Invited Paper
New insights in static and dynamic properties of Gibbs monolayer ¨ V. Fazio, J. Ortegren, P. Koelsch, A. Andersen, D. Wantke, H. M¨ ohwald and H. Motschmann 1 Max-Planck-Institute of Colloids and Interfaces, Am M¨ uhlenberg 1, 14424 Golm/Potsdam (Germany)
Abstract In this paper we discuss selected equilibrium and dynamic properties of adsorption layers of soluble surfactants. The surface state has been investigated by nonlinear optical techniques based on second order χ(2) effects which exhibit a high surface specificity and suppress bulk contributions. The surface tension isotherm σ(c) of the homologous series of n-alkyldimethylphosphine (n = 8 − 12) can be described by Frumkin’s equation of state which yields the surface interaction parameter, surface coverage and the corresponding area per molecule A. The comparison of the surface tension σ at a given area per molecule A reveals a strong alternation within the homologous series. Odd C2n±1 layers show a lower surface tension than the adjacent even members C2n of the homologous series. This effect is also present at low surface coverage (A = 1.4nm2 ) and cannot be attributed to a differences in the chain-packing within a crystalline state. Infrared-Visible Sum-Frequency Generation Spectroscopy (SFGS) has been used to monitor the orientation and chain order within the aliphatic tail. SFGS spectra have been recorded for different chain lengths and at different areas per molecule. The analysis of the spectra yields an order parameter G which is proportional to the number of gauche defects within the aliphatic tail. The odd-even effect in the surface tension turned out to be accompanied by an odd-even effect in the order parameter G. The data suggest that an ordered structure has a bigger impact on the surface tension than an unordered structure. The odd-even effect is also observed in the orientation of the terminating methyl group as retrieved by polarization dependent SFGS measurements. The data shed some light in the relation between molecular and macroscopic properties. Furthermore surface dilatational viscoelastic properties of a fluorinated amphiphile have been measured by a novel version of the oscillating bubble. The oscillating bubble method generates a non-equilibrium state by a harmonic compression and expansion of the surface layer formed at the tip of a capillary. The surface state is monitored by Surface Second Harmonic Generation (SHG). This technique is highly surface specific and discriminates between monolayer and subsurface coverage. Our set-up allows to measure the monolayer coverage under dynamic conditions and to relate this to surface dilatational viscosity and elasticity. For a purely elastic surface layer the prediction of the Lucassen van den Temple model (LvdT) are fulfilled. keywords: Surface Second Harmonic Generation, Infrared-Visible sum frequency spectroscopy, odd-even effects, soluble surfactants, liquid-air interface, dilatational viscoelastic properties
1
Introduction
are the decrease in the surface tension or the changes of the surface elasticity upon adsorption. Adsorption layers of soluble surfactant governs a variety of pheSoluble surfactant assembles two structural elements: nomena in surface and colloidal sciences such as the a hydrophilic head group and a hydrophobic tail. The stability of foams or emulsions [1, 2]. For this reaprevailing molecular asymmetry causes a spontaneous son the study of Gibbs monolayer is one of the central self-organisation of an amphiphile at the liquid-liquid themes in colloidal science. Indeed, many groups try or liquid-air interface. A so-called Gibbs monolayer is to understand and control the structure of interfaces at formed and as a consequence the macroscopic proper- the molecular level and investigate the consequences on ties of the interface are altered. Prominent examples 1 corresponding
38
author:
[email protected]
Proceedings of SPIE Vol. 5223 Physical Chemistry of Interfaces and Nanomaterials II, edited by Tianquan Lian, Hai-Lung Dai (SPIE, Bellingham, WA, 2003) · 0277-786X/03/$15.00
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forces, dynamics and friction [3, 4, 5, 6] In the last decade, several linear and nonlinear optical techniques have been established for investigating the structure and the dynamics of surfactants at interfaces. [7, 8]. Optical techniques combine attractive features for the study of these systems. Each technique probes different structural elements and the combination of linear and nonlinear optical techniques leads to a profound picture of the prevailing interfacial architecture [9, 10]. Nonlinear optical techniques based on a second order χ(2) effect possess a high and intrinsic surface specifity [11]. A symmetry consideration reveals that all second-order nonlinear effect cannot occur in centrosymmetric media. Hence, the bulk of a liquid does not contribute to the signal. However, at the interface, since the inversion symmetry is broken, the signal comes from the adsorbed amphiphiles. Second harmonic generation probes electronic properties and gives information on the