Interfacial Viscoelasticity in Emulsions and Foams

Both the generation and the stability of emulsions and foams depend on interfacial properties induced by adsorbed surfactants. The essential function of surfac tants during emulsification and foaming is not that they reduce the equilibrium interfacial tension, but they impart specific dynam ic properties to the in te rface. A su rfactant-covered interface behaves as a two-dimensional body with its own elasticity and viscosity, which a re re lated to the non-equilibrium values of the interfacial ten sion. Such viscoelasticity is a significant factor in any liquid flow near the interface, and in the stabi lity of the thin films separating emulsion drops or foam bubbles. Numerically , values of the viscoelastici ty va ry over a wide range, depending on surfactant parameters, on rate of su rface deformation and on any relaxation processes driving the interfacial tension to its equilibrium. A quantitative framework is ava il able for the evaluation of these rheological pa ramete rs, and of the dynamic surface tension of surfactant solution s, if the relaxation mechanism is diffusional interchange between surface and sol ution. We review both theoretical and experimental in formation on the dynamic surface behaviour of surfactants , and indi cate possible routes to a quantitative model fo r their functiona li ty in food emulsions and foams. Key \Vords: Emulsification, emulsifier, emulsion stability, dynamic surface properties, diffusional relaxation, Gibbs elasticity, surface dilational modulus, surface dilational elasticity, su rface dilational viscosity. Initial paper received September 15, 1992 Manuscript received February 17, 1993 Direct inquiries to E. Lucassen-Reynders Telephone number: 31-10-460-5377 Fax number: 31-10-460-5236 Introduction The very existence of emulsions and foams has long been known to depend on the presence of emulsifiers and foamers, i.e., surface active molecules which are adsorbed at the interface from the continuous liquid phase. The primary effect of such molecular adsorption is that it reduces the tension of the interface. However , reduction of the inter facial tension cannot in itself explain the formation of emulsions and foams with more than transient stabi lity. If this were the case , it should be possible to prepare emulsions in the absence of surface active solu tes , from pure low-tension liqu ids. In practice, it is impossible to obtain emulsions with any degree of stabi lity in this way. The essential stabilisi ng function of surface active molecules during emulsification is to enable the interface to res ist tangential stresses from the adjoining flowing liquid s (van den Tempel, 1960). An emu lsifier-covered in te rface , therefore, can behave as a two-dimensional body with its own rheologi cal properties. These provide the liquid films separati ng emu lsion drops with a mechani sm for dynamic stabi lisation, without which any two drops just formed would be liable to immediate coalescence during the emulsification process. The present paper reviews background and existing data on the rheology of interfaces and indicates how rheological paramete rs can be estimated in areas where measurements are difficult. Relevance of Interfacial Rheology for Liquid Film Stability The main mechanism by which a surfactant aids emulsification and foaming is that its presence can lead to gradients in interfacial tension , which enable the interface to resis t tangential stresses. Figure I illustrates how such gradients can produce a resistance against local thinning of the liquid film separating two emulsion drops or foam bubbles, and so prevent coalescence. Without surfactant, such a liquid film has a uniform interfac ial tension (u), and is insufficiently stable against rupture induced by any sudden local disturbance of its surface. In the presence of surfactant, however, the E.H. Lucassen-Reynders same disturbance will create a momentary gradient in ad sorption and hence a gradient in interfacial tension . Interfaces can be likened to elastic membranes , in which regions of high tension tend to contract more strongly than do low-tension regions. Film areas with high interfacial tension, therefore, exert a pull on adjacent lowertension areas, which results in a flow of surface with ad hering liquid towards the thin spot in the film. In this manner, surfactants added as emulsifiers or foaming agents can make a surface resist any deformation that would create a gradient surface tension, and so they provide the film with a self-healing mechanism. Moreover, a sufficiently rigid adsorption layer can slow down drainage of the liquid film to a considerable degrees , and thus contribute to the long-term stability of emulsions and foams. Qualitatively , the effect of a surface's resistance against tension gradients has been described more than a century ago by Marangoni (1872) and Gibbs (1878) . Quantitatively, the measurement of this resistance is from the last few decades , and so is the interpretation of the results in terms of surfactant properties. Definition of Interfacial Rheological Parameters Interfacial or two-dimensional rheology, like its more familiar three-dimensional counterpart, defines the functiona l relationship between stress , deformation and rate of deformation in terms of coefficients of elasticity and viscosity. In both two and three dimension s, different types of deformation of fluid elements are possible, with different elastic and viscous coefficients. Two main types are illustrated in Figure 2 for a two-dimensional sys tem : (i) shear , i.e., changes in shape at con stant area or volume; (ii) compression/dilation , i.e. , changes in area or volume at constant shape. However , the analogy between 2-D and 3-D rheology is far from complete and can be misleading , as has been pointed out (van den Tempel, 1977). One major difference is that the interface is not an autonomous system: it exists only as a boundary between two bulk phases . Its motion is always coupled to that of the adjoining 3-D phases, and "intrinsic" surface properties cannot be separated from substrate effects without arbitrariness. Because of this non-autonomy , contents and composition of an interface can change during an experiment. This will be shown to result in unusual rheological behaviour, especially in emulsions and foams. Another important difference between twoand three-dimensional systems is that, in many instances of practical application, 3-D liquids can be considered incompressible, i.e . , their molecular volume is constant. In the 2-D case, however, surfactant-covered interfaces do not have constant molecular areas: such interfaces can be compressed and expanded over a range of areas stretching in practice from 0.3 to more than 10 nm2/molecule. Accordingly, reported viscosities of 3-D liquids general ly 2 do not refer to compressional but to shearing deformation , but in 2-D systems "surface viscosity" must be specified as measured either in shear or in compression/ dilation. Surface shear viscosity has traditionally enjoyed the greater popularity with experimentalists (Goodrich eta/. , 1975; Mohan eta/. , 1976), especially in food systems (Dickinson er a/., 1988; MartinezMendoza and Sherman, 1990; Kiosseoglou, 1992). The quantitative determination of surface dilational elasticity and viscosity has come into focus during the last few decades (Lucassen and van den Tempel, 1972; Wasan et a/., 1979). The emphasis in the present paper will be on these dilational properties. In the case of compression/dilation , the surface stress , ! \u , resulting from small area variations , ! \A, can be expressed as the sum of an elast ic and a viscous contribution , with the latter depending on the relative rate of the area variation: . £\q = Ect! \lnA + lJct dinA dt (I) The elastic coefficient Ect is a measure of the recoverable energy stored in the interface , whil e the viscous coefficient '1ct reflects the loss of energy through relaxation processes, i.e. , any spontan eous processes which affect the surface tension within the time scale of the experiment. Contrary to what is often considered normal practice in shearing deformation , the elastic component cannot be ignored in compression/dilation . In fac t, the elasti c contribution to the surface stress generall y predomi nates over that of vi scosi ty in many emulsions and foams . The two coeffi cients Ect and 71 ct can be combined into a single modulu s of viscoelas tic ity , E, defined as the increase in surface ten sion for a unit of relati ve increase in surface area: