Vibrational Sum Frequency Research Articles

Comparative Thermodynamic and Structural Analysis of Polyfluorinated Dodecylphosphonic Acid Adsorption to Distilled and River Water Interfaces.

As concerns rise about the health risks posed by per- and polyfluoroalkyl substances (PFAS) in the environment, there is a need to understand how these pollutants accumulate at environmental interfaces. Untangling the details of molecular adsorption, particularly when there are potential interactions with other molecules in environmental systems, can obscure the ability to focus on a particular contaminant with molecular specificity. Often adsorption studies of environmental interfaces require a reductionist approach, where laboratory experiments may not be fully tractable to environmental systems. In this work, we study polyfluorinated dodecylphosphonic acid (F21-DDPA) at the aqueous surfaces of distilled water (the most reduced "environmental" surface) and river water to explore the use of vibrational sum-frequency (VSF) spectroscopy as an experimental probe of fluorinated contaminants at natural environmental surfaces. We demonstrate how VSF spectroscopy offers advantages over nonspecific surface tension measurements when measuring PFAS adsorption isotherms at river water surfaces. VSF spectra of the C-F stretching region selectively probe the presence of F21-DDPA and can be used to extract meaningful structural insights and calculate surface concentrations, even at the complex river water surface. This study highlights the potential for VSF spectroscopy to be developed as a probe of fluorinated contaminants at natural environmental interfaces.

Playing Favorites: Preferential Adsorption of Nonionic over Anionic Surfactants at the Liquid/Liquid Interface.

While many studies have investigated synergic interactions between surfactants in mixed systems, understanding possible competitive behaviors between interfacial components of binary surfactant systems is necessary for the optimized efficacy of applications dependent on surface properties. Such is the focus of these studies in which the surface behavior of a binary surfactant mixture containing nonionic (Span-80) and anionic (AOT) components adsorbing to the oil/water interface was investigated with vibrational sum-frequency (VSF) spectroscopy and surface tensiometry experimental methods. Time-dependent spectroscopic studies reveal that while both nonionic and anionic surfactants initially adsorb to the interface, anionic surfactants desorb over time as the nonionic surfactant continues to adsorb. Concentration studies that vary the ratio of Span-80 to AOT in bulk solution show that the nonionic surfactant preferentially adsorbs to the oil/water interface over the anionic surfactant. These studies have important implications for applications in which mixed surfactant systems are used to alter interfacial properties, such as pharmaceuticals, industrial films, and environmental remediation.

Twist and Stretch: Assignment and Surface Charge Sensitivity of a Water Combination Band and Its Implications for Vibrational Sum Frequency Spectra Interpretations.

The vibrational spectrum of water at hydrophobic/hydrophilic interfaces is crucial to understanding complex surface chemistry phenomena. Vibrational sum frequency (VSF) spectroscopy is a valuable nonlinear spectroscopic technique for exploring the details of vibrational spectra of molecules at surfaces. However, spectral assignments and analysis of VSF spectra are often more nuanced than in linear spectroscopy. This study is aimed at understanding the source of the broad VSF signal in the oil-water surface water spectrum at energies slightly higher than traditionally examined, beyond the unbound free OH oscillator of surface water molecules. Analyzing isotopic dilutions of the aqueous solvent with VSF spectroscopy, we demonstrate that this signal is due to a combination band of water stretch and libration motions. The spectral characteristics of this band are found to be highly sensitive to the sign and magnitude of the surface charge induced by adsorption of both anionic and cationic surfactants. The results have implications for VSF measurements of the C-H stretching vibrations of various adsorbates when studied with D2O as the aqueous solvent. Because the vibrational signal from the water combination band is dependent on surface charge, it is imperative to include the presence of the combination band when fitting surface spectra.

Molecular Recognition at Mineral Interfaces: Implications for the Beneficiation of Rare Earth Ores.