interfacial symmetry, the surface coverage of the adsorbed species and their corresponding orientation. Infrared-visible sum frequency generation provides information on the chain order, the number of gauche defects and the orientation of selected functional groups. A puzzling feature is an odd-even effect observed within a homologous series of soluble surfactants [12]. The only difference between subsequent members of a homologous series is the number of carbon atoms Cn ; n = 8, 9, 10.. of the hydrophobic tail which are attached to the very same headgroup. The measurement of selected thermodynamic parameters, such as the surface tension, reveals an alternation with the chain length [13]. For instance the surface tension of the C9 representative is at a given area per molecule lower than the one measured with the C8 and C10 representative of the homologous series. This effect can also be observed at large areas per molecule and cannot be attributed to different chain packing within a crystalline state. This problem has been tackled by Infrared-visible sum frequency generation spectroscopy. This spectroscopy is sensitive for monitoring the orientation of the methyl end group and the number of gauche defects within the alkyl chain. These data provide a detailed molecular picture for the observed differences between even and odd chains and it sheds some light in the relation between surface energy and molecular conformation. . Finally we report on a novel experiment allowing one to measure and record the exchange dynamics of adsorbed and dissolved amphiphiles [14]. For the purpose of this study, we combined the technique of the oscil-
lating bubble with second harmonic generation. The technique of the oscillating bubble generates a nonequilibrium state by a periodic compression and expansion of the surface in a very well defined fashion. The design of our chamber gives access to the time range of 1 s to 1 ms, the chosen geometry suppresses Marangoni flow and allows a direct measurement of the surface elasticity modulus. The state of this bubble is then further investigated by SHG yielding the molecular exchange rate of surfactants with the adjacent sublayer as the major unknown parameter in the theoretical models describing the surface rheology.
2 2.1
Background Sum frequency generation SFG
In an SFG experiment a visible ωvis and a infrared laser ωvis beam are focussed on the interface of interest. The spatial and temporal overlap of both pulses generates new light oscillating at the sum frequency ωvis + ωIR . The frequency of the infrared beam is changed and the SFG intensity is detected as a function of the wavelength and the polarization of the incoming beam. The spectrum contains information about the vibrational modes of adsorbed species.
Fig. 1 Sketch of the experimental arrangement for an SFG spectrum. SFG is a nonlinear optical process of the second order χ(2) . The appealing feature of all χ(2) processes is the intrinsic surface specificity which is not matched by any linear optical techniques. The signal stems from the topmost monolayer while the bulk of a fluid does not contribute. The number of photons N ωvis + ωIR generated at the Proc. of SPIE Vol. 5223
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39
sum frequency is proportional to the energy of both pulses and inverse proportional to the area and the pulse width.
A major improvement of the originally proposed design has been achieved by using fresh light for each non-linear process instead of using the retro reflected pump light (532 nm) in the second pass through the BBO amplifier. This additional alignment work pays 2 W W ωIR ωvis Nωvis +ωIR ∝ χ(2) (1) off by a better mode profile and higher conversion efAt ficiency. Doughnut mode profiles, which are likely to be observed in the fundamental after any conversion The structural information of the monolayer is conprocess, are avoided. The idler output of the amplitained in the nonlinear susceptibility of the second orfier is tuneable within the range of 1.1-1.5µm. The der. It can be separated into two contributions, a nonidler is separated by dichroic mirrors and difference (2) resonant term χR which varies little with a scan of frequency mixed with the YAG fundamental (1.06µm) (2) the infrared frequency, and a resonant term χN R which in a AgGaS2 crystal. A delay line ensures the correct contains the desired information about the vibrational timing of both pulses. The output covers the spectral modes of the molecules. The latter can be expressed range of 2.9- 5.5 µm with about 40-80 µJ per pulse. in terms of the Raman and infrared transition dipole moments of the molecule, Mlmν and Anν
(2)
χR
lmn
=
v
N Anν Mlmν ων − ωIR − iΓν
(2)
where N is the number of adsorbed molecules , ων is the resonant frequency of the vibrational mode with the Lorentzian halfwidth Γν driven by the external infrared frequency ωIR . As a consequence of eqn. (2) a vibrational mode must be infrared and Raman active in order to be observed in a SFG spectra. This simplifies the analysis of the spectra and limits substantially the number of allowed modes.
