Experimental study of oil-water interface layers dilatation rheological properties

NOTESExperimental study ofmental methods, so that experimental methods will haveimportant significance in measuring oil-water interfaceoil-water interface layersdilatational properties.In the previous paper7, we reported a new methoddilatation rheologicalthat can measure interface dilatational elastic module (E)and dilatational viscosity (η) for all surfactants based onpropertiesthe surface pressure relaxation process, as well as therelationship between air-liquid surface film dilatationZHU Hong1-, ZHAO Feng', TANG Ji an, LI Jingru,properties and the stability of foam. In this note, by meansLI Xingchang? & JIANG Long2of the liquid-liquid interface film pressure measurement1. School of Science, Northem Jiaotong University, Beijing 100044,apparatus based on the principles of Langmuir film bal-ance, we measure the dilatation rheological properties of a2. Institute of Chemistry, Molecular Science Center, Chinese Academyseries of alkane-water interface layers, and go into theof Sciences, Beijing 100101, China;stability of interface film properties.3. School of Chemical Engineering and Technology. China University of1 ExperimentalMining and Technology , Xuzhou 221008, ChinaCorrespondence should be addressed to Jiang Long (e -mail: jianglng@public.net.cn)(- ) Chemicals. n-hexane, analytically pure，pro-duced by Beijing Chemical Factory, σ= 18.9 mN/m (atAbstract Oil-water interface layers dilatation rheological18.. 7); n-octane, analytically pure, bought from Beijingproperties have been measured with the liquid-liquid inte r-face film pressure measurement apparatus, which was basedChemical Factory, σ= 21.3 mN/m (at 2..); n-decane,on the principles of Langmuir film balance. Experimentalimported in terms of bath dispatching by Shanghai Rea-results show that it is possible to form the interface -associ-gent Company, σ= 23.9 mN/m (at 18..7); n-hexadecane,ated material caused by the attraction of the dispersionchemically pure, produced by Shanghai Reagent lst Fac-forces at the pure alkane-water interface. The type of mate-tory, and was purified as follows: NaOH solution washing,rial is sensitive to the interfacial pressure. Under the influ-followed by CaCl2 siccating in the absence of water andence of the interfacial pressure, the stability of the interfacevacuum rerun, σ= 27.3 mN/m (at 20.. 7). Bidistilled waterassociated materials decreases with the increase of the num-ber of alkane carbons.was used. The above values of σare corresponding to theKeywords: liquid-liquid interface film pressure measurement appa-reported values.ratus, interface dilatational elasticity, dispersion forces, interface(-i) Interface film pressure measurement appaa-associated material.us. The determination of the liquid-liquid interfacialtension has much more difficulty than that of the liquid-airIn recent years, surface rheology developed rapidlyinterfacial tension. In Langmuir Balance experiments,in surface science, and its theory has important applicationpeople often used the Wilhelmy plate method for meas -to the actual production'. More detailed reviews on sur-uring σ at the air-liquid interface. And the accurate valuesface rheology have been made in literature [2.3]. There areof σ are quite simple to be obtained. However, themany methods available for the measurement of surfaceWilhelmy plate method for measuring σ at the liquid-rheology[4-*61, however, many of them are sufficientlyliquid interface is far from simple. As we know, with thecomplicated. Analogous to bulk rheology, surface rheolo-change of time，pressure, and adsorption of surfactants ongy involves in the relationship among the stress, the strainthe Wilhelmy plate, etc., a continual variation of interfa-and the rate of the strain. But the analogies are merelycial tension and interfacial pressure (σ。