Adding an amphiphilic material to a clear solvent is fundamentally different than adding a non-amphiphilic material to the same solvent. Use water as solvent. Adding a non-amphiphilic substance such as a salt (NaCl) results in a clear solution that has equal concentrations of sodium and chloride ions dissolved in it. Now consider an amphiphilic material such as sodium lauryl sulfate (SLS). After adding it you get a clear “solution”, but this time the concentration is different throughout the solvent. This is because the C12 alkyl groups contained in it are breaking the hydrogen bonds between water molecules. The high energy of the system causes the SLS to saturate the air/water interface (lowering surface tension) and then forms aggregates called micelles. The concentration at which micelles form is the “critical micelle concentration” (CMC). This phenomenon is shown in Figure 1. It is important to note that before the addition of SLS, the surface tension of water is approximately 72 dynes/cm, and after reaching the critical micelle concentration, its surface tension is 30 dynes/cm. Surfactants based on hydrophobic agents and oils with alkyl groups have surface tensions in the range of 30 dynes/cm to 35 dynes/cm. This is due to the presence of methylene groups (–CH2-) primarily in the molecules of such materials. Surfactants with siloxane groups as hydrophobes have lower surface tensions in the range of 20 dynes/cm2. This happens because the silicone hydrophobic groups are predominantly methyl (CH3-) present in the molecule. This observation is critical to understanding how silicone surfactants reduce surface tension in oil-based systems.
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