surfactant activity and application

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Questions and Answers

What is the required HLB of the oil phase when combining soybean oil and cetyl alcohol?

8.75

11.50

10.00

9.13 (correct)

What does the variable Γ represent in Gibbs' equation?

Surface excess concentration (correct)

Surface tension

Thermodynamic temperature

Bulk concentration

What is the result of increasing surfactant concentration in a solution before reaching the critical micelle concentration (cmc)?

Increased surface tension

Significant fall in surface tension (correct)

Dynamic equilibrium of monomers

Formation of solid aggregates

What happens to surfactant molecules above the critical micelle concentration (cmc)?

They form micelles in dynamic equilibrium Signup and view all the answers

What is the bulk concentration denoted as in the Gibbs adsorption equation?

Cb Signup and view all the answers

What does the critical micelle concentration (cmc) indicate?

The concentration at which micelles start to form Signup and view all the answers

How is the available area per molecule calculated from surface excess concentration (Γ)?

1 divided by Γ multiplied by NA Signup and view all the answers

Which statement accurately describes surfactant behavior at the surface?

A significant proportion of surfactant remains in the bulk Signup and view all the answers

What is the primary reason hydrophobic groups tend to withdraw from the aqueous phase in surfactants?

They disrupt the structured arrangement of water molecules. Signup and view all the answers

Which factor determines the solubility of surfactants in an aqueous solution?

The ability of the polar group to hydrogen bond to water. Signup and view all the answers

How do surfactant molecules behave when added to water?

They orientate at the surface or interfaces. Signup and view all the answers

What occurs to the surface tension of a liquid when surfactants are added?

Surface tension decreases due to the displacement of water molecules. Signup and view all the answers

Why are ionic and polar materials soluble in water?

They can hydrogen bond with water molecules. Signup and view all the answers

What is the effect of increasing the length of the hydrocarbon chain in surfactants?

It favors adsorption at surface/interfaces. Signup and view all the answers

Which of the following statements best describes the structure of water molecules?

They have a high degree of structure due to hydrogen bonding. Signup and view all the answers

What occurs when non-polar regions interact with water?

Water requires extra structured clusters to surround non-polar regions. Signup and view all the answers

What does a high HLB value indicate about a surfactant?

It is water soluble Signup and view all the answers

Which HLB range is associated with poor solubility?

1-3 Signup and view all the answers

How is the HLB of a surfactant calculated?

HLB = percentage of hydrophilic part / 5 Signup and view all the answers

What does a required HLB of 7 for soybean oil suggest about suitable emulsifiers?

Emulsifiers should have an HLB between 6 and 8. Signup and view all the answers

If the hydrophilic part of a non-ionic surfactant is calculated as 281 and the total mass is 422, what is the HLB value?

13.3 Signup and view all the answers

Which of the following is a characteristic of emulsifiers with an HLB of 8-10?

They facilitate o/w emulsification. Signup and view all the answers

In an oil phase containing 15% soybean oil and 5% cetyl alcohol, what is the weight percentage of soybean oil in the oil phase?

75% Signup and view all the answers

What happens to the contractile nature of a solution when surfactants are added?

It reduces surface tension. Signup and view all the answers

What occurs when the concentration of surfactants is too high?

Surfactant molecules become restrained by water structure. Signup and view all the answers

Which type of surfactant generally forms micelles at lower concentrations?

Non-ionic surfactants Signup and view all the answers

What is the effect of increasing the chain length of the hydrophobic group in surfactants?

Decreases cmc and increases micellar size. Signup and view all the answers

Which of the following has a higher critical micelle concentration (cmc)?

Non-ionic surfactants with shorter hydrocarbon chains Signup and view all the answers

What defines the shape of non-ionic micelles compared to ionic micelles?

Non-ionic micelles can be ellipsoidal in shape. Signup and view all the answers

What does an increase in the hydrophilic portion of a surfactant molecule result in?

Increase in cmc. Signup and view all the answers

How does the presence of bound counter ions affect ionic micelles?

They make ionic micelles spherical and bind around 70-80% of counter ions. Signup and view all the answers

What is the relationship between the aggregation number and cmc in non-ionic surfactants?

Higher aggregation number is associated with lower cmc. Signup and view all the answers

What effect does the addition of electrolytes have on ionic micelles?

Decreases micellar size and cmc Signup and view all the answers

At what condition do non-ionic surfactants typically become cloudy?

At their cloud point temperature Signup and view all the answers

What is the Krafft point related to regarding ionic surfactants?

It is the temperature above which the solubility of ionic surfactants increases significantly. Signup and view all the answers

How does the presence of bound counter ions influence ionic micelles?

They reduce the overall charge and exert a retardation force. Signup and view all the answers

What happens to the conductivity of ionic micelles during micellization?

Conductivity initially increases then may decrease due to drag effects. Signup and view all the answers

What role do larger mass counter ions play in micellar systems?

They can absorb into the micellar surface decreasing charge repulsion. Signup and view all the answers

What is the significance of the critical micelle concentration (cmc)?

It is where surfactant begins to saturate and micelles form. Signup and view all the answers

How does temperature affect the solubility of ethoxylate surfactants?

Solubility decreases with temperature rise. Signup and view all the answers

Study Notes

Surfactant Activity & Application

Surfactants are amphipathic molecules, possessing both hydrophobic and hydrophilic groups.
Hydrophobic groups (typically carbon chains) repel water, while hydrophilic groups (often ionic or polar) attract water.
Water's structured arrangement due to hydrogen bonding can be disrupted by solutes.
Surfactants compensate for this disruption, allowing ionic and polar materials to readily dissolve in water.
Non-polar substances do not readily dissolve in water as there is no compensating force for disruption of water structure.
Solubility with surfactants depends on the polar group's effectiveness in bonding with water to counteract hydrophobic repulsion.
Surfactant molecules orient at interfaces, with hydrocarbon chains facing away from the aqueous phase.
The longer the surfactant hydrocarbon chain, the more energetically favorable adsorption at surfaces/interfaces is. This leads to higher surfactant concentration at the surface.
Surfactant adsorption reduces surface tension.
The HLB (hydrophilic-lipophilic balance) value indicates the surfactant's relative affinity for oil or water; low HLB suggests greater oil solubility.
Different HLB ranges correspond to different surfactant applications; e.g., HLB values ≥13 are for solubilization, while 8-10 suggest o/w emulsification.
The HLB system relates all fats and oils to a required HLB value.
Appropriate emulsifiers or blends thereof are selected based on the required HLB of the oil phase.
Gibbs derived an equation to calculate the extent of surfactant adsorption.
Surface excess concentration accounts for surfactant in excess of that present if the bulk phase extended to the surface.
Calculation of the available surface area per molecule involves considerations of the surface excess concentration, Avogadro's number and bulk concentration.
Although some surfactant is at the surface, most is within the bulk of the solution.
The addition of surfactants leads to a significant reduction in surface tension, followed by a less pronounced decrease as the critical micelle concentration (cmc) is reached.
Micelles are aggregates of surfactant molecules at concentrations above the cmc.
The formation of micelles occurs to reduce the surface area and thus the free energy of the system.
The structure of the hydrophobic group affects cmc and micelle size; longer chains result in a decreased cmc and a larger micelle size.
The hydrophilic group also impacts cmc; a larger hydrophilic group results in an increased cmc.
Non-ionic surfactants have lower cmc and higher aggregation numbers than ionic surfactants.
The addition of electrolytes to ionic micelles decreases micelle size, reducing the repulsive forces between head groups.
At higher temperatures, solutions of some surfactants can become cloudy; conversely, solubility can decrease, this may be due to micellization.
The Krafft point represents the temperature above which the solubility of ionic surfactants significantly increases.

Micelle Formation

Micelle formation minimizes the system's free energy.
Water structuring around the hydrophobic group is energetically unfavorable.
Molecules self-aggregate for optimal packing of the hydrophobic groups, in minimizing contact with water.
Molecules in micelles exhibit freedom of movement compared to those constrained by water structure.
Micelle formation is dynamic; micelles continually break down into monomers and reform.
Non-ionic surfactants form micelles at lower concentrations than ionic surfactants.
Ionic micelles are spherical, with approximately 70-80% counter ions bound to them.
Non-ionic micelles are much larger than ionic micelles and are often asymmetrical, typically ellipsoidal in shape.
Oxyethylene groups in some non-ionic surfactants hydrate and entrap water within the micelles.

Factors Affecting CMC & Micelle Size

Chain length: increase in chain length leads to a decrease in CMC and an increase in micelle size.
Hydrophilic group: an increase in the hydrophilic portion leads to an increase in cmc.

CMC Determination

Plot conductivity versus surfactant concentration.
Two linear phases are observed, with a less steep second phase above the critical micelle concentration (cmc).

Electrical Conductivity of Ionic Micelles

Movement of ions is affected by solvent drag, and micellization reduces drag, meaning it should increase conductance.
Ionic atmosphere around micelles breaks down water structuring, meaning it should decrease conductance.
Bound counter-ions reduce overall charge, reducing the flow of counter ions and affecting conductance.

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