Below is the general formula for the Brij family of surfactants. CH,-(CH2)-(CHCH

Question

Below is the general formula for the Brij family of surfactants. CH,-(CH2)-(CHCHO-OH (a) Using the concept of the hydrophilic-lyophilic balance, briefly explain how the character of the surfactant will change by varying the values of n and m. (b) Calculate the HLB values for surfactants with values of Brij 52: n 16, m 2 and Brij 98: n 18, m = 20. (c) For oil-water emulsions which phase would you expect to be the continuous phase when using these surfactants. Give a reason for your choice. (d) The HLB value of 11.2 was required for the preparation of a stable oil-in-water emulsion. Calculate the proportions of Brij 52 and Br98 required to achieve a HLB value of 1 12.

Explanation / Answer

Answer (a): By increasing the value of "n" in the given surfactant, hydrophobic character will increase and hydrophilic character will decrease due to increase in the alkyl chain length [-(CH2)n-]. However, By increasing the value of "m" in the given surfactant, hydrophobic character will decrease and hydrophilic character will increase due to increase in the polar chain length [-(CH2CH2O)m-].

Answer (b): Use Griffin's Mathematical method for HLB value calculation as given below:

HLB = 20 x [Mh]/M where,  [Mh] = molecular weight of hydrophilic group;  [M] = molecular weight of whole molecule

i) HLB value of Brij52:

[Mh] = 88 [when m = 2; Molecular wieght of (CH2CH2O)2 is 88]

[M] = 344 [when n = 16 and m = 2; Molecular wieght of CH3(CH2)16(CH2CH2O)2OH is 344].

HLB of Brij52 = 20 x 88/344

HLB of Brij52 = 5.11

ii) HLB value of Brij98:

[Mh] = 88 [when m = 20; Molecular wieght of (CH2CH2O)20 is 881]

[M] = 344 [when n = 18 and m = 20; Molecular wieght of CH3(CH2)16(CH2CH2O)2OH is 1165].

HLB of Brij98 = 20 x 881/1165

HLB of Brij98 = 15.12

Answer (c): Emulsions can be of two types: i) water in oil (W/O type) and ii) oil in water (O/W type).

HLB system is particularly useful to identify surfactants for oil and water emulsification. if a surfactant has low HLB value than it is considered very oil soluble, while a surfactant with high HLB is considered to be water soluble. HLB in the range of 3.5 to 6.0, surfactants are more suitable for use in W/O emulsions. Surfactants with HLB values in the 8 to 18 range are most commonly used in O/W emulsions.

Hence, Brij52 with and HLB value 5.11 will be suitable for W/O emulsions in which oil is a contineous phase. However, Brij98 with and HLB value 15.12 will be suitable for O/W emulsions in which water is a contineous phase.

Answer (d):

% of Brij52 = 100 X (Target HLB - HLB of Brij98)/HLB of Brij52 - HLB of Brij98

% of Brij52 = 100 X (11.2 - 15.12)/5.11-15.12

% of Brij52 = 39.2 %

% of Brij98 = 100 X (Target HLB - HLB of Brij52)/HLB of Brij98 - HLB of Brij52

% of Brij98 = 100 X (11.2 - 5.11)/15.12-5.11

% of Brij98 = 60.8 %

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