An empirical approach to the bond additivity model in quantitative interpretation of sum frequency generation vibrational spectra

Knowledge of the ratios between different polarizability betai'j'k' tensor elements of a chemical group in a molecule is crucial for quantitative interpretation and polarization analysis of its sum frequency generation vibrational spectroscopy (SFG-VS) spectrum at interface. The bond additivity model (BAM) or the hyperpolarizability derivative model along with experimentally obtained Raman depolarization ratios has been widely used to obtain such tensor ratios for the CH3, CH2, and CH groups. Successfully, such treatment can quantitatively reproduce the intensity polarization dependence in SFG-VS spectra for the symmetric (SS) and asymmetric (AS) stretching modes of CH3 and CH2 groups, respectively. However, the relative intensities between the SS and AS modes usually do not agree with each other within this model even for some of the simplest molecular systems, such as the air/methanol interface. This fact certainly has cast uncertainties on the effectiveness and conclusions based on the BAM. One of such examples is that the AS mode of CH3 group has never been observed in SFG-VS spectra from the air/methanol interface, while this AS mode is usually very strong for SFG-VS spectra from the air/ethanol interface, other short chain alcohol, as well as long chain surfactants. In order to answer these questions, an empirical approach from known Raman and IR spectra is used to make corrections to the BAM. With the corrected ratios between the betai'j'k' tensor elements of the SS and AS modes, all features in the SFG-VS spectra of the air/methanol and air/ethanol interfaces can be quantitatively interpreted. This empirical approach not only provides new understandings of the effectiveness and limitations of the bond additivity model but also provides a practical way for its application in SFG-VS studies of molecular interfaces.     

A Surface Rheological Study of Silwet L-77 Surfactant at the Air/Water Interface

The dilational viscoelastic properties of Silwet L-77, a trisiloxane surfactant of the general formula (Me₃SiO)₂SiMe(CH₂)₃(OCH₂CH₂)_7.5OH, at the air/water interface were investigated. Aqueous solutions of Silwet L-77 were spread onto a pendant drop and the dynamic surface tensions were measured by means of axisymmetric drop shape analysis. The surface dilatational elasticity, viscosity, and phase angle of the adsorption monolayer were also determined using the oscillating drop technique. The influences of frequency and concentration on the surface dilational properties were expounded. It is shown that the surface dilational modulus and elastic modulus increased with the increase of frequency, the viscous modulus was complex, while the phase angle decreased with the increase of frequency. Surface dilational modulus, elastic modulus, and viscous modulus passed through the maximum with the increase of bulk concentration and the phase angle increased with the increase of concentration.     

Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

Here we report a detailed study on spectroscopy, structure, and orientational distribution, as well as orientational motion, of water molecules at the air/water interface, investigated with sum frequency generation vibrational spectroscopy (SFG-VS). Quantitative polarization and experimental configuration analyses of the SFG data in different polarizations with four sets of experimental configurations can shed new light on our present understanding of the air/water interface. Firstly, we concluded that the orientational motion of the interfacial water molecules can only be in a limited angular range, instead of rapidly varying over a broad angular range in the vibrational relaxation time as suggested previously. Secondly, because different vibrational modes of different molecular species at the interface has different symmetry properties, polarization and symmetry analyses of the SFG-VS spectral features can help the assignment of the SFG-VS spectra peaks to different interfacial species. These analyses concluded that the narrow 3693 cm(-1) and broad 3550 cm(-1) peaks belong to C(infinityv) symmetry, while the broad 3250 and 3450 cm(-1) peaks belong to the symmetric stretching modes with C2v symmetry. Thus, the 3693 cm(-1) peak is assigned to the free OH, the 3550 cm(-1) peak is assigned to the singly hydrogen-bonded OH stretching mode, and the 3250 and 3450 cm(-1) peaks are assigned to interfacial water molecules as two hydrogen donors for hydrogen bonding (with C2v symmetry), respectively. Thirdly, analysis of the SFG-VS spectra concluded that the singly hydrogen-bonded water molecules at the air/water interface have their dipole vector directed almost parallel to the interface and is with a very narrow orientational distribution. The doubly hydrogen-bonded donor water molecules have their dipole vector pointing away from the liquid phase.     

