Neutron Reflectometry on Superspreading and Non-Superspreading Trisiloxane Surfactants
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Neutron Reflectometry on Superspreading and Non-Superspreading Trisiloxane Surfactants
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Langmuir

Cite this: Langmuir 2025, 41, 47, 31839–31848
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https://doi.org/10.1021/acs.langmuir.5c03781
Published November 20, 2025

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Abstract

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Certain trisiloxane surfactants have the remarkable property of being able to superspread: small volumes of the surfactant solution rapidly wet large areas of hydrophobic surfaces. The molecular properties of the surfactants that govern this technologically relevant process are still under debate. To gain a deeper understanding, the surfactant behavior during the spreading process needs to be studied at molecular length scales. Here, we present neutron reflectivity analyses of two trisiloxane surfactants of similar chemical structure, of which only one exhibits superspreading properties. We present an approach to determining the composition of the adsorbed surfactant layer in spread surfactant films at the solid–liquid interface, accounting for contributions from attenuated back-reflections of the neutron beam in films with thicknesses in the range of several tens to hundreds of micrometers. Differences between superspreading and non-superspreading surfactants with regard to their volume fraction profiles at the solid/liquid interface obtained in the self-consistent analysis of the reflectivity curves are in agreement with a simple explanation of the difference in spreading behavior based on thermodynamics.

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Introduction

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A number of trisiloxane (TSO) surfactants exhibit the capability to cause small volumes of diluted aqueous solution to spread to surprisingly large diameters on sufficiently hydrophobic surfaces, a phenomenon known as superspreading. When placed onto a hydrophobic surface such as a polypropylene film, a small droplet of 50 μL solution of 0.1 wt % superspreader surfactant can be expected to spread to an area of 70–80 mm diameter after 1 min, (1) as shown in the Supporting Information (Figure S1). This behavior is not only limited to TSO surfactants, but other surfactants also show this behavior. (2) Superspreading has also been observed in mixtures of cationic and anionic surfactants. (3) This exceptional wetting ability has significant practical applications, particularly as an agricultural adjuvant, where it ensures fast, uniform wetting of a plant’s hydrophobic leaves, increased penetration of active ingredients into the plant, and allows for stomatal flooding, all while lowering spray volumes by 30%. (4)
Superspreading is well documented and is explained in terms of the system’s free energy, expressed as the spreading coefficient,
S=γS−(γSL+γL)
(1)
where γS is the surface tension of the bare solid, γL is the tension of the surfactant solution’s liquid surface, and γSL is the interfacial tension between the solid and the solution. Spreading, i.e., complete wetting, occurs as long as S is positive. (5)
One such TSO surfactant is BREAK-THRU S 240 (see Figure 1, abbreviated as S240 in the following), which has the structure M(D′R)M where M represents a trimethylsiloxy group, (CH3)3SiO1/2–, D′R represents −O1/2Si(CH3)(R)O1/2–, and R represents a polyether chain consisting of a statistical mixture of ethylene oxide (p) and propylene oxide (q) units, in this case p = 6 and q = 3. Interestingly, there is a structurally quite similar TSO surfactant, BREAK-THRU S 233 (abbreviated as S233, see also Figure 1), which does not exhibit superspreading. S233’s polyether is longer and consists of more alkylene oxide units (p = 10, q = 2). Therefore, the difference in behavior is dictated only by the R polyether monomer composition and length (see Figure 1).

Figure 1

Figure 1. Chemical structure of S233 (p = 10, q = 2) and S240 (p = 6, q = 3). The structure consists of a hydrophobic trisiloxane group with a hydrophilic polyether chain whose monomer composition differs between the two molecules in terms of the number of ethylene oxide (p) and propylene oxide units (q).

