Thickening High Concentration MgSO4 Solution

Regarding your question about using Hydroxyethyl Cellulose for thickening a 71% MgSO4 solution:

While Hydroxyethyl Cellulose is more tolerant to electrolytes than Carbomer or Acrylate Crosspolymer (like Pro Polymer), a concentration of 71% MgSO4 is an extremely high salt environment. It is possible that Hydroxyethyl Cellulose's thickening efficiency would be significantly reduced or unstable under such conditions.

Xanthan Gum, as previously recommended, is generally known for its excellent stability and thickening performance in very high salt concentrations, making it a more reliable option for your specific formula.

Regarding your concern about skin penetration: Both Hydroxyethyl Cellulose and Xanthan Gum are large polymer molecules and will not penetrate the skin. Your requirement for the thickener itself not to penetrate is met by both. The previous discussion about penetration enhancers related to helping the MgSO4 penetrate the skin, not the thickener.

In summary, while you could test Hydroxyethyl Cellulose, Xanthan Gum is more likely to successfully provide the desired slight thickening in your high-concentration MgSO4 solution due to its superior electrolyte tolerance.