不对称Gemini表面活性剂在气/液界面的吸附动力学

合成出由1个亚甲基联接羟基和季铵基头基, 且带两根不同长度烷烃链的不对称Gemini表面活性剂CmH2m+1OCH2CH(OH)CH2N+(CH3)2C8H17Br(记为CmOhpNC8, m=10, 12, 14). 用最大泡压法研究了浓度低于临界胶团浓度时, CmOhpNC8在气/液界面上的吸附动力学. 结果表明, CmOhpNC8表现出很明显的吸附动力学效应. CmOhpNC8向新鲜气/液界面吸附时由扩散过程控制; 当界面上已具有一定吸附量时, 显示出吸附能垒Ea. 随着烷烃链的增长而明显降低, 表明长烷烃链的分子到达亚层后更容易插入表面层,这被归结为分子烷烃链间的疏水相互作用随着链增长而增强所致.     

Consistency in the sum frequency generation intensity and phase vibrational spectra of the air/neat water interface

Substantial progress has been made in the quantitative understanding and interpretation of the hydrogen bonding and ordering structure of the air/water interface since the first sum-frequency generation vibrational spectroscopy (SFG-VS) measurement by Q. Du et al. in 1993 (Phys. Rev. Lett. 1993, 70, 2312-2316). However, there are still disagreements and controversies on the consistency between the different experimental measurements, as well as in the theoretical and computational results. One critical problem lies in the lack of consistency between the SFG-VS intensity measurements and the recently developed SFG-VS phase spectra measurements of the neat air/water interface, which has inspired various theoretical efforts. In this report, the reliability of the SFG-VS intensity spectra of the air/neat water interface is to be quantitatively examined, and possible sources of inaccuracies in the SFG-VS phase spectral measurement are to be discussed based on the nonresonant SHG phase measurements. Solid evidence is shown indicating that the SFG-VS intensity spectra from different laboratories are now quantitatively converging and in agreement with each other. However, the possible inaccuracies and inconsistencies in the SFG-VS phase spectra measurements need to be carefully examined against a properly corrected phase standard to take full advantage of this powerful experimental tool.     

Determination of structure and energetics for gibbs surface adsorption layers of binary liquid mixture 2. Methanol + water

Vapor/methanol and vapor/methanol-water mixture interfaces have been among the benchmark liquid interfaces under extensive experimental and theoretical investigation. In this report, we studied the orientation, structure and energetics of the vapor/methanol-water interface with newly developed techniques in sum frequency generation vibrational spectroscopy (SFG-VS). Different from the interpretations in previous SFG-VS studies for a more disordered interface at higher bulk methanol concentrations, we found that the methanol-water mixture interface is well ordered in the whole concentration region. We are able to do so because direct polarization null angle (PNA) measurement allowed us to accurately determine the CH3 orientation at the interface and to separate the orientational and interface density contributions to the SFG-VS signal. We found that the CH3 groups at the interface pointed out almost perpendicularly from the interface. We further found that this well-ordered vapor/methanol-water mixture interface has an antiparallel structure. With the double layer adsorption model (DAM) and Langmuir isotherm, the adsorption free energies for the first and second layer are obtained as -1.7 +/- 0.1 kcal/mol and 0.5 +/- 0.4 kcal/mol, respectively. Therefore, the second layer adsorption is slightly negative, and this means that replacement of the second layer water molecule with methanol molecule is energetically unfavorable. Comparing this interface with the vapor/acetone-water mixture interface reported previously, we are able to correlate the second layer adsorption free energy with the work of self-association using the pairwise self- and mutual interaction energies between the water and solute molecules. These results provided detailed microscopic structural evidences for understanding of liquid interfaces.     