3
Surface SFG
The tuneable IR light is incident at an angle of 38 degrees and the visible (532 nm) at 52 degrees defined form normal of the reflecting surface. The direction of the SFG light is given by the momentum conservation and close to the reflected visible light. It can be separated by a pinhole in conjunction with a suitable filters combination. The signal is detected by a photomultiplier, amplified and subsequently processed by a digitising oscilloscope.
Experimental
3.1
Laser system
The experiment requires the simultaneous generation of high power visible and tuneable infrared radiation with a bandwidth that should be close to the transform limit. This is achieved an optical parametric generator/amplifier pumped by the frequency doubled light of an active-passive mode locked Nd-YAG laser (Continuum, USA). The laser emits 35 ps pulses at a fixed repetition rate of 10 Hz. The frequency-doubled light (532 nm) of the YAG fundamental is used to pump an optical parametric generator consisting of two BBO crystals mounted in rotary stages. The signal beam of the optical parametric generator is narrowed down in bandwidth by a grating and amplified in a second pass. This design is based on elements suggested by Lauberau et.al. [15] and Zhang et. al. [16]. 40
3.2
3.3
Oscillating bubble
The technique of the oscillating bubble generates a nonequilibrium state in the monolayer by periodic compression and expansion of a monolayer [17, 18]. The design of our chamber gives access to a frequency range up to 500 HZ, the piezo transducer drives bubble via the incompressible liquid. A critical issue is the manufacturing of the capillary which should maintain the three phase contact line at the tip of the capillary during the oscillation. This has been achieved by a breaking a freshly silanized capillary leading to the desired sharp hydrophilic/hydrophobic phase boundary. Typical capillary diameter d are for the optical measurements d = 0.8 ± 0.1mm and for the pressure measurements d = 0.3 ± 0.1 mm.
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3.4
Surface Second Harmonic Generation, SHG
Second harmonic generation experiments were carried out in total reflection mode using the frequency doubled light λ = 532nm of an active-passive mode locked Nd-YAG laser (BMI industries, Paris) with a pulse width of τ = 30ps and a repetition rate of 10Hz. The laser system has been synchronized with the state of the rapidly oscillating bubble allowing to record several hundreds of shots at a freely chosen phase interval between bubble oscillation and laser pulse. Details can be found in [14].
3.5
Sample preparation
An aqueous solution of the surfactant at a concentration close to the critical micelle concentration (CMC) was prepared using bi-distilled water. This solution was then purified using an apparatus described in [19]. This applied purification scheme removes all surface active impurities by repeated cycles consisting of a) compression of the surface layer, b) its removal with the aid of a capillary, c) dilation to an increased surface and d) formation of a new adsorption layer. This procedure is repeated until a constant surface tension is established [20]. All subsequent solutions were prepared by diluting the stock solution.
3.6
Fig. 2: Surface tension versus bulk concentration of aqueous n-alkyldimethylphosphine (n=8-12). Filled circles correspond to the concentrations chosen for the SFGS experiment. The surface tension isotherm can be described by Frumkins equation of state [13]. This equation is a mean field approach which successfully describes a great variety of surfactant system [22]. The equation of state yields the area per molecule and the surface interaction parameter.
Surface tension measurement
Surface tension was determined by a Lauda tensiometer (Model TE 1C) with a slightly modified arrangement in order to meet the requirement imposed by surfactant solutions [21]. Surface tension was recorded until a constant equilibrium value, σe , was established.
4
Results and discussions
The surface tension versus solution concentration isotherms of the homologous series of nalkyldymethylphosphine are shown in Figure 2.
