Surface and Interfacial Phenomena

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Questions and Answers

In a scenario where a pharmaceutical scientist aims to enhance the dissolution rate of a tablet, which modification would likely produce the MOST desirable outcome?

Increasing the liquid-air surface tension to promote droplet formation on the tablet's surface.
Selecting an excipient that increases the contact angle between the dissolving medium and the tablet.
Reducing the surface tension of the dissolving medium to facilitate wetting and penetration into the tablet's pores. (correct)
Applying a hydrophobic coating to the tablet to prevent rapid disintegration.

A researcher observes a contact angle of approximately 150° between a liquid and a solid surface. Which of the following interpretations is the MOST accurate regarding the intermolecular forces at play?

The adhesive forces between the liquid and solid are significantly stronger than the cohesive forces within the liquid.
The liquid has a strong affinity for the solid surface, leading to complete wetting.
The liquid and solid are chemically inert, resulting in a neutral contact angle.
The cohesive forces within the liquid are dominant, causing it to minimize contact with the solid surface. (correct)

Considering the principles of surface tension and contact angle, which scenario would MOST likely benefit from the addition of a surfactant?

Increasing the viscosity of a liquid suspension to prevent settling.
Improving the structural integrity of a compressed tablet during manufacturing.
Facilitating the uniform spreading of a topical ointment on the skin. (correct)
Enhancing the miscibility of two highly polar solvents in a chemical reaction.

In the context of pharmaceutical formulation, what is the MOST significant implication of a liquid exhibiting a contact angle close to 0° on a solid substrate?

The liquid will readily wet the solid, promoting spreading and penetration. (D) Signup and view all the answers

A formulator is attempting to create a stable suspension of a hydrophobic drug in an aqueous medium. Based on the principles of surface tension and contact angle, which strategy would be the MOST effective in achieving this goal?

Adding a wetting agent to reduce the contact angle between the drug particles and the aqueous medium. (C) Signup and view all the answers

A liquid with a negative spreading coefficient when placed on another liquid will most likely:

Create globules or a floating drop on the surface. (A) Signup and view all the answers

Which scenario would prevent a liquid from spreading on a sublayer, according to the spreading coefficient?

The combined surface tension of the spreading liquid and interfacial tension exceeds the surface tension of the sublayer liquid. (A) Signup and view all the answers

How does surface tension influence the behavior of detergents?

By decreasing the surface tension of water, allowing it to spread more easily and wet surfaces. (A) Signup and view all the answers

In the context of liquid behavior, what primarily determines the contact angle between a liquid and a solid surface?

The interactions across the liquid/vapor interface and the solid surface. (B) Signup and view all the answers

What is the significance of achieving a zero or positive spreading coefficient?

It is a prerequisite for one liquid to spread over another. (B) Signup and view all the answers

Consider two liquids, A and B. Liquid A has a surface tension of 50 dyne/cm, and liquid B has a surface tension of 30 dyne/cm. If the interfacial tension between them is 25 dyne/cm, will liquid B spread over liquid A, and what is the spreading coefficient?

No, S = -5 dyne/cm (C) Signup and view all the answers

How does surface tension contribute to capillary rise in narrow tubes?

It contributes to the curvature of the liquid's surface, resulting in an upward force. (C) Signup and view all the answers

In a system involving a liquid, vapor, and solid, under what conditions would you expect to observe a high contact angle?

When the cohesive forces within the liquid are dominant, leading to poor wetting of the solid surface. (B) Signup and view all the answers

Interfacial tension is typically lower than surface tension. Which of the following BEST explains this difference?

Adhesive forces between two immiscible liquids are greater than those between a liquid and a gas. (B) Signup and view all the answers

In the capillary rise method, what factors directly influence the height a liquid will rise in the capillary tube?

The surface tension of the liquid, the density of the liquid, the radius of the tube, and the acceleration due to gravity. (C) Signup and view all the answers

A Du Noüy ring tensiometer is used to measure the surface tension of a liquid. If a ring with a larger radius is used, and the detachment force remains the same, what can be concluded about the liquid's surface tension?

Surface tension will decrease. (D) Signup and view all the answers

If the detachment force measured by a Du Noüy ring tensiometer is found to be unusually high, what is the MOST likely cause, assuming the instrument is calibrated correctly?

The formation of a meniscus around the ring during detachment adds to the measured force. (C) Signup and view all the answers

In the drop weight method, how does an increase in the number of drops formed by a given volume affect the derived surface tension, assuming the liquid density remains constant?

The derived surface tension decreases. (C) Signup and view all the answers

During an experiment using the drop weight method, it is observed that a different liquid forms drops with significantly smaller volumes than the reference liquid. Assuming identical experimental conditions, what can be inferred about the surface tension of the new liquid?

