Probing specific ion effects at air-aqueous dibutyl phosphate interfaces using vibrational sum frequency generation spectroscopy Open Access

Interfaces at air/liquid and liquid/liquid boundaries are molecularly thin spaces that the most active chemical reactions occur where the reactants reside in immiscible phases and the reaction leads to molecular transfer across phase boundaries. The interaction mechanisms at the interface directly determines the mass transfer efficiency of many natural and engineered chemical processes, such as cloud formation, marine carbon cycling, and material separation using solvent extraction techniques. Particularly, liquid–liquid extraction (LLE), which leverages the varying constituent solubility between two insoluble solvents, has been broadly developed across the chemical, metallurgical, nuclear, pharmaceutical, petroleum, and petrochemical fields for material separation and purification.¹ In the nuclear industry, LLE has been successfully implemented to produce plutonium and uranium for nuclear power generation and weapons production with high recovery yields through the plutonium uranium redox extraction process (PUREX).^2,3 In the PUREX process, an aqueous phase containing the irradiated nuclear fuel dissolved in nitric acid is contacted with an organic phase consisting of the surfactant tributyl phosphate (TBP) diluted with a hydrocarbon such as kerosene. The uranium, plutonium, and TBP partition to the aqueous–organic interfacial region from their respective bulk phases, where they chemically react to form neutral adducts. The adducts then partition from the interface to the organic phase due to their neutral charge, thus effectively separating uranium and plutonium from the other contents of the irradiated nuclear fuel. However, despite its widespread implementation, the fundamental mechanisms control the transfer rates of species traversing the aqueous–organic interface. Even less is known about the effect of TBP degradation products, such as dibutyl phosphate (DBP), monobutyl phosphate (MBP), and phosphoric acid due to hydrolysis, nitrolysis, and radiolysis.^4–11 Among the degradation products, the rate of formation of DBP is much greater than that of MBP or phosphoric acid, and its build-up is significant enough to cause adverse effects, such as material loss and crud formation.^2,8–14

While most DBP studies are focused on its interactions with heavier metals, there have not been many investigations of DBP and its interactions with lighter cations. Lighter cations are of interest because they can be present in PUREX processes as salting agents. The salting out of species in the PUREX process was historically explored for advantageous purposes, often to enhance the recovery of certain metals such as Th, Ce, Eu, Sm, Nd, Am, Cm, and Cf. Among the cations explored were Al, K, Mg, Be, Li, Cr, and Na nitrates.^5,15 These experiments were mainly limited to TBP systems and did not explore how the choice of salting agents would affect the partitioning behavior of DBP and its subsequent complexes or their tendencies to emulsify. In order to further our understanding of how changes in bulk phase environments influence the partitioning behavior of DBP to and within the interfacial region, direct information at the liquid–liquid interface is required. Recently, vibrational sum frequency generation spectroscopy (vSFG) has emerged as a powerful tool to study many interfaces as long as they are accessible to both visible and infrared (IR) light, including liquid–liquid interfaces. For liquid interfaces, this is one of the only available techniques that has the ability to provide surface vibrational spectra, from which molecular level structural information at the interface can be obtained.

The fundamental principles of vSFG have been described and reviewed in the literature.^16–21 Briefly, vSFG spectroscopy is a second-order nonlinear optical technique that selectively analyzes molecules at interface where molecules do not possess centrosymmetry. The phenomena arises as the result of a three-photon process, typically with a visible beam with a fixed frequency $ωV is$ and a tunable infrared beam $ωIR$ overlap on a surface in both space and time, producing a sum frequency (SF) beam at a frequency ω_SF = ω_Vis + ω_IR. During the scanning across IR frequencies, the vSFG intensity is resonantly enhanced when the IR source is resonant with a vibrational mode of an interfacial species. The intensity of the SF beam is directly proportional to the square of the second-order nonlinear susceptibility χ⁽²⁾. As a second-order nonlinear process, by the dipole approximation, vSFG is forbidden in bulk phases due to the symmetry restriction of centrosymmetric media. As a result, χ⁽²⁾ = 0. Interfaces introduce natural breaks in centrosymmetric environments, leading to χ⁽²⁾ ≠ 0. As such, vSFG is inherently a surface selective spectroscopic technique, mainly measuring the first molecular layers of an interface.