Fig. 3 Alternation of the surface tension with the length of the alkyl chain for three different areas-permolecule in the monolayer (top: 0.8nm2 /molecule; center: 1.0nm2 /molecule; bottom: 1.4nm2 /molecule). The comparison of the surface tension at a given area Proc. of SPIE Vol. 5223
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per molecule is shown in Figure 3. The data reveal an alternation of the surface tension with the length of the aliphatic tail of the surfactants at three different molecular coverages. For the same area-per-molecule odd layers always present a lower surface tension than even layers. This effect is also observed at low surface coverage and cannot be attributed to chain packing in a crystalline state. In order to gain further insights in the chain order, SFGS measurements were performed for selected solution concentrations (as indicated by the filled dots in Figure 2. The purified surfactant solution was allowed to equilibrate before the SFG measurements were performed. SFG spectra were measured for two polarization combinations, SSP and PPP, where the first polarization is referred to ωSFG , the second to ωvis and the third to ωIR .
two resonant peaks in the SFG spectra allows a complete characterization of the chain conformation. For instance, in a perfectly all-trans chain, the CH2 groups are arranged in a zig-zag configuration with adjacent groups pointing in opposite directions. This near inversion symmetry leads to a cancellation of the CH2 contribution. The presence of gauche defects in the chain breaks this near-centrosymmetry and enables the CH2 mode to contribute in SFG. It is important to note here that this effect is local and thus it does not matter if the chain contains an even or odd number of alkyl groups. While the intensity of the CH2 mode, I SFG (d+), increases with the number of gauche defects, the CH3 mode, I SFG (r+), generally decreases because of the decrease of order in the chains [24]. However, I SFG (r+) is less sensitive to the orientational disorder than I SFG (d+), and the ratio, G=
I SFG (d+) I SFG (r+)
(3)
which is derived from one single spectrum, is a semiquantitative measure of conformational order [25]. The conformational order parameter G as determined from the PPP spectra in Figure 4 is plotted in Figure 5 as a function of the solution concentration. Generally, for all surfactants, G increases when the solution concentration decreases because molecules occupy larger areas-per-molecule and thus have more freedom to adopt gauche conformations.
Fig. 4 SFG spectra of adsorption layers of nalkyldimethylphosphine for different concentrations versus IR light wavenumber. Dots correspond to SSP and squares to PPP polarization combinations. The corresponding SFG spectra are shown in Figure 4. In the range 2900- 3000 cm−1 the spectra are dominated by the CH2 and CH3 symmetric stretching modes d+, 2840 cm−1 and r+, 2875 cm−1 , respectively. In some cases their overlap turn out to be strong, which required a fitting procedure [23]. The analysis of the 42
FIG. 5: Conformational order parameter of the the surfactant monolayers as estimated from the PPP spectra in Figure 4. One immediately notices that even-chain layers always present a higher value of G than odd-chain layers. This is especially evident in Figure 6, where G has been
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plotted as a function of the length of the alkyl chain for three different areas-per-molecule: at equal surface coverage, odd-chain layers always present less defects, and thus a more ordered conformation than even-chain ones.
air, and of the interface [27]. Θ as estimated from the SSP and PPP SFGS spectra is plotted in Figure 8 as a function of the solution concentration and of the areaper-molecule. Here, we used r = 2.2 ± 0.3 [29].