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While considerable effort has been made to determine the exact mechanism of superspreading, the process remains challenging to elucidate due to the rapid speed of spreading, which makes real-time observation with molecular resolution difficult. Reflection anisotropy spectroscopy has provided some insights by comparing surfactant adsorption on hydrophobic substrates, where it has revealed that superspreading S240 solutions exhibit anisotropic structures, while non-superspreading S233 behaves isotropically. (6) However, this technique was unable to determine specific anisotropic structural differences.
Many studies have focused on superspreading TSO surfactants, such as S240 only, often attributing their superspreading properties to their compact T-shaped TSO group. However, this will lead to oversimplified conclusions when ignoring the structurally similar non-superspreading TSO surfactants. Efforts to compare these surfactants directly yielded valuable insights. Wang et al., for example, used high-speed imaging to investigate the wetting kinetics of TSO surfactants and other organic surfactants, such as C12TAB and SDS. (7−9) Kovalchuk et al. slowed down the spreading process by adding glycerol to the surfactant solution, allowing for more detailed observations. (10)
Simulations and theoretical models have shed further light on the mechanics of superspreading. Computational studies, while resource-intensive due to the need to capture molecular-level interactions over macroscale timeframes, suggest that superspreading involves a unique interfacial behavior. Large-scale molecular dynamics simulations by Isele-Holder et al. indicate that the three-phase contact line (TPCL) in superspreading systems exhibits a “rolling” transition rather than a sharp corner. This rolling transition minimizes energy at the interface and supports the rapid movement of the TPCL at approximately 500 μm/s. (11) This behavior contrasts with non-superspreading systems, where a fixed contact angle limits motion.
Marangoni flow has often been proposed as the driving mechanism for superspreading, with many authors attributing it to surface tension gradients along the droplet surface. (10,12−14) One commonly cited piece of evidence for this theory is the observation that superspreading is most effective at a surfactant concentration of 0.1 wt %, while higher or lower concentrations are less effective, suggesting a role for Marangoni-driven redistribution. However, it has been well-known for decades that this peculiar concentration dependence (“a lot does not help a lot”) is not present when preventing evaporation. Also, there is sufficient experimental evidence that surface tension gradients do not play a role in superspreading. (9) While Marangoni effects might contribute to the initial redistribution of surfactant molecules, they are unlikely to sustain the rapid and prolonged spreading characteristic of superspreading surfactants. (15)
An alternative hypothesis that we consider more convincing is that of a rolling mechanism, in which superspreading surfactants, like S240, form bilayers (16) with minimal spontaneous curvature. These bilayers could “unzip” at the surface of the spreading drop, effectively transferring surfactant molecules from the bulk solution to the air–liquid interface. This unzipping action provides a continuous supply of surfactants to maintain rapid spreading, as illustrated in Figure 6B of ref (15). Supporting evidence comes from foam film studies, which demonstrate distinct differences between superspreading and non-superspreading surfactants in their ability to form stable bilayer structures under dynamic conditions. (17)
Irrespective of the specific spreading dynamics, it is important to acknowledge the gaps in our understanding of the equilibrium situation, which is described by eq 1 and ultimately determines whether or not superspreading can occur.
Both of these surfactants have almost identical liquid surface tensions at the typical spreading concentration of 0.1 wt % (γL = 21.5 mN/m vs γL = 22.5 mN/m, respectively (18)). Since γS in eq 1 is surfactant-independent, the difference determining the sign of S (and thus the occurrence of superspreading) must therefore be in the value of γSL. While γSL is not directly accessible experimentally, it can, however, be safely assumed that this interfacial tension is governed by the organization and coverage of surfactants adsorbed at the solid–liquid interface. This can be rationalized in the following way: the more a surfactant layer can minimize unfavorable contact of water and the hydrophobic surface, the lower γSL will be. Consequently, a method that can provide information about the molecular organization at the interface is necessary.
Specular neutron reflectivity (NR) has been widely used to study the structure and organization of lipid and surfactant layers at solid/liquid interfaces. (19−21) The ability to resolve the structural details of molecular-sized films down to nanometer spatial resolution makes it an ideal technique in studying the interfacial behavior of these surfactant molecules by determining the laterally averaged density profiles perpendicular to the surface. In addition, NR is nondestructive and has the advantage of using selective deuteration or heavy water to create contrast between chemical components. The technique has already been applied to TSO surfactants to study the temperature-dependent adsorption structure of TSO surfactants on a titanium oxide surface. (22)
In the present work, the behavior of superspreading and non-superspreading TSO surfactants at the solid/liquid interface is systematically characterized using NR with the aim of shedding light on the molecular organization in such films. Measuring NR on any liquid after spreading is difficult due to the nature of a circular puddle not being able to fully cover the rectangular neutron beam footprint. Furthermore, the superspreading results in films that are too thin to neglect the back reflection from the upper surface and, at the same time, too thick to neglect beam attenuation. Accounting for attenuated reflectivity contributions from the air–water interface on the top side of the film is therefore required when analyzing the reflectivity data.

Results

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This study systematically characterizes the behavior of TSO surfactants at the solid–liquid interface, alongside complementary measurements of their behavior at the air–water interface. Building on the hypothesis introduced earlier, we aim to clarify the role of surfactant interfacial organization, particularly at the solid interface, in determining the spreading behavior of superspreading and non-superspreading surfactants. To prepare the solid surfaces for superspreading, the silicon blocks were hydrophobically functionalized with chlorotrimethylsilane (CTMS) by creating a molecularly thin layer of organosilyl groups from −Si(CH3)3 attached to the surface, thereby increasing the water contact angle to ≈77°. All experiments involving solid substrates were performed under conditions of saturated humidity, achieved with a large water bath (D2O) surrounding the sample inside a closed measurement chamber. An illustration of this setup can be found in the Supporting Information (Figure S2).
The reflectivity curves obtained with superspread and non-superspread droplets were modeled together with reference reflectivity measurements of bare solid surfaces and surfactant-loaded air/water interfaces in a self-consistent manner involving a number of common parameters. To maximize comprehensibility, the results are presented starting from the simplest (reference) configurations and then moving to the configurations that are more complicated to describe.

Reference Measurements

Newly silanized blocks were characterized by NR first, to determine a number of basic reference parameters for the models used in subsequent analyses. These reflectivity curves, obtained in air (close to 100% humidity) and under water, are shown in Figure 2A,C, respectively, with solid red lines indicating the best-matching fits, which provide the volume fraction profiles shown alongside. The reflectivity curves in air and under water were modeled with a common parameter set using a volume-fraction-based approach in the spirit of our earlier work. (23,24) In this model, the SLD profile as a function of depth z, ρ(z), is calculated from the volume fraction profiles of all chemical components ϕj(z) and from their SLDs ρj, where j identifies the chemical component:
ρ(z)=ρSiϕSi(z)+ρoxiϕoxi(z)+ρsilϕsil(z)+ρwatϕwat(z)
(2)

Figure 2

Figure 2. Neutron reflectivity data (A, C, E) and deduced volume fraction profiles (B, D, F) of the reference systems: (A, B) Bare silanized silicon block in air, (C, D) bare silanized silicon block in water, and (E, F) air–water interface of a 0.1 wt % surfactant solution (shown is S240). Solid lines in panels (A, C, E) indicate the best fits to the data that correspond to the volume fraction profiles in panels (B, D, F).

