Liquid interface adhesion

Chapter 1 is a view of the potential of surface forces apparatus (SFA) measurements of two-dimensional organized ensembles at solid-liquid interfaces. At this level, information is acquired that is not available at the scale of single molecules. Chapter 2 describes the measurement of surface interactions that occur between and within nanosized surface structures—interfacial forces responsible for adhesion, friction, and recognition. 

Food typically is a complicated system with diverse interfaces. Stable air-water or oil-water interfaces are essential for the production of food foams and emulsions. Interface phenomena, therefore, attract great interest in the food industry. AFM provides enough resolution to visualize the interface structures, but it cannot be directly applied on air-liquid or liquid-liquid interfaces. Fortunately, the interface structure can be captured and transferred onto a freshly cleaved mica substrate using Langmuir-Blodgett techniques for AFM scan. Images are normally captured under butanol to reduce adhesion between the probe and the sample. Then, sample distortion or damage can be avoided (Morris et al, 1999). 

In bringing together the surfaces of two liquids to form a liquid-liquid interface heat is either evolved or absorbed during the process. This heat change may be termed with Harkins the heat of adhesion. The sum total of the surface energies of the two surfaces before contact less that of the interface after contact represents this total amount of energy given off or absorbed if the process be carried out isothermally or... 

Thus, fundamentally the interest is in testing the limits and theory of polymer behavior in end-tethered systems, e.g., viscoelastic behavior, wetting and surface energies, adhesion, shear forces relevant to tribology, etc. It should be noted that relevant surfaces and interfaces can also refer to polymers adsorbed in liquid-liquid, liquid-gas, solid-gas, and solid-liquid interfaces, which makes these polymer systems also of prime importance in interfacial science and colloidal phenomena (Fig. 2). Correspondingly, a wide number of potential applications can be enumerated ranging from lubrication and microelectronics to bioimplant surfaces. 

Combination with equation 2 for eneigy of adhesion of a solid—liquid interface (eq. 3) gives equation 5 ... 

The work of adhesion between two immiscible liquids is equal to the work required to separate unit area of the liquid-liquid interface and... 

Synthetic surfactants and polymers are probably most often used to modify the characteristics of a solid surface, i.e., they function at the solid - liquid interface, such as in the processes of detergency, lubrication, or the formation of adhesive bonds. The performance of modem FT - IR spectrometers is such that many new applications to the characterization of the solid - liquid interface, particularly in kinetics studies, are possible. Reflection - absorption spectroscopy and attenuated total reflectance (ATR) techniques have been applied to "wet" interfaces, even the air - water interface, and have figured prominently in recent studies of "self -assembled" mono - and multilayers. 

Polymer Adsorption. A review of the theory and measurement of polymer adsorption points out succinctly the distinquishing features of the behavior of macromolecules at solid - liquid interfaces (118). Polymer adsoiption and desorption kinetics are more complex than those of small molecules, mainly because of the lower diffusion rates of polymer chains in solution and the "rearrangement" of adsorbed chains on a solid surface, characterized by slowly formed, multi-point attachments. The latter point is one which is of special interest in protein adsoiption from aqueous solutions. In the case of proteins, initial adsoiption kinetics may be quite rapid. However, the slow rearrangement step may be much more important in terms of the function of the adsorbed layer in natural processes, such as thrombogenesis or biocorrosion / biofouling caused by cell adhesion. 

The adsorption of surfactants at the liquid/air interface, which results in surface tension reduction, is important for many applications in industry such as wetting, spraying, impaction, and adhesion of droplets. Adsorption at the liquid/liquid interface is important in emulsification and subsequent stabilization of the emulsion. Adsorption at the solid/liquid interface is important in wetting phenomena, preparation of solid/liquid dispersions, and stabilization of suspensions. Below a brief description of the various adsorption phenomena is given. 

Adhesion between two liquids. The attraction exerted by one liquid across the interface on the other requires that work must be done to separate them. It may easily be shown that this work, called the adhesional work9 between the liquids, is equal to the sum of the surface tensions of the liquids singly, less the interfacial tension of the liquid-liquid interface. Suppose A and B (Fig. 1) are the liquids in a column one square centimetre in cross-section then when they are in contact the energy of the interface is yAB when they have been separated by a direct... 

The work of adhesion between two liquids orientation at liquid-liquid interfaces. Much the clearest conclusions at present available as to the structure of the surfaces of liquids (except those covered by films of another substance) concern the interface between two liquids. They have been obtained by considering the work of adhesion between the liquids, or the perpendicular attraction across the interface. This work of adhesion can be ascertained by measurement of the surface tensions of the two liquids separately, and of the interfacial tension between them, and applying Dupre s equation (Chap. I, 8) ... 