While vSFG has been successfully applied to liquid–liquid interfaces,²² these are among the most challenging and elusive interfaces to study due to their “buried” nature. Because of this, it is common for investigators to simplify their systems to air–aqueous interfaces. The air–aqueous interface is the simplest of these systems modeling liquid–liquid interfaces, where the air acts as a hydrophobic phase to emulate an organic–aqueous interface. Alternatively, surfactant monolayers formed at the interface such that the tails of the surfactants protrude into the air phase has also been utilized to simulate a hydrophobic liquid phase.^16,23–28 These types of simplified hydrophobic–aqueous interfaces have been extensively studied within the nonlinear optical spectroscopy community due to their wide range of applicability to numerous fields.^29–52 In the work presented here, vSFG spectroscopy and surface tension measurements are used to probe the air–aqueous interfaces of DBP solutions containing nitrate salts. Conveniently, some of the cations from the salting agents previously explored for the PUREX process are also part of the Hofmeister cation series: Na⁺, Li⁺, and Mg²⁺.^49–51 While specific ion effects (SIEs) are unique to each individual system, the Hofmeister series still provides a good reference for foundational studies. The Hofmeister cation series specifically studied here includes Cs⁺, Na⁺, Li⁺, and Mg²⁺. Each cation will be introduced as a nitrate salt to the bulk aqueous phase of samples containing 0.24 mM dibutyl phosphoric acid (HDBP) in water. For clarity, the abbreviations HDBP and DBP⁻ will be used to refer to the undissociated and dissociated form of the molecule, respectively, while the abbreviation DBP will be used to refer to dibutyl phosphate molecules in general, where their interactions with other components from the aqueous phase are not specified. Two ionic strengths, 1 mM and 1M, adjusted by the corresponding nitrate salts concentrations, respectively, are explored. At 1 mM, the charge present at the interface, the interfacial potential in essence, is expected to modulate interactions between ions at the air/aqueous DBP interface. At 1M, the interfacial potential is screened, and the specific ion effect (SIE) is expected to occur between the DBP head group and each individual cation. The objective of this study is to establish fundamental qualitative relationships between the SIE of counter cations and the partitioning of DBP at air–aqueous interfaces. These results will form a foundation for future SIE studies of DBP with heavier metals at air–aqueous and will be broadly applicable to the understanding of molecular transport processes across hydrophilic and hydrophobic interfaces.

Dibutyl phosphoric acid (HDBP, 97% pure) and magnesium nitrate hexahydrate (99+%) were purchased from Acros Organics and used without further purification. Crystalline sodium nitrate and ethanol (certified ACS reagent) were purchased from Fisher Chemical. Lithium nitrate (99.999%, metals basis) and both Fisherbrand^TM Sparkleen^TM 1 detergent and Epredia^TM SoftCIDE^TM hand soaps were purchased from Thermo Fisher Scientific. Cesium nitrate (ultrapure) was purchased from Spectrum. Unless otherwise stated, all water used was nanopure water, which was sourced from an in-house U.S. filter ion exchange system.

For all sample containers and covers used in this work, rigorous cleaning procedures were followed as described previously (see the supplementary material).⁵² Batch solutions of the liquid samples were prepared in KIMAX bottles. All salts were baked in a Thermo Scientific Heratherm oven prior to use. Appropriate amounts of the salts were first poured into PYREX^TM Petri dishes and enclosed in aluminum foil dust covers to prevent possible contamination during the baking process. NaNO₃, CsNO₃, and LiNO₃ were baked at 100 °C while Mg(NO₃)₂ · 6H₂O at 80 °C for at least 1 h to remove organic contaminants and physisorbed water from the salts. Calculated amounts of the salts were then weighed into the KIMAX bottles using an OHAUS balance. Aluminum foil that had been treated in a Novascan PSD Pro Series Digital UV ozone system for 30 min was used in place of weighing paper. Calculated volumes of nanopure water were then added to the KIMAX bottles. The sample solutions were then sonicated for 30 min in a VWR Company’s ultrasonic cleaner to fully dissolve the salts into solution. Finally, HDBP was added to the solutions by volume using a Hamilton^TM syringe. The solutions were then gently mixed and allowed to settle prior to being used for measurements. The HDBP was always added last right before the batch solutions were used for any measurements in order to minimize any degradation of the HDBP from hydrolysis or nitrolysis. Any time the batch solutions were set aside to settle prior to taking measurements, they were either wrapped in aluminum foil or placed in a dark cabinet to minimize any radiolysis of the HDBP from laboratory ambient light. Our previous work showed no significant HDBP degradation occurred over the scanning time of the samples.⁵² 

The vSFG experiments were performed using the EKSPLA picosecond scanning spectrometer in the Environmental Spectroscopy Laboratory within the Environmental Molecular Science Laboratory, a US DOE BER user facility operated by Pacific Northwest National Laboratory. Details of the spectrometer have been described previously.^53–56 Briefly, the SFG-VS spectrometer utilizes the output of a Nd:YAG laser (1064 nm, EKSPLA PL2251A-50) running at 10 Hz with a 29 ps pulse width. A portion of the fundamental output is converted into a visible beam at 532 nm after passing through a KD*P crystal. The remaining of the fundamental output was used to pump an optical parametric amplifier (OPA) and enter a difference frequency generation system (EKSPLA PG401/DFG), producing an infrared (IR) beam that is tunable between 1000 to 4300 cm⁻¹. The optical layout is configured with the visible and IR laser beam incident angles at 65° and 55°, respectively, from the horizontal plane for the IR beam, where the two beams overlap at the sample surface. The SFG signal beam is collected in specular reflection geometry with a spectral resolution of ∼6 cm⁻¹. All the measurements were performed at ambient laboratory temperature (72–73 °F) and humidity (25%–34%). For each sample, 2 scans were performed with an increment of 5 cm⁻¹ and 150 laser pulses per increment. All spectra intensities were normalized by the visible and IR laser powers. All vSFG spectra presented in this manuscript were recorded using the SSP-polarization combination, where the polarization of the sum frequency (SF), visible, and IR beams are in the S-, S-, and P-polarization, respectively.

