Question

During micelle formation in the solution of surfactants (a) enthalpy change is positive but entropy change is negative (b) both the enthalpy and entropy are negative (c) enthalpy change is negative but entropy change is positive (d) both the enthalpy and entropy changes are positive

Short answer

(c) Enthalpy change is negative but entropy change is positive.

Step-by-step solution

Understand Micelle Formation

Micelle formation occurs when surfactant molecules aggregate in an aqueous solution, typically in response to hydrophobic interactions. Surfactants have hydrophobic tails and hydrophilic heads. When reaching a critical concentration, they form micelles, with the hydrophobic tails inside and the hydrophilic heads outside, minimizing the system's free energy.

Analyze Enthalpy Change

Analyze the enthalpy change associated with micelle formation. This process is usually exothermic due to the release of energy from new intermolecular interactions, such as van der Waals forces between the hydrophobic tails. Thus, the enthalpy change is typically negative.

Analyze Entropy Change

Consider the entropy change. Initially, water molecules are ordered around separate hydrophobic tails. When micelles form, these water molecules are released, gaining freedom, and increasing the system's overall entropy. Therefore, the entropy change is positive.

Conclusion from Analysis

From the analyses in Steps 2 and 3, during micelle formation, enthalpy change is negative (system releases energy) and entropy change is positive (system gains disorder). Thus, the correct answer is (c).

Key concepts

The Role of Surfactants in Micelle Formation

Surfactants are fascinating molecules that play a crucial role in reducing surface tension between two liquids or a liquid and a solid. These molecules are often amphiphilic, which means they possess both a hydrophilic (water-attracting) head and a hydrophobic (water-repelling) tail.

When dissolved in water, surfactant molecules arrange themselves in a way that reduces the unfavorable interactions between water molecules and their hydrophobic tails. This arrangement leads to the formation of structures called micelles. In these micelles, the hydrophobic tails come together in the interior, away from water, while the hydrophilic heads face outward, interacting with the surrounding water.

This self-assembly into micelles occurs because it minimizes the free energy of the system. At a certain concentration, known as the critical micelle concentration (CMC), the formation of micelles becomes favorable. This fascinating property of surfactants is widely utilized in cleaning products and in the formation of emulsions.

Understanding Enthalpy Change During Micelle Formation

Enthalpy change, represented by \(\Delta H\), refers to the heat change during a process at constant pressure. In the context of micelle formation, this change is often exothermic. This means the process releases energy.

As surfactant molecules come together to form micelles, new intermolecular interactions, such as van der Waals forces between the hydrophobic tails, are established. The formation of these new stable interactions releases energy, resulting in a negative enthalpy change.

• Exothermic processes, like micelle formation, help drive the reaction forward by making it energetically favorable.
• These interactions allow the surfactant molecules to settle into a lower-energy state, stabilizing the newly formed micelles.

Recognizing this aspect of enthalpy in micellar systems helps understand why such transformations are spontaneous under certain conditions.

Exploring Entropy Change in Micelle Formation

Entropy, symbolized by \(\Delta S\), is a measure of disorder or randomness in a system. During micelle formation, entropy change is typically positive.

Initially, water molecules are relatively ordered around the hydrophobic tails of individual surfactant molecules, as they attempt to shield themselves from the water. However, when micelles form, these water molecules are released from their ordered state, gaining freedom, and thus increasing entropy.

This increase in disorder is a key driver of the micelle formation process. A positive entropy change signifies that the system has become more random, which is favorable in terms of thermodynamics, as systems naturally progress towards greater disorder if allowed.

• Greater entropy corresponds to a higher degree of disorder.
• Micelle formation releases more water molecules into a less ordered state, contributing to increased system entropy.

This positive change in entropy complements the negative enthalpy change to make micelle formation a spontaneous process.

Hydrophobic Interactions in Micelle Formation

Hydrophobic interactions are a primary force driving the aggregation of surfactant molecules into micelles. This phenomenon occurs due to the water-repelling nature of hydrophobic tails present in surfactants.

In the aqueous environment, water molecules arrange themselves to minimize contact with hydrophobic tails. This arrangement is not energetically efficient, as it constraints the water molecules into a structured, ordered state. To alleviate this energy demand, the hydrophobic tails congregate together inside micelles to minimize their contact with water.

• This clustering results in a reduction of unfavorable water-tail interactions, contributing significantly to the stability of micelles.
• Hydrophobic interactions are non-covalent and play a vital role in biological systems, influencing protein folding and cellular membrane formation.

Through these interactions, micelle formation reduces the system's overall free energy, showcasing the elegance of molecular self-assembly.