表面張力は液体分子同士の引力の平均値と考えると理解しやすいと思います。非極性な炭化水素ならファンデアワールス力だけの結合ですし、水の場合は更に強い水素結合が加わります。有機化合物の溶解（水和）によって水分子同士の水素結合が一部切れますから、その分平均値としての分子同士の相互作用が減って表面張力が弱くなることになります。
I think it is more understandable if you consider the surface tension as an average value of attraction between the liquid molecules. A nonpolar hydrocarbon is a bond of Van der Waals force only, and a stronger hydrogen bond will be added in case it is water. Since a part of the hydrogen bond between water molecules will be cut by the dissolution (hydration) of the organic compounds, the coaction between the molecules as an average will decrease and the surface tension will be weakened.