Prior to taking any measurements, a known thin quartz standard was used to optimize the vSFG spectrometer. A PYREX Petri dish containing 20 ml of nanopure water was then used for the alignment and height optimization of the air–water interface at the IR frequency of 3705 cm⁻¹. After completion of the alignment process, two scans of the air–water interface were taken for reference purposes. For each sample, 20 ml of the batch solution was transferred to a PYREX Petri dish using a Hamilton syringe, and two scans of the air–aqueous solution interface were then performed. For each salt concentration, two samples were taken, totaling four vSFG spectra per batch solution. Scaling between samples from experiments performed with a different alignment was achieved using spectra that had the highest CH₃ SS peak intensities. All vSFG spectra presented in this work are on the same scale and are, therefore, directly comparable.

Surface tension measurements were taken with a LB film system (KSV Instruments, Finland). The Teflon trough and Wilhelmy plate were rinsed with ethanol, copious amounts of nanopure water, and then dried with nitrogen. The Wilhelmy plate was placed in a plasma chamber for 2 min prior to use. A Hamilton syringe was used to transfer 75.6 ml of the sample batch solution to the Teflon trough. The Wilhelmy plate was then wetted using the same sample solution prior to be placed over the trough. The system was equilibrated for 30 min before taking the first measurement. For each sample solution, a total of three measurements were taken. Unless otherwise stated, all measurements were taken at ambient laboratory temperature (70–73 °F) and under humidity (21%–27%) conditions.

The air–aqueous interface SSP vSFG spectra of samples containing 0.24 mM in 1 mM and 1M ionic salt concentrations are shown in Fig. 1.

The Debye length (λ_D) for the electrolyte solution is given by $λD=ε0εrRT2∗103F2I$⁠, where ɛ₀ is the permittivity of free space, ɛ_r is the dielectric constant, R is the molar gas constant, T is temperature, F is Faraday’s constant, and I is ionic strength. At 0.24 mM HDBP in solution with 1:1 electrolytes NaNO₃, LiNO₃, and CsNO₃ at a temperature of 300 K, this yields Debye lengths of 3.04 and 86.4 Å at 1M and 1 mM, respectively. For 1:2 salt solution of Mg(NO₃)₂, the Debye lengths would be 1.76 and 49.9 Å for salt concentrations 1M and 1 mM respectively.

The spectral range 2800–3800 cm⁻¹ captures the frequency regions for both the C–H vibrational modes of the interfacial DBP molecules and the O–H vibrational modes of the interfacial water molecules. The frequency range 2800–3000 cm⁻¹ is the C–H region of the spectra in which there are four readily visible peaks pertaining to the butyl chains of the DBP molecules. The peaks appearing around the frequencies 2855 and 2885 cm⁻¹ are assigned to symmetric stretches (SS) of the CH₂ and CH₃ vibrational modes, respectively,^6,57 referred hereafter as CH₂ SS and CH₃ SS. The peak appearing around the frequency of 2920 cm⁻¹ is assigned as the asymmetric stretch (AS) of the CH₂ vibrational mode, referred hereafter as CH₂ AS. Finally, the peak appearing around the frequency of 2950 cm⁻¹ is assigned as the CH₃ Fermi resonance (FR) vibrational mode,⁵⁷ referred hereafter as CH₃ FR. The frequency range 3000–3800 cm⁻¹ is the O–H region of the spectra in which there are three readily visible peaks.^{16–18,25,58,59} The broad peaks appearing at around 3200 and 3400 cm⁻¹ are assigned to O–H stretching vibration modes of water molecules hydrogen bonding with one another. The strength of these hydrogen bonds varies. In general, the peak around the frequency of 3200 cm⁻¹ is interpreted as a region with strong hydrogen bonding interactions and the peak around the frequency of 3400 cm⁻¹ is interpreted as a region with weaker hydrogen bonding interactions. The sharper peak at around 3700 cm⁻¹ is assigned as the free OH peak, which is interpreted as the O–H mode of water molecules dangling across the air–aqueous interface.