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The last term, ρwatϕwat(z), applies only to the measurement under water because air has negligible SLD. In this case, ϕwat(z) follows from the condition that the sum of all volume fractions amounts to 1 at each z-position:
∑jϕj(z)≡1
(3)
The silicon substrate (j = “Si”) is modeled as a semi-infinite continuum with a constant SLD ρSi. The oxide layer (j = “oxi”) was represented as a rough homogeneous layer with SLD ρoxi and was allowed to have a finite water fraction Φoxihydr, which resulted in a value of 17% after fitting, which is the case for silicon blocks under water and in water-saturated air. The thickness, doxi ≈ 16–19 Å, and the Si/oxide roughness, σSi/oxi ≈ 5 Å, obtained in the fit are in agreement with earlier work. (24,25) Silane (j = ‘sil’) was also represented as a rough homogeneous slab. Its thicknesses and roughnesses were obtained in the fit as dsil ≈ 3.5 Å, σoxi/sil ≈ 1.5 Å, and σsil/air = σsil/wat ≈ 1.5 Å. These values agree with the expected molecularly thin organosilyl layer. The volume fraction profiles corresponding to the solid surfaces in air and under water are shown in Figure 2B,D, respectively. We did not detect any water adsorbed to the hydrophobic surface. This suggests that the air was not oversaturated with water and that condensation on the surface was avoided.
The third reference examined the air–water interface of a 0.1 wt % surfactant solution using a Langmuir trough. The corresponding SLD profile is
ρ(z)=ρTSOϕTSO(z)+ρPOLϕPOL(z)+ρwatϕwat(z)
(4)
The adsorbed surfactants were modeled with two rough slabs in direct contact, a hydrophobic TSO group (j = ‘TSO’) and a hydrophilic polyether chain (j = “POL”) as shown in Figure 1. These had adjustable thicknesses (dTSO and dPOL) and theoretical maximal volume fractions, ΦTSO0 and ΦPOL0, valid in the limit of negligible interface roughness. The associated water fractions are assumed to occupy any remaining space up to the TSO/POL interface, as shown in the corresponding volume profiles in Figure 2F, so that the water fraction in the polyether slab is ΦwatPOL = 1 – Φ0POL. The maximum volume fractions of TSO and POL after accounting for interfacial roughness, denoted as ΦmaxTSO and ΦmaxPOL, are slightly lower than ΦTSO0 and ΦPOL0 and can be visually extracted from the volume fraction profile in the figure. The parameters of ϕTSO(z) and ϕPOL(z) were not entirely independent but constrained such that each of the two moieties occupies an overall volume consistent with the ratio of their known molecular volumes, which are given in Table 2. The scattering length densities ρTSO and ρPOL were fixed at the calculated values, as also shown in Table 2.
For superspreading S240, the fits yield thicknesses of dTSO ≈ 7.5 Å and dPOL ≈ 13.5 Å. Both values align with expectations, considering, on one side, the diameter of the Si(CH3)3 groups and, on the other side, a polyether with 9 repeat units (p = 6 and q = 3) of ≈4 Å length, assuming a considerably contracted, quasi-random conformation. In the volume fraction profiles in Figure 2F the surfactants are shown to form a compact layer with the hydrophobic moiety reaching ΦmaxTSO ≈ 74% and the hydrophilic chain reaching ΦmaxPOL ≈ 63%. The two interfaces involving TSO are relatively sharp, but the transition of the polyether layer toward water is rather gradual, which reflects that this is not a dense layer but a bit like a hydrated polymer brush, for which similar profiles have been reported, albeit typically at even higher hydration levels. (23−26) The key parameters of the surfactant layers at the air–water interface are summarized in Table 1. For the non-superspreader S233 shown in the Supporting Information (Figure S3), we obtain the same TSO thickness (dTSOS233 ≈ 7.5 Å) and comparable values of ΦmaxTSO and ΦmaxPOL, but the polyether profile is significantly more extended (dPOLS233 ≈ 16.0 Å), as expected for a slightly longer polyether contour length (12 units, p = 10 and q = 2).
Table 1. Parameters for the Surfactants S240 and S233 Were Obtained from Fits of the Reflectivity Data at the Air/Water and Solid/Liquid Interfacesa
 surfactant
parameterS240S233
dTSO (±1.0)7.5 Å7.5 Å
dPOL (±1.0)13.5 Å16.0 Å
air/water interface
ΦPOL0 (±0.05)0.770.81
ΦPOLmax (±0.05)0.630.70
ΦPOLwat (±0.05)0.230.19
solid/liquid interface
ΦPOL0 (±0.05)0.770.67
ΦPOLmax (±0.05)0.630.59
ΦPOLwat (±0.05)0.230.34
ΦTSO0 (±0.05)1.000.83
ΦTSOmax (±0.05)0.830.67
ΦTSOwat (±0.05)0.000.17
a

Φj0 is the maximal volume fraction of layer j under hypothetical “no-roughness” conditions, Φjmax is its maximal volume fraction after roughness is applied, Φjwat is its water fraction, and dj is its thickness parameter.

