Why does viscosity decrease when divalent cations are dissolved into aqueous solutions of sodium carboxymethylcellulose?

Sodium Carboxymethylcellulose (CMC/NaCMC/Aqualon) is a common product used in many instances for its unique properties, especially its ability to make solutions viscous. It is well known that the charge of metal ions directly affects CMC's viscosity. 


According to the pdf: [AQUALON Sodium Carboxymethylcellulose](https://encrypted.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjM4teHkejUAhVLChoKHTPCCYMQFggnMAA&url=https%3A%2F%2Fwww.researchgate.net%2Ffile.PostFileLoader.html%3Fid%3D551d8944f15bc717108b467d%26assetKey%3DAS%253A273748727402502%25401442278190421&usg=AFQjCNG-UpYNKzgD6Z7rs4C2nK9hI9_6CA) (ResearchGate), it states that: 



> 
> Compatibility of CMC with inorganic salt solutions depends largely on
>  the ability of the added cation to form a soluble salt of
>  carboxymethylcellulose.
> 
> 
> 


Then it proceeds with: 



> 
> As a general rule, monovalent cations from soluble salts of
>  carboxymethylcellulose, divalent cations are borderline, and trivalent
>  cations form insoluble salts.
> 
> 
> 


It then continues on to discuss the effect of this on the viscosity of aqueous solutions with these cations mixed with CMC, but doesn't explain why. 


It says that divalent cations (which are "borderline" yet still soluble into CMC solutions) decrease the viscosity of CMC solutions, yet monovalent cations (which fully dissolve) do not decease the solution's viscosity (or not to the same extent). Why is this? 