Ce-bastnäsite is the single largest mineral source for light rare-earth elements. In view of the growing industrial importance of rare-earth minerals, it is critical to develop more efficient methods for separating the valuable rare-earth-containing minerals from the surrounding gangue. In this work, we employ a combination of periodic density functional theory (DFT) and molecular mechanics (MM) calculations together with the de novo molecular design program HostDesigner to identify bis-phosphinate ligands that preferentially bind to the (100) Ce-bastnäsite surface rather than the (104) calcite surface. DFT calculations for a simple phosphinate ligand were employed to qualitatively understand key behaviors involved in ligand-metal, ligand-solvent, and solvent-metal interactions. These insights were then used to guide the search for flexible, rigid, and semirigid hydrocarbon linkers to identify candidate bis-phosphinate ligands with the potential to bind preferentially to Ce-bastnäsite. Among the five most promising bis-phosphinate ligands suggested by theoretical studies, three ligands were synthesized and their adsorption characteristics to bastnäsite (100) interfaces were characterized using vibrational sum-frequency (vSFG) spectroscopy, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and isothermal titration calorimetry (ITC). The efficacy of the selective interfacial molecular binding was demonstrated by identifying a bis-phosphinate ligand capable of providing an overall higher surface coverage of alkyl groups relative to a monophosphinate ligand. The results highlight the interplay between adsorption binding strength and maximum surface coverage in determining ligand efficiency to render the mineral surface hydrophobic. DFT calculations further indicate that all tested ligands have higher affinity for Ce-bastnäsite than for calcite. This is consistent with the ITC data showing stronger adsorption enthalpy to bastnäsite than to calcite, making these ligands promising candidates for selective flotation of Ce-bastnäsite.

Coming to Order: Adsorption and Structure of Nonionic Polymer at the Oil/Water Interface as Influenced by Cationic and Anionic Surfactants.

Polymer-surfactant mixtures are versatile chemical systems because of their ability to form a variety of complexes both in bulk solution and at surfaces. The adsorption and structure of polymer-surfactant complexes at the oil/water interface define their use surface chemistry applications. Previous studies have investigated the interactions between charged polyelectrolytes and surfactants; however, a similar level of insight into the interfacial behavior of nonionic polymers in mixed systems is lacking. The study herein uses vibrational sum frequency (VSF) spectroscopy to elucidate the molecular details of nonionic polyacrylamide (PAM) adsorption to the oil/water interface in the presence of surfactant. The polymer's adsorption and conformational structure at the interface is investigated as it interacts with cationic and anionic surfactants. Where the polymer will not adsorb to the interface on its own in solution, the presence of either cationic or anionic surfactant causes favorable adsorption of the polymer to the oil/water interface. VSF spectra indicate that the cationic surfactant interacts with PAM at the interface through charge-dipole interactions to induce conformational ordering of the polymer backbone. However, conformational ordering of polymer is not induced at the interface when anionic surfactant is present.

Vibrational spectra of dissociatively adsorbed D2O on Al-terminated α-Al2O3(0001) surfaces from ab initio molecular dynamics.

Water can adsorb molecularly or dissociatively onto different sites of metal oxide surfaces. These adsorption sites can be disentangled using surface-sensitive vibrational spectroscopy. Here, we model Vibrational Sum Frequency (VSF) spectra for various forms of dissociated, deuterated water on a reconstructed, Al-terminated α-Al2O3(0001) surface at submonolayer coverages (the so-called 1-2, 1-4, and 1-4' modes). Using an efficient scheme based on velocity-velocity autocorrelation functions, we go beyond previous normal mode analyses by including anharmonicity, mode coupling, and thermal surface motion in the framework of ab initio molecular dynamics. In this way, we calculate vibrational density of states curves, infrared, and VSF spectra. Comparing computed VSF spectra with measured ones, we find that relative frequencies of resonances are in quite good agreement and linewidths are reasonably well represented, while VSF intensities coincide not well. We argue that intensities are sensitively affected by local interactions and thermal fluctuations, even at such low coverage, while absolute peak positions strongly depend on the choice of the electronic structure method and on the appropriate inclusion of anharmonicity.

Resolving the chemical identity of H2SO4 derived anions on Pt(111) electrodes: they're sulfate.

Understanding how electrolyte composition controls electrocatalytic reactions requires molecular-level insight into electrode/electrolyte interaction. Perhaps the most basic aspect of this interaction, the speciation of the interfacial ion, is often controversial for even relatively simple systems. For example, for Pt(111) in 0.5 M H2SO4 it has long been debated whether the adsorbed anion is SO42-, HSO4- or an H3O+SO42- ion pair. Here we apply interface-specific vibrational sum frequency (VSF) spectroscopy and theory to this problem and perform an isotope exchange study: we collect VSF spectra of Pt(111) in H2SO4(H2O) and D2SO4(D2O) as a function of bias and show that at all potentials they are identical. This is the most direct spectroscopic evidence to date that SO42- is the dominant adsorbate, despite the fact that at 0.5 M H2SO4 bulk solution is dominated by HSO4-. This approach is based on the unique selection rule of the VSF spectroscopy and thus offers a new way of accessing general electrode/electrolyte interaction in electrocatalysis.

Vibrational spectroscopy of hydroxylated α-Al2O3(0001) surfaces with and without water: An ab initio molecular dynamics study.

Using gradient- and dispersion-corrected density functional theory in connection with ab initio molecular dynamics and efficient, parametrized Velocity-Velocity Autocorrelation Function (VVAF) methodology, we study the vibrational spectra (Vibrational Sum Frequency, VSF, and infrared, IR) of hydroxylated α-Al2O3(0001) surfaces with and without additional water. Specifically, by considering a naked hydroxylated surface and the same surface with a particularly stable, "ice-like" hexagonal water later allows us to identify and disentangle main spectroscopic bands of OH bonds, their orientation and dynamics, and the role of water adsorption. In particular, we assign spectroscopic signals around 3700 cm-1 as being dominated by perpendicularly oriented non-hydrogen bonded aluminol groups, with and without additional water. Furthermore, the thin water layer gives spectroscopic signals which are already comparable to previous theoretical and experimental findings for the solid/(bulk) liquid interface, showing that water molecules closest to the surface play a decisive role in the vibrational response of these systems. From a methodological point of view, the effects of temperature, anharmonicity, hydrogen-bonding, and structural dynamics are taken into account and analyzed, allowing us to compare the calculated IR and VSF spectra with the ones based on normal mode analysis and vibrational density of states. The VVAF approach employed in this work appears to be a computationally accurate yet feasible method to address the vibrational fingerprints and dynamical properties of water/metal oxide interfaces.

Model Behavior: Characterization of Hydroxyacetone at the Air-Water Interface Using Experimental and Computational Vibrational Sum Frequency Spectroscopy.

Small atmospheric aldehydes and ketones are known to play a significant role in the formation of secondary organic aerosols (SOA). However, many of them are difficult to experimentally isolate, as they tend to form hydration and oligomer species. Hydroxyacetone (HA) is unusual in this class as it contributes to SOA while existing predominantly in its unhydrated monomeric form. This allows HA to serve as a valuable model system for similar secondary organic carbonyls. In this paper the surface behavior of HA at the air-water interface has been investigated using vibrational sum frequency (VSF) spectroscopy and Wilhelmy plate surface tensiometry in combination with computational molecular dynamics simulations and density functional theory calculations. The experimental results demonstrate that HA has a high degree of surface activity and is ordered at the interface. Furthermore, oriented water is observed at the interface, even at high HA concentrations. Spectral features also reveal the presence of both cis and trans HA conformers at the interface, in differing orientations. Molecular dynamics results indicate conformer dependent shifts in HA orientation between the subsurface (∼5 Å deep) and surface. Together, these results provide a picture of a highly dynamic, but statistically ordered, interface composed of multiple HA conformers with solvated water. These results have implications for HA's behavior in aqueous particles, which may affect its role in the atmosphere and SOA formation.

Interfacial Structure and Hydration of 3D Lipid Monolayers in Aqueous Solution.

Three-dimensional (3D) phospholipid monolayers at hydrophobic surfaces are ubiquitous and found in nature as adiposome organelles or in man-made materials such as drug delivery systems. However, the molecular level understanding of such monolayers remains elusive. Here, we investigate the molecular structure of phosphatidylcholine (PC) lipids forming 3D monolayers on the surface of hexadecane nanodroplets. The effects of acyl chain length, saturation, and number of acyl tails per lipid were studied with vibrational sum frequency and second harmonic scattering techniques. We find that 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC) lipids form tightly packed monolayers. Upon shortening the tail length to 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), more gauche defects are observed. Monolayers of unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and single acyl chained 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (lyso-PC) contain more disorder. Despite these variations in the packing, the headgroup orientation remained approximately parallel to the nanodroplet interface. Remarkably, the lyso-PC uniquely forms more diluted and "patchy" 3D monolayers.

