Surface Phenomena and Adhesion to Tooth Structure

Surface Energy

Atoms and molecules at the surfaces of liquids and solids possess more energy (surface energy) than do those in the interior. In the case of liquids, this energy is called surface tension. As illustrated in Fig 5-1, the molecules at the surface are farther apart due to loss of molecules by evaporation. From Fig 5-2 it can be seen that this greater average separation leads to a higher net energy of attraction. This reaction results in a surface contractile force or surface tension, which causes the liquid to form drops and to exhibit a surface skin that resists extension or penetration.

The surface energies of oxides and metals are greater than those of liquids (Table 5-1). In general, the higher the bond strength of a substance, the greater the surface energy. Since metallic bonds are much stronger than the van der Waals bonds of liquids, metals have higher surface energies. The units of surface energy are ergs/cm2, but the surface tension of liquids is often expressed in the equivalent units of dynes/cm. The total surface energy of a system is the product of the surface energy of the material and the total area. Therefore, a high total surface energy exists if a material is finely divided (powder or colloid) to provide a large surface area, especially if the material has a high surface energy per unit area (eg, metals or ionic crystals).

Sintering

The firing of porcelain is a sintering process used for forming denture teeth and porcelain-fused-to-metal crowns. Sintering is a densification process in which finely divided particles are heated in contact. The driving force for this method is the reduction of total surface area and, thus, total surface energy.

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Fig 5-1 Distribution of vacancies (open circles) within isolated capillary liquid. More vacancies are present at (a) the surface as compared to (b) the bulk. Fewer vacancies are found at (c) the liquid-solid interface. (Modified with permission from O’Brien.1)

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Fig 5-2 Relations between intermolecular forces, intermolecular energies, and distance as affected by distance (rij) between molecules. Departure from equilibrium distance rij causes repulsion or attraction. (Reprinted with permission from O’Brien.1)

Table 5-1 Surface energies of various substances

SUBSTANCE SURFACE ENERGY (ERG/CM2) TEMPERATURE (°C)

Water

72

20

Benzene 29 20

Olive oil

36

20

Saliva 56 23

NaCl crystal

300

25

Dental porcelain 365 1,000

Copper, solid

1,430

1,080

Silver, solid 1,140 750

Table 5-2 Contact angles of liquids on solids

SOLID LIQUID CONTACT ANGLE (DEGREES)

Amalgam alloy

Water

80

Silicate cement Water 10

Acrylic resin

Water

75

Teflon Water 110

Ag3Sn

Mercury

140

Gold alloys Porcelain enamel 40–50

Nickel alloys

Porcelain enamel

80–100

Hydron Water 0

Etched enamel

Pit and fissure sealants

0

Image Wetting

The degree of spreading of a liquid drop on a solid surface is called wetting. The contact angle (θ), formed by the liquid surface and the interface separating the liquid and solid, is used as a measure of the degree of wetting (Fig 5-3). A 0-degree contact angle indicates complete wetting, and low values correspond to good wetting. Values above 90 degrees indicate poor wetting. Table 5-2 lists contact angles for several systems. Good wetting promotes capillary penetration and adhesion and indicates strong attraction between the liquid and solid surface molecules. Good wetting is important in soldering and is a factor in better denture retention. A more natural appearance is achieved if restorative materials are wetted by a thin film of saliva. Hydrophobic substances are those that exhibit high contact angles with water (eg, Teflon, silicone coatings) (Figs 5-4 and 5-5).

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Fig 5-3 Wetting. Low contact angle indicates good wetting (left); high contact angle indicates poor wetting (right). (Re-printed with permission from O’Brien and Ryge.2)

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Fig 5-4 Hydrophobic film of fluorinated polymer on enamel giving high contact angle. (Reprinted with permission from O’Brien.3)

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Fig 5-5 Poor wetting shown by porcelain enamel drops on nongold alloy at 1,040°C. The furnace thermocouple is seen above the drops. (Reprinted with permission from O’Brien and Ryge.4)

Image Adsorption

To reduce surface energy, atoms and molecules that are mobile will concentrate at high-energy surfaces. For example, finely divided charcoal will adsorb quantities of several gases, and detergent (or soap) molecules will concentrate at the surface of water, leading to a large reduction in surface energy. Adsorption is strongest when there is large energy saving (high surface energy and large surface area) and slows down as the surface is covered.

