10^6/mol m^−2	A/nm^2	pC20	CMC/C20	/kJ mol^−1	/kJ mol^−1
bC7-bC9EO15.8	0.0108	26.6	2.60	0.640	6.71	55.8	−55.8	−38.3
C8-C8EO16.2	0.00444	28.8	2.93	0.567	6.78	26.8	−55.2	−40.5
C16.8EO15.5	0.00416	38.6	3.17	0.524	6.17	6.14	−51.2	−40.7

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The and values for these nonionic surfactants can be calculated using the following equations:^40,41

where π_CMC denotes the surface pressure at the CMC (π_CMC = γ₀ − γ_CMC; where γ₀ and γ_CMC denote the surface tensions of the water and surfactant solution at the CMC, respectively). The CMC values had the order C_16.8EO_15.5 < C₈-C₈EO_16.2 < bC₇-bC₉EO_15.8. This finding indicates that both the introduction of additional hydrophobic chains and the presence of branched structures raise the CMC and is consistent with a previous report.³⁵ Similarly, the absolute value of decreased in the same order: C_16.8EO_15.5 > C₈-C₈EO_16.2 > bC₇-bC₉EO_15.8, suggesting that micelle formation becomes less favorable with increased molecular complexity.

The γ_CMC values followed the order C_16.8EO_15.5 > C₈-C₈EO_16.2 > bC₇-bC₉EO_15.8. As methyl groups (–CH₃) have lower surface energies than methylene groups (–CH₂–),⁴² the abundant terminal methyl groups in the branched structure of bC₇-bC₉EO_15.8 likely contributed to its superior ability to lower surface tension. Higher pC₂₀ values were obtained for bC₇-bC₉EO_15.8 and C₈-C₈EO_16.2 than for C_16.8EO_15.5, indicating enhanced interfacial adsorption. This trend was consistent with the decrease in γ_CMC; that is, surfactants with higher pC₂₀ tended to exhibit lower γ_CMC because the adsorption at the air/liquid interface was more efficient. It is noteworthy that the γ_CMC of bC₇-bC₉EO_15.8 was lower than that of bC₇-bC₉EO_12.0 (28.8 mN m⁻¹),³⁵ despite having a longer and polydisperse EO chain. Although longer EO chains are generally associated with higher hydration and reduced surface activity, this result suggests that the EO chain length and its distribution may enhance the interfacial adsorption for certain branched architectures.

The surfactant performance per carbon atom in the hydrophobic group can be evaluated using the C_γ value:⁴³

where γ₀ is the surface tension of pure water (72 mN m⁻¹) and C_tot is the total number of carbon atoms in the hydrophobic group per molecule. The C_γ values of bC₇-bC₉EO_15.8, C₈-C₈EO_16.2, and C_16.8EO_15.5 were 2.7, 2.5, and 2.1, respectively. These results indicate that the multi-branched surfactant had the highest efficiency as regards reduction of the surface tension per hydrophobic carbon atom. Alexander et al.⁴³ investigated the influence of the hydrophobic chain structure on C_γ for sodium sulfate-type surfactants, and reported values of 2.1 and 2.5–2.6 for linear single-chain and multi-branched double chain surfactants, respectively. The C_γ of bC₇-bC₉EO_15.8 exceeded these values, showing a similar trend even among nonionic surfactants. Mohamed et al.⁴⁴ also reported a decrease in surfactant surface tension with the number of branched methyl groups: anionic single-chain surfactants with 3 and 9 branched methyl groups exhibited surface tensions of 29.8 and 29.7 mN m⁻¹, respectively, whereas a double-chain surfactant with two 2-ethylhexyl chains exhibited a higher value of 31.8 mN m⁻¹. Thus, the high surface tension-reducing performance of bC₇-bC₉EO_15.8, which has 8-branched methyl groups, is noteworthy.

The CMC/C₂₀ ratio, which represents the competition between adsorption and micelle formation, had the order C_16.8EO_15.5 < C₈-C₈EO_16.2 < bC₇-bC₉EO_15.8. This indicates that the multi-branched surfactant preferentially adsorbs at the air/liquid interface as opposed to micelle formation occurring in the bulk. Correspondingly, the absolute values of increased in the same order, highlighting the contribution of the branched structures to the interfacial adsorption stability.