Computational Vibrational Sum Frequency Spectra of Formaldehyde and Hydroxymethanesulfonate at Aqueous Interfaces

The identity and arrangement of aqueous species at the interface of atmospheric aerosol impacts aerosol properties including albedo and propensity to uptake additional gas phase species. Formaldehyde and sulfur dioxide are two common atmospheric species that alter (individually and in concert) aqueous atmospheric aerosol interfaces. Vibrational sum frequency (VSF) spectroscopy studies of planar aqueous formaldehyde solution surfaces have shown alteration during exposure to sulfur dioxide gas. Additional changes were observed once exposure was ceased. The results suggested the formation of a new organic species, hydroxymethanesulfonate (HMS), which acts as a thermodynamic sink for aqueous sulfur dioxide and lead to acidification of the aerosol particles. The coherent nature of the VSF response and its strong dependence on surface species present and their orientations, however, made definite vibrational assignments for exact species notoriously difficult. The focus of this paper is on elucidating the speci...

Unusual Structure and Dynamics at Silica/Methanol and Silica/Ethanol Interfaces--A Molecular Dynamics and Nonlinear Optical Study.

Vibrational sum frequency (VSF) spectroscopy and molecular dynamics simulations are used to investigate ethanol-silica and methanol-silica interfaces. We describe the subtle differences in molecular organization that result in the observed differences in the VSF spectra for methanol and ethanol at the alcohol-silica interface. Alcohol molecules hydrogen-bonded to the silica surface induce orientational opposition in an adjacent low-population region, which implies VSF signal reduction. This low population region is essentially of zero density in the ethanol system, implying less signal cancelation. Simulated silica defect sites increase the population of this region in both systems. Interestingly, the induced orientation in this region influences subsequent molecular orientation only in the ethanol-silica system, preserving the interfacial anisotropy. These effects suggest a stronger VSF response from the ethanol-silica system versus the methanol-silica system, where more methanol molecules reside in the low-population region, and this region does not induce order in subsequent solvent layers.

Twist and turn: effect of stereoconfiguration on the interfacial assembly of polyelectrolytes.

Understanding the conditions that promote the adsorption, assembly, and accumulation of charged macromolecules at the interface between aqueous and hydrophobic liquids is important to a multitude of biological, environmental, and industrial processes. Here, the oil-water interfacial behavior of stereoisomers of polymethacrylic acid (PMA), a model system for both naturally occurring and synthetic polyelectrolytes, is investigated with a combination of vibrational sum-frequency (VSF) spectroscopy, surface tension, and computations. Syndiotactic and isotactic isomers both show rapid adsorption to the oil-water interface with a net orientation indicative of a high degree of ordering. The stereoconfiguration is found to affect whether only a single layer or multiple layers assemble at the interface. Surface tension measurements show additional adsorption for syndiotactic PMA over time. The additional layers do not contribute to the VSF spectrum indicating disorder in all but the initial layer. The isotactic isomer shows no evidence of accumulation at the interface beyond the single ordered layer. Molecular dynamics calculations show marked differences between the two isomers in the orientation of their substituent groups at the interface. The hydrophilic and hydrophobic moieties in the isotactic isomer are easily partitioned to the water and oil phases, respectively, whereas a fair portion of hydrophobic groups remain in the water phase for the syndiotactic PMA. The available hydrophobic contacts in the water phase at the interface are credited with allowing further adsorption.

Doubling down: delving into the details of diacid adsorption at aqueous surfaces.