σ) will appear,superficial, and far away from the essence. For example,which causes the poor measuring accuracy and reproduci-for an incompressible bulk liquid, its compressible volu -bility, and sometimes to such an extent that the measuringme is zero, while monolayers of surfactants are co m-process cannot go on. Consequently, at the liquid-liquidpressible so that it is possible to change the area ofinterface, we did not adopt the Wilhelmy plate as experi-monolayers. One more example is displayed as follows:mental assembly to measure σ, but directly measured theinterfacial shear viscosity is different from interfacialforces exerted on the thread by molecules, i.e. interfacialdilatational viscosity in essence, the former is produced by(surface) pressure, according to the principle proposed bythe intermolecular interactions at interfaces, while theLangmuir8]. We designed an apparatus to measure oil-latter is caused by the resistance to enter the bulk solutionwat中国煤化工as shownin fig. 1. Thefrom the interface, and is different from the traditionaltrougCNMHF0 mm long. In the expeni-definition of viscosity. Traditional theory involves in sur-mentGsses the area of the inter-factant molecular desorption , reorientation, etc. In theface, molecules in the interface exerts force on a thread instudy of shear viscosity, the measurements of those proc-the interface, and the thread would move forward, accom-esses mentioned above are closely related to the experi-panied by the displacement of the balance stick ahead of2056Chinese Science Bulletin Vol. 47 No. 24 December 2002NOTESthe thread. A mirror is placed at the summit of the balanceesses are similar to the above compression processes .stick, so it would be inclined with the moving of the bal-When the interfacial pressure increases to a certain value,ance stick. According to the relation between the dis-stop the compression and observe the relation of the dis-placement of the reflecting light spot and the force equi-tance that the light spot goes backward versus time. Thelibrium, we could know that the displacement of the lightinterfacial pressure gradually decreases with the diffusionspot is proportional to the increase of the interfacial (sur-of molecules from the interface to the bulk phase. Then,face) pressure (within the lineal scope). After being modi-plot the interfacial pressure relaxation curves by the for-fied, this apparatus is so sensitive that its light spot movesmulation transformation.1 mm meaning the increase of the interfacial pressure for 2 Results and discussion0.35 mN/m The moving barrier is accomplished by anelectric motor with various velocities from 0.01 to 20We had an experiment of the pure n-hexadecane-mm/min. By means of the CCD, the light spot and thewater interface compression when studying properties ofscreen of an electronic stop watch are copied into the tapeoil-water interface dilatational theology. Results show that,simultaneously, so we know the variation of interfacialat the pure n-hexadecane-water interface in the absence ofsurfactant molecules, interfacial pressure slowly increasedpressure with time.when we compressed the area of interface. Under thesame condition, we compressed the n-hexadecane-airsurface and the water-air surface, respectively. Experi-5mental results show that their surface pressures all do notincrease. It is only at the oil-water interface that this phe-nomenon occurs.目-4感EFig.1. Sthematic ilustration of liquid-liquid interface film pressuremeasurement apparatus.1, Oil-trough; 2, liquid liquid interface; 3, water2trough; 4, rigid metal stick; 5, reflecting mirror; 6, laser; 7，thin thread; 8,barrier; 9, barrier motor.(.1) Compression and relaxation experiment of oil-water interface. Experiment of interfacial pressure areacurves: Put bidistilled water into the trough of the LBbalance. The thread must be intimate contacted with the100 150200250300 350400surface of water to prevent leak. Then we drop oil in theArea/cm?direction of the thread in order to make the thread wetted,and put the oil phase into the trough carefully, wait for 0.5Fig. 2. Hexadecane-water interface pressure versus area curves.h, then press the interface layers slowly with a barrier at aWe argue that the increase of the interfacial pressurespeed of 3.1302 cm/s. If there are molecules or associatedis related to the interaction between oil molecules andmaterials at the interface, the soluble molecules will be-water molecules at the interface. The phenomenon has nothave as if they were insoluble ones. Because the speed ofbeen reported in literature. According to the Gibbs equa-compression is far higher than the diffusion rate of solubleionl9]molecules, the molecules have no time to diffuse betweenthe interface and the bulk phase. The pressure exerted byr=-_dσ CdC RTinterfacial molecules on the thread increases as the area ofthe interface is compressed, accompanied by the increasehe decrease of the interfacial tension is related to theof the concentration of molecules at the interface. By theconcentrationmolecules. at the interface, i.CCD, the displacement