FIG. 8: Tilt of the terminal methylene group as a function of the length of the alkyl chain for three different surface coverages of the monolayers
For the refractive index of the surface monolayer we used nvis = nIR = nSFG = 1.4 [23]. Θ increases slightly Fig. 6 The order parameter G which is proportional to the with the decrease of the concentration and/or with the number of gauche defects shows an alternation with increas- increase of the area-per-molecule as qualitatively exing chain lengths N . G is plotted for three different areas pected. As in the case of the conformational order parameter, one notices immediately that even-chain layper molecule. ers always adopt a larger tilt than odd-chain layers. The orientation of the terminal methyl group can be ◦ also determined from the SFG spectra [25, 26]. The A tilt angle of ≈ 35 for the methyl group corresponds methyl group has a local C3v symmetry which reduces to a nearly upright oriented aliphatic tail [30]. Thus, the number of independent elements of the hyperpolar- in the odd-chain monolayers molecules are in a higher izibility tensor to the following two: βx x y = βy y z = ordered and upright configuration, as also confirmed rβz z z where r is the hyperpolarizibility ratio. The by the corresponding small value of G . On the other dash indicates that β is defined in the corresponding hand, in even-chain monolayers, chains present a more molecular frame of reference. The polar angle of the disordered conformation (larger G) which leads to a C-CH3 symmetry axis, z , with respect to the labora- larger average tilt of the methyl group. tory z axis, Θ, can then be determined from the ratio For the same area-per-molecule odd-chain layers are of the SFG intensities of the symmetric CH3 modes, more ordered than even-chain ones. Surface tension r+, for at least two polarization combinations. measurements reveal that the presence of the odd-chain This ratio corresponds to that of the effective second- layers on the water surface lowers the surface tension order susceptibilities which is a function of r and Θ. more than the presence of the even-chain layers. This For SSP and PPP polarization combinations Θ can be suggests the general feature that ordered monolayers influence the interface more than disordered ones. extracted [27, 28] from χeff,SSP(r+) cosΘ(1 + r) + cos3 Θ(1 − r) = χeff,PPP(r+) cosΘ(A + Br) + cos3 Θ(C + Dr) (4) where A, B, C, and D are experimentally accessible constants which depend on the angles of incidence and on the refractive indices of aqueous surfactant solution, Proc. of SPIE Vol. 5223
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techniques [32] utilize this principle. The data analysis relies on the precise knowledge of the surface age at a given location, however, the latter may be ill defined due to a mass transport caused by a gradient in the surface tension. An experimental arrangement which overcomes all these problems is the technique of the oscillating bubble which measures the dilatational properties of the surfactant layer in the mid frequency range [1 Hz to FIG. 8: Tilt of the terminal methyl group as a function of 500 Hz]. The layout of the experiment is sketched in the length of the alkyl chain Figure 9. A bubble is formed at a tip of a capillary In conclusion, by SFGS we have obtained direct exper- which is inserted in the aqueous surfactant solution. imental evidence of odd-even effects in the chain con- At first the bubble is adjusted to a half sphere geomeformation of adsorption monolayers of soluble surfac- try and then forced by a Piezo transducer to a well detants. To our knowledge, this is the first time that odd- fined oscillation leading to a periodic compression and even effects in the molecular conformation of monolay- expansion of the surface layer. The amplitude can be ers have been directly measured. From the results, one adjusted by the voltage applied to the Piezo transducer can deduce the general feature that surfactants with and is monitored by a microscope and CCD camera. the same headgroup adopt different conformations at interfaces depending on the odd or even number of groups in the chains. The aliphatic tails influences the macroscopic characteristics of the films, as the surface tension. In particular, our data suggest that ordered layers influence the interface more that disordered ones, as they lower the surface tension more than the disordered ones do. Finally, these results brought light into an old and important problem: the relation between surfactant conformation and interfacial energy. It is desirable that the Monte Carlo simulation community addresses this problem and obtains insights on a molecular level. The previous experiments investigates equilibrium properties but it is actually the dynamics which governs many phenomena and which provides the key for the understanding of the equilibrium properties. The last section of this paper describes a unique experiment which allows the measurement of the exchange dynamics between adsorbed and dissolved amphipiles at the air-water interface.