The neat air–water spectrum (green) shown in Fig. 1 is in good agreement with the published literature.^16–18,60 For the 0.24 mM HDBP solution (black), at least some of the DBP molecules have partitioned to the air–aqueous interface. This is apparent from the appearance of the C–H peaks, the CH₃ SS and CH₃ FR peaks appearing with the highest vSFG intensities. It is noted that a prominent CH₂ SS peak is not observed and it seems to appear only as a spectral shoulder at ∼2855 cm⁻¹ for this sample, indicating the interfacial DBP molecules are in a relatively high order of conformation.^{16,19,25,26,61} The intensities of the 3200 and 3400 cm⁻¹ peaks also increase for this spectrum, which is evidence of a charge interface likely due to the dissociation of HDBP into H⁺ and DBP⁻ ions.^16,23,24 As a moderately strong acid,^62,63 HDBP is expected to deprotonate to an extent when it is introduced to a bulk aqueous phase. In particular, the pKa for HDBP in water is estimated to have a value of 1,^62–64 corresponding to an equilibrium constant of K_a = 0.1, which suggests >99% ionization of the HDBP molecules in the 0.24 mM HDBP sample. This estimated value of K_a = 0.1 for HDBP in water aligns with its experimentally reported value of K_a = 0.08 ± 0.03.⁶⁵ Given that most HDBP molecules dissociate (into H⁺ and DBP⁻ ions), it is likely that the DBP at the air–water interface is primarily in its dissociated form (DBP⁻), which induces a negative charge at the air–aqueous interface. This negative charge, rather than the electronegativity of the oxygen atoms from the DBP head groups alone, is responsible for the observed increase in the 3200 and 3400 cm⁻¹ peak intensities when neat air/water spectra are compared to the air/0.24 mM aqueous HDBP spectra shown in Fig. 1. When a charge is present at air–aqueous interfaces, an electrostatic field commonly referred to as an interfacial potential is induced in the double layer. This causes the restructuring of the interfacial water molecules, with negatively charged surfactants typically orienting water molecules in the H-up direction, while positively charged ones orient them in the H-down direction.^25,26 The water molecules that become oriented align in such a way that they break centrosymmetric environments in the interfacial region. The oriented water molecules are SFG active as opposed to disoriented water molecules of the bulk and contribute to the increase in vSFG intensity. In addition, random water molecules in subsequent molecular layers from the interface are partially reoriented to become SFG active and can also contribute to the vSFG intensity. As a result, enhancement of the 3200 and 3400 cm⁻¹ peaks is observed. The results from our previous work^5,52 also support that at a bulk concentration of 0.24 mM, nearly all the DBP molecules occupying the interface are in deprotonated form (DBP⁻) molecules. Indeed, in our recent work,⁵² we also observed an increase of roughly threefold in the 3200 and 3400 cm⁻¹ OH peak intensities when mM concentrations of HDBP were added to the neat air/water interface. 1 mM HDBP in water has a measured pH of 4.4. Based on the measured pH of 1 mM HDBP in water and the ionic strengths of the samples, we can safely say that all our samples lie in the acidic pH range, most likely between pH 4–5. For the neat air/water interface, the charge density is positive and on the order of μC/m²; thus, we can estimate the expected nearly 100% dissociation of HDBP into DBP⁻ and H⁺ ions, which leads to a negative surface charge density ranging from micro to milli Coulombs/m².

In the case of DBP⁻, the negative charge promotes stronger hydrogen bonding within the interfacial water network than of the neutral air–water interface. This is evident from the greater increase in the 3200 cm⁻¹ peak intensity compared to that of the 3400 cm⁻¹ peak intensity. It is noted that a full monolayer of DBP⁻ molecules is not likely formed at this concentration, as indicated by only partial suppression of the dangling O–H peak at around 3700 cm⁻¹. A decrease in peak intensity could be indicative of less water molecules straddling the air–aqueous interface, a loss of relative order in these water molecules, or a combination of both. Regardless, some water molecules continue to straddle the air–aqueous interface for this sample. The appearance of the free OH peak of water fully confined in lipid Langmuir monolayers has been observed previously although at slight lower frequencies due to water interaction with the lipid hydrophobic tail groups.⁶⁶ 

The addition of 0.24 mM HDBP to the bulk aqueous phase results in a negatively charged interface. When counter cations are added to the bulk aqueous phase in the form of nitrate salts, the counter cations partition to the interface to screen the negative charge. This can affect both the partitioning of DBP to the interface as well as its orientation once present there. When 1 mM ionic strength salt concentrations are added, the largest change observed in the vSFG spectra in Fig. 1 (red vs black) is the increase in C–H peak intensities. The increase in C–H peak intensities could indicate that more DBP molecules have partitioned to the interface, that the DBP already present at the interface have undergone orientation changes such that they are more favorable for the SSP polarization combination, or that there is a combination of both an increased number of DBP molecules and orientation changes. Based on our previous work⁵² and published literature,^62,63 the last case is the most likely scenario. In general, as the counter cations partition to the negatively charged interface, they are expected to neutralize the charge of some of the DBP⁻ molecules. This would reduce the electrostatic repulsion between other interfacial DBP⁻ molecules, facilitating higher packing and likely instigating orientation changes. Despite these changes, the CH₂ SS peak intensity remains unimpressive for all counter cations studied, indicating that a high order of conformation is maintained for the interfacial DBP molecules even after the addition of 1 mM ionic strength salt concentrations to the bulk aqueous phase. Overall, there are no drastic line shape changes in this region for all salts studied when compared to that of the 0.24 mM HDBP in the water sample. The intensities of these peaks are also mostly retained, with the greatest change observed being the small attenuation for the Mg²⁺ sample shown in Fig. 1(d). This indicates that the structure of the interfacial water network is fairly maintained after the addition of the 1 mM ionic strength nitrate salts to the bulk aqueous phase. Interestingly, the free OH peak at around 3700 cm⁻¹ slightly increases for all 1 mM ionic strength salt concentrations studied. This may be due to disruption of the partial DBP monolayer at the interface. As 1 mM ionic salts are added to the bulk aqueous phase, the orientation of the interfacial DBP molecules may change, which could allow for either more water molecules to straddle the interface or promote an increase in the relative order of these water molecules.