of the light spot and the screen of0δ中国煤化工s not occur when com-an electronic stop watch are simultaneously copied intopressYHC N M H Gthat is, we cannot observethe tape. So the curve of interface pressure to area is plot-the molecular condensation. This phenomenon is easilyted.understood when the traditional theory is considered:Interface relaxation experiment: The beginning proc-because there are no surfactants at the surface. However,Chinese Sg好内数letin Vol. 47 No. 24 December 20022057NOTESat the liquid-liquid interface, even though there are noTable 1 Values of σ AB and AG for five alkanes forming interfaces withwater"(all values in mN/m )surfactants, we could observe the increase of π, i.e. thecondensation of molecules, as if there were surfactants atAlkaneσaσσABAGinterface. In order to study the interaction between oil andn-hexane18.451.5-2.9water molecules further, we measured interfacial pressuresn-octane21.850.9of a series of alkane-water interfaces. Results show thatn-decane23.81.9the same phenomenon did occur.n-dodecane25.550.1n-hexadecane 28.128.151.36.83.a)σg=σwr=72.8, σ =21.9.■n-hexane-water3.0●n-octane waterBut when going into the measurement of interface▲n-dodecane-waterdilatation rheological properties, we found that our ex-2.5perimental results were corresponding to the change of the2.attraction interaction energy at the interface. One couldfind in fig. 3 that the compressible trend of the surfacepressure apparently decreases with the increase of thenumber of alkane carbons, that is, we could test the theory1.0of the variation of interaction energy at the interface.0.5Furthermore, from interface relaxation experimental e-sults, we could characterize the intermolecular interaction0.0energy at the n-hexane-water interface.-0.5.0r100 150200250300 350 40Fig. 3. Three plots of interface compression.Interaction energy of the various types of liquid-2.0 tliquid interfaces is described in terms of the work of cohe-sion (w) and the work of adhesion (w2) in classic booksll0l.If w。> w。, the attractive forces between interfacial no-lecules are too weak to promote the process of spreading;if w。< Wa, the attraction forces are strong. The total at-1.0 rtractive interaction energy (the spreading coefficient) is另given byOG= W- W。=σAB+σ -σp,(2)where σAB is the oil-water interfacial tension, σA is the-202040608010012014surface tension of oil, σp is the surface tension of pureTime/swater.According to the principle that the interaction of theFig.4. Relaxation curves of the interfacial pressure with time.dispersion force is always present between any molecules,According to the equation represented in [7], table 2the Girifalco-Good-Fowkes equation is put forward byshows values of the interface relaxation time for the dif-Fowkesl]1 as follows:ferent hexane-water interfaces. From table 2, one knowσAB=σ +σp - 2VσXog ,(3)that Tu-hexan-water > Troctane-water Turdecane-waer < -dodeane -water Theinterface relaxation time is related to the diffusion speedwhere σj and σg are the dispersion components of σof molecules from interfaces into the bulk phases. Thefor A and B, respectively. Table 1 lists the data and cal-larger the relaxation time is, the longer the residence timeculated results for five alkanes forming interfaces withof molecules at interfaces will be. Therefore, we couldwater according to eqs. (2) and (3). .demonstrate the effect of the length of the alkane chainsFrom table 1, one could observe that the potentialon the stress relaxation time. In addition, table 1 displaysenergy of attraction at a series of alkane-water interfaces中国煤化工-octane-water and the AGgradually decreases (the higher the OG values are, theof nrelaxation time becomessmaller the attractive interaction energy at two-phase in-shorlYHCNMHG that the intermolecularterfaces will be, which means that no process of spreadinginteraction energy becomes weak at the interface. There-occurs). However, at present there are few quantitativefore, by the experimental approach we could quantita-experimental methods to characterize the variation of OG.tively characterize the interfacial intermolecular interac-2058Chinese Science Bulletin Vol. 47 No. 24 December 2002NOTEStion energy, i.e. the longer the chains are, the poorer thetions. However, further information of its molecules, theinteraction will be.relationship between the structure and the behavior, etc.Table 2 Values of interface relaxation timewill be completely explored in the following