Fig. 9 Schematic crossectional view of the oscillating bubble cell. A piezo transducer forces the bubble in an oscillation. The state of the bubble can be monitored by a pressure transducer or a by SHG in total reflection mode
The relative area changes ∆A/A0 of the surface layer are obtained by image analysis. The experiment conA general problem for studying surfactant dynamics is sists of a homogeneous dilation and compression of the the design of an experiment with well defined bound- surface layer and gives access to the time regime regime ary conditions allowing a reliable mathematical mod- where the molecular exchange processes occur between elling of the underlying transport equations [3]. A fre- adsorbed and dissolved amphiphiles occur. Precise quently pursued approach is the maintenance of a non- measurement in this time regime are of utmost imporequilibrium state under steady state conditions. Usu- tance to understand the contribution of the molecule ally a fresh surface is formed at a given location and a exchange on the thermodynamic and rheological propnon-equilibrium state is maintained by a flow pattern. erties. The prevailing surface coverage is monitored by surface At present we have two ways to monitor the nontension or optical measurements. The technique of an equilibrium state within the adsorption layer: by anaoverflowing cylinder [31], the inclined plate and all jet 44
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lyzing the signal of the pressure sensor and by SHG. It can be shown that the pressure sensor at the button of the cell records the dynamic portion of the surface tension. This information combined with the relative area changes yields the surface dilational elasticity modulus E [33]. A important quantitity in this respect is the measurement of the phasehift between position of the piezo transducer and the pressure sensor. There is no phaseshift observed for a purely elastic surface layer. A surface viscosity shows off as a phase shift between position of the piezo and pressure response.
with a rapidly oscillating bubble imposes severe experimental hurdles. All relevant experimental details are discussed in great detail in a recently submitted paper [14]. In this contribution we focus on the experimental findings. Surface second harmonic generation requires molecules with a high hyperpolarizibility β. Our model amphiphile 6-((2-Hydroxy-ethyl)-2-[4-(4-trifluoromethylphenylazo) -phenoxy]-ethyl-amino)-hexane-1,2,3,4,5pentaol (F381) possesses a azobenzene chromophore with the Trifluormethyl substituent which provides a sufficient hyperpolarizibility for the purpose of this study.
The surface dilatational elasticity modulus of aqueous surfactant solutions of F381 have been measured in the frequency range of 10-500 Hz. The calibration, data analysis and the fitting procedure are described in [34]. The surfactant system F381 turned out to be purely elastic following the prediction of the Lucassen- van den Tempel model [35]. The elasticity modulus levels off to a plateau at frequencies exceeding 20 Hz. The high frequency limit of the surface elasticity modulus Fig. 10 The widely accepted model of a Gibbs monolayer E depends on the prevailing bulk concentration and describes the interface as an absorbed amphiphilic monoincreases with the bulk concentration. layer and an adjacent sublayer. The latter separates bulk and monolayer and is the region where the molecular exchange occurs. The SHG experiment measures only the monolayer coverage Γm
Quite recently we introduced a new version of the oscillating bubble where the surface state is monitored by the non-linear optical technique SHG [14]. Our setup ensures that laser system and bubble oscillation remains well synchronized in the course of the experiment. The phase difference between bubble oscillation and laser pulse can be freely chosen allowing to monitor the surface state in a stroboscopic manner in a similar fashion as the corresponding equilibrium experiments are carried out. The appealing feature of this arrangement is again the intrinsic surface specificity of SHG. SHG probes only the region with polar order of the interfacial region. The interface consists of the amphiphilic monolayer and a sublayer with negligible polar ordering. The sublayer separates the amphiphilic layer from the bulk phase. All exchange processes between adsorbed and dissolved molecules occur in that region. SHG measure only the surface coverage Γm of the monolayer. Hence this arrangement can be used to follow the surface coverage Γm under dynamic conditions. The combination of the nonlinear optical technique
Fig. 11 The dilatational elasticity modulus of aqueous surfactant solutions of the fluor tenside F381 measured at different bulk concentration 30µM, 100µM, 200µM, in depedence of the frequency
The amplitude of the sinusoidal voltage over the piezoelectric translator leads to a relative area change of ∆A/A = 0.18 ± 0.04. We define the phase angle Φ of the bubble during oscillation as 90◦ , and 270◦ , when the bubble obtains its minimum and maximum volume, respectively. Consequently, at the phase angles 0◦ and 180◦ , the bubble is at its equilibrium volume. Proc. of SPIE Vol. 5223