Adsorption occurs only at the surface, whereas absorption involves penetration and uptake by the interior of the material (as in swelling of hydrocolloid impression materials in water). Gold foil adsorbs gases readily and must be degassed before use.

Image Colloids

Colloids contain material that is present in particles larger than ordinary atoms or molecules but still invisible to the unaided eye (ie, 10 to 10,000 Å). There are three types of colloidal systems.

Insoluble dispersed particles

Lyophobic is the term used to describe materials that are insoluble in a liquid medium (eg, sulfur in water) (Table 5-3). These fine particles acquire electric charges that keep them in suspension. Colloidal gold, for example, is used to form a gold coating on alloys to be bonded to porcelain. Although lyophobic systems may last for many years, they are unstable and can be precipitated by electric methods (smokes) or gravity (emulsions).

Large molecules

These systems are solutions in which the dispersed molecules are of colloidal dimension. They are stable but the large size of the macromolecules gives the solutions properties similar to those of lyophobic systems.

Dental materials consisting of such solutions include agar and alginate hydrocolloid impression materials. These materials aggregate to form gels through van der Waals bonding between the long chain molecules, and the gels undergo imbibition and syneresis (absorption and exudation of solvent) with resulting swelling and shrinkage, respectively. This behavior is responsible for the dimensional instability of these impression materials. As with other colloidal systems, the liquid state is called the sol state.

Association colloids

Surface active agents, such as soaps and detergents, are examples of association colloids, which are aggregates of smaller molecules that achieve colloidal size. Each molecule consists of a long-chain hydrocarbon with a small charged polar group at one end (eg, sodium palmitate). The aggregates formed by these molecules, called micelles, often are spheric; in aqueous systems, the hydrocarbon ends gather in the center of the micelle, and the polar groups are exposed on the outside. This system is useful in cleaning because grease and other organic films are dissolved in the interior of the micelles and held in suspension.

Table 5-3 Classification of lyophobic colloids

DISPERSED PHASE CONTINUOUS PHASE TYPE

Solid

Liquid

Sol

Solid Gas Aerosol (smoke)

Liquid

Liquid

Emulsion

Liquid Gas Aerosol (fog)

Gas

Liquid

Foam

Gas Solid Foam

Image Capillary Penetration

The surface energy of a liquid creates pressure that drives the liquid into crevices, narrow spaces, and thin tubes. Saliva penetration around restorations (leakage) and around teeth are important examples. Capillary penetration of saliva is also partially responsible for denture retention.

Capillary rise

If a glass tube is immersed in a liquid (Fig 5-6a), the capillary rise, h, is given by the formula:

h = 2γ cos θ/rdg

where γ is the surface tension, θ is the contact angle, r is the tube radius, d is the liquid’s density, and g is the gravitational constant of 980 dynes/g. If the contact angle is less than 90 degrees, as in water-glass and saliva-enamel systems, elevation of the liquid occurs. However, if the contact angle exceeds 90 degrees (eg, mercury on glass or water on Teflon), depression takes place and pressure must be applied to force the liquid into the space (Fig 5-6b). If the capillary liquid is not connected with a reservoir of liquid, isolated capillaries (isocaps) are formed (Fig 5-7). The liquids in isocaps exert adhesive force on the walls of the capillary, which can be strong for thin layers. This capillary adhesion is a factor in denture retention.

To increase the penetration of saliva around acrylic dentures, surface coatings of silica are applied to reduce the contact angle. The main factor here, however, is maintenance of a small gap distance (ie, close adaptation between denture and mucosa), as shown in Fig 5-8.

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Fig 5-6 Effect of contact angle on capillary penetration. (a) Capillary elevation with concave meniscus (low contact angle). (b) Capillary depression with convex meniscus (high contact angle). ΔP is the capillary pressure. (Modified with permission from O’Brien et al.5)

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Fig 5-7 (a) Capillary liquid in contact with a reservoir. (b, c)

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May 28, 2016 | Posted by in Dental Materials | Comments Off on Surface Phenomena and Adhesion to Tooth Structure
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