A method for removing a solute from an aqueous solution or for concentrating a dilute aqueous solution via modulating the ionic strength of an aqueous solution, the method comprising: (a) contacting a switchable water comprising water and an additive switchable between a first form and a second form, wherein said second form of the additive is water-miscible or water soluble and includes at least one ionized functional group that is neutral in said first form of the additive, such that switching the additive from the first form to the second form increases the ionic strength of the switchable water, with an ionizing trigger to ionize at least one functional group in the additive and thereby increase the ionic strength of the switchable water; wherein the additive is a polymer, and the polymer has at least one amine nitrogen-containing functional group that is sufficiently basic to be protonated when the polymer is in the presence of the ionizing trigger; and wherein the ionizing trigger is: a Bronsted acid, such as hydrochloric acid, formic acid, sulfuric acid or carbonic acid; or a gas that when dissolved in water reacts with water to liberate hydrogen ions, such as CO 2, NO 2, SO 2, SO 3, CS 2, or COS; (b) providing a semi-permeable membrane that is selectively permeable for water and has on one side a draw solution that comprises said switchable water, wherein the step of contacting the switchable water with the ionizing trigger to switch the additive to the second form is performed before or after association of the switchable water with the semi-permeable membrane to increase the osmotic pressure of the draw solution; (c) contacting the semi-permeable membrane with a feed stream of the aqueous solution to permit water to flow from the aqueous solution through the semi-permeable membrane into the increased ionic strength draw solution; and (d) optionally, removing the additive from the resulting diluted draw solution. (a) contacting a switchable water comprising water and an additive switchable between a first form and a second form, wherein said second form of the additive is water-miscible or water soluble and includes at least one ionized functional group that is neutral in said first form of the additive, such that switching the additive from the first form to the second form increases the ionic strength of the switchable water, with an ionizing trigger to ionize at least one functional group in the additive and thereby increase the ionic strength of the switchable water; wherein the additive is a polymer, and the polymer has at least one amine nitrogen-containing functional group that is sufficiently basic to be protonated when the polymer is in the presence of the ionizing trigger; and wherein the ionizing trigger is: a Bronsted acid, such as hydrochloric acid, formic acid, sulfuric acid or carbonic acid; or a gas that when dissolved in water reacts with water to liberate hydrogen ions, such as CO 2, NO 2, SO 2, SO 3, CS 2, or COS; a Bronsted acid, such as hydrochloric acid, formic acid, sulfuric acid or carbonic acid; or a gas that when dissolved in water reacts with water to liberate hydrogen ions, such as CO 2, NO 2, SO 2, SO 3, CS 2, or COS; (b) providing a semi-permeable membrane that is selectively permeable for water and has on one side a draw solution that comprises said switchable water, wherein the step of contacting the switchable water with the ionizing trigger to switch the additive to the second form is performed before or after association of the switchable water with the semi-permeable membrane to increase the osmotic pressure of the draw solution; (c) contacting the semi-permeable membrane with a feed stream of the aqueous solution to permit water to flow from the aqueous solution through the semi-permeable membrane into the increased ionic strength draw solution; and (d) optionally, removing the additive from the resulting diluted draw solution.