Measurement of the kinetic rate constants for the adsorption of superspreading trisiloxanes to an air/aqueous interface and the relevance of these measurements to the mechanism of superspreading

Super-spreading trisiloxane surfactants are a class of amphiphiles which consist of nonpolar trisiloxane headgroups ((CH3)3-Si-O)2-Si(CH3)(CH2)3-) and polar parts composed of between four and eight ethylene oxides (ethoxylates, -OCH2CH2-). Millimeter-sized aqueous drops of trisiloxane solutions at concentrations well above the critical aggregate concentration spread rapidly on very hydrophobic surfaces, completely wetting out at equilibrium. The wetting out can be understood as a consequence of the ability of the trisiloxanes at the advancing perimeter of the drop to adsorb at the air/aqueous and aqueous/hydrophobic solid interfaces and to reduce considerably the tensions of these interfaces, creating a positive spreading coefficient. The rapid spreading can be due to maintaining a positive spreading coefficient at the perimeter as the drop spreads. However, the air/aqueous and solid/aqueous interfaces at the perimeter are depleted of surfactant by interfacial expansion as the drop spreads. The spreading coefficient can remain positive if the rate of surfactant adsorption onto the solid and fluid surfaces from the spreading aqueous film at the perimeter exceeds the diluting effect due to the area expansion. This task is made more difficult by the fact that the reservoir of surfactant in the film is continually depleted by adsorption to the expanding interfaces. If the adsorption cannot keep pace with the area expansion at the perimeter, and the surface concentrations become reduced at the contact line, a negative spreading coefficient which retards the drop movement can develop. In this case, however, a Marangoni mechanism can account for the rapid spreading if the surface concentrations at the drop apex are assumed to remain high compared to the perimeter so that the drop is pulled out by the higher tension at the perimeter than at the apex. To maintain a high apex concentration, surfactant adsorption must exceed the rate of interfacial dilation at the apex due to the outward flow. This is conceivable because, unlike that at the contact line, the surfactant reservoir in the liquid at the drop center is not continually depleted by adsorption onto an expanding solid surface. In an effort to understand the rapid spreading, we measure the kinetic rate constants for adsorption of unaggregated trisiloxane surfactant from the sublayer to the air/aqueous surface. The kinetic rate of adsorption, computed assuming the bulk concentration of monomer to be uniform and undepleted, represents the fastest that surfactant monomer can adsorb onto the air/aqueous surface in the absence of direct adsorption of aggregates. The kinetic constants are obtained by measuring the dynamic tension relaxation as trisiloxanes adsorb onto a clean pendant bubble interface. We find that the rate of kinetic adsorption is only of the same order as the area expansion rates observed in superspreading, and therefore the unaggregated flux cannot maintain very high surface concentrations at the air/aqueous interface, either at the apex or at the perimeter. Hence in order to maintain either a positive spreading coefficient or a Marangoni gradient, the surfactant adsorptive flux needs to be augmented, and the direct adsorption of aggregates (which in the case of the trisiloxanes are bilayers and vesicles) is suggested as one possibility.     

和频振动光谱研究多晶金电极/溶液界面乙腈分子取向的flip-flop行为

用和频振动光谱研究乙腈/金电极界面,观测到乙腈的甲基振动峰强度随施加的电极电势而变化.当电极电势越过零电荷电势（pzc）时,甲基振动峰符号发生反转,这意味着基团取向发生反转（flip-flop）.由此推断出乙腈分子在金电极界面的吸附构型.即在零电荷电势下,电极界面吸附的乙腈分子构型为甲基靠近电极表面而腈基远离电极表面;而高于零电荷电势则电极界面吸附的乙腈分子构型发生反转,变为腈基靠近电极表面而甲基远离电极表面的构型.     