The highest Γ was obtained for C_16.8EO_15.5, with a slightly lower value being obtained for bC₇-bC₉EO_15.8. In contrast, the largest A was obtained for bC₇-bC₉EO_15.8, suggesting that the bulky structure of this surfactant yielded a more dispersed adsorption conformation at the air/liquid interface. In general, bulky molecules have higher tight packing but promote surface tension reduction because of greater disruption of interfacial water. These trends agree with the findings of Alexander et al.⁴³ and Sagisaka et al.,⁴⁵ and elucidate the structure–function relationships governing interfacial adsorption for branched and multi-chain surfactants.

Wettability

The wetting behavior of aqueous solutions containing EO-type nonionic surfactants was evaluated based on contact angle measurements on various solid substrates. Fig. 3 shows the time-dependent changes in the contact angles of bC₇-bC₉EO_15.8, C₈-C₈EO_16.2, and C_16.8EO_15.5 solutions (0.0100–10.0 mmol dm⁻³) more than 180 s after deposition on a PET surface. For all surfactant solutions, a gradual decrease in contact angle from that of pure water (82°) was observed, with the degree and rate of decrease being strongly dependent on the surfactant concentration and hydrophobic group structures. For bC₇-bC₉EO_15.8, a sharp decrease was observed immediately following deposition at concentrations exceeding 0.0200 mmol dm⁻³, with the contact angle reaching 46° at 10.0 mmol dm⁻³. For C₈-C₈EO_16.2, a similar decreasing trend was observed for concentrations exceeding 0.0100 mmol dm⁻³; however, the contact angles were slightly higher than those of bC₇-bC₉EO_15.8. In contrast, C_16.8EO_15.5 exhibited a gradual reduction in contact angle across the entire concentration range. That is, the contact angle decreased gradually regardless of the concentration. The contact angles of C₈-C₈EO_16.2 and C_16.8EO_15.5 at 0.0100 mmol dm⁻³ were 82° and 83°, respectively, and decreased to 59° and 61°, respectively, at 10.0 mmol dm⁻³. Although the final contact angles were similar, a more rapid decrease over time was observed for C₈-C₈EO_16.2, indicating faster wetting kinetics. These findings indicate that the spreading rate and extent on PET followed the order C_16.8EO_15.5 < C₈-C₈EO_16.2 < bC₇-bC₉EO_15.8, and that an increased number of alkyl chains and branching enhanced the wettability.

Fig. 3 Time-dependent variations in contact angles of nonionic surfactants (top) and droplet morphology images at concentration of 0.100 mmol dm⁻³ (bottom) over 180 s following dropping onto PET surface at 23 °C: (a) bC₇-bC₉EO_15.8, (b) C₈-C₈EO_16.2, and (c) C_16.8EO_15.5.

On paraffin surfaces, the contact angle also decreased rapidly at a specific concentration, before leveling off (Fig. S2); this behavior was similar to that on PET. On PU surfaces, the contact angles at low concentrations (∼0.0100 mmol dm⁻³) remained close to that of water (104°). However, decreases were observed for concentrations exceeding 0.0100, 0.100, and 0.200 mmol dm⁻³ for bC₇-bC₉EO_15.8, C₈-C₈EO_16.2, and C_16.8EO_15.5, respectively (Fig. S3). This suggests that wetting was promoted by an increased number of alkyl chains and branched methyl groups on the PU surface, similar to the PET surface. On the PU surface, the contact angles decreased once the concentrations exceeded the CMC, unlike the results for the other substrates. This outcome indicates that a saturated air/liquid interface enhances PU wetting. On glass, a transient increase in contact angle was observed for bC₇-bC₉EO_15.8 at low concentrations, which was followed by a sharp decrease over time. In contrast, immediate decreases in contact angle with no initial peak were observed for C₈-C₈EO_16.2 and C_16.8EO_15.5 (Fig. S4). This time-dependent variation followed the same order as the PET case, again suggesting that branched and multi-chain structures facilitate rapid spreading, regardless of substrate polarity. He et al.⁴⁶ reported that increased branching in hydrophobic chains enhances the surface packing density of –CH₃ and –CH₂– groups, rapidly reducing the surface energy. This supports the observed high wettability of bC₇-bC₉EO_15.8, which is likely due to faster and denser adsorption on solid surfaces.