The surfactant-loaded liquid/air interfaces were also investigated with total reflection X-ray fluorescence (TRXF) and grazing-incidence X-ray off-specular scattering (GIXOS). (27−35) The results and a more detailed discussion can be found in the Supporting Information. Most notably, TRXF revealed a slightly higher packing density (≈8% higher) for S240 than that for S233.

S240 Layers Adsorbed to the Solid/Solution Interface

To isolate the structure of surfactant layers adsorbed to the solid/liquid interface, we first measured reflectivity from a thick droplet of S240 solution obtained by the addition of 200 μL of D2O to a superspread puddle of S240 (0.1 wt % 10 μL). The surfactant concentration dropped to approximately 0.05 g/L, which is only slightly below the critical aggregation concentration (CAC) of S240 (approximately 0.07 g/L). (36) The addition of D2O did not lead to further spreading. The total number of surfactant molecules is sufficient to create two monolayers of surfactant (one at the solid/liquid interface and one at the air/water interface) covering the area of the initial puddle that was created by the spreading of the 10 μL drop. This amount is, however, insufficient to cover the area newly created by increasing the droplet volume when water is added. The puddle, therefore, slightly retracted laterally. It was then equilibrated for at least 30 min prior to the measurement. The resulting scenario is illustrated in Figure 3A and simplifies the analysis because, in contrast to a thin film, one does not have to account for reflectivity contributions from the air/water interface, as discussed further below.

Figure 3

Figure 3. Illustration of a “thick” droplet of S240 solution obtained by the addition of 200 μL D2O to a superspread puddle of S240 (0.1 wt % 10 μL) (A). Neutron reflectivity data (B) and deduced volume fraction profiles (C) of this system. Solid lines in panel (B) indicate the best fits to the data that correspond to the volume fraction profiles in panel (C).

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Figure 3B shows the reflectivity curve obtained with this sample. Again, the solid red line indicates the simulated reflectivity curve corresponding to a volume-fraction-based model.
In this experiment, the droplet is smaller than the beam footprint, necessitating an adjustable surface coverage parameter, xc, in the reflectivity model. The reflectivity is thus modeled as a superposition of covered and uncovered regions:
R(qz)=xcRc(qz)+(1−xc)Rnc(qz)
(5)
where Rnc(qz) represents the reflectivity of the non-covered silanized silicon block in air, as shown in Figure 3A and described with eq 2 without the term ρwatϕwat(z).
Rc(qz) represents the reflectivity of the covered regions. Here, we integrate eqs 2 and 4 into one expression for the description of the surfactant-loaded solid/solution interface:
ρ(z)=ρSiϕSi(z)+ρoxiϕoxi(z)+ρsilϕsil(z)+ρTSOϕTSO(z)+ρPOLϕPOL(z)+ρwatϕwat(z)
(6)
The volume fraction profiles of each component were described in the same way as described in the previous paragraphs; however, several parameters associated with the surfactants were allowed to assume different values at the solid/liquid interface than at the air/water interface. These were the roughness parameters as well as the coupled parameters for the maximal volume fractions of TSO and POL, in order to generally allow for different surfactant coverages at the two different interface types. The water volume fraction profile was again obtained through the constraint of eq 3.
The model reproduces the experimental data well, as seen in Figure 3B. The volume fraction profiles obtained from the fit are shown in Figure 3C, and the key parameters are summarized again in Table 1. For S240, we observe a very densely packed surfactant layer. While the polyether layer is strongly hydrated, the TSO forms a compact and water-free layer on the silanized surface, indicating a space-filling configuration of the hydrophobic moieties at the interface. Attempts to model the data with water present in the TSO layer were less effective, as detailed in the Supporting Information (Figure S6).

Superspread S240 Film

A concentration of 0.1 wt % S240 solution is sufficient to cause superspreading of a deposited droplet. In this section, we describe the reflectivity results of a 10 μL of 0.1 wt % solution that has superspread into a thin film on the hydrophobic substrate, as illustrated in Figure 4A. The resulting film has a radius of approximately 15 mm, corresponding to a calculated thickness of 15 μm using dW = V/(r2π).

Figure 4

Figure 4. Illustration of a “thin” droplet of a (0.1 wt % 10 μL) superspread S240 solution (A). Neutron reflectivity data (B) of this system. Solid lines in (B) represent the best fit as described in the text.

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This thickness range of several tens of μm presents experimental challenges, as the NR not only includes additional contributions from the liquid/air interface on the film’s back side, but this contribution is also subject to attenuation effects, as noted earlier. (37) As the film-internal path lengths, and thus the attenuation strengths, are different for the two incident angles θi used for the measurements, we are confronted with two non-overlapping reflectivity portions obtained at the two angles, as seen in Figures 4B and 5B, which have to be treated separately.

Figure 5

Figure 5. Demonstration of a “thick” droplet of non-superspread 0.1 wt % S233 solution (A). Neutron reflectivity data (B) and deduced volume fraction profiles (C) of this system. Solid lines in panels (B) indicate the best fits to the data that correspond to the volume fraction profiles in panels (C).