Fluorescence evidence that a phase transition causes the induction time in the reduction in dynamic tension during surfactant adsorption to a clean air/water interface and a kinetic-diffusive transport model for the phase-induced induction

An aqueous soluble surfactant adsorbing from solution onto an initially clean air/water interface often exhibits an induction period in the surface tension relaxation in which, as the adsorption begins, the tension remains near the clean interface value for an extended period of time before decreasing rapidly to the equilibrium value. In this study, using a model nonionic soluble surfactant, C14E6(CH3(CH2)13-(OCH2CH2)6-OH), we present direct fluorescence evidence that this induction is due to a first-order phase transition from a gaseous (G) to a liquid expanded (LE) phase that the assembling monolayer undergoes at constant surface pressure. An open channel flow cell is initially filled with water, and onto its air/water interface is spread an insoluble amphiphilic dye that fluoresces upon irradiation in the LE phase and whose fluorescence is quenched in the G phase. An aqueous solution of C14E(6) is then allowed to flow through the channel. We observe the immediate appearance of bright islands of the LE phase growing in a dark (G) background, confirming the presence of the G/LE phase transition. These islands eventually occupy the entire surface, after which the interface remains uniformly bright. We correlate this phase transition to the induction period by simultaneously measuring the tension of the interface of the open channel, and verifying that as the islands grow the tension remains at the clean value until the bright LE phase occupies the entire surface, whereupon the tension rapidly decreases. We further develop a phase transition surfactant transport model for the induction period in which surfactant diffuses toward and kinetically adsorbs onto the surface, and then rapidly equilibrates between the G and LE phases. For our model surfactant C14E6, we independently measure the surface concentration of the nucleating LE phase, the LE phase surfactant equation of state, the kinetic rate constants for adsorption into the LE phase, and the bulk diffusion coefficient. Using these measurements, we predict induction times for adsorption onto a clean surface without convection. We also measure these induction times in tension relaxation for adsorption onto a pendant bubble using axisymmetric shape analysis, and demonstrate agreement with the simulations with no adjustable constants.     

Mixed monolayers of Gemini surfactants and stearic acid at the air/water interface

The properties of mixed monolayers composed of the cationic Gemini surfactant ([C(18)H(37)(CH(3))(2)N(+)(CH(2))(3)N(+)(CH(3))(2)C(18)H(37)],2Br(+), abbreviated as 18-3-18,2Br(-1)) and stearic acid (SA) at the air/water interface were investigated by using a Langmuir film balance. The excess areas at the different mixed monolayer compositions were obtained and used to evaluate the miscibility and nonideality of mixing. Due to the electrostatic attractive interactions between 18-3-18,2Br(-1) and SA, the excess areas indicated negative deviations from ideal mixing. Moreover, 18-3-18,2Br(-1) and SA were miscible at the air/water interface, as was confirmed by atomic force microscopy (AFM) images of the LB films transferred onto mica substrates. The attenuated total reflectance (ATR) infrared spectra showed that SA in the mixed monolayers was ionized completely at a composition X(SA)=0.67 and formed a "cationic-anionic surfactant," i.e., the carboxylate, with 18-3-18,2Br(-1) owing to the electrostatic interaction between the head groups.     

Vibrational frequencies and structural determination of triethynylmethylstannane

The normal mode frequencies and corresponding vibrational assignments of Triethynylmethylstannane (SnCH(3)(CCH)(3)) are examined theoretically using the Gaussian 98 set of quantum chemistry codes. Each of the vibrational modes was assigned to one of nine types of motion predicted by a group theoretical analysis (Sn-C stretch, C[triple bond]C stretch, C-H stretch, C[triple bond]C-H bend, Sn-C[triple bond]C bend, C-Sn-C bend, H-C-H bend, CH(3) wag, and CH(3) twist) utilizing the C(3v) symmetry of the molecule. A set of uniform scaling factors was derived for each type of motion. Predicted infrared and Raman intensities are reported.     