The surface tension of the new liquid is likely lower than that of the reference liquid. (A) Signup and view all the answers

How would using a Stalgmometer with a tip of non-uniform radius impact the accuracy of surface tension measurements obtained via the drop weight method?

It would decrease the accuracy because the non-uniform radius would lead to inconsistent drop formation. (C) Signup and view all the answers

When measuring interfacial tension between oil and water using the ring method, what methodological adjustment would improve the accuracy of the measurement, given the potential for emulsification?

Ensure the ring is perfectly clean and free of any contaminants that could promote emulsification. (D) Signup and view all the answers

Which scenario exemplifies a system where adhesive forces significantly outweigh cohesive forces?

The dissolution of sugar in water. (A) Signup and view all the answers

Consider a scenario where a new polymer coating is applied to a glass slide. If the coating causes water to spread out evenly in a thin film rather than forming droplets, what can be inferred about the surface energies at play?

The adhesive forces between the water and the coating are greater than the cohesive forces within the water. (B) Signup and view all the answers

In the context of capillarity, how does the interplay between adhesive and cohesive forces dictate whether a liquid will rise or fall within a narrow tube?

Liquids rise when adhesive forces are greater, forming a concave meniscus; they fall when cohesive forces dominate, creating a convex meniscus. (B) Signup and view all the answers

A researcher observes that a particular liquid forms a convex meniscus in a glass tube and exhibits a high contact angle. Which of the following conclusions is most justified?

The cohesive forces within the liquid are dominant, leading to poor wetting of the glass surface. (C) Signup and view all the answers

How does the concept of surface free energy relate to the phenomenon of a liquid's wettability on a solid surface?

Wettability is enhanced when the solid's surface free energy is higher, promoting greater interaction with the liquid. (D) Signup and view all the answers

Consider two immiscible liquids, A and B, in contact. If the interfacial tension between them is very high, what does this imply about their miscibility and the forces at play?

A and B are immiscible, with strong cohesive forces within each liquid and weak adhesive forces between them. (A) Signup and view all the answers

A surfactant is added to a system of oil and water. How does the surfactant modify the interfacial properties, and what is the underlying mechanism?

It reduces the interfacial tension by positioning itself at the interface, reducing the energy needed to expand the surface area. (D) Signup and view all the answers

In the context of surface phenomena, what distinguishes an 'interface' from a 'surface,' and where are these terms typically applied?

An 'interface' is the boundary between any two phases, whereas a 'surface' specifically refers to a boundary where one phase is a gas (or vapor). (A) Signup and view all the answers

A scientist observes that a small droplet of liquid maintains a near-perfect spherical shape. Which statement best explains this phenomenon in terms of surface free energy and molecular forces?

The spherical shape minimizes the surface area, thereby minimizing the overall surface free energy due to inward cohesive forces. (A) Signup and view all the answers

Consider two immiscible liquids in contact. How does interfacial tension differ fundamentally from surface tension?

Interfacial tension arises from the interaction between two liquid phases, while surface tension arises from the interaction between a liquid and a gas phase. (B) Signup and view all the answers

A researcher aims to reduce the surface tension of a liquid to improve its spreadability on a solid surface. Which action would be most effective?

Adding a surfactant to the liquid to decrease the cohesive forces between surface molecules. (A) Signup and view all the answers

Which of the following scenarios best demonstrates the effects of surface tension in everyday life?

The beading of water droplets on a freshly waxed car. (D) Signup and view all the answers

Consider the equation $W = \gamma \Delta A$, where $W$ represents work, $\gamma$ represents surface tension, and $\Delta A$ represents the change in area. Under what conditions would a minimal amount of work be required to increase the surface area of a liquid?

When increasing the surface area of a liquid with low surface tension. (A) Signup and view all the answers

If a liquid has a surface tension of 25 dynes/cm, what force must be applied to maintain equilibrium on a 5 cm wire lying on the surface of the liquid, opposing the liquid's inward pull?

250 dynes (B) Signup and view all the answers

Why do smaller droplets evaporate faster than larger droplets, considering the principles of surface tension and vapor pressure?

Smaller droplets have a larger surface area to volume ratio, enhancing evaporation. (B) Signup and view all the answers

A researcher observes a contact angle greater than 90 degrees between a liquid and a solid surface. What can be inferred regarding the relative magnitudes of cohesive and adhesive forces?

Cohesive forces are much stronger than adhesive forces, leading to non-wetting. (A) Signup and view all the answers

Flashcards

Contact Angle

The angle formed between a liquid and a solid surface at the contact point.

Surface Tension

The cohesive force between liquid molecules at the surface that causes it to behave like a stretched elastic membrane.