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Taken together, modeling the reflectivity requires accounting for both covered and uncovered regions, as well as contributions from the liquid/air interface, whose attenuation depends on the incident angle θi:
R(qz,θi)=xcRc(qz,θi)+(1−xc)·Rnc(qz)
(7)
where
Rc(qz,θi)=RSL(qz)+TSL(qz)·RLA(qz′)·TLS(qz′)·α(qz,θi)
(8)
RSL is the reflectivity of the solid/water interface (corresponding to the volume fraction profile in Figure 3C), TSL is the transmittivity of the solid/water interface, and RLA is the reflectivity of the water/air interface (when starting from the water), which is derived from previously described air/water measurements, Figure 2E,F. TLS is the transmittivity of the solid/water interface in the reverse direction. The attenuation effect is incorporated through a factor of α(qz,θi), as described in the Methods section. Finally, qz′ denotes the refraction-corrected magnitude of the scattering vector that applies to the film-internal reflectivities and transmittivities,
qz′=qz2−16π(ρwat−ρSi)
(9)
Since the back side of a spread film is not necessarily as perfectly flat and smooth as a free air/water interface, RLA was not directly calculated from the SLD profiles deduced from the reference measurements at the free air water interface. Instead, we allowed for an elevated roughness through analytical convolution of the previously determined SLD profiles with a Gaussian function of adjustable width σLA.
The model of best fit reproduces the experimental reflectivity data relatively well, as shown in Figure 4B. However, there are some discrepancies: the initial decay region does not match perfectly, and slight deviations are observed in the overlapping q region. Nevertheless, the fit supports the methodology used. From the spread amount of aqueous solution and the puddle’s diameter, the average film thickness can be estimated to be about 15 μm. This value is, however, inconsistent with the observed level of attenuation of the back reflection when assuming that the H2O content is ≈5%, as reported by the position of the critical angle of total reflection, which probes the immediate vicinity of the interface. Instead, a larger “effective film thickness” of 58 μm is required to reproduce the reflectivity data. Possible reasons for this discrepancy include a significantly higher H2O content further away from the solid surface, heterogeneities in the water layer thickness caused by a large diffuse meniscus, heterogeneities in the alignment of the backside with the solid surface, which contribute differently for the different incident angles having different footprints, and uncertainties in the exact relative scaling between first and second angles.
The additional roughness of the air/water interface was obtained as σLA = 15 Å, while overall the surface coverage was determined to be xc = 0.79. Moreover, it is crucial to emphasize that a water-free TSO layer at the solid–liquid interface, as shown in Figure 3C, was again necessary to reproduce the experimental data accurately. Attempts to model the system with water present in the TSO layer were consistently ineffective, as detailed in the Supporting Information. Irrespective of the discrepancy between the nominal and effective film thickness observed for the superspread S240 film, the low hydration level of the TSO layer can be considered robust because it reproduces the entirety of the NR curves best within the self-consistent model that simultaneously describes all data sets that belong together.

Non-Superspreading Surfactant S233

Figure 5B,C shows the reflectivity and volume fraction profiles for the non-superspreading surfactant S233. A larger volume of solution (0.1 wt % 500 μL) was required to produce a sufficient amount of wetted area due to the significantly reduced spreading behavior of S233. Therefore, in this experiment, the entire silicon block was covered by S233 solution with the intention of making a simple system to measure only the solid–liquid interface.
However, as shown in Figure 5B, attenuated reflection contributions from the solution’s back side were clearly observed also in this experiment, as evidenced by the non-overlap of data points corresponding to the two different incident angles. The most likely reason for this result is that the wetting of the container walls again resulted in a rather thin water layer, as schematically depicted in Figure 5A, leading to the formation of a meniscus. Consequently, the reflectivity data for S233 were processed using the same methodology (eqs 7–9) that was used for the S240 reflectivity data in Figure 4B, involving a reference measurement from the air–water interface. Note, however, that xc was fixed at 1 for S233, because of the complete liquid coverage, such that the modeled reflectivity curve in Figure 5B is already fully described by eq 8. The model also fits well to the reflectivity data, as shown in Figure 5B. The obtained additional roughness of the water–air interface for S233, σLA ≈ 7 Å, is notably smaller than that for S240 (σLA ≈ 15 Å) in the thin film, which suggests that the characteristics of the surfactant-loaded backside interface of the thicker film are more similar to those of a meniscus-free macroscopic air/solution interface. The critical angle of total reflection in this measurement indicates a negligible H2O fraction. With this assumption, the observed attenuation corresponds to a water layer thickness of dW ≈ 230 μm, which appears plausible. The volume fraction profiles of the chemical components at the solid–liquid interface are shown in Figure 5C. The fits here follow the same constraints as in the previously discussed models. The water volume fraction profile was again obtained through the constraint of eq 3, and the surfactant thicknesses dTSO and dPOL were again constrained to the same values for the air–water and solid/water interfaces. The obtained volume fraction profiles shown in Figure 5C reveal a maximal polyether volume fraction of ΦPOLmax = 0.58 (ΦPOL0 = 0.67) and a hydrophobic TSO layer with a maximal volume fraction of ΦTSOmax = 0.70 (ΦTSO0 = 0.83). These values are considerably lower than at the air/water interface (see Table 1 and the Supporting Information, Figure S3), suggesting that S233 struggles much more than S240 to form dense layers at the solid surface. As a consequence, we find ≈17% hydration (ΦTSOwat = 1 – ΦTSO0 = 0.17) in the hydrophobic TSO layer, which is in contrast to the water-free TSO layer for S240. These findings are consistent with a second data set for S233, which is included in the Supporting Information (Figure S7). Attempts to fit the reflectivity data while forcing a water-free TSO layer failed to converge on a suitable model, further supporting the observed hydration. Detailed comparisons of these alternative fits are also available in the Supporting Information.