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In measuring SHG at different phase angles, two obstacles must be overcome: First: the detected SHG signal is proportional to the square of the intensity of the fundamental light. This means that as the bubble oscillates, a small spatial deviation from focus at various phase angles may cause a change in the detected SHG signal. Second: the curved surface in combination with a spatially limited area of the lens collecting the out coming light may lead to a loss of some of the SHG intensity. Therefore, the shape of the bubble ◦ at minimum volume (Φ = 90 ), having a less curved surface, improves the reproducibility. These problems were solved by adjusting the equilibrium volume of the bubble in the following way: in the case of measuring at a phase angle of 90◦ , the equilibrium volume of the bubble was made slightly larger than a half-sphere, so that the bubble was exactly a half-sphere as it was hit by light. Analogically, in the case of measuring at a phase angle of 270◦ , the equilibrium volume of the bubble was made slightly smaller than a half-sphere, again so that the bubble was a half-sphere as it was hit by light. In the case of the phase angles 0◦ and 180◦ the bubble adopts a half-sphere as it was hit by light. Consequently, at all measurements the laser pulse hit the bubble at exactly the very same position and same size. The result of the measurement are displayed by the filled squares in figure 12. The SHG response has been measured as a function of the phase between bubble position and laser pulse at fixed frequency of 60 Hz. The filled circles refer to the measurement of pure water under identical conditions. The amplitude of the oscillation in this plot gives the ∆Γm , the variation of the monolayer coverage within the topmost adsorption layer upon a compression and expansion cycle. The change in the concentration of surfactants in the monolayer during oscillation is obtained by differentiating Γ = Nm /A at constant Nm which leads to: ∆A ∆Γm =− Γm A
Fig. 12 SHG measured on an oscillating bubble at a fixed frequency of 60 Hz in dependence of the phase shift between laser pulse and bubble oscillation. The amplitude as indicated by the arrow yields the change in the surface coverage ∆Γm . All measurements were carried out at the very same drop shape and about 200-300 pulses have been recorded in order to obtain a reliable signal to noise ratio. The error bars indicate the reproducibility of the data by independent runs. The filled circles have been measured using pure water in absence of surfactant.
5
Conclusion:
(5)
From the SHG data in Figure 12, ∆Γm can be calculated. The result is 0.17, which is within experimental accuracy, equal to the relative change in bubble area ∆A/A = 0.18. This means that equation 5 is fulfilled, and that the SHG data presented in Figure 12 are in accordance with the dynamic elasticity measurements and the LvdT model. Moreover, SHG measurements performed in the frequency range 10-60 Hz gave the same behavior. To conclude, we were able to directly 46
measure the change in the concentration of surfactants in the monolayer at oscillation frequencies between 10 and 60 Hz. The presented data confirms the predictions made from dynamic elasticity measurements, and by the LvdT model.
We report on odd-even effects observed in adsorption layers at the air-water interface within a homologous series of a the soluble surfactant nalkyldimethylphosphine (n = 8 − 12). The observational fact is that at a given area per molecule odd layers adopt a higher surface tension than the adjacent even layers. The alternation in the surface tension at given area per molecule can be attributed to the chain order as retrieved by sum frequency generation spectroscopy. The analysis of the spectra suggest that an
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ordered structure has a bigger influence on the surface tension than unordered structure. This statement should be further addressed by Monte Carlo simulations. Our data relate the surface energy to molecular conformation and address a fundamental question of interface science. Furthermore a novel version of the oscillating bubble technique has been introduced. The device gives access to the surface dilatational viscosity and elasticity in the mid frequency range 10 Hz to 500 Hz. This time window is of particular importance as the molecular exchange processes between adsorbed and dissolved amphiphiles occur there. The evaluation of a pressure response yields the surface elastic modulus, the evaluation of the phase shift between pressure response and driving piezo gives the surface viscosity. Our new version of the oscillating bubble follows the monolayer coverage during expansion and compression cycles by Surface Second Harmonic Generation. This technique records only the surface coverage within the absorbed monolayer while contribution from the sublayer and bulk are suppressed. Hence, the measurement yields the surface coverage under dynamic conditions. It could be demonstrated that the assumptions of the Lucassen van Temple model for the purely elastic component are valid. Further experiments are dedicated to the investigation of the viscoelastic systems in order to clarify the role of the surfactant exchange within the dissipative process leading to the surface viscosity.
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