The interfacial tension (excess surface energy) in a liquid-liquid system will be lower than in a liquid-gas interface. The condensation of liquid molecules to form very small droplets will be slowed down by a relatively high energy barrier due to the curvature. z When a new surface is created, it is reasonable to assume that a finite amount of time will be required before new molecules will form and diffuse to the surface and bring the system back into equilibrium. Time for diffusion of liquid molecules to the surface. self-diffusion constant, 10-6 cm2/s), about milliseconds--- difficult to measure surface tensions in such a short time.
Most often, the effect is a lowering of surface tension, but the opposite effect can also be found. For an ideal binary mixture, the surface tension of the solution is σmix : σmix = σ1X + σ2(1-X) , X: mole fraction of component 1. There will usually be some positive or negative deviation from linearity. z When the second component is an inorganic electrolyte. The addition of an inorganic electrolyte always results in an increase in the surface tension of the solution. the effect is not dramatic and requires a rather high salt concentration) ---- fig.
The effect is due to the structuring of water molecules on the surface due to the salvage phenomenon. The surface tension will decrease with the addition of an organic solution and the extent of such decrease depends on: (1) the hydrophilic head, (2). If the vapor phase is replaced by a condensed phase, which has a higher molecular density and more opportunities for attractive interaction between molecules in the interfacial region, the interfacial tension will decrease.
Octanol-water – (polar group interacts more specifically with water): to 8.5 mN/m. z Any change in the nature of the molecules that make up the surface is expected to result in a lowering of the surface energy of the system. --- effect of surfactant to lower surface tension. z Most surfactants do not affect the surface tension of organic liquid.
Adsorption Effectiveness
A large value of Ccmc/C20 indicates that the available free surfactant is being used more efficiently.---Table 8.2. The effectiveness and efficiency of surfactants may not run parallel.--- It is commonly observed that the materials that produce a significant lowering of the surface tension at low concentrations (i.e. are more effective) are less effective (i.e. have a smaller Γm) . The maximum number of molecules that can fit into a given area (Γm) depends on the cross-sectional area of.
For the straight-chain ionic surfactant, the head group always predominates—so σmin varies only slightly with hydrocarbon chain length (Table 8.2). The sign of the charge on the ionic surfactant has only a small effect---indicating that the geometric requirements are fairly constant from one set of heads to another. In the presence of electrolyte - the electrostatic repulsion between adjacent molecules decreases - the effective area becomes smaller - slight increase in the effectiveness of the surfactant.
Introduction of other polar groups, which are quite distant from the main group (Figure 8.10) Æ Change in orientation of adsorbed molecules Æ Substantial decrease in efficiency and. Effects of counterions: tight binding of ions reduces electrostatic repulsion between adsorbed molecules Æ tighter packing. The head group may be less important for hydrophilics, which are closely related in size and charge character.
Insoluble Monomolecular Films
- Surface pressure (π)
- Surface Potential
- Surface Rheology
The dipole moments of these groups are partially oriented relative to the interface when the molecules adsorb at the interface. V = n µ cosθ/ε (8.13) n: number of molecules in the films (known amount) µ: dipole moment of the head group (known or available) ε: permittivity of the film (dielectric constant x permittivity of . a vacuum) ). Eq (8.13) makes it possible to estimate the tilt angle (θ) of the dipole to the surface normal Æ infer the orientation of the entire molecules.
For a mixed monolayer, the surface film potential can be used to estimate the film homogeneity or film composition (when the values of pure films are known). It can be used to study the penetration of an insoluble layer by materials injected just below the surface. In the presence of a film with an adsorbed monolayer (Gibbs monolayer or Langmuir monolayer), the rheological properties (surface viscosity, elasticity) of the surface may change.
The Physical States of Monolayer Films
- Gaseous Films
- Liquid Films
- Condensed Films
- Some Factors Affecting the Type of Film Formed
- Mixed-Film Formation
- Surface Films of Polymers and Proteins
- Monolayer Films at Liquid-Liquid Interface and on Nonaqueous Liquids
- Deposited Monolayer and Multilayer Films
The critical molecular area for stearic acid is about 0.205 nm2, as is that for palmitic and myristic, and other members of the series with more than 12 carbons. In the condensed film state, when the area is further reduced, the pressure drops suddenly. If the head group has higher solubility, (eg, the -OH of the alcohol, C18OH), it can be compressed into the L2 film, but not into the solid film.
If the tail is branched or has two or more hydrocarbon chains (eg esters): the cross-sectional area becomes larger and a more extended film is formed. For molecules containing two hydrophilic groups: the interaction between molecules and water is increased due to the second hydrophilic group Æ the molecules tend to lie more flat on the water surface at low surface pressure. As the pressure increases, the energy required to force these groups away from the surface Æ π will be greater.
The process of raising the tails in such a material is gradual, so that a curve characteristic of an expanded film will arise. Trans double bond --- hydrocarbon chain will be more straight Æ high lateral interactions and good packing efficiency Æ solid or L2 film. Effects of changes in the nature of the substrate (Subphase) z Nature of subphase: pH, concentration and valence of dissolved ions.
Each cation associates with two acid molecules (forming metal salt) leading to a tighter packing and formation of solid or condensed film. Two normal chain carboxylic acid (Fig. 8.19a), the properties of the mixed monolayer lie between those of the pure materials. Increasing the surface pressure can cause one component to be removed from the surface (Fig. 8.19c).
Film penetration: soluble surfactants in the subphase penetrate the surface film to significantly change its nature or to undergo physical or chemical processes. Typical penetration experiment --- formation of an insoluble monolayer at a surface pressure π, then a soluble surfactant is injected under the monolayer and changes in surface pressure (at constant area) due to penetration or inclusion of the new material into the monolayer. Reason: the presence of non-polar liquid reduces the lateral cohesive interaction between adjacent tails (swelling of the monolayer).
将气液界面上的单分子层转移到固体基质上----一般称为“Langmuir-Blodgett技术”，转移的薄膜称为“Langmuir-Blodgett薄膜”。 (a)亲水性固体基质从被单层覆盖的液体表面移动；