Discussion

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This study demonstrates that TSO surfactants adsorbed at the hydrophobic solid–liquid interface form well-defined layers, comprising two distinct regions: an inner, poorly hydrated layer formed by the hydrophobic TSO moiety and an outer, strongly hydrated layer formed by the hydrophilic polyether moiety. This organization is summarized in Figure 6, which illustrates the structural differences between superspreading S240 and non-superspreading S233 surfactants.

Figure 6

Figure 6. Visual representation of how adhered surfactant layers can affect the amount of water in contact with the hydrophobic surface. A closely packed surfactant monolayer in (A) minimizes water contact with the hydrophobic surface, efficiently reducing the interfacial energy. Less dense packing or weak adsorption in (B) leads to a smaller reduction of interfacial energy.

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The clearest difference between S240 and S233 lies in the hydration of the hydrophobic TSO layer directly in contact with the solid surface. For S240, the TSO layer is water-free, indicating efficient packing that minimizes water contact with the hydrophobic surface as seen in Figure 6A. In contrast, S233 shows significant hydration within the TSO layer, suggesting less efficient packing and the presence of water in unfavorable contact with the hydrophobic surface, which can be seen in Figure 6B. As also described in the Introduction and eq 1, the more densely packed water-free TSO layer of the S240 ensures a lower interfacial tension γSL for the solid/liquid layer, which leads to a positive spreading coefficient S, enabling superspreading. For S233, the hydrated TSO layer results in a higher γSL, flipping the sign of S to negative and thereby allowing only partial wetting instead of superspreading. The inability of S233 to form a water-free TSO layer is in line with a larger steric footprint of its polyether chain, which is also the reason behind the formation of spherical micelles in water. (16,38) As shown in Figure 1 and Table 1, the polyether portion of S233 is slightly longer, with 12 total monomer units (p = 10, q = 2), than that of S240s 9 units (p = 6 and q = 3), as reflected in the best fits. This increased length increases the volume requirement and also the area requirement of the polyether moiety, disrupting the efficient packing of the TSO layer at the solid–liquid interface. Consequently, water cannot be prevented from penetrating into the interfacial region, coming into contact with the hydrophobic surface, and increasing γSL. On the other hand, S233 is almost equally good as S240 at efficiently packing at the air/water interface, as shown in the Supporting Information S3, which leads to the question of why it can do this at the air/water but not at the solid/water interface. One reason may be the higher tension of the bare air/water interface in comparison to that of the bare solid/water interface, so that dense molecular packing at the air/water interface may be free-energetically so favorable that it justifies entropically unfavorable stretching of the polyether chains. Another possible explanation may be based on differences between the two interface types with regard to the in-plane mobility of the adsorbed TSO groups. At the fluid air/water interface, the in-plane mobility is generally high as it allows for collective movements, so that stretched-out polyether chain conformations that optimize packing can be rapidly assumed upon increasing surface adsorption. At the solid/liquid interface, by contrast, the in-plane mobility is comparatively low and rearrangements that would lead to denser packing take more time and may not occur on the time scale of the spreading process. Therefore, dense packing may occur immediately only for surfactants with a balanced headgroup area requirement, as for S240, whereas the looser packing observed for S233 may reflect a state that does not represent equilibrium even on the time scale of the NR experiments. This hypothesis could be tested by studying the behavior of these surfactants at fluid oil/water interfaces, (39,40) where the in-plane mobility is high. We note that liquid/liquid interfaces are nowadays accessible also to NR, (41) but the solubility of the surfactants in oil poses an additional challenge. Complementary insights may further be gained through the interpretation of the adsorption kinetics and the surface tension isotherms of these surfactants, (36) which can reveal the occupation of different configurational states. (42) Finally, molecular dynamics simulations could provide more detailed insights into the molecular structure and behavior of the surfactant layers, (43) while a combination of neutron reflectometry (NR) and X-ray reflectometry (44) could help further refine the structural models developed in this work. It is important to note that a positive spreading coefficient is only the thermodynamic prerequisite for superspreading. Whether fast superspreading can occur is a matter of efficient surfactant transport, which seems to be related to the phase behavior of the surfactant aggregates. In this context, the importance of the L3-phase was proposed decades ago. (1,16,38) NR on stationary samples is well-suited to investigating the surfactant adsorption layer in equilibrium. Time-resolved investigations of the wetting kinetics currently appear difficult with the neutron sources and instruments available. But such measurements may become feasible in the future.

Experimental Section

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Chemicals and Sample Preparation

All chemicals were purchased from Merck and used as received without any further purification unless otherwise stated. In addition, H2O was ultra-pure Milli-Q water (18 MΩ cm–1) in this study. BREAK-THRU S233 and BREAK-THRU S240 were provided by Evonik Operations GmbH (Essen, Germany). The chemical structures of these surfactants are listed in Figure 1. The product purity was 100% according to the specification sheet by the supplier. Surfactant solutions were prepared immediately before use by mixing 0.1 wt % of surfactant and D2O water with light shaking to the desired concentration. Silicon single crystal blocks (5 cm × 5 cm × 1 cm), purchased from Korth Kristalle (Altenholz, Germany), were polished on both sides and had a thin layer of native oxide (SiO2) on their surface. These were functionalized to be hydrophobic prior to the experiments by first cleaning with a solvent cascade of chloroform (99.8%), acetone (99.8%), ethanol (99.9%), and pure water, for 15 min each, then dried with nitrogen (N2) gas and UV-ozone treated for 25 min. Second, the functionalization was done by placing them in a sealed N2 environment with a 15 mL beaker of CTMS (≥98%) and leaving them overnight to vapor deposit, then washing them with pure water when removed. (45) Surfactant solutions were deposited onto the block surface by placing the tip of an Eppendorf pipette, or for nanoquantities, the NanoLiter2020 (WPI Instruments, Friedberg, Germany), close to the surface and gently depositing the liquid in a single droplet.