The behavior of complex interfacial systems is central to an ever-increasing number of applications. Vibrational sum frequency (VSF) spectroscopy is a powerful technique for obtaining surface specific structural information. The coherent nature of VSF that provides surface specificity, however, also creates difficulty in spectral interpretation especially as the system complexity increases. Computations of VSF spectra shed light on the molecular level source of the experimental VSF signal, allowing for the analysis of more complicated systems. Unfortunately, the majority of calculations of VSF spectra look at the response of the solvent or of rigid molecules and therefore often poorly reflect the experimental environment of most VSF spectroscopic measurements. In this work, flexible solute molecules at interfaces are investigated by doubling down, obtaining and comparing experimental and theoretical spectra, to determine a more accurate computational treatment. The surface behavior and VSF spectra of glutaric acid and adipic acid at the air/water interface are determined experimentally and calculated using a combination of classical molecular dynamics and density functional theory. Both diacids are found to be surface active. At high concentrations, glutaric acid forms dimers altering its VSF response and acidic properties. Calculated VSF spectra are found to be sensitive to vibrational mode frequencies, with ordering and spacing affecting relative intensities, as well as molecular conformation. A proper description requires consideration of multiple conformers and anharmonic effects on the molecular vibrational energies.

The Free OD at the Air/D2O Interface Is Structurally and Dynamically Heterogeneous

Air/water interfaces are both ubiquitous in the environment and technology and a useful model for hydrophobic solvation more generally. Previous experimental and computational studies have highlighted that molecular level markers of such an extended hydrophobic surface are broken hydrogen bonds and, as a result, OH groups that are not hydrogen bond donors: free OH. Understanding both the time-averaged structure and structural dynamics of these free OH thus plays a critical role in developing a quantitative, molecular level understanding of hydrophobic solvation. Here we show, by combining polarization-dependent vibrational sum frequency (VSF) spectroscopy and molecular dynamics simulation, that the free OD of D2O at the air/D2O interface is structurally and dynamically heterogeneous: that longer lived free OD groups tend to point closer to the surface normal, have a narrower orientational distribution, and are closer to the vapor phase. Knowledge of this structural heterogeneity should help link existing descriptions of hydrophobic solvation that focus either on the termination of the bulk hydrogen bond network or local density fluctuations. In addition the results of this study clarify that schemes to increase signal-to-noise ratios in VSF measurements by delaying the visible pulse relative to the infrared should be used only with independent constraints on the system's structural dynamics.

Tuning the Molecular Order of C60 Functionalized Phosphonic Acid Monolayers

Mixed self-assembled monolayers (SAM) of alkyl phosphonic acids and C(60) functionalized octadecyl phosphonic acids (C(60)C(18)-PA) are deposited on alumina substrates from solution and are shown to form well-ordered structures with an insulating layer of alkyl chains and a semiconducting layer that comprises mainly C(60). Such an ordered structure is a necessity for the application of SAMs in organic transistors but is difficult to obtain since C(60)C(18)-PA without additional support do self-assemble in dense packaging but not in a well-ordered fashion. To avoid disordering of the SAM and to gain a better control of the interfacial properties we have investigated the stabilizing effects of fluorinated dodecyl phosphonic acids (FC(12)-PA) on the C(60)C(18)-PA monolayer. Vibrational sum-frequency (SFG) spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, and electrical measurements were applied to study the mixed monolayers. Here, we make use of the differently labeled PA to determine surface coverages and molecular properties of the two species independently. Adsorption of FC(12)-PA gives rise to vibrational bands at 1344 cm(-1) and 1376 cm(-1) in SFG spectra, while a pronounced vibrational band centered at 1465 cm(-1) is attributable to C(60) vibrations. The coexistence of the bands is indicative for the presence of a mixed monolayer that is composed of both molecular species. Furthermore, a pronounced maximum in SFG intensity of the C(60) band is observed for SAMs, which are deposited from solutions with ~75% C(60)C(18)-PA and ~25% FC(12)-PA. The intensity maximum originates from successful stabilization of C(60) modified C(60)C(18)-PA by FC(12)-PA and a significantly improved molecular order. Conclusions from SFG spectra are corroborated by electric measurements that show best performance at these concentrations. Our results provide new information on the morphology and composition of C(60) modified SAMs and establish a route to fabricate well-defined layers for molecular scale organic electronics.