When the bulk aqueous phase salt concentrations are increased to 1M ionic strengths, considerable changes in the spectra are observed (blue compared to red spectra). For all cases studied, the C–H peak increased in intensity. Again, this is likely due to a combination of both DBP partitioning and orientation changes of the interfacial DBP molecules. The increased partitioning of DBP at the interface may be attributed to a salting out effect, which is known to occur for systems related to the PUREX process.^67–69 CH₂ SS peaks at around 2855 cm⁻¹ become prominent in the spectra of these samples, suggesting that the higher ionic strength salt concentrations instigate a relatively lower order of conformation of the interfacial DBP molecules, where they are no longer in a predominantly all-trans configuration and gauche defects are present.^19,25,26,61 It is noted that the negative charge resulting from the interfacial DBP⁻ molecules is fully neutralized for these samples. This is evident from the large attenuation in the 3200 and 3400 cm⁻¹ vSFG peak intensities, which are lowered to values close to those of the neutral air–water interface. In all cases, the 3200 cm⁻¹ peak is suppressed further than the 3400 cm⁻¹ peak, indicating a relative loss of stronger hydrogen bonds within the interfacial water network. Further suppression of the free OH peak at around 3700 cm⁻¹ also occurs for all cases studied, indicating a reduction of water molecules straddling the interface and or a loss in their relative order. A likely explanation for this is that the DBP⁻ molecules push the dangling water molecules down toward the aqueous phase as they occupy the interface.

Complementary surface tension measurements were taken of each solution to aid in the interpretation of the vSFG spectra (Table I and Fig. 2). Surface tension is a thermodynamic property that relates directly to the surface excess free energy. The surface excess free energy of the surfactant is proportional to the interfacial potential or charge as defined by the modified Frumkin isotherm.⁷⁰ However, we note that when a surfactant is added to the air/water interface, it is known to decrease the surface tension by adsorbing at the interface.^71–73 

Indeed, compared to the previously reported surface tension of 68.9 ± 1.2 dyn/cm for a sample containing 0.24 mM HDBP in water,⁵² the surface tension decreases to around 66.0–67.2 dyn/cm when 1 mM salts are added to the bulk aqueous phase. It further decreases to around 65.6–66.1 dyn/cm when the bulk ionic strength is increased to 1M. Increasing ionic strength of the neat air/water interface is expected to increase surface tension;^70,74 therefore, the decrease in surface tension at air/aqueous DBP interface from 72 dyn/cm to about 65.6–66.1 dyn/cm is a strong indicator that the addition of more salt is promoting surfactant adsorption or partitioning of more DBP molecules to the interface. Pure water is known for having a high surface tension of about 72 dyn/cm.⁷⁵ The presence of 1 mM and 1M ionic strength salt samples at the interface would account for the surface tension drop from 72 dyn/cm to about 65.6–66.1 dyn/cm. The decrease in surface tension is, therefore, most likely due to an increased number of interfacial DBP molecules. However, given that the difference between the 1 mM and 1M ionic strength samples is less than 1 dyn/cm, the overall number of interfacial DBP molecules between these samples may be similar. Friedman analysis between the 1 mM and 1M datasets (Table S1) indicated that at the 15% significance level, these populations are considered different, meaning there is a slight difference in means dependent on the concentration between the 1 mM and 1M ionic strength salt samples. The surface tension values reported in Table I are consistent with the range of surface tension values reported in our previous work⁵² and those reported by Lyle and Smith^62,63 for similar HDBP systems. For the 1 mM ionic strength salt concentration, there is a drop of about 2 dyn/cm compared to the surface tension of 0.24 mM HDBP in the water sample.

Given the minimal changes in the O–H region of the vSFG spectra for the 0.24 mM HDBP in water and 1 mM ionic strength samples, the decrease in surface tension is not attributed to change in the surface excess free energy that is affected by change in the interfacial water network (Fig. 1). Instead, it is due to the presence of DBP molecules at the interface. Since it is unlikely that that changes in the surface excess free energy which is affected by interfacial potential and orientation of water molecules alone would account for the total decrease in surface tension, it is reasonable to suspect that an increase in the number of interfacial DBP molecules significantly contributes to the lowered surface tension values.

When the ionic strength of the salts is increased to 1M, there are considerable changes in both the C–H and O–H regions of the vSFG spectra; however, the decrease in surface tension between the 1 mM and 1M samples is less than 1 dyn/cm. While the decrease in surface tension could be due to both an increased number of interfacial DBP molecules and a loss of the long-range order of the interfacial water network, it is important to note that the vSFG O–H intensities and line shapes of these samples are fairly similar to that of the neat air–water sample. It is clear from Figs. 1 and 2 that there are distinct differences between the samples with 1 mM and 1M ionic strength salt concentrations; however, only small changes occur between samples of the same ionic strength among salts of the four metal ions. Indeed, the modest specific ion trends are generally consistent with many specific ion studies performed with sum frequency generation and second harmonic generation spectroscopies at mineral/aqueous and organic/aqueous interfaces.^76–83 The following sections address these smaller SIE trends.

The averaged SSP vSFG spectra of the samples containing 0.24 mM HDBP and 1 mM ionic strength salt concentrations are plotted together in Fig. 3. The results of the relevant statistical analysis are shown in Tables S2–S4.