NR Experiments

Specular NR was performed on the horizontal time-of-flight reflectometer FIGARO at the Institut Laue-Langevin (Grenoble, France). (46) In the experiments, the incident beam reaches the interface with an adjustable incident angle θ. The reflectivity, i.e., the intensity ratio R between reflected and incident beams, is recorded as a function of the scattering vector component perpendicular to the interface, qz = (4π/λ)sin(θ), where λ is the neutron wavelength. The reflectivity R(qz) is imposed by the scattering length density (SLD) profile ρ(z) along the direction perpendicular to the interface, z. The SLD profile, in turn, originates from the interfacial distributions of all chemical components (see Results section) having their characteristic SLDs, where bk is the coherent scattering length of atomic nuclei of type k and Nki is the number of such nuclei in the chemical component i occupying the volume vi.
ρi=1vi∑kNkibk
(10)
With that, the depth distribution of the molecular constituents in a sample can be reconstructed from the analysis of the R(qz) curves. In order to maximize the SLD contrast and thus the reflected intensity, deuterated water (D2O, ρD2O = 6.35 × 10–6 Å–2) was used in addition to H2O (ρH2O = −0.56 × 10–6 Å–2). No signs of off-specular scattering were observed.

Solid Surfaces

All experiments involving solid surfaces were conducted in an airtight sample cell with internally heated water reservoirs, allowing for humidity control. Both humidity and temperature were monitored in real time using a sensor (B+B Thermo-Technik, Donaueschingen) with relative humidity kept above 95% and temperature at around room temperature. The measurements were carried out using two incident angles, θ1 = 0.71° and θ2 = 2.41°, with a wavelength range of 2 Å < λ < 22 Å, and the full width at half-maximum qz-resolution, Δqz/qz, was qz-dependent and ranged between 4 and 12%.

Liquid Surfaces

Measurements were performed at room temperature on bulk solutions of 0.1 wt % surfactant in both D2O and air contrast-matched water (ACMW, 92:8 D2O/H2O v/v, ρACMW = 0) inside a Langmuir trough installed at FIGARO. The neutron beam approached through the air at two incident angles, θ1 = 0.72° and θ2 = 3.86°, with a wavelength range of 2 Å < λ < 20 Å and Δqzqz≈7−8%.

Scattering Length Densities

The SLD values of all chemical components (excluding water) are pre-established and constant. These are silicon (ρSi = 2.07 × 10–6 Å–2), silicon oxide (ρoxi = 3.47 × 10–6 Å–2), and silane (ρsil = −0.30 × 10–6 Å–2, approximated from the known mass density of HSi(CH3)3 (0.635 g/cm3) (47) and its coherent scattering length when accounting for the hydrogen not present in the covalently bound silane layer. Regarding the surfactants, we distinguish between their hydrophobic (TSO) and hydrophilic (polyether, POL) parts for modeling purposes. Their SLDs were calculated as described in the Supporting Information, based on independent densiometry experiments using the allylpolyethers (ALP0620 for S233, ALP0540 for S240). The calculated SLDs are shown in Table 2.
Table 2. Summary of Calculated Molecular Volumes (Vi) and Scattering Length Densities (ρi) for the Surfactant Molecules and Polyether Chains
moietyV [nm3]ρ [10–6 Å–2]
ALP06200.8220.59
ALP05400.6640.47
HMTS0.476–0.09
S2331.2940.31
S2401.1450.23

Calculation of Theoretical Reflectivity Curves

The SLD profiles ρ(z) were discretized into thin layers of 2 Å thickness with constant SLD. The qz-dependent reflection intensities were then determined by computing Fresnel’s reflection coefficients at each interface between the slabs and using Parratt’s iterative method for their phase-correct superposition. (48) To match the finite experimental qz resolution, the theoretical reflectivity curves were convoluted with Gaussian functions, representing the experimental resolution. Finally, all unconstrained model parameters were adjusted to achieve the best agreement with all experimental reflectivity curves, characterized by the minimal chi-square deviation, χ2. Interface roughness parameters were constrained to have a lower limit of 1.5 Å and an upper limit of half the thickness of the slabs they belong to. The estimated parameter uncertainties are ±1 Å for thicknesses and roughness parameters, and ±0.05 for volume fraction parameters. These estimates are larger than the statistical uncertainty alone because they also account for systematic uncertainties, which are typically the dominant contribution as discussed previously. (24)

Attenuation of the Neutron Beam Inside a Thin Water Film

For thin aqueous films of intermediate thicknesses in the range of several to several tens of micrometers, the reflection from the back side experiences significant attenuation that has to be taken into account when modeling the reflectivity curves. The attenuation factor α that applies to the back side reflection is given by the law of Lambert–Beer,
α(qz,θi)=e−L(qz,θi)/Latt
(11)
and depends on the attenuation length Latt and on the path length
L(qz,θi)=2dW/sin[θ(qz,θi)]
(12)
where θ follows from qz and θi according to Snell’s law, as shown in the Supporting Information.