Interpretation of the water surface vibrational sum-frequency spectrum

We propose a novel interpretation of the water liquid-vapor interface vibrational sum-frequency (VSF) spectrum in terms of hydrogen-bonding classes. Unlike an absorption spectrum, the VSF signal can be considered as a sum of signed contributions from different hydrogen-bonded species in the sample. We show that the recently observed positive feature at low frequency, in the imaginary part of the signal, is a result of cancellation between the positive contributions from four-hydrogen-bonded molecules and negative contributions from those molecules with one or two broken hydrogen bonds. Spectral densities for each of these subgroups span the entire relevant spectral range. Three-body interactions within our newly developed E3B water simulation model prove to be critical in describing the proper balance between different hydrogen-bonded species, as (two-body) SPC/E, TIP4P, and TIP4P/2005 models fail to reproduce the positive feature. The results clarify the molecular origin of the VSF signal, and highlight the importance of many-body interactions for water in heterogeneous situations.

Structural Inhomogeneity of Interfacial Water at Lipid Monolayers Revealed by Surface-Specific Vibrational Pump−Probe Spectroscopy

We report vibrational lifetime measurements of the OH stretch vibration of interfacial water in contact with lipid monolayers, using time-resolved vibrational sum frequency (VSF) spectroscopy. The dynamics of water in contact with four different lipids are reported and are characterized by vibrational relaxation rates measured at 3200, 3300, 3400, and 3500 cm(-1). We observe that the water molecules with an OH frequency ranging from 3300 to 3500 cm(-1) all show vibrational relaxation with a time constant of T(1) = 180 ± 35 fs, similar to what is found for bulk water. Water molecules with OH groups near 3200 cm(-1) show distinctly faster relaxation dynamics, with T(1) < 80 fs. We successfully model the data by describing the interfacial water containing two distinct subensembles in which spectral diffusion is, respectively, rapid (3300-3500 cm(-1)) and absent (3200 cm(-1)). We discuss the potential biological implications of the presence of the strongly hydrogen-bonded, rapidly relaxing water molecules at 3200 cm(-1) that are decoupled from the bulk water system.

Elucidating the Association of Water in Wet 1-Octanol from Normal to High Temperature by Near- and Mid-Infrared Spectroscopy

Near- (NIR) and mid-infrared (MIR) absorption spectra of pure and water-saturated 1-octanol were measured along the liquid-gas coexistence curve from ambient temperature and pressure up to 300 degrees C and 10 MPa. Density of the mixture as a function of temperature was assessed by spectral analysis in the NIR region. Two distinct regimes of temperature were identified in the evolution of solvent-subtracted spectra: at normal conditions and up to 180 degrees C, water is organized in multimeric H-bonded aggregates, while at higher temperature, mainly dimers and monomers exist. Water-water and alcohol-alcohol H-bonding interactions play a major role in wet octanol at low temperature, with resulting microheterogeneity and water segregation at a molecular level; on the other hand, water-alcohol interactions are relevant at higher temperature, as also revealed by the estimated density. At low temperature, water dissolved in octanol shows features which are comparable to those of interfacial water obtained by vibrational sum frequency (VSF) spectroscopy, while at high temperature the spectra generally reproduce those of water in the supercritical phase. Overall spectral assignment was supported by ab initio calculations at the MP2 level on small water clusters.

Molecular Restructuring of Water and Lipids upon the Interaction of DNA with Lipid Monolayers

Understanding the molecular mechanism of DNA/lipid interaction is critical in optimizing the use of lipid cofactors in gene therapy. Here, we address this question by employing label-free vibrational sum frequency (VSF) spectroscopy to study the interaction of DNA with lipid monolayers of the cationic lipids DPTAP(1,2-dipalmitoyl-3-trimethylammonium-propane) and diC14-amidine as well as the zwitterionic lipid DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) in the presence and absence of calcium. Our approach has the advantage both of allowing us to explicitly probe intermolecular interactions and of providing insight into the structure of water and lipids around DNA at the lipid interface. We find, by examination of the OD stretch of interfacial D(2)O, that water structure differs markedly between systems containing DNA adsorbed to cationic and those that contain DNA adsorbed to zwitterionic lipid monolayers (in the presence or absence of Ca(2+)). The spectral response of interfacial water in the cationic system is consistent with a highly structured, undercoordinated, structural 'type' of water. Further, by investigation of CH stretch modes of the diC14-amidine lipid tails, we demonstrate that the adsorption of DNA to this lipid leads to increased ordering of lipid tails.