It is clear from Fig. 3(a) that all four cations instigate very similar behavior of the interfacial DBP when they are present in the bulk aqueous phase at 1 mM ionic strengths. In fact, there is no statistically relevant trend observed for the CH₃ SS peak intensities (Fig. 3), as indicated by the results from one way ANOVA (Table S2), where the null hypothesis is retained and there is likely no difference in the mean CH₃ SS peak intensities. This not only indicates that the 1 mM ionic strength counter cations instigate a high order of conformation in the interfacial DBP molecules but also that this order is similar among all counter cations studied.

It is evident from Fig. 3 that the largest changes in the vSFG spectra of these samples occur around the 3200 and 3400 cm⁻¹ region of the O–H stretch vibration [Fig. 3(b)]. It is noted that there is almost no attenuation of the free OH peak with increasing charge density of the counter cations. This indicates that the number and orientation of the interfacial water molecules dangling across the air–aqueous interface are similar among the different counter cations studied. Attenuation of the 3200 and 3400 cm⁻¹ O–H vibrational modes occurs in a direct Hofmeister order, where counter cations with higher charge densities instigate greater attenuation. This trend is reflected in the O–H areas plotted in Fig. 3(d). The smallest attenuation is observed for the monovalent cation Cs⁺, which has the lowest charge density among the counter cations studied; while the greatest attenuation occurs for the divalent cation Mg²⁺, which has the highest charge density. The results of one-way ANOVA for the OH region area analysis show that the null hypothesis is rejected at the 95% confidence level (a significance threshold of 0.05) and there are statistically significant differences between the mean OH area analysis values (Table S3). The results of a multiple comparison Tukey test (Table S4) show that when considering a confidence level of 85% (a significance threshold of 0.15), which is selected as the significance of a soft trend, there is a difference in mean area values between Cs⁺ and Na⁺. Table S4 also indicates there is not likely a difference in mean area values between Na⁺ and Li⁺, and there is a statistically significant difference between the mean area values of Mg²⁺ and the other cation species at a 95% confidence level (a significance threshold of 0.05). In other words, while there may be a soft trend between the monovalent cations according to their charge densities, there is certainly a statistically relevant difference between the monovalent cations and the divalent cation Mg²⁺.

The attenuation of the 3200 and 3400 cm⁻¹ peaks with counter cations of increasing charge density is consistent with previous findings.^84,85 At low ionic strengths of 1 mM where electrostatic interactions are expected to dominate, counter cations with higher charge densities have been shown to preferentially partition and adsorb to negatively charged interfaces compared to those with lower charge densities. As the higher charge density counter cations partition to the interface, they screen the negative charge more effectively and neutralize the electrostatic field present there. This results in a greater loss of the long-range order of the interfacial water network in the subsequent molecular layers compared to counter cations with lower charge densities. Overall, fewer water molecules contribute to the vSFG intensity and greater attenuation of the 3200 and 3400 cm⁻¹ peaks is observed. It is noted that for the case of DBP and the counter cations studied, the relative intensities and line shape of the 3200 and 3400 cm⁻¹ peaks are consistent at the 1 mM ionic strength salt concentrations. This implies that the structure of the interfacial water network is similar for all counter cations studied, and that the relative ratio of stronger and weaker hydrogen bonds within these networks at the first molecular layers of the interface is also similar. Thus, at low ionic strengths of 1 mM, the identity of the Hofmeister counter cations studied mainly affects the screening of negatively charged interfaces of DBP solution but does not largely alter the interfacial water structure at the first few molecular layers of the interface or the relative ratios of the stronger and weaker hydrogen bonds present there.

The results of the surface tension measurements of the samples containing 0.24 mM HDBP in 1 mM ionic strength salt concentrations (Table I, Fig. 2) show consistent values among the counter cations studied, ranging from 66.0 to 67.2 dyn/cm. The relevant statistical analysis (Table S5) shows that the surface tension data fall in a small range of 66.0–67.2 dyn/cm among the counter cations studied. The null hypothesis is maintained at the 95% confidence level (a significance threshold of 0.05), and there is no statistically relevant difference in means observed for these data. This implies that the cohesive forces among the interfacial molecules for these samples are consistent, regardless of the identity of the counter cations studied. This result is supportive of the CH₃ SS peak analysis of the vSFG data shown in Fig. 3(a), which indicated that the overall number and conformation of the interfacial DBP molecules are similar between these samples. The screening of the counter cations at this ionic strength does not disrupt the cohesive forces among the interfacial molecules enough for there to be a measurable difference in the surface tension values.

The averaged SSP vSFG spectra of the samples containing 0.24 mM HDBP and 1M ionic strength salt concentrations (Fig. 4) show that at higher ionic strength concentrations of 1M, the interfacial charge or potential is effectively screened, and SIE are observed in the C–H region of the vSFG spectra shown in Fig. 4(a).