Conclusions

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This study characterized the interfacial behavior of superspreading and non-superspreading TSO surfactants using NR. The initial focus was the measurement and analysis of reflectivity data from thin spreading films, which turned out to be more complicated than the analysis of data obtained with large solution volumes. However, we maximized the robustness of the interpretation of the available data through the use of a self-consistent model with common parameters that simultaneously describe all of the data sets that belong together. The results highlight key differences between S240 and S233, particularly in how polyether chain variations affect hydration at the solid–liquid interface and determine superspreading ability. S240 forms a compact, water-free hydrophobic layer, while S233 cannot prevent partial hydration of the hydrophobic substrate due to less efficient packing and therefore less reduction of interfacial energy. These findings reinforce the link between molecular organization, interfacial energy, and spreading behavior. While challenges remain, such as handling attenuation effects in NR for thin layers, this work demonstrates the utility of NR in probing surfactant behavior with molecular precision.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.5c03781.

  • Images of S240 and S233 droplets; Experimental setup; Reflectivity of S233 at the air/water interface; Synchrotron-based X-ray scattering and X-ray fluorescence measurements; Comparing different scenarios of water fractions in TSO for S240 and S233; Calculation of water-layer-internal incident angles; Further neutron reflectivity measurements of a 0.1 wt % 200 μL sample of S233; Calculation of surfactant SLD using densiometry measurements; Tabulated neutron reflectivity fitting parameters (PDF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • Séforah Carolina Marques Silva - Institute for Technical Thermodynamics, TU Darmstadt, Peter-Grünberg-Str. 10, 64287 Darmstadt, Germany
    • Philipp Gutfreund - Institut Laue-Langevin, 71 Av. des Martyrs, 38000 Grenoble, France;  Orcidhttps://orcid.org/0000-0002-7412-8571
    • Tatiana Gambaryan-Roisman - Institute for Technical Thermodynamics, TU Darmstadt, Peter-Grünberg-Str. 10, 64287 Darmstadt, Germany;  Orcidhttps://orcid.org/0000-0003-0259-129X
  • Author Contributions

    J.R.: investigation, methodology, formal analysis, writing – original draft preparation; S.C.M.S.: investigation, formal analysis, writing – review and editing; P.G.: methodology, formal analysis, writing – review and editing; J.V.: conceptualization, investigation, writing – original draft preparation, acquisition of funding, supervision; T.G.-R.: conceptualization, investigation, writing – review and editing, acquisition of funding, supervision; E.S.: conceptualization, methodology, investigation, formal analysis, writing – original draft preparation, acquisition of funding, supervision.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors would like to thank the Institut Laue-Langevin (ILL) for beam time allocation (DOI:10.5291/ILL-DATA.9-10-1740), and the ILL Soft Condensed Matter laboratories for their support. We would also like to thank Hacer Yalcinkaya for the densiometry measurements. The research leading to these results received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement number 955612 (NanoPaInt).

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  • Abstract

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    Figure 1

    Figure 1. Chemical structure of S233 (p = 10, q = 2) and S240 (p = 6, q = 3). The structure consists of a hydrophobic trisiloxane group with a hydrophilic polyether chain whose monomer composition differs between the two molecules in terms of the number of ethylene oxide (p) and propylene oxide units (q).

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    Figure 2

    Figure 2. Neutron reflectivity data (A, C, E) and deduced volume fraction profiles (B, D, F) of the reference systems: (A, B) Bare silanized silicon block in air, (C, D) bare silanized silicon block in water, and (E, F) air–water interface of a 0.1 wt % surfactant solution (shown is S240). Solid lines in panels (A, C, E) indicate the best fits to the data that correspond to the volume fraction profiles in panels (B, D, F).

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    Figure 3

    Figure 3. Illustration of a “thick” droplet of S240 solution obtained by the addition of 200 μL D2O to a superspread puddle of S240 (0.1 wt % 10 μL) (A). Neutron reflectivity data (B) and deduced volume fraction profiles (C) of this system. Solid lines in panel (B) indicate the best fits to the data that correspond to the volume fraction profiles in panel (C).

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    Figure 4

    Figure 4. Illustration of a “thin” droplet of a (0.1 wt % 10 μL) superspread S240 solution (A). Neutron reflectivity data (B) of this system. Solid lines in (B) represent the best fit as described in the text.

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    Figure 5

    Figure 5. Demonstration of a “thick” droplet of non-superspread 0.1 wt % S233 solution (A). Neutron reflectivity data (B) and deduced volume fraction profiles (C) of this system. Solid lines in panels (B) indicate the best fits to the data that correspond to the volume fraction profiles in panels (C).

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    Figure 6

    Figure 6. Visual representation of how adhered surfactant layers can affect the amount of water in contact with the hydrophobic surface. A closely packed surfactant monolayer in (A) minimizes water contact with the hydrophobic surface, efficiently reducing the interfacial energy. Less dense packing or weak adsorption in (B) leads to a smaller reduction of interfacial energy.

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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.5c03781.

    • Images of S240 and S233 droplets; Experimental setup; Reflectivity of S233 at the air/water interface; Synchrotron-based X-ray scattering and X-ray fluorescence measurements; Comparing different scenarios of water fractions in TSO for S240 and S233; Calculation of water-layer-internal incident angles; Further neutron reflectivity measurements of a 0.1 wt % 200 μL sample of S233; Calculation of surfactant SLD using densiometry measurements; Tabulated neutron reflectivity fitting parameters (PDF)


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