X-ray reflectivity investigation of the foam bilayer film formed by hexaethylene glycol dodecyl ethe

REPORTS .Chinese Science Bulletin 2003 Vol. 48 No. 17 18000 --1802can not be explained by the theory of lyophobic colloidstability of Deryaguin, Landau, Verwey and OverbeckX-ray reflectivity(DLVO) as that of CBF1231. The stability of NBF involvesinvestigation of the foammuch su btle and complex repulsive short-range forcesassociated with the local structure of water (hy dration,bilayer film formed bysteric and protrusion forces) instead of electrostaticforcesl2.31.hexaethylene glycolThe five-layer organization (air/surfactant/water/surfactant/air) of the NBF can be used as models of foamdodecyl etherfilm, biological membrane, etc. to explore their physico-chemical properties such as cell fusion, membrane inter-SHEN Qiang, J. J. Benattar, LI Xin & LIU Shaojieaction, stability of colloidal particles and food emu lsion,intermolecular interaction between surfactant and poly mer.1. Key Laboratory for Colloid and Interface Chemistry of State EducaIn this contribution, X-ray reflectivity method was used totion Ministry, Shandong University, Jinan 250100, China;. Service de Physique de I Etat Condense, CEA/Saclay, F-91191 Gifinvestigate the structural properties of the NBF of Ci2E。sur-Y vette Cedex, Francemolecules.Correspondence should be addressed to Shen Qiang (e-mail: qshen@1 Expermentalsdu.edu.cn)The C2E (Sigma Co.) is of A. R. grade and usedAbstract Structural parameters for the free-standingfoam film of hexaethylene glycol monododecyl ether (C1E)without further purification. The water is purified from ahave been measured by using X-ray reflectometry. The re-milli-Q system and its conductivity is less than 18 MS/cm.sults indicate that a“five-laminae”model corresponds to thisAt first the prepared aqueous CiE solution was sealed ininverted bilayer and divides it into five regions. The thick- a box in order to maintain water vapor saturation. Afternesses of the aliphatic chain regions, the polar head-groupbeing immersed into the soap solution for a while, a rec-regions, and the central aqueous core are 0.90, 1.35 and 131tangular frame made of stainless steel was withdrawn tonm, respectively; and their corresponding electron densitiesmake a macroscopic vertical foam film (7 mm X 40 mm)are 2.4x 10 , 2.6x 103 and 2.x3 103 electron/nm', respec-suspended on its central part. The X -ray reflectivity meas -tively; the interfacial roughness between adjacent regions is0.34 hm. The central core of this foam flm does not Contain urements were not conducted until the two-dimensionalfree water, its thickness decreased 0.40 nm under the irradia-foam bilayer became black (shown in Fig. 1).tion of i nfrared rays due to the loss of structural water.Keywords: X-ray reflectivity, foam bilayer film, Newton black film,surfactant, hexaethylene glycol dodecyl etherFrameDOI: 10.1360/0 3wb0054X-ray beamWhen the thickness of soap foam film varies in aNBFdefinite region, color changes of a soap bubble will beobserved due to the interference of light. Black soap filmsare the final stages of thinning of soap films due to thedrainage of water in the absence of evaporation and areTroughgenerally formed from solutions of an ionic surfactant inthe presence of salt. After a bubble is formed, in thebeginning the soap film is relatively thick and it appearswhite; then the film thins gradually and finally reaches theFig. 1. Schematic presentations for the formation of the free standingvertical foam film and the reflected X-ray beam of NBF.thickness smaller than the wavelength of visible light.However, these black films are“visible" under the X-rayThe X-ray reflectometry is a self-made four-circlebeams. There are two different types of black films ac-diffractometer suitable for vertical surface. A conven-cording to the film thickness: common black film (CBF)tionaland Newton black film (NBF)". The CBF is the metasta-ble state of foam bilayer containing free water in betweenfine-focus copper tube is used as an X-ray source. Thethe two monolayers, its thickness ranges from 10 to 100monochrom中国煤化，工lects the Cu Ka,nm and is determined by a balance between van der Waalsline (λ= 0.15:ty measurementsattraction and the double layer repulsion forces. While theshow the ratidfYHCN M H Gis the intensity ofNBF has an equilibrium thickness less than 10 nm, andthe incident beam at the incident angle of0; I(θ ) is thethe depend ence of its thickness upon hydrostatic pressureintensity of the specular beam reflected by the NBF at anangle of θ(Fig.1). The reflectance angle θ varies from 618明9方数据Chinese Science Bulletin Vol. 48 No. 17 September 2003REPORTSto 45 mrad with a low divergence of 0.1 mrad, and the θethylene glycol monoalkyl ether possess favorable inter-range corresponds to the variation of the wave-vector facial activity and biological compatibility, and their ad-transfer Q2 along z axis between 0.6 and 3.6 nm.sorption at the air/water interface has almost no relationAn incandescence lamp of 60 W was put 15 cm .with electrolytesl6l. However, the maximum interfacialaway from the airtight experimental cell. In this opticalamount of adsorption should occur above their criticalcircuit directed to the NBF, various high-quality colored micellization concentrations (CMCs). Concerning theglass filters can be set up to check which wavelength ofstructure of this macroscopic foam bilayer and the CMClight has iradiative effect on this NBF structure. Actually,value of G2E (ca. 0.039 mg/mL), the concentration ofin the blank experiment white light and others with vari- this nonionic surfactant was fixed at 0.50 mg/mL in bulkous wavelengths had no influence on the photon count of phase without addition of electrolytes. It is usual that inX-ray reflectivity because of the vacuum path of X-raythe beginning the foam bilayer of C2E6 was white, then itbeam.became colorized after 15-17 min drainage, and finallyIn the whole process the temperature of the sealedit turned to black. The type of this black foam film can besystem was kept constant at 25土1..1determined quickly according to the following equation:2 Results and discussionhotal ≈nX/2sinθ,(1)Although the atomic force microscopy (AFM)where hotal is the total thickness of this foam black bilayer;provides the microscopic structure of surface with resolu-n is the serial number of diffraction peaks in the relation-tion on the order of nanometers, the accuracy and sensi-ship between Re and Q; λ is wavelength (0.154 nm), andtivity of X-ray reflectivity are higher than these of AFM.θ is the incident angle at the corresponding peak. Here,Due to the large number of measuring points, such athe obtained black film of q2E possesses about 6.4 nmmethod usually takes 10 h to acquire a full reflectivityfor its total thickness and at least 48 h for its lifetime,curve. It could, therefore, only be applied to very stablewhich belongs to NBF. While, for the brine systems offilms but greatly increase the accuracy to the extent asionic surfactants, the foam black films usually belong tolow as 107 order6-1o. It implies us to screen out an φ-CBF at low concentration of salt".The electron density of the C2E NBF can be givenpropriate kind of amphiphilic substances, as well as theby the state equation of ideal film of foam bilayer listed insuitable kind and concentration of electrolytes, for prep aeq. (2):ration of foam black film. The structure and the relativeproperties of a foam bilayer measured by means of X-rayRgl(pp(z)eQuz|(2)reflectivity are shown in Fig. 2. From this schematic fig-θ4 aure, it can be speculated that the NBF consists of two4Langmuir monolayers of surfactant molecules with theirwhere the vector Qsinθ is perpendicular to thepolar head-groups standing face to face. So, the surfactantconcentration should be higher than the double value ofsurface of foam film; and the ρ(z) is the mean electroncritical aggregation concentration. However, this verticaldensity along z axis. Eq. (2) exhibits a strong decay andfoam black film can be detected by means of its visibleKiessig fringes (iffraction peaks) in the reflectivity curvemeniscus connecting with the bulk reservoir.resulting from the interference due to large density gradi-In contrast to anionic surfactants, the molecules ofentsl6.7. To fit the reflectivity curve, this kind of dimen-sional systems should be treated as a succession of homo-geneous regions due to the molecular structure of surfac-tant and the mean electron density in different regions.Then, the mean interfacing roughness of these adjacenta)regions will be given out by simulation software of40G60SPEED.Fig. 3 shows X-ray reflectivity curve of the ratio Rb) 日varying with the wave-vector Q2 and its simulation results.The experimental data coincide well with the simulationcurve, which indicates the homogeneity of the Ci2E NBF.If C2E molecule was divided into two parts of the hydro-carbon chain中国煤化Iqpcording 'NBFmean electroFig. 2. Schematic presentation for the structure and properties of NBF:consists of fiThe thickness of the hydrophobic regions (a and e) is 0.90YHCNMHGshowninFig.2.(a) electron density; (b) structure divided intoa, b, C, d and e parts; (C)rm, which indicates that the hydrocarbon chains of sur-interfacial roughness.factant molecules are not parallel to the bulk surface andCh明數捨ience Bulletin Vol. 48 No. 17 September 20031801REPORTS .have a degree of 40° sloping to the horizon. The meanof the airtight system in a range of +0.5°C, but the thick-electron density of these hydrophobic regions is 2.4x 103ness of the C2E NBF has almost no changes from 20 toelectron/nm3 corresponding to 0.7 that of water (the elec-30°C. So, the thinning phenomenon of this film is due totron density of water is 3.56x 10-3 electron /nm3)1.the fact that the IR irradiation broke up the hydrogenbonds between terminal hydroxide groups of C2E and02water and decreased the amount of bound water.0~0.63 Conclusions10~In summary, X-ray reflectivity experiments can describe the unambiguous structure of the foam film of052.04.0 6.0CiE and give the relative parameters. The sandwich-likeZnmstructure of the NBF is composed of two monolayers of心10-surfactant molecules (Ca. 4.50 nm) in the liquid crystalline107state and a central hydration layer (ca. 1.31 nm). Underthe irradiation of infrared rays, the X-ray reflectivity can0-also be explored to investigate its application in the09structural water in foam bilayer.10-0Acknowledgements This work was partly supported by the Scientific052.02.53.03.54Research Foundation for the Returned Overseas Chinese Scholars fromthe Ministry of Education of China (Grant No.2002247), the ScientificQ:/mmResearch Foundation for the Honor of the Excellent Y oung Scientists ofFig. 3. The X-ray reflectivity experimental data ( ) and the simulationShandong Province (Grant No. 02BS115) and the Scientific Researchresults( ) of the G2E。NBF, the inserted curve is the electron densityFoundation for the Young Scientists of Shandong University (Grant No.Y34029).changing along Z axis.ReferencesThe two hydrophilic regions (b and d) have a thick-1. Guenoun, P., Schlachli, A., Setenac, D. et al, Free-standing blackness of 1.35 nm and a high electron density of 2.6 x 103films of polymers: A model of charged brushes in interaction,Phys. Rev. Lett., 1995, 74(18): 3628 "5631.electron/nm'. These regions correspond basically to theJacob, N. I, Hakan, W., Hydration or steric forces between am-ethylene glycol ether groups. Due to the presence of anphiphilic surfaces, Langmuir, 1990, 6(4): 873“-876.oxygen atom between adjacent C "-C bonds, these regions3. Klitzing, R. V., Espert, A., Asnacios, A. et al, Forces in foamfilms containing polyelectrolyte and surfactant, Colloids and Sur-have higher values of thickness and electron density thanfaces A, 1999, 149(1 "3): 131 "140.these of hydrophobic regions. The central region, namelyDubois, M, Zemb, Th., Swelling limits for bilayer microstruc-tures: the implosion of lamellar structure versus disordered hregion C, of the film is 1.31 nm thick and has a lowermellae, Current Opinion in Colloid & Inerface Sci, 2000, 5(1“electron density of 2.3 X 10-3 electron/nm (0.6 that of2): 27“37.water), which suggests that there is no free water but onlyLalchev, z. L, wilde, p. J.. Clark, D. c, Efet of lipid phase statethe structural water. This also indicates that the structuraland foam flm type on the properties of DMPG stabilized foams, J.properties of region C depend only upon the repulsiveBenattar, J. J., Schalchli, A., Sentenac, D. et al,Study of theshort-range forces associated with the local structure ofpolymer-surfactant interaction in black films and monolayers,water. Nevertheless, the total thickness of this NBF isProgr.Colloid Polym.Sel.1997, 105():113 17...about 5.81 nm and the interfacial roughness between twoBenattar,J. J, Nedyalkov, M, Prost, J. et al, Insertion process ofadjacent regions including the two air/hydrophobic regionip spshs layer. wibi. a Newton blaclinterfaces is about 0.34 nm.Kago, K., Matsuoka, H, Yoshitome, R. et al, The importance of aThe ultraviolet ray (X< 400 nm) and the visible lightdirect in situ evaluation of an amphiphilic diblock copolymermonolayer; the similarity and difference between its nanostruc-(400 --700 nm) have no irradiative effect on the GzEtures on water and on solid substrates examined by X-ray reflec-NBF. But, the infrared rays 6 700 nm) made the Kiessigtometry and atomic force microscopy, Langmuir, 1999, 15(12):fringes in the Rg -Q. curve to move right-handed in 304295 "43019. Barthelemy, P, Curillier, N, Chaudier, Y. et al, Stability ofmin, that is to say, the IR made the C2E NBF be thinning.Newton balck flms of highly fluorinated non- ionic surfactants, J.The simulation results show that the IR irradiation hasFluorine Chem., 2000, 105(1): 95“-102.influence only on the structural properties of the central0. Belorgey, O., Benattar, J. J, Structural properties of soap black1 roflortivi, Phys. Rev. Ltt, 1991,core of this film. At this moment, the thickness and elec-66(3): 31中国煤化工tron density of region c changed to 0.91 nm and2.2x 1031. Ciuillier,jtructure of freestand-electron/nm, respectively. If the incandescence lamp hading phospMHC N M H G2000, 16(11): 5029-5035.been switched off for 8 min, the thickness of the NBFchanged from 5.41 nm back to 5.81 nm again.(Received March 19, 2003; accepted May 8, 2003)Actually, these irradiations modify the temperature18月方数据Chinese Science Bulletin Vol. 48 No. 17 September 2003