Methylamine and trimethylamine have gas-phase proton affinities of 899 and 942 kJ/mol respectively, so trimethylamine is the stronger base in the gas phase. In water the pKa values of the conjugate acids are 10.6 for $\mathrm{CH}{}_3\mathrm{NH}{}_3{}^{+}$ and 9.8 for $(\mathrm{CH}{}_3){}_3\mathrm{NH}{}^{+}$. The reversal in water is mainly because

1. A$(\mathrm{CH}{}_3){}_3\mathrm{NH}{}^{+}$ has only one $\mathrm{N}-\mathrm{H}$ for hydrogen bonding to water, so it loses more solvation stabilisation than $\mathrm{CH}{}_3\mathrm{NH}{}_3{}^{+}$
2. Btrimethylamine has no lone pair in water
3. Cthe $+I$ effect of methyl groups switches to $-I$ in water
4. Dwater is a stronger acid than the trimethylammonium ion