Surface chemistry studies such phenomena as damping, adhesion, sorption and surface tension.
Surface tension (σ) is defined as energy per unit area, [J/m2], or the force that causes the surface of a liquid to contract [N/m]. Among the pure liquids, Hg has a maximum value of σ. Among the biological fluids, H2O has a maximum value of σ, and dissolved substances may either increase or decrease this constant, forming a new system.
As Gibbs’ energy (G) tends to the minimum (ΔG <0 for the spontaneous process), the surface Gibbs’ energy GS = σ ∙ S → min, and for pure liquids S → min is the only way to decrease free surface energy, that is why liquids form drops. The greater σ — the bigger drop may be formed, thus the number of drops in the certain volume is less. This method is often used to find the value of σ in either pure solvents or solutions including biological fluids and drugs. Surface tension (σ) depends on t°, pressure, the polarity of phases, concentration and nature of solutes (or admixtures). Here is another way to decrease free surface energy (GS): the component with a lesser value of σ is pushed out, onto the surface.
There are three groups of substances by the influence on surface tension (σ):
SAA, surface active agents, or surfactants — decrease σ, being pushed out on the surface (e.g.: alcohols, carboxylic acids, thiols, and so on).
SNA, surface nonactive agents, have no influence on σ (e.g.: sucrose, glucose, and so on).
SIA, surface inactive agents, increase σ, hide in the bulk (e.g.: NaCl, H2SO4, KOH, and other inorganic electrolytes).
To emphasize SAA (surfactants), we should mention the features of their structure. Surfactants have diphillic nature, contain both hydrophilic and hydrophobic parts (●───).
Hydrophilic “heads”(●) include polar groups (–СООН, –ОН, –NH2, –NO2, –HO, –SO3H, …). Hydrophobic “tails” (───) — hydrocarbon chains. For example:
CH3 — (CH2 )14 — COONa
(sodium palmitate, the main component of solid soap).
Washing power of soaps, detergents (C17H35COONa, C12H25SO4Na, etc.) is based on SAA diphillic nature.
SAA are used to prepare drugs and drug delivery systems, emulsions (including food), antiseptic agents against bacteria and fungi, household products such as toothpastes, shampoos, shaving foams, etc.
Surfactant (mixture of SAA) covers the surface of alveoli inside, prevents lung collapse, stimulates assimilation of oxygen, particularly important in newborn children.
Some features of adsorption on a mobile border (in system “Liquid/Gas”)
On the surface “water–air”: nonpolar tails interact with nonpolar air, polar heads are hidden in polar water. In the bulk: to reduce the interaction of the hydrophobic tail with water, SAA molecules form hydrophilic micelles. Self-assembled micelles are used for drug delivery in modern medicine.
Adsorption in this system may be found with the help of Gibbs’ formula:
A = g ∙ c/(RT), (6.1)
where g = (–Δσ/Δc) is called surface activity and defined as ability to change the value of surface tension σ; c is the molar concentration of the solute, mol/L; comparing two solutions, we use the arithmetic mean value of “c”; R is the gas constant, R = 8.31 J/mol·K; T is the temperature of the solution on the Kelvin scale, K; T = 273 + t°.
Surface activity (g) depends on the structure of the molecule. For example, g becomes 3–3.5 times greater with each added –CH2-group in organic surfactants (Duclos–Traube’s rule); therefore, g (valeric acid) > g (butyric acid).
When we add SAA to pure water (c2 – c1 > 0, Δc >0), surface tension σ decreases (σ2 – σ1 < 0, Δσ < 0); thus g = (–Δσ/Δc)> 0; A > 0. Consequently, SAA is positively absorbed, the surface “takes” this component. Positive surface activity g, positive adsorption A. But adsorption in this system is limited: when monomolecular layer is formed, all the places on the surface are “occupied”, the value of A reaches its maximum, and s gets minimum and does not change any more.
For SIA, vice versa: σ2 – σ1 > 0, Δσ > 0; g < 0; A < 0. SIA is negatively adsorbed, and tends to the bulk, the surface “removes” this component. Negative surface activity, negative adsorption.
Emulsions (systems “Liquid/Liquid”)
Two immiscible liquids form an emulsion in presence of emulsifying agent (ESA, stabilizer). Polar liquid is denoted “water”, nonpolar – “oil”. In spite of diphilic nature, ESA always has a predominance of either hydrophilic or hydrophobic properties. There are two possible types of emulsions: “oil–in–water” (o/w), when small droplets of nonpolar phase are dispersed throughout polar dispersion medium, and vice versa, an emulsion “water–in–oil” (w/o), if the small droplets of polar phase are dispersed throughout nonpolar dispersion medium.