Polymerizable cationic gemini surfactant

A polymerizable cationic gemini surfactant, [CH(2)=C(CH(3))COO(CH(2))(11)N(+)CH(3))(2)CH(2)](2).2Br(-), 1 has been synthesized and its basic interfacial properties were investigated (in water and in the presence of 0.05 M NaBr). For comparison, the properties of monomeric surfactant corresponding to 1, CH(2)=C(CH(3))COO(CH(2))(11)N(+)(CH(3))(3).Br(-), 2, were also investigated. Parameters studied include cmc (critical micelle concentration), C(20) (required to reduce the surface tension of the solvent by 20 mN/m), gamma(cmc) (the surface tension at the cmc), Gamma(cmc) (the maximum surface excess concentration at the air/water interface), A(min) (the minimum area per surfactant molecule at the air/water interface), and cmc/C(20) ratio (a measure of the tendency to form micelles relative to adsorb at the air/water interface). For the polymerizable gemini surfactant, 1, the methacryloxy groups at the terminal of each hydrophobic group in a molecule have no contact with the air/water interface in the monolayer, whereas for the corresponding monomeric surfactant, 2, the methacryloxy group contacts at the interface forming a looped configuration like a bolaamphiphile. Polymerized micelles of the gemini surfactant are fairly small monodisperse and spherical particles with a mean diameter of 3 nm.     

Phases confirmation of cloudy Silwet L-77 aqueous solution by dielectric relaxation spectroscopy

<正>The phase transition of Silwet L-77(a kind of siloxane surfactant) dilute aqueous solution with temperature was investigated by dielectric relaxation spectroscopy.In the initial heating process,a dielectric relaxation was found at about 10~6 Hz,which was considered as the interface polarization ascribed to the interface between water and liquid phases(W and L_1).With the temperature increasing further,a new dielectric relaxation was observed at about 10~4 Hz,which was assigned to the appearance of liquid crystal phase(L_α).According to the dielectric parameters fitted by Cole-Cole equation,the coexistence temperature of W,L_1 and L_αwas determined at about 37.0℃.     

X-ray reflectivity and interfacial tension study of the structure and phase behavior of the interface between water and mixed surfactant solutions of CH3(CH2)19OH and CF3(CF2)7(CH2)2OH in hexane

The interface between water and mixed surfactant solutions of CH(3)(CH(2))(19)OH and CF(3)(CF(2))(7)(CH(2))(2)OH in hexane was studied with interfacial tension and X-ray reflectivity measurements. Measurements of the tension as a function of temperature for a range of total bulk surfactant concentrations and for three different values of the molal ratio of fluorinated to total surfactant concentration (0.25, 0.28, and 0.5) determined that the interface can be in three different monolayer phases. The interfacial excess entropy determined for these phases suggests that two of the phases are condensed single surfactant monolayers of CH(3)(CH(2))(19)OH and CF(3)(CF(2))(7)(CH(2))(2)OH. By studying four different compositions as a function of temperature, X-ray reflectivity was used to determine the structure of these monolayers in all three phases at the liquid-liquid interface. The X-ray reflectivity measurements were analyzed with a layer model to determine the electron density and thickness of the headgroup and tailgroup layers. The reflectivity demonstrates that phases 1 and 2 correspond to an interface fully covered by only one of the surfactants (liquid monolayer of CH(3)(CH(2))(19)OH in phase 1 and a solid condensed monolayer of CF(3)(CF(2))(7)(CH(2))(2)OH in phase 2). This was determined by analysis of the electron density profile as well as by direct comparison to reflectivity studies of the liquid-liquid interface in systems containing only one of the surfactants (plus hexane and water). The liquid monolayer of CH(3)(CH(2))(19)OH undergoes a transition to the solid monolayer of CF(3)(CF(2))(7)(CH(2))(2)OH with increasing temperature. Phase 3 and the transition regions between phases 1 and 2 consist of a mixed monolayer at the interface that contains domains of the two surfactants. In phase 3 the interface also contains gaseous regions that occupy progressively more of the interface as the temperature is increased. The reflectivity determined the coverage of the surfactant domains at the interface. A simple model is presented that predicts the basic features of the domain coverage as a function of temperature for the mixed surfactant system from the behavior of the single surfactant systems.     

用红外光谱研究具有互穿网络结构的pH/温度双重敏感性微凝胶的相转变行为

采用傅立叶变换红外光谱仪测试了由聚(N-异丙基丙烯酰胺)(PNIPA)和聚丙烯酸(PAA)两种聚合物网络形成的具有互穿聚合物网络结构(IPN)的pH/温度双重敏感性微凝胶D2O分散液,通过差谱技术对不同pH值和温度条件下的红外吸收光谱进行处理,研究微凝胶相转变过程中分子链微环境的变化.结果表明,随着D2O介质的pH值增...     

Infrared plus vacuum ultraviolet spectroscopy of neutral and ionic methanol monomers and clusters: new experimental results

We present new observations of the infrared (IR) spectrum of neutral methanol and neutral and protonated methanol clusters employing IR plus vacuum ultraviolet (vuv) spectroscopic techniques. The tunable IR light covers the energy ranges of 2500-4500 cm(-1) and 5000-7500 cm(-1). The CH and OH fundamental stretch modes, the OH overtone mode, and combination bands are identified in the vibrational spectrum of supersonic expansion cooled methanol (2500-7500 cm(-1)). Cluster size selected IR plus vuv nonresonant infrared ion-dip infrared spectra of neutral methanol clusters, (CH(3)OH)(n) (n=2,[ellipsis (horizontal)],8), demonstrate that the methanol dimer has free and bonded OH stretch features, while clusters larger than the dimer display only hydrogen bonded OH stretch features. CH stretch mode spectra do not change with cluster size. These results suggest that all clusters larger than the dimer have a cyclic structure with OH groups involved in hydrogen bonding. CH groups are apparently not part of this cyclic binding network. Studies of protonated methanol cluster ions (CH(3)OH)(n)H(+) n=1,[ellipsis (horizontal)],7 are performed by size selected vuv plus IR photodissociation spectroscopy in the OH and CH stretch regions. Energies of the free and hydrogen bonded OH stretches exhibit blueshifts with increasing n, and these two modes converge to approximately 3670 and 3400 cm(-1) at cluster size n=7, respectively.     

Nanoscale aggregate structures of trisiloxane surfactants at the solid-liquid interface

The self-associating structures at the solid-liquid interface of three nonionic trisiloxane surfactants ((CH3)3SiO)2Si(CH3)(CH2)3(OCH2CH2)n OH (n = 6, 8, and 12), or BEn, are studied as a function of substrate properties by atomic force microscopy (AFM) imaging and force measurement. These trisiloxane surfactants are known as superwetters, which promote rapid spreading of dilute aqueous solutions on low-energy surfaces. This study also attempts to relate the BEn surface aggregate structures at the solid-liquid interface to their superwetting behavior. Four substrates are used in the study: muscovite mica, highly oriented pyrolytic graphite, and oxidized silicon wafer with and without a full monolayer of self-assembled n-octadecyltrichlorosilane (OTS). The concentration of BEn is fixed at 2 times the critical aggregation concentration (CAC). The BEn surfactants are only weakly attracted to hydrophilic surfaces, more on oxidized silicon than on mica. All three form ordinary planar monolayers on HOPG and OTS-covered oxidized silicon. The significance of surfactant adsorption on the AFM tip is investigated by comparing the force curves obtained by tips with and without thiol modification. The surface aggregate structures of the BEn surfactants correlate with their bulk structures and do not exhibit anomalous adsorption behavior. The adsorption behavior of the BEn superwetters is similar to that of the CmEn surfactants. Thus, our results confirm previous work showing that superwetting shares its main features with other classes of surfactants.     

Synthesis and Characterization of Tetrametal Cluster [Co2(CO)6][μ-HC2CH2OCH2C2H-μ][Co2(CO)6]

The treatment of dipropargyl ether HC2CH2OCH2C2H with Co2(CO)8 resulted in the formation of a novel cluster [Co2(CO)6][μ-HC2CH2OCH2C2H-μ][Co2(CO)6]. The cluster was characterized by C/H analyses, IR and 1H NMR spectroscopy and X-ray crystal structure determination. It belongs to monoclinic system, space group P21/c with the following crystallographic parameters: a=18.149(2), b=7.0111(7), c=20.897(2)(A), β=115.318(7)°, V=2403.7(5)(A)3, Z=4, Mr=665.97, Dc=1.840 g·cm-3, F(000)=1304.00, μ(Mo-Kα)=27.76 cm-1 and R=0.030 for 2372 observed reflections.     

Methanol Perturbing Modeling Cell Membranes Investigated using Linear and Nonlinear Vibrational Spectroscopy (cited: 3)

Cell membranes play a crucial role in many biological functions of cells. A small change in the composition of cell membranes can strongly influence the functions of membrane-associated proteins, such as ion and water channels, and thus mediate the chemical andphysical balance in cells. Such composition change could originate from the introduction of short-chain alcohols, or other anesthetics into membranes. In this work, we have applied sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), to investigate interaction between methanol and 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (d54-DMPC) lipid bilayers. Lipid's hydrocarbon interior is deuterated while its head group is hydrogenated. At the same time, CH₃ symmetric stretch from methanol and lipid head amine group has different frequency, thus we can distinguish the behaviors of methanol, lipid head amine group, and lipid hydrocarbon interior. Based on the spectral feature of the bending mode of the water molecules replaced by methanol, we determined that the methanol molecules are intercalated into the region between amine and phosphate groups at the lipid hydrophilic head. The dipole of CH₃ groups of methanol and lipid head, and the water O-H all adopt the same orientation directions. The introduction of methanol into the lipid hydrophilic head group can strongly perturb the entire length of the alkyl chains, resulting that the signals of CD₃ and CD₃ groups from both leaflets can not cancel each other.     

Adsorption of sodium dodecyl sulfate at the hydrophobic solid/aqueous solution interface in the presence of poly(ethylene glycol): dependence upon polymer molecular weight

The polar orientation and degree of conformational order of sodium dodecyl sulfate (SDS) adsorbed at the hydrophobic octadecanethiol/aqueous solution interface in the presence of poly(ethylene glycol) (PEG) has been investigated as a function of the surfactant concentration and the molecular weight of the polymer. Sum frequency generation (SFG) vibrational spectroscopy was employed to obtain spectra of interfacial surfactant; weak SFG signals from interfacial polymer were also detected for polymer molecular weights of 900 and above. The phase of the SFG spectra indicated that both the surfactant and polymer had a net orientation of their CH2 and/or CH3 groups toward the hydrophobic surface. Spectra of SDS in the presence of mixed polymer/surfactant solutions showed increasing conformational order as the surfactant concentration was raised. At the lowest surfactant concentrations, the spectra of SDS were weaker in the presence of the polymer than in its absence. All PEG molecular weights investigated, with the exception of PEG 400, gave rise to significant inhibition of ordered surfactant adsorption below the critical micelle concentration. The greatest inhibitory effect was noted for PEG 900. Probing interfacial PEG specifically through the use of perdeuterated SDS revealed that the polymer spectral intensity decreased monotonically as the surfactant concentration was increased for all polymer molecular weights where a PEG spectrum was apparent. These findings are interpreted in terms of the displacement of preadsorbed polymer as the surfactant concentration increases. This result is compatible with observations of adsorption from SDS/PEG solutions at solid/solution and solution/air interfaces made using other techniques.