Fig. 4 shows contact angles measured 10 s after droplet deposition as functions of concentration on PET, paraffin, PU, and glass surfaces. For glass, in all cases except bC₇-bC₉EO_15.8 on glass, the contact angle decreased with increasing concentration in all cases except that of bC₇-bC₉EO_15.8. Above a certain threshold concentration, a sharp decrease was observed, which was followed by a gradual decrease. These profiles suggest a two-step wetting mechanism: initial saturation of the air/liquid interface followed by enhanced adsorption at the solid/liquid interface.⁴⁷ Notably, the concentrations at which the contact angle decreased significantly followed the order bC₇-bC₉EO_15.8 < C₈-C₈EO_16.2 < C_16.8EO_15.5; this trend is opposite to that of the CMC values. This outcome suggests that, while the CMC represented micellization onset, enhanced wettability occurred at concentrations exceeding CMC and was more closely related to γ_CMC. Surfactants with lower γ_CMC tend to adsorb more effectively at air/liquid and solid/liquid interfaces, which contributed to a more pronounced reduction in contact angle even at lower concentrations. According to Young's equation, as the interfacial tension at the solid/liquid interface (γ_SL) decreases, the contact angle (θ) decreases accordingly. Consequently, interfacial saturation likely occurred for bC₇-bC₉EO_15.8 at lower concentrations than those of C₈-C₈EO_16.2 and C_16.8EO_15.5, which yielded a significant reduction in contact angle. In particular, the enhanced wettability of bC₇-bC₉EO_15.8 is thought to stem from effective lowering of the surface free energy through dense adsorption of the branched methyl chains in the alkyl group. The marked decrease in contact angle observed for bC₇-bC₉EO_15.8 in this study suggests that the enhanced wettability was supported by not only monomer adsorption but also the formation of micelle-like aggregates and multilayered structures at the solid/liquid interface, as facilitated by the unique branched structure of this surfactant.^45,46,48,49

Fig. 4 Variation in contact angle with concentration for EO-type nonionic surfactants 10 s following dropping onto (a) PET, (b) paraffin, (c) PU, and (d) glass surfaces at 23 °C.

On glass surfaces, for bC₇-bC₉EO_15.8, the maximum contact angle was obtained at 0.0200 mmol dm⁻³. This was followed by a rapid decrease, and the minimum value was reached at a lower concentration than those of the linear analogs. The initial increase was attributed to EO chain adsorption on the hydroxy groups of the glass surface by the hydrogen bonds, which exposed hydrophobic alkyl chains to the water side, causing temporary hydrophobization. As the concentration increased, multilayer adsorption and aggregate formation reversed the surface to a hydrophilic state, drastically lowering the contact angle through the adsorption of hydrophobic groups of many surfactant molecules. Similar maxima have been reported for ionic surfactants^48,50,51 and for certain nonionic surfactants such as mixtures of p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethyleneglycol) of Triton X-100 and Triton X-165 (where X-100 and X-165 represent EO₁₀ and EO_16.5, respectively)^47,49,52 and silicone-based surfactants.⁵³ These suggest the occurrence of temporary monolayer formation followed by multilayer reorganization. Such behavior supports the conclusion that bC₇-bC₉EO_15.8 first forms a monolayer exposing hydrophobic groups, then rearranges into a multilayer, causing the sharp decrease in contact angle. This behavior suggests that the hydrophobic effect of the hydrophobic groups of the multi-branched surfactant facing the water side exceeds that of the linear-type surfactants.