years.n-hexane- n-octane- n-decane- n-dodecane-Acknowledgements We thank Prof. Han Dakuang for his valuablewatersuggestion and help. This work was supported by the China NationalRelaxation time/s35.76.367.33Petroleum Corporation and the Climbing Project of Northern JiaotongUniversity (Grant Nos. NJTUPD169 and NJTUPD170).Experimental results show that the decrease of theReferencesinterfacial tension or the increase of the interfacial pres-1. Dorshow, R. B., Swofford, R. L, Application of surface laser-sure was present, even though this was in the absence oflight scattering spectroscopy to photoabsorbing systems: Thesurfactants at liquid-liquid interfaces. We argue that theassociated materials caused by the attraction of the disper-measurement of interfacial tension and viscosity in crude oil, J.sion force are present at the interfaces, which are sensitiveAppl. Chem., 1989, 65: 5637.to the interfacial pressure. First， the Fowkes theoryl!"l. Lemaire, C. Langevin, D.. Longitudinal surface waves at liquiddemonstrates that the attraction of the dispersion forceinterfaces: measurement of monolayer viscoelasticity, Colloidsbetween two phases may make surface tension lower,and Surfaces, 1992, 65: 101.which provides the theoretical evidence for the existenceof the associated materials. There is also the possibility3. Miller, R., Wistneck, R., Kriger, J. et al, Dilational and shearthat the interfacial associated materials may exist accord-rheology of adsorption layers at liquid interfaces, Colloids anding to the iceberg theoryl12.131 that the water order structureSurface A, 1996, 111:75.is formed around the organic molecules. At oil-water in-，Lucassen, J., Longitudinal capillary waves, Trans. Faraday. Sterfaces, the reactions of the dispersion forces between oil1968, 64:2221.molecules and water molecules form the so-called am-phipathic interfacial associated material. This material is5. Kretzschrnar, G, Miller, R., Dynamic properties of adsorptiononly formed on special interfaces, and is a kind of dy-layers of amphiphilic substances at fluid interface, Adv. Colloidnamic interfacial associated material. Not being a comp O-Interface Sci, 1991, 36: 65.nent, the interfacial associated material merely dissociates,6. Jiang, Q, Chiew, Y. C., Valentini, J. E, Damping of cylindricalinstead of diffusing from interfaces into bulk phases inpropagating capillary waves on monolayer-covered surface,terms of the total component when the interface is com-pressed, and the dissociated oil molecules and water mD-Langmuir, 1992, 8: 2747.lecules diffuse into their own bulk phases respectively.. Jiang, L, Zhao, F, Tang, J. A. et al, Determination of interfaceThe dissociating process of the associated material nD-(surface) dilational rheological properties, Chinese Science Bulle-lecules is so slow that it is difficult for them to disappeartion, 2001, 46(9): 736.from interfaces, thus strengthening the viscoelasticity ofinterfacial layers. The higher the two-phase intermolecular8. Gaines, G. L. Jr, Insoluble Monolayers at Liquid-Gas Interfaces,attractive energy is, the longer the interfacial pressureNew York: Interscience, 1966relaxation time will be, as has been shown by the study of9. Zhou,Z. K., Gu, X. R.. Ma, J. M.. Principles of Colloid and Sur-the interface dilatation rheology.face Chemistry (in Chinese), Bejjing: Peking University Publish-3 Conclusioning Company, 1984.The liquid-liquid interface has not been clearly0. Hiemenz, P. C, Rajagopalan, R., Principles of Colloid and Sur-studied in the interface science because of the complexityface Chemistry , 3rd ed, New York: Marcel Dekker, 1997: 270”of the system and the lack of experimental methods. In271, 289 -290.this note we provide a good method for measuring prop-erties of the interfacial rheology. By the measurement of11. Fowkes, F. M., Attractive forces at interfaces, Ind. Eng. Chem,interface dilatation rheological properties, the field of1964, 56(40).liquid-liquid interface that is known very little by people2.中国煤化工’olume and entopy in condensedmay be deeply and semi-quantitatively studied, which isimpossible by the static measurement. We propose theCCHCNMHGi:507.I 3. Snunoua, n., rmmcapies 0l sulution and solubility, New York:concept of amphipathic interfacial associated material toMarcel Dekker, 1978: 123 "133.interpret theological measurement results, and character-ize directly the essence of interfacial molecular interac-(Received August 7, 2002)Chinese Sg宇内数剧/letin Vol. 47 No. 24 December 20022059