Dobry and Boyer-Kawenoki (14) concluded that for polymers, compatibility is the exception and incompatibility is the rule. For a heterogeneous system, incompatibility is an advantage for reinforcement, provided that the adhesion at the interface is strong enough to withstand the applied stresses. The nature of adhesion depends on the type of interface—e.g., liquid-liquid, liquid-solid, or solid-solid. In the case of two polymers at the liquid-liquid interface, a rubbery polymer-to-rubbery polymer adhesion (R-R adhesion) (35, 36) is likely to determine the... 

Prior to this discovery, in 1954 Silberberg and Kuhn (62) were first to study the polymer-in-polymer emulsion containing ethylcellulose and polystyrene in a nonaqueous solvent, benzene. The mechanisms of polymer emulsification, demixing, and phase reversal were studied. Wetzel and Hocks discovery would then equate the pressure-sensitive adhesive to a polymer-polymer emulsion instead of a polymer-polymer suspension. Since the interface is liquid-liquid, the adhesion then becomes one type of R-R adhesion (35, 36). According to our previous discussion, diffusion is not operative unless both resin and rubber have an identical solubility parameter. The major interfacial interaction is physical adsorption, which, in turn, determines adhesion. Our previous work on the wettability of elastomers (37, 38) can help predict adhesion results. Detailed studies on the function of tackifiers have been made by Wetzel and Alexander (69), and by Hock (20, 21), and therefore the subject requires no further elaboration. 

Contact angle measurements provide information on the wettability of the sample, the surface energetics of the solid, and the interfacial properties of the solid-liquid interface. The samples were immersed in water and captive air and octane bubbles were determined by measuring the bubble dimensions. By measurement of both air and octane contact angles the surface free energy (.y) of the solid-vapor ( > ) interface may be calculated by use of Young s equation and the narmonic mean hypothesis for separation of the dispersive and polar components of the work of adhesion. This method for determination of surface and interfacial proper-... 

Knowledge of interfacial tensions is important in the practical problems of lubrication and adhesion. Successfiil lubrication occurs for a system where a lubricant completely wets a sohd and maintains complete coverage under tribological conditions. The degree of adhesion of a lubricant to a solid or the strength of an adhesive can be determined by the extent to which the free energy of the system is lowered by the adsorption of the lubricant or adhesive. The energy necessary to separate the sohd liquid interface (in vacuum)... 

Wolfram, E. Adhesion at Solid-Liquid Interface. Lecture Course at University of Bristol, England, 1977/1978. 

Some of the areas where interfacial protein layers dominate the boundary chemistry are reviewed, and we introduce some nondestructive armlytical methods which can be used simultaneously and/or sequentially to detect and characterize the microscopic amounts of matter at protein or other substrates which spontaneously acquire protein conditioning films. Examples include collagen and gelatin, synthetic polypeptides, nylons, and the biomedically important surfaces of vessel grafts, skin, tissue, and blood. The importance of prerequisite adsorbed films of proteins during thrombus formation, cell adhesion, use of intrauterine contraceptives, development of dental adhesives, and prevention of maritime fouling is discussed. Specifics of protein adsorption at solid/liquid and gas/liquid interfaces are compared. 

It is desirable to find a simple surface property that will induce an equally simple and hence predictable behavior of plasma and its proteins and then of blood and its platelets. Are simple guidelines for building non-thrombogenic materials available or even possible Wettability (96), flow (97, 98y 99,100), and the effects of air/liquid interfaces (101) all seem to be relatively simple, physical factors with a clear effect on platelet adhesion. Physical, hydrophobic bonding, e.g., a force imposed... 

Experiments with solids immersed in cell culture media, with and without living cells present, convincingly demonstrate the absolute condition that protein-dominated films accumulate at the solid-liquid boundaries prior to cell adhesion. The reality and speed of this spontaneous, adsorptive event are documented by using infrared-transmitting, multiple internal reflection plates as the immersed solid with care to prevent film transfer from gas-liquid interfaces. 

In the case of a liquid-gas interface, molecules of the liquid in the boundary can only develop attractive cohesive forces with molecules situated below and adjacent to them. They can develop attractive adhesive forces with molecules of the gaseous phase. However at the gas-liquid interface, these adhesive forces are quite small. The net effect is that molecules at the surface of the liquid have potential energies greater than those of similar molecules in the interior of the liquid and experience an inward force toward the bulk of liquid. This force pulls the molecules of the interface together and the surface contracts. 

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