In particular, statistically relevant differences are observed for the CH₃ SS peaks of the spectra shown in Fig. 4(c). The results of a one-way ANOVA for CH₃ SS mean peak intensities (Table S6) indicate that the null hypothesis is rejected at a confidence level of 95% (a significance threshold of 0.05), which means that there are statistically significant differences in the mean CH₃ SS peak intensity values. The results of a multiple comparison Tukey test for CH₃ SS mean peak intensities (Table S7) show that there is a statistically significant difference at the 95% confidence level (a significance level of 0.05) in the CH₃ SS mean peak intensities of Na⁺ and Mg²⁺. Lowering the confidence level to 85% (a significance threshold of 0.15) indicates soft trends between the mean values for Na⁺ and Cs⁺ as well as Li⁺ and Mg²⁺; however, there is no statistical difference between Cs⁺ and Li⁺.

The trend in CH₃ SS peak intensities appear to be related to the specific interactions between each individual cation and the DBP⁻ headgroup, where the interactions are driven by the general “like seeks like” rule.^50,86 It has been previously reported that for the general case of a phosphate head group, the counter cation Na⁺ has a charge that is most similar to that of the phosphate head group in terms of “hard” and “soft” charges.⁸⁷ The counter cations Li⁺ and Cs⁺ have charges that are less similar, where Li⁺ has a harder charge and Cs⁺ has a softer charge. In theory, the divalent counter cation Mg²⁺ would have a harder charge than Li⁺ and would have the least similar charge compared to the phosphate head group. According to the general “like seeks like” rule, Na⁺ would be expected to interact the most with the phosphate head group and neutralize the negative charge of the DBP⁻ molecules. This would result in the more effective salting out of the DBP molecules, which would explain the higher vSFG intensity observed for the CH₃ SS peak for the Na⁺ sample compared to the other counter cations. The counter cations Li⁺ and Cs⁺, having charges less similar, would be expected to less effectively neutralize the negative charge of the DBP⁻ molecules and, therefore, result in less salting out. This would explain the lower vSFG intensities of the CH₃ SS peaks compared to that of the Na⁺ sample. Since the charge of the Mg²⁺ is the least similar, it would be expected to interact with the interfacial DBP⁻ molecules the least. While it may neutralize the negative charge at the interface more effectively than the monovalent cations, it is the least effective in salting out of the DBP molecules. This would explain why the vSFG intensity of the CH₃ SS peak for the Mg²⁺ sample is the lowest out of the counter cations studied. It is also noted that among the four cations studied, Mg²⁺ has the single largest hydration enthalpy (in kJ/mol): −520 (Li⁺), −406 (Na⁺), −276 (Cs⁺), and −1921 (Mg²⁺). Thus, the tight binding of hydration waters in case of Mg²⁺, may soften the influence of DBP⁻, resulting the lowest area in the O–H region of the SSP vSFG spectra among the four salt solutions.

While the negative charge at the interface is effectively screened at 1M ionic strength salt concentrations, the direct Hofmeister trend previously observed in the O–H region of the vSFG spectra for the 1 mM ionic strength samples (Fig. 3) persists. The exception is Na⁺, which diverges from the other cations and yields a higher area calculated for the O–H region [Fig. 4(d)]. The results of the one-way ANOVA analysis for the mean OH region area analysis values (Table S8) indicate that at the 95% confidence level (a significance threshold of 0.05), the null hypothesis is rejected and there is a statistically relevant difference in the mean OH region area analysis values. Multiple comparison analysis results for mean OH region area analysis values (Table S9) confirm the observed Na⁺ divergence from the other⁸⁸ to form aggregates.^89–91 If aggregates are starting to form at 1M NaNO₃, then this could instigate a variety of slightly different orientations of the water molecules within the interfacial region. This could explain the larger O–H area calculated for the Na⁺ sample. Lowering the confidence level to 85% (a significance threshold of 0.15) indicates soft trends between the mean values for Cs⁺ and Li⁺ as well as Li⁺ and Mg²⁺. It should be noted that this trend is overall significantly smaller than that observed for the 1 mM ionic strength samples, with the range in average areas being 0.036–0.045 a.u. for 1M ionic strengths compared to the larger range of 0.092–0.13 a.u. observed for the 1 mM ionic strength samples. Nonetheless, this trend is a soft trend, persisting with 85% confidence. Consistent with our observations, Tyrode and Corkery studied the specific ion effects on charging of the arachidic acid Langmuir monolayer as a function monovalent ion concentration below 100 mM in the subphase, and they found that the charging of the monolayer was experimentally found to be independent of the identity of the monovalent cation (i.e., Li⁺, Na⁺, Rb⁺) or anion (i.e., F⁻, Cl⁻, I⁻) at low salt concentration.⁹² At charged interface, two types of water populations give rise to the vSFG signal, the chemically bound water molecules close to surface and aligned water molecules in the diffused layer. For 0.24 mM HDBP in water (surface charge density without ion-pairing in the range of μC–mC/m²), the diffused layer water molecules primarily contribute to the vSFG signal in the H-bonded OH stretching region. Formation of ion pairs between the phosphate head group and counter cation decreases the effective surface charge density and attenuates the vSFG intensity in the OH stretching region. The extent of attenuation of vSFG intensity can be related to the ion specific effects. Judd et al. investigated the effect of ion specific effects on the vSFG intensity of the eicosyl sulfate (ESO₄) Langmuir monolayer at air–water interface with LiCl, NaCl, and KCl subphases and showed that OH stretching region vSFG intensity increases in the order K⁺ < Na⁺ < Li⁺, opposite order to binding affinity of the cations to the sulfate head group.⁸⁸ At ESO₄ mole fraction of 0.33 and lower, all cations were concluded to have formed contact ion pairs. Above this concentration, Li⁺ formed contact ion pairs with the sulfate head groups whereas K⁺ was found to only form solvent shared ion pairs with the head groups. Unfortunately, our vSFG measurements in the OH stretch region cannot provide direct insight into whether the cation binds to the phosphate head group of the DBP⁻ through contact ion pair formation or via solvent shared ion paring.

Overall, the present results appear to indicate that counter cations still partition to the interfacial region based on their charge densities; however, once within the interfacial region, the counter cations interact with the DBP⁻ molecules based on the “like seeks like” rule.

Surface tension data of the samples containing 0.24 mM HDBP in 1M ionic strength salt concentrations (Table I, Fig. 2) and the corresponding one-way ANOVA results (Table S10) indicate that there is not likely a difference in the mean surface tension measurements between the various counter cations. This implies the cohesive forces at the interface still remain similar between all counter cations studied. These results highlight the sensitivity of vSFG spectroscopy compared to surface tension instruments. There are statistically significant differences in both the C–H and O–H regions of the vSFG spectra with 95% confidence, and soft trends at 85% confidence are also observed for both regions. However, these differences do not manifest measurable changes in the surface tension results.

Fundamental qualitative relationships among bulk nitrate salts of Cs⁺, Na⁺, Li⁺, and Mg²⁺ as well as HDBP molecules at the air–aqueous interface have been investigated at 0.24 mM HDBP in the presence of either 1 mM or 1M ionic strengths. At a low bulk HDBP concentration of 0.24 mM, nearly all the HDBP molecules have deprotonated, supplying a negatively charged air–aqueous interface for the counter cations to partition to. The addition of counter cations via nitrate salts in general appears to result in the salting out of DBP molecules. At low salt concentrations of 1 mM ionic strength, the counter cations partition to the air–aqueous interface in a direct Hofmeister order and partially screen the negative charge from the DBP⁻ molecules. Due to their neutralized charge, some of the DBP molecules partition to the interface as they prefer the hydrophobic air phase. This would also reduce the electrostatic repulsions between them, allowing for a higher packing of DBP molecules at the interface. As the salt concentrations are increased to an ionic strength of 1M, the negative charge from the remaining DBP⁻ molecules is fully neutralized and a higher degree of salting out appears to occur. This results in a relative loss in the order of conformation of the interfacial DBP molecules as gauche defects occur, evident from the prominent CH₂ SS peak in the vSFG spectra for these samples. As more DBP molecules partition to the interface and higher packing is achieved, the butyl chains from the interfacial DBP molecules likely contact other neighboring DBP molecules and push them out of an all-trans conformation. The degree of salting out at these higher 1M ionic strength salt concentrations appear to be dependent on the specific interactions between each individual cation and the DBP⁻ head group. As the negative charge of the DBP⁻ is neutralized, SIE between the counter cations and the DBP⁻ head group can occur. Despite the soft trends observed in the vSFG spectra of both the 1 mM and 1M samples, no statistically relevant trends were observed for the surface tension measurements of counter cation samples with the same ionic strength. This demonstrates the sensitivity of vSFG spectroscopy and its ability to provide more detailed insights into interfacial regions. This work forms a foundation for future investigations of DBP at hydrophobic–aqueous interfaces, such as those involving heavy metal ion dependence, and provides one of the first steps toward potentially utilizing vSFG to investigate third phase materials, such as crud at hydrophobic-aqueous interfaces.

See the supplementary material for information about the sample container clean procedures and all the result tables of the statistical data analysis.

This research was supported at Washington State University (WSU) by the National Nuclear Security Administration (NNSA) under Award No. DE-NA0003763 for C.L. and S.C. while partially supported by the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM), an Energy Frontier Research Center (Grant No. FWP68932), and the Geosciences Program at Pacific Northwest National Laboratory (PNNL) (FWP No. 56674) both funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), for other co-authors. A portion of this research was performed using EMSL, a DOE national user facility at PNNL sponsored by the Office of Biological and Environmental Research. PNNL is a multi-program national laboratory operated for the DOE by Battelle Memorial Institute under Contract No. DE-AC05-76RL01830. C.L. is thankful for the support from the PNNL-WSU DGRP program.

The authors have no conflicts to disclose.

Christina Louie: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Writing – original draft (equal). Narendra Adhikari: Formal analysis (equal); Methodology (equal); Writing – review & editing (equal). Mavis D. Boamah: Formal analysis (equal); Methodology (equal); Writing – review & editing (equal). Sue Clark: Conceptualization (equal); Funding acquisition (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal). Aashish Tuladhar: Data curation (equal); Funding acquisition (equal); Methodology (equal); Resources (equal); Writing – review & editing (equal). Carolyn I. Pearce: Conceptualization (equal); Funding acquisition (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal). Kevin M. Rosso: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Methodology (equal); Resources (equal); Supervision (equal); Writing – review & editing (equal). Zheming Wang: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Methodology (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.