As regards the time-dependent contact angle behavior on EO-type surfactants (Fig. S5–S8), the order C_16.8EO_15.5 > C₈-C₈EO_16.2 > bC₇-bC₉EO_15.8 was found. This result indicates that the wettability was enhanced by both an increased chain number and branching. This behavior was independent of the surface polarity: bC₇-bC₉EO_15.8 exhibited excellent wettability even on highly hydrophobic paraffin and PU surfaces. Although the contact angles of the EO-type surfactants decreased with water evaporation, the experiments were conducted under uniform conditions such as uniform concentration, droplet volume, temperature, and dropping rate. Thus, comparison of the wetting behavior was possible, and predominant differences were observed depending on the structures of the hydrophobic groups. According to Yoon and Ravishankar,⁵⁴ the surface hydrophobicity correlates with the water contact angle, with 90° as a critical boundary. In this study, the water contact angles were ordered as follows: paraffin (111°) > PU (104°) > PET (82°) > glass (12°) (Fig. S9), reflecting increasing polarity. For C_16.8EO_15.5 at 1.00 mmol dm⁻³, contact angle was in the order glass < PET < paraffin ≃ PU, whereas for bC₇-bC₉EO_15.8 and C₈-C₈EO_16.2 the order was glass < PET ≃ PU ≃ paraffin, indicating superior spreading on hydrophobic substrates as well. In conclusion, these results reveal excellent wetting and spreading ability even on more hydrophobic substrates for the surfactants with multi-branched and linear double chains compared to the single-chain surfactants. This highlights the significant impact of the hydrophobic-group architecture on the wettability and provides important insights for the design of advanced surfactant systems.

Jiang and Sandler⁵⁵ conducted Monte Carlo simulations of the adsorption and separation of linear and branched alkanes and reported that adsorption and desorption are reversible for linear alkanes, with the adsorbed amount decreasing with an increasing degree of branching. Compared to n-pentane and isopentane, neopentane exhibited a considerably lower absolute value of ΔS°, indicating that the alkane configuration of neopentane is a pseudospherical molecule, unlike the linear alkanes.⁵⁵ Furthermore, Dickson et al.⁵⁶ investigated the interface behavior via molecular simulation and reported that stubby surfactants with multi-branched methyl chains block interactions between water and CO₂ at the CO₂/water interface, thereby lowering the surface tension to favor formation of W/CO₂ microemulsions.⁵⁶ These findings are consistent with the results of the present study. The pseudospherical structure of the multi-branched alkyl groups caused formation of micelle-like aggregates and multilayered structures, along with temporary hydrophobization on glass surface. Note that the multi-branched surfactant efficiently reduced the surface tension and exhibited excellent wettability, despite the small amount of adsorption (small surface excess concentration Γ) due to the bulkiness.

The surface free energy change (ΔG) related to wetting was calculated using the Extrand equation:⁵⁷

Table 2 summarizes the ΔG values of the EO-type nonionic surfactants derived from the contact angle at 10 s. As the concentration increased from 0.100 to 1.00 mmol dm⁻³, the absolute values of ΔG (|ΔG|) increased, indicating enhanced wettability. Compared to values reported for ionic surfactants (−1.01 and −1.10 kJ mol⁻¹ on glass for didodecylammonium bromide and the anionic surfactant sodium bis(2-ethylhexyl)sulfosuccinate, respectively⁵⁰), larger |ΔG| were obtained for the EO-type surfactants, supporting their superior wetting properties. At 1.0 mmol dm⁻³ concentration, |ΔG| followed the order C_16.8EO_15.5 < C₈-C₈EO_16.2 < bC₇-bC₉EO_15.8 across all substrates. For C₈-C₈EO_16.2 and C_16.8EO_15.5, the |ΔG| order was PU < paraffin < PET < glass, whereas for bC₇-bC₉EO_15.8, the order was PET ≃ paraffin ≃ PU < glass, indicating greatly enhanced wettability on paraffin and PU for the branched surfactant. Finally, time-resolved contact angle data indicated rapid spreading of bC₇-bC₉EO_15.8 upon deposition, likely due to fast adsorption promoted by the bulky, branched structure of this surfactant. These findings provide important insights into the relationship between the molecular structures of surfactants and their wettability behavior on solid surfaces. In particular, the incorporation of multi-branched alkyl chains and an increase in the number of hydrophobic chains significantly enhanced the interfacial adsorption, yielding improved wettability regardless of the substrate polarity. These results suggest that the wettability of EO-type nonionic surfactants depends not only on the thermodynamic equilibrium of adsorption but also on the adsorption dynamics and orientation of surfactant molecules at the interfaces. The unique molecular structures of these surfactants, including their chain numbers and branching, strongly govern their interfacial behaviors, providing valuable insights for the future design of high-performance surfactants for industrial applications.

Table 2 Wetting free energies of EO-type nonionic surfactants 10 s after dropping onto PET, paraffin, PU, and glass surfaces at 23 °C

Surfactant	Concentration/mmol dm^−3	ΔG/kJ mol^−1
		PET	Paraffin	PU	Glass
bC7-bC9EO15.8	0.100	−1.61	−1.61	−1.25	−7.97
	1.00	−2.87	−2.69	−2.99	−14.4
C8-C8EO16.2	0.100	−1.08	−1.14	−0.540	−7.77
	1.00	−2.08	−1.89	−1.68	−10.4
C16.8EO15.5	0.100	−1.23	−0.628	−0.430	−6.97
	1.00	−1.33	−0.890	−0.807	−9.13

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Conclusions

In this study, the effects of the number and branched structure of the alkyl chains of EO-type nonionic surfactants–specifically, multi-branched chain bC₇-bC₉EO_15.8 with an EO chain length distribution-on interfacial adsorption and wetting behavior were systematically investigated, through comparison with corresponding linear surfactants (double-chain type C₈-C₈EO_16.2 and single-chain type C_16.8EO_15.5). Although bC₇-bC₉EO_15.8 exhibited a high CMC, an excellent ability to lower the surface tension and achieve efficient adsorption and orientation at the air/liquid interface was observed. The methyl-rich and bulky branched structure of this surfactant facilitated dense interfacial adsorption and orientation, significantly improving the wettability on solid surfaces compared to those of the linear chain surfactants.

The wetting behavior on a glass surface was particularly noteworthy: bC₇-bC₉EO_15.8 exhibited a transient maximum in the contact angle with increasing concentration, followed by rapid wetting and spreading. This phenomenon likely arises from temporary hydrophobization of the glass surface due to initial EO-chain adsorption and hydrophobic-group exposure, followed by the formation of micelles or multilayer structures that promote hydrophilicity and drastic wetting and spreading. Furthermore, analysis of the time-dependent variation in the contact angle and the associated surface free energy changes revealed that an increased number of alkyl chains and the introduction of branched structures promote adsorption at the solid/liquid interface; hence, the wettability of nonionic surfactants is dramatically enhanced. The adsorption and micelle-forming behaviors of multi-branched EO-type surfactants can be discussed thermodynamically in more detail by examining the effect of temperature. We will investigate this aspect in future research, to clarify the properties induced by the multi-branched chains.

These findings deepen understanding of the structure–function relationships of surfactants at both air/liquid and solid/liquid interfaces. They also provide valuable insights for the molecular design of highly functional surfactants for use in a wide range of applications, including cleaning, dispersion, coating, and adhesion, and in electronic materials. In particular, these surfactants have potential application in high-performance coating and cleaning formulations, for which rapid wetting and low surface tension are essential.

Author contributions

Risa Kawai: investigation, writing – original draft. Nana Tsutsui: investigation. Kotoha Ueno: investigation. Shiho Yada: resources, validation. Masashi Ohno: resources, validation, Toshinari Koda: resources, validation. Tomokazu Yoshimura: conceptualization, supervision, writing – review and editing.

Conflicts of interest

The authors declare that they have no competing financial interests or personal relationships that could have influenced the work reported in this study.

Data availability

The data supporting this article have been included as part of the SI.

Additional experimental data, including cloud point; and contact angle. See DOI: https://doi.org/10.1039/d5cp02321a

Acknowledgements

The authors thank Editage (https://www.editage.